action 3: report on waste treatment technologies
TRANSCRIPT
NTUA
Establishment of Waste Network for sustainable solid waste management
planning and promotion of integrated decision tools in the Balkan Region
(BALKWASTE)
LIFE07/ENV/RO/686
ACTION 3: REPORT ON WASTE TREATMENT TECHNOLOGIES
REPORT ON WASTE TREATMENT TECHNOLOGIES
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INDEX
TABLES .................................................................................................................... 5
FIGURES ................................................................................................................... 7
PICTURES ................................................................................................................ 9
1. BIOLOGICAL TREATMENT................................................................................ 10
1.1. COMPOSTING ............................................................................................................ 10
1.1.1. Introduction to composting process ........................................................................................ 10
1.1.2. Biology of composting ............................................................................................................. 10
1.1.3. Factors affecting the composting process ............................................................................. 12 1.1.3.1. Feedstock and nutrient balance............................................................................................................... 12 1.1.3.2. Particle size ............................................................................................................................................ 15 1.1.3.3. Moisture content ..................................................................................................................................... 15 1.1.3.4. Oxygen flow ........................................................................................................................................... 16 1.1.3.5. Temperature ........................................................................................................................................... 16
1.1.4. Composting Systems Classification ....................................................................................... 19 1.1.4.1. Windrow Systems ................................................................................................................................... 19 1.1.4.2. Turned Windrow System ......................................................................................................................... 23 1.1.4.3. In-Vessel Systems .................................................................................................................................. 30
1.1.5. Post-Processing....................................................................................................................... 37
1.1.6. Mass and energy balances ..................................................................................................... 37
1.1.7. Market potential for products .................................................................................................. 39 1.1.7.1. Limitations .............................................................................................................................................. 40
1.1.8. Environmental impacts ............................................................................................................ 45
1.1.9. Economic data ......................................................................................................................... 46
1.2. ANAEROBIC DIGESTION .............................................................................................. 47
1.2.1. Introduction to anaerobic digestion process .......................................................................... 47
1.2.2. Biology of anaerobic digestion ................................................................................................ 48
1.2.3. FEEDSTOCK OF ANAEROBIC DIGESTION ..................................................................... 50
1.2.4. Procedures of Anaerobic Waste Fermentation ...................................................................... 52 1.2.4.1. Delivery and Storage .............................................................................................................................. 54 1.2.4.2. Pre-processing ....................................................................................................................................... 55 1.2.4.3. Anaerobic Fermentation .......................................................................................................................... 55 1.2.4.4. Post-processing ...................................................................................................................................... 56
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1.2.5. Process Engineering of Anaerobic Fermentation of Biowaste ............................................. 56 1.2.5.1. Dry and Wet Fermentation ...................................................................................................................... 58 1.2.5.2. Continuous and Discontinuous Operation ................................................................................................ 60 1.2.5.3. Thermophilic and Mesophilic Operation ................................................................................................... 61 1.2.5.4. Agitation ................................................................................................................................................. 62
1.2.6. Anaerobic Digestion Products ................................................................................................ 62 1.2.6.1. Biogas .................................................................................................................................................... 62 1.2.6.2. Digestate ................................................................................................................................................ 65
1.2.7. Market potential for products .................................................................................................. 66
1.2.8. Mass and energy balances ..................................................................................................... 68
1.2.9. Parameters effecting anaerobic digestion process ............................................................... 71 1.2.9.1. Organic Loading Rate ............................................................................................................................. 71 1.2.9.2. Biomass Yield......................................................................................................................................... 72 1.2.9.3. Specific Biological Activity ....................................................................................................................... 75 1.2.9.4. Hydraulic Retention Time and Solids Retention Time ............................................................................... 76 1.2.9.5. Start-Up Time ......................................................................................................................................... 77 1.2.9.6. Microbiology ........................................................................................................................................... 78 1.2.9.7. Environmental Factors ............................................................................................................................ 78 1.2.9.8. Reactor Configuration ............................................................................................................................. 82
1.2.10. Environmental impacts .......................................................................................................... 82
1.2.11. Economic data ....................................................................................................................... 83
1.3. MECHANICAL BIOLOGICAL TREATMENT (MBT) .............................................................. 86
1.3.1. Mechanical sorting component ............................................................................................... 86
1.3.2. Biological processing compartment ........................................................................................ 87
1.3.3. Mass and energy balances ..................................................................................................... 88
1.3.4. Market potential for products .................................................................................................. 91
1.3.5. Environmental impacts ............................................................................................................ 91
1.3.6. Economic data ......................................................................................................................... 92
1.4. CASE STUDIES OF BIOLOGICAL TREATMENT SYSTEMS IN THE TARGET AREA .................. 93
1.4.1. General ............................................................................................................................ 93
1.4.2. Mechanical Biological Treatment Plant in the West Attica Region, Greece ................ 94
1.4.3. Mechanical Biological Treatment Plant in Chania, Greece .......................................... 96
1.4.4. Mechanical Biological Treatment Plant in Kalamata, Greece ...................................... 97
1.4.5. Composting Plant for Solid Waste at the Landfill Site in Piatra Neamt, Romania ...... 97
1.4.6. Composting Plant in Vrhnika, Slovenia ......................................................................... 98
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1.4.7 Composting Plant in Puconci, Slovenia......................................................................... 99
THERMAL TREATMENT TECHNOLOGIES ......................................................... 100
2.1. GENERAL ............................................................................................................. 100
2.2. INCINERATION .......................................................................................................... 107
2.2.1. General................................................................................................................................... 107
2.2.2. Types of incinerators ............................................................................................................. 111
2.2.3. Air emissions.......................................................................................................................... 123
2.2.4. Wastewater ............................................................................................................................ 133
2.2.5. Solid residues ........................................................................................................................ 135
2.2.6. Mass and energy balances ................................................................................................... 137
2.2.7. Market potential for products ................................................................................................ 137
2.2.8. Environmental impacts .......................................................................................................... 138
2.2.9. Economic data ....................................................................................................................... 139
2.2.10. Applicability in the target area............................................................................................. 139
2.3. GASIFICATION.......................................................................................................... 146
2.3.1. General................................................................................................................................... 146
2.3.2. Feedstock............................................................................................................................... 155
2.3.3. Gasifier ................................................................................................................................... 155
2.3.4. Oxygen plant .......................................................................................................................... 156
2.3.5. Gas Clean-Up ........................................................................................................................ 156
2.3.6. Mass and energy balances ................................................................................................... 156
2.3.7. Market potential for products ................................................................................................ 159
2.3.8. Environmental impacts .......................................................................................................... 159
2.3.9. Economic data ....................................................................................................................... 160
2.3.10. Applicability in the target area............................................................................................. 160
2.4. PYROLYSIS .............................................................................................................. 164
2.4.1. General................................................................................................................................... 164
2.4.2. Mass and energy balances ................................................................................................... 166
2.4.3. Market potential for products ................................................................................................ 167
2.4.4. Environmental impacts .......................................................................................................... 167
2.4.5. Economic data ....................................................................................................................... 167
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2.4.6. Applicability in the target area............................................................................................... 168
2.5. PLASMA GASIFICATION TECHNOLOGY ........................................................................ 169
2.5.1. General................................................................................................................................... 169
2.5.2. Mass and energy balances ................................................................................................... 174
2.5.3. Market potential for products ................................................................................................ 174
2.5.4. Environmental impacts .......................................................................................................... 175
2.5.5. Economic data ....................................................................................................................... 175
2.5.6. Applicability in the target region ............................................................................................ 175
REFERENCES ...................................................................................................... 183
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TABLES
Table 1: Indicative C/N ratios of various organic waste streams .................................. 14
Table 2: Indicative relationships between temperature and time duration during composting for the sanitization of the final product ........................................................................ 18
Table 3: Heavy metal limits for European compost standards (mg/kg dm)................... 41
Table 4: Organic pollutants standards for compost and stabilised biowaste ................ 44
Table 5: Impurities standards for compost and stabilised biowaste ............................. 44
Table 6: EU requirements on pathogens/weeds in compost ........................................ 45
based on the EC eco-label ......................................................................................... 45
Table 7: Characteristics of anaerobic waste treatments (Rilling, 1994) ........................ 57
Table 8: Comparison of one- and two-stage processes .............................................. 58
Table 9: Comparison of wet and dry fermentations ..................................................... 60
(Jördening and Winter, 2005). .................................................................................... 60
Table 10: Comparison of continuous and discontinuous feed...................................... 61
Table 11: Comparison of mesophilic and thermophilic process operation (Jördening and Winter, 2005) ............................................................................................................. 62
Table 12: Mean composition and specific yields of biogas in relation to the kind of substances degraded (Rilling, 1994) ............................................................................................. 63
Table 13: Energy input and output from various biomass resources, (EUBIA) ............. 70
Table 14: Biomass yield coefficients for different biological treatment processes and stages (Young and McCarty,1969; Henze and Harremoes, 1983; van Haandel and Lettinga, 1994) 73
Table 15: Biomass yield coefficients for different types of substrate (Pavlostathis and Giraldo, 1991) 73
Table 16: HRTs of anaerobic systems needed to achieve 80% COD removal efficiency at temperature >20 C (Van Haandel and Lettinga, 1994) ................................................ 77
Table 17: Inhibition of anaerobic digestion by heavy metals (Konzell-Katsiri and Kartsonas,1986)......................................................................................................... 81
Table 18: Operation incineration facilities in the USA ................................................ 115
Table 19: Electrical Energy production from Renewable Energy Sources in USA in 2002 (except hydroelectric) (DOE-EIA, Annual Energy Outlook 2002) ............................... 116
Table 20: Daily average values of air emission limit values ....................................... 123
(Directive 2000/76/EC on the incineration of waste) .................................................. 123
Table 21: Half-hourly average values of air emission limit values .............................. 124
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(Directive 2000/76/EC on the incineration of waste) .................................................. 124
Table 22: Average values over the sample period of a minimum of 30 minutes and a maximum of 8 hours ................................................................................................. 124
(Directive 2000/76/EC on the incineration of waste) .................................................. 124
Table 23: Limit values of CO concentrations ............................................................. 125
(Directive 2000/76/EC on the incineration of waste) .................................................. 125
Table 24: Existing technologies for the management and treatment of gaseous pollutants 129
Table 25: Emission limit values for discharges of waste water from the cleaning of exhaust gases 134
(Directive 2000/76/EC on the incineration of waste) .................................................. 134
Table 26: Summary of quantities of solid waste, wastewater and gases produced during the operation of an incineration plant .............................................................................. 136
Table 27: Synopsis on data on incineration units in Romania .................................... 140
Table 28: Summary of solid waste, wastewater and air emissions generated during the operation of a gasification unit .................................................................................. 152
Table 29: Commercial Plasma Waste Processing Facilities (Circeo, 2007) ............... 173
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FIGURES
Figure 1: Composting process flow chart (Wastesum, 2006) ....................................... 11
Figure 2: The main stages of the composting process (Wastesum, 2006) ................... 12
Figure 3: Schematic diagram of aerated static pile composting ................................... 20
(Diaz et al., 2002) ....................................................................................................... 20
Figure 4: Approximate dimensions for an aerated static pile (Diaz et al., 2002) ........... 21
Figure 5: Examples of a self-propelled and a towed windrow turners .......................... 27
(Diaz et al., 2002; Recycle & Composting Equipment Pty Ltd, 2010) ........................... 27
Figure 6: Schematic diagram of vertical plug-flow reactors ......................................... 32
Figure 7: Typical examples of Channels or Trenches (Turovskiy and Mathai, 2006; Diaz et al., 2002) 33
Figure 8: Typical mass flow diagram of composting .................................................... 37
Figure 9: Heavy metals limit values for compost in European countries ...................... 42
Figure 10: Anaerobic digestion flow chart (Wastesum, 2006) ...................................... 47
Figure 11: The stages of anaerobic digestion (Wastesum, 2006) ................................ 50
Figure 12: Suitability of waste for aerobic composting and anaerobic digestion (Kern et al., 1996) 52
Figure 13: Possible treatment steps used in anaerobic digestion process of biodegradable organic waste (Rilling, 1994) ...................................................................................... 54
Figure 14: Capacity Range of engines in relation to their electrical efficiency .............. 68
Figure 15: Typical mass balance for an anaerobic digestion system ........................... 69
(Ostrem K., 2004) ...................................................................................................... 69
Figure 16: Typical energy balance for an anaerobic digestion system (Ostrem K., 2004)71
Figure 17: (a) Capital cost and (b) M&O costs curves for European MSW digesters (CIWMB, 2008) 84
Figure 18: Mechanical Biological Treatment flow chart ............................................... 86
Figure 19: Schematic presentation of inputs and outputs of a typical mechanical sorting component with aerobic digestion (Juniper, 2006) ...................................................... 89
Figure 20: Schematic presentation of inputs and outputs of a typical mechanical sorting component with anaerobic digestion ........................................................................... 90
Figure 21: Schematic presentation of inputs and outputs of a typical mechanical sorting component with biodrying ........................................................................................... 90
Figure 22: MBT plant in Ano Liosia ............................................................................. 95
Figure 23: MBT plant in Chania, Crete........................................................................ 96
Figure 24: MBT plant in Kalamata, Peloponnese ........................................................ 97
Figure 26: Composting plant in Vrhnika, Slovenia ....................................................... 99
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Figure 27: Composting plant in Puconci, Slovenia ...................................................... 99
Figure 28: Pyramid of the priorities in the waste management sector ........................ 100
Figure 29: Management practices for municipal waste in the EU countries (Eurostat 2008) 101
Figure 30: Incineration process flow chart................................................................. 109
Indicative incineration facilities operating at European level can be seen in the photos below. 110
Figure 31: Diagrammatic configuration of incineration plant (with energy recovery) in Paris 111
Figure 32: Typical mass-fired waste incineration plant .............................................. 112
(with energy production) ........................................................................................... 112
Figure 33: Typical RDF-fired incineration facility ....................................................... 114
Figure 34: Three types of incinerators: (a) fixed grate (left), (b) rotary kiln (middle), (c) fluidized bed (right) (Finbioenergy, 2006).................................................................. 117
Figure 35: Dioxin emissions in USA (Themelis & Gregory 2002) ............................... 126
Figure 36: Dioxins emission in the USA (Deriziotis, 2004). ........................................ 126
Figure 37: Cyclones (left), electrostatic precipitators (middle) & bagfilters (right) ....... 130
Figure 38: Typical bagfilters ..................................................................................... 130
Figure 39: Typical Electrostatic precipitator .............................................................. 131
Figure 40: dry or semi-dry absorption towers (scrubbing) .......................................... 133
Figure 41: Gasification process flow chart ................................................................ 147
Figure 42: Process for converting waste into energy ................................................. 148
(Vasudevan & Mathew 2007) ................................................................................... 148
Figure 43: Vertical steady bed gasification plants ..................................................... 149
Figure 44: Horizontal steady bed gasification plants ................................................. 150
Figure 45: ITI gasification plant flow diagram ............................................................ 154
Figure 46: Schematic presentation of inputs and outputs of a typical gasification process 157
Figure 47: Mass and energy balance of the ITI gasification plant .............................. 158
Figure 48: Flow diagram of the EfW plant in Celje .................................................... 162
Figure 49: Process scheme of the EfW plant in Celje................................................ 163
Figure 50: Energy and mass balance of the EfW plant in Celje ................................. 163
Figure 51: Pyrolysis process flow chart ..................................................................... 166
Figure 52: Plasma gasification process flow chart ..................................................... 170
Figure 53: Plasma torch operation ............................................................................ 172
Figure 54: Plasma Gasification / Vitrification Process ............................................... 178
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PICTURES
Picture 1: MSW incineration plant in Amsterdam ...................................................... 110
Picture 2: MSW incineration plant in Brescia............................................................. 110
Picture 3: MSW incineration plant in Vienna ............................................................. 110
Picture 4: Incineration Plant in Zorbau, Germany for municipal solid waste and industrial waste 111
Picture 5: Incineration plant in Thun, Switzerland for municipal solid waste and dewatered sludge 111
Picture 6: MSW gasification plant in Chiba (Japan) ................................................... 151
Picture 7: Celje Waste to Energy CHP Plant ............................................................. 161
Picture 8: Molten slag pouring from plasma waste gasification reactor (Pyrogenesis Inc, Montreal, Canada) ................................................................................................... 171
Picture 9: Final inert slag residue can be used in construction applications ............... 171
Picture 10: General view of the demonstration gasification / vitrification unit ............. 176
Picture 11: Another general view of the demonstration gasification / vitrification unit . 177
Picture 12: Feeding system ...................................................................................... 177
Picture 13: Gasification / vitrification furnace............................................................. 178
Picture 14: Cyclone Picture 15: Secondary Combustion Chamber ........... 180
Picture 16: Quench Vessel Picture 17: Scrubber ........................ 181
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1. BIOLOGICAL TREATMENT
1.1. COMPOSTING
1.1.1. Introduction to composting process
Composting is defined as the aerobic, or oxygen requiring process during which
the organic matter is decomposed by micro-organisms under controlled conditions
to a biologically stable end product. During composting the microorganisms
consume oxygen for the bio-oxidation of the organic matter resulting in the
generation of heat, carbon dioxide and water vapor, which are released into the
atmosphere (Ipek et al., 2002; Epstein, 1997). At the same time, the volume and
mass of the organic raw material is reduced significantly transforming it into a
stable organic final product which can be used as soil conditioner, improver as well
as for land reclamation (Hogg et al., 2009; Epstein, 1997; Engeli et al., 1993; Carry
et al., 1990; Toffey, 1990). The rate of the organic matter decomposition depends
upon the evolution of the environmental conditions (e.g. temperature, moisture,
oxygen) which regulate the growth of aerobic micro-organisms. Therefore,
composting is the “controlled” aerobic biodegradation of most organic (biologically
derived carbon-containing) solid matter meaning that the environmental conditions
are controlled throughout the process. In that way composting differentiates from
the decomposition which occurs in nature (Gidarakos, 2007). Nevertheless, the
biochemical process in composting and in the natural decomposition of the organic
matter is the same.
1.1.2. Biology of composting
The composting process can generally be divided into two major stages. The first
stage comprises of the ‘‘active phase’’ of the process which mainly involves the
development of bio-oxidation reactions. Therefore, the readily available organic
matter is used as energy source by microorganisms for their metabolic activities.
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The second phase of the composting process, known as ‘‘curing phase’’, involving
the production of organic macromolecules humus-like substances for the formation
of mature compost (Cooperband, 2000). All reactions are based on numerous
biological, thermal and physicochemical phenomena and involve oxygen
consumption, as well as heat, water and carbon dioxide production. A schematic
representation of the composting process is shown in Figure 1.
Figure 1: Composting process flow chart (Wastesum, 2006)
In the process of composting, presented in Figure 1, microorganisms decompose
the biodegradable organic fraction in order to produce carbon dioxide, water, heat,
and humus the biological stable organic end product. Considering that the aerobic
degradation is carried out under optimal conditions, than the composting process
can be classified into through three separate phases (Figure 2) which may have
considerable overlap based on temperature gradients and differential temperature
effects on microorganisms. The first phase incorporates a mesophilic or moderate-
temperature phase (<40-45oC), which has a duration of a couple of days. The
second stage, named thermophilic phase (>40-45oC), involves the development of
Feedstock (organic matter including carbon,
chemical energy, protein, nitrogen)
----------------------------------- Minerals (including nitrogen and other
nutrients) -----------------------------------
H2O ----------------------------------
Micro-organisms
Organic matter
Minerals
H2O
Micro-organisms
Finished-compost
Oxygen O2
H2O Raw Material
Compost site
Heat CO2
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elevated-temperatures due to enhanced decomposition of the organic compounds
which can last from a few days to several weeks depending on the availability of
organic matter to be exposed to aerobic degradation. The last stage includes the
cooling and maturation phase which results to the biological stabilisation of the
end product. The factors affecting the composting process include: the physical
and chemical properties of the raw material, the level of oxygen, the moisture
content, the temperature and the time over which the composting process takes
place.
Figure 2: The main stages of the composting process (Wastesum, 2006)
1.1.3. Factors affecting the composting process
The main parameters which regulate the composting process include: the physical
and chemical properties of the raw material, the level of oxygen, the moisture
content, the temperature and the particle size of the substrate.
1.1.3.1. Feedstock and nutrient balance
In the composting process it is essential to determine the nutrient content of the
feedstock, since aerobic micro-organisms responsible for the biodegradation of the
organic raw material require various nutrients in order to grow. The main nutrients
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involved are the following in a decreasing order of importance: carbon (C),
nitrogen (N), phosphorous (P) and potassium (K) (Gajalakshmi and Abbasi, 2008).
Apart from micro-organisms growth, nitrogen, potassium and phosphorous are
also primary nutrients for plants and their concentrations influence the quality of
the composted organic material. Of the nutrient elements required for microbial
decomposition, carbon and nitrogen (C:N ratio) are considered to be the most
important, since the majority of organic materials contain ample quantities of
nutrients (EA, 2001). Therefore, the amounts of carbon and/or nitrogen are the
substances most likely to affect the composting process by their presence in
insufficient or excessive quantities. Carbon is both the energy source and the
basic microbial building block of microorganisms, whereas nitrogen is a crucial
component of proteins, nucleic acids, amino acids, enzymes and co-enzymes
needed for microbial cell growth (Gajalakshmi and Abbasi, 2008). Considering the
above, the nutritional balance during composting is mainly de ned by the carbon
to nitrogen ration i.e. C/N ratio. According to literature the optimum C/N ratio for
composting is in the range 25–35 (Gaur, 2000; Golueke, 1992), because it is
considered that the microorganisms require 30 parts of C per unit of N (Alexander
1977; Bishop and Godfrey, 1983). High C/N ratios make the process very slow as
there is an excess of carbonated degradable substrate for the microorganisms
(Bernal et al., 1998; Verdonck, 1988). On the other hand, in low C/N ratios there is
an excess of N in the organic material which in turn provides excess of inorganic N
due to the nitrification process leading to ammonia losses through volatilisation or
nitrogen losses by leaching from the composting mass (Pagans et al., 2005;
Sanchez-Monedero et al., 2001; Reddy et al., 1979). Controlling the C/N ratio,
when mixing different organic materials, is crucial for the successful
implementation of the composting process. In general, "green" organic materials
(e.g., grass clippings, food scraps, manure), contain large amounts of nitrogen,
whereas "brown" organic materials (e.g. dry leaves, wood chips, branches),
contain large amounts of carbon but little nitrogen. Therefore a proper balance of
"green" and "brown" organic waste shall result in appropriate C/N ratios for the
initiation of the composting process. Indicative C/N ratios of various organic waste
are presented in Table 1.
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Table 1: Indicative C/N ratios of various organic waste streams
organic waste
C/N Diaz et
al. (2002)
Diaz and Savage (2007)
Trautmann and Krasny
(1998)
IWMI (2003)
Cornstalks 60-73 - -
Fruit waste 20-49 - 20-50
Rice hulls 113-
1120 - -
Vegetable waste 11-13 - 10-20 13
Poultry litter (broiler) 12-15 15 - 10-18
Cattle manure 11-30 18 -
Horse manure 22-50 25 20-50
Garbage (food waste) 14-16 - 15 10-16
Paper (from domestic
refuse) 127-178 - 100-200
Newspapers - - 400-900
Refuse 34-80 - - 23-66
Sewage sludge 5-16 11 - 6-10
Primary sludge - - - 7-11
Activated sludge - 6 - 6-8
Grass clippings 9-25 - 10-25
Leaves 40-80 - 40-80
Shrub trimmings 53 -
Tree trimmings
Bark
- hardwood
- softwood
16
-
-
-
-
-
-
100-400
100-1200 170-500
Wood chips or shavings
- hardwood
- softwood
-
-
-
-
450-800
200-1300
Sawdust 200-750 200-500 200-750
Straw - 128-150 50-150
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1.1.3.2. Particle size
The particle size of the substrate and more specifically the surface area of the
organic material exposed to microorganisms is another factor which affects the
rate of the composting process. The raw material which is grinded, chipped and/or
shredded acquire reduced particle size or otherwise increased surface area on
which the microorganism can feed thus waste is degraded more rapidly (EA,
2001). In addition, smaller particles also produce a more homogeneous mixture
and improve substrate insulation which promotes the maintenance of optimum
temperatures (O’Leary and Walsh, 1995). However, if the particles are too small,
they might prevent air diffusion through the substrate (Gidarakos, 2007).
According to Diaz et al. (2002), an average particle size of 10mm to 50mm
generally produces the best results. However, certain composting methods that do
not include a turning process require a more robust physical structure to resist
settling (e.g. due to gravity and biodegradation process), so larger particles are
necessary (greater than 50mm) (Diaz and Savage, 2007). GTZ (2000)
recommends chopping all materials to be composted to the length of about 50-
100mm, whereas Obeng and Wright (1987) reported that typical particle sizes
should be approximately 10mm for forced aeration composting and 50mm for
passive aeration and windrow composting.
1.1.3.3. Moisture content
Moisture supports the metabolic and biodegradation processes of the micro-
organisms, since water is the medium for biochemical reactions, transportation of
nutrients and allows the microorganisms to move about (Gajalakshmi and Abbasi,
2008). Generally, the ideal moisture content is considered to be between 40% and
65% for the optimal biodegradation of the raw material. However, the optimal
moisture level is depended upon the composted material and more specifically on
its porosity (Diaz and Savage, 2007). Organic mix with a low porosity requires
higher moisture content than a substrate with a higher porosity level (Diaz and
Savage, 2007). Moisture content which is lower or higher than the optimum range
results in the inhibition of the microbial activity due to early dehydration and the
formation of anaerobic conditions respectively (Gajalakshmi and Abbasi, 2008; de
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Bertoldi et al., 1983). When the moisture content exceeds 70%, O2 movement is
inhibited and the process tends to become anaerobic because the air spaces of
the substrate are filled with water obstructing the sufficient oxygen diffusion within
the organic mass (Tiquia, et al., 2002, 1996). On the other hand, if the moisture
content is lower than required, the microorganisms’ growth and subsequently the
decomposition rate of organic matter are significantly reduced creating a final
product that is physical but not biologically stabilized (Diaz and Savage, 2007a; de
Bertoldi et al., 1983).
1.1.3.4. Oxygen flow
The oxygen that is required for the composting process is essential for the aerobic
metabolism and respiration by the microorganisms, but also for the bio-oxidation of
the organic molecules present in the substrate. Oxygen consumption during
composting is directly proportional to the microbial activity providing a direct
relationship between oxygen consumption, temperature, moisture and aeration
(EA, 2001). Therefore, aeration is a key factor for composting, since proper
aeration controls the temperature, removes excess moisture and CO2 and
provides O2 for the biological processes. According to Miller, (1992) the optimum
O2 concentration is between 15% and 20%. If there is insufficient oxygen, the
process can become anaerobic involving a different set of micro-organisms and
different biochemical reactions which result in the production of methane gas and
malodorous compounds, such as hydrogen sulfide gas and ammonia. Aeration of
the organic substrate is achieved through agitation, active aeration (air blowing)
and/or passive aeration (natural diffusion of air though negative pressure) (IWMI,
2003).
1.1.3.5. Temperature
Temperature is one of the main control parameters of the composting process and
constitutes a by-product of the microbial activity during organic matter
biodegradation. The importance of temperature monitoring lies on the fact that it
reflects the activity of microorganisms in the substrate and it represents an
indicator of the proper evolution and occurrence of the composting process (Diaz
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and Savage, 2007a). According to Hassen et al. (2001) substrate temperature
determines the rate at which biological processes take place and plays an
important role in the evolution and succession of the micro-organisms population.
Microorganisms require a certain temperature range for optimal activity. de
Bertoldi et al. (1983) state that the optimum temperature range for the
maximization of the decomposition rate is 40–65°C. According to Epstein (1997)
and Miller (1992), thermophilic microorganisms become less active at elevated
temperatures between 60-70°C and thus the microbial activity is reduced. At even
higher levels (>70°C) Mena et al. (2003), Fermor et al. (1989) and Finstein et al.
(1986) indicate that the microorganisms suffer the effects of high temperatures
(inactivation or elimination) and the process slows down. At these temperatures
many micro-organisms die or become dormant and the process effectively stops
until the micro-organisms can recover.
While high composting temperatures may inhibit or slow down the biodegradation
of organic waste, elevated temperatures are desirable for the destruction of
pathogens and weed seeds that may be contained in the substrate. According to
Hogg et al. (2002) the key parameter for the sanitization of the substrate is the
temperature-time regime. Indicative relationships between temperature and time
duration during composting for the sanitization of the final product is given in Table
2.
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Table 2: Indicative relationships between temperature and time duration during composting for the sanitization of the final product
Country Composting System
Temperature – time regime
Canada
(CCME,
2005)
Windrows 55°C for 15 days
Static piles 55°C for 3 days
In-vessel 55°C for 3 days
USA
(USEPA,
2003)
Class
compost
B Class compost
Windrows 55°C for 15
days
40°C for 5 days. For 4
hours during the 5 day
period, the temperature
must exceed 55°C
Static piles 55°C for 3
days
40°C for 5 days. For 4
hours during the 5 day
period, the temperature
must exceed 55°C
In-vessel 55°C for 3
days
40°C for 5 days. For 4
hours during the 5 day
period, the temperature
must exceed 55°C
UK
(DoE,
1996)
Windrows or Aerated
piled
The compost must be maintained at 40°C for
at least 5 days and for 4 hours during this
period at a minimum of 55°C within the body
of the pile followed by a period of maturation
adequate to ensure that the compost reaction
process is substantially complete
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1.1.4. Composting Systems Classification
The currently available compost systems can be generally classified into two
broad categories the “windrow” and the “in-vessel” composting systems. The main
feature of windrow technology is the accumulation and formation of the organic
substrate into piles. Typically, the piles are usually shaped into more or less
elongated windrows with specified width and height. With respect to the in-vessel
composting systems, the aerobic decomposition of the organic matter takes place
in a bioreactor. It should be noted that many of the current in-vessel systems
involve the parallel use of windrow systems for the curing and maturation phase of
the end product (Dziejowski and Kazanowska, 2002).
1.1.4.1. Windrow Systems
Windrow systems are further subdivided on the basis of the aeration method of the
substrate into “turned windrow” and “forced air windrow or static pile”. The
windrows may or may not be sheltered from the elements. In the windrow
composting process, the mixture to be composted is stacked in long parallel rows
or windrows. The cross section of the windrows is usually trapezoidal or triangular,
mainly depending on the characteristics of the equipment used for the agitation or
aeration of the piles. A variety of factors combine to determine the dimensions of
the area requirement. Among them are total volume of material to be
accommodated during all stages of the compost process, i.e., from the
construction of the windrows through disposal of the stored product, the
configuration of the windrows, space required for the associated materials
handling equipment and the maneuvering thereof, and the aeration system (forced
or turning).
1.1.4.1.1. Forced air windrow – Aerated static system
In a forced air windrow or aerated static composting system, air is either forced
upwards through the composting mass or is pulled downwards and through it
(Shammas and Wang, 2009). In both instances, the composting mass is not
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disturbed. The forced aeration composting systems usually involves a combination
of drawing air into and through the pile, followed by air forcing upward through the
pile. The air that leaves the substrate is either discharged directly into the
environment, or is forced through a cone-shaped biofilter (e.g. finished compost or
other “stable” organic matter). The use of biofilter serves as a mean for the
deodorization of the effluent air stream. According to Bidlingmaier (1996) and
Schlegelmilch et al. (2005) finished compost and other organic materials can
effectively serve as an odor filter. The basic arrangements of an aerated static pile
are shown in Figures 3 and 4. The system includes the following six steps:
1. the mixing of the raw material with of a bulking agent
2. the construction of the windrow (width and height),
3. the decomposition process of the substrate (composting),
4. the screening of the end product and the removal of the bulking agent,
5. the curing phase and
6. the storage of the final product.
The construction of the windrow involves the longitude and parallel placement of a
series of perforated longitudinally orientated air pipes (e.g. 10.2–15.2 cm diameter)
along each compost pile (Diaz and Savage, 2007).
Figure 2: Schematic diagram of aerated static pile composting
(Diaz et al., 2002)
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Figure 4: Approximate dimensions for an aerated static pile (Diaz et al., 2002)
In order to avoid short circuiting of air during air suction or forced aeration, the
perforated pipes must be placed in an appropriate distance from the edges of the
windrow. According to Diaz and Savage (2007) this distance is between 1.5–2.7m.
The pipes are connected to a blower through a length of non-perforated pipe. The
piping network is covered with a layer of bulking agent or finished compost that
extends over the area of the pile in which the raw material will be composted. The
formation of a bulking agent layer is used to facilitate the movement and uniform
distribution of air throughout the organic mass during composting. In addition, the
formed layer enables the absorption of excess moisture resulting from the
composted raw material and thereby minimizing leachate runoff. The organic
waste is stacked on the network piping and to bulking agent layer in order to form
the windrow pile, as shown in Figure 3. According to the specifications provide by
Diaz and Savage (2007), the finished pile should be about 20–30m long, about 3–
6m wide, and about 1.5–2.5m high. Additionally, on top of the formulated pile a
layer of matured compost or synthetic materials (about 15-20 cm thick) is placed in
order to absorb emitted odors from the composting mass and to ensure the
homogeneous distribution of temperature throughout the organic matter. The
aforementioned arrangement aims at achieving the desired temperature level to
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optimize the decomposition rate of the organic material and to obtain the required
temperature-time regime for the sanitization of the organic mass throughout the
pile. Leachate control is provided by sloped and sealed or impervious composting
pads with a surrounding drainage system (Diaz and Savage, 2007).
1.1.4.1.2. Extended Aerated Pile
In case where large amounts of organic material are to be composted, the
extended aerated pile method can be adopted. The extended aerated pile has the
following arrangement: On the first day, a pile is constructed in the same way as
described in for the conventional aerated static pile, with the exception that only
one side and the two ends of the pile are covered with the a finished compost
layer. The exposed side of the pile is covered with a thin layer of compost in order
to prevent the escape of odors. On the second day, a second piping network and
bulking layer is set parallel to the exposed side of the pile erected on the first day,
and the pile is erected in the same manner as was the first pile. This procedure is
repeated for 28 days. The first pile is removed after 21 days; the second pile on
the day after, and so on. An important advantage of this approach is a substantial
reduction in spatial requirements. The land area requirement for systems that use
a single pile is about 7–11 tonnes d.w. of organic waste treated per hectare. The
estimate of about 7 tonne/ha allows for sufficient land area to accommodate
leachate collection, administration, and storage (e.g. raw material, end product).
1.1.4.1.3. Economics
The aerated static pile method is probably the least expensive method of all of the
various types of composting technologies that are currently available. Aerated
static pile method is a low technology which requires minimum capital investment
in terms of equipment (the amount of material handling is limited) as well as in the
operation and maintenance cost (Shammas and Wang, 2009). It is difficult to
present a generally applicable capital cost for static pile composting technology
since the treatment process and the developed compost markets are usually site
specific. With respect to material and operational costs, Diaz et al. (2007) state
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that the cost for composting a mixture of sludge and woodchips is about $50 per
tonne (2005 US dollars), of which about $10 per tonne is for woodchips whereas
according to Mavropoulos et al. (2008) the windrow composting cost of green
waste ranges between 20 to 35 € per tonne.
1.1.4.1.4. Limitations
The aerated static pile method is not the most suitable for all types of raw
materials and under all conditions. Since aerated static pile method does not
acquire a mechanism for the agitation of the substrate during the composting
process, the material used requires having relatively uniform particle size not
exceeding 3.5–5 cm in any dimension (Diaz and Savage, 2007). Granular
materials such as sludge are the most appropriate. In case of organic mixtures of
large particle size that exhibit a wide spectrum of dimensions the composting
process shall probably result in uneven distribution and movement of air through
the pile. This uneven distribution of air through the pile promotes air short-
circuiting and the development of anaerobic pockets of decomposing material.
1.1.4.2. Turned Windrow System
The turned windrow method is the one that traditionally and conventionally has
been associated with composting. The term “turned” applies to the method used
for aeration. Aeration of the windrow is achieved by agitation of the substrate using
tractors with front end loaders or any other appropriate machinery which tears
down the piles and reconstructs them. Turning not only promotes aeration, but it
also ensures uniformity of decomposition by exposing at one time or another all of
the composting material to the particularly active interior zone of a pile. In addition,
the mechanical agitation of the substrate reduces to some extend the particle size
of the organic material, whereas water loss due to evaporation (elevated
temperatures) is accelerated (Cornell University, 2010). The water loss can be
considered as a benefit of the composting method in cases where the moisture
content of the substrate is too high. However, in low moisture content the turning
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of the substrate can be potentially disadvantageous and water addition is required
during the turning process.
1.1.4.2.1. Construction of Piles
As in the case of static pile method, forced windrow piles are constructed by
stacking the prepared feedstock in the form of an elongated pile. The cross section
of the windrows is trapezoidal or triangular depending on the conditions of the area
in which the system is cited (e.g. geared to climatic conditions and efficient use of
the composting working area.). According to Diaz and Savage (2007) in dry and
windy areas the piles usually acquire a trapezoidal shape because the ratio of
exposed surface area to volume is lower with such a configuration. In addition, the
volume of the overall hot zone is greater in a trapezoidal shaped pile in relation to
a triangular or conical cross section, since heat loss is less and windrow volume
per unit pad area is greatest. On the other hand, during wet weather the flattened
top is a disadvantage because water is absorbed into the composting mass
changing the moisture conditions within the organic mass. Although the climate
condition might influence windrow geometry, in practice, the determinant is mainly
the turning equipment (e.g. manual, mechanical type).
In operations in which the turning is carried out mechanically, the pile configuration
that results will obviously be the one imparted by the machine. Ideally, the windrow
should be about 1.5–2.0m high (Diaz et al., 2002). In situations in which it is
practical to perform the turning manually, the height should be roughly that of the
average laborer. At most, it should not be higher than that easily reached with the
normal pitch of the equipment used in turning. Another factor that impacts the
maximum height is the tendency of stacked material to compact. The height for
mechanical turning depends on the design of the turning equipment — generally, it
is between 1.5 and 3.0 m (Diaz et al., 2002). The pile’s width is a function of
convenience and expediency, since it has little effect to the diffusion of oxygen into
a pile and, therefore, it does not contribute significantly to meeting the oxygen
requires within the composting mass. With manual turning, a width of about 2.4-
2.7m is considered to be suitable, whereas the width of the pile with mechanical
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turning depends upon the design of the mechanical equipment (usually 3.0 to 4.0
m) (Diaz et al., 2002). In theory, the length of the windrow is indeterminate. For
example, the length of a 180 tonne conical shape windrow of material at a height
of 1.8m and width of 2.5m would be about 46.0 m. A nearly continuous system can
be set up by successively adding each day’s input of raw waste to one end of the
windrow. The continuous windrow system is employed by adding fresh material to
one end of the pile and removing material from the other as it reaches stability.
1.1.4.2.2. Arrangement of the Windrows
The specific arrangement of the windrows at a composting facility depends upon
the availability of land and the accessibility of the equipment used. Whatever the
arrangement, the windrows should be positioned such that each day’s input can
be followed until the material is completely composted. An important requirement
is the space that is required to perform the turning of each day’s input material,
whether the turning is done manually or mechanically. With manual turning, the
total area requirement is at least two times that of the original windrow depending
upon the type of machinery used, since turning varies with type of machine (Diaz
and Savage, 2007). It is worth mentioning that some machines accomplish turning
in such a manner that as the original windrow is torn down, the new windrow is
reconstructed directly behind the machine. This type of machine requires little
more area than that of the original windrow, whereas other types of machines
rebuild the windrow adjacent to its original position the area requirement of which
is comparable to that described for manual turning. According to Diaz et al. (2002)
a considerable degree of advancement has been made in the design of
mechanical turning machines. Emphasis has been placed on the comfort of the
operator, on the size of the windrow, and on the overall space requirements.
Indicative types of turners are the auger turner, the elevating face conveyor, and
the rotary drum with flails.
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1.1.4.2.3. Methods of Turning
With respect to manual turning of the piles, the most common and convenient
equipment is the pitchfork with four or five tines. In the manual turning of the pile,
when reconstructing building the pile, material from the outside layers of the
original windrow should be placed in the interior of the rebuilt windrow. In this way
during the compost cycle every particle of the material is at one time or another in
the active interior zone of the pile. If this ideal situation is not practical to attain, the
deficiency can be compensated by increasing the frequency of turning. Finally, it is
important not to compact the raw material when constructing the original windrow,
and when rebuilding the pile.
1.1.4.2.4. Frequency of Turning
The turning frequency of the pile is strongly related to the rate of oxygen uptake by
the active microbial population. Practically, there is a compromise between
required turning frequency and technical and economic feasibility of meeting that
the required frequency. Structural strength and moisture content of the substrate
are the most important physical characteristics when it comes to the determination
of the turning frequency. Other parameters involved with the effectiveness of the
turning procedure are the pathogen elimination and the uniformity of substrate
decomposition. Another variable factor is the decomposition duration desired by
the operator. High-rate composting requires very frequent turning, since the rate of
degradation is directly proportional to the turning frequency. The lower the
moisture content of the organic matter and the firmer the structure of the particles,
the less frequent will be the required turning. For instance, when straw, rice hulls,
dry grass, dry leaves, woodchips, or sawdust are used as bulking material and the
moisture content of the mixture is about 60% or less, turning on the third day after
constructing the original pile and every other day thereafter, for a total of about
four turnings, is sufficient to accomplish “high rate” composting. After the fourth
turning, the frequency can be reduced to once each 4 or 5 days. The same
program is applicable if paper is the bulking material, provided that the moisture
content does not exceed 50% (Golueke and McGauhey, 1953). If the composting
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mass gives off fouling odors, it means that the composting process has become
anoxic, which in most cases it is due to the presence of excessive moisture. To
overcome the anoxic condition additional turning, at least once each day, is
required to foster evaporation until odors are disappeared.
1.1.4.2.5. Equipment Used for Turning
There are several types of windrow turners available on the market with higher
capacity than rototillers which are quite satisfactory for relatively small composting
operations (Diaz and Savage, 2007). Windrow turners can be generally
categorized into three main groups divided according to the design of the turner
mechanism. The three groups include the auger turner, the elevating face
conveyor, and the rotary drum with flails. Some types of turners are designed to be
towed and others are self-propelled (Figure 5). The self-propelled types are more
expensive than the towed types. An advantage of the towed type is the fact that
the tractor can be used for other purposes between turnings. In addition, the self-
propelled type requires much less space for maneuvering and, therefore, reducing
the required area, since windrows can be closer to each other. The turning
capacity of the machines ranges from a few tonnes per hour to as much as 3,000
tonnes/h depending on model, whereas the costs (2005 US dollars) of the self-
powered machines range from about $200,000 to 300,000 (Diaz et al., 2002).
Figure 5: Examples of a self-propelled and a towed windrow turners (Diaz et al., 2002; Recycle & Composting Equipment Pty Ltd, 2010)
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1.1.4.2.6. Site Preparation
The composting piles should be placed on a hard paved surface. The pad should
be sufficiently rugged to support the combined weight of the composting mass and
associated materials handling equipment, as well as the maneuvering of the
machines. The main reasons for the paving are: (1) to facilitate materials handling,
(2) to control any leachate that may be formed, and (3) to prevent fly larvae from
escaping the area. In summary, preservation of sanitation and materials handling
are the two key factors. In operations processing less than about 10 tonnes/day,
the paving may consist simply of well-compacted clay as a base with a layer of
packed gravel or crushed stone on the surface. In the event that crushed stone
and gravel are not available, a layer of soil can be used. The soil should be firmly
packed on top of the clay. Of course, when soil is used as the top layer, a problem
arises during the rainy season. Paving is especially essential if mechanical turners
are utilized. The machines are fairly heavy and, accordingly, can operate properly
only on a firm footing. Paving materials in addition to gravel and crushed stone are
asphalt and concrete. Special provisions should be made for collecting the
leachate that might be generated. The fresh leachate has an extremely
objectionable odor and unless controlled, it can lead to the development of
problems. In desert regions, the windrows should be protected from the wind so as
to reduce moisture loss through evaporation. In regions of moderate to heavy
rainfall, the windrows should be sheltered from the rain. If shelters are not
available, the possibility of the windrows taking in an excessive amount of
moisture would be particularly high.
1.1.4.2.7. Economics
The cost of windrow composting depends on factors such as the type and particle
size distribution of the organic feedstock (whether preprocessing is required for
size reduction), the contamination level of the raw material, the requirements for
permits, the labor costs and the use and value of the produced compost. Generally
windrow composting facilities are relatively low-cost processes. If the composted
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raw material is yard waste, then the cost of the process using turned windrow
technology is in the range of $15–30 per input tonne inclusive of capital and
operation and maintenance expenses (Diaz et al., 2007). According to recent data
provided by ARCADIS & EUNOMIA (2010) the windrow composting costs ranges
marginally higher than 20 to 40 €/tonne (net of compost sales) depending upon the
specifications of the composting facilities. Additionally the operating costs is
estimated at 6.5 €/tonne whereas the annual maintenance costs is considered at
3.15 €/tonne.
1.1.4.2.8. Limitations
The major limitation of turned windrow systems probably is related to public health
issues. The limitations are particularly applicable to operations that involve the
processing of sewage sludge or animal waste which incorporate pathogenic
microorganisms. This limitation stems from two basic features when operating
turned windrows. The first feature is related to the fact that elevated temperatures,
which favor pathogens elimination, do not generally prevail throughout a windrow,
since in its outer layers the temperatures are lower than in the active interior zone
of a pile. The second feature involves the recontamination of the sanitized material
when turning the pile. In case when outer layers of the pile do not acquire the
desired temperature level there is a risk of pathogen exposure of sanitized organic
material (Bustamante et al., 2008). However, repeated turning eventually reduces
the pathogen populations to concentrations that are less than infective. This latter
condition is reached by the time the material is ready for final processing and use.
Improper and/or insufficient turning might lead to the generation of fouling odors.
Even with a suitable protocol, some odors are certain to be generated. This
situation is typical for all composting system that involves handling and processing
of organic waste, regardless the method employed (e.g. static, turned windrow, or
mechanized composting). The generation of objectionable odors is mainly
occurring, in nuisance proportions, during the preparation and the active process
of composting. Therefore, appropriate preventing measures can be taken only
during that time. A slower then optimum rate of organic matter decomposition and
the subsequent larger land requirement often have been alleged against the use of
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the turned windrow as contrasted to the “highspeed” composting which is claimed
for mechanical composting. With respect to the composting rate, it should be
emphasized that enhanced composting is achieved only when high-priced land
area and costly machinery usage are involved. If machinery means are not
involved and land area is not critical, rapid composting loses its advantage.
Furthermore, under those conditions, the intensity and frequency of turning can be
reduced. The reason is that very little odor is emitted from a pile of composting
material that is not disturbed. It is mainly during the turning process that foul odors,
if present, are released from the pile. However, it must be emphasized that this
relaxation in terms of turning frequency of the pile is safe only when no human
habitations are nearby (i.e. distance >150m).
1.1.4.3. In-Vessel Systems
In-vessel composting occurs within a contained vessel, enabling the operator to
maintain closer control over the process in comparison with other composting
methods. The in-vessel systems are designed to minimize odors (e.g. biofilter) and
process time by controlling environmental conditions such as air ow, temperature,
and oxygen concentration. In this section the term “in-vessel” or “reactor” is
applied to the unit or set of units in which the “active” stage of composting takes
place. These units are also called bioreactors, since composting essentially is a
biological process. There are several in-vessel systems on the market. The growth
in new designs is partly related to the regulatory requirements enacted by some
European member states and by the EU. The primary objective of the in-vessel
design is to provide the best environmental conditions, particularly aeration,
temperature, and moisture. Nearly all in-vessel systems use forced aeration in
combination with stirring, tumbling, or both.
In general, bioreactors can be divided into two main types (1) vertical and (2)
horizontal (Haug, 1993). Horizontal bioreactors are further categorized into (1)
channels, (2) cells (3) containers (4) tunnels and (5) “inclined” reactors or rotating
drums (Crowe et al., 2002). In-vessel bioreactors can also be classified as a
function of the movement of the material. Consequently, the reactors can be
denoted as static and dynamic.
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The retention time, during which the active phase of composting takes place within
the bioreactors, generally lasts from 7 to 15 days and largely depends upon the
type of substrate used. Since the detention time is rather short, upon completion of
the rapid degradation phase, the material that exits the bioreactor most of the
times is placed in windrows for further maturation. A brief description of each type
of bioreactor is presented to the following paragraphs.
1.1.4.3.1. Vertical Reactors
Generally in vertical in-vessel bioreactors the organic material is introduced
through the top of the system and removed from the bottom of the unit as shown in
Figure 6. The end product is usually is discharged out the bottom of the bioreactor
by a horizontally rotating screw auger. As such, the bioreactors acquire the
configurations to operate in a continuous basis. Air is introduced in these systems
by forced aeration either from the bottom, by means of aeration pipes, traveling up
through the composting mass where it is collected for treatment or through air
lances hanging from the top of the bioreactor. The emitted gas is removed from
the reactors is transported to a gas treatment system. Typically, vertical in-vessel
bioreactors involve some type of cylindrical container or tank and they are
manufactured from steel and concrete, whereas they are thermally insulated. The
capacity of these systems ranges from a few cubic meters to more than 1500m3
(Diaz et al., 2007). It must be stated that most vertical bioreactors are used for
composting solid waste and sewage sludge and they have been plagued by a
number of operational difficulties (Diaz et al., 2002).
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Figure 6: Schematic diagram of vertical plug-flow reactors
1.1.4.3.2. Horizontal Reactors
Horizontal reactors are units that, as their classifications suggest, operate in the
horizontal position. Horizontal reactors can be further classified into: channels,
cells, containers, tunnels and rotating drums.
Channels or Trenches These designs are similar to windrow composting facilities, since the organic
material is placed in piles. The main difference between the channels and
windrows is that in channel composting, the material to be treated is placed
between walls, whereas in most cases the facility is housed inside a building. The
walls vary in height from 1-3m with a distance of approximately 6m apart from
each wall, whereas the piles are about 50m long. The air is introduced within the
organic mass through forced aeration or air suction, while at the same time the
substrate is agitated with mechanical turning. In order to manage the potentially
negative impacts of the emissions from the composting mass, the processing
building is kept under negative pressure. The intake emitted gas is removed from
the building to a biofilter (Misra et al., 2003) or other air pollution control device
(e.g. bioscrubbers) (Shammas and Wang, 2009). Generally, the raw material is
loaded into the trenches by means of a conveyor belt or with automated units that
use Archimedean screws or front-end loaders, whereas in similar manner the end
product is unloaded. Channels can be operated either on a batch basis or on a
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continuous basis. On a batch basis, the incoming material is loaded into the
channel as soon as the first phase of the composting process is finished and
treated material has been removed. In channels which operate in a continuous
mode, the incoming material is loaded on a daily basis. Channels operating on a
continuous basis can be further classified into longitudinal and lateral channels
according to the movement direction of the organic material treated.
Longitudinal Channels: In this type of channel the substrate is gradually moved
from one end to another by the turning machine. Therefore, the processing time is
related to the design of the turner which in turn specifies the movement rate of the
composting mass. Indicatively, during the intensive phase of decomposition (the
first phase of the process), the processing time is about 4 weeks (Diaz et al.,
2007). Longitudinal channels incorporate different channel shapes the most
common of which are the straight, elliptical and U-shaped.
Lateral Movement Channels: In this type of channel the substrate is transferred by
the turning machine laterally to the next row. The loading operations are usually
performed through the use of conveyors. Loading is carried out approximately
every 2–3 days, depending upon the volumetric capacity of the channel. Most of
the designs include forced aeration and rely on the use of conveyors to remove
the composted material and transport it to the maturation area (Diaz et al., 2007).
Figure 7: Typical examples of Channels or Trenches (Turovskiy and Mathai, 2006; Diaz et al., 2002)
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Cells Cells, or biocells, are hermetically enclosed units, generally rectangular in shape,
in which the composting takes place. In that way the environmental conditions of
the composting process can be fully controlled and optimized, whereas the outside
surfaces are thermally insulated to minimize thermal losses during composting.
Biocells operate on batch basis and they can be built onsite or can be
prefabricated. In a typical operational sequence, the substrate is introduced into
the cell by means of front-end loaders or conveyors. Once the unit has been filled,
the biocell is closed and the composting process begins. Typically, the period of
intensive composting lasts for approximately 14 days depending on the type of
treated material. Air supply is provided to the organic matter by means of a forced
aeration system (pipes or channels) through the bottom layer of the cell forcing the
air to move upwards through the organic mass. The gas emitted during the bio-
oxidation phase is removed at the top of the biocell and usually directed to a
biofilter or partially recirculated. Some biocell systems incorporate a heat
exchanger to pre-heat the air prior to introduction into the composting mass. Water
addition to the substrate is performed by means of a hydration system (nozzles
and pipes) which is typically installed at the top of the biocell, whereas the
generated leachate (excess moisture) is collected and recirculated to regulate the
moisture content. Furthermore, some biocell models incorporate screw conveyors
and moving floors aiming to agitate the substrate, while being in the container.
After the end of the intensive composting process, the organic material is removed
from the biocell with a front-end loader. The biocells capacity ranges between 100
and 1000m3, whereas typical dimensions are: 6m wide, 4m high and more than
50m long. The height of the material inside the container must be carefully chosen
in order to limit substrate’s compaction and enhance proper air diffusion
throughout the composting mass.
Containers Typically the top of the container is opened or removed and the raw material is
loaded into the container by means of a conveyor belt or a front-end loader.
Containers usually are rectangular in shape, with volumetric capacities ranging
from 20 to 40m3. Air is supplied to the substrate via force aeration from the bottom
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of the system (e.g. nozzles), while the exhaust gas is removed from the container
and directed to an air control system (i.e biofilter). The containers are usually
equipped with a hydration system for the moisture content control of the substrate,
whereas generated leachate is gravitationally removed through perforated pipes
located at the bottom of the system. The processing time is about 8–15 days, at
the end of which the organic material is discharged from an opening located at the
one end of the container by means of a roll-off collection truck. The containers are
usually installed in parallel system/modules with which a nearly continuous system
can be set up. Each module includes approximately 6 to 8 containers with a total
capacity which ranges from 3,000 to 5,000 tonnes/year of organic matter.
Tunnels Tunnels or biotunnels are insulated, rectangular shaped structures which are
made out of metal, concrete or brick and their typical measurements are: 4–5m
wide, 3-4m high, and up to 30m long. The tunnels operate on a continuous basis
and the substrate is introduced at the one end of the tunnel on a daily basis. The
organic mass is forced forward toward the opposite end of the tunnel by means of
a hydraulic piston or through the reciprocating motion of moving floors. Moisture
and oxygen levels are monitored at all times, whereas water and air are introduced
whenever deemed necessary. Aeration is provided to the organic mass via
compressors which deliver the air through the floor while in some tunnel systems
air is provided by the use of centrifugal fans in order to reduce the noise level that
is generated. Pipes placed on the roof of the unit remove the discharge air through
negative pressure. Some of the units recirculate the process air through reversing
aeration systems which can reach up to 80%. The entire process is automated
and controlled by means of a computer. The retention time of the organic material
is approximately 14 days whereas the overall treatment process lasts about 2
weeks.
Inclined Bioreactor or Rotating Drum Another type of in-vessel composting system is the inclined bioreactor or rotating
drum consisting of a rotating cylinder. The cylinder is built at a slight inclination so
that the substrate is moved from one end to another. The rotating drums usually
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incorporate internal vanes which, combined with the rotating action of the drum,
contribute to the size reduction as well as to the agitation of the organic matter.
This type of reactor normally is used for the active phase of composting and by
carefully controlling the oxygen and moisture contents and maintaining at optimum
or near-optimum levels, the composting process can be accelerated. The cylinder
is approximately 45m long and 2–4m in diameter, whereas the rotational speed is
about 0.2–2 rpm (Diaz et al., 2002). Under normal operating conditions, the
bioreactor is filled to about two-thirds, while the retention time of the substrate is
about 1 week. After the active phase of composting within the rotating drum, the
organic material is cured for a few weeks in windrows. (Diaz et al., 2002).
1.1.4.3.4. Economic Limitations
As has been mentioned in-vessel composting systems reserve the bioreactor for
the active stage of the composting process and rely upon windrow systems for the
curing and maturation phase of the organic matter. The rationale of these systems
is to maintain conditions at optimum levels during the active stage of the process
and thus accelerating the microbial activity rate and consequently shortening the
active phase. The economic gain of in-vessel systems in comparison to windrow
composting is the reduction of residence time and the increase of its processing
capacity as well as the better quality of the end product, since the conditions
during the process are usually optimized and controlled at all times. On the
contrary the capital and O&M costs of in-vessel composting systems are
significantly than those of windrow systems. According to Diaz et al. (2007), in the
early 1970s, capital costs for compost plants in the USA were of the order of
$15,000–20,000 per tonne of daily capacity. The operational costs were about
$10–15 per tonne processed. In 2005 the costs range from about $25,000 to about
$80,000 per tonne of daily capacity weeks (Diaz et al., 2007). According to recent
data the financial cost of in-vessel biowaste treatment in EU member states
ranges from 30 to 41 € per tonne treated. The unit cost includes annualized
capital, O&M expenditure as well as other specific costs or revenues (e.g.
revenues from sale of the energy) without considering taxes or any form of
subsidization of the end products (EC, 2010).
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1.1.5. Post-Processing
Post-processing practices involve the various stages that are employed to in order
to refine the produced compost and to meet market and regulatory standards.
Among the main processing units that are included in compost post-processing
practices are: size reduction, screening, air classification, and de-stoning. It must
be stated that the post processing techniques achieve adequate separation in
cases where the moisture content of the orgaic end product is lower than 30%.
1.1.6. Mass and energy balances
A typical mass balance diagram of the composting process is shown in Figure 8.
According to the figure, for 100 tn of processing MSW, 95tn is assumed to be the
input for the composting process after the mechanical pretreatment. The
degradation process of the organic matter causes the loss of 63tn DS and water,
thus producing a final end compost which accounts to 30tn and 2 tn of residues.
Figure 8: Typical mass flow diagram of composting
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In composting, energy is generated during the biodegradable organic solids
oxidation in the form of heat release (enthalpy of reaction). Also, energy enters the
composting system through the ambient air (sensible and latent heat), and exits
the system through the flue gas (sensible and latent heat and the heat of reaction
associated to the presence reduced gaseous species, namely ammonia (NH3) and
hydrogen sulphide (H2S)). The enthalpy accumulated (EAc) in the composting
materials along the processing time can be evaluated through the following
equation (Neves et al., 2007):
where :
NI is input rate of energy associated to the inflow rate of ambient air:
NE is the output rate of energy associated to the composting flue gas rate:
is the molar saturation ratio of water vapor at the atmospheric pressure p1 (=
1.013 105 Pa); pvs is the water saturation vapor pressure calculated from the
Clausius–Clapeyron equation at the actual temperature of the composting
materials (TR), considering that flue gas is moisture saturated.
NG is the rate of energy released by the biological oxidation of the
biodegradable materials,
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NU is the rate energy transfer between the composting materials at temperature
TR and the environment, at temperature TS (considered constant and equal to
298 K),
where AR is the estimated superficial area of each load of waste ( 0.24 m2) and U
is the global heat transfer coefficient. It was considered that heat is transferred
though conduction, based in a compost thermal conductivity of 0.1 W m-1 K-1.
The temperature of the composting materials can be calculated at any moment by:
1.1.7. Market potential for products
As mentioned above the end product of the composting process is the compost.
Compost has the potential to be used as a soil amendment in various applications.
Compost can substantially improve the fertility, texture, aeration, nutrient content
and water retention capacity of the soil. Due to its beneficial characteristics,
compost has a variety of potential applications and can be used by several market
segments. Some of the markets include:
• agriculture (small- and large-scale);
• landscaping;
• gardening (residential, community);
• nurseries;
• top dressing (e.g. golf courses, parks, median strips);
• land reclamation or rehabilitation (landfills, surface mines, and others); and
• erosion control.
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The markets or uses listed in the previous paragraph are constrained by: (1) the
characteristics of the compost, (2) the limitations applicable to its use, and (3)
pertinent laws and regulations (Alexander, 2000; Harrison et al., 2003).
Results of marketing studies and surveys conducted in several countries have
demonstrated that some of the most critical elements in the use and marketability
of the compost products are: (1) quality, and (2) consistency. The quality of a
specific type of compost is a function of its chemical, biological, and physical
characteristics. Assuming that a composting process is properly carried out, the
quality of the finished product is determined by: (1) the composition and
characteristics of the input material used in the production of the compost, and (2)
the type and thoroughness of the process used to remove impurities. Some of the
physical characteristics that are normally desired for a particular compost product
are color, uniform particle size, earthy odor, absence of contaminants, adequate
moisture, concentration of nutrients, and amount of organic matter (Eggerth et al.,
1989). The size of a particular market for compost depends, to a large extent, on
the quality of the compost and on the types of uses for the material. Composts
from different types of substrates (e.g., yard waste, source-separated MSW) have
different characteristics and consequently have different potential markets
(Franklin Associates et al., 1990).
1.1.7.1. Limitations
The use of compost and therefore the limitation of its use on land is depended on
the potential adverse effects on human health and safety, animal & livestock
health and safety, crop production and the quality of the air, water, and land
resources. The significance of the aforementioned limitation is related to whom or
what is affected and the extent to which they are affected. The limitations
associated with the use of the compost with respect to the health and safety of
humans are related to the harmful substances that may be present in the product.
Among the main substances that are being regulated in orde to prevent potential
adverse effects include the pathogenic organisms, heavy metals, persistent
organic pollutants (POPs) and level of contaminants (e.g. plastic, glass). The level
of their appearance is mainly associated of to the feedstock material to the
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composting process. Although the output from processing mixed MSW is not
considered compost in several countries that are members of the EU, this practice
is still being conducted and considered in other countries.
EU initiatives with respect to the heavy metals maximum consentration in compost
are also seen in Table 3. The second draft EU Working Document on the
Biological Treatment of Biowaste lays down heavy metal limit values for two
classes of compost both towards the high quality end product, while a third class
of material, stabilised biowastes, which is still considered as ‘waste’. In addition
according to the commission decision 2001/688/EC (EC Eco-label) on
“establishing ecological criteria for the award of the Community eco-label to soil
improvers and growing media” environmental performance in regard to the heavy
metal concentration of soil improvers1 and growing media2 have been set. For the
utilization of compost as fertilizer or soil conditioner within organic farming (Eco-
agric), specific compost quality standards for heavy metal concentration are also
provided. Within the Eco-agric (EC 2092/91 - EC 1488/97) only composted source
separated household waste containing only vegetable and animal waste is
accepted.
Table 3: Heavy metal limits for European compost standards (mg/kg dm)
Policy measures Cd Crtot CrVI Cu Hg Ni Pb Zn As
EC Draft W.D.
Biological
Treatment of
Biowaste (class 1)
0.7 100 100 0.5 50 100 200
Draft W.D.
Biological
Treatment of
Biowaste (class 2)
1.5 150 150 1 75 150 400
Stabilised
Biowaste** 5 600 600 5 150 500 1500
1 soil improvers: materials to be added to the soil in situ primarily to maintain or improve its physical properties, and which may improve its chemical and/or biological properties or activity 2 growing media: material, other than soils in situ, in which plants are grown
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EC/'eco-label'
2001/688/EC 1 100 100 1 50 100 300 10
EC/'eco-agric'
2092/91 EC-
1488/97 EC 0.7 70 0 70 0.4 25 45 200
Figure 9 gives a comparative survey on heavy metal limit and guide values for
composts in European countries expressed as relative mean limits as compared to
the maximum concentration of the EC Eco-Label for soil improver (= 100 %).
Figure 9: Heavy metals limit values for compost in European countries
[mean percentage relative to threshold values of the EC Ecolabel for soil
improver]. Countries with more than one compost category or quality class
referring to PTE thresholds are indicated with ‘I / II /III’]
There are thousands of chemically synthesised compounds that are used in
products and materials commonly used in our everyday life. Many of them are
potential contaminants of biowaste, although, due to their low concentration or
easiness to be broken down by micro-organisms, as to the buffering capacity of
soils, they do not cause a threat to the environment. However, there are some
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organic compounds that are not easily broken down during waste treatment and
tend to accumulate and be the source of concern due to their eco-toxicity, the eco-
toxicity of the products resulting from their degradation or to their potential for bio-
accumulation. There are usually three main reasons why an organic compound
may be subject to preventive action:
(a) the break down by soil micro-organisms of the compound concerned is slow
(from some months to many years) and, therefore, there is an actual risk of build-
up in the soil;
(b) the organic compound can bio-accumulate in animals and, therefore, it poses a
serious threat to man;
(c) the degradation products of the organic compound are more toxic than the
initial compound.
Therefore, there is likelihood a very high number of organic contaminants to be
found in compost made from collected and treated biodegradable organic waste.
Each year, the use of new compounds increases by a few thousand. Some of
these compounds break down or undergo a transformation during the composting
operations, while others remain stable. The presence of organic contaminants in
compost used on soils could represent a potential risk to the environment and to
the quality of crops intended for human or animal consumption.
Limits for organic contaminants were proposed only within the second draft of the
Working Document on biological treatment of biowaste, concerning the
polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs)
only (Table 4), and their concentration was set to be in consistence with the
Sewage Sludge Directive (86/278/EEC). In general, organic contaminants are
expected to be at low levels in composts derived form source separated materials
and, therefore, in most European countries there are no set limit values for organic
contaminant in composts.
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Table 4: Organic pollutants standards for compost and stabilised biowaste
Parameter
(mg/kg dm)
Compost Class 1 Compost Class 2 Stabilised
Biowaste
PCBs (mg/kg dm) - - 0.4
PAHs (mg.kg dm) - - 3
Threshold values for these organic pollutants to be set in consistence with the
Sewage Sludge Directive.
Impurities or any inert non organic contraries may be found in composts from
biodegradable municipal waste. The better the performance of separate collection
from households or small enterprises the higher the purity. When developing an
industry standard for compost quality, the presence of foreign matter in compost
should be taken into consideration, since it has a negative impact on consumers
and on the composting industry in general. The consumers look for compost free
of visible foreign matter or otherwise harmful foreign matter. Table 5 presents the
classification of compost according to the level of impurities in compost as has
been laid down by the second working document on Biological Treatment of
Biowaste in EU.
Table 5: Impurities standards for compost and stabilised biowaste
Parameter Compost Class
1
Compost Class
2
Stabilised Biowaste
Impurities >2mm <0.5% <0.5% <3%
Gravel and stones
>5mm
<5% <5% -
From the very beginning of the implementation of compost standards hygienic
aspects have been addressed in order to “guarantee a safe product” and to
prevent the spreading of human, animal and plant diseases. Provisions for the
exclusion of potential pathogenic microorganisms within process and quality
requirements are established at two levels:
• direct methods by setting minimum requirements for pathogenic indicator
organisms in the final product
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• indirect methods by documentation and recording of the process showing
compliance with required process parameters (HACCP concepts,
temperature regime, black and white zone separation,
hygienisation/sanitisation in closed reactors etc.).
Table 6 shows the requirements of the Decision of EC ECO-label.
Table 6: EU requirements on pathogens/weeds in compost based on the EC eco-label
Pathogens /Weeds Approval of technology (AT)
Salmonella sp. absent in 25 g
E. coli < 1000 MPN (most probable number)/g
Helminth Ova absent in 1.5 g
weeds/propagules germinated plants: 2 plants /l
1.1.8. Environmental impacts
Significant environmental benefits can be obtained by the use of compost as a soil
conditioner, a fertilizer, or a growth medium. Those benefits are related to the
nutrients recycling back to the soil which enables the reduction of synthetic
fertilizers. When it is used as daily cover at landfills, it replaces other materials that
would otherwise be used for that purpose.
However, there are also negative impacts on the environment associated with the
production and utilization of compost. These impacts depend both on the
operational composting process and to a large extent to the waste composition of
the input organic streams. Mixed MSW and sewage sludge composting pose
greater risks because these materials typically contain higher concentrations of
heavy metals, POPs and pathogenic microorganisms than do source separated
organic waste.
Negative impacts of composting on the environment can also be caused from
gases that are released due to the improper operation and maintainance of the
compost piles. More specifically, in cases when composting piles are not properly
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operated in that they do not deliver the required oxygen within the organic mass,
anaerobic bacteria are being developed which result in the production of
methane.. The release of methane significantly contributes to the problem of
greenhouse gases in the atmosphere. Additionally, poorly operated composting
facilities are always associated with unpleasant odors and the creation of
nuisance. Other air emissions are generated by the combustion engines used to
power windrow turning machines and grinders.
During composting leachate production is another matter of concern especially in
open composting systems. Leachates are produced from water runoff and
condensation at the compost facilities due to the increased moisture content of the
feedstock material as well as due to the need for water addition to the organic
mass in order to maintain the moisture content at an acceptable level during the
biodegradation process. Leachates occasionally acquire increased levels of
biological oxygen demand (BOD) that may exceed the acceptable discharge limits.
Leachates runoff to surface water can reduce significantly the amount of dissolved
oxygen in the aquatic ecosystem resulting in eutrophic conditions. Sound practice
here is to avoid discharge to water and to capture or direct all leachate to
absorption in sand or soil.
1.1.9. Economic data
The capital costs of the in-vessel composting varies significantly according to the
scale of the facility, the input material that is being treated, characteristics of the
exhaust gases that are being treated and the retention time of the organic matter.
According to the World Bank, the capital cost for the development of an in vessel
composting system is approximatetely of the order of 240€ per tonne of capacity
(35-55 million € for a capacity of 500 tonnes/day) while the operation and
maintenance cost is 20-40 €/tonne. According to recent data ASCARDIS &
EUNOMIA (2010) typical capital costs are of the order of 190€ per tonne of
capacity and suggest operating and maintenance cost of 12.5 and 10.25€ per
tonne respectively.
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1.2. ANAEROBIC DIGESTION
1.2.1. Introduction to anaerobic digestion process
Anaerobic digestion (Figure 10) is defined as the biological process during which
the organic material is decomposed by anaerobic microorganisms in the absence
of dissolved oxygen (i.e. anaerobic conditions). Anaerobic microorganisms digest
the input organic material which is converted through anaerobic degradation into a
more stabilized form, while a high energy gas mixture (biogas) consisting mainly of
methane (CH4) and carbon dioxide (CO2), is generated. Biogas is collected and
utilized as a source of energy, since it can be combusted in a cogeneration unit
and produce green energy. Apart from CO2, CH4 is also considered as a gas
which contributes significantly to the greenhouse effect and hence to global
climate change. The organic material can originate from industrial or municipal
waste, agricultural residues or sludge generated from wastewater treatment plants
(Pavlostathis and Giraldo-Gomez, 1991).
Figure 3: Anaerobic digestion flow chart (Wastesum, 2006)
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1.2.2. Biology of anaerobic digestion
One of the key factors in the success of microbial-mediated processes is an
adequate understanding of process microbiology, more speci cally the study of
microorganisms involved in organic waste decomposition and the subsequent by-
product formation. The anaerobic fermentation process is much more complex
than composting due to the involvement of a diverse group of microorganisms and
a series of interdependent metabolic stages which demand meticulous process
control for stable operation. The anaerobic digestion of organic material is
accomplished by a consortium of microorganisms (bacteria) working
synergistically in the absence of oxygen. These microorganisms use up the initial
feedstock as an energy and biomass source through various biological and
chemical reactions transforming the input organic matter to intermediate molecules
such as sugars, hydrogen and acetic acid before finally being converted to biogas.
The anaerobic digestion process can be generally classified into four distinct
stages which are related to the biological and chemical phases of anaerobic
treatement of biodegradable organic waste as shown in Figure 9:
1. Hydrolysis
2. Acidogenesis
3. Acetogenesis
4. Methanogenesis
Generally the organic input material is composed of large macromolecules which
need to be broken down into smaller chemical components so that the anaerobic
bacteria will be able to access the energy potential of the substrate. Hydrolysis is
the first stage of the anaerobic digestion process in which complex and large
organic polymers are decomposed and dissolved to constituent monomers
(Ostrem, 2004). Therefore the hydrolysis stage involves the breakdown of complex
organic molecules such as polysaccharides, proteins, and lipids into simple
compounds namely sugars, amino acids, and fatty acids by extracellular enzymes
(e.g. cellulase, protease and lipase) and then to soluble products of small enough
size to allow their transport across the cell membrane. Hydrolysis can be a rate-
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limiting step in the overall anaerobic treatment processes for waste containing
lipids and/or a signi cant amount of particulate matter (e.g., sewage sludge,
animal manure, and food waste) (Henze and Harremoes 1983; van Haandel and
Lettinga 1994).
In the hydrolysis phase, acetate and hydrogen are produced which can be directly
taken up by methanogens. Additionally, chemical compounds such as volatile fatty
acids which acquire greater chain length than acetate (e.g. propionic, formic,
lactic, butyric, or succinic acids) must be further broken down into constituents that
can be used by methanogenic microorganisms (Khanal, 2008). The biological
process in which the products resulting from hydrolysis are further breaking down
is called acidogenesis and it takes place by acidogenic bacteria. Volatile fatty
acids are generated along with ammonia, carbon dioxide and hydrogen sulfide as
well as other by-products. The specific concentrations of products formed in this
stage vary with the type of bacteria as well as with culture conditions, such as
temperature and pH (United Tech 2003). The third stage of the anaerobic
digestion is the acetogenesis. During this stage, simple molecules which have
been produced through the acidogenesis stage (e.g. acetate) are further degraded
by acetogens to produce mainly acetic acid as well as carbon dioxide and
hydrogen (Khanal, 2008). The final stage of anaerobic digestion of the
biodegradable organic waste is the methanogenesis phase during which
methanogenic bacteria use the intermediate products resulting from the preceding
stages for the production of methane, carbon dioxide and water. Methanogens are
sensitive to changes and prefer a neutral to slightly alkaline (between pH 6.5 and
pH 8) (Wastesum, 2006). The remaining non-digestable organic material, which
the anaerobic microbes cannot decompose along with the dead bacterial,
constitutes the digestate.
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Figure 4: The stages of anaerobic digestion (Wastesum, 2006)
1.2.3. FEEDSTOCK OF ANAEROBIC DIGESTION
The feedstock material that is used in anaerobic digestion constitutes the most
important initial parameter when considering the application of anaerobic
treatment. Feedstock include any substrate that can be converted to methane by
anaerobic bacteria thereby it can range from readily degradable organic waste
(e.g. wastewater) to complex high-solid waste. Anaerobic digesters typically can
accept any biodegradable material, but the level of biodegradability is the key
factor for its successful application. Anaerobic microorganisms can dissolve
organic matter to varying degrees of success. More specifically sugars which are
short chain hydrocarbons can be used readily whereas the decomposition process
of cellulose and hemicellulose compounds is significantly longer. Anaerobic
microorganisms are unable to break down long chain woody molecules such as
lignin (Wastesum, 2006). Therefore it can be inferred that the characteristics of the
input material determine in a large extent the methane yield and production rates
within the anaerobic digesters. In order to improve the methane potential in
anaerobic digestion several techniques are being applied by determining various
characteristics of the organic feedstock. Additional variables such as solids
content as well as elemental and organic analyses are considered as useful
methods in the design and the operation of anaerobic digesters.
Another parameter that needs to be considered when it comes to anaerobic
treatment is the moisture content of the feedstock material. Generally, the higher
the moisture content of feedstock the easier its handling and conveyance since
standard pumps can be applied instead of concrete pumps and physical means of
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movement which are more energy demanding practices. However, the increased
moisture content of the organic input material results in higher volume and area
that is required in comparison to the levels of biogas that are generated.
Therefore, the type of anaerobic system that will be developed and applied is
depended on the moisture level of the input material different. Another key
consideration in anaerobic digestion is the C/N ratio of the initial substrate that is
subjected to anaerobic decomposition. The C/N ratio represents the relationship
between the amount of carbon and nitrogen present in the organic material thus
regulating the nutrients take up of microbes and balancing their growth. Optimum
C/N ratios in anaerobic digesters are between 20-30 (Verma, 2002). A high C/N
ratio is an indication of rapid consumption of nitrogen by methanogens and results
in lower gas production. On the other hand, a lower C/N ratio causes ammonia
accumulation and pH values exceeding 8.5, which is toxic to methanogenic
bacteria (Verma, 2002).
The impurities level of the organic input material is another key parameter when
considering the deployment of anaerobic treatment. In cases when the substrate
acquires increased quantities of impurities such as plastic, glass or metals, then a
pre-treatment stage is required in order to increase the purity level of the feedstock
and to prevent potential malfunctions and inefficiencies of the anaerobic digesters
processes (Wastesum, 2006).
The feedstock for an anaerobic digestion plant can be organic waste that has been
separately collected and delivered to the plant ready for processing or, municipal
solid waste or its fraction from a mechanical sorting plant in which the other
fraction is the refuse-derived fuel. A further source of organic waste is ‘green
wastes’ collected at centralized collection points. At least the purity of the raw
material fed into the anaerobic digestion process dictates the quality of the product
coming out at the end of the process. The range of application of the anaerobic
digestion process is very broad. In principle, any organic material can be digested
a list of which is given below (Rilling, 1994):
organic municipal solid waste
waste from central markets (e.g. fruit, vegetable and flower residuals)
slaughterhouse waste (paunch manure)
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residues from the fish processing industry
food waste from hotels, restaurants, and canteens
bleaching soil
drift materials such as seaweed or algae
agricultural waste
manure
beer draff
fruit or wine marc
sewage sludge.
However, the treatment method for each waste stream might be depended on its
moisture level as shown in Figure 12, since composting is widely used for waste
containing high amounts of dry matter, whereas anaerobic digestion has turned
out to be a good alternative for treating wet organic waste.
Figure 5: Suitability of waste for aerobic composting and anaerobic
digestion (Kern et al., 1996)
1.2.4. Procedures of Anaerobic Waste Fermentation
The anaerobic treatment of organic waste generally follows specific steps as
presented below (Rilling, 1994):
1. delivery and storage of biodegradable organic waste
2. preprocessing of the incoming biodegradable organic waste
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3. anaerobic fermentation
4. storage and treatment of the digester gas
5. treatment of the process water
6. post-processing of the digested material.
Figure 13 shows the possible treatment phases carried out in anaerobic digestion
process. Typically, all fermentation processes can be described as a combination
of a selection of these treatment phases. The process technology demanded for
the implementation of the different phases of the treatment varies significantly and
depends on the anaerobic process chosen. In general, the gas production
increases and the detention time decreases with increasing energy input for
preparation of the material and the fermentation itself (mesophilic/thermophilic).
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Figure 6: Possible treatment steps used in anaerobic digestion process of
biodegradable organic waste (Rilling, 1994)
1.2.4.1. Delivery and Storage
The organic substrates are quantitatively and qualitatively recorded by weighing,
are visually inspected at an acceptance station, and are unloaded into a flat or
deep bunker or a collecting tank that serves as a short-term intermediate storage
place and permits continuous feeding to the subsequent pretreatment plant.
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1.2.4.2. Pre-processing
In the pretreatment stage pollutants and inert material are removed from the
organic material, whereas the substrate is homogenized and conditioned. The
pretreatment type depends on the specific system of the anaerobic fermentation
process. More specifically, dry fermentation processes use dry preprocessing, in
which sieves, grinders, shredders, metal separators, homogenization drums,
ballistic separators, and hand sorting sections can be combined. On the other
hand, in wet fermentation processes the organic substrate is additionally mixed
with water, homogenized, and shredded. Organic material that has larger surface
area is more easily broken down by the bacteria.
1.2.4.3. Anaerobic Fermentation
Once the pre-processing procedure has been elaborated any recyclable or
unwanted materials is separated from the incoming waste, whereas the organic
material is shredded and supplied to the digester. In case when organic waste with
high water content, e.g. sewage sludge, is used as raw material the addition water
is not required, whereas for dry substrates, e.g. household organic waste, water is
usually necessary to be added in order to dilute the solids. Waste with low
structure and high water content are best for wet fermentation. On the other hand,
substrates with high structural strength can also be anaerobically decomposed
through dry fermentation processes (RISE-AT, 1998). For anaerobic fermentation
to take place, heat is needed to be adjusted to the required process temperatures
to about 35°C (mesophilic operation mode) or 50-55°C (thermophilic operation
mode), and in some cases water addition is prerequisite. During substrate
digestion the decomposition of the organic matter is carried out in the absence of
oxygen i.e. anaerobically, in closed, temperature-regulated bioreactors. Depending
on the process operation, the material consistency may vary between well-
structured and fluid suspension organic matter. The output of the anaerobic
digester is a wet, organically stabilized residue (digestate) and biogas. After
dewatering of the digestate, a compost like material can be obtained by aerobic
post-treatment. In addition, the wastewater produced during draining can be partly
recirculated into the pre-processing stage to adjust the water content of the initial
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substrate, whereas surplus wastewater has to be treated accordingly (e.g. purified
in specially designed purification ponds) and discharged. Biogas, which constitutes
the main product of anaerobic, is used as an energy source. Biogas is generally
used in decentralized fuel-burning power stations for the production of electricity
and heat in order to cover the energy requirements of the fermentation process
and thus enabling the system to operate in an energy-neutral manner. The excess
energy is marketed by supplying the public heat power needs. In cases when only
easily degradable organic waste components are used in the anaerobic digestion,
the energy is produced with minimal technical expenditure, whereas the energy-
intensive pretreatment stages can be omitted (RISE-AT, 1998).
1.2.4.4. Post-processing
The solid by-product of the anaerobic digestion has to be stabilized, sanitized and
refined prior to its application for agricultural or horticultural uses. Typically, after
dewatering and/or drying, the digestate is composted and matured in order to
become a good quality, marketable compost. The biogas, after drying and, if
required, purification, can be used as an energy source.
1.2.5. Process Engineering of Anaerobic Fermentation of Biowaste
In comparison to the commonly applied composting treatment, anaerobic
fermentation of biowaste is a relatively new and dynamic biological process. With
great scientific expenditure, process developments and optimizations are being
pursued, so it may be assumed that the technological potential of biowaste
fermentation has not yet been fully exhausted. Anaerobic digestion is generally
suitable for the biological treatment of readily degradable substances which
acquire low structure and high water content (e.g. kitchen waste). The anaerobic
fermentation processes of organic solid waste differ in number depending on (i)
the biodegradation stages (one or two-stage), (ii) separation of liquid and solids
(one or two-phase), (iii) water content (dry or wet fermentation), (iv) feed method
(continuous or discontinuous), and (v) agitation method (Rilling, 1994). The most
important characteristics of anaerobic digestion are presented in Table 7.
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Table 7: Characteristics of anaerobic waste treatments (Rilling, 1994)
Stages of biodegradation
One-stage Two-stage
Separation of liquid and
solids
One-phase dry
fermentation
Two-phase wet
fermentation
Total solids content 25%-45% <15%
Water content 55%-75% >85%
Feed method Discontinuous Continuous
Agitation None Stirring, mixing,
percolation
Temperature Mesophilic (30-37oC) Thermophilic (55-65oC)
As stated above the anaerobic fermentation of biowaste can be operated by one-
stage or two-stage fermentation. In the one-stage process (Table 8) all
fermentation stages (e.g. hydrolysis, acidification, acidification and
methanogenesis) take place in one reactor; therefore, optimum reaction conditions
for the overall process are not achieved, due to the different environmental
requirements during the various stages of the fermentation. Therefore, the
degradation rate is reduced and consequently the retention time increases. The
basic advantage of one stage process operation is the relatively simple technical
installation and operation of the anaerobic digestion plant, whereas the costs are
lower. In two-stage processes (Table 8), the hydrolysis and acidification-
acidification take place in one bioreactor, while methanogenesis is carried out in a
separate reactors thus providing flexibility to optimize each of these reactions so
that e.g. mixing and adjustment of the pH can be optimized separately, permitting
higher degradation degrees and loading rates. In two-stage processes the
retention time of the substrate is significantly decreased. However, such systems
involve more sophisticated technical design and operation and subsequently
higher costs.
In the first reactor, organic fraction is hydrolyzed producing dissolved organics,
organic acids, CO2 and low concentrations of hydrogen. The reaction rate in the
first reactor is limited by the rate of hydrolysis of cellulose. In the second stage the
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highly concentrated water is supplied to an anaerobic fixed-film reactor, sludge
blanket reactor, or other appropriate system where methane and CO2 are
produced as final products. In the second reactor the rate of reaction is limited by
microbial growth (Verma, 2002).
Table 8: Comparison of one- and two-stage processes Process Operation One-stage Two-stage
Operational reliability In the same range
Technical equipment Relatively simple Very complex
Process control Compromise solution Optimal
Risk of process instability High Minimal
Retention time Long Short
Degradation rate Reduced Increased
1.2.5.1. Dry and Wet Fermentation
According to the moisture level of the substrate the anaerobic digestion systems
can be classified as dry fermentation processes or “high-solids systems” (dry
solids content >15-20%) and wet fermentation processes “low-solids systems” (dry
solids content <15%) (Jördening and Winter, 2005). However, there is no
established standard for the cutoff point. Table 9 shows advantages and
disadvantages of dry and wet fermentation. With the dry fermentation process,
little or no water is added to the biowaste. As a consequence, the material streams
to be treated are minimized. The resulting advantages are smaller reactor volumes
and easier dewatering of the digestate thus less costly reactors. Operating in dry
fermentation processes places higher requirements on mechanical pretreatment
and conveyance (e.g. pumping denser material), on the gas-tightness of charge
and discharge equipment and on mixing the substrate. Due to the low substrate
mobility in high solid content fermentation, a defined residence time can be
reached by approximating plug flow, which is particularly important for the
sanitization of the organic solid end product in the thermophilic operation process.
The degradation rates in dry fermentation processes are lower than in wet
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fermentation, due to the larger particle size and reduced substrate surface
availability (Jördening and Winter, 2005).
In the case of wet fermentation processes, the organic waste is shredded into
small particle size whereas specific amount of water is added aiming to formulate
sludges or suspensions. The low solid content of the substrate enables the use
standardd mechanical conveyance techniques (pumping) while the removal of
interfering substances can be achieved by sink–float separation (Jördening and
Winter, 2005). Additionally, the agitation of the organic substrate is easily
operated; allowing controlled degassing and defined concentration equalization in
the digester which in turn optimizes the degradation performance of the
microorganisms. The mean substrate concentrations and thus also the related
degradation rates are lower than in plug flow systems, since for completely mixed
systems the concentrations in the system are equal to the outlet concentrations.
Mixing is limited due to the increased sensitivity of methanogenic microorganisms,
while mixing at a lower degree may result in floating and sinking layers.
Homogeneity and a fluid consistency permit easier process control. However, by
fluidizing biowaste, the treated substrate increases and it requires larger area and
volume (aggregates and reactors) for the treatment process to be performed.
Fluidization and dewatering of the fermentation suspension are costly procedures,
since they require considerable technical and energetic expenditures. However, if
the degrees of degradation are the same, recycling the liquid phase from the
dewatering step to the fluidization of the initial substrate, makes it possible to
reduce the wastewater quantity to an amount comparable to that used in dry
fermentation and to keep a considerable part of the required thermal energy within
the system (Jördening and Winter, 2005).
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Table 9: Comparison of wet and dry fermentations (Jördening and Winter, 2005).
Process Mode Dry Wet
Total solids content High 25-45% Low 2-15%
Reactor volume Minimized Increased
Conveyance technique Expensive Simple
Agitation Difficult Easy
Scumming Little risk High risk
Short circuit flow Little risk High risk
Solid-liquid separation Simple Expensive
Variety of waste components Small Great
1.2.5.2. Continuous and Discontinuous Operation
In case when the anaerobic digestion process is in continuous operation mode,
the bioreactor is fed and discharged regularly. Completely mixed and plug flow
systems are operating in that mode during which sufficient substrate is fed into the
reactor to replace the putrefied material as it is discharged. Therefore, in such
systems the substrate must be flowable and uniform to allow its unobstructed
movement, whereas steady provision of nutrients in the form of raw biodegradable
waste enables stable process operation and constant biogas yield. Depending on
the bioreactor type, design and the means of substrate mixing, short circuits may
occur. In such occasions the retention time cannot be guaranteed for the whole
substrate in completely mixed systems. In the discontinuous-batch operation
mode, the digester is completely filled with raw organic material mixed with
digestate provided by another bioreactor and then discharged after a specified
retention time. Batch mode bioreactors are easier to design with a relative lower
cost than plug and flow systems, while they are suitable for dry as well as for wet
fermentation (Table 10) (Jördening and Winter, 2005).
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Table 10: Comparison of continuous and discontinuous feed
Process Operation Continuous Discontinuous
Retention time Shorter Longer
Technical equipment Complex Simple
1.2.5.3. Thermophilic and Mesophilic Operation
In anaerobic digestion, the optimal process conditions are in the mesophilic
temperature range (about 35oC) and in the thermophilic temperature range (about
50-55oC) (RISE-AT, 1998). At this temperatures process condition the methane
fermentation is maximized. Bioreactors designed to operate on mesophilic levels
are heated to 30 - 40oC and this type of systems acquire high stability process,
whereas small temperature deviations have minor effect on the microorganisms.
This is attributed to the fact that a great variety of mesophilic methane bacteria
exists that shows low sensitivity to temperature variation. The main advantage of
mesophilic process operation is the lower amount of energy (e.g. heat) required to
be supplied and the subsequent higher net energy production (RISE-AT, 1998).
On the other hand, thermophilic operation requires temperatures between 50 and
60oC. Under certain conditions the thermophilic process operation enables higher
substrate decomposition rates with subsequent lower retention times. However,
this operation type requires larger amounts of energy to maintain the process
temperature and thus higher energy expenditure. Therefore, the net energy
production is lower than mesophilic operation, whereas the temperature sensitivity
of the thermophilic microorganisms reduces the process stability. In addition,
under thermophilic conditions the sanitization of the substrate might be achieved
for a fixed retention time; otherwise, sanitation has to be achieved in a separate
treatment step or by composting (RISE-AT, 1998). Table 11 lists the advantages
and disadvantages of mesophilic and thermophilic process operations.
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Table 11: Comparison of mesophilic and thermophilic process operation (Jördening and Winter, 2005)
Process Operation Mesophilic (35oC) Thermophilic
Process stability Higher Lower
Temperature sensitivity Low High
Energy demand Low High
Degradation rate Decreased Increased
Detention time Longer or the same Shorter or the same
Sanitation No Possible
1.2.5.4. Agitation
For a high degradation activity of the bacteria, it is necessary to provide the
microorganisms with sufficient degradable substrate, whereas the metabolic
products of the organisms have to be removed (Dauber, 1993). The
aforementioned requirements can be met by mechanical mixing or other agitation
of the bioreactor’s substrate. Another way in achieving the required mixing of the
organic matter is to install a water recirculation system, by which the process
water, which ensures nutrient provision and the removal of metabolic products,
trickles through the biowaste in the reactor (Rilling and Stegmann, 1992). Other
processes use compressed biogas for total or partial mixing of the material.
1.2.6. Anaerobic Digestion Products
The main products resulting from the anaerobic digestion process are the biogas,
the solid end product (digestate) and water.
1.2.6.1. Biogas
Biogas is a mixture of various gases. Independent of the fermentation
temperature, a biogas is produced which consists of 60%–70% methane and
30%–40% carbon dioxide, whereas trace components of ammonia (NH3) and
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hydrogen sulfide (H2S) can be detected. However, the yield biogas depends on
several factors such as temperature, pH and alkalinity, hydraulic and organic
loading rates, toxic compounds, substrate type, and total solids (TS)/volatile solids
(VS) content (Pavlostathis and Giraldo-Gomez, 1991). The caloric value of the
biogas is about 5.5–6.0 kWh m–3 which corresponds to about 0.5 L of diesel oil.
According to Symons and Buswell (1933) the yield and composition of the biogas
can be estimated from the following equation, when the chemical composition of
the substrate is known
242 48248224CObanCHbanOHbanOHC ban
Table 12 shows the mean composition and specific quantity of biogas as
dependent on the kind of degraded substances. For anaerobic digestion of the
organic fraction of municipal solid waste, an average biogas yield of 100 m3 t–1 wet
biowaste and having a methane content of about 60% by volume may be
assumed. The highest yield of methane is accomplished after the bacterial
population has reached its peak and it begins to decrease due to the gradual
depletion of the organic load.
Table 12: Mean composition and specific yields of biogas in relation to the kind of substances degraded (Rilling, 1994)
Substance Gas Yield
(m3kg-1TS)
CH4 Methane
content (Vol. %)
CO2 Carbon Dioxide
Content
(Vol. %)
Carbohydrates 0.79 50 50
Fats 1.27 68 32
Proteins 0.70 71 29
Municipal Solid Waste
(MSW)
0.1-0.2 55-65 35-45
Biowaste 0.2-0.3 55-65 35-45
Sewage Sludge 0.2-0.4 60-70 30-40
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Manure 0.1-0.3 60-65 35-40
The biogas is generally stored in an inflatable bubbles located on top of the
system while in other cases the gas is collected and stored in biogas holders
near to the facility.
1.2.6.1.1. Electricity Supply
The mode of operation of a gas engine depends on type of its use e.g. covers
peak load, covers basic load, supplies its own needs and only feeds the surplus
into the network. The electricity supply mode is determined by the local conditions
as well as the price of electricity. Different plant designs are needed for covering a
constant basic load and for covering peak loads for certain periods of the day.
Peak load covering requires complex and expensive gasholders for longer periods
and larger and more expensive power stations. The worldwide ongoing system of
promoting renewable energy, as from biogas, does not especially consider
whether the power is generated for basic or for peak load and at what time of day
the current is fed into the network. Therefore, biogas plants are normally designed
to cover the basic load, although the produced power depends on the activity of
the microorganism and, as a result, varies. Biogas plants are usually constructed
at places, where the power network is not available and special efforts are
required to connect the central heat and power to the public power network
(Deublin and Steinhouser, 2008)
1.2.6.1.2. Heat Supply
Generally, the economics of biogas industry largely depends on the utilization and
exploitation of the generated heat from biogas combustion. It must be borne in
mind that the heat is produced over the whole year and not only in the winter,
when it can be easily used. The heat could be used for the following purposes
(Jördening and Winter, 2005):
• heating swimming pools and/or industrial plants
• heating stables
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• heating greenhouses
• cleaning and disinfection of the milking equipment
• transformation of warmth in cold e.g. for milk cooling.
1.2.6.2. Digestate
Digestate is the solid end product of the process and contains organic compounds
which are not susceptible to anaerobic microorganisms attack (e.g. lignin). It also
consists of the inorganic remains of the dead bacteria population that have been
developed during the anaerobic treatment (Wastesum, 2006). Digestate is
produced in three different physical states depending on the type of feedstock
material and digestion process. The three forms are namely fibrous, liquor or a
sludge-based combination of the two aforementioned fractions. More specifically in
the case where two-stage anaerobic system is applied, fibrous and liquor
digestates are produced from the different digestion tanks. In single stage
digestion systems the two digestate fractions are being combined and need to be
further processed in case when separation of the two forms is required.
When the digestion is complete, the residue slurry is removed and dewatered to
produce a liquid stream and a drier solid. The water content is filtered out and re-
circulated to the digester, and the filter cake is cured aerobically, to form compost.
The final product is screened for any undesirable materials, (such as glass shards,
plastic pieces etc) before being used on the land and sold as organic soil
amendment to condition and improve soil (Ostrem, 2004). It must be noted that the
produced digestate may contain ammonia a compound which is potential
phytotoxic to plants while testing for pathogenic microorganisms shall be provided
especially in case where the time temperature regime in not sufficient for their
inactivation. However, pathogen destruction can be guaranteed at thermophilic
temperatures with a high SRT (Ostrem, 2004). Therefore, the digestate is
generally composted after the digestion in order to produce high quality end
product. It must be stated that anaerobic digestion does not reduce nutrient
content (NPK value), making the digestate more valuable as a fertilizer (Mahony
and O'Flaherty, 2002).
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1.2.6.3. Wastewater
This wastewater resulting from the anaerobic digestion process comes from the
moisture content of the produced digestate that is treated (e.g. dewatered) as well
as the water that is being produced during the biological reactions within the
digesters.. The wastewater that is being collected is generally recirculated to the
system aiming to adjust and regulate the water content of the feedstock material
whereas its excess is treated accordingly. Typically, wastewater contains high
levels of organic load which is mainly not biodegradable and often it is required to
be further processed (Wastesum, 2006).
1.2.7. Market potential for products
The methane in biogas is utilized as a renewable energy source to co-generate
heat and electricity, using generally a reciprocating engine and/or microturbine.
The energy generated (electricity & heat) is used to supply the energy requirement
of the system in order to operate in an energy sufficient and energy neutral
manner whereas the excess electricity can be either provided the local grid or sold
to potential suppliers . The biogas that is produced during the anaerobic treatment
of organic waste is considered to be biogenic meaning that it does not contribute
to increasing atmospheric carbon dioxide concentrations since it is not released
directly into the atmosphere while the emitted carbon dioxide originates from
organic sources with a short carbon cycle.
Biogas may require treatment to refine it in order to use it as an energy source.
Hydrogen sulfide is among the main chemical components that need to be
removed from biogas since it constitutes a toxic product and it is released as a
trace component of the biogas. Environmental legislation puts stringent limits on
hydrogen sulfide concentration of biogases. Therefore, gas scrubbing and
cleaning is required when the levels of hydrogen sulfide in the gas are high. The
primary challenge associated with the use of biogas as a fuel is the need for gas
cleaning to ensure that the gas meets the quality requirements for the utilization
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equipment. Biogas cleaning is a capital-intensive, multistage operation that can
also carry high maintenance costs due to media replacements and/or power costs.
However, if the gas impurities are left untreated, they can increase the
maintenance requirements of the equipment fueled by the gas and thus reducing
equipment duration. Therefore, gas cleaning to reduce condensation, lower H2S
levels, and removal siloxanes is a prerequisite for effective gas utilization. Any
foam and sediments entrained in the gas stream are separated using a foam
separator in the digester gas piping, while for scrubbing H2S from biogas the most
commonly used methods include the use of iron sponge or chemical scrubbers
and the addition of ferric (Fe3+) salts to the feed. Finally, for removing seloxanes
there are two common types of systems (1) low-temperature drying systems and
(2) graphite molecular sieve scrubbers.
As has been stated, biogas can be used either for the production of heat only or
for the co-generation of heat and power. Alternatively, a stirling engine or gas
turbine, a micro gas turbine, high - and low - temperature fuel cells, or a
combination of a high - temperature fuel cell with a gas turbine can be used.
Biogas can also be used to produce steam by which an engine is driven, e.g., in
the Organic Rankine Cycle (ORC), the Cheng Cycle, the steam turbine, the steam
piston engine, or the steam screw engine (Deublin and Steinhouser, 2008).
Another very interesting technology for the utilization of biogas is the steam and
gas power station. Figure 14 shows the range of capacities for the power
generators which are available on the market as pilot plants or on an industrial
scale. The electrical efficiency indicates the ratio of electrical power to the total
energy content in the biogas.
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Figure 7: Capacity Range of engines in relation to their electrical efficiency
The generated current and heat can supply the anaerobic bioreactor itself, the
associated buildings, and neighboring industrial companies or houses. The surplus
energy can be fed into the public electricity network, and the heat into the network
for long - distance heat supply.
The acidogenic digestate is a stable organic material comprised mainly of
lignocellulosic compounds, but also of a variety of inorganic elements resulting
from the dead bacterial population. The material can be used as compost or to
make low grade building products e.g. fibreboard. The methanogenic digestate is
rich in nutrients and can be used as a fertilizer dependent on the quality of the
material being digested. Levels of potentially toxic elements should be chemically
assessed. This will be dependent upon the type and composition of the initial
substrate.
1.2.8. Mass and energy balances
A typical mass balance of the anaerobic digestion process is shown in Figure 15.
From the diagram it can be stated that from 1 tonne of organic fraction of
Municipal Solid Waste (OFMSW) that is treated, 120 kg is the produced biogas,
423Kg is the digestate, 437kg is wastewater, while the remaining 10kg is the inert
material of the OFMSW that is removed and disposed to the landfill prior to the
biological treatment process. It must be mentioned that the mass balance
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considers a water recirculation of 370kg to the mixing tank, from the dewatering
process of the generated solid digestate. The recycled process water aims to
adjust/regulate the water to solid ratio of the feedstock, while at the same time
saving money and resources during the operation of the anaerobic treatment
plant.
Figure 8: Typical mass balance for an anaerobic digestion system
(Ostrem K., 2004) One tonne of waste produces between 80 and 130 m3 of biogas, depending on the
process, as has been reported by several large AD design firms treating MSW
(e.g. BTA, Valorga, WAASA, DRANCO, Linde, Kompogas).
The net energy output in anaerobic digestion systems using different raw materials
varies depending on transportation distance, means of transportation, conversion
techniques and needs for handling of raw materials and digested residues. For
Swedish conditions, from a life-cycle perspective, it appears that for transportation
distances up to 50 km, the energy needed for running the biogas systems typically
corresponds to 30-50 % of the energy content in the produced biogas. All raw
materials could be transported more than 150 km, some dry waste streams up to
700 km, before the energy balance turns negative. The higher the water content in
the raw material, the more sensitive the net energy output is to the transportation
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distance. There is great variability in data on biogas yield from different raw
materials, thus estimations about biogas yield strongly affect the net energy
output. Despite inherent uncertainties, the overall conclusion is that the net energy
input in the studied biogas systems normally significantly exceeds the energy
output in the form of produced biogas.
Energy input in various handling and transportation operations for bringing
different raw materials to the biogas plant and for transportation of digested
residues, and the biogas yield from various biomass resources. Values within
parentheses indicate interval found in the literature (EUBIA).
Table 13: Energy input and output from various biomass resources, (EUBIA)
A typical energy flow diagram of the product of anaerobic digestion is presented in
Figure 16 in which the methane content of biogas is 60%3, whereas the remainder
of the gas is predominately CO2, with trace elements of other gases, such as H2S,
NH3 and water vapor. According to Figure 16, 100m3 of biogas (60% CH4) acquire
an energy content of 560kWh which can give 336kWh of thermal and 224 of
electric energy, while the remaining 56.0kWh is attributed to energy losses. The
energy generated can be used within the plant for heat or electric supply purposes
or to sell the electric energy to the local electric network. Anaerobic digestion
plants large enough to produce electricity in a cogeneration unit can be self-
sufficient on the power generation from their produced biogas. The low
3 methane content of biogas ranges from 50% to as high as 75%, though most plants report values close to 60%.
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temperatures required for anaerobic digestion (less than 110°F), allow the heat to
be supplied entirely from the biogas as well.
Figure 9: Typical energy balance for an anaerobic digestion system (Ostrem
K., 2004)
1.2.9. Parameters effecting anaerobic digestion process
naerobic fermentation processes are affected by the changes in environmental
conditions; therefore, it is important to examine some of the important factors that
govern the anaerobic bioconversion process. These include organic loading rate,
biomass yield, substrate utilization rate, hydraulic retention time (HRT) and solids
retention time (SRT), start-up time, microbiology, environmental factors and
reactor configuration. The following sections elaborate on these factors.
1.2.9.1. Organic Loading Rate
In anaerobic process the loading parameters are expressed in terms of organic
loadings. More specifically, the organic loading rate of solid waste and organic
sludge is based on volatile solids (VS), while for wastewater it is expressed as
BOD or COD. Conventional environmental engineering practice has been to
express digester loadings on a weight to volume basis per unit time (kilograms of
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VS per day per cubic meter of volume - kg/d/m3). The stability of the anaerobic
fermentation and the biogas production rate are dependent upon organic loading
rates. In cases where organic loading rate is higher than normal, the digestion
process often becomes unbalanced due to the excessive production of volatile
acids to inhibitory concentrations. CO2 production under these conditions often
causes foaming of the digester and contributes to operating problems.
Maintenance of uniform or near uniform loading rates based on frequent or
continuous additions of substrate to the digester yields the most consistent
digester operation (Deublin and Steinhouser, 2008).
1.2.9.2. Biomass Yield
Biomass yield is a quantitative measure of cell growth in a system for a given
substrate which is represented by the yield coefficient (Y), given by the following
equation (Khanal, 2008).
Y = X/ S
where
X biomass concentration (mg VSS/L),
S substrate concentration (mg COD/L).
The biomass yield per mole of ATP totals 10.5 g volatile suspended solids for both
aerobic and anaerobic processes (Henze and Harremoes 1983). With respect to
the metabolic processes of microorganism, the total aerobic ATP generation is 38
mol, whereas for anaerobic digestion it is only 4 mol ATP/mol glucose. Therefore,
the biomass yield for the anaerobic treatment process is significantly lower
compared to the aerobic one. Anaerobic degradation of organic matter is
accomplished through a number of metabolic stages in a sequence by several
groups of microorganisms working synergistically. The yield coefficients for
different biological treatment processes and stages are presented in Table 14.
According to the table it can be seen that the yield coefficient of acid-producing
bacteria is significantly higher than that of methane-producing bacteria (Henze and
Harremoes, 1983), whereas in the aerobic treatment process for biodegradable
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COD irrespective of the type of substrates the yield coefficient is fairly constant
(van Haandel and Lettinga, 1994). In anaerobic digestion treatment process, the
yield coefficient depends not only on COD removed but also on the different
substrate conditions being metabolized as shown in Table 15. More specifically,
carbohydrate and protein compounds have relatively high yield coefficients, since
both microorganism groups, acidogens and methanogens, are involved in the
decomposition process of the above mentioned substrates to produce methane.
Therefore, the yield coefficients of the aforementioned compounds result from the
summation individual yield coefficient of acidogenic and methanogenic bacteria.
On the other hand, chemical compounds, such as acetate and hydrogen, have
lower yield coefficients, since only methanogenic bacteria are involved in the
metabolism of these substrates.
Table 14: Biomass yield coefficients for different biological treatment processes and stages (Young and McCarty,1969; Henze and Harremoes,
1983; van Haandel and Lettinga, 1994)
Process Yield Coefficient (Kg VSS/kg COD)
Acidogenesis 0.15
Methanogenesis 0.03
Overall 0.18
Anaerobic filter (mixed culture)
(carbohydrate + protein as substrate) 0.115-0.121
Anaerobic treatment process 0.05-0.15
Table 15: Biomass yield coefficients for different types of substrate (Pavlostathis and Giraldo, 1991)
Type of substrate Yield Coefficient (Y)
(Kg VSS/kg COD)
Carbohydrate 0.350
Proteins 0.250
Fats 0.038
Butyrate 0.058
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Propionate 0.037
Acetate 0.032
Hydrogen 0.038
The estimation of the attainable methane production depends on various
parameters among which the organic matter composition, the granulation of
waste, the proportions of the involved substrates, the level of microbial
degradability of the biomass, the relationship between nutrients (e.g. C/N ratio)
and the moisture and organic matter content. Additional factors which affect
methane yield are related to the anaerobic digestion method employed. Among
these factors are the number of stages, the temperature level (i.e. mesophilic,
thermophilic), the retention time of the substrate in the bioreactor, the type and
frequency of substrate agitation, and the quantity and frequency of the substrate
addition. These parameters must be analyzed in a laboratory test as well as in a
pilot scale to confirm the obtained results prior to the construction of a production
plant. The degradability of the substrates, the biogas yield, the maximum
recommendable volume load, the possible and practical substrate mixtures and
the changes of the concentrations of certain materials are important if large scale
anaerobic digestion plants are to be operated (continuous or batchwise mode) and
they can be determined through laboratory test (Reher, 2003). Before a large -
scale plant is constructed, the results from the laboratory test should be confirmed
in a pilot plant for the preliminary test of the fermentation process. According to
Reher (2003) and Tiehm and Neis (2002) a pilot plant should consists of a
hydrolyzer, a methane reactor, and a storage tank which should be individually
equipped with arrangements for maintaining moderate temperatures, and with
filling and cleaning devices. The recommended measurements evaluated and
monitored in the laboratory and/or pilot plant tests are the following (Deublin and
Steinhouser, 2008):
• Temperature
• pH value and redox potential
• dry matter, water content
• Content of organic dry matter (Loss on Ignition)
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• Degradability as total content of organic acids/acetic acid equivalent and
inhibitors
• Salt content
• Total content of N, P, K, Mg and S
• Availability of plant nutrients such as NO3-, NH4
+, P2O5, K2O, and Mg
• Granulation (maximum grain size), gross density
• Heavy metals (e.g. Pb, Cd, Cr, Cu, Ni, Zn, Hg)
• Content of short - chain fatty acids, principally acetic acid, propionic acid,
butyric acid, and iso - butyric acid
• C/N ratio.
The above mentioned measurements give important information on the biogas
yield, the level of nutrients, the extent of decomposition of the biomass which can
be provided during the fermentation, the fertilization value of the residue, and also
the preferable type, dimensions and mode of operation of the production plant.
1.2.9.3. Specific Biological Activity
Specific biological activity indicates the ability of microorganisms/biomass to utilize
and metabolize the substrate. According to Khanal (2008) specific biological
activity is usually reported as:
Specific substrate utilization rate = (kg CODremoval)/(kg VSS·day)
The anaerobic digestion process has a substrate utilization rate between 0.75–1.5
kg COD/kg VSS day, which is significantly higher than of composting (Henze and
Harremoes, 1983; Khanal, 2008). The reason of this difference is due to the fact
that oxygen transfer and diffusion limitation is not an issue in an anaerobic
digestion processes as it is in aerobic treatment plants. Additionally, when high
concentration of different substrates in close proximity through biomass
immobilization or granulation is maintained, a good balance of synergetic relation
between acidogens and methanogens can be obtained. Finally the improvement of
the understanding of the trace nutrient requirements of methanogens has
significantly increased the specific activity of anaerobic systems (Speece, 1983).
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1.2.9.4. Hydraulic Retention Time and Solids Retention Time
Hydraulic Retention Time (HRT) and Solids Retention Time (SRT) and are two
important design parameters in anaerobic digestion treatment processes. The
HRT is the ratio of the reactor volume to the flow rate of the influent substrate.
Therefore, it is the time that substrate spends in the bioreactor in contact with the
biomass (Cecchi et al., 2003). The time required to achieve a given degree of
treatment depends on the rate of microbial metabolism and subsequently on the
type and composition of input organic material. Waste containing readily available
biodegradable compounds such as sugar, require low HRT, whereas complex
waste, e.g. lignin organic compounds, is slowly degradable and needs longer HRT
for their decomposition.
SRT is the average residence time of solids into the reactor and it estimated by the
ratio between the content of total solids in the reactor and the solids flow rate
extracted from the reactor (Cecchi et al., 2003). Therefore, SRT controls the
biomass in the reactor to achieve a given degree of waste stabilization, whereas it
determines the permissible organic loading rate in the anaerobic process. If the
quantity of biomass extracted from the reactor is equal to the biomass produced in
the reactor, then the solids concentration in the reactor, as biomass, will be
constant in a given time and it can be said that the reactor is operating in steady-
state conditions. SRT is a measure of the biological system’s capability to achieve
specific effluent standards as well as to maintain a satisfactory biodegradation rate
of pollutants.
According to Speece (1996) the HRT is considered in process design especially
for complex and slowly degradable organic pollutants, whereas the SRT is a
design deciding factor for easily degradable organics. In case of methanogenic
bacteria, which are slow-growing microorganisms, special attention shall be given
to prevent their washout from the reactor in order to achieve a longer SRT.
Elevated HRTs require a bigger reactor volume thus increasing capital
expenditure. An early attempt to maintain a long SRT irrespective of HRT was the
use of the clarigester or anaerobic contact process, where the anaerobic sludge
was allowed to settle in the settling tank and was then returned back to the
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reactor. A wide variety of high-rate anaerobic reactors have been able to maintain
extremely high SRTs due to biomass immobilization or agglomeration. Such
systems operate under short HRTs without any fear of biomass washout. The
empirical HRTs for different anaerobic systems to achieve the same degree of
treatment are presented in Table 16.
Table 16: HRTs of anaerobic systems needed to achieve 80% COD removal efficiency at temperature >20 C (Van Haandel and Lettinga, 1994)
Anaerobic System HRT (h)
UASB 5.5
Fluidized/expanded bed 5.5
Anaerobic filter 20
Anaerobic pond a 144 (6 days) a : BOD removal efficiency.
1.2.9.5. Start-Up Time
Start-up is the initial operational period during which the process is brought to a
point where normal performances of the biological treatment system can be
achieved with continuous substrate feeding. Start-up time is important parameter
in anaerobic digestion processes due to the slow growth rate of anaerobic
microorganisms, especially methanogens, and their susceptibility to changes in
environmental factors. Anaerobic treatment systems often need quite a long start-
up time, which may weaken their competitiveness with aerobic treatment systems.
A start-up time between 2–4 months is generally obtained at a mesophilic
operational mode (35 C), whereas under thermophilic conditions (55 C) start-up
exceeds 12 months because of the high decay rate of biomass (Khanal, 2008).
The start-up time could be significantly reduced in case when the exact microbial
culture for the waste treated is used as a seed, thus leading to increased
generation time of the anaerobic microorganisms. To further reduce start-up time,
substrate loading rates and environmental factors such as nutrient availability, pH,
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temperature and redox potential should be maintained at levels favoured by
microbes during the start-up.
1.2.9.6. Microbiology
The microbiology of the anaerobic treatment system is much more complex than
composting, since digestion involves a sequential multistep process in which a
consortium of microorganisms working synergistically degrades the organic
matter. The stability of an anaerobic treatment plants constitutes a challenge due
to the sensitivity of anaerobic microorganisms, especially methanogenic bacteria,
to potential changes of the environmental factors such as pH, temperature, redox,
sufficiency of nutrients and trace elements. Special focus should be given to
anaerobic digestion operation to maintain suitable for the microorganism
conditions, since in case of system failure (e.g. unfavorable environmental
condition and/or biomass washout from the reactor) it may take significant time for
the system to return to a normal operating conditions due to the slow growth rate
of methanogenic bacteria.
1.2.9.7. Environmental Factors
It has been pointed out earlier that anaerobic processes are severely affected by
the changes in environmental conditions. Anaerobic treatment system is much
more susceptible than the aerobic one for the same degree to deviation from the
optimum environmental conditions. The successful operation of anaerobic
reactors, therefore, demands a meticulous control of environmental factors close
to the comfort of the microorganisms involved in the process. The effect of
environmental factors on treatment efficiency is usually evaluated by the methane
yield because methanogenesis is a rate-limiting step in anaerobic treatment of
wastewater. Hence, the major environmental factors are usually governed by the
methanogenesis. Brief descriptions of the important environmental factors are
outlined here.
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1.2.9.7.1. Temperature
Anaerobic processes, like other biological processes, strongly depend on
temperature. There are mainly two temperature ranges that provide optimum
digestion conditions for the production of methane – the mesophilic and
thermophilic ranges. The anaerobic conversion of organic matter has its highest
efficiency at a temperature 35–40 C for mesophilic conditions and at about 55 C
for the thermophilic conditions (van Haandel and Lettinga 1994). Anaerobic
processes, however, can still operate in a temperature range of 10–45 C without
major changes in the microbial ecosystem. Generally, anaerobic treatment
processes are more sensitive to temperature changes than the aerobic treatment
process. Temperature variations of ±3°C have minor effect on the fermentation
(Winter, 1985). In the thermophilic range (between 55 and 65°C), a constant
temperature level has to be maintained, since small deviations may cause a
drastic reduction of the degradation rates and thus of biogas production. Igoni et
al. (2008) and Tchobanoglous et al. (1991) proposed that the optimal temperature
ranges are the mesophilic, namely 30–38°C, and the thermophilic 44–57°C (Igoni
et al., 2008), respectively. It has been observed that higher temperatures in the
thermophilic range reduce the required retention time.
1.2.9.7.2. pH
The optimum operating pH depends upon the anaerobic fermentation stage and
subsequently on the associated bacteria namely acid-producing and methane-
producing bacteria. During digestion, the two processes of acidification and
methanogenesis require different pH levels for optimal process control. More
specifically, acidogenic bacteria prefer a pH between 5.5 and 6.5, while
methanogenic bacteria prefer a range of 7.8–8.2. In an environment where both
cultures coexist (e.g. one stage process), the optimal pH range is 6.8–7.4. A
favorable pH range for methanogenic bacteria is between 6 and 8 with an optimum
pH for the group as a whole near 7.0. In case where the process takes place in a
single bioreactor, methanogenesis is considered to be the rate-limiting step and it
is necessary to maintain the reactor pH close to neutral. Normally, acid and
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ammonia production vary only slightly due to the buffering effect of carbon
dioxide/bicarbonate (CO2/HCO3–) and ammonia/ammonium (NH3/NH4
+), which are
formed during fermentation, and the pH normally stays constant between 7 and 8.
1.2.9.7.3. Water Content
Bacteria consume the available organic substrate in dissolved form. Therefore, the
production biogas and the water content of the initial organic matter are
interdependent. When the water content is below 20% by weight, biogas
production is significantly limited, whereas increasing water content biogas
production is enhanced, reaching its optimum at 91%–98% water by weight
(Kaltwasser, 1980).
1.2.9.7.4. Oxidation–reduction potentials (ORP)
Methane bacteria are very sensitive to oxygen and have lower activity in the
presence of oxygen thus reducing biogas yield. According to Mudrak and Kunst
(1991) the anaerobic process shows a certain tolerance to limited, even
continuous, quantities of oxygen. The redox potential can be used as an indicator
of the process of methane fermentation, since methanogenic bacterial growth
requires a relatively low redox potential. Hungate (1966) found that -300 mV is the
minimum redox value, whereas Morris (1975) reported that the optimum ORP
value for the growth of anaerobic microorganisms in any medium, is between -200
to -350 mV at pH 7. Finally, according to Archer and Harris (1986) and Hungate
(1967) it is has been established that methanogens require an extremely reducing
environment, with redox potentials as low as 400 mV.
1.2.9.7.5. Nutrients and Trace Metals
All biological treatment methods require nutrients and trace elements during waste
processing. Nutrients and trace metals are not directly involved in waste
processing and stabilization but they are the essential components of existing
microbial cells growth and synthesis of new cells. Therefore, the presence of
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nutrients and trace metals provide the needed physicochemical conditions for the
optimum growth of microorganisms. If the digested organic material does not have
one or more of the important nutrients and trace elements, the waste degradability
is severely affected, since microbial cells are unable to grow at optimum rate and
to produce new cells.
1.2.9.7.6. Toxicity and Inhibition
Anaerobic microorganisms are inhibited by the substances present in the influent
waste stream and by the metabolic byproducts of microorganisms. Ammonia,
halogenated compounds, heavy metals and cyanide are examples of the former,
while ammonia, volatile fatty acids, and sulfide are examples of the latter.
Toxicants, components in the feed material causing adverse effects on bacterial
metabolism, are responsible for the occasional failure of anaerobic digesters. With
reference to investigations of Konzeli-Katsiri and Kartsonas (1986), Table 17 lists
the limit concentrations (mg L–1) for inhibition and toxicity of heavy metals in
anaerobic digestion.
Table 17: Inhibition of anaerobic digestion by heavy metals (Konzell-Katsiri and Kartsonas,1986)
Heavy Metal Inhibition (mg L–1) Toxicity (mg L–1)
Copper (Cu) 40-250 170-300
Cadmium (Cd) - 20-600
Zinc (Zn) 150-400 250-600
Nickel (Ni) 10-300 30-1000
Lead (Pb) 300-340 340
Chromium III (Cr) 120-300 200-500
Chromium VI (Cr) 100-110 200-420
1.2.9.7.7 Volatile Fatty Acids (VFAs)
Volatile fatty acids accumulation during process imbalance directly reflects a
kinetic uncoupling between acid producers and consumers (Switzenbaum et al.,
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1990). The volatile fatty acids concentration has been most suggested for
monitoring of anaerobic digester (Hill and Holmberg, 1988; Lahav et al., 2002;
Feitkenhauer et al., 2002; Mechichi and Sayadi, 2005). In a low buffered system,
pH, partial alkalinity and VFA measurements are useful for process monitoring,
whereas in highly buffered system only VFA is reliable for indicating process
imbalance (Murto et al., 2004).
1.2.9.8. Reactor Configuration
The configuration of the anaerobic digester is of paramount importance in
anaerobic fermentation processes. The relatively low biosynthesis rate of
methanogens in an anaerobic system demands special consideration for
bioreactor design. The selection of bioreactor types depends on the requirement of
a high SRT/HRT ratio, in order to prevent the washout of slow-growing and
sensitive methanogenic bacteria. Therefore, the anaerobic digestion performance
of the bioreactors is based on their capability to maintain a high SRT/HRT ratio
and thus to retain biomass. Another approach for reactor configuration selection is
based on required effluent quality. Because of relatively high half-saturation
constants for anaerobic microorganisms, continuous stirred tank reactors may not
be suitable, as immediate dilution of the waste leads to low concentrations of
organic matters, but still too high to meet the effluent discharge standards, which
are below the range of anaerobic degradation. Under such conditions, a staging or
plug flow type reactor would be more appropriate (Khanal, 2008).
1.2.10. Environmental impacts
The technologies used for the anaerobic digestion appear to be more ecological
than those of composting. The three categories of greenhouse effect, acidification
and heavy metals play an important role in the environmental impact assessment.
Carbon dioxide emission cannot be prevented, if biogenic matter is degraded. On
the other hand, methane is freed in nature as soon as biomass is piled up into
heaps. For an aerobic treatment after anaerobic digestion, there is the
disadvantage that the organic matter is well inoculated with anaerobic bacteria.
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Even if just a very small share of the organic matter is degraded during
composting after anaerobic digestion, methane emissions may be larger than
those caused by composting alone. As far as energy is concerned, digestion
plants are very good from an ecological point of view, mainly because they do not
need external fossil and nuclear energy. The production of renewable energy has
positive consequences on nearly all impact categories, because of savings in or
compensation for non-renewable energy.
1.2.11. Economic data
It is difficult to discuss in detail the economics of deploying an anaerobic digestion
plant for biowaste, because of the many factors that affect the costs and the
variation in circumstances and costs between different countries. When comparing
systems costs, one must consider which of the following cost items are included in
the analysis (1) Predevelopment costs (Siting and permitting, Land acquisition,
Environmental impact assessment, Engineering planning and design,
Hydrogeological investigation), (2) Construction costs (Infrastructure e.g. access
roads, piping, utility connections, Cleaning and excavation, Buildings and
construction, Equipment e.g. tanks, machinery, electronics, Labor), (3) Operating
costs (Maintenance fees, Labor, Materials, Water and energy, Supervision and
training, Insurance, Overheads, Wastewater disposal, Solid residuals disposal,
Regulatory fees. Figure 17(a) presents the capital cost curves for European MSW
digesters, while Figure 17(b) presents their maintenance and operational costs
incorporating the above mentioned parameters (CIWMB, 2008).
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Figure 10: (a) Capital cost and (b) M&O costs curves for European MSW
digesters (CIWMB, 2008)
In general, when looking at the treatment cost per tonne of MSW for the large
facilities built in Europe, it is clear that over the last few years the trend is for a
reduction in overall treatment costs making anaerobic treatment systems more
competitive. However, economies of scale mean that the complex industrial
systems need to process many thousands of tonnes of MSW per year to have a
reasonable treatment cost per tonne. According to the Carbon Finance Unit of the
World Bank in 2008, the capital cost of an anaerobic digestion treatment system
with a capacity of 300 tonnes/day is around 15-55 million € (319€ per tonne capital
cost), while the operation and maintenance cost is 40-70 € per tonne.
Another source estimates the relevant treatment costs to 70 – 100 €/t (Neamt
Master Plan, 2008). Based on recent EU data, the financial cost of anaerobic
digestion in EU member states (including annualized capex, opex, maintenance
expenditures and other specific costs or revenues) varies between 48 - 107
€/tonne for the various end uses of biogas namely electricity, CHP, supply to the
grid or as vehicle fuel (EC, 2010). Finally according to the different uses of biogas
the report provided by ARCADIS & EUNOMIA (2010) suggests the following costs
for anaerobic digestion:
AD with Electricity Only: capital costs 375€ per tonne, operating costs
37.50€ per tonne for facilities with a capacity ranging from 20,000 to 30,000
tonnes with appropriate post-treatment of the produced digestate.
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AD with combined heat and power: capital cost of 478 € per tonne, and an
operating cost of 38.75 € per tonne
AD with Gas Upgrading for Use as Vehicle Fuel: capital cost of 440 € per
tonne and operating expenditures of 45.25 € per tonne
AD with Gas Upgrading for Use in Grid: capital cost of 275 € per tonne
whereas for operating expenditures no data were available.
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1.3. MECHANICAL BIOLOGICAL TREATMENT (MBT)
A mechanical biological treatment system is a waste processing facility that
combines a waste sorting facility with biological treatment methods e.g. anaerobic
digestion and/or composting. MBT plants are designed to process mixed
household waste as well as commercial and industrial waste. Therefore, MBT is
neither a single technology nor a complete solution, since it combines a wide
range of techniques and processing operations (mechanical and biological)
dictated by the market needs of the end products. Thus, MBT systems vary greatly
in their complexity and functionality. Figure 18 presents a process diagram of a
Mechanical Biological Treatment facility.
Figure 11: Mechanical Biological Treatment flow chart
1.3.1. Mechanical sorting component
The "mechanical" element is usually an automated mechanical sorting stage. This
either removes recyclable elements from a mixed waste stream (such as metals,
plastics, glass and paper) or processes them. MBTs typically involve a
combination of screens, magnetic separation, eddy current separation, optical
separation and air classification.
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The mechanical sorting processes recover a part of MSW as recyclable materials,
while another part formulates a combustible product known as ‘Refuse Derived
Fuel (RDF) which covers a wide range of materials sorted in such a manner in
order to obtain high calorific value. RDF can be incinerated in power stations,
pyrolysis and gasification systems, co-incinerated in other industrial combustion
processes for energy production.
1.3.2. Biological processing compartment
The "biological" element includes the biological treatment of the biodegradable
organic materials that has been sorted, since after the mechanical sorting stage.
Therefore, the biological processing compartment refers to the following methods:
Aerobic treatment (composting)
Anaerobic digestion
Biodrying
By applying composting, the organic materials are treated with aerobic
microorganisms. The microorganisms break down the organic compounds into
carbon dioxide and a stabilized solid end product (compost). More details on the
aerobic treatment of organic waste are given in section 1.1.
Anaerobic digestion breaks down the biodegradable organics to produce biogas
(mainly methane) and a stabilized solid end product which has similar
characteristics and potential applications with compost. The biogas can be used,
after cleaning, to generate electricity and heat. More information on the anaerobic
digestion of organic waste are given in section 1.2.
Biodrying of organic waste material involves the rapid heating of waste through the
action of aerobic microbes. During this partial composting stage the heat
generated by the microbes result in rapid drying of the waste. These systems are
often configured to produce a refuse-derived fuel where a dry, light material is
advantageous for later transport combustion.
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Some MBT systems incorporate both anaerobic digestion and composting
treatment methods. This may either take the form of a full anaerobic digestion
phase, followed by post - composting of the produced digestate. Alternatively a
partial anaerobic digestion phase can be induced on water that is percolated
through the initial substrate, dissolving the readily available organic matter, with
the remaining material being sent to a windrow composting facility.
1.3.3. Mass and energy balances
A typical mass balance diagram of an MBT process with aerobic digestion is
shown in Figure 19. According to the figure, for 100 tn of processing MSW, 46kg is
assumed to be the biodegradable organic fraction which is mixed with additives
producing a final end compost which accounts to 18tn (39% of the biodegradable
waste treated). During the process of composting, 18tn are the mass losses due to
the leachate and emissions production, while 9.6tn is the residue that remains
after refining the mature compost.
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Figure 12: Schematic presentation of inputs and outputs of a typical
mechanical sorting component with aerobic digestion (Juniper, 2006)
Below a mass balance of an MBT with anaerobic digestion is presented. This is
the process whereby only a fraction of 50% to 65% of the total organic fraction is
actually digested, while the remaining 50 to 35% is bypassed and is not subjected
to anaerobic decomposition. The digested residue is then intensively mixed with
the non-digested organics. The dry matter concentration of 45% in the resulting
mixture of the two fractions allows for efficient aeration and rapid aerobic
decomposition. A plant treating 100,000 ton per year of residual solid waste is
recovering recyclables and producing burnable fractions. About 28,000 tonnes per
year of organics are diverted to digestion, to which also about 7,000 ton per year
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of non-digested dewatered sewage sludge is added. No wastewater is generated
at the plant.
Figure 13: Schematic presentation of inputs and outputs of a typical
mechanical sorting component with anaerobic digestion
A typical mass balance diagram of an MBT process with biodrying is shown in
Figure 21. According to the figure, for 1 tn of processing MSW, during the
biodrying process 250kg of gas products are produced, while the remaining
material is separated mechanically to 550kg of Solid Recovered Fuel (SRF), 35kg
metals and 165kg residues.
Figure 14: Schematic presentation of inputs and outputs of a typical
mechanical sorting component with biodrying
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The energy for reaching high temperatures and for drying during the aerobic
phase is mainly provided by the fraction that was not digested. The energy
balance for an MBT treatment process can be calculated through the energy
balances of the composting and the anaerobic digestion process.
1.3.4. Market potential for products
The products of the Mechanical Biological Treatment technology are:
Recyclable materials such as metals, paper, plastics, glass etc.
Unusable materials (inert materials) safely disposed to sanitary landfill
Biogas (anaerobic digestion)
Organic stabilized end product
refuse derived fuel - RDF (High calorific fraction).
MBT systems can form an integral part of a region's waste treatment
infrastructure. These systems are typically integrated with curbside collection
schemes. In the event that a RDF is produced as a by-product then a combustion
facility would be required. Alternatively MBT practices can diminish the need for
home separation and curbside collection of recyclable elements of waste. This
gives the ability of local authorities and councils to reduce the use of waste
vehicles on the roads and keep recycling rates high.
1.3.5. Environmental impacts
The environmental impacts produced from the MBT can be drawn account being
taken of the respective environmental impacts of the composting and the
anaerobic digestion process.
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1.3.6. Economic data
The treatment cost of a mechanical biological treatment system with a capacity of
150,000 tonnes per annum is around 45 €/tn (Neamt Master Plan, 2008).
According to ARCADIS & EUNOMIA (2010) a typical mechanical sorting
component with biodrying acquires a capital cost of €250 per tonne, with operating
costs of €21 per tonne before residue disposal.
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1.4. CASE STUDIES OF BIOLOGICAL TREATMENT SYSTEMS IN THE TARGET AREA
1.4.1. General
As proved by the results of the analyses of the composition of municipal solid
waste that took place in both Romania and Bulgaria by the SUROVINA working
team, the generated waste is characterized by high organic content. Therefore, the
potential for the application of biological methods in Romania and Bulgaria, as well
as the whole Balkan Region is very high. Specific case studies of biological
methods applied in Greece, Romania and Slovenia are described below. More
specifically, information is provided about indicative mechanical biological
treatment facilities in Greece and indicative composting facilities in Slovenia and
Romania.
In Bulgaria currently no composting, anaerobic digestion or MBT treatment plants
are operating and only home composting takes place in 25 municipalities including
5,500 households.
Greece is considered to rely heavily on MBT/mixed waste composting to deliver its
Landfill Directive obligations, as shown by the relevant infrastructure that has
already been developed (Kalamata, Ano Liosia, Chania, Heraklion and Kefalonia).
The composting unit in Kalamata was the first that was built and operated in
Greece from 1997 – 2002 with certain problems and difficulties mainly originating
from the fact that no schemes for the separate collection of organic waste were
applied. Nowadays, the idea of trying to put this unit in operation phase again is
also seriously discussed. Regarding the unit in Chania, according to 2008 data, 34
tones of biodegradable waste are composted every day (8,900 tones on annual
basis). In Athens, there is one MBT plant in Ano Liossia, covering approximately
20% of the whole waste produced in the area, and produces RDF and low quality
compost. During 2009 a biodrying facility started its operation in Heraklion in Crete
(75,000 tn/year) and a MBT facility in the island of Kefalonia (25,000 tn/year).
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Currently, eight (8) aerobic treatment (composting) facilities for bio-waste and two
(2) anaerobic treatment facilities are operating in Slovenia. Both anaerobic
digestion plants are processing organic waste including bio-waste. The anaerobic
digestion plant of Bioenerg has an installed electrical capacity of 1,460 kWe and
the one of Koto d.d. 526 kWe. Totally 13 anaerobic digestion plants are operating
in Slovenia treating various feedstocks. Apart from the composting and biogas
plants some mechanical – biological treatment units are also in operation.
Currently five composting facilities are operating in Romania. The existing
facilities are treating mainly green waste from parks and gardens and a small part
of household organics. Additionally, one composting facility in Region 3 South and
one composting facility in Region 2 South-East are under construction.
1.4.2. Mechanical Biological Treatment Plant in the West Attica Region, Greece
The Ano-Liosia Integrated Waste Management Scheme is situated in the Western
suburbs of Athens in Greece. The entire processing plant comprises of a landfill,
an industrial unit of incineration of hospital waste and an MBT scheme (Figure 22)
for waste. The latter includes a large composting facility. The MBT part for
mechanical recycling of waste is the largest one in Europe and one of the largest
in the world. It receives waste from the Attica region. Currently, the population of
Attica exceeds 4.5 million people. The plant was designed and constructed after
an international tender, which was procured by the Association of Communities
and Municipalities of the Attica Region (ACMAR). ACMAR is the Public Authority
responsible for the management (treatment, recycling and disposal) of Solid
Waste of about 95% of the population of the Attica Region. The construction of the
factory of Mechanical Recycling was funded by the European Union and by the
Greek government.
The MBT plant constructed by ENVITEC is located in Ano Liosia, next to the
sanitary landfill of West Attica and it occupies an area of 178,000m2. It has a
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treatment capacity of 1,200 tn of MSW and 50 tonnes of green waste in a daily
basis which equals to 330,000 tonnes/year. The plant recycles packaging material,
namely ferrous and aluminum materials and solid fuel (RDF: paper and plastic
waste) as well as compost, while the remaining residues are landfilled.
Figure 15: MBT plant in Ano Liosia
The MBT consists of the following components:
A. Entrance Facilities – Weighting of waste, Unit for the Reception of Waste
B. Unit of Mechanical Separation
C. The Composting Unit
D. The refinery unit
E. Curing Unit
F. Packaging Unit
G. Wastewater Treatment Unit
H. Unit for Treatment of Air Emissions from the Composting Unit
I. Unit for Treatment of Air Emissions from the Mechanical Separation Unit.
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1.4.3. Mechanical Biological Treatment Plant in Chania, Greece
The plant of Mechanical Separation and Composting located in Chania was
designed to treat municipal solid waste from a number of municipalities, which
amount to 70,000 tn/yr and 10,500 tn/yr of green waste. The recyclable waste is
separated from mixed waste and sorted into organic, plastic, paper, ferrous and
aluminum materials, while RDF and compost is produced. Overall 65% of this
quantity is recovered and put into the market, while the remaining 35% is disposed
in the nearby sanitary landfill for treatment residues.
The plant is designed to operate six hours/day, 260 days/yr and the installed
capacity is 2.3 MW. The total operational cost of the plant is estimated at 40€/ tn.
The revenues from the sale of the compost product and of the recovered materials
are approximately 15€ / tn. The net operation cost 25 - 30€/ tn.
Figure 16: MBT plant in Chania, Crete
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1.4.4. Mechanical Biological Treatment Plant in Kalamata, Greece
The MBT plant in Kalamata constitutes of a mechanical separation unit and a
composting plant. It processes the total quantity of municipal solid waste
generated in the municipality of Kalamata as well as a portion of the sewage
sludge produced in the Waste Water Treatment plant of the town. The design
capacity of the plant is 400 tn/week mixed MSW and 40 tn/week sewage sludge.
In peak periods, the plant has treated up to 30% more load (520 tn/week).
Figure 17: MBT plant in Kalamata, Peloponnese
1.4.5. Composting Plant for Solid Waste at the Landfill Site in Piatra Neamt, Romania
The primary objective of the Composting Plant for the Organic fraction of Solid
Waste in Piatra Neamt is to compost as much as possible of the municipal
biodegradable waste (organic fraction) aiming at reducing the needed landfill
capacity. The coarse material necessary for the enhancement of the composting
process include wood waste in form of wood chips from the industry in Piatra
Neamt. The capacity of the composting plant is 12,000 tn/yr of organic fraction
(biodegradable), with available space for a possible future extension of 5,000 tn/yr.
This relates to the collected biodegradable organic fraction of the MSW. Structure
fraction for the composting process to operate a structure material is necessary to
ensure C/N ratio, air distribution, etc. The necessary amount is approximately
equal to the organic fraction i.e. 13,000 tn/yr (structure fraction) and with an
extension of 5,000 tn/yr to 18,000 tn/yr. The total design capacity of the
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composting plant in Piatra Neamt is 25,000 tn/yr (12,000 tn biodegradable +
13,000 tn structure fraction).
Figure 18: Composting plant in Piatra Neamt, Romania
1.4.6. Composting Plant in Vrhnika, Slovenia
At the composting plant in Vrhnika about 10,000 tn of separately collected
household waste as well as green and garden waste from the region around
Ljubljana are processed. The plant comprises of an enclosed composting vessel
with wheel loader, turning negative aerating curing area and a positive aerated
compost storage. The material is placed in four closed boxes, turned weekly and
composted for a total of four weeks. For a better control of odour emissions, the
exhaust air from the curing area is treated via a biofilter. After about eight to twelve
weeks, the finished compost is screened and marketed. A further increase in
product quality is added by the storage of the finished compost on the pressure
aerated storage area where there are also aerobic conditions until is placed on the
market (mainly landscaping and horticulture). The operational cost for the
composting plant in Vrhnika is 18 €/tn of waste treated.
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Figure 19: Composting plant in Vrhnika, Slovenia
1.4.7 Composting Plant in Puconci, Slovenia
The composting plant in Puconci treats 4,000 tn/yr of biowaste and green waste.
The plant has an open windrow composting platform with negative aeration and
positive aerated curing area.
Figure 20: Composting plant in Puconci, Slovenia
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THERMAL TREATMENT TECHNOLOGIES
2.1. GENERAL
This section deals with the description of the alternative thermal practices for
municipal solid waste management. Thermal methods for waste management aim
at the reduction of the waste volume, the conversion of waste to harmless
materials and the utilization of the energy that is hidden within waste as heat,
steam, electrical energy or combustible material.
According to the New Waste Framework Directive 2008/98/EC, the waste
treatment methods are categorized as “Disposal” or “Recovery” and the thermal
management practices that are accompanied by significant energy recovery are
included in the “Recovery”. In addition, the pyramid of the priorities in the waste
management sector clearly shows that energy recovery is more desired option in
relation to the final disposal.
Figure 28: Pyramid of the priorities in the waste management sector
That is why more and more countries around the world develop and apply Waste-
to-Energy technologies in order to handle the constantly increasing generated
municipal waste. Technologically advanced countries in the domain of waste
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management are characterized by increased recycling rates and, at the same
time, operation of a high number of Waste-to-Energy facilities (around 420 in the
27 European Member-States).
At European level, there are great variations in the municipal solid waste
management practices applied in the 27 EU Member – States. It can be said that
on average, 40% of the generated municipal waste is landfilled, 40% is recycled
and 20% is incinerated.
Figure 29: Management practices for municipal waste in the EU countries
(Eurostat 2008)
There are EU countries where more than 90% of the generated municipal waste is
landfilled, while the rest 10% is recycled or energetically recovered, while in other
EU countries that are advanced in the waste management field only 10% of
municipal waste is disposed at landfills, 65% is recycled and the rest is subjected
to thermal treatment methods. Most specifically, in relation to the thermal waste
treatment the public opinion is against this alternative and there are EU countries
where no thermal management practices are applied for the management of the
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generated municipal waste (Bulgaria, Estonia, Cyprus, Latvia, Lithuania, Malta,
Romania, Greece), while in other EU countries thermal management practices
are applied at very limited degree (Slovenia (1%), Poland (1%), Ireland (3%).
European countries that are characterized as advanced in the field of solid waste
management have already achieved very high recycling rates and at the same
time use thermal methods for a large part of the generated municipal waste. More
specifically, the percentage of thermal waste treatment is 54% in Denmark, while
the relevant figure for Sweden, Holland, Luxembourg Belgium, Germany, France
(Autret et al. 2007), Austria and Portugal are 49%, 39%, 36%, 36%, 35%, 32%,
27% 19% respectively. It has also to be noted that the green cities in Europe
(Stockholm Hamburg, etc) have incorporated thermal treatment facilities in their
planning for effective solid waste management.
Thermal treatment of municipal solid waste includes all processes that result in the
conversion of the waste content in gas, liquid and solid products with release of
thermal energy.
The thermal waste management technologies can be categorized as follows:
Combustion - Incineratyion
Pyrolysis
Gasification
Plasma technology.
Additional innovative thermal waste management techniques combine
incineration, pyrolysis and gasification, which constitute the three basic thermal
treatment options for solid waste. The application units include typical
constructions of conventional methods. The main reasons justifying the rapid
expansion of new methods include the benefits from the applications. These
benefits can be ecological (environmentally friendly air emissions, low quantities of
inert solid residues), energetic (production of energy and less use of fossil fuels)
and economic (lower capital cost).
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The main targets of the application of thermal management practices in the field of
solid waste management are:
The minimization of the waste quantities that end up at landfills.
The conversion to inert materials that are less harmful for human health and
the environment.
The reduction of the environmental pollution and particularly the avoidance
of the generation and release of volatile substances, such as furans and
dioxins.
The utilization of the colorific value of waste for energy production (heat,
power, fuel).
At this point it should be noted a lot of effort has been made to develop models in
order to correlate of the net calorific value of the waste to be treated with different
characteristics, such as the elemental waste content. For the case of the municipal
solid waste, a large numbers have been proposed, but there is no model that
could be characterized as generally acceptable and reliable. Indicatively, the
following are mentioned:
Dulong: HHV = 81C +342,5 (H – O/8)+22,5 S – 6(9H –W)
Steuer: HHV = 81 (C – 3 x O/8) +57 x 3 x /8 +345 ( – /10) +25S – 6(9H+W)
Scheurer – Kestner: HHV = 81 (C – 3 x O/4) + 342,5H + 22,5 S+ 57x3x O/4 -
6(9H+W)
Chang: HHV=8561,11 C+179,72 H-63,89 S-111,17 O-91,11 Cl-66,94N
Wilson: HHV = 7831 Corg+35,932(H-O/8)+2212S -3545Cinorg +1187 O +578N
In general, it can be expressed that the calorific value of a material depends on the
content in the basic combustible elements, which are C and H and S at lower
degree. Moisture and ash are also main parameters for the potential of energy
utilization of a material. The moisture included in waste constitutes an obstacle for
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the easy thermal treatment, since it requires a significant amount of energy so as
to be removed allowing for waste to be combusted and provide the thermal load
included within the waste. On the other side, ash involves inorganic constituents
included in waste (metals, glass and other inert materials, such as soil), which
cannot act as energy sources (Kathiravale et al. 2003, Menikpura & Basnayake
2009, Rao et al. 2004, Minutillo et al. 2009, Friedl et al. 2005).
The thermal waste treatment facilities have to be combined with collection systems
of the generated waste, sanitary landfills, plants for recovery of materials from
waste, composting facilities, etc.
It has to be noted that Directive 2008/98/EC of the European Parliament and of the
Council of 19 November 2008 on waste and repealing certain Directives
(commonly known as Waste Framework Directive) clarifies when the incineration
of municipal solid waste is energy effective and, therefore, can be considered are
recovery process and not disposal one. More specifically, the Waste Framework
Directive considers incineration facilities for municipal solid waste as recovery
plants in the case that the energy efficiency is above or equal to:
0.60 for incineration installations in operation and permitted in accordance
with legislation before 1st January 2009
0.65 for incineration installations permitted after 31st December 2008.
The energy efficiency is calculated using the following formula:
Energy efficiency R1 = [Ep - (Ef + Ei)] / [0,97 × (Ew + Ef)], where:
Ep = annual energy produced as electricity or heat, expressed in GJ/year. It is
determined by multiplying heat produced for commercial use by the coefficient 1.1
and the energy in the form of electricity by the coefficient 2.6
Ef = energy input to the system from fuels leading to steam production on annual
basis, expressed in GJ/year
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Ew = annual energy contained in the treated waste calculated by the use of the net
calorific value of the waste, also expressed in GJ/year
Ei = energy imported excluding Ew and Ef on annual basis, expressed in GJ/year
0.97 = is a mathematical factor accounting for energy losses due to bottom ash
and radiation.
In the way, the planning of incineration facilities in the future is encourages to go
towards installations characterized by high energy efficiency. The assessment of
the energy efficiency of 231 Waste-To-Energy Plants that represent 70% of the
relevant total capacity at European level showed that 169 plants out them were
characterized by values of energy efficiency higher than 0.60, while 251 plants had
energy efficiency lower than the value of 0.60 or did not reply to the question in
relation to the energy efficiency. Consequently, 40% of the Waste-toEnergy Plants
that are operating within the European Union, Switzerland and Norway already
satisfy the criterion of the Waste Framework Directive so that their operation can
be considered as recover and not disposal (Stengler 2010).
It is noted that the formula referring to the energy efficiency does not constitute
performance indicator of the plant. On the basis of the Waste Framework
Directive, the aforementioned limits are not committing, since the climatological
conditions should also be taken into consideration, since they can influence the
energy efficiency of the plants. The limits on the energy efficiency of waste-to-
energy plants that are set by EU can be satisfied according to the Confederation of
European Waste-to-Energy Plants (CEWEP, www.cewep.eu), even in the case of
exclusive production of electrical energy. In order to confirm the aforementioned
estimation, a thermal waste treatment medium-size unit with capacity 300,000
Tonnes per year produces 25 MW of electrical power with an indicative
performance degree 26.5%, while the value of the energy efficiency is estimated
about R1 = 0,697 (www.wtert.gr).
The satisfaction of the energy efficiency target from a waste-to-energy plant does
not only depend on the energy included within the waste to be treated, but also on
other factors, such as the input to the system from fuels leading to steam
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production and the amount of the other energy forms that is introduced to the
installation. Therefore, it is not possible to determine the energy efficiency just
using data on the net calorific values of waste. Nevertheless, in the case that the
net calorific value is relatively high, then the energy efficiency target can be
achieved more easily.
Thermal waste management methods should be applied together with separation
at source of all materials that can be recycled in order to maximize material
recovery from waste. The advantages of thermal methods in waste treatment are
summarized as follows:
Reduction of the weight and volume of the treated waste: The final solid
residues have weight that varies from 3 to 20% in relation to the initial
weight of waste, depending on the technology that is used. Gasification and
pyrolysis result in lower quantities of solid residues comparing to
incineration.
Absence of pathogenic factors in the products:
The products of thermal treatment, due to the high temperatures that
are developed, are characterized from complete absence of
pathogenic factors.
Demand for limited areas:
The thermal treatment units are characterized by low demands for
land for their installation.
The pyrolysis and gasification processes require less space in
relation to incineration.
Utilization of the energy content of waste:
Through the thermal treatment technologies, the exploitation of the
energy content of waste is possible.
This energy can be either electric or thermal energy.
Reduction of the burden paused to the landfill sites and consequent
increase of their lifetime.
Extraction of the organic fraction of municipal waste from landfill sites, as
required by the relevant legislative framework (Directive 1999/31/EC).
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Indicative disadvantages of the application of thermal methods are the following:
Relatively high capital cost:
Higher than that of other technologies for the management of
municipal waste.
Significant part of the total capital cost, especially for the case of
incineration, is spent on antipollution measures.
Increased operation cost
In general, the thermal management techniques are characterized by
relatively high operation cost. The cost is reduced substantially as
the capacity of the plant increases.
Demand for high quantities of waste:
Especially for the case of incineration – combustion, a minimum
capacity is required so that the units are financially feasible.
Estimated minimum served population from incineration facilities is
100,000 inhabitants (around 50,000 tones of waste annually).
Gasification and pyrolysis can be applied for much lower waste
quantities (around 15,000 tones of waste per year)
Need for specialized personnel.
2.2. INCINERATION
2.2.1. General
Incineration, which is commonly referred as combustion, is the oxidization of the
chemical compounds with oxygen (O2) in order to transform the chemical energy
of solid waste organic matter into thermal energy. The incineration of carbon-
based materials can be implemented in an oxygen-rich environment (greater than
stoichiometric), typically at temperatures higher than 850oC. The incineration of
waste is one of the oldest thermal treatment technologies and the most commonly
used worldwide.
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The process produces a waste gas comprised primarily of carbon dioxide (CO2)
and water gas (H2O). Air emissions also include nitrogen oxides, sulphur dioxide,
etc. The most important factor during the process is the presence of oxygen.
During the full combustion there is oxygen in excess and, consequently, the
stoichiometric coefficient of oxygen in the combustion reaction is higher than the
value “1”. In theory, if the coefficient is equal to “1”, no carbon monoxide (CO) is
produced and the average gas temperature is 1,200°C. The reactions that are
then taking place are:
C + O2 CO2 + 393.77J
CxHy + (x+ y/4) O2 xCO2 + y/2 H2O
In the case of lack of oxygen, the reactions are characterized as incomplete
combustion ones, where the produced CO2 reacts with C that has not been
consumed yet and is converted to CO at higher temperatures.
C + CO2 +172.58J 2CO (3)
The scope of this thermal treatment method is the reduction of the volume of the
treated waste with simultaneous utilization of the contained energy. The recovered
energy can be used for:
• heating
• Steam production
• Electric energy production
In order to achieve the complete incineration of solid waste, a number of
preconditions have to be satisfied. These include the following:
• adequate fuel material and oxidation means at the combustion heart
• achievable of the ignition temperature
• suitable mixture proportion
• continuous removal of the gases that are produced during combustion
• continuous removal of the combustion residues
• maintenance of suitable temperature within the furnace
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• turbulent flow of gases
• adequate residence time of waste at the combustion area (Gidarakos,
2006).
The method could be applied for the treatment of mixed solid waste, as well as for
the treatment of pre-selected waste. It can reduce the volume of the municipal
solid waste treated by 90% and its weight by 75%. The incineration technology is
viable for the thermal treatment of high quantities of solid waste (more than
100,000 tn/year).
Figure 21: Incineration process flow chart
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Indicative incineration facilities operating at European level can be seen in the
photos below.
Picture 1: MSW incineration plant in Amsterdam
Picture 2: MSW incineration plant in Brescia
Picture 3: MSW incineration plant in Vienna
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Picture 4: Incineration Plant in Zorbau, Germany for municipal solid waste and
industrial waste
Picture 5: Incineration plant in Thun, Switzerland for
municipal solid waste and dewatered sludge
Figure 22: Diagrammatic configuration of incineration plant (with energy
recovery) in Paris
2.2.2. Types of incinerators
There are two main types of incinerators (combustion units). The facilities which
need minimum pre-treatment of waste before they are placed for the incineration
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process (mass-fired) and the facilities which operate with specific waste fractions
that are derived from pre-treatment of municipal solid waste (Refused-Derived
Fuel (RDF) and Solid Recovered Fuel (SRF).
Mass-burn incineration (Figure 32) is currently the most widely deployed thermal
treatment option, with almost 90% of incinerated waste being processed through
such facilities. As the name implies, waste is combusted with little or no sorting or
other pre-treatment.
Figure 23: Typical mass-fired waste incineration plant
(with energy production)
There are, of course, many dangers that this process may face, such as the
introduction of dangerous and high-volume objects. These dangers, though, can
be handled with careful monitoring of the whole process by the facility personnel or
by interrupting the process manually when/if needed. The energy contained in
waste also depends on the period of the year, climate and waste composition.
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The second type of facilities described above are those which use a mixture of
specific waste fractions from the whole municipal solid waste mixture that derive
from the pre-treatment of waste in MBT facilities (MBT: mechanical biological
treatment) and is comprised of materials, such as: organics, paper, textiles,
leather, rubber materials, etc. The facilities under this type are less (in terms of
number worldwide) than the mass-fired facilities because of the fact that a pre-
treatment facility for the production of (RDF/SRF) is needed.
Their advantages compared to the mass-fired units are:
1. Faster boiler response than mass burn.
2. Units can be designed for cofiring.
3. Higher thermal boiler efficiency because of the lower excess air
requirements.
4. Boiler and overall plant costs are generally lower than those referring to
mass burn units.
5. Potential for sewage sludge disposal.
6. RDF-fired units are easier in use
7. Less space is needed for their installation
8. Finally, the pre-treatment of municipal solid waste gives the chance to
remove materials such as PVC and metals that contribute to the production
of dangerous gases which are transferred along with the gases produced
from the combustion unit.
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Figure 24: Typical RDF-fired incineration facility
These facilities have the following disadvantages:
1. The need to build, own, and operate a prepared fuel system is required.
2. Because of the required processing facility, the overall facility horsepower
requirements may be higher than those for mass bum units.
3. Depending on the process, some combustible material may be lost in
processing.
The process target is the production of a final mixture with high calorific value. This
is why there are quality specifications which the produced RDF must come in line
with. More specifically:
The minimum of the calorific value should be equal to 4,000 kcal/g (16.744
MJ/kg)
The moisture content should not exceed 20%
The paper and plastic content should exceed 95% (dry weight).
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The process in this type of facilities takes place in special combustion chambers
with total capacity between 8 and 25 t/h (Vehlow 2006).
In Germany the total waste capacity of the first category was 21.5 million tons of
waste in 2009 in addition to the 3.3 million tons of waste of the second category. In
terms of economy the municipal solid waste incinerators are better in terms of
mean energy values (as today), while the second type of incinerators provide
better economic results in the case of high energy values when the pre-treatment
cost is relatively small (Fiege & Fendel 2010).
The incineration facilities represent the main municipal solid waste thermal
treatment method used today. In these units the lignite and cement industries in
which specific waste fractions are used are also included (Fiege & Fendel 2010).
In Table 18 the operational incineration facilities in the United States of America
can be seen:
Table 18: Operation incineration facilities in the USA
Type Number of Units Capacity,(tons/day) Capacity Million
(tons/year)
Mass-fired 65 78,489 24.3
RDF Incineration 15 22,022 6.9
The total population that the incineration facilities serve is estimated to 31 million.
This estimation is based on the assumption that each inhabitant generates 1.3
tons of municipal solid waste on annual basis.
Energy and environmental advantages of waste incineration By using 1 ton of municipal solid waste in a modern incineration facility
approximately 550 kWh are produced, while the use of at least 250 kg of carbon or
the use of 160 liters of oil is prevented. Furthermore, this technology is the only
solution against landfilling of non-recyclable waste, since landfilling produces CH4
which is an important greenhouse gas, where the 40% of methane produced is
being released to the atmosphere even in modern landfills. The methane that
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escapes has a dynamic (GHG - Greenhouse Gas Potential) 23 times greater than
that of the same CO2 volume.
Considering the electricity produced and the CH4 emissions minimization (due to
the minimization of waste being landfilled), the incineration facilities help the
reduction of «Greenhouse gases» by 1.1 to 1.3 tons CO2 per ton of municipal solid
waste being incinerated instead of being disposed at landfills.
Besides the energy advantages that this treatment method provides, thermal
treatment contributes to the minimization of greenhouse gases. This reduction for
just the case of the USA is estimated to 40 million tons CO2.
In 2004 the incineration facilities produced 13.5x109 kWh of electricity, an energy
amount which was more than any other existing source of renewable energy
(besides hydroelectric and geothermal facilities). For instance, wind energy
produced 5.3x109 kWh, while solar energy produced 0,87x109 kWh (Table 19).
Table 19: Electrical Energy production from Renewable Energy Sources in USA in 2002 (except hydroelectric) (DOE-EIA, Annual Energy Outlook 2002)
Energy Source Production in 109 kWh % Energy from RES
Earth (Geothermic) 13.52 28.0%
Waste* 13.50 28.0%
Biogas* 6.65 13.8%
Wood/Biomass 8.37 17.4%
Sun (Heat) 0.87 1.8%
Sun (Photovoltaic) 0.01 0.0%
Wind 5.3 11.0%
Total 48.22 100.0%
* http://www.eia.doe.gov/cneaf/solar.renewables/page/mswaste/msw.html
At European level incineration facilities provided the electricity network with 19 x
109 KWh in 2007, amount of energy which is capable of providing the necessary
electricity for the operation of 148 million lambs (15W) for a whole year. If these
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lambs were set in a line, they would cover the distance from Brussels to Honolulu
(11,800 km).
There are various types of incinerators with various advantages and
disadvantages. The most widely used are: The moving grate, the fixed grate,
rotary-kiln, fluidized bed, etc.
(a) (b) (c)
Figure 25: Three types of incinerators: (a) fixed grate (left), (b) rotary kiln (middle), (c) fluidized bed (right) (Finbioenergy, 2006)
Moving grate incinerator
The grates are placed to the combustion chamber wall and they implement the
following operations:
Movement of the solid waste stream throughout the facility
Provision of air amount at quite steady rate
Material stirring at the main combustion zone
Transfer of the ash produced during the combustion process
The grates must be covered with materials with high tolerance to mechanical
movements, thermal and chemical reactions. Emphasis must be given to the
materials tolerance to S and Cl which are corrosive when combined with high
temperatures. The stages of the main process are:
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Drying: evaporation of the moisture contained within waste with the use of
fire, heated air, eradiation
Vaporization of most of the VOCs (volatile organic compounds) with
temperature rise.
Combustion: The heat needed is provided with the use of irradiation from
the flame and the flame chamber wall.
Gasification and burning: Because of the waste combustion, a great
number of substances turn into gas. The remaining C is fully oxidized, while
the gases from the combustion and gasification process are burnt.
Combustion completion: the end solid product is obtained at the end of the
grate.
Modern incinerators consist of turbines for the hot combustion gases that pass
through the heat exchange sections of the combustion chamber, to be turned into
electric energy or heat.
Rotary kiln Incinerators
This type of incinerator processes a variety of waste streams that other
technologies cannot. This design of incinerator has two chambers: a primary
chamber and a secondary chamber. The primary chamber in a rotary kiln
incinerator consists of an inclined refractory lined cylindrical tube. The movement
of the cylinder on its axis facilitates the movement of waste. In the primary
chamber, there is conversion of the solid fraction to gases, through volatilization,
destructive distillation and partial combustion reactions. The secondary chamber is
necessary to complete gas phase combustion reactions. The unit consists of a
system of gaseous emissions control.
The clinkers spill out at the end of the cylinder. A tall flue gas stack, fan, or steam
jet supplies the needed draft. Ash drops through the grate, but many particles are
carried along with the hot gases. The particles and any combustible gases may be
combusted in an "afterburner".
The chamber has to be covered by materials with tolerance to high temperatures,
while a continuous flow of waste is necessary. The temperature inside the
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chamber is between 800 and 1,400 C, while the effective combustion is achieved
through absolute control of the temperature and the waste movement inside the
facility. Generally, the higher the temperature, less time is needed for the waste to
remain in the combustion chamber.
Because of the fact that the gases produced inside the chamber remain for a short
period in order to achieve full combustion, a chamber for afterburning is placed.
The residues of the chamber are then led to the cooling system.
The agitation of waste inside the chamber depends on the time remaining,
because this is where the agitation takes place. The gases must be low in CO and
H/C considering that the chamber operates with excess of O2 (Gidarakos 2006).
The main parameters of this type of incinerators that should be considered
include:
The temperature of the rotary kiln which leads to the combustion of waste
The internal pressure of the chamber that must be negative in order to
avoid gaseous emissions and particles to the atmosphere
The provision rate for O2 and the waste flow rate so that the process
conditions are suitable.
Fluidized bed Incinerators
A strong airflow is forced through a sand bed. The air seeps through the sand until
a point is reached where the sand particles separate to let the air through and
mixing and churning occurs, thus a fluidized bed is created and fuel and waste can
now be introduced.
The sand with the pre-treated waste and/or fuel is kept suspended on pumped air
currents and takes on a fluid-like character. The bed is thereby violently mixed and
agitated keeping small inert particles and air in a fluid-like state. This allows all of
the mass of waste, fuel and sand to be fully circulated through the furnace.
Evaporation takes place due to the O2 provided, the mixing and the high
temperature.
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Temperature is the main operation parameter for this type of incinerators. It
depends on the waste treated, the gases produced, while the temperature varies
from 750°C to 850°C.
The O2 needed and the retention time are the most important parameters inside
the chamber. The determination of these parameters depends on the waste
provided for processing. The O2 concentration is controlled in order to achieve a
perfect combustion. With this incinerator, temperature variations inside the
chamber can be avoided and, as a result, the production of gases due to
incomplete combustion can also be avoided.
Fuels rich in ash and moisture can also be used for the production of energy. The
rate that fuel is turned to energy becomes even higher and, thus, the need for air
becomes less (55% compared to the common 100%) (Yassin et al. 2009).
Typical incineration facility
A typical incineration facility includes the following parts:
A Weighing System
This system is for weighing solid waste for the better control and recording of the
incoming waste streams and, thus, it is designed to be practical in order to
minimize the time that vehicles remain at this point.
A Reception Site
Due to the fact that waste does not arrive on continuous basis (contrary to the
feeding of the facility), the existence of waste reception and temporary storage site
is considered essential. The design of the site is made in a way that the following
are ensured:
• The unloading time is as little as possible
• All transferred waste is received
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• The homogeneity of the waste that will be used as feeding material is
achieved
• The smooth feeding of the facility is ensured.
Moreover, the design of the reception site should be based on the minimization of
the environmental impact. For instance, the solid waste should remain for a
maximum of two days in order to avoid the relevant odors, while the bottom of the
site has to be characterized by weathering to allow the leachates and washing
wastewater to go away.
Feeding System
The feeding system has to be adapted to the feeding rate and velocity of the
installation.
Combustion Hearths
The ignition of solid waste at incineration facilities is achieved through the use of
specific burner, which operates with secondary fuel. Basic parameters for the
appropriate operation of the combustion hearths are:
• Achievement of the minimum desired temperature
• Adequate combustion time
• Achievement of turbulence conditions / homogenous waste incineration.
Boiler
The boiler is the system with which the energy content of the fuel material (hot off-
gases) can be utilized in a suitable way through steam production (e.g. at
neighboring industrial facilities or for the heating of the neighboring urban areas.
Pressure, temperature and steam production rate are basic parameters for the
effective operation of the boiler. Its construction has external insulation in order for
the system not to lose temperature during the process. The materials used for its
construction must also be tolerant to high temperatures (and temperature
differences) between the inside and the outside of the facility.
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System for the removal of residues
Residues represent 20 - 40% of the weight of the initial waste and are categorized
as:
• Residues that go out of the grates: 20 - 35%
• Residues that go through the grates: 1 - 2%.
The residues are collected at hoppers where they are transferred with the use of
specific cooling system.
Emission control system
The role of the emission system control focuses on particles, HCl, HF, SO2,
dioxins and heavy metals and is discussed below (Niessen, 2002). After the
emissions pass through the boiler, the gaseous emissions pass through a cleaning
facility and, then, they are emitted to the atmosphere. In the cleaning systems a
large number of different technologies for the removal of flying particles, NOx,
x, etc. that are considered to be safe and secure can be applied.
The incineration process can be presented with a mass balance diagram for a
typical incineration facility. The waste streams percentages depend on the
composition of the incoming waste and the emissions control system that is used.
In particular for the production of energy from waste incineration it is estimated
that 1 ton of waste produces approximately 300kWh electricity and 600kWh of
thermal energy.
Air emissions, wastewater and solid waste are the result of the process of waste
incineration. A detailed analysis of the composition and properties is provided
below.
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2.2.3. Air emissions
The gases produced by incineration facilities contain N2 and excess O2, dust
particles, the typical products of combustion and other harmful substances,
depending on the composition of the incoming waste. The main ones are: SO2,
NOx HCl, HF, heavy metals and polycyclic H/C, which are the most dangerous
pollutants in the exhaust gases, such as dioxins and furans.
An average of 4,000 – 5,000 m³ per tonne of waste gas with a temperature of
1,000°C are generated from the incineration, which in the first phase of cleaning of
the produced gas drops sharply to 350°C.
The limit values of air emissions are listed in Tables 20, 21, 22 and 23.
Table 20: Daily average values of air emission limit values (Directive 2000/76/EC on the incineration of waste)
Total Dust 10 mg/m3
Gaseous and vaporous organic substances, expressed as
TOC 10 mg/m3
HCl 10 mg/m3
HF 1 mg/m3
SO2 50 mg/m3
NO & NO2, expressed as NO2, for existing incineration
plants with a nominal capacity exceeding 6 tonnes /hour or
new incineration plants
200 mg/m3
NO & NO2, expressed as NO2, for existing incineration
plants with a nominal capacity 400 mg/m3
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Table 21: Half-hourly average values of air emission limit values (Directive 2000/76/EC on the incineration of waste)
(100 %) (97 %)
Total Dust 30 mg/m3 10 mg/m3
Gaseous and vaporous organic substances,
expressed as TOC
20 mg/m3
10 mg/m3
HCl 60 mg/m3 10 mg/m3
HF 4 mg/m3 2 mg/m3
SO2 200 mg/m3 50 mg/m3
NO & NO2, expressed as NO2, for existing
incineration plants with a nominal capacity
exceeding 6 tonnes /hour or new incineration
plants
400 mg/m3 200 mg/m3
Table 22: Average values over the sample period of a minimum of 30 minutes and a maximum of 8 hours
(Directive 2000/76/EC on the incineration of waste)
Cadmium and its compounds (Cd) total
0,05 mg/m3 Thallium and its compounds (Tl)
Mercury and its compounds (Hg) 0,05 mg/m3
Antimony and its compounds (Sb)
Total
0,5 mg/m3
Arsenic and its compounds (As)
Lead and its compounds (Pb)
Chromium and its compounds (Cr)
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Cobalt and its compounds (Co)
Copper and its compounds (Cu)
Manganese and its compounds (Mn)
Nickel and its compounds (Ni)
Vanadium and its compounds (V)
Dioxins and Furans 0,1 ng/m3
Table 23: Limit values of CO concentrations (Directive 2000/76/EC on the incineration of waste)
Daily average value 50 mg/m3 of combustion gas
at least 95 % of all measurements
determined as 10-minute average
values
150 mg/m3 of combustion gas
all measurements determined as half-
hourly average values taken in any
24-hour period
100 mg/m3 of combustion gas
The concentration of CO in combustion gases (excluding the start and stop) does
not exceed the above limit values.
The competent authority may grant exemptions for incineration plants using
fluidised bed technology, provided that the authorized emission limit value for CO
equals to 100 mg/m3 hourly maximum.
Then, further reference is made to dioxins and furans, which are among the most
dangerous pollutants because characterized by high toxicity (Allsopp et al. 2001).
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Dioxins: They include two aromatic rings joined with a pair of individuals O.
The toxic effects of dioxins (Yang & Kim 2004) and furans had not been
understood worldwide until the late 80's. With the implementation of MACT
regulations, a "toxic equivalent» (TEQ-Toxic equivalent) of dioxin emissions from
Waste-to-Energy Facilities in the U.S.A. has decreased since 1987 1,000 times in
a value of less than 10 gr TEQ per year (Figure 35). On the other side and in direct
relation to the above, the main source of dioxins, as recorded by US EPA (Figure
36) is the uncontrolled burning of waste, which emits about 600 gr per year.
Figure 26: Dioxin emissions in USA (Themelis & Gregory 2002)
Figure 27: Dioxins emission in the USA (Deriziotis, 2004).
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In Germany in 1990 1/3 of dioxin emissions originated from incinerators, and in
2000 the relevant figure was less than 1% although the amount of MSW
incinerated more than doubled in relation to 1990. Overall the level of dioxins from
400gr in 1990 was limited to less than 0.5 gr in 2000.
Furans: They differ from the dioxins only in that the two aromatic rings are joined
by an atom of O.
A total of 75 compounds known as polychlorinated dibenzo-p-dioxins (PCDD) and
135 dibenzofurans (PCDF) are classified in this group of compounds.
Reference to exposure to dioxins is meant exposure to a mixture of PCDD and
PCDF, the toxicity of which is determined by the toxic equivalency factors (Toxic
Equivalent Factors, 1 - TEF), which are calculated in relation to the toxic effects of
2,3,7,8 TCDD, also known as the Seveso poison.
Dioxins and furans are produced in almost all processes of combustion in the gas
phase, while the exact mechanism of their formation remains unknown. It is known
that a temperature of formation is 300°C, in which two reactions are possible, the
formation and decomposition. The presence of chlorinated organic compounds in
the waste to be incinerated and the increased levels of O encourage their creation.
The operating conditions of the incinerator affect decisively the creation of dioxins
and not the composition of the waste and the quantity of PVC contained in them.
They can affect humans through respiration or absorption through the skin, in case
they are released to close proximity to the recipients. In other cases their
introduction in the body is caused by consumption of food and particularly fruit and
vegetables.
There is evidence for the contribution of dioxins and furans to processes of
carcinogenicity in humans, making it necessary to take measures to reduce their
concentrations in the air emissions.
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Primary measures
- Improvement of the incineration of waste and suspended particles (products of
incomplete combustion).
- Optimization of the requirement of O2.
- Improvement of thermal control systems to ensure control of the combustion air
- The use of improved grates.
- Adaptation of appropriate systems of grates to changes in the composition of
the waste (e.g. calorific value).
- Control of the temperature of crossing through the filter at a level lower than
that of the formation of dioxins (200°C).
Secondary Measures
- Improvement of the cleaning of steam boilers (continuous cleaning).
- Preliminary collection of particulate phase before cooling (high temperature
removal of particulate matter).
- Interference in the temperature of the electrostatic filter to reduce the formation
of dioxins.
- Improvement of the systems for the purification of gases by improving the
collection of particulate matter and pollutants.
- Removal of PPDD / PCDF by adsorption of active C.
Gas cleaning systems
The existing technologies on the management of air pollutants are summarized in
Table 24.
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Table 24: Existing technologies for the management and treatment of gaseous pollutants
Pollutant Abatement Technology
Suspended Solids
Cyclones
Electrostatic filters
(Wet - dry)
Bag Filters
Acid Gases
Dry Adsorption
Semi-Dry Adsorption
Wet Sparying
Nitrogen Oxides Selective non-catalytic reduction
Selective Catalytic Reduction
In order to achieve the removal of particulate and gaseous pollutants different
methods of cleaning are employed. These include deposition chambers, which
remove 40% of airborne particles, wetting screens (efficiency 95%), cyclones
(efficiency 60-80%), fluid absorption towers (efficiency 80-95%), electrostatic
precipitator (efficiency 99-99.5%) and bag filters (efficiency 99.9%).
.
Next, the main systems for determining the composition of the gas produced
during the incineration of waste are described (Figure 37).
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.
Figure 28: Cyclones (left), electrostatic precipitators (middle) & bagfilters (right)
Bag filters: The gases pass through porous materials, where the particulates are
trapped. Depending on the requirements, hardware filters are made of natural
fibers, plastic fibers, glass, minerals, etc. The dust which is concentrated in cells of
the filter is removed by vibration or shock or air in countercurrent (Figure 38).
Figure 29: Typical bagfilters
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Electrostatic precipitator (electric filters): The electrostatic precipitator (Figure 39)
consists of the cathode, which may be a simple thin wire and the anode, i.e. the
inner casing of the electric filter. Another device consists of a system of parallel
plates with a potential difference between them. Between cathode and anode
voltage develops at 30-80 kV. When particles enter the field of the cathode, they
become electrically charged and those negatively charged move towards the
positive pole (anode). The speed of the particles depends on their mass and the
Coulomb forces that are developed.
Figure 30: Typical Electrostatic precipitator
Cyclones: The cyclones are based on the development of centrifugal force in the
entry of gases in a symmetric space, which at the bottom is cone-shaped.
Particles due to centrifugal force and the rotational flow are driven to the wall and,
then, removed to the bottom. The cyclones are often deployed along with
electrostatic precipitators.
Besides the removal of suspended solids, the removal of other pollutants is often
necessary, e.g. acid gas, if their content is higher than the relevant acceptable
limits. Emphasis is given on the HCl, generated mainly from the combustion of
PVC, and oxides of nitrogen, sulfur, phosphorus. The only effective and
appropriate way is in this case the operation of towers of wet and dry absorption
(scrubbing). The liquid absorption towers are necessary in any case for
incinerating toxic and hazardous waste.
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The process of liquid absorption is based on absorption of gaseous pollutants
using a selected washing fluid (solvent). The effectiveness of the process depends
mainly on the available surface of the solvent, which controls the mass transfer
from gas to liquid phase. To this end, various techniques are employed, such as:
• Venture type scrubbers
• filling Towers
• disc Towers
• absorption-type film Towers (thin layer).
Most incinerators in central Europe are using the same technology of liquid
absorption. The process takes place in different units consisting of two phases, an
initial acid absorption phase and a second phase, neutral or slightly alkaline. The
configuration of the acid absorption is often spray or venturi type and in that phase
reduction of the temperature of flue gas from 180-200oC to 63-65oC is achieved.
For the second phase (neutral or slightly alkaline) filling towers are mainly used.
Commercially available absorption tower systems operate with or without
producing waste.
Such two-phase systems are quite effective in removing waste gases from the
incineration of halogen hydrides, HF, HCl, HBr, the Hg and SO2. With this
technology the initial concentrations of these compounds in the waste gases are
reduced well below the statutory limits.
The dry or semi-dry absorption towers (Figure 40) based on simple and low cost
technologies exist and are operational in many facilities in the world. In most
cases, the adsorbent medium is either injected directly into the flue gas duct or
through spray towers in dry or semi-dry form. The products of absorption are
removed, in a second phase through a membrane filter. The process of absorption
can be performed with various reagents (limestone, CaCO3, calcium oxide, CaO,
lime, Ca(OH)2, etc.).
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Figure 40: dry or semi-dry absorption towers (scrubbing)
Today, the technology of absorption towers using dry CaCO3 is gradually
abandoned, as the composition of air emissions produced by the treatment does
not comply with strict statutory limits (Dvorak et el. 2009).
2.2.4. Wastewater
Wastewater is generated by the use of water during the incineration process and
in particular:
• ash quenching (0.1 m3 H2O / t waste)
• gas cooling (2 m3 H2O / t waste)
• liquid absorption towers (2 m3 H2O / t waste)
• In some electrostatic precipitator to remove particulates from the collection
points.
The wastewater contains suspended particles, inorganic and organic in solution.
They are toxic and need treatment before discharge into drains. The most
common methods of treatment are the precipitation and then adjustment of the pH.
Wet gas cleaning system: The liquid comes in contact with the gases where
migration of substances from gases in the liquid phase takes place. The
absorption depends on surface transport, residence time and type of fluid. The
fluid system is developed so that it ensures the removal of ultrafine particles that
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are not easy to remove through the application of dry systems, e.g. filters. The
main wet cleaning systems are:
• flush Towers (scrubber)
• Ventouri scrubbers
• Rotating sprinklers.
The limit values of pollutant parameters for discharging wastewater from the
cleaning of exhaust gases are summarized in Table 25.
Table 25: Emission limit values for discharges of waste water from the cleaning of exhaust gases
(Directive 2000/76/EC on the incineration of waste)
Polluting Substances Emission limit values
expressed in mass
concentrations for unfiltered
samples
Total suspended solids as defined by
Directive 91/271/EC
95% /
30mg/L
100% / 45
mg/L
Mercury and its compounds (Hg) 0.03 mg/L
Cadmium and its compounds (Cd) 0.05 mg/L
Thallium and its compounds (Tl) 0.05 mg/L
Arsenic and its compounds (As) 0.15 mg/L
Lead and its compounds (Pb) 0.2 mg/L
Chromium and its compounds (Cr) 0.5 mg/L
Copper and its compounds (Cu) 0.5 mg/L
Nickel and its compounds (Ni) 0.5 mg/L
Zinc and its compounds (Zn) 1.5 mg/L
Dioxins and furans 0.3 ng/L
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2.2.5. Solid residues
The secondary solid residues that are generated during incineration can be
categorized as follows:
• Fly ash: The ash is composed of the lightest part of the ash, which drifted
from the exhaust and is collected by special filters. The ash has high
concentrations of heavy metals, soluble salts, organic and the higher
content of all residues of chlorinated organic compounds.
• Bottom ash: This is the residue that is collected at the bottom of the
furnace.
• Ash from the boilers
• dust filter cleaning
• solid residues from flue gas purification process (WASTESUM, 2006).
The solid residues stream must be treated before its final disposal, while a main
portion of their quantities could be recycled by applying specific processes.
If the bottom ash is not used, it may be released under the same conditions as
MSW without any problem.
Technologies for the inactivation of fly ash, which is considered hazardous waste,
are in development. Most common is the conversion to material useful for road
construction, structural applications, etc. The use of ash in road construction -
paving is common practice in Europe. The disposal in a landfill must take into
account the leachability of the different components. If a method of inactivation is
not implemented, it should be placed in hazardous waste disposal site.
For the treatment of filters dust various systems are used such as heat (high
temperature). The purpose of working at high temperatures is to melt the filter dust
and transform it into material that is glassy state, which may be allocated to
different uses or placed as inactive.
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In order to ensure the complete control of emissions, sampling and analysis is
required for the determination of the composition of the incoming waste, the
generated solids (residue - fly ash), the produced gas and wastewater generated
during the processing of waste gases.
Quantifying the environmental impact of the implementation of incineration
The method of combustion can cause a variety of environmental impacts taking
into account that there are emissions into the environment as gas, liquid and solid
pollutants. Table 26 summarizes all the amounts of solid waste, wastewater and
air emissions during operation of combustion plants.
Table 26: Summary of quantities of solid waste, wastewater and gases produced during the operation of an incineration plant
Solids (ash, metals, glass, other non-
combustible materials)
25-40% by weight of
waste
Gases: Dust, CO, CO2, H2O, NOx, SO2,
dioxins, furans)
4-5,000m3 gas /
tonne of waste
Wastewater: (suspended particles, organic-
inorganic breakdowns) ~ 4m3 water / tonne waste
The efficiency of the removal of hazardous components of waste treated in
incinerators must be at least 99.99% and is defined as:
DRE = Win - Wout / Win * 100%
Where DRE = efficiency of the incinerator
Win = rate of supply of a particular substance waste
Wout = rate of emission of that substance in the waste gas
The legislation for hazardous waste incinerators does not allow the production of
gas with a concentration of solid particles greater than 180 mg / dscm, for O2
content of 7%. To monitor compliance with that restriction in a variety of
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conditions, current conditions are extrapolated to the conditions of the legislation,
according to the formula below:
Pc = Pm * 14 / (21 - Y)
Where: Pc = corrected concentration of particulate matter (mg / dscm)
Pm = the measured concentration of particulate matter (mg / dscm)
Y = the measured concentration of O2 in the flue gas chimney (%)
The emission factors vary depending on the type of treatment of the produced
gases. Also, heat and electricity are considered to have separate emissions
(Gidarakos 2006, Niessen 2002).
2.2.6. Mass and energy balances
According to a typical mass balance of the incineration process, for one tonne of
input MSW, 200-350 kg is assumed to be bottom ash (10% by volume and
approximately 20 to 35% by weight of the solid waste input), 35-45kg flue gas
cleaning residue including fly ash and 25-30 kg metals.
The rest of the input is converted into energy. The typical amount of net energy
that can be produced per tonne of domestic waste is about 0.7 MWh of electricity
and 2 MWh of district heating.
2.2.7. Market potential for products
Produced heat and electricity may be exploited with a reciprocating engine or
microturbine often in a cogeneration arrangement in order to feed the incineration
system. Excess electricity can be sold to suppliers or put into the local grid.
Electricity produced by incineration systems is considered to be renewable energy.
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The produced bottom ash may be used in construction of roads, embankments
and landfills, in accordance with the local legislation. At present, the ways to use
bottom ash and fly ash as additives in cement plants are under research. The
metals may be used in metal recycling industry.
2.2.8. Environmental impacts
The incineration of solid waste generates pollutant emissions (gases, ash, dust
and smoke), wastewater, slag and odors. The possible presence in the waste of
Cl, F, S, N and other elements could contribute to toxic or corrosive gases. The
wastewater that originates from the treatment of exhaust gases and quenching of
incinerator ash contains heavy metals and inorganic materials with increased
acidity or alkalinity and temperature, while its disposal is permitted only after
pertinent treatment and compliance with applicable regulations. Dioxins and furans
can be products of incomplete combustion and may be decomposed completely
by pyrolysis. The particulate emissions, acid gases (HCl, HF, SO2) and heavy
metals (Hg, Cd, Pb) are of significant importance. Novel incineration technologies
completely decompose dioxins and furans, neutralize toxicity and stabilize the
residue, which could be used in the construction sector. Released aerosols mainly
consist of ash absorbing other toxic pollutants; these reduce atmospheric visibility,
resulting in public complains, and minimization of aerosol emissions is a must.
If electricity and heat generated from incineration is used, the waste replaces
natural resources used for conventional production of energy. The production of
energy from renewable sources has positive consequences on nearly all
environmental impact categories, because of savings in or compensation for non-
renewable energy.
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2.2.9. Economic data
According to the Carbon Finance Unit of the World Bank in 2008, the capital cost
of an incineration system with a capacity of 1,300 tonnes per day is 20-120 €/tone,
while the operation and maintenance cost is 55-80 €/tonne. For the case of
Romania, the Neamt Master Plan of the year 2008 considers that an average
value for the application of the relevant thermal technology is 120-140 €/t for a
capacity of 150,000 t/a. According to EU data the financial cost of incineration,
including the annualized capital, O&M expenditures as well as other specific costs
or revenues from sale of the energy, ranges between 58 to 104 €/tonne (EC,
2010).
2.2.10. Applicability in the target area
In general, thermal management practices are characterized by higher cost in
relation to other management solutions. This together with the existing economic
crisis can be a reason why Romania, Bulgaria and Greece have not yet decided to
apply any thermal waste management method exclusively for municipal waste.
Furthermore, in many cases the public opinion is not really ready to accept the
option of incineration being afraid mainly for the air emissions, but it is considered
that these fears and skepticisms will be gradually overcome. On the other hand, it
has to be noted that incineration is a well tested method which is widely applied in
all advanced in waste management issues European countries that at the same
time achieve high recycling rates. Furthermore, the modern incineration plants can
be located even at the centre of big cities like in Paris, Vienna, etc. provided that
their operation is monitored as required by the relevant Directive on the
incineration of waste (2000/76/EC). It is also important to ensure that incineration
units have high energy performance so that the relevant treatment can be
considered as recovery and not disposal. Finally, energy production through
incineration plants can contribute to reducing CO2 emissions.
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Most specifically, in Romania the incineration practices are already applied but
there are still no incineration facilities just for treating municipal solid waste. The
relevant data for incineration units in Romania are summarized in Table 27.
Table 27: Synopsis on data on incineration units in Romania No. Name and address
of facility Capacity (tones/year)
Cost Waste streams incinerated
Public opinion
1. OLTCHIM- VALCEA
county
11.445 tones
/year. Co-
incineration of
the own waste
generated
It was
accepted by
the
population.
2. S.C.STEMAR
S.R.L.Vaslui -
Vaslui
966 tones
/year. Co-
incineration of
the own waste
generated
It was
accepted by
the
population.
3. VRANCART S.A.-
Adjud, Vrancea
County
17.199 tones
/year. Co-
incineration of
the own waste
generated
It was
accepted by
the
population.
At national level, the total capacity of Co-incineration of the own waste generated is of 29 610 tones/year.
4. CARPATCEMENT HOLDING SA – Sucursala Bicaz
419 432
tones/year
Co-
incineration
in cement
kilns
It was
accepted by
the
population.
5. SC LAFARGE
ROMCIM SA -
MEDGIDIA,
Constanta County,
203 000
tones/year
Co-
incineration
in cement
kilns
It was
accepted by
the
population.
6. SC REPA does It was
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CARPATCEMENT
HOLDING SA –
Bucuresti- Fieni,
Dambovita County
not have
information
about capacity
accepted by
the
population.
7. HOLCIM
(ROMANIA) SA
CIMENT –
CAMPULUNG,
Arges County
26 208
tones/year
Co-
incineration
in cement
kilns
It was
accepted by
the
population.
8. SC
CARPATCEMENT
HOLDING SA
BUCURESTI -
Deva- Cement
Factory
Chiscadaga,
Hunedoara County
131.400
tones/year
Co-
incineration
in cement
kilns
It was
accepted by
the
population.
9. Holcim (Romania)
SA - Ciment Alesd,
Bihor County
77.000
tones/year Co-
incineration
in cement
kilns
It was
accepted by
the
population.
10. LAFARGE CIMENT
(ROMANIA) S.A.
BUCURESTI –
Hoghiz, Brasov
County
20. 788
tones/year
Co-
incineration
in cement
kilns
It was
accepted by
the
population.
Total authorized capacity of co-incineration in Romania is of 907,438 tones/year.
Incinerators
11. SC
CHIMCOMPLEX SA
Borzesti –- Bacau
County
680 tones/year incineration
of their own
waste
It was
accepted by
the
population.
12. SC ANTIBIOTICE 432 tones/year incineration It was
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SA IASI - Iasi
County
of their own
waste
accepted by
the
population.
13. S.C.KOBER SRL –
Turturesti- Neamt
County
1.248
tones/year incineration
of their own
waste
It was
accepted by
the
population.
14. COMPANIA
NATIONALA,,
IMPRIMERIA
NATIONALA"
SA,BUCURESTI-
Bucuresti
28 tones/year incineration
of their own
waste
It was
accepted by
the
population.
15. SC CHIMESTER
BV.SA
Bucuresti
126 tones/year incineration
of their own
waste
It was
accepted by
the
population.
At national level, the total authorized capacity of incineration of facilities’ own
waste generated is 2,514 tonnes/year.
Incinerators for hazardous contaminated packaging waste
16. S.C. MONDECO
SRL, Suceava-
Suceava County
10.800 tones/year
hazardous
medical
waste and
hazardous
industrial
waste
It was
accepted by
the
population.
17. SC PROD IMPORT
CDC SRL ALTAN
TEPE,
COM.Stejaru-
TULCEA County
1.500
tones/year hazardous
medical
waste and
hazardous
industrial
waste
It was
accepted by
the
population.
18. Compania 2.628 hazardous It was
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Nationala
Administratia
Porturilor Maritime
Constanta SA -
Constanta County
tones/year medical
waste and
hazardous
industrial
waste
accepted by
the
population.
19. SC PRO AIR
CLEAN SA,
TIMISOARA-
Timisoara County
3.577 tones/year
hazardous
medical
waste and
hazardous
industrial
waste
It was
accepted by
the
population.
20. SC IF TEHNOLOGII
SRL CLUJ- Cluj
County
1.430
tones/year hazardous
medical
waste and
hazardous
industrial
waste
It was
accepted by
the
population.
21. SC IRIDEX GROUP
IMPORT EXPORT
SRL- Bucuresti
6.000 tones/year
hazardous
medical
waste and
hazardous
industrial
waste
It was
accepted by
the
population.
22. S.C. AVAND S.R.L.,
Iasi- Iasi County
11.300
tones/year hazardous
medical
waste and
hazardous
industrial
waste
It was
accepted by
the
population.
23. SC SUPERSTAR
COM SRL-
SUCEAVA County
2.010 tones/year
hazardous
medical
waste and
hazardous
It was
accepted by
the
population.
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industrial
waste
24. S.C. ECO FIRE
SISTEMS SRL-
CONSTANTA
County
10.080 tones/year
hazardous
medical
waste and
hazardous
industrial
waste
It was
accepted by
the
population.
25. SC GUARDIAN
SRL- Dolj County
4.620 tones/year
hazardous
medical
waste and
hazardous
industrial
waste
It was
accepted by
the
population.
26. ENVISAN NV
BELGIA-
SUSCURSALA
PITESTI- Arges
County
93.312 tones/year
hazardous
medical
waste and
hazardous
industrial
waste
It was
accepted by
the
population.
27. SC ECOBURN
SRL- PLOIESTI-
Prahova County
4.000 tones/year
hazardous
medical
waste and
hazardous
industrial
waste
It was
accepted by
the
population.
The Romanian total authorized capacity of waste incineration is 156,789
tonnes/year.
In Greece, no incineration techniques have been applied for the case of municipal
solid waste. In fact, there is only one incineration facility which operates in the
Attica Region in order to treat hospital waste. It should also be noted that less
hospital waste treated than its capacity allows. Referring to municipal solid waste,
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it has to be noted that the academic society organizes events in order to deal with
the skepticisms about the application of incineration (www.wtert.gr). Furthermore,
some peripheral waste management plans foresee the construction and operation
of incineration units, but have not yet been implemented. For instance, the
relevant scheme for Crete Region foresees the operation of an incineration unit
with capacity to treat 170,000 tonnes of treated municipal solid waste (SRF and
RDF). There are also supporters of the incineration option for the case of Attica
Region who have organized events in order to explain the reasons for doing so
and the relevant benefits.
In Bulgaria, there is total absence of thermal treatment facilities and no significant
progress is anticipated in the years to come due to the unwillingness to adopt such
practices yet, as well as the economic crisis. Unfortunately, landfilling is expected
to continue to be the main treatment method used for the management of
municipal solid waste.
In Slovenia there is only one incinerator for municipal solid waste. Furthermore,
there were two cement kilns having environmental permit. Within 2011 one cement
kiln lost the environemental permit for co-incineration of municipal solid waste, so
now there is only one cement kiln with the environmental permit for co-
incineration. Regarding the incinerator for municipal solid waste, there can be
incinerated only treated municipal solid waste. The capacity for the incinerator is
25.000t of treated municipal solid waste. It can be said that the public opinion does
not accept the installation and operation of incineration facilities until now.
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2.3. GASIFICATION
2.3.1. General
Gasification is a process of incomplete combustion of solid waste (under
stoichiometric conditions regulating the supply of oxidant). A variety of processes
take place, while the gas is formed at temperatures above 700°C and is rich in H2,
CH4, H2O, N2, CO, CO2 and small amounts of high H/Cs. The purpose of this
method is the maximum release of CO and H2. The mixture of CO and H2 is known
as synthesis gas (or syngas).
It is theoretically the next stage of pyrolysis. At this stage the residual coke is
oxidized at high temperatures (> 800oC). As a gasification agent steam, CO2, O2 or
air are used.
The main reactions taking place during the process of gasification are:
(1) Oxidation (exothermic)
C + O2 CO2
(2) Reaction of water evaporation (endothermic)
C + H2O CO + H2
(3) CO + H2O CO2 + H2 (exothermic)
(4) Boudouard Reaction (endothermic)
C + CO2 2CO
(5) Reaction of formation of CH4 (exothermic)
C + 2H2 CH4
The heat to keep the process going derives from the exothermic reactions, while
the combustion products are mainly produced by the endothermic reactions.
It is likely that other reactions take place at low temperatures where with the
addition of H2O CO2 is formed and at higher temperatures CO.
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The gasification reaction rate depends on temperature, porosity, internal structure
of the fuel source and diameter of the pores. Specifically, untreated waste is
harder to gasify than that from cracking. Similarly, the loose material is brittle
compared to the coherent solid material. Solid materials readily allow the passage
of air through the plating reactor.
Figure 31: Gasification process flow chart
The difference in the gasification of pyrolysis is that during gasification additional
fuel gas is fed for further conversion of organic residues into gaseous products
(Figure 42) (Gidarakos 2006, Girods et al. 2009, Klein 2002).
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Figure 32: Process for converting waste into energy (Vasudevan & Mathew 2007)
Syngas generally has a heating value of 250-300 Btu/scf, compared to natural gas
at approximately 1,000 BTU/scf. Typically, 70–85% of the carbon in the feedstock
is converted into the syngas. The ratio of carbon monoxide to hydrogen depends
in part upon the hydrogen and carbon content of the feedstock and the type of
gasifier used.
Typical gasification plant
The main types of gasification facilities are:
• Vertical fixed bed
• Horizontal fixed bed
• Fluidized bed
• Multiple foci
• Rotating furnace.
From all five of these types of facilities (Rezaiyan & Cheremisinoff 2000), the
vertical fixed bed facilities (Figure 43), the horizontal fixed bed (Figure 44) (Dalai et
al. 2009) and the fluidized bed (Groi et al. 2008) are more widespread.
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The vertical fixed bed plants have advantages, such as simplicity and low
investment costs, but are directly affected by fluctuations in the composition of
incoming waste (it is preferred to be homogeneous, e.g. the RDF in concentrated
form - pellets). The gas product of the plant is of low calorific value and
simultaneously small quantities of liquid and important quantities of solid products
are produced.
Based on the results of pilot applications for units operating at 650 - 820oC, it has
been proved that:
• The resulting solids have great adsorptive capacity and can be used in
tertiary treatment facilities and sewage water.
• The gas product can be used as fuel in engines burning oil at a ratio of 4:1,
the performance of the machine can reach 76% of the performance that it
would have if there was exclusive use of oil.
The gases resulting from the treatment of the gaseous product (high performance
cyclones) are comparable in composition to the gases produced by incineration
and in some cases contain less polluting load (Bebar et al. 2005).
Figure 43: Vertical steady bed gasification plants
Regarding facilities of horizontal fixed bed, they are the type widely used in
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commerce. The facility consists of two parts: (a) the main gasification chamber
and (b) the combustion chamber. The first stage carries the process of gasification
and gas produced is burned completely in the second chamber with excess air at
650 - 900°C. The exhaust gases of the combustion chamber are products through
complete combustion that have temperatures ranging from 650oC - 900oC and can
be exploited through the recovery of energy contained in them. The exhaust gases
are driven through heat recovery to produce steam or hot H2O. The low velocity
and turbulence in the first chamber minimize the entry of particles into the gas
stream and lead to lower particulate emissions than conventional combustion
chambers. Such units are commercially available from different manufacturers in
standard sizes capacity 200 – 1,700 kg / h.
Figure 44: Horizontal steady bed gasification plants
Finally, the fluidized bed plants are still at pilot level. With minor modifications, the
fluidized bed combustion with excess air can act as a gasification plant fluidized
bed with air flow below the stoichiometric ratio.
But other than the horizontal bed units, the other systems have not been
developed at full-scale and additional research is required towards this direction.
The produced gas can be utilized in various ways, including:
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Combustion to produce steam. The advantages compared to incineration, is
that the gases are cleaned before combustion, thus enabling operation at
higher pressure boiler and superheater of steam at higher temperatures, to
achieve improved performance and electricity, which can approach 30%.
Power internal combustion engine which drives electrical generator. The
electrical energy can exceed 40% but requires a very thorough cleaning of
gas before feeding the machine.
Movement of Steam turbine and combined cycle. And this method, which
also requires a good cleaning before the gas supply, can result in yields of
40% in electricity.
Feeding into the city gas network. Requires good cleaning and stable
quality.
Provision of gas to the industry, such as cement for direct combustion in
burner. In this case a significantly reduced cleaning is required.
Supply of the gas to an industry where it is used for Steam generation. The
cleaning requirements are a function of boiler operating conditions (Ahmed
& Gupta 2009, Belgiorno et al. 2003, Bjorklunda et al. 2001, Brothier et al.
2007, Ganana et al. 2006, He et al. 2009).
An indicative waste gasification plant is shown in Picture 6.
Picture 6: MSW gasification plant in Chiba (Japan)
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Emissions from gasification units
The end products of gasification are:
• gas rich in CO, H2, CO2 and saturated H / C (mainly CH4) that can be used
as fuel (Scheidl et al. 1991)
• Solid waste material consisting of C and aggregates.
Table 28 summarizes all types of solid waste, wastewater and off-gases generated
during the operation of a gasification unit.
Table 28: Summary of solid waste, wastewater and air emissions generated during the operation of a gasification unit
Solids pure C embedded in various inert materials
Gases CO, H2, saturated H / C
The gasification plant can operate with either supply of air or supply of pure O2. In
the case that there is supply of air, because of the presence of atmospheric
nitrogen, the calorific value of the gas product is relatively low and is around 5.6
MJ/m3. A typical composition is: 10% CO2, 20% CO, 15% H2, 2% CH4, 53% N2.
If the supply is pure O2, the standard composition is: 14% CO2, 50% CO, 30% H2,
4% CH4, 1% CxHy, 1% N2 and energy content between 10 and 11.2 MJ / m3.
Based on the principle of the gasification processes (this also accounts for the
case of pyrolysis) there are limited air emissions comparing with the
implementation of the incineration process due to the less air used (US
Department of Energy 2000, Radian International LLC 2000). In each case,
regarding the permissible levels of emissions generated during gasification, they
are identical with all techniques of thermal processing of solid waste and what has
already been described about the limits of the combustion – incineration process.
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In the following figure, a schematic flow diagram of the Gasification Plant in
Caribbean of the ITI Energy Limited is presented.
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Figure 45: ITI gasification plant flow diagram
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2.3.2. Feedstock
Gasification enables the capture — in an environmentally beneficial manner — of
the remaining “value” present in a variety of low-grade hydrocarbon materials
(“feedstocks”) that would otherwise have minimal or even negative economic
value. Gasifiers can be designed to run on a single material or a blend of
feedstocks:
Solids: All types of coal and petroleum coke (a low value byproduct of
refining) and biomass, such as wood waste, agricultural waste, and
household waste.
Liquids: Liquid refinery residuals (including asphalts, bitumen and other oil
sands residues) and wastewater from chemical plants and refineries.
Gas: Natural gas or refinery/chemical off-gas.
2.3.3. Gasifier
The core of the gasification system is the gasifier, a pressurized vessel where the
feed material reacts with oxygen (or air) and steam at high temperatures. There
are several basic gasifier designs, distinguished by the use of wet or dry feed, the
use of air or oxygen, the reactor’s flow direction (up-flow, downflow, or circulating),
and the gas cooling process. Currently, gasifiers are capable of handling up to
3,000 tonnes/day of feedstock throughput and this will increase in the near future.
After being ground into very small particles — or fed directly (if a gas or liquid) —
the feedstock is injected into the gasifier, along with a controlled amount of air or
oxygen and steam. Temperatures in a gasifier range from 1,400-2,800 degrees
Fahrenheit. The heat and pressure inside the gasifier break apart the chemical
bonds of the feedstock, forming syngas. The syngas consists primarily of H2 and
CO and, depending upon the specific gasification technology, smaller quantities of
CH4, CO2, H2S and water vapor.
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2.3.4. Oxygen plant
Most gasification systems use almost pure oxygen (as opposed to air) to help
facilitate the reaction in the gasifier. This oxygen (95–99% purity) is generated in a
plant using proven cryogenic technology. The oxygen is then fed into the gasifier
through separate co-feed ports in the feed injector.
2.3.5. Gas Clean-Up
The raw syngas produced in the gasifier contains trace levels of impurities that
have to be removed prior to its ultimate use. After the gas is cooled, the trace
minerals, particulates, sulfur, mercury and unconverted carbon are removed to
very low levels using commercially proven cleaning processes common to the
chemical and refining industries.
For feeds (such as coal) containing mercury, more than 95% of the mercury can
be removed from the syngas using relatively small and commercially available
activated carbon beds (WASTESUM, 2006).
2.3.6. Mass and energy balances
A typical mass balance of the gasification process is shown in Figure 46. On the
basis of the diagram it can be stated that 1 tonne of treated feedstock leads to
680-810 kg produced syngas, 170-300 kg carbon char and ash that can be
recycled or disposed of at a landfill, while the remaining 20 kg is the residue from
the flue gas treatment that must be sent to a hazardous waste landfill.
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Figure 46: Schematic presentation of inputs and outputs of a typical gasification process
In the following figure (Figure 47), the energy and mass balances of the
Gasification Plant in Caribbean of the ITI Energy Limited are presented.
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Figure 47: Mass and energy balance of the ITI gasification plant
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Assuming 100% chemical energy in the fuel feedstock, the typical gasifier converts
this fuel to 70-80% chemical energy in the gaseous phase, 15-20% heat and some
heat loss and unconverted fuel, which depends on the type of gasifier and the fuel.
2.3.7. Market potential for products
Syngas can be combusted to produce electric power and steam or used as a
building block for a variety of chemicals and fuels. Most solid and liquid feed
gasifiers produce a glass-like byproduct called slag, which is non-hazardous and
can be used in roadbed construction or in roofing materials. Also, in most
gasification plants, more than 99% of the sulfur is removed and recovered either
as elemental sulfur or sulfuric acid.
Hydrogen and carbon monoxide, the major components of syngas, are the basic
building blocks of a number of other products, such as chemicals and fertilizers. In
addition, a gasification plant can be designed to produce more than one product at
a time (co-production or “polygeneration”), such as the production of electricity,
steam, and chemicals (e.g. methanol or ammonia). This polygeneration flexibility
allows a facility to increase its efficiency and improve the economics of its
operations (Rezaiyan & Cheremisinoff, 2005; Klein, 2002; Radian International
LLC, 2000; Belgiorno et al., 2003).
2.3.8. Environmental impacts
The environmental impacts of the use of gasification systems are generally much
milder than incineration. They focus on air emissions and solid residues, as in all
thermal technologies. At high temperatures used in gasification, toxic metals
including cadmium and mercury, acid gases including hydrochloric acid and
ozone-forming nitrogen oxides could be released. Also, dioxins and furans may be
generated in the cooling process following the burning of ordinary paper and
plastic in case that the operation of the unit is not made and controlled properly.
Using municipal solid waste for fuel releases into the atmosphere the carbon
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which is in the paper, cardboard, food waste, yard waste and other biological
materials, plus the carbon in plastic products and containers made from
petroleum. The gasification of petroleum-based plastics adds to greenhouse gases
in the same way as burning fossil fuels, such as coal, oil or natural gas.
Gasification may reduce solid waste volume by 85 to 92%. In addition, the use of
gasification processes reduce methane emissions produced from the disposal to
landfill sites, while being a waste to energy treatment method, it enables the
displacement of CO2 that would have been emitted if the electricity had been
generated from fossil fuels.
2.3.9. Economic data
According to the Carbon Finance Unit of the World Bank in 2008, the capital cost
of a gasification system with a capacity of 900 tonnes per day is 10-115 €/tonne,
while the operation and maintenance cost is 55-100 €/tonne. The relevant cost is
approximately 130 Euro/t on the basis of the estimations of Neamt Master Plan.
2.3.10. Applicability in the target area
The application potential of gasification and plasma gasification is also considered
high, since these methods have recently proved that they are effective and
flexible, since they can also be used for the treatment of other waste streams (e.g.
sludge, hospital waste, etc.) apart from municipal waste. That is why the
gasification practices are considered as suitable alternative especially in the case
of isolated areas, such as islands. The relevant cost is similar to that of other
thermal management practices, higher than that of biological options, the relevant
land demand is limited and the energy yield is also considered of vital importance.
The experience from the operation of such plants is less than that from
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incineration units. Without doubt, the existing economic crisis in the whole Balkan
Region is a significant obstacle for such attempts today. An example on the
application of this technology is Slovenia, a country with less economic problems
comparing to Romania, Greece and Bulgaria, is provided below.
EfW Gasification CHP Plant in Celje, Slovenia
The Energy from Waste (EfW) Gasification plant in Celje, Slovenia constructed by
KIV constitutes part of an integrated waste management scheme for the city of
Celje and the surrounding districts.
Picture 7: Celje Waste to Energy CHP Plant
The waste treatment facilities comprise kerbside recycling, Eco Islands, a picking
station with baling machinery, MBT and the EfW. The remaining MSW goes
through a MBT facility, where loose RDF output from the residual waste becomes
the fuel supplying the KIV EfW plant. Additionally belt pressed sewage sludge is
mixed into the RDF just prior to being fed into the KIV capacity gasifier.
The waste treatment plants (MBT + EfW) are designed to cope with the waste
from up to 240,000 people across 24 Municipalities. The plant has been designed
to divert the waste away from landfill. It is a town / city sized solution, only
requiring short waste shipments thereby minimising carbon footprint.
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The mixed RDF (80%) and Sewage Sludge (20%) has a combined net calorific
value (CV) of 13.6MJ/kg. With this CV the 18MWth capacity gasifier plant is
capable of handling up to 37,000 tpa, based on guaranteed operational hours of
7,800. 15MWth of high pressure superheated steam produces 2.1MWe of power
as it passes through a steam turbo alternator. The plant is ‘heat led’ and feeds the
recovered energy of up to 13MWth into the existing District Heating scheme as hot
water at 110 C. If the scheme was ‘electricity led’, it would produce 3.8MWe of
gross power.
Figure 48: Flow diagram of the EfW plant in Celje
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Figure 49: Process scheme of the EfW plant in Celje
Figure 50: Energy and mass balance of the EfW plant in Celje
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2.4. PYROLYSIS
2.4.1. General
Pyrolysis is the method during which natural and chemical decomposition of the
thermally unstable organic substances included in waste is taking place under
temperature in the area of 400 - 800°C, with the absence of air or O2. The great
difference of pyrolysis comparing with incineration and gasification is that it is a
highly endothermic process and requires outer energy source so as to take place.
In fact, it is difficult to have conditions of complete absence of O2, so in practice
pyrolytic systems are operating with oxygen quantities less than the stoichiometric
ones.
The reactions that are taking place are initially decomposition ones, when organic
constituents characterized by low volatility are converted into other more volatile
substances:
CxHy CcHd + CmHn
Furthermore, in the primary stages of the pyrolysis stage condensation reactions,
hydrogen removal reactions and reactions forming rings are taking place that lead
to the formation of a solid residue containing carbon from organic substances of
low volatility:
CxHy CpHq + H2 + coke
Then, other reactions of the organic pollutants occur. In the case of O2 existence,
CO and CO2 are formed or the interaction with 2 is possible. The produced
coke can be gasified into 2 and CO2.
The pyrolysis products can be liquid, solid or gaseous. The exact amounts depend
on the nature of the waste to be treated, the heating conditions, the temperature
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and the treatment duration (contact time) (Institution of Mechanical Engineers,
2007; Gidarakos, 2006).
The main advantages of pyrolysis comparing to incineration include the following:
The decomposition temperature is much lower than that of incineration.
The decomposition is taking place in reducing atmosphere and not oxidizing
one, like in the case of incineration. The requirement for lower O2 quantity
also results in limited air emissions.
The content of ash in C is much higher than in the incineration.
Metals included in waste are not oxidized during pyrolysis and, therefore,
can be exploited more easily.
The combustion of the pyrolysis gas does not produce ash and the cleaning
process of the off-gasses is easier.
The initial waste volume is reduced at higher degree comparing with
incineration.
The main disadvantages of pyrolysis include:
The biggest problem of pyrolysis is that the waste to be treated has to be
cut down in small pieces sorted prior to the pyrolysis process and this can
substantially increase the cost for the installation and operation of pyrolysis
units.
The pyrolysis products have certain problems and in no case they can be
disposed at the environment as they are.
The systems for the cleaning of the generated gases and wastewater are
characterized by high cost.
At present, the application of the method at full scale is very limited.
The pyrolysis method has several different variations, one of which is thermolysis.
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2.4.2. Mass and energy balances
A typical mass balance of the pyrolysis process is shown in Figure 51. On the
basis of this diagram it can be stated that 1 tonne of treated carbon-based
materials of Municipal Solid Waste (MSW) produces 380 kg syngas, 220 kg
wastewater, 240kg char, while the remaining 150kg are other residues (metals,
inert, salt).
Figure 51: Pyrolysis process flow chart
Considering the complexity of biomass composition, pyrolysis and the absence of
the thermodynamic parameter, it is difficult to determine the conversion of energy.
In general it can be considered:
Q = CP T + QP,
where Q is absorption of heat during pyrolysis process, kJ·kg-1,
CP is specific heat capacity of substance, kJ· (kg·K)-1,
T is change of temperature,
QP is enthalpies of reactions during the process, kJ·kg-1.
The proposed energy balance model equation is described below: For 1,000kg
feedstock, 320kg biomass fuel is needed. The Lower Heating Value (LHV) of the
biomass and biomass fuel is 3,900 kcal·kg-1 and the LHV of the process products
is as follows:
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• The output of the gas is 250m3, Lower Heating Value (LHV) is 15.18MJ·m-3
• The output of the charcoal is 300kg and its Lower Heating Value (LHV) is
7,100 kcal·kg-1 (WASTESUM, 2006).
2.4.3. Market potential for products
The pyrolysis gas produced from the pyrolysis process can be utilized in boilers,
gas turbines or internal combustion engines in order to generate electricity, while
part of the pyrolysis ash can be used for manufacturing brick materials.
2.4.4. Environmental impacts
The environmental impacts of the pyrolysis process focus on air emissions and
solid residues, as in all thermal technologies. Due to the high temperatures used in
pyrolysis, toxic metals including cadmium and mercury, acid gases including
hydrochloric acid and ozone-forming nitrogen oxides can be released. On the
other hand, pyrolysis enables fossil fuel substitution by the MSW and, in addition,
slow pyrolysis may stabilize a portion of the C in these effects of biochar remain
for 10 years after initial application. Furthermore, the methane emissions produced
from the disposal of MSW to landfill sites are reduced.
2.4.5. Economic data
According to the Carbon Finance Unit of the World Bank in 2008, the capital cost
of a gasification system with a capacity of 70-270 tonnes per day is 30-60 €/tonne,
while the operation and maintenance cost is 55-100 €/tonne. In general, the
application of the pyrolysis process can be considered viable for smaller waste
quantities in relation to incineration.
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2.4.6. Applicability in the target area
Pyrolysis is one of the innovative thermal waste management methods with limited
application in full scale. The majority of the existing pyrolysis units in operation are
pilot ones. This is the main reason why it is not expected soon to have this option
available in the Balkan Region within the near future. Nevertheless, it is expected
that there will also be some developments in this field later, perhaps next decade.
The relevant cost is considered as preventive factor for the development of such
systems for Romania, Bulgaria, Greece and Slovenia for the time being.
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2.5. PLASMA GASIFICATION TECHNOLOGY
2.5.1. General
Plasma refers to every gas of which at least a percentage of its atoms or
molecules is partially or totally ionized. In a plasma state of matter, the free
electrons occur at reasonably high concentrations and the charges of electrons
are balanced by positive ions. As a result, plasma is quasi-neutral. It is generated
from electric discharges, e.g. from the passage of current (continuous, alternate or
high frequency) through the gas and from the use of the dissipation of resistive
energy in order to make the gas sufficiently hot. Plasma is characterized as the
fourth state of matter and differs from the ideal gases, because it is characterized
by ‘collective phenomena’. ‘Collective phenomena’ originate from the wide range
of Coulomb forces. As a result, the charged particles do not interact only with
neighboring particles through collisions, but they also bear the influence of an
average electromagnetic field, which is generated by the rest charges. In a large
number of phenomena, collisions do not play important role, as ‘collective
phenomena’ take place much faster than the characteristic collision time (Blachos,
2000).
Plasma Technology can be used as a tool for green chemistry and waste
management (Mollah et al., 2000). Thermal plasmas have the potential to play an
important role in a variety of chemical processes. They are characterized by high
electron density and low electron energy. Compared to most gases even at
elevated temperatures and pressures, the chemical reactivity and quenching rates
that are characteristic of these plasmas is far greater. Plasma technology is very
drastic due to the presence of highly reactive atomic and ionic species and the
achievement of higher temperatures in comparison with other thermal methods. In
fact, the extremely high temperatures (several thousands degrees in Celsius
scale) occur only in the core of the plasma, while the temperature decreases
substantially in the marginal zones (Gomez et al., 2009).
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Five distinct categories of processes are used as the basis for the plasma systems
catering for waste management (Juniper, 2006). These are:
Plasma pyrolysis (Huang & Tang, 2007; Sheng et al., 2008)
Plasma combustion (also called plasma incineration or plasma oxidation)
Plasma vitrification
Plasma gasification in two different variants (Malkow, 2004)
Plasma polishing using plasma to clean off-gases
Plasma gasification is the most common plasma process. It is an advanced
gasification process which is performed in an oxygen-starved environment to
decompose organic solid waste into its basic molecular structure. Plasma
gasification does not combust the waste as incinerators do. It converts the organic
waste into a fuel gas that still contains all the chemical and heat energy from the
waste. Also, it converts the inorganic waste into an inert vitrified glass (Moustakas
et al., 2005; Moustakas et al., 2008).
Mixed solid waste is shredded and fed into a reactor where an electric discharge
similar to a lightning (the plasma) converts the organic fraction into synthesis gas
and the inorganic fraction into molten slag. Typically temperatures are greater than
7,000°F achieving complete conversion of carbon-based materials, including tars,
oils, and char, to syngas composed primarily of H2 and CO, while the inorganic
materials are converted to a solid, vitreous slag. The syngas can be utilized in
boilers, gas turbines, or internal combustion engines to generate electricity while
the slag is inert and can be used as gravel.
Figure 52: Plasma gasification process flow chart
Waste
Plasma Energy
Usable Inert Slag
Synthesis Gas CO, H2, CO2, N2
Controlled Air Feed
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Picture 8: Molten slag pouring from plasma waste gasification reactor
(Pyrogenesis Inc, Montreal, Canada)
The advantages of the process include: Good environmental performance,
production of about 400 KWh net of electricity per tonne of waste treated, no by-
products going to landfill.
Picture 9: Final inert slag residue can be used in construction applications
The disadvantages of the process include: Relatively high cost, high level of
maintenance and skilled labor required for operations.
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Electricity is fed to a torch, which has two electrodes, creating an arc. Inert gas is
passed through the arc, heating the process gas to internal temperatures as high
as 25,000 degrees Fahrenheit. The following diagram illustrates how the plasma
torch operates.
Figure 53: Plasma torch operation
The temperature a few feet from the torch can be as high as 5,000-8,000oC. Due
to these high temperatures, the input waste is completely destroyed and broken
down into its basic elemental components. At these high temperatures all metals
become molten and flow out the bottom of the reactor. Inorganics, such as silica,
soil, concrete, glass, gravel, etc. are vitrified into glass and flow out the bottom of
the reactor. There is no ash remaining to go back to a landfill and the produced
vitrified residue called slag is the only material that can end up at landfills if no
suitable markets (e.g. as construction material) are found for that.
The plasma technology is flexible, since it can be used for the thermal treatment of
a variety of waste streams. The only variable is the amount of energy that it takes
to destroy the waste. Consequently, no sorting of waste is necessary and any type
of waste, except nuclear waste, can be processed.
The plasma reactor operates at a slightly negative pressure, meaning that the feed
system is simplified, because the gas does not want to escape. The gas has to be
pulled from the reactor by the suction of the compressor. Because of the size and
the negative pressure, the feed system can handle bundles of material up to 1
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meter in size. This means that sizeable waste can be fed directly into the reactor
and pre-processing of the waste is not needed. Also, the performance of the
plasma gasifier is not affected by the moisture of the waste (during incineration,
the moisture of waste consumes energy to vaporize and can impact the capacity
and economics of the process) (WASTESUM, 2006).
An indicative list of initiatives to apply plasma technology in the field of waste
treatment is given in the table below.
Table 29: Commercial Plasma Waste Processing Facilities (Circeo, 2007)
Location Waste Capacity (TPD) Start Date
Mihama-Mikata,
JP
MSW/WWTP
Sludge
28 2002
Utashinai, JP MSW/ASR 300 2002
Kinuura, JP MSW Ash 50 1995
Kakogawa, JP MSW Ash 30 2003
Shimonoseki, JP MSW Ash 41 2002
Imizu, JP MSW Ash 12 2002
Maizuru, JP MSW Ash 6 2003
Iizuka, JP Industrial 10 2004
Osaka, JP PCBs 4 2006
Taipei, TW Medical &
Batteries
4 2005
Bordeaux, FR MSW ash 10 1998
Morcenx, FR Asbestos 22 2001
Bergen, NO Tannery 15 2001
Landskrona, SW Fly ash 200 1983
Jonquiere, Canada Aluminum dross 50 1991
Ottawa, Canada MSW 85 2007
(demonstration)
Anniston, AL Catalytic
converters 24 1985
Honolulu, HI Medical 1 2001
Hawthorne, NV Munitions 10 2006
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Alpoca, WV Ammunition 10 2003
2.5.2. Mass and energy balances
In general, the mass and energy balances have similarities with the respective
ones referring to gasification. A typical energy balance assumes that from one
tonne of waste treated more than 400 KWh net electricity is produced.
2.5.3. Market potential for products
There are a number of applications for the plasma gasification syngas. For
example, it can be utilized as fuel source to produce electric power (e.g. in a
simplified steam-cycle configuration consisting of a conventional boiler/steam
generator with steam turbine) or in a gas engine, configured to accept lower heat
value gas. The gas can be used in a gas turbine, both in simple cycle and in
combined cycle operations. It can also be used as a feedstock for chemical
processes, e.g. the production of methanol.
The use of lower heat value plasma gasification syngas as a fuel source for gas
engines has been successfully demonstrated with syngas generated from various
feedstocks, including the gasification of biomass. Other applications for the
utilization of the plasma gasification syngas are as follows: separation of hydrogen
from the syngas, which can provide an excellent source of hydrogen for use with
fuel cells, using the syngas as a feedstock for the production of liquid fuels, such
as ethanol.
Applications for the glassy product include roadbed/fill construction and concrete
aggregate. Any reclaimed valuable metal could be sold to metal dealers and
processors. Metal alloy is bought and sold based on a commodity-based pricing
system.
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2.5.4. Environmental impacts
Plasma gasification uses an external heat source to gasify the waste. Almost all of
the carbon is converted to fuel gas. Plasma gasification is the closest technology
available to pure gasification. Because of the temperatures and drastic conditions
involved all the tars, char and dioxins are broken down. The exit gas from the
reactor is cleaner and there is no ash at the bottom of the reactor, while there are
no by-products that end up to landfills provided that there are available markets for
the produced slag. On the other hand, the use of plasma gasification processes
reduce methane emissions produced from the disposal to landfill sites, while as a
waste to energy treatment method, enables the displacement of CO2 that would
have been emitted had the electricity been generated from fossil fuels.
2.5.5. Economic data
According to the Carbon Finance Unit of the World Bank in 2008, the capital cost
of a plasma gasification system with a capacity of 900 tonnes per day is 40-60
€/tonne, while the operation and maintenance cost is 55-100 €/tonne.
Nevertheless, most sources estimate that the cost is a little bit higher than other
thermal methods due to the use of electrical energy.
2.5.6. Applicability in the target region
The application potential of gasification and plasma gasification is also considered
high, since these methods have recently proved that they are effective and
flexible, since they can also be used for the treatment of other waste streams (e.g.
sludge, hospital waste, etc.) apart from municipal waste. That is why the
gasification practices are considered as suitable alternative especially in the case
of isolated areas, such as islands. The relevant cost is similar to that of other
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thermal management practices, higher than that of biological options, the relevant
land demand is limited and the energy yield is also considered of vital importance.
The experience from the operation of such plants is less than that from
incineration units.
The first attempt to apply gasification process in the target region and more
specifically in Greece was made he National Technical University of Athens, with a
unit that was installed in Mykonos in order to treat all types of waste generated on
the island with emphasis on municipal solid waste. The unit had been initially
designed and developed in the framework of the LIFE project entitled:
“Development of a demonstration plasma gasification / vitrification unit for the
treatment of hazardous wastes” and later was modified in order to cater for the
treatment of municipal solid waste, too. The scope was to investigate the use of
this innovative technique in an isolated area like an island in order to provide a
solution to the overall management of waste. General views of the whole
demonstration facility are available below:
Picture 10: General view of the demonstration gasification / vitrification unit
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Picture 11: Another general view of the demonstration gasification /
vitrification unit
The primary waste feeding system consists of a hopper intended for feed of solid
material having maximum moisture content of 50% and a maximum particle size of
2.5 cm. The screw conveyor solid feeder has a maximum capacity of about 85
kg/h of waste and the feeding capacity varies depending on the feed waste bulk
density. The feed rate is adjustable by varying the speed of the screw conveyor.
Waste is manually loaded into the hopper connected to the screw conveyor. The
feed rate is continuous and very steady, compared to a hydraulic feeder.
Waste is fed from a hopper through a screw feeder to the top of the furnace and
dropping down is passing through the very hot and free of oxygen region between
the two electrodes.
Picture 12: Feeding system
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The furnace is comprised of a crucible, with approximately 130 litters capacity. It
also includes a start-up natural gas burner for preheating and idle operation, a port
for gasification air injection, a water-cooling mechanism for the graphite
electrodes, an external surface water-cooling for the furnace walls and a tapping
hole for periodical or continuous slag removal. During the operation of the plasma
unit, the bottom part of the furnace contains the molten slag, while the upper
section of it contains the process gases and is lined with a suitable high-
temperature refractory. The required gasification air fed to the furnace is supplied
by a compressed air system. Adjusting the valves on the compressed air line can
control the flow rate.
Picture 13: Gasification / vitrification furnace
Figure 54: Plasma Gasification / Vitrification Process
Synthesis Gas Waste
Molten Slag
Iron Heel
+ -
Untreated Waste
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In the pilot unit the furnace in which waste gasification is taking place is preheated
at 600-800°C by burning propane in its interior. After preheating, two cylindrical
graphite electrodes are inserted in the furnace and their ends are approached to a
close distance. Two graphite electrodes are used to supply an electrical arc to the
furnace. The current flows from the anode (+) to the molten bath and from the bath
to the cathode (-). The cathode is grounded at zero (0) potential.
Graphite electrodes with male/female threads are used. The electrode dimensions
were 7.6 cm in diameter and 106.7 in length. Electrodes are installed with the
female end down, in order to avoid dust accumulation in the threads. Two
electrodes were screwed together on each side (anode and cathode) and are
mounted on flexible joints, which allow them to be moved over the slag pool and
improve mixing. The mechanism also permits the electrodes’ extension into the
furnace to be adjusted during operation (Carabin & Holcroft, 2005; Carabin et al.,
2004; Gagnon & Carabin, 2006).
.
The DC power supply for the electrodes has a maximum power output of 200 KVA
(Plasma arc power supply, input: 600 VAC-3 -60HZ, 3 X 200A fuses).
Then, a high voltage is applied between them producing an electrical arc which is
raising locally the temperature up to values as high as 5,000°C and creating a
plasma atmosphere. Air is not permitted to enter the furnace. Under these
conditions it is ensured that from the volatile part of the waste syngas is produced
consisting mainly of H2, CO, CO2 and H2O and containing in very low proportions
H2S and HCl, but without significant presence of NOx. A camera is installed in
front of a window on the top of the furnace, connected with a laptop, by which we
can watch or make video recording of the electrical arc and the decomposition of
the organic matter taking place in the interior of the furnace.
The slag could be tapped out periodically from the tap hole located on the front
side of the crucible, close to the bottom of the furnace. The slag was either poured
in a slag mold to form ingots or quenched in a water tank to produce granulated
slag.
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The inorganic part of the waste used is melted, drops to the bottom of the furnace
and from time to time is removed through a hole in the lower part of the furnace, is
collected to a fire resistant pan and is taken to the laboratory for analysis and
investigation of its toxicity.
The hot cyclone was designed to remove dust in the synthesis gas. The produced
gases, while entering the cyclone, are put in circular movement and the centrifugal
force makes particulate matter contained in the gases to be removed to a high
degree.
Picture 14: Cyclone Picture 15: Secondary Combustion Chamber
The result of its operation is the oxidation of the components of the furnace gases.
The secondary combustion chamber was designed to combust H2 and CO in the
synthesis gas. In order to combust CO and H2 into CO2 and H2O, air is added into
the secondary combustion chamber. Propane burners are used to maintain the
chamber temperature at 1,100oC. The operator can check local regulations to
determine the required temperature in secondary combustion chamber. This
temperature is required to fully combust CO and H2 in a region where no
hazardous by-products are created. In normal operation, the gas residence time in
the secondary combustion chamber is about two seconds. A single blower
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provides the combustion air for the burners and the combustion air for the
synthesis gas.
It is located at the outlet of the secondary combustion chamber. Its role is to cool
the combustion gases quickly to approximately 75oC so as to minimize any
production of dioxins, furans or other organic compounds. The shock-like cooling
avoids the formation of the aforementioned compounds from elementary
molecules in the synthesis gas due to the de novo Synthesis back reactions
(Calaminus & Stahlberg, 1998). These reactions are known to occur in waste heat
boilers where a slow cooling in the range from 400oC to 250oC of flue gases with
chlorine compounds, non combusted organic molecules and catalysts such as
dust will result in dioxin formation. The quench vessel uses two atomizing nozzles
to quench the gas from the secondary combustion chamber. These nozzles are
capable of providing 2 litters per minute of flow. Regulating the amount of the
quenching water can control the gas temperature exiting the vessel.
Picture 16: Quench Vessel Picture 17: Scrubber
It removes water-soluble components of the off-gas including hydrochloric acid
and most oxides of sulphur, prior to discharge. Since the synthesis gas may
contain acid gases (such as HCl or SO2), a packed tower type wet scrubber uses
caustic soda to neutralize the acid gas from the quench vessel. The pH of the
scrubbing solution is controlled at 9.0. The scrubber liquor is re-circulated through
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a wet bagfilter in order to remove suspended particles. The bagfilter is a cartridge
unit having series of cylindrical filters that are cleaned periodically by an automatic
sequence using pulses of compressed gas.
The pilot unit has a maximum hourly capacity of only 50 kg of waste and the
quantity of the syngas produced is too low for a gas engine to convert it in
electrical energy; therefore, the syngas has to be released in the atmosphere but
in a safe way. Hence, CO and H2 have to be transformed to CO2 and H2O and for
this purpose a Secondary Combustion Chamber (SCC) has been added in the
installation, which is maintained at high temperature (around 700-800°C) by
combusting propane with air and in which CO and H2 are burnt to CO2 and H2O.
The SCC in our installation is situated after the furnace and between the two units
is interceded a cyclone to remove the solid particles. After the SCC the flue gases
are objected to quenching by coming in contact with a big quantity of cold water
and this takes place in a pipe where flue gases and cooling water are moving
opposite each other. After quenching, the flue gases are passing for cleaning
through a scrubber with NaOH solution, then through a filter and finally before they
are released to the atmosphere via a stack are cooled in a heat exchanger to
condense and recirculate the maximum quantity of water vapors. The results of
the pilot application were positive and encouraging for future applications using
this technology. It is hoped that a full scale unit will operate soon in Mykonos and
other Greek islands using gasification or plasma gasification technology. However,
it is true that the existing severe economic crisis in Greece will cause significant
delays is these management plans.
No other similar applications have been made in Romania, Bulgaria or Slovenia.
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