ESTIMATING RENEWABLE THERMAL DRYING POTENTIAL OF SEWAGE SLUDGE

Julio Cezar Rietow1,2

*, Bruno Galvão Cavalieri1,2

, Charles Carneiro1,3

,

Anderson Cardoso Sakuma2, Gustavo Rafael Collere Possetti

1,3

1Water and Sanitation Company of Paraná State (Sanepar, Brazil) - Research and Development Consulting

2Pontifícia Universidade Católica do Paraná (PUCPR, Brazil) - Polytechnic School - Environmental Engineering

3Instituto Superior de Administração e Economia do Mercosul (ISAE, Brazil) - Professional Master’s Program in

Governance and Sustainability

*julio.rietow@gmail.com

ABSTRACT

The Brazilian government, through the National Sanitation Plan, intends to invest more than US$ 500

billion in the sanitation sector until the year 2033, especially in the areas of collection and treatment of

wastewater. Given this situation, sanitation companies tend to look for simplified technologies and

already established treatment, for example, the UASB reactors. A major advantage of these systems is

the production of biogas, a by-product of high energy potential that contains methane in its

composition. Additionally, these systems also produce sludge, a by-product that typically has high

moisture content and pathogenic microorganisms. So it is necessary to promote the dewatering and the

hygienization of the sludge before its proper disposal. Thus, this study aimed to evaluate the possibility

of using the biogas generated in the anaerobic reactors of a midsize domestic wastewater treatment

plant (WWTP) as a source of energy for drying and hygienization the sewage sludge produced in it.

The estimates were made from a mathematical model that took into account the conversion routes of

organic matter and losses of methane in the reactor. The biogas produced by the UASB reactors in

WWTP studied was estimated, on average, to be equal to (2,250.0 ± 330.1) Nm³.d-1

, with a potential

energy associated with this gas of (15,755.0 ± 102.3) kWh.d-1

, since the excess sludge production was

estimated on average to be equal to (11.50 ± 0.04) m³.d-1

. Using a study of thermal dryer patterns with

a yield of 80% biogas burning, it was estimated that the potential energy available for use in WWTP

2

during the study period was (4,374.5x103

± 27,996.8) kWh.year-1

. Meanwhile, the estimate of the

necessary energy demand for sludge drying to total solids (TS) content of 85% was

(3,710.7x103 ± 23,744.0) kWh.year

-1. Thus, the results show that the biogas generated by the UASB

reactors provide the full amount of energy required for the drying and thermal hygienization of the

sludge.

Keywords: biogas, thermal hygienization, sewage sludge, UASB reactor.

1 INTRODUCTION

The Brazilian government, through the National Sanitation Plan (PLANSAB), plans to invest more

than US$ 500 billion in the sanitation sector until the year 2033, especially in the areas of collection

and treatment of wastewater. Given this scenario, sanitation companies are looking for simplified

technologies and already established treatment methods from a sustainable perspective that consider

environmental, social and economic aspects.

Thus, anaerobic systems, especially those based on Upflow Anaerobic Sludge Blanket (UASB)

reactors, are widely used in Brazil. The financial resources required for their construction and operation

are typically less expensive than those required for systems with aerobic nature. Additionally, UASB

reactors enable the production of a by-product such as biogas, a compound that can be used for

renewable energy purposes within the same wastewater treatment system (CHERNICHARO, 2007).

Currently, most Brazilian sanitation companies that have UASB reactors technologies in their sewage

treatment systems do not use the produced biogas. Because it is composed mainly of methane (CH4),

one of the main drivers of greenhouse gases, the captured biogas is typically sent to an open flare

where it is then thermally destroyed with low efficiency. According to the data of the

3

Intergovernmental Panel on Climate Change (IPCC, 2013), CH4 has a global warming potential 25

times that of carbon dioxide (CO2).

However, when the biogas is destroyed in open flares, the methane chemical energy is wasted. Thus,

even if premature, tests and studies involving the use of biogas are being conducted by sanitation

companies in Brazil, especially regarding heat and electrical generation.

In addition to biogas, another by-product originating from the wastewater treatment in a UASB reactor

is the sludge. The sludge, which contains pathogens and is continuously produced in wastewater

treatment plants (WWTP), is one of the main problems of sanitation companies in Brazil. The costs of

handling, treatment, transportation and disposal may reach 20-60% of all operating costs of the

treatment plant (ANDREOLI et al, 2001).

Thus, one of the possible solutions to reduce costs involving sludge management is the thermal drying

energy from biogas utilization. Additionally, the use of this gas promotes the reduction of CH4

emissions to the atmosphere, contributing to environmental sustainability and supporting renewable

energy for WWTPs. However, to validate it an alternative, it is necessary to know the amount of biogas

available for use and, as a result, compare it to the amount required for a given sludge drying system.

In this context, this study aimed to investigate the possibility of energy biogas utilization to promote

both drying and thermal hygienization of the sludge produced in a Brazilian medium-sized WWTP.

The estimation of biogas production and its respective energy potential occurred using a model that

considers the mass balance of chemical oxygen demand (COD) and losses of methane within a UASB

reactor. A similar model was used to estimate the sludge production. The balance of energy in the

sludge dryer was based on experimental data recently obtained.

4

2 BIOGAS PRODUCTION IN UASB REACTORS

The improved source of UASB reactors occurred in the Netherlands in the 1970’s after research

conducted by Wageningen University, headed by Professor Gatze Lettinga. The UASB reactor

technology quickly spread through tropical countries like Colombia, India and Brazil. In the latter, the

acceptance of UASB reactors was well-known, putting it in its current position as a worldwide leader

(JORDÃO and PESSÔA, 2011).

The operational principle inherent to wastewater treatment in a UASB reactor is shown in Figure 1.

The domestic wastewater in upflow sludge is mixed with a preformed blanket or inoculated, given its

richness in anaerobic microorganisms. The organic matter present in the sewage sticks to the sludge

blanket, which then is degraded and stabilized by means of the microbiological activity of the system,

becoming more stable products such as water, biogas, sludge and scum (CAMPOS, 2000).

Figure 1: Schematic representation of the operation of a UASB reactor filled with domestic sewage.

Source: Chernicharo, 2007.

According to Lettinga (2005), the use of UASB reactor can be understood as a treatment alternative for

sustainable development. Pointing towards self-sufficiency and resource saving, the UASB reactor

Collect the effluent Biogas output

Three-phase separator

Deflector gases

Gas bubbles

Influent

Decantation compartment

Opening for the decanter

Sludge particles

Digestion compartment

5

generates by-products such as excess sludge for agriculture and biogas for bioenergy production, which

can be reused by sanitation companies.

The biogas originating from the UASB reactor consists mainly of CH4 (60 to 85% v/v), nitrogen (N2 -

10 to 25% v/v) and CO2 (5 to 15% v/v). In a smaller volume, biogas still has small amounts of

hydrogen (H2), hydrogen sulfide (H2S), ammonia (NH3) and other trace gases. The biogas energy

potential is largely related to the amount of CH4 present in its composition. Thus, the lower heating

value (LHV) of CH4 is around 8,580 kcal.m-3

, since biogas with a concentration of 60% CH4 has a

LHV of the order of 5,138 kcal.m-3

(LOBATO, 2011).

In addition, the thermal efficiency arising from the burning of biogas has been studied as an alternative

to the sludge drying process in thermal dryers. The thermal efficiency of the biogas combustion in

dryers is approximately 80% and is thus an energy efficient drying process (WELLINGER et al.,

2013). In addition, studies by Possetti et al. (2014), as part of an experimental investigation of a

thermal sludge drying pilot system powered by biogas, showed that the energy required to evaporate 1

kilogram (kg) of water present in the sludge is approximately 1.2 kWh.

Although decision-making intrinsic to the energy use of biogas should be guided by measurement

analysis, this is often not feasible. In these cases, decisions must be guided by mathematical model

estimates. Studies by Silva et al. (2014) in a large WWTP showed that the biogas estimation model in

UASB reactors proposed by Lobato (2011) presents the results with smaller deviations from those

measured. This must consider the model in question in its COD balance mass, as shown in Figure 2.

All possible routes conversion of organic matter in an UASB reactor and the loss of methane to liquid

medium for the gaseous medium and also related to the reduction of sulfate (SO4) can not be neglected.

In general, models of biogas estimates that do not take into account the mass balance of COD and

losses of methane in UASB reactors overestimate the production of biogas and the chemical energy

6

accumulated in it. Models such as the IPCC (2006) and the Framework Convention of the United

Nations on Climate Change (UNFCCC, 2013), may overestimate the production of biogas in UASB

reactors by up to seven times, putting out financially and economically unfeasible projects over the

energy use of this gas (SILVA et al., 2014).

Figure 2 – Schematic representation of the biogas estimation model in a UASB reactor based on COD balance and methane losses.

Source: Lobato, 2011.

2 METHODOLOGY

The methodology in this paper was applied at medium-sized WWTP located in Curitiba-Paraná, Brazil.

This WWTP has the capacity to treat up to 38,000 m³.d-1

of domestic sewage. Before being released

into the river, sewage is treated physically and biologically. For the biological sewage treatment, the

treatment plant has six UASB reactors and two aeration ponds. The biogas generated by UASB reactors

is captured by a pipe structure and delivered into open flares, where CH4 is partially destroyed. The

excess sludge removed from the UASB reactors is sent to a centrifuge mechanical dewatering system,

which induces a sludge total solids (TS) concentration of about 20% in the end the process.

The mathematical model proposed by Lobato (2011) estimated the potential energy generation

associated with biogas and sludge production from a WWTP. Responsible for COD mass balance in the

7

process, the model considered all the conversion modes of COD inside a UASB reactor, as well as

losses of methane in the gas and liquid phases.

Thus, to implement its model, it is important to highlight some parameters: the population served by

WWTP (Pop), the contribution rate per capita of COD (QPCCOD), the contribution rate per capita

sewage (QPC), COD removal efficiency in the reactors (ECOD = 60%), the SO4 concentration in raw

wastewater (CSO4 = 80 gSO4.m-3

), the efficiency to reduce SO4 into the reactors (ESO4 = 80%), the

operating temperature of the reactors (T = 25ºC), the percentage of CH4 present in biogas

(CCH4 = 70%), the sludge production rate in the system (Yobs = 0.213 kgCODsludge.kgCODremov-1

), the

total solids volatile (TSV) conversion factor into COD (KTSV-COD = 1.42 kgCODsludge.kgSTV-1

), the

percentage of TSV in the system (CTSV = 40%), the sludge density ( = 1,020 kg.m-3

), the COD factor

that is consumed in the SO4 reduction process (KDQO-SO4 = 0.667 kgCOD.kgSO4converted-1

), the

percentage loss of CH4 in the gas phase with the residual gas (pw = 7.5%) and the loss coefficient

related to CH4 dissolved in the liquid medium (pL = 25 mg.L-1

).

For a better understanding of the biogas and sludge produced at WWTP by UASB reactors based on

Lobato’s method (2011), a large amount of operational information during the year of 2013 was used.

Therefore, calculations of Pop, QPCCOD and QPC were made from the historical mean of discharge

data to treated sewage in UASB reactors (Qmed) and COD of the wastewater influent to the reactors

(CODinfluent). The mean standard deviation of the estimates were calculated and considered as standard

uncertainty. Then, this uncertainty was expanded for a confidence level of 95.5%.

To estimate the energy required for drying the sludge produced, the value proposed by Possetti et al.

(2014) was used, and then 1.2 kWh energy is required to remove 1 kg of water present in the sludge.

The estimate also took into account a final concentration of 85% of TS and a thermal efficiency of

biogas burning into the dryers of 80%.

8

For a better understanding of the methodology used, Figure 3 shows the layout of the sludge drying

process using biogas as a source of renewable energy in a thermal dryer.

Figure 3 – Schematic representation of the thermal sludge drying process using biogas.

Source: Authors.

The estimated result of the potential energy generation available for use was then compared with the

power required to dry sludge in a thermal dryer; thus, self-sustainability energy of the thermal sludge

drying system could be evaluated.

4 RESULTS AND DISCUSSIONS

During the evaluation period, sewage influent discharge to WWTP UASB reactors followed a variable

performance. The mean was equal to (26,000.0 ± 37.1) m³.d-1

, and sewage COD average was equal to

(733.4 ± 3.1) mg.L-1

. The variable performance of COD is probably related to the significant rain that

occurred during the study period, which is also responsible for higher dilution of organic matter in

sewage. By multiplying discharge and COD, the average value obtained from COD load applied to the

system was equal to (18,555.0 ± 77.2) kgCOD.d-1

.

9

The chart in Figure 4 shows the mass balance of COD in UASB reactors studied. The COD removal

efficiency used was equal to 60%, so the percentage of COD solubilized with treated effluent was 40%.

About (13.0 ± 1.8)% of COD removed in the process was converted into sludge. The COD used to

reduce SO4 was on average (8.2 ± 3.0)%. Regarding COD converted into methane and dissolved in the

effluent, the value was equal to (16.4 ± 6.0)%. On the other hand, COD that was converted into

methane and lost from the gas phase was (3.9 ± 0.6)%. Lastly, COD that was converted into methane

and recovered from the biogas was equal to (19.0 ± 11.8)%.

Figure 4 - Chart of approximate values of COD mass balance within the UASB reactors.

Source: Authors.

The normalized production of biogas was on average equal to (2,250.0 ± 330.1) Nm³.d-1

. Assuming

that biogas is composed by 70% of CH4, the production normalized of CH4 in the system was

(1,575.0 ± 231.4) Nm³.d-1

. Thus, the WWTP produced about (781,165.1 ± 114,050.1) Nm³ of biogas

and (546,815.6 ± 79,834.9) Nm³ of CH4 in 2013.

The production of biogas associated with potential energy generation was on average equal to

(15,755.0 ± 102.3) kWh.d-1

. During the study period, the WWTP produced an estimated

10

(5,468.1x106 ± 34,995.2) kWh.year

-1. Taking into account a yield of 80% from biogas burning in the

sludge thermal dryer, the potential of energy available for use was (12,604.7 ± 81.8) kWh.d-1

or

(4,374.5x103 ± 27,996.8) kWh.years

-1.

The sludge production estimation into UASB reactors is about (11.50 ± 0.04) m³.d-1

of this sub-

product. Considering 20% of TS after the mechanical dewatering step, it is estimated that the WWTP

has produced about (4,043.7 ± 16.4) m³ of sludge in 2013. To achieve a final sludge content of 85% TS

it would be necessary to remove, on average, (8,763.8 ± 36.3) kg.d-1

of water in this material. Thereby,

the energy required to dry the sludge through sludge thermal drying system was on average

(10,516.6 ± 43.6) kWh.d-1

or (3,710.7x103 ± 23,744.0) kWh.years

-1.

Therefore, comparing the potential energy available for use with the required energy of the sludge

thermal drying system, as shown in the Figure 5, it is clear that the energy potential associated as

biogas produced by the UASB reactors would achieve in full the necessary energy demand for sludge

drying.

0 50 100 150 200 250 300 3500

10.000

20.000

30.000

40.000

50.000

60.000

70.000

80.000

En

erg

y (

kW

h.d

ia-1)

Time (days)

Potential energy available for use

Required energy of the sludge thermal drying system

Figure 5 – Comparison between the potential of energy from biogas available for use and the energy demand from sludge drying system.

Source: Authors.

11

5 CONCLUSIONS

The results reported in this study indicate that the biogas produced by the UASB reactors can be used

as a renewable energy source for the drying and hygienization of sewage sludge produced in the

WWTP evaluation. The estimates showed that the use of the energy potential of biogas would fully

support the heat required for sludge thermal drying until 85% of TS. It is important, however, to verify

the biogas quantities available in WWTP by mean of field measurements.

The energy use of biogas and contributions to environmental issues regarding the reduction of

greenhouse gases emissions stands out as a promising alternative to reduce the costs inherent to the

management of sewage sludge produced in WWTPs. With the investments to be made in the Brazilian

sanitation sector, the energy use of biogas should be fully explored, favoring not only the process of

thermal drying for sludge but also the generation of electricity and cogeneration of energy. Thus, the

issue of sanitation in Brazil will soon be viewed with new perspectives, moving toward a more

sustainable, socially just, environmentally friendly, safe and economically viable model.

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