Mechanical-Biological Pre-Treatment of Waste – Hope and Reality

Stephanie, Thiel and Karl Joachim, Thomé-Kozmiensky, vivis CONSULT GmbH

CONTACT Dr.-Ing. Stephanie Thiel, Professor Dr. Dr. h.c. Karl J. Thomé-Kozmiensky vivis CONSULT GmbH Dorfstraße 51 D-16816 Nietwerder Germany Tel.: 0049 - 33 91 / 45 45 – 0 Fax: 0049 - 33 91 / 45 45 – 10 E-Mail: info@vivis.de EXECUTIVE SUMMARY At present in Germany there are 61 plants for Mechanical(-Biological) Treatment (M(B)T) of residual wastes with a total capacity of about 6.4 million tons per year. They can be classified in four process concepts: material stream separation, mechanical-biological stabilization (with a biological drying process), mechanical-physical stabilization (with a thermal drying process) and mechanical(-biological) pre-treatment prior to incineration. M(B)Ts are complex waste treatment plants with a wide ranging variety in terms of their technical configuration, which comprises mechanical processing, mostly biological treatment, possibly processing of the high-calorific fractions for use as solid recovered fuel (SRF), flue gas purification and possibly waste water treatment. Most of the processes subsumed under the generic term mechanical(-biological) treatment have proven their functional capability – despite of the short developmental period of development. In many cases various development stages, usual in process engineering, were skipped. Therefore it does not astonish, that during the years of their development the M(B)T plants had to scope with various problems, which meanwhile are partly solved and partly still subject of optimization: Technical problems are for example the wear of processing and conveying aggregates, blocking of the Regenerative Thermal Oxidation (RTO) through siloxanes and corrosion. This results in reduction of time availability and throughput as well as increased personnel requirement for cleaning, maintenance and repairs and therefore higher operating costs. The main ecological problem is that landfill fractions from MBT have a higher organic proportion (TOC-value) and a higher biological activity than ash/slag from waste incineration plants (WIPs). This creates disadvantages in terms of climate protection and pollutant-mobilisation in the leachate. As it was not possible to comply with the assignment criteria specified for the landfilling of ash/slag from WIPs, for the landfill disposal of secondary waste from MBT other, less strict criteria were defined. From the technical and economic as well as environmental point of view there is continued need for development. The advantages of the MBT system may include lower transport costs in very sparsely populated areas, in individual cases a higher energy efficiency and the potential for separation and recycling of further value added materials – the efficiency and ecological relevance of those additional processes have to be checked for each individual case.

The initial political goal of M(B)T was equivalence with the incineration of waste. This is obviously not the case: MBT cannot replace waste incineration, it is only a pre-treatment of waste prior to its incineration. In all MBT plants solid recovered fuel is produced. Therefore incineration capacities have to be created in SRF power stations, an alternative for limited amounts may be co-incineration in lignite- and hard coal-fired power stations as well as cement plants. Thus incineration is not and cannot be replaced by MBT, but it is simply delayed in the scope of a more complex system – with more material streams and treatment steps. Altogether in Germany almost 60 weight percent of the waste input of MBTs finally are incinerated in SRF power stations, coal-fired power stations, cement plants and conventional waste incineration plants. The waste disposal costs with mechanical(-biological) waste treatment plants and waste incineration plants are similar. In Germany, they amount to approximately 100 Euro per ton. INTRODUCTION The first mechanical-biological waste treatment plants (MBT) in Germany were aimed at producing a material suitable for landfilling. They were low level plants with rather simple technical standard. In response to changes in legislative and market requirements over the years, the objectives and the technology have developed considerably. Modern high-level MBT plants were regarded as an alternative to the long-established and effective waste incineration plants (WIP), which however were unwanted for political reasons and due to lacking acceptance on one part of the population. So the initial idea was to avoid combustion by mechanical-biological treatment. Increasingly, the production of high-grade solid recovered fuel (SRF) and the separation of value added materials for recycling became important objectives. MBT seemed to be a key technology to achieve the political target of full recovery of waste from human settlements till 2020. Furthermore, the production of SRF and recyclable materials should make disposal of municipal solid waste more cost efficient thanks to price reductions or even revenues for these fractions. Besides from very sparsely populated areas, local MBT concepts provided the advantage of lower transport distances and costs in comparison to large central WIPs. With regard to environmental impacts and legal compliance of landfilling secondary waste from MBT, a compromise has to be made: the MBT landfill fractions could not comply with the disposal criteria specified for ash/slag from WIPs. To allow the implementation of MBT technology, laid down in the Ordinance on Environmentally Compatible Storage of Waste from Human Settlements and on Biological Waste-Treatment Facilities (in German: Abfallablagerungsverordnung – AbfAblV) the German legislator defined special assignment criteria for MBT landfill fractions which are far less strict than those for WIP residues. Thus, especially in the last years before the ban on landfilling of untreated waste from human settlements became effective on 1st June 2005, numerous plants were built, which vary widely in their process configuration and its technical realization. Meanwhile MBT plants have established in Germany. The numerous modifications and retrofits show that the development of the MBT technology is still far from complete.

This paper first of all provides an overview of the number and capacities of MBT plants operated in Germany. On the basis of the analysis of their processes it describes the characteristics of the different process concepts and their processing configurations. The evaluation of the experiences gained during commissioning and operation points out the various technical and economic problems which the M(B)T plants had and partly still have to scope with. Finally the output streams of MBT plants are examined – both qualitatively in regard to their nature and disposal as well as quantitatively. The production of SRF and other value added material and secondary waste streams from the MBT plants throughout Germany are estimated. PLANTS AND CAPACITIES IN GERMANY At present, 61 mechanical(-biological) waste treatment plants with a total capacity of around 6.4 million tons per year are operated in Germany (cf. [2, 28, 12]). 42 of them are equipped with a biological treatment stage. Two plants – in Heilbronn and in Buchen (capacity about 90 000 and 150 000 t/a) –, which were commissioned in 2005, were already closed in 2007. Most of the plants (i.e. altogether thirteen) are located in the German state of Lower Saxony, followed by North Rhine Westphalia with eleven and Brandenburg with nine plants. The plant throughputs range between 25 000 t/a (Lindenberg/Gardelegen) and 300 000 t/a (Cröbern near Leipzig) respectively. Figure 1 shows the general distribution of the individual plants amongst the four process concepts and their variants as well as their share in the overall capacity. Altogether 52 plants were examined in detail with regard to their processing configuration and output streams [31]. The results form the basis for the remarks below.

Figure 1: Distribution of the number and total capacity of mechanical(-biological) waste treatment plants in Germany based on the four process concepts and their variants

PROCESS CONCEPTS The generic term Mechanical(-Biological) Treatment Plants – M(B)T plants – in this paper comprises those plants, in which household waste (in the most cases with other wastes like commercial waste, bulky waste, sorting waste) is treated by applying mechanical processing, mostly in combination with biological treatment. The mechanical(-biological) waste treatment includes four different process concepts in which mechanical, biological and thermal process stages with different objectives are combined with each other: • Material Stream Separation (MSS), • Mechanical-Biological Stabilization (MBS) – with biological drying, • Mechanical-Physical Stabilization (MPS) – with thermal drying, • Mechanical(-Biological) Pre-treatment prior to incineration (MBPT). Material Stream Separation In plants based to the concept of Material Stream Separation (MSS) the mixed waste is separated by mechanical processing into: • a concentrated high-calorific fraction for use as solid recovered fuel, • value added materials for material recycling (especially metals) and • a fraction with a depleted calorific value that is biologically treated and then

landfilled. To comply with landfilling criteria, in some cases coarse particle screening is also installed downstream. Depending on the specific design of the process, mineral waste constituents for landfilling can be discharged at various points in the process chain. Material stream separation is the most common process concept applied in Germany. Mechanical-biological Stabilization The aim of Mechanical-Biological Stabilization (MBS) is to stabilize the carbon as the main source of energy contained in the waste – especially the biologically degradable components from the organic waste fraction – by biological drying (generation of heat by microorganisms) and to transform it as far as possible into the high calorific fraction for use as SRF. The drying stage is an important precondition for the efficiency of the subsequent separation of the remaining waste into combustible, other value and inert materials. The Herhof [9], Nehlsen [15] and Lurgi processes follow this process concept. Mechanical-biological stabilization forms the basis for ten plants in Germany. The most commonly applied process is the Herhof dry stabilate process. It comprises mechanical processing (MP I = coarse processing), biological drying and further, much more sophisticated, mechanical processing (MP II = fine processing). Apart from dry stabilate and metallic value added materials, only an inert fraction of mineral waste components is produced as output, which can be landfilled without further treatment. Between the three above-mentioned processes, there are differences in terms of management of the material streams (full stream and sub-stream stabilization) as well as the output fractions and their channels of disposal (e.g. in individual cases separation of hard materials for construction processes and of fines for thermal treatment in a WIP). Mechanical-physical Stabilization In contrast to the MBS, in the Mechanical-Physical Stabilization (MPS) the waste is dried

with the supply of external thermal energy (fossil fuels) instead of self-heating as a result of biological degradation. Currently there are three such MPS plants in Germany. Coarse processing (e.g. pre-sorting by hand or with an excavator, multi-stage comminution and sorting) in which the value added materials (Fe, NF metals, in some cases a high-calorific fraction) are removed is followed by thermal drying (full stream or sub-stream stabilization). Then comes fine processing (sorting, classification) for the separation of fractions for SRF production and mineral fractions suitable for landfilling. Mechanical(-biological) Pre-treatment Prior To Incineration In the Mechanical(-Biological) Pre-Treatment prior to incineration (MBPT), high-calorific components for use as SRF and metals are separated from the waste. All residual material is supplied to a waste incineration plant (WIP). Any biological drying stage that may be included is used primarily for drying the low-calorific residual fraction for improvement of the fuel technical properties – and, if required, secondarily for the production of a second high-calorific fraction (stabilate). With this process, the amount of waste to be incinerated in a WIP is reduced considerably compared to direct incineration of the untreated residual waste. The process concept is implemented purely mechanically in seven plants and in three plants with a biological drying stage. PROCESSING CONFIGURATIONS Mechanical-biological waste treatment can be divided in the following modules: • mechanical processing prior to biological treatment (MP I), • biological treatment, • mechanical processing after biological treatment (MP II), • solid recovered fuel processing, • flue gas purification and • waste water treatment. Some of the modules do not exist in all plants or in some cases are outhoused in external plants. The individual modules can have different functions depending on the processing concept. They also differ in respect of the technical equipment used, – quite substantially in some cases. In the following the processing configuration is presented for the four process concepts mentioned before. Mechanical Processing Prior To Biological Treatment In material stream separation as well as in mechanical(-biological) pre-treatment prior to incineration, mechanical processing prior to biological treatment is used for separation of the fraction with enriched calorific value and other value added materials. The plant configurations are very different and range from restriction to single-stage comminution, screening and magnetic separation (which are common components in almost all mechanical processing modules) to complex combinations with sophisticated classification and sorting methods. In addition to separation in fine and coarse fractions by screening and separation of Fe scrap, in the majority of plants the waste is also separated by air classifiers and/or ballistic separators into high-gravity and low-gravity fractions. Partly in ballistic separation, besides the high-gravity and low-gravity fraction, a fine fraction is produced by combined gravity separation and screening. In many plants, eddy current separators are installed for the removal of NF scrap.

For the production of high-quality SRF, sorting by means of near-infrared spectroscopy (NIR) increasingly gains importance. This technique is used on the one hand for positive sorting, i.e. to selectively remove low-pollutant high-calorific material groups from different waste sub-streams. On the other hand it is used for negative sorting, i.e. for the removal of unwanted materials. For instance, polyvinyl chloride (PVC) is removed from the enriched calorific fraction or a sub-stream of it, so that the content of harmful chlorine is reduced substantially. In isolated cases, NIR positive and negative sorting are combined in one plant. In the MBS plants, mechanical processing prior to the biological stage (MP I) tends to be simple, as the materials are separated either solely or mainly after drying. This process stage includes: • in full stream MBS plants: single-stage comminution and, if required, magnetic

separation and screening, • in sub-stream MBS plants: comminution, magnetic separation, screening and air

classification. Processing steps in MPS plants are comminution, screening, magnetic and eddy current separation as well as air classification and NIR sorting in some cases. Further information about mechanical processing, sorting technologies and production of SRF are published e.g. in [16, 17, 18, 22]. Biological Treatment 42 out of the 61 mechanical(-biological) waste treatment plants in Germany are equipped with a biological treatment stage. In plants for material stream separation • rotting, • sub-stream fermentation, • full-stream fermentation as well as • the percolation process are used. The rotting processes consist of intensive rotting and secondary rotting. In the intensive rotting, as rotting system mainly tunnels and occasionally even rows, windrows, boxes and containers are used. In the secondary rotting the windrows predominate. In the plants with sub-stream fermentation a sub-stream of the low-calorific fraction is fermented in a single-stage dry fermentation process. The digestate is fed together with the remaining sub-stream of the low-calorific fraction first into the intensive and subsequently into the secondary rotting. The allocation to the anaerobic-aerobic or purely aerobic treatment depends on the nature, particle size and material properties of the waste input or on the organisational and operating situation. In the plants with full stream fermentation, the whole low-calorific fraction first passes through the anaerobic stage and subsequently through the aerobic stage. The conventional configuration consists of dry fermentation and secondary rotting of the digestate. In five plants a new concept with wet fermentation and subsequent aeration of the digestate in sludge activation tanks was realized. The aerated digestate is dewatered, dried and landfilled. During the commissioning, however serious operational errors occurred.

The only plant based on a percolation process is operated in Kahlenberg. The biological treatment in the so-called ZAK process comprises three stages: (1) the percolation and aerobic hydrolysis of the low-calorific fraction, (2) the purification and fermentation of the percolation water and (3) the biological drying of the percolation solids with subsequent complex mechanical material separation. The details are described in literature (e.g. [19]). Both of the percolation plants according to the ISKA process, which were operated in Buchen and Heilbronn, were decommissioned in 2007. In MBS plants, biological drying is performed in boxes, containers and tunnels with a sliding floor. The residual water content totals < 15 to 20 wt. %, depending on the system. Seven out of ten MBS plants are designed as full stream stabilization units. In the other three plants, a high-calorific fraction is separated before the residual fraction with depleted calorific value is dried biologically (sub-stream stabilization). Finally the fine fraction of the stabilized waste is treated in a secondary rotting stage. For the analogue thermal drying in MPS plants rotary dryer fuelled with natural gas are applied. In mechanical(-biological) pre-treatment prior to incineration the biological drying stage – providing this exists – is designed as a rotting tunnel. Mechanical Processing After Biological Treatment Mechanical processing after biological treatment (MP II) is oriented to the prior treatment stage and the aspired qualities of the output streams – especially SRF and landfill fraction –, and therefore also differs from one plant to another. In MSS plants in isolated cases, coarse particle screening of the rotten output is installed downstream of the biological treatment stage to ensure compliance with the AbfAblV assignment values. There are, however, also cases with more complex MP II, e.g. coarse particle screening combined with air classification or even the Biodegma process in Neumünster and the ZAK process in Kahlenberg with multi-stage classification and sorting process, but these remain the exception. The coarse particles removed are added to the fraction with concentrated calorific value for use as SRF or sent to a waste incineration plant. The basic MP II processes typically found in MBS plants include screening, air classification, magnetic and eddy current separation. Here multi-stage, complex classification and sorting processes are used to separate the biologically dried waste into stabilate/SRF and value added materials for material recycling as well as – as the case may be – inert materials for landfilling or a residual fraction for further biological after-treatment. In individual cases, the processes listed above are supplemented with hard material separation or optical sorting (MBS Asslar). The thermally dried waste of the MPS is also processed with more or less complex combinations of screening and air classification stages. The waste is separated into stabilate and inert material fractions. Sometimes optical sorting processes (X-ray sensors and colour sensors in the range of visible light) are integrated (MPS Berlin-Reinickendorf and -Pankow). Of the three MBPT plants equipped with a biological drying stage, only in one case (Wiewärthe) the dried sub-stream of the waste, by means of downstream screening, is further separated into higher calorific stabilate/SRF and low-calorific stabilate for waste

incineration. In the other two plants – Neuss und Erftstadt – the dried sub-stream of the waste is delivered to waste incineration without further processing. Solid Recovered Fuel Processing Processing of the high-calorific fractions and/or the stabilate to suitable SRF not necessarily has to be a process stage at a mechanical(-biological) waste treatment plant, it can also be performed in an external plant. In addition, clear-cut separation between mechanical processing and SRF processing is not always possible. In any case the aim of this process stage is an improvement of the SRF quality, such as to meet the quality criteria stipulated by the buyer. The higher these requirements are, the greater the processing effort and expense required. Typical elements of SRF processing are further comminution as well as magnetic and eddy current separation for the removal of residual scrap particles. Additionally possible processing steps are air classification for further removal of high-gravity and inert fractions, NIR systems for reduction of chlorine content, thermal drying and pelletization (press agglomeration), for example for the production of green or hard pellets. In Figure 2, 3 and 4, different qualities of SRF are shown.

Figure 2: Coarse fraction Figure 3: Pelletized Figure 4: SRF hard pellets with enriched calorific value SRF < 10 mm Flue Gas Purification Systems for flue gas purification are key components of MBTs and have substantial impact on plant costs. In MBT plants flue gas streams with different forms and loads of contaminations result and are to be purified. Low contaminated flue gas streams, e.g. from waste delivery or mechanical processing, are passed through a dust filter and/or a biofilter to reduce dust and odour. By a sophisticated flue gas management system – circuitry and multiple-shift usage of sub-streams – the demand for fresh air, the amounts of flue gas and the sizing of the purification aggregates are minimized. The flue gas stream from the intensive rotting process is very high contaminated, especially with organic compounds (VOC), nitrogen compounds like ammonia (NH3) and nitrous oxide (N2O) as well as odour. In the first step, in an acid scrubber ammonia is segregated to the greatest possible extent. Afterwards in a Regenerative Thermal Oxidation (RTO) reactor with ceramic honeycomb structure the organic compounds are oxidized. The purified gas from the RTO is released into the atmosphere through a stack. (e.g. [1, 14, 20, 25, 36]) Waste Water Treatment In MBT plants, waste water can be generated in the following areas: • bunker, • fermentation unit, • intensive rotting, secondary rotting, drying unit, • biofilter and scrubber,

• cleaning of halls, machines and vehicles, • sanitary facilities, • run-off water from roofs and traffic areas. The concentrations in waste water from MBT plants for almost all parameters are several times higher than those in landfill leachate. In the assessment of process waste water from mechanical-biological waste treatment, in particular the following parameters are relevant: • chemical oxygen demand (COD), • nitrogen compounds, especially inorganic nitrogen compounds, • salt content, • adsorbable organohalogens (AOX), • heavy metals like chromium, lead, copper and zinc. For MBT waste waters there is no universal treatment concept. The diverse composition of the waste water pollutants requires a process combination of different cleaning stages. An overview of the various processes used for the purification of waste water from MBT plants can be found in [23, 13, 6] for example. TECHNICAL, ECONOMIC AND ECOLOGICAL PROBLEMS During the years of their development, the M(B)T plants had to scope with various technical as well as economic problems and also induced ecological disadvantages, which meanwhile are partly solved and partly still subject of optimization. In the literature there are lots of reports about experiences made during commissioning and operation of M(B)T plants (e.g. [7, 8, 9, 10, 11, 15, 21, 29, 33, 34]). Mechanical Processing • high level of wear and tear with processing and conveying aggregates (e.g.

comminution, pelletization, chain belt conveyor), • blockages and contamination, e.g. during screening and ballistic

separation, • increased time and effort for cleaning, maintenance and repairs, • consequence: necessity of longer and/or more frequent stillstand and

maintenance periods and thereby reduction of time availability and throughput,

• personnel requirement frequently was significantly underestimated – less in terms of plant operation but primarily of cleaning, maintenance and repairs (see above),

• energy requirement partly was underestimated. Fermentation • strongly fluctuating production of biogas due to discontinuous

substrate-entry, • partly much higher amount of waste water generated than assumed in the

planning, which requires a complex and very costly treatment. At five plants operating with the new concept of wet fermentation and aeration of the digestate in sludge activation tanks, various – partly very serious – operational errors occurred during the commissioning, like: • need for optimization in process control for stable plant operation in the

interaction between fermentation and aeration,

• blockages, • formation of thick and hardened internal floating roofs, • deflagration and fire, as far as • destruction of the biological treatment stage through bursting of a fermentation reactor. Flue Gas Purification – Regenerative Thermal Oxidation (RTO) • blocking of the ceramic honeycomb structure through siloxanes in the flue

gas, which requires laborious cleaning in periodic intervals (e.g. every six weeks),

• dimensioning was frequently too small, especially regarding the stillstand periods for RTO cleaning,

• in many cases lacking redundancy, • corrosion in the casing of the RTO and the gas pipes, • energy requirement was frequently underestimated, Corrosion • corrosion affects – besides the RTO – also buildings, ventilation system of

the rotting system ect. Many of the points mentioned above contribute to reduced time availability and throughput of the plant as well as higher effort for cleaning, maintenance and repair work combined with higher personnel requirement – and therefore lead to increased operating costs. The waste disposal costs with M(B)T plants are similar to those with waste incineration plants. Besides construction and operation of the M(B)T plant, they also comprise the costs for combustion of SRF and, if applicable, further combustible fractions, landfilling of landfill fractions as well as transports. In Germany, the waste disposal costs for municipal solid waste amount to approximately 100 Euro per ton. Landfill Fraction • As it was not possible to comply with the assignment criteria specified for

the landfilling of ash/slag from waste incineration plants, for landfill disposal of secondary waste from MBT other, less strict criteria were defined.

• The landfill fractions from MBT have a higher organic proportion (TOC value), and therefore a higher biological activity than ash/slag from waste incineration plants. This may cause climate-damaging methane emissions and increased mobilisation of pollutants such as heavy metals.

Conclusion Most of the processes subsumed under the generic term mechanical(-biological) treatment have proven their functional capability – despite of the short developmental period. In many cases development stages usual in process engineering were skipped. Therefore it does not astonish, that during plant operation problems occurred, that caused considerable economic drawbacks. From the technical and economic as well as environmental point of view, there is continued need for development. OUTPUT STREAMS AND THEIR DISPOSAL

In mechanical(-biological) waste treatment plants the following main output streams are produced: • solid recovered fuel (SRF), • combustible fractions, which don’t meet SRF quality, • value added materials and • landfill fractions. Solid Recovered Fuel For Energy Recovery In all M(B)T plants fractions with enriched calorific value – solid recovered fuels – are produced. Depending on the process concept they can be separated in one or several process modules. The energy recovery of the SRF is possible as: • (mono-)incineration in SRF power stations, • co-incineration in industrial plants and • conventional waste incineration. Incineration in SRF power stations SRF power stations in principle are waste incineration plants, which – compared to those plants for untreated municipal solid waste – are designed for differing fuel technical properties, e.g. bulk density, calorific value and ignition behaviour. In Germany, currently thirty SRF power stations with a total capacity for SRF from municipal and commercial solid wastes of 3.6 million t/a are in operation. Another six plants with a capacity of 1.2 million t/a are under construction (as at November 2009) [26]. Finally there are a number of projects, whose realisation is uncertain. As firing system the water cooled grate firing dominates. Some plants are designed as fluidised bed combustion (FBC) units, prevailing those for energy supply of paper mills, because in the FBC reactor at the same time the residues from paper manufacturing – e.g. rejects – can be incinerated. [27] Co-incineration in industrial plants By co-incineration of SRF in industrial combustion plants a part of the fossil fuels can be substituted. Preconditions are, that the plant disposes of the permission and that the technical equipment – especially for SRF storage, feeding and flue gas treatment – exists or is retrofitted. The co-incineration of SRF from MBT plants is operated in lignite-fired and hard coal-fired power plants [31, 30] as well as cement plants [35, 24] – in each case both, directly and with upstream thermal processing (pyrolysis, gasification). SRFs are difficult fuels when compared to fossil fuels [4], in particular regarding the following fuel technical properties: energy conversion density, ignition and burnout behaviour, slag formation and corrosion potential [3, 5]. The co-combustion of SRF from MBT plants, i.e.: • in coal fired power plants (about 650 000 t/a SRF from municipal solid

waste and specific process waste) and • in the cement industry (about 186 000 t/a processed fractions from

municipal solid wastes) is of subordinate significance according to the amounts.

Conventional waste incineration A part of the SRF from MBT plants is recovered for energy in conventional waste incineration plants [32]. In the bunker, the SRF is mixed with the untreated residual waste and subsequently incinerated together with it. Some new plants a priori were designed for a mixture of untreated residual waste and fractions with enriched calorific value from mechanical processing of municipal and commercial solid wastes. Combustible Fractions For Waste Incineration In about one-third of the M(B)T plants combustible fractions are separated in the mechanical processing modules. Predominantly it contains high-gravity fractions from ballistic separation or air classification, which are neither suitable for SRF production nor comply with the criteria for landfilling and therefore are disposed in waste incineration plants. A special case is the process concept of mechanical(-biological) pre-treatment prior to incineration, that just targets the combustion of all residual waste constituents remaining after separation of SRF fractions and value added materials. Therefore the quantity of waste disposed in WIPs is much larger than in other process concepts. Independent of the particular process also during pre-sorting in the bunker e.g. bulky waste components and contaminated materials, can be separated and delivered to a WIP. Value Added Materials For Material Recycling In all M(B)T plants, magnetic separators for the separation of Fe scrap are applied. Also eddy current separators for the separation of NF scrap are widely used. Partly already during waste receipt and pre-sorting, large pieces of metal scraps are taken out of the delivered waste, e.g. with a grab dredger. In several plants, a wood fraction for energy recovery in biomass power stations is produced from wooden mono-charges, bulky wastes, wood-containing commercial wastes as well as coarse wooden components from waste pre-sorting. Only one single plant recovers white, green and brown glass from household and commercial waste by optical sorting. In another plant a paper fraction for material recycling is separated by NIR sorting. In particular cases mineral fractions are recovered for material recycling. For example in one MBT plant stones, glass and ceramic components are separated. This mineral fraction is further mechanically processes in another extern plant and then recycled, e.g. for construction of roads. Landfill Fractions There are two types of landfill fractions: • on the one hand organic fractions, which were treated in biological

treatment processes to comply with the assignment criteria for landfilling and

• on the other hand inert materials separated by mechanical processing. MASS BALANCE OF M(B)T PLANTS THROUGHOUT GERMANY – AN ESTIMATION

The quantitative partitioning of the waste input into different output streams particularly depends on: • the material composition of the waste input, • the process concept and • the objective and correspondent technical equipment of the M(B)T plant

(e.g.: production of high quality SRF, or: maximizing the SRF amount with simultaneous lowering of quality).

Within the scope of a study the SRF production as well as the other value added materials and secondary waste streams from the mechanical(-biological) waste treatment plants in Germany were estimated. On the basis of a balance scheme the yearly mass flows of the different output streams were calculated (Figure 5). The methodical procedure applied, the assumptions, and data taken as a basis and the derived balance scheme are described in the study [31].

Figure 5: Mass balance of M(B)T plants throughout Germany (estimation, 11/2007) Accordingly the total SRF production in German M(B)T plants amounts to about three million tons per year. Additionally about 0.7 million tons of combustible fractions (including extraneous materials) are recovered in waste incineration plants. Thus altogether nearly sixty percent of the M(B)A input material finally are incinerated – partly by mono-incineration in SRF power stations, partly by co-incineration in coal-fired power plants and cement plants and partly by conventional waste incineration. Beyond that, about 160 000 tons of Fe scrap and about 32 000 tons of NF scrap are separated annually for material recycling. Finally about 1.4 million tons per year of secondary wastes from M(B)T plants remain for landfilling. The difference between the mentioned output streams and the waste input amounting to 6.4 million tons per year consists partly of mass lost in rotting processes and partly of water separated in biological or thermal drying processes (together about 1.1 million tons per year).

SUMMARY AND CONCLUSIONS M(B)Ts are complex waste treatment plants with a wide ranging variety in terms of their technical configuration. Most of the processes subsumed under the generic term mechanical(-biological) treatment have proven their functional capability – despite of the short period of development. In many cases various development stages, usual in process engineering, were skipped. Therefore it does not astonish, that during the years of their development the M(B)T plants had to scope with various problems, which meanwhile are partly solved and partly still subject of optimization: Technical problems are for example the wear of processing and conveying aggregates, blocking of the Regenerative Thermal Oxidation (RTO) through siloxanes and corrosion. This results in reduction of time availability and throughput as well as increased personnel requirement for cleaning, maintenance and repairs and therefore higher operating costs. The main ecological problem is that landfill fractions from MBT have a higher organic proportion (TOC-value) and a higher biological activity than ash/slag from waste incineration plants (WIPs). This creates disadvantages in terms of climate protection and pollutant-mobilisation in the leachate. As it was not possible to comply with the assignment criteria specified for landfilling of ash/slag from WIPs, for the landfill disposal of secondary waste from MBT other, less strict criteria were defined. From the technical and economic as well as environmental point of view there is continued need for development. The advantages of the MBT system may include lower transport costs in very sparsely populated areas, in individual cases a higher energy efficiency and the potential for separation and recycling of further value added materials – the efficiency and ecological relevance of those additional processes have to be checked for each individual case. The initial political goal of M(B)T was equivalence with the incineration of waste. This is obviously not the case: MBT cannot replace waste incineration, it is only a pre-treatment of waste prior to its incineration. In all MBT plants solid recovered fuel is produced. Therefore incineration capacities have to be created in SRF power stations, an alternative for limited amounts may be co-incineration in lignite- and hard coal-fired power stations as well as cement plants. Thus incineration is not and cannot be replaced by MBT, but it is simply delayed in the scope of a more complex system – with more material streams and treatment steps (Figure 6). Altogether in Germany almost 60 weight percent of the waste input of MBTs finally are incinerated in SRF power stations, coal-fired power stations, cement plants and conventional waste incineration plants. The waste disposal costs with mechanical(-biological) waste treatment plants and waste incineration plants are similar. In Germany, they amount to approximately 100 Euro per ton.

Figure 6: Comparison of the systems of MBT and WIP

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