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U P D A T E 3 3

Mechanical Biological Treatment of Municipal Solid Waste

This Technology Report from Task 36 was preparedfor ExCo58 in Sweden in October 2006 by NicoleJaitner and Jim Poll

Abstract

The IEA Bioenergy Agreement is set up to examineand develop bioenergy solutions. One potentialsource of energy is municipal wastes, and whilstmass burn energy from waste solutions exist, thereare many barriers to its implementation. Onealternative approach is to pre-sort waste usingcombinations of mechanical and biologicalprocesses (so call MBT processes) so that energyand additional recycling can be recovered, often atscales unattractive for mass burn systems andovercoming some of the barriers inherent with massburn energy from waste.

This report reviews mechanical biological treatmentprocesses. These typically split the residual wastestream into 3 fractions: a recyclable stream (glass,metals, etc.), a biological stream (for composting oranaerobic digestion), and a fuel stream for energyrecovery. There are about 50 such facilities inoperation in Europe mainly in Germany, Italy, andSpain. There is considerable interest in thesetechnologies throughout the world, but particularlyin Europe, as a means of achieving resource recovery(materials and energy) and landfill diversion.However, while the technology has its limitationsand is still developing it offers potential EfWsolutions in circumstances where other traditionalapproaches would be difficult to implement.

Introduction

One of the Topics in the Programme of Work beingundertaken by Task 36 in the 2004-2006 triennium isMechanical Biological Treatment (MBT) ofmunicipal solid waste (MSW). The aim of this workis to review the status of MBT systems and theirpotential for integrating energy recovery processes.

One of the drivers for this technology in Europe isthe Landfill Directive [1]. The Directive imposestargets on EU member states to reduce the amount ofbiodegradable waste going to landfill compared tothe waste generated in 1995 to: 75% by 16 July2006; 50% by 16 July 2009; and 35% by 16 July2016. An even stronger driver is the complete ban onorganic material entering landfills in countries suchas Sweden, Switzerland, Austria, and Germany.

Compliance with the EC and national directivesrequires increased deployment of recycling andrecovery operations for waste. One of the options isto segregate the biodegradable waste and treat it inone of a number of ways. The interest in MBT isincreasing as a potential option for achieving therequirements of the landfill directive.

In other parts of the world the key driver is theadditional resource efficiency achieved throughsorting and treatment to optimise the embeddedenergy recovery through maintaining the energycontained in materials and products that arerecycled, rather than combusted, for the recovery ofthe inherent energy [2].

The use of MBT also alleviates some of the publicantipathy for Energy from Waste (EfW) incinerationtechnologies [3]. MBT allows the recovery of energy

from municipal wastes with reduced problems ingaining permits to build such plants.

MBT Process and Products

MBT is a generic term that encompasses a widerange of technologies that aim to process waste by amixture of biological treatment and mechanicalseparation to achieve waste stabilisation, recyclingand energy recovery. The details of a range ofsystems are provided in system reviews e.g., thosecarried out by Juniper [4], UK Environment Agency[5, 6] and the Swedish Association of WasteManagement [7]. In MBT the biodegradable fractionis treated post sorting, whilst in BiologicalMechanical Treatment (BMT) the biologicaltreatment occurs first.

Mixed residual waste (i.e., waste remaining aftersource separation, or “grey” waste) is first sorted viaa series of mechanical treatment options thatseparate out recyclable materials (e.g., metal andglass) and fractions for biological treatment and orrefuse derived fuel. All systems have sortingprocesses that separate various fractions andmechanically degrade the organic fractions throughshredding, wetting, and tumbling, or through theaddition of steam. The main effect is to concentratethese fractions for further processing. The keydifference between various systems is the choiceadopted for processing the higher calorific valuematerials. Options include producing a substitute for

fossil fuels (refuse derived fuel - RDF), or removingthe higher calorific components such as plastics forlandfilling and processing the residue to producecompost. The main biological process can be carriedout either aerobically (composting) or anaerobically(digestion - AD). Whilst biologically these aredifferent processes, the final degraded solid productsare similar, with anaerobic digestion having theadded benefit of generating a gas with a highmethane content that can be used as a fuel.

BMT is a special case of MBT where the whole ofthe waste is treated biologically prior to sorting. Thisbiological treatment is principally to dry the wastethus making subsequent mechanical separation forRDF more effective. Waste is aerated withincomposting vessels; as temperature rises so themoisture is driven off. After one to two weeks thewaste is dried and undergoes mechanical separationto generate a fuel (RDF) fraction. The fuel is thenprepared for market. The reject waste is still high inorganics and can undergo further composting togenerate a poor quality compost for landfill cover,but typically this fraction is simply landfilled as themost readily degradable materials are lost in theinitial composting stage.

The main outputs from the various MBT/BMTprocesses are shown in the Table below. Thefractions of these outputs vary between the differentproprietary processes but generally, depending onthe feedstock, are within the ranges shown.

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Main Outputs from MBT/BMT Processes Fractions

Recyclables: e.g., metals aggregate substitutes and plastics 4 - 14%

Organic/Compost: the organic rich fraction that is then composted 38 - 70%or digested to generate a compost product and biogas

Fuel (RDF): the fuel fraction that is either burnt on-site or sent for 0 - 75%combustion in a remote combustion facility

Rejects: the residues and rejects that have to be landfilled. 10 –25%

There are five main types of MBT process:

• Plants which stabilise waste prior to landfill

• Plants which produce a compost product

• Plants which produce a RDF product

• Plants which produce both a compost product anda RDF product

• Plants which incorporate anaerobic digestion togenerate biogas for electricity production

MBT processes also enable metals and othermaterials to be recovered for recycling.

Task 36 has compiled a database of MBT facilities[8]. The number of MBT plants has risen rapidlyfrom around 20 in 1990 to 65 in 2000 and over 170by 2005. Figure 1 shows the number of plants bycountry. Germany has the largest number of MBTplants, followed by Italy and Spain. The totalcapacity of these plants is over 15 million tonnes peryear, and the capacities range from under 10,000tonnes per year to 300,000 tonnes per year.

Typical examples of well established processes arethe Horstmann process which produces RDF andcompost products and/or stabilised waste forlandfilling, and the Eco-Deco process whichprincipally produces a RDF product. The totalcapacity of Eco-deco plants installed in Italy andSpain is over 750,000 t/yr, and the three plantsplanned for the UK (two are built) will have a totalcapacity of over 400,000 t/yr.

An anaerobic digestion (AD) process can be used torecover energy, and one example of this type ofprocess is the 3R-UR process at Eastern Creek,Sydney, Australia which has a current capacity of175,000 t/yr. An example of a developing technologyis the autoclave (steam treatment) process forproducing a RDF product which has not beencommercially demonstrated for MSW applicationsbut there are several demonstration facilities andprojects in planning.

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GermanyItaly

Spain UKFrance

Austria

The NetherlandsFinland

AustraliaUSA

China

BelgiumU.A.E.

Canada

Czech RepublicTurkey

JapanPoland

LuxembourgIsra

el

SwedenLibya

Figure 1: Number of plants in each country(based on data from the Task 36 MBT database)

Tonnesper

aunnum

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The Challenges for MBTAlthough a significant number of MBT processeshave been developed, and a large number ofcommercial plants have been constructed, thepotential for future plants will depend on a numberof commercial and technical challenges, particularlythe availability of markets and/or uses for theproducts that MBT plants produce.

Three approaches for using MBT plants to recoverenergy are anaerobic digestion, the use of the RDFproduct as a secondary fuel in an industrial facility,and dedicated combustion facilities.

Anaerobic digestion: AD systems can recoversubstantial energy from the degradable fractions ofthe waste and are inherently biological in origin asthe plastics and fossil derived wastes do not degrade.The amount of energy that can be derived from thewaste is dependant on the feedstock composition andthe process configuration. However, depending onthe particulars of the process design and operation,gas yields are between 40 and 140 m3/t of inputwaste. This corresponds to electrical production ofbetween 100 and 360 kWh/tonne of waste. Withplant consumption being between 100 and 300kWh/t the export energy is between 0 and 200 kWh/tof electricity with additional heat energy availablewhich may find use in heating schemes

RDF as a secondary fuel: The use of RDF in anindustrial combustion facility as a replacement forother fuels may be constrained by waste specificlegislation (such as the Waste Incineration Directivein the EU). There are also a number of technicalissues. For example, RDF has a lower carbon contentthan coal requiring altered air injection patterns, andthe higher levels of alkali metals can result in higherlevels of fouling and corrosion. One common use forRDF is in cement kilns, but as the chlorine contentcan affect the quality of the cement product, thecement industry has set limits for the maximumchlorine content of the RDF product, and someplants have not been able to meet this limit.Furthermore, there is often some public opposition

to the use of RDF (and other fuels) as a waste fuel inany combustion facility. The calorific value of RDFvaries with processes from products that are similarto the input waste (but are dried or have some inertmaterials, e.g., glass, metals etc. removed) at 10-12MJ/kg, to highly refined plastics rich fuels with a CVup to 20 MJ/kg. The efficiency of combustion andenergy conversion will vary with the combustionplant with incinerators achieving electric efficienciesof around 20% but co-combustion in power plantswill give much higher efficiencies similar to thoseachieved on the primary fuel.

Dedicated combustion facilities: These can eitherburn the RDF directly, or further process it using agasification or pyrolysis technology. The technicalissues for direct combustion of RDF will be similar tothose for combustion in an industrial facility.Gasification technologies are well established forprocessing some biomass and waste materials such aswood, but there are currently few commerciallyoperating plants that treat municipal wastes. RDF ismore homogeneous than municipal waste as it hasgone through mechanical sorting and pre-treatmentprocess, and is easier to process, and there is also areadily available market for the electricity that wouldbe generated. However, the combined costs for theMBT plant and the thermal treatment facility meanthat this is likely to be a much less economicallyattractive option than conventional incinerationsystems, but still may be more appropriate to aparticular location due to the local political, legislative,or structural circumstances. It can also apply to areaswhere smaller satellite MBT facilities can support acentral combustion facility where higher efficienciescan be exploited through larger scale plant, improvedtechnology, or locating near a heat user.

The RDF produced by a MBT process will have tocompete with other “renewable” or “non-fossil”fuels such as tyres, biomass products, and energycrops. Potential users may view fuels with a highrenewable content as a more attractive fuel, but theRDF produced by many MBT processes will have ahigh plastic content diluting the renewable content,

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and thus would complicate the accounting of CO2benefits or may not count as a renewable fuel undersome regulatory systems.

Research suggests that the RDF produced by steamtreatment (autoclave) processes will have a muchhigher biomass content (the RDF contains a muchlower percentage of plastic), and thus this type ofprocess, may be able to produce a more marketableRDF product. It should be noted that these processesare still in development and no full scale processesare operating on MSW and the initial promise ofthese system may not be fully realised.

The development of markets for these fuels is beingassisted by the development of standards for solidrecovered fuels (SRF) through the Europeanstandards organisation (CEN /TC342). Thedevelopment of these standards will allowdevelopers and operators to optimise their facilitiesfor the fuel qualities available with the confidencethat the fuel will not compromise operation of theirenergy production plant. However, the existence ofstandards will not of itself develop markets and otherfactors will be required to fully facilitate the sale ofRDF/SRF.

Where AD is a component of the MBT system thenthe biogas produced will contribute to the energybalance of the plant. The opportunities for gasutilisation have been extensively addressed by Task37 and include direct combustion in a boiler, engine,or turbine to generate steam or electricity orconversion to pipeline quality for injection to the gasnetwork or use as vehicle fuel [9].

MBT processes may also produce a compost productfrom the mixed residual waste input which is likelyto have higher levels of contamination (glass, metal,plastics) and lower nutrient levels than productsproduced from source separated feedstocks. Thus,whilst there may be opportunities to use thecompost/digestate produced by MBT processes as asoil improver, it will be very difficult for it tocompete with the compost produced from sourceseparated materials in many of the current marketsfor waste derived compost products. Most of the

compost being produced by MBT plants is currentlybeing landfilled or used as landfill cover, althoughsome is being used in Australia, Spain, and Israel.The use of compost from mixed residual wastedepends entirely on the legislation implemented inthe various countries.

MBT plants can also recover dry recyclable productsincluding ferrous and non-ferrous metals, glass, anda mixed polymer plastics product. These productswill have to compete with source separated products,and whilst the glass product may well be suitable foruse in aggregate substitute applications, markets forthe mixed polymer product are currently limited.

The original concept for MBT was to developprocesses that reduce the biodegradable content ofresidual waste, by stabilising it through the use of acomposting process so that it could be landfilledwith lower environmental impacts. This approach issupported by environmental pressure groups and aprinciple benefit of this approach is that the publicopposition associated with mass burn systems can beminimised. This allows the development of EfWsystems in locations where for example waste heatcould be fully utilised and where permittingprocesses may prevent traditional systems.

In Germany, the first MBT plants which wereinstalled were able to identify markets (cement kilnsand power stations) for their RDF product, but thelimited total size of these markets, together with theissues regarding potential use of the compostproduct, mean that the new plants which are beingconstructed in Germany in order to enable thelandfill Directive targets to be met are onlyproducing a stabilised product (with a organiccarbon content of less than 5%) which is thenlandfilled. Recent changes in German legislationhave also reduced the capacity in EfW facilities,which means that finding capacity to burnadditional RDF will be more difficult. It should benoted that there on-going substantial increases inwaste incineration capacity in Germany. Swedenand the Netherlands to cope with waste divertedfrom landfill.

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The Future for MBTConventional EfW (incineration) will continue tohave a key role to play in treating residual wastes.MBT type processes may complement EfW systemsand offer a number of potential benefits but faceconsiderable challenges in realising these. Factorsthat make MBT attractive include the following:

• MBT plants may be financially viable when thereject fraction can be landfilled and landfillcharges are low.

• MBT plants have the potential to operateeconomically at smaller scale and thus have theability to process waste locally, producing a RDFproduct that can be burnt in more remotecombustion facilities.

• The RDF product should be more suitable for heatrecovery as well as electricity generation as thescale should better match the smaller demands forheat in areas without district heating networks.

• MBT integrated with anaerobic digestion offers thepotential for renewable energy recovery comparedto composting based systems.

However, a number of factors can make MBTunattractive:

• The overall amount of energy, as electricity, whichcan be recovered using a MBT process is likely tobe lower than that from using conventionalincineration. This is due to the energy used in theprocess and the residues sent to landfill and notburnt.

• Lack of markets for the RDF and compost productscan prove a barrier for the commercial viability ofthe MBT facility. Processes which might producehigher biomass content RDF product (such asautoclaving) are still being developed and mayprovide some competition for MBT in future.

• The poor public perception of any facilitycombusting waste derived fuels (including theRDF produced by MBT plants and industrialprocesses burning the RDF product) can be abarrier for getting permits to operate.

• Greater landfill capacity will be required forprocess rejects (and for any products which cannot be marketed or used).

Some MBT technologies are well established, andwhilst markets have been found for the RDFproducts that the initial plants produced, the currentmarket capacity is limited. A large potential marketfor RDF is use in power stations, but unless co-firingsolutions can be further developed successfully (bothcommercially and technically), the capacity forusing RDF products to generate electricity may wellbe restricted to either MBT plants which incorporatean AD facility or a dedicated thermal conversionfacility. There will be limited opportunities torecover heat at these plants as waste facilities areoften sited away from other industry and housingdue to concerns over odour and emissions.Additional development work could be conducted toproduce better quality compost products and developsuitable markets for them and to improve the qualityand range of other recyclates to enhance theconservation of embedded energy. Otherwise, thefuture role of MBT may well be just to treat smalllocal arisings of waste in order to recover recyclablesand stabilise the remaining waste prior to landfill,but this will depend on the availability of suitablelandfill capacity.

Task 36 participants visiting the OMRIN MBT plant in the

Netherlands. This plant features mechanical sorting and anaerobic

digestion as the biological treatment. The residual fraction is

partially land filled but mainly incinerated

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References

1 European Commission. 1999. Council Directive1999/31/EC of 26 April 1999 on the landfill ofwaste, Official Journal of the European Union, L182, 16.7.1999, p. 1–19.

2 Australian Government ProductivityCommission. 2006. Waste Management,http://www.pc.gov.au/inquiry/waste/draftreport/waste.pdf

3 Hogg, D. 2006. Dirty Truths.http://www.foe.co.uk/resource/briefings/dirty_truths.pdf

4 Juniper Consulting. 2005. MBT: A guide forDecision Makers – Processes Policies andMarkets. http://www.juniper.co.uk/Publications/mbt_report.html

5 Environment Agency. Waste Technology DataCentre, http://www.environment-agency.gov.uk/wtd/679004/?version=1&lang=_e

6 DEFRA. 2005. Mechanical Biological Treatmentand Mechanical Heat Treatment of MunicipalSolid Waste. http://www.defra.gov.uk/environment/waste/wip/newtech/pdf/mechbiotreat.pdf

7 Wannholt, L. 1999. Biological treatment ofdomestic waste in closed plants in Europe - Plantvisit reports’. RVF Report 98:8. ISSN 1103-4092.RVF - The Swedish Association of WasteManagement and RVF Service AB, Malmö.321pp.

8 Wheeler, P. 2007. MBT facilities, a review ofsystem. In press.

9 IEA Bioenergy Task 37. 2006. Biogas Productionand Utilisation, http://www.iea-biogas.net/

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