municipal solid waste: what to do with the biodegradables?

BACAS BELGIAN ACADEMY COUNCIL OF APPLIED SCIENCES

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Table of content

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Résumé . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Samenvatting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Chapter 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Chapter 2.Legal constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10European Union . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Belgium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Flanders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Wallonia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Brussels-Capital Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Chapter 3.General considerations on Municipal Solid Waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Preliminary remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Data and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Final remarks – Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Chapter 4.Best available techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18General considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18High temperature processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Incineration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Gasification and pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Plasma processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Low temperature processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Composting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26Anaerobic digestion (biomethanisation) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Chapter 5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Preliminary remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Scientific, technological, economical and environmental considerations . . . . . . . . . . . . . . . . . . . . . . . . . . 34Legal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Chapter 6.Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Further readings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39


This report was prepared according to the BACASobjectives to inform responsible authorities and torecommend the measures to deal with the concernedtopic.

The paper deals with the biodegradable part of wastegenerated by citizens in urban environments. This isessentially household waste and gardening waste.Assimilated to this category of waste is almost all thatcomes from restaurants, canteens and food shopsinasmuch as its composition is similar to that ofhouse hold waste.

In the European Union, people generate currently523 kg per inhabitant and per year of municipal solidwaste (MSW). Hazards and nuisances associatedwith dumping are deemed unacceptable. Very specificand mandatory regulations make landfilling very diffi-cult to manage. The trend is, accordingly, to reduce asfar as possible the residual amount of waste to dump.Today, in most developed countries, local programsaim to separate household hazardous wastes (chemi-cal cleaners , pesticides, paints, batteries, oils, etc…)and to recover certain materials (metals, paper, card-board, plastics, glass, textiles, etc…) at the source.There remains however currently 204 kg/inhabitant.year of biodegradable waste in MSW, and it is respon-sible for most of the waste’s related disturbances inurban environments. For the European Union with its500 millions inhabitants, this makes 102 million Mg(1Mg = 1 metric tonne) of biodegradable MSW, i.e.approximately 20% of all biodegradable waste gene-rated by economic activities each year in the EU. Thisjustifies fully the present report.

From a legal standpoint, the European Union adoptsdirectives which must be transposed by MemberStates in their own legislation within a given period oftime. This report includes a short analysis of the maindirectives of interest for the subject treated. The newDirective 2008/98/EC is examined in detail; it introdu-ces a waste hierarchy in 5 points: prevention; prepa-ring for re-use; recycling; other recovery, e.g. energyrecovery; disposal. The present status of legislation inBelgium is also described briefly.

The best available techniques for the treatment of biodegradables contained in MSW are examined,restricting the scope to techniques that “have beendeveloped and tried with success on an industrialscale allowing implementation in the relevant indus-trial sector, under economically and technically viableconditions” as defined in Directive 2008/1/EC.

Accordingly, R&D processes and pilot plants are notincluded. However, some processes which stilldepend on subsidies for survival are discussed in thereport.

Among the high temperature processes, incinerationmay be considered as pertaining to the best availabletechniques for the treatment of MSW, because it com-plies with all the conditions imposed by the relevantEU directives, including environment protection.Biodegradables contained in MSW are easily proces-sed in mass-burn, modular or fluidized-bed incinera-tors. There is no need for separate collection or pre -liminary sorting out. They can be burnt as such, evenin cardboard or plastics packaging. A preparation stepis however required before introduction in a fluidizedbed incinerator. If the incinerator plant generatesenough energy to comply with the requirements defi-ned in Annex II/R1 of the Directive 2008/98/EC, it canbe called “Energy recovery plant”.

Other high temperature processes (pyrolysis, gasifica-tion and plasma processes) are not considered todayas “best available techniques” in the European Unionfor various reasons mentioned in the report. Theyshould be monitored for their technical and economi-cal progresses. The low temperature processes arenot able to treat MSW as such. They can only copewith the biodegradables fraction contained in MSW,and necessitate either separate collection of the bio-degradables at the source or adequate sorting ofMSW before loading in the process. There are twopossibilities: aerobic treatment (composting) andanaerobic digestion (biomethanisation). The two pro-cesses depend on microorganisms for their correctfunctioning. Only part of the carbon is converted toCO2.

Composting can be operated over a large range ofscale from very small (home and backyard) to largecentralized composting plants. At the small scale,there is the advantage that biodegradables are re -moved from the waste stream. At the large scale, theprocess becomes more difficult to operate because ofthe need to feed correctly air and water to the loadand also because the liquids must be recovered andt-reated. There is no energy recovery. Good qualitycompost can be considered as a fertilizer and a soilamendment. However, a true market for this compostdoes not seem to exist.

Biomethanisation received a lot of attention during thepast decades, because it generates methane that can

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be used to recover energy. There are various ways tooperate the process, and potential feedstocks fromdifferent origins may be envisaged (agricultural origin;industrial origin; MSW and sewage sludge). There aredifficulties to control properly the process especially ifthe characteristics of the load change with time.Energy recovery is much lower than with incineration.Good quality digestate can be considered as a fertili-zer. Again, a true market for this digestate does notseem to exist. The process is economically viable onlywhen subsidies are available.

A detailed discussion is included in the report. In pre-liminary remarks it is stated that: any chemical ele-ment present in the incoming stream will anyhow bepresent in the outputs in the same quantity (this holdsespecially for heavy metals); no process may claimany “greenhouse effect” advantage (after decomposi-tion, compost and digestate end up with CO2 andH2O); for energy recovery, when the global process issplit into two partial processes, with the first of the par-tial exothermic, the net calorific effect of the second

partial is reduced (this is the case for the combustionof methane from anaerobic digestion); most flawedwaste policies forget and leave out thermodynamics;“not in my backyard” emotional reactions are ruled outif the technology does not justify them. The discussionis split in two parts: the first one is limited to scientific,technological, economical and environmental consi-derations; the second to legal considerations.

Finally, the conclusions present the necessary ele-ments for the authorities to make correct decisions.After reducing by all possible means the amount ofbiodegradables contained in MSW, the main decisiondeals with proceeding or not to the separate collectionof the biodegradables remaining in MSW at thesource , taking all elements in consideration. There arealso recommendations. Among them appears theneed for new European directives and BREF docu-ments for composting and anaerobic digestion: thiscould help in generating markets for compost anddigestate.

Ce document a été rédigé selon les objectifs duBACAS d’informer les autorités responsables et demettre à leur disposition tous les éléments nécessai-res pour prendre les bonnes décisions.

Ce rapport concerne la partie biodégradable desdéchets générés par les citoyens dans un environne-ment urbain. Il s’agit essentiellement de déchetsménagers et de jardinage. Sont aussi assimilés àcette catégorie de déchets presque tous ceux quiviennent des restaurants, des cantines et des épice-ries pour autant que leur composition soit analogue àcelle des déchets ménagers.

Dans l’Union Européenne, la population génère actu-ellement 523 kg par habitant et par an de déchetsmunicipaux solides (MSW). Les risques et nuisancesassociés à leur mise en décharge sont jugés inaccep-tables. La tendance est donc à réduire autant quepossible la quantité résiduelle de déchets à mettre endécharge.

Aujourd’hui, dans la plupart des pays développés, desprogrammes locaux permettent de séparer à la sour-ce les déchets ménagers dangereux (nettoyants chi-miques, pesticides, peintures, batteries, huiles, etc…)et de récupérer certains matériaux (métaux, papiers,cartons, plastiques, verre, textiles, etc…). Il restenéanmoins actuellement 204 kg/habitant.an dedéchets biodégradables dans les MSW, et ce sont euxqui sont responsables de la plupart des nuisancesdues aux déchets en milieu urbain. Considérant envi-ron 500 millions d’habitants dans l’Union Européenne,

cela fait au total 102 millions Mg (1 Mg = 1 tonne mét-rique) de déchets biodégradables dans les MSW,c’est-à-dire environ 20% de l’ensemble des déchetsbiodégradables générés par l’activité économiquedans l’UE. Ceci justifie pleinement le présent rapport.

D’un point de vue légal, l’Union Européenne édictedes directives qui doivent être transposées par cha-que pays membre dans leur propre législation dansun délai limité. Ce rapport comprend une courte ana-lyse des principales directives d’intérêt pour le sujettraité. La nouvelle Directive 2008/98/EC est examinéeen détails; elle introduit une hiérarchie en 5 points:prévention; préparation pour la réutilisation; recycla-ge; autre récupération, par exemple récupération d’é-nergie; mise en décharge. L’état actuel de la législa-tion belge est aussi inclus.

Les meilleures techniques disponibles pour le traite-ment de la partie biodégradable des déchets conte-nus dans les MSW sont examinées, en se limitant à“celles qui ont été développées et essayées avecsuccès à l’échelle industrielle de façon à permettreleur incorporation dans le secteur industriel adéquat,dans des conditions économiquement et techni-quement viables”, ainsi que prévu dans la Directive2008/1/EC. Les procédés qui sont au stade de la R&Dainsi que les installations pilotes ne sont pas inclus.Toutefois, certains procédés dont la survie est assu -rée par des subsides sont discutés dans ce rapport.

Parmi les procédés fonctionnant à températureélevée, l’incinération peut être considérée, au sein

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de l’Union Européenne, comme appartenant auxmeilleures techniques disponibles pour le traitementdes MSW, parce qu’elle satisfait à toutes le conditionsimposées par les directives européennes y relatives, ycompris pour la protection de l’environnement. Lesdéchets biodégradables contenus dans les MSW sontaisément traités dans les incinérateurs à combustionde masse, modulaires ou à lit fluidisé. Il n’est pasnécessaire de procéder au préalable à une collecteséparée ou à un tri en usine. Ils peuvent être brûléstels quels, même dans leur emballage en carton ou enplastique. Une certaine préparation est cependantnécessaire pour les incinérateurs à lit fluidisé. Si l’usi-ne d’incinération satisfait à la formule définie dansl’Annexe II/R1 de la Directive 2008/98/EC, elle méritel’appellation “Centre de valorisation énergétique”.

Les autres procédés fonctionnant à températureélevée (pyrolyse, gazéification et procédés plasma) nepeuvent être considérés actuellement comme figurantdans les meilleures techniques disponibles pourl’Union Européenne pour diverses raisons mention-nées dans le rapport. Il convient évidemment de suivreles progrès techniques et économiques qu’ils feront.

Les procédés fonctionnant à basse température nesont pas capables de traiter les MSW tels quels. Ils nepeuvent traiter que la partie biodégradable des MSW,et nécessitent soit une collecte séparée de cette par-tie, soit un tri adéquat à l’entrée de l’installation detraitement. Il existe deux possibilités: le traitementaérobie (compostage) et la digestion anaérobie(biométhanisation). Les deux procédés dépendent demicroorganismes pour leur fonctionnement. Seule unepartie du carbone est convertie en CO2.

Le compostage peut être réalisé depuis une échelletrès petite (en appartement ou au fond du jardin) jus-qu’à grande échelle (compostage centralisé). A petiteéchelle, il présente l’avantage de voir les déchetsbiodégradables retirés du circuit global des déchets. Agrande échelle, le procédé devient plus difficile àexploiter à cause de la nécessité d’alimenter correc-tement la charge en air et en eau, et aussi parce qu’ilfaut récupérer et traiter les effluents liquides. Aucuneénergie n’est récupérée. Un compost de bonne qualitépeut être considéré comme un engrais et comme unagent d’amélioration des sols. Cependant, il ne sem-ble pas qu’un réel marché existe pour ce compost.

La biométhanisation a fait l’objet d’une attention parti-culière au cours des décennies écoulées parce qu’el-le génère du méthane qui peut être utilisé comme

source d’énergie. Il y a différentes manières d’exploi-ter le procédé, et des matières d’origines diversessont suceptibles d’être traitées (agricole, industrielle,MSW et boues d’épuration). On rencontre desdifficultés dans le contrôle du processus notammentlorsque les caractéristiques de l’alimentation chan-gent au cours du temps. La récupération d’énergie estnettement inférieure à celle obtenue par incinération.Un digestat de bonne qualité peut être considérécomme un engrais. A nouveau, il ne semble pas yavoir de réel marché pour ce digestat. Le procédén’est économiquement viable que lorsque des subsi-des sont disponibles.

Le rapport comprend une discussion détaillée. Desremarques préliminaires font observer que: toutélément chimique présent dans le flux entrant doit seretrouver dans le flux sortant dans les mêmes quan-tités (ceci vaut spécialement pour les métaux lourds);aucun des procédés ne présente un avantage marquéen ce qui concerne les gaz à effet de serre (aprèsdécomposition, les composts et digestats terminentleur vie avec production de CO2 et de H2O); en cequi concerne la récupération d’énergie, quand un pro-cessus global est scindé en deux parties dont lapremière est exothermique, le dégagement dechaleur de la seconde partie en est diminué d’autant( c’est le cas pour la combustion du méthane produitpar digestion anaérobie); l'oubli de la thermodynami-que explique la plupart des défauts de pas mal depolitiques des déchets; les réactions émotionnellesdu type “pas dans mon jardin” ne sont pas prises enconsidération si la technique ne les justifie pas. Ladiscussion est scindée en deux parties: la premièrene prend en compte que les considérations scienti -fiques, technologiques, économiques et environne-mentales; la seconde partie couvre les considérationslégales .

Finalement, les conclusions reprennent tous les élé-ments nécessaires aux autorités pour prendre desdécisions correctes. Après avoir réduit par tous lesmoyens disponibles la quantité de déchets biodégrad-ables contenus dans les MSW, la décision principaleporte sur la collecte séparée ou non de ces déchets àla source, prenant en compte tous les éléments à con-sidérer. Les conclusions comprennent aussi desrecommandations. Parmi elles figure la nécessitépour l’Union Européenne d’édicter de nouvelles direc-tives et des documents BREF pour le compostage etpour la digestion anaérobie: ceci pourrait aider à lacréation de marchés pour les composts et pour lesdigestats.

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In overeenstemming met de door BACAS geformu-leerde doelstellingen werd dit rapport opgemaakt omde bevoegde overheden te informeren en aanbeve -lingen te formuleren voor een gerichte aanpak van deerin beschreven problemen.

Deze tekst handelt over het biologisch afbreekbaredeel van het afval geproduceerd door de bevolking ineen stedelijke omgeving. Het gaat hierbij voornamelijkom huishoudelijk en tuinafval. Ook het afval van res-taurants, eetgelegenheden en voedingswinkels, voorzover de samenstelling ervan vergelijkbaar is metdie van huishoudelijk afval, wordt hiermee geassi -mileerd.

In de Europese Unie produceert de bevolking van-daag 523 kg vast huishoudelijk afval (MSW = munici-pal solid waste) per jaar en per inwoner. Risico’s enhinder als gevolg van het storten ervan worden alsonaanvaardbaar beschouwd. Zeer specifieke en bin-dende regelgeving maakt het storten erg moeilijkbeheerbaar. Als gevolg hiervan is er een tendens ont-staan om de residuele hoeveelheid te storten afvalmaximaal te beperken.

In de meeste ontwikkelde landen bestaan er nulocale programma’s om risicohoudend huishoudelijkafval (chemische schoonmaakmiddelen, pesticiden,verven, batterijen, oliën etc.) van de rest van het MSWte scheiden en bepaalde materialen (metalen, papier,karton, kunststoffen, glas, textiel etc.) aan de bron terecupereren. Toch blijft er vandaag nog 204 kg bio -logisch afbreekbaar materiaal per jaar en per inwonerin het MSW, en dat is meteen de hoofdoorzaak vanoverlast door afval in stedelijke milieus. Voor deEuropese Unie, met haar 500 miljoen inwoners,betekent dit 102 miljoen Mg (1 Mg = 1 metrieke ton)biologisch afbreekbaar MSW, d.i. nagenoeg 20% vanal het biologisch afbreekbaar afval gegenereerd dooreconomische activiteiten in de E.U. Deze vaststellingwas voldoende aanleiding voor het schrijven van ditrapport.

Wettelijk gesproken vaardigt de Europese Unie richt-lijnen uit die door de lidstaten binnen een bepaaldetijd moeten omgezet worden in nationale wetgeving. Indit rapport wordt een korte analyse gemaakt van debelangrijkste richtlijnen die betrekking hebben op deafvalproblematiek. De nieuwe richtlijn 2008/98/ECwordt in detail besproken; zij definieert, in volgordevan prioriteit, vijf stappen in de afvalbehandeling,namelijk voorkoming, voorbereiding voor hergebruik,recycling, andere vormen van recuperatie, bv. ener-gierecuperatie en tenslotte storten. De bestaandeBelgische wetgeving ter zake wordt ook kort geanaly-seerd.

De best beschikbare technieken voor de verwerkingvan biologisch afbreekbare materialen in MSWworden beschreven, zij het met een beperking tot dietechnieken die “met succes op industriële schaalontwikkeld en getest zijn en in relevante industriëlesectoren op economisch en technisch duurzame wijzeinzetbaar zijn”, zoals bepaald in richtlijn 2008/1/EC.Processen die nog in een onderzoeks- of pilootfaseverkeren werden daarom niet opgenomen. Welworden enkele processen, die nog steeds alleenmaar mits subsidiëring leefbaar zijn, in dit rapportbesproken.

Wat de hogetemperatuursprocessen betreft mag ver-branding als een van de best beschikbare techniekenvoor de behandeling van MSW beschouwd worden,omdat zij voldoet aan alle voorwaarden opgelegd inde relevante E.U.-richtlijnen, met inbegrip van deeisen m.b.t. milieuzorg. Biologisch afbreekbare stoffenin MSW kunnen makkelijk mee verwerkt worden inmodulaire of wervelbedovens voor massaverbran-ding. Het is niet nodig het biologisch afbreekbaarmateriaal apart in te zamelen of vooraf uit te sorteren.Het kan als dusdanig verbrand worden, zelfs in zijnkartonnen of plastic verpakking. Bij verwerking in eenwervelbedoven is echter wel een voorbereidende stapnodig. Indien de verbrandingsoven voldoende energieproduceert volgens de vereisten gedefinieerd inbijlage II/R1 van richtlijn 2008/98/EC, kan over een“energieterug-winningsinstallatie” worden gesproken.

Andere hogetemperatuursprocessen (pyrolyse, ver-gassing en plasmavorming) worden vandaag in deEuropese Unie om uiteenlopende redenen, die verderin dit rapport besproken worden, nog niet als bestbeschikbare technieken bestempeld. Hun technischeen economische vooruitgang moet wel opgevolgdworden.

Lagetemperatuursprocessen zijn niet geschikt vooreen ongeconditioneerde verwerking van MSW. Zij zijnenkel van toepassing voor de verwerking van de bio-logisch afbreekbare fracties in MSW en vergen ofweleen aparte inzameling aan de bron of een doelmatiguitsorteren van het MSW voor het in het proces inge-bracht wordt. Er zijn twee mogelijkheden: aerobebehandeling (compostering) of anaerobe gisting (bio-methanisering). Voor beide processen geldt dat hungoede werking afhangt van micro-organismen.Slechts een deel van de aanwezige koolstof wordtomgezet in kooldioxide.

Compostering kan zeer kleinschalig gebeuren (in deachtertuin) of in grote gecentraliseerde compost -fabrieken. Toepassing op kleine schaal biedt hetvoordeel dat biologisch afbreekbare materialen uit de

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afvalstroom verwijderd worden. Op grote schaal wordthet proces een stuk moeilijker omdat lucht en waterheel precies in de lading gedoseerd moeten wordenen ook omdat het afvalwater moet gerecupereerd engezuiverd worden. Er is geen energieterugwinning.Compost van goede kwaliteit kan als meststof ofgrondverbeteraar gebruikt worden. Toch lijkt er nietecht een markt voor compost te bestaan.

Biomethanisering kreeg de voorbije decennia heelwat aandacht omdat in dit proces methaan geprodu-ceerd wordt dat kan gebruikt worden om energie terecupereren. Er zijn verschillende mogelijkheden omdit proces uit te voeren en het kan met materiaal vandiverse oorsprong gevoed worden (afval van land-bouw of industrie, MSW en slib uit waterzuiveringsin-stallaties). Een adequate procesbeheersing is nieteenvoudig, in het bijzonder wanneer de samenstellingvan de lading varieert in de tijd. Energierecuperatie isveel geringer dan bij verbranding. Residu’s van hogekwaliteit kunnen als meststof ingezet worden. Ookhier lijkt er niet echt een markt voor dit soort reststof-fen te bestaan. Het proces is economisch alleen maarleefbaar als het gesubsidieerd wordt.

Het rapport omvat een gedetailleerde bespreking vande diverse processen. In de inleidende bemerkingenwordt onder meer het volgende gesteld: elk chemischelement aanwezig in de ingangsstroom zal hoe danook in gelijke hoeveelheid aanwezig zijn in de uit-gangsstroom (dit is in het bijzonder het geval voorzware metalen); geen enkel proces kan bogen op enig

“broeikasgasvoordeel” (na ontbinding worden ookcompost en residu’s van gistingsprocessen omgezetin CO2 en water); wanneer het globale proces opge-splitst is in twee deelprocessen, waarvan het eersteexotherm is, daalt het netto energetisch rendementvan het tweede deelproces (dit is het geval met deverbranding van methaan verkregen uit anaerobe gis-ting); wanneer afvalbeleid mislukt is het meestalwegens een miskenning van de thermodynamischewetten; emotionele “not in my backyard” reacties wor-den niet besproken tenzij ze werkelijk op technischeoverwegingen gestoeld zijn. De bespreking bestaat uittwee delen: het eerste beperkt zich tot wetenschap-pelijke, technologische, economische en milieutechni-sche aspecten van afvalverwerking terwijl het tweedegaat over wettelijke overwegingen.In de besluiten vindt men alle noodzakelijke elemen-ten terug die de overheden in staat moeten stellencorrecte beslissingen te nemen. Na in eerste instantiemet alle mogelijke middelen het restaandeel aanbiologisch afbreekbare stoffen in MSW verkleind tehebben, moet men beslissen of men al dan niet wilovergaan tot een aparte inzameling aan de bron vande biologisch afbreekbare stoffen die nog in het MSWaanwezig blijven, rekening houdend met alle aspectendie men in aanmerking moet nemen. De besluitenomvatten ook aanbevelingen, met name de nood aannieuwe Europese richtlijnen en BREF -documentenvoor compostering en anaerobe gisting: dit zoumoeten bijdragen tot de ontwikkeling van een marktvoor compost en restmateriaal uit vergistingspro -cessen.

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1 BREF = best available technique (“BAT”) reference.


This paper deals essentially with biodegradables inmunicipal solid waste (MSW) generated by citizens inurban environments. This is essentially householdwaste and gardening waste. Assimilated to this cate-gory of waste is refuse from restaurants, canteensand food shops as far its composition is similar to thatof household waste.

Not included are agricultural and forestry waste,sewage sludge, industrial waste of any kind, hazar-dous substances, waste electric and electronic equip-ment (WEEE), medical waste, waste oils, end-of-lifevehicles, batteries and accumulators, mining waste,nuclear waste, etc… Most of these wastes are subjectto distinct legislation and Codes of practice.

Modern ways of life in developed countries have ledpeople to generate more and more MSW: currently523 kg per inhabitant and per year in the EuropeanUnion. At the same time, landfilling has become moreand more difficult to manage for many reasons:hazards and nuisances associated with dumping aredeemed unacceptable and meet strong opposition,leading to very specific and compulsory regulationswith, as a result, a dearth of suitable sites.

This has created a very complex political, techno -logical and economical problem for decision makersinvolved in waste management, especially underurban conditions.

Sometimes, dramatic situations arise where waste issimply piling up in the streets with the associated dis-advantages and health hazards. A situation of thiskind occurred recently in Naples (Italy).

European Union Directives aim at reducing not onlythe amount of MSW but also the residues remainingafter treatment to be dumped in dedicated landfills . Aswill appear in Chapters 2 and 3, the management ofbiodegradables in MSW needs some explicit deci-sions from the authorities, which are not so easy tomake.

BACAS decided to set up a working group to gatheras much information as possible on proven technolo-gies that could be applied to these biodegradablesand to consider their position in MSW treatment. Theaim was to produce a document describing briefly theavailable technologies with their inherent advantagesand disadvantages and including some cost estimateswherever available. This last exercise is risky, sincecost factors are all but static, predictable and homo-geneous.

Finally, some guidelines helping in decision-makingshould be proposed.

It should be emphasized that this report is notintended to waive the need for consulting appropriatespecialists or to ask for competitive tenders for eachspecific situation.

At the outset, it immediately appeared that the amountof literature on the subject was far too large to beconsulted and analysed in a reasonable timeframe.Experts were accordingly associated to the group,either as members or as lecturers on specific topics.

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Chapter 1

INTRODUCTION


2.1. European Union

In order to address properly the problem of biode-gradables in MSW in the European Union at large andin Belgium in particular, it is necessary at first to enlar-ge the legislative scope to waste management ingeneral, because part of the broader legislation isconcerned with MSW. It must be emphasized that theEuropean Union adopts directives, which must betransposed in the legislations of the Members Stateswithin a given time period. Each Member State mustalso issue a report on how this task was implemented.This general procedure was decided because thecontext and starting point are too different betweenthe Members States. This is especially true for wastemanagement and in particular for the biodegradablesin the municipal waste.

Although the E.U. directives have played an essentialrole in shaping the administrative structures respon -sible for waste management and in issuing rules forspecific categories of waste, the biodegradables haveuntil recently received only limited E.U. attention.

The European Union is currently working on ThematicStrategies in the frame of the Sixth EnvironmentAction Programme of the European Community 2002-2012 (http://ec.europa.eu/environment/newprg/strate-gies_en.htm). The thematic strategies are key ele-ments for better regulation and are all accompaniedby economic, social and environmental impact asses-sments and extensive stakeholder consultations.

The fields covered are Air, Waste prevention and recy-cling, Maritime environment, Soil, Pesticides, Naturalresources and Urban environment. The waste preven-tion and recycling strategy is of special interest for thisreport.

The European Union’s approach to waste manage-ment was based until 12 December 2008 (see belowDirective 2008/98/EC) on three basic principles(http://ec.europa.eu/environment/waste/index.htm):

– waste prevention: reducing the amount of wasteas well as its hazardousness (improve manufactu-ring methods, stimulate consumers to demandgreener products and less packaging)

– recycling and reuse: if waste cannot be prevented,recover as much matter as possible, mainly by

recycling. Specific waste streams are defined (bat-teries, packaging waste, end-of-life vehicles, elec-trical and electronic waste, etc…)

– improving final disposal and monitoring: wherepossible, waste that cannot be recycled or reusedshould be safely incinerated, with landfill only usedas a last resort. Strict guidelines must be followedin any case for incineration and for landfill.

An interesting background document entitled “EUWaste Policy – The story behind the strategy” may bedownloaded for free on the site (http://ec.europa.eu/environment/waste).

As to legislation, most of it may be found on the site(http://europa.eu/legislation_summaries/environ-ment/waste_management/index_en.htm).

Concerning Waste Disposal, Directive 2006/12/EC,which entered into force on 17 May 2006 aims to limitthe generation of waste and to optimise the organiza-tion of waste treatment and disposal. Member Statesmust prohibit the abandonment, dumping or uncon-trolled disposal of waste, and must promote wasteprevention, recycling and processing for re-use.Annex I defines categories of waste (MSW is notspecified ). Annex IIA lists disposal operations (incine-ration is considered here as a disposal operation).Annex IIB lists recovery operations (R1: use principal-ly as a fuel or other means to generate energy).

Concerning a Strategy on the Prevention andRecycling of Waste, there is no specific directive.There is however a Commission Communication of21 December 2005 COM (2005) 666. Emphasis isgiven on biodegradable waste, “two-thirds of whichmust be redirected to be disposed of using methodsother than landfill as required under Directive1999/31/EC”(see below specific directives afterDirective 2008/98/EC).

Directive 2008/1/EC of the European Parliament andof the Council of 15 January 2008 deals withIntegrated Pollution Prevention and Control (theIPPC Directive). It defines the obligations with whichindustrial and agricultural activities with a high pollu-tion potential must comply. These activities requirehaving a permit that can only be issued if certain envi-ronmental conditions are met, so that the companiesthemselves bear responsibility for preventing and

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Chapter 2

LEGAL CONSTRAINTS


reducing any pollution they may cause. This Directivereplaces the earlier Directive 96/61/EC. Its Article 2(Definitions, point 12) is of special interest for thisreport: “best available techniques” means the mosteffective and advanced stage in the development ofactivities and their methods of operation which indica-te the practical suitability of particular techniques forproviding in principle the basis for emission limit valu-es designed to prevent and, where that is not practi-cable, generally to reduce emissions and the impacton the environment as a whole:

– “techniques” shall include both the technologyused and the way in which the installation isdesigned, built, maintained, operated and decom-missioned;

– ”available techniques” means those developed ona scale which allows implementation in the relevantindustrial sector, under economically and techni-cally viable conditions, taking into considerationthe costs and advantages, whether or not the tech-niques are used or produced inside the MemberState in question, as long as they are reasonablyaccessible to the operator;

– ”best” means most effective in achieving a highgeneral level of protection of the environment as awhole.

In determining the best available techniques, specialconsideration should be given to the items listed inAnnex IV (notably “comparable processes, facilitiesor methods of operation which have been triedwith success on an industrial scale”) “.

The IPPC Directive is completed with so-called“BREF” documents (Best Available TechniquesREFerence documents); among these various BREFDocuments, two are devoted to waste: the BREF onWaste Incineration and that on Waste Treatment faci-lities.

The new Directive 2008/98/EC of the EuropeanParliament and of the Council of 19 November2008 on waste and repealing certain Directiveswas published in the Official Journal of the EuropeanUnion of 22 November 2008 and entered into force on12 December 2008. The deadline for transposition inthe Member States is 12 December 2010. Its contentis of special interest for this report.

– In the introducing preamble (Whereas), it isstated that: “(6) The first objective of any waste poli-cy should be to minimise the negative effects of thegeneration and management of waste on humanhealth and the environment. Waste policy shouldalso aim at reducing the use of resources, andfavour the practical application of the waste hierar-chy”; “(8) It is (therefore) necessary to reviseDirective 2006/12/EC in order to clarify key con-cepts such as the definitions of waste, recovery and

disposal, to strengthen the measures that must betaken in regard to waste prevention, to introduce anapproach that takes into account the whole life-cycle of products and materials not only the wastephase, and to focus on reducing the environmentalimpacts of waste generation and waste manage-ment, thereby strengthening the economic value ofwaste. Furthermore, the recovery of waste and theuse of recovered materials should be encouragedin order to conserve natural resources...”;“(14)…encourage a harmonised classification ofwaste and ensure the harmonised determination ofhazardous waste within the Community”; “(15) It isnecessary to distinguish between the preliminarystorage of waste pending its collection, the collec-tion of waste and the storage of waste pending tre-atment…”; “(18) Definitions of prevention, re-use,treatment and recycling should be included in thisDirective, in order to clarify the scope of these con-cepts.”; “(20) This Directive should also clarify whenthe incineration of municipal solid waste is energy-efficient and may be considered a recovery opera-tion”; “(22) There should be no confusion betweenthe various aspects of waste definition, and approp-riate procedures should be applied, where neces-sary, to by-products that are not waste, on the onehand, or to waste that ceases to be waste, on theother hand…”; “(28)… waste should be separatelycollected if technically, environmentally and econo-mically practicable, before undergoing recoveryoperations that deliver the best overall environmen-tal outcome…”; “(31) The waste hierarchy gene-rally lays down a priority order of what consti-tutes the best overall environmental option inwaste legislation and policy, while departingfrom such hierarchy may be necessary for spe-cific waste streams when justified for reasonsof, inter alia, technical feasibility, economic via-bility and environmental protection.”; “ (35) It isimportant, in accordance with the waste hierarchy,and for the purpose of reduction of greenhouse gasemissions originating from waste disposal on land-fills, to facilitate the separate collection and propertreatment of bio-waste in order to produce environ-mentally safe compost and other bio-waste basedmaterials…”; “ (45) Member States should providefor effective, proportionate and dissuasive penaltiesto be imposed on natural and legal persons respon-sible for waste management, such as waste produ-cers, holders, brokers, dealers, transporters andcollectors, establishments or undertakings whichcarry out waste treatment operations and wastemanagement schemes, in cases where they infrin-ge the provisions of this Directive…” ; “ (47) In par-ticular, the Commission should be empowered toestablish criteria regarding a number of issues suchas the conditions under which an object is to beconsidered a byproduct, the end-of-waste statusand the determination of waste which is considered

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as hazardous, as well as to establish detailed ruleson the application and calculation methods for ver-ifying compliance with the recycling targets set outin this Directive. Furthermore, the Commissionshould be empowered to adapt the annexes totechnical and scientific progress and to specifythe application of the formula for incinerationfacilities referred to in Annex II, R1…”

– In Chapter I - Subject matter, scope and defini-tions, are worth to be mentioned: in Article 3Definitions 4. “Bio-waste” means biodegradablegarden and park waste, food and kitchen wastefrom households, restaurants, caterers andretail premises and comparable waste fromfood processing plants. 12. “Prevention” meansmeasures taken before a substance, material orproduct has become waste, that reduce: (a) thequantity of waste, including through the re-use ofproducts or the extension of the life-span of pro-ducts; (b) the adverse impacts of the generatedwaste on the environment and human health; or(c) the content of harmful substances in materialsand products. 13. “Re-use” means any operation bywhich products or components that are not wasteare used again for the same purpose for which theywere conceived. 14. “Treatment” means recoveryor disposal operations, including preparation priorto recovery or disposal. 15. “Recovery” means anyoperation the principal result of which is waste ser-ving a useful purpose by replacing other materials,which would otherwise have been used to fulfil aparticular function, or waste being prepared to fulfilthat function, in the plant or in the wider economy.Annex II sets out a nonexhaustive list of recoveryoperations. 16. “Preparing for re-use” meanschecking, cleaning or repairing recovery opera-tions, by which products or components of productsthat have become waste are prepared so that theycan be re-used without any other pre-processing.17. “ Recycling” means any recovery operation bywhich waste materials are reprocessed intoproducts , materials or substances whether for theoriginal or other purposes. It includes the reproces-sing of organic materials but does not includeenergy recovery and the reprocessing into mate-rials that are to be used as fuels or for backfillingoperations. 19 “Disposal” means any operation,which is not recovery even where the operation hasas a secondary consequence the reclamation ofsubstances or energy. Annex I sets out a non-exhaustive list of disposal operations. 20. “Bestavailable techniques” means best available tech-niques as defined in Article 2(11) of Directive96/61/EC.In Article 4. Waste hierarchy1. The following waste hierarchy shall apply asa priority order in waste prevention and manage-ment legislation and policy:

(a) prevention; (b) preparing for re-use; (c) recycling; (d) other recovery, e.g. energy recovery; and(e) disposal.

2. When applying the waste hierarchy referred to inparagraph 1, Member States shall take measuresto encourage the options that deliver the bestoverall environmental outcome. This may requi-re specific waste streams departing from thehierarchy… In Article 5 By-Products. 1.A substance or object,resulting from a production process, the primaryaim of which is not the production of that item, maybe regarded as not being waste… but as being aby-product only if the following conditions are met:(a) further use of the substance or object is certain;(b) the substance or object can be used directly wit-hout any further processing other than normalindustrial practice; (c) the substance or objectis produced as an integral part of a productionprocess ; and (d) further use is lawful, i.e. the sub-stance or object fulfils all relevant product, environ-mental and health protection requirements for thespecific use and will not lead to overall adverseenvironmental or human health impacts.In Article 6 End-of-waste status 1. Certain speci-fied waste shall cease to be waste within the mea-ning of point (1) of Article 3 when it has undergonea recovery, including recycling, operation andcomplies with specific criteria to be developed inaccordance with the following conditions: (a) thesubstance or object is commonly used for specificpurposes; (b) a market or demand exists for sucha substance or object; (c) the substance or objectfulfils the technical requirements for the specificpurposes and meets the existing legislation andstandards applicable to products; and (d) the use ofthe substance or object will not lead to overalladverse environmental or human health impacts…criteria shall include limit values for pollutants… In Article 7 List of waste 4. The reclassificationof hazardous waste as non-hazardous wastemay not be achieved by diluting or mixing thewaste with the aim of lowering the initial concentra-tions of hazardous substances to a level below thethresholds for defining waste as hazardous.

– In Chapter II General Requirements. Article 13.Protection of human health and the environ-ment. Member States shall take the necessarymeasures to ensure that waste management iscarried out without endangering human health,without harming the environment and in particular:(a) without risk for water, air, soil, plants or animals;(b) without causing a nuisance through noise andodours; and (c) without adversely affecting thecountryside or places of special interest.

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– In Chapter III Waste Management. Article 22.Bio-waste. Member States shall take measures,as appropriate… to encourage: (a) the separatecollection of bio-waste with a view to the com-posting and digestion of bio-waste; (b) the treat-ment of bio-waste in a way that fulfils a high level ofenvironmental protection; (c) the use of environ-mentally safe materials produced from bio-waste.The Commission shall carry out an assessmenton the management of bio-waste with a view tosubmitting a proposal if appropriate. The asses-sment shall examine the opportunity of settingminimum requirements for bio-waste manage-ment and quality criteria for compost anddigestate from bio-waste, in order to guarantee ahigh level of protection for human health and theenvironment.

– In Chapter VII Final Provisions. Article 41 Repealand transitional provisions: Directives 75/439/EEC,91/689/EEC and 2006/12/EC are hereby repealedwith effect from 12 December 2010.

– In Annex II Recovery operations. R1 Use princi-pally as a fuel or other means to generate ener-gy: this includes incineration facilities dedicated tothe processing of municipal solid waste only wheretheir energy efficiency is equal to or above… (acertain value obtained by applying a formula givenin the Directive).

Concerning Waste Management Statistics, there isonly an EC Regulation n° 2150/2002 of the EuropeanParliament and of the Council of 25 November 2002.Statistics are to be produced using a nomenclatureset out in Annex III.

There is also a Communication from the Commissionof July 1998 (COM(98)463) dealing with theCompetitiveness of the recycling industries. Thiscommunication lists the major difficulties encounteredby recycling businesses in achieving or maintainingviability, and proposes a package of measures capa-ble of solving these problems.

Landfill of waste is the object of Council Directive1999/31/EC of 26 April 1999, intended to prevent orreduce the adverse effects of the landfill of waste onthe environment, in particular on surface water,groundwater, soil, air and human health.

It defines the different categories of waste (Article 2(b)“municipal waste” means waste from house-holds, as well as other waste which, because of itsnature or composition, is similar to waste fromhousehold”). Landfills are divided into three catego-ries: landfills for hazardous waste; landfills for non-hazardous waste; landfills for inert waste. Waste mustbe treated before being dumped. The following wastesmay not be accepted in a landfill: liquid waste; flam-mable waste; explosive or oxidizing waste; used tyres,

with certain exceptions; any other type of waste whichdoes not meet the acceptance criteria defined inAnnex II. Control and monitoring procedures in opera-tion and after-care phases are described in Annex III.Article 5 states that “Member States shall set up anational strategy for the implementation of the reduc-tion of biodegradable waste going to landfills, not laterthan two years after its entry into force”. This strategyshall ensure that, not later than 5 years thereafter, bio-degradable municipal waste going to landfill mustbe reduced to 75% of the total amount (by weight) ofbiodegradable municipal waste produced in 1995.This proportion must be reduced to 50% not later than8 years (2009) and to 35% not later than 15 years(2016) after the entry into force. The Directive alsostates (article 6) that only waste that has been subjectto treatment can be dumped. Treatment means thephysical, thermal, chemical or biological processes,including sorting, which change the characteristics ofthe waste in order to reduce its volume or hazardousnature, to facilitate its handling or to enhance reco-very.

Directive 2000/76/EC of the European Parliament andof the Council of 4 December 2000 deals with WasteIncineration. This Directive introduces measures toprevent or reduce air, water and soil pollution causedby the incineration or co-incineration of waste, aswell as the resulting risk to human health. There is aprior authorization requirement for incineration andco-incineration plants, and emission limits are givenfor certain pollutants released to air or to water. TheDirective does not cover experimental plants forimproving the incineration process and which treatless than 50 Mg of waste per year. Nor does it coverplants treating only: vegetable waste from agricultureand forestry, the food industry or the production ofpaper; wood waste; cork waste; radioactive waste;animal carcases; waste resulting from the exploitationof oil and gas and incinerated on board offshoreinstallations. To guarantee complete waste combus-tion, all plants are required to keep the gases at atemperature of at least 850°C for at least 2 seconds.If hazardous waste with a content of more than 1% ofhalogenated organic substances, expressed as chlo-rine, is incinerated, the temperature has to be raisedto 1100°C for at least 2 seconds. The heat generatedhas to be put to good use as far as possible: this is thereason why incineration is nowadays mostly called“Energy recovery” technology. Annex V gives limitvalues for incineration plant emissions to air.Corresponding limit values for co- incineration(notably cement kilns) are given in Annex II.Monitoring systems must be installed.The Directive deals also with wastewater from thecleaning of exhaust gases as well as with residuesresulting from the operation. Member States had toadopt appropriate measures before 28 February2002. Since the end of 2005 all existing plants in

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the European Union must satisfy to this Directive.The BREF Document on Waste Incineration introdu-ces complementary recommendations, e.g. achieva-ble emission values, sometimes more severe than thelimit values of the Incineration Directive. In addition tothis, the new Directive 2008/98/EC imposes evenmore severe constraints.

Although this falls out of the scope of this report, it isalso interesting to mention Council Directive91/689/EEC of 12 December 1991 on the ControlledManagement of Hazardous Waste. The Directivedoes not cover domestic waste.

The Member States are to ensure that hazardouswaste is recorded and identified.

They must also ensure that different categories ofhazardous waste are not mixed and that hazardouswaste is not mixed with non-hazardous waste, savewhere the necessary measures have been taken tosafeguard human health and the environment.

2.2. Belgium

With a few exceptions, environmental policy is regio-nalized in Belgium. Thus EU directives are transposedin each Region separately, with the consequence thatdifferences may appear between the actual regula-tions applying in each of the three Regions as well astheir practical implementation. The 2008 wasteframework directive (Directive 2008/98/EC) has to betransposed no later than 12 December 2010.

Flanders

The Vlaams Reglement inzake Afvalvoorkomingen Afvalbeheer (VLAREA) was promulgated on17 December 1997. Implementation plans for house-hold waste management were established successi-vely, the last one dating from 14 December 2007 andcovering the 2008-2015 period. Among its objectivesare a limitation of waste production to 560 kg/inhabi-tant. year, with a maximum of residual waste of180 kg/inhabitant.year by 2010 and 150 kg/inhabi-tant.year by 2015. This will require expanding the rateof selective collection to 75%.

According to the Order of the Flemish government of5 December 2003, the following waste categoriesmust be separated from household waste: hazardouswaste, glass, paper and cardboard, metals, vegeta-bles, textiles, electrical and electronic waste, tyres anddemolition waste. The Order also stipulates that it isforbidden to dump unsorted household waste.Derogations are allowed under special circumstances.

Wallonia

The first waste decree dates from 27 June 1996. Itwas modified on 22 March 2007.

It acknowledges the waste management hierarchy ofthe EC, and prescribes among others the establish-ment of implementation plans (“Plans wallons desdéchets”).

On 30 March 2006, the Walloon government definednew strategic orientations in order to reach the objec-tive of a maximum annual per capita waste generationof 471 kg/inhabitant.year.

In compliance with the EC landfill Directive, by1 January 2010, biodegradable waste will no longerbe allowed in controlled landfills (“Centre d’enfouisse-ment technique” or CET), except under exceptionalcircumstances such as the unavailability of treatmentfacilities.

A new regulation (“Arrêté”) on waste management isalmost ready, supposed to take effect on 1 January2010. It deals with the use on or in soils of compostscoming from aerobic and anaerobic decomposition,and fermentation residues coming from anaerobicdecomposition of waste.

Another “Arrêté” was issued on 18 June 2009 (publis-hed in the “Moniteur belge” on 11 September 2009). Itdeals with specific conditions to be applied to com-posting facilities having storage capacities equal orhigher to 500 m3.

Brussels–Capital Region

From the on-line Documentation Centre of theBrussels Capital Region (Droit bruxellois de l’envi -ronnement): http://www.bruxellesenvironnement.be/Templates/DroitContent.aspx?langtype=2060 it appe-ars that since 1991, 7 “Arrêtés” were taken concerningincineration. The last one was issued on23 November 2000 and published in the “Moniteurbelge” on 22 December 2000. It was dealing withmaximum concentrations of dioxins and furans in exitgases taking into account the toxicity equivalent factorfor each specific molecule. Concerning landfills,the “Arrêtés” of 18 April 2002 and of 13 November2003, respectively published in the “Moniteur belge” of17 May 2002 and of 18 December 2003 implementDirective 1999/31/EC and modify Annex II accordingto Council decision 2003/33/CE.

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3.1. Preliminary remarks

The waste statistics mentioned in this chapter origina-te from Eurostat and/or from OECD sources. Therespective National Statistical Institutes and Ministriescollect data on the environment each year. Theseinstitutions complete the section on waste in a jointEurostat/OECD questionnaire. This questionnairealso contains data collected in previous years.Member States send updated tables to Eurostat andthe OECD. Differences between statistics given byEurostat and OECD should accordingly occur onlywhen no data is available for a certain country andyear since an estimate must be established to fill thegap in order to calculate aggregates at EU level.

At the Eurostat website http://epp.eurostat.ec. europa.eu/portal/page/portal/waste/data/ data is alsoaccessible concerning the generation of waste, sortedout by economic activities according to the NACERev.1.1. Code. An entry Waste from households isavailable.Municipal waste consists of waste collected by or onbehalf of municipal authorities. For areas not coveredby a municipal waste collection scheme the amount ofwaste generated is estimated. The term ”Municipal” isused in different ways in the various countries due todifferent waste management practices. The bulk ofmunicipal waste is originating from households,though similar wastes from sources such as commer-ce, offices and public institutions are partly included,depending on local collection practices, which mayvary from place to place. Differences between coun-tries are mainly the result of differences in the covera-ge of these “assimilated wastes”.

According to the OECD/Eurostat Joint Questionnairemunicipal waste includes the following types of mate-rials: paper, paperboard and paper products, plastics,glass, metals, food and garden waste and textiles.There are also data concerning the treatment ofmunicipal waste: landfill is defined as deposit ofwaste into or onto land and thus covers both internalsites (where a waste generator is carrying out itsown waste disposal at the place of generation) andexternal sites. Incineration covers incineration plantsand co-incineration plants as defined in Directive2000/76/EC.

Data for the year 2006 are used because they areavailable for every item considered. Data for the

European Union come from Eurostat. For other coun-tries, they come from OECD.

As a final remark, it should be noted that in the statis-tical classification used in the OECD/Eurostat JoinedQuestionnaire, the Waste section is currently underrevision in the frame of the implementation of theWaste Statistics Regulation.

3.2. Data and discussion

Each year the 27 countries in the European Union dis-card approximately 3 billion Mg (1 Mg = 1 metrictonne) of waste generated by economic activity andconsumption.Among these some 60 million Mg are hazardouswaste. In total, we throw away almost 6 Mg/inhabi-tant.year of waste. Among these, 523 kg/inhabitant.year are MSW (representing more than 260 millionMg), of which 204 kg/inhabitant.year are biodegrada-ble waste: the fraction this report deals with essential-ly. Considering some 500 millions inhabitants, thatmakes 102 million Mg of biodegradable MSW in theEuropean Union. A rough estimate of the total amountof biodegradable waste included in the 3 billion Mg oftotal waste falls in the range between 500 and 700 mil-lion Mg a year. Accordingly, this paper deals withapproximately 20% (102 divided by 500) of the biode-gradable waste generated by economic activities eachyear in the European Union. It should also be remem-bered that biodegradable MSW are responsible formost of the waste’s related disturbances in urbanenvironments. This brief evaluation justifies fully thepresent paper.

As far as the composition of MSW is concerned, alt-hough there are differences between countries andcities, the following approximate percentages comeout from an analysis of the literature on the subject:

– food and garden 39% – paper and cardboard 25% – plastics 7% – glass 8%– metals 5% – textiles 1% – others 15%

The percentage of biodegradable waste in MSWvaries largely among the European Union MemberStates with a minimum in the UK (22%) and a maxi-

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Chapter 3

GENERAL CONSIDERATIONS ON MUNICIPAL SOLID WASTE


mum in Greece (49%). The percentage of gardenwaste (citizens and local authorities) also varieswidely from 20-30 kg/inhabitant.year in cities to 150 kg/inhabitant.year and more in rural environment. Theamount of kitchen waste (citizens, canteens, catering,retailers and supermarkets) is more constant andremains between 60 and 90 kg/inhabitant.year.

According to Eurostat, in 2006, 221 kg/inhabitant.yearof MSW were landfilled in the EU27, decreasingfrom 289 in 1996. The amounts landfilled were verydifferent from one country to another: between amaximum of 652 kg/inhabitant.year for Cyprus and aminimum of 4 kg/inhabitant.year in Germany (Belgiumis among the “good” countries with 24 kg/inhabitant.year coming from 169 kg/inhabitant.year in 1996).There is considerable pressure to decrease landfillingin the European Union.

Eurostat gives also statistics for MSW incineration. In2006, 100 kg/inhabitant.year were incinerated in theEU27, increasing from 66 kg/inhabitant.year in 1996.Again, large differences are observed between thecountries: between 0 kg/inhabitant.year for some ofthe new members to a maximum of 394 kg/inhabi-tant.year in Denmark.Belgium is at 162 kg/inhabitant.year to compare with182 kg/inhabitant.year for France, 184 kg/inhabitant.year for Germany and 233 kg/inhabitant.year forSweden.

Today, the general trend is to enforce the waste hier-archy established by the European Union (Directive2008/98/EC, Article 4):

– prevent waste in the first place – preparing the product for re-use – recycle – other recovery (e.g. energy recovery by incinera-tion)

– environmentally sound disposal of the waste (in alandfill)

Much is already done in this direction: paper and card-boards, plastics, glass, metals, textiles, batteries,electrical and electronic equipment, construction anddemolition debris, waste clothing, medication, fluores-cent tubes, paints, chemicals, spray cans, fertilizerand pesticide containers, shoe polish are sorted out atthe source and recovered separately either on a door-to-door basis or in special collection centres (civicamenities). A big part of it is recycled or re-used afterreconditioning. There is also an effort to decrease thepackaging (plastics and paper or cardboards), eventu-ally coming back to glass or metals. Of course,complete recycling is not possible: there is always aneconomic limit. For instance, in a document entitled“Support in the Drafting of an ExIA on the ThematicStrategy on the Prevention and Recycling of Waste

(TSPRW)”, Final Report submitted by EPEC (London)in preparation to the IPPC – Directive 2008/1/EC, onpage 51 is given a Table with the optimal recyclingrates for packaging materials:

– plastics: 28 to 38% – steel: 60 to 75% – aluminium: 25 to 31% – wood: 47 to 65% – paper and board: 60 to 74% – glass: 53 to 87% – composites: 0%

Taking this into consideration, what is left is tooptimize the treatment of the biodegradables in MSW.

This is not an easy task because its content is not welldefined:

– for food, there are fruits and vegetables residueseither free or packed, the same already cooked orprepared with some kind of sauce, fast food packedmeals eventually not even opened, various packedmilk products, eggs, meat, bread, cakes, chocola-tes, ice cream residues, and various packages notaccepted in the plastic recycling stream

– for garden waste, there are grass and bush or eventree residues (greens and browns) in variableamounts according to the season.

The question arises as to whether to impose a sourceseparation at home. This is already done partially forthe garden residues, and citizens can either deliverthemselves the residues in containers parks, or askto the municipal authorities to have these residuescollected at their door at their own expense.

It remains finally to sort out very carefully the foodresidues in order to prepare separate uncooked fruitsand vegetables residues that could be treated eitherby aerobic (compost) or anaerobic digestion (bio -methanisation). A rough estimate is that the maximumamount that could be recovered this way is around50 kg/inhabitant.year. The issues involved are to makesure that the citizens will agree to do so (some trainingwill be necessary because for instance fruits that aretreated chemically should not be introduced incomposting facilities), to transform all collecting trucksin order to manage a separate bin for this new kindof residue, and to take into account the expensesassociated with the new waste stream to treat andtreatment properly speaking.

In the following chapters, the main high temperature(incineration, gasification, pyrolysis, and certainplasma processes) and low temperature processes(composting and anaerobic digestion) will be con -sidered. After a short technical description, the maincharacteristic data will be given including economicswhere available, and their advantages and disadvan-

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tages will be discussed. Finally, after some generaldiscussion, a scheme will be presented, aimed athelping decision makers in their final choice.

3.3. Final remarks – Definitions

Difficulties were encountered while writing these firstthree chapters, due to imprecise definitions of waste.This can lead to wrong statistics data: some residuescould be counted twice (or more) or be deleted. It isevident that collecting data for residues or waste isvery difficult anyway. Although an effort has already bedone to better define “waste” in Directive 2006/12/ECand recently in Directive 2008/98/EC, a further effortcould be done to improve definitions in the field ofMSW. Here are some examples:

– In Directive 1999/31/EC, “municipal waste” isdefined as “waste from households, as well asother waste which, because of its nature or com -position, is similar to waste from households”. Asthere is no specification concerning neither thenature nor the composition of this kind of waste,and as Directive 2008/98/EC does not give anyfurther information with this respect, the content ofthis paper could remain very imprecise.

– In the Glossary of statistical terms – OECD(http://stats.oecd.org/glossary/download.asp.)«municipal wastes are wastes produced byresidential , commercial and public sectors that arecollected by local authorities for treatment and/ordisposal in a central location”. Another definitionconcerns “municipal waste (for energy) consists ofproducts that are combusted directly to produceheat and/or power and comprises wastes producedby the residential, commercial and public servicessectors that are collected by local authorities fordisposal in a central location. Hospital waste isincluded in this category”. Another definition isgiven for “household waste: refers to waste materialusually generated in the residential environment.Waste with similar characteristics may be genera-ted in other economic activities and can thus betreated and disposed together with householdwaste”.

– In the OECD/Eurostat Joint Questionnaire, “munici-pal waste” includes the following types of materials:paper, paperboard and paper products, plastics,glass, metals, food and garden waste and textiles.

In this paper, MSW are considered as produced byhouseholds, commercial and public services sectorsand collected by local authorities. They include paperand cardboard, plastics, glass, metals, textiles, foodand garden waste. Hospital waste is not includedexcept if especially mentioned.

Another quite imprecise definition concerns biomassand bio-waste:

– In the abovementioned Glossary, “biologicalwaste is waste containing mostly natural organicmaterials (remains of plants, animal excrements,biological sludge from waste-water treatment plantsand so forth)”; “biomass is the quantity of livingmaterial of plant or animal origin, present at a giventime within a given area”; “solid biomass is definedas any plant matter used directly as fuel or conver-ted into other forms before combustion. Includedare wood, vegetal waste (including wood waste andcrops used for energy production), animal mate-rials/wastes, sulphite lyes, also known as “blackliquor” (an alkaline spent liquor from the digesters inthe production of sulphate or soda pulp during themanufacture of paper where the energy contentderives from the lignin removed from the woodpulp) and other solid biomass. Charcoal is includ -ed.”

– Directive 2006/12/EC did not give any precision onthe subject.

– In Directive 2008/98/EC, Article 3, Definitions, 4.“bio-waste” means biodegradable garden and parkwaste, food and kitchen waste from households,restaurants, caterers and retail premises andcomparable waste from food processing plants.

There are large discrepancies between thevarious estimates of the amount of bio-waste thatcould be used to produce energy or fuel of anykind, by a factor of at least 5 or 10!

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4.1. General considerations

In this chapter, “best available techniques” are con-sidered as defined in Directive 2008/1/EC (the IPPCDirective). They must have been developed andtried with success on an industrial scale allowingimplementation in the relevant industrial sector,under economically and technically viable condi-tions.Accordingly, research and development as well aspilot plant processes will not be discussed. Further,will only be considered techniques that are of interestfor the treatment of biodegradables in MSW. Selectivecollection, recycling and manual or electromechanicalsorting will only be briefly discussed, where appro -priate.

A distinction can be made between processes operat-ing at high temperature and those where the boilingpoint of water at atmospheric pressure is not exceed-ed.

4.2. High temperature processes

They include incineration, gasification and pyrolysisdepending on the amount of air introduced in the heat-ing chamber. Incineration is a combustion processconducted with an adequate amount of excess air toensure that all carbonaceous materials introducedinto the system are completely burnt to CO2. In gasifi-cation, only part of these carbonaceous materialsintroduced in the system is burnt, delivering enoughheat to allow endothermic reactions between carbonand H2O, CO2 and eventually H2 to occur, and gener-ating low calorific power gases containing CO, H2,CO2, H2O and CH4. In pyrolysis, no oxygen at all isintroduced into the externally heated chamber con-taining the waste material, and carbonaceous materi-als undergo only thermal decomposition without anyoxidization or gasification reactions. As gasesproduced by these processes must still be purified,plasma generators were tested, introducing heat byelectricity instead of by combustion reactions in orderto decrease the volume of gases to be cleaned.

4.2.A. Incineration

Incineration has been practiced already for centuries:old timers knew how to “purify by fire” especially incase of an outbreak of contagious disease. By the endof the nineteenth century, incineration of urban waste

started to take place at a large scale in order toreplace exports of MSW to the countryside. The firstfurnaces were operated manually with heavy air pol-lution caused by grit and smoke, resulting in strongpublic opposition. This should no more be the case asall incineration plants must comply with the EuropeanDirectives on the subject, as mentioned in Chapter 2.

Incineration in the European Union may accord-ingly be considered as pertaining to the best avail-able techniques for the treatment of MSW. When theincineration plant generates energy and satisfies tothe formulae defined in Annex II/R1 of the Directive2008/98/EC, it can be called “Energy recoveryplant”.

Without going into the details, here are some techno-logical points of interest in the frame of this report.

According to the Solid Waste ManagementSourcebook edited by the United Nations in the frameof the United Nations Environment Programme (avail-able through the following website address:http://www.unep.or.jp/ietc/estdir/pub/msw/), there arefour environmentally sound technical options: mass-burn systems, modular systems, fluidized-bed sys-tems and refuse-derived fuel (RDF) firing.

Mass-burn systems

They are predominant in Europe and the USA forMSW incineration. They consist generally of two orthree incineration units ranging in capacity from 50 Mgto 1,000 Mg per day, so that the facility capacityranges from about 100 Mg to 3,000 Mg per day. Theycan accept waste almost without pre-processingexcept for removal of oversized items that could getstuck in the ash extractor. Anyway, in mostEuropean countries, local programs aim to sepa-rate household hazardous wastes (chemicalcleaners, pesticides, paints, batteries, oils, etc…)and to recover certain materials (metals, paper,cardboard, plastics, glass, textiles, etc…) at thesource.

Although the following description is general, specificsare inspired from the Bruxelles-Energie plant.

Trucks enter directly the intake area and tip theirwastes either on the tipping floor (USA) or, evenbetter , directly in the pit. From a feed chute, MSW is

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Chapter 4

BEST AVAILABLE TECHNIQUES


continuously fed to a grate system, which moves thewaste through a combustion chamber using a tum-bling motion. There are many grate systems: movinggrates, reciprocating grates, reverse reciprocatinggrates, roller grates, etc… As the lower heating valueof the waste has steadily increased, water-cooledgrates are becoming more common. The residencetime of the waste on the grate is large enough toensure full combustion: more or less 50 minutes. Mostsystems are presently waste-to-energy plants and thecombustion chamber walls are equipped with watertubes hidden from the flames by refractory silicon-car-bide tiles to protect the steel tubes from the aggres-sive combustion gases and wastes projections. Waterin the tubes is converted to steam that can either beused for heating (urban heating, swimming pools,industrial heating, etc…) or fed to turbines to generateelectricity. In the latter case, superheating is neces-sary, achieved by fuel or gas injection in a separateboiler.Waste combustion occurs with large excess air, inject-ed at high velocity to mix the combustion gases andavoid any production of CO. When introduced on thegrate, the waste is dried and dehydrated due to radi-ant heat coming from the walls of the combustionchamber: the surface of the refractory tiles is at leastat 850°C.Decomposition and combustion processes startaccordingly, generating the necessary heat to thewhole system. To avoid any gas leak to the ambientatmosphere, the whole system is under depression.Part of the combustion air is introduced underneaththe grate as primary air while secondary air is intro-duced above the grate, enhancing turbulence.Retention time of the gases in the turbulent high tem-perature zone is higher than 2 seconds at more than850°C to complete combustion and ensure decompo-sition of any dioxins. If the gases temperature doesnot exceed 950°C, NOx production remains verysmall. In any case, flue gases treatment is compulso-ry and constitutes somehow a plant in the plant allow-ing the separation of fly ash, the neutralization ofacids (HCl, SOx, HF), the abatement of heavy metals,the reduction of NOx, and the trapping of any remain-ing dioxins or furans by adsorption on activated car-bon or oxidation on a DeNOx-catalyst. Flue gas treat-ment sharply increases the investment costs. Also,although the thermal efficiency of the boiler is betterthan 80%, global thermal efficiency falls to 60% due toall annex treatments. In many plants, activated carboncontaminated by dioxins and furans is reintroduced inthe combustion chamber, and any water used for fluegas purification is recycled after treatment.

The main advantages of mass-burn MSW incinera-tion are as follows:

– volume reduction of at least 90% – weight reduction of 75% – residues are sterile

– heat of combustion is recovered either to generateelectricity or for heating

– there is a potential recovery of metals and con-struction materials

– the amount of final residue to dump in a hazardouswaste landfill is low

– the process is capable to destroy most undesirableorganic molecules

Disadvantages are as follows:

– capital intensive – technically complicated plant, mainly due to gaspurification

– special precautions must be taken during mainte-nance (dust, heavy metals, various hazardousorganic materials)

– down time for maintenance is from 8 to 15% on ayearly basis

– cans, tarry materials, bulky tree roots and largepieces of plastics should be avoided

Here are some quantitative data (partly from expertsand from CCE “Livre vert sur la gestion desbiodéchets dans l’UE“ COM(2008)811 final –SEC(2008)2936, paragraph 4.2:

– investment costs: 300 to 360 Euros per annual Mgof waste

– total treatment cost, including capital charges:100 Euros per Mg of waste

– power requirements: 40 to 120 kWh per Mg ofwaste (including some thermal energy)

– after deduction of the power requirements, thereremains between 450 kWh/Mg and 500 kWh/Mg ofwaste for the electricity network

– after treatment, about 20% of the initial waste arerecovered as bottom ashes partly sintered used forroad embankment after removing metal inclusions,1.5% are recycled as metals, 1.5% are recoveredas fly ash to incorporate in porous cement blocksor in cement insulating products, 0.2% to 3% arefinally dumped or retreated as hazardous sludge

– at the Brussels Region, the minimum acceptablelower heating value is 4 GJ/Mg of waste (i.e. onetenth of the lower heating value of fuel oil). In 1996,the actual value was 7.6 GJ/Mg. It increased steadi-ly to the current value of 9 GJ/Mg. In 1986, thedesign value of the plant was for a lower heatingvalue of 7.5 GJ/Mg of waste. The maximum accept-able lower heating value is 14.7 GJ/Mg of waste butit must be mixed with other waste to bring the aver-age value below 12.6 GJ/Mg, acceptable on thegrate of the furnace-boiler. In that case, the input ofwaste must be proportionally reduced.

Modular incinerators

Modular incinerators are usually prefabricated unitswith relatively small capacities, of between 5 to

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120 Mg of solid waste per day. With one to four units,a typical facility reaches a total capacity of about15 Mg to 400 Mg per day. Steam is usually the soleenergy output.

They are used for smaller communities or for com-mercial or industrial operations. On average, capitalcosts per Mg of capacity are lower than for other incin-eration options. Gases generated in the primary com-bustion chamber flow to an afterburner secondarycombustion chamber that ensures complete combus-tion and serves as the primary means of pollutioncontrol . Modular incinerators become less commonbecause of concerns over the consistency andadequacy of air pollution controls.

Fluidized-bed incinerators

Fluidized-bed incinerators are used most extensivelyin Japan (currently more than 167 facilities). Japaneseplants are typically of medium scale (50 Mg to 150 Mgof waste per day). The process is gaining interest onthe European MSW incineration market, because ofpotential co-firing of other waste, although mass-burnstill dominates.

In a fluidized-bed incinerator, a bed of limestone orsand, or other inert materials that can withstand hightemperatures, is fluidized by air. Fluidisation causesthe bed to expand and behave more or less like a boil-ing liquid. There are broadly two types of fluid-bedtechnologies: a bubbling bed and (for large plants) acirculating bed.

Fluidized-bed incinerators require front-end process-ing, also called fuel preparation.

They are also generally associated with sourceseparation because glass and metals are not reallycompatible with the system: their removal requirescirculating about ten times more bed material, in orderto subtract them from the bed. However, they can suc-cessfully burn wastes of widely varying moisture andheat content, making them more compatible with highrecovery-recycling practices. They can also be veryeffectively controlled to achieve higher energy con -version efficiency, less residual ash, and lower airemissions. Cost comparisons with mass-burn areinconclusive; despite their apparent simplicity fluidbed units tend to be more expensive. This couldbecome a fully commercially proven practice inEurope in the near future.

Refuse-derived fuel

Refuse-derived fuel (RDF, sometimes called waste-derived fuel WDF) refers to solid waste in any formthat is used as fuel. However, it refers primarily tosolid waste that has been mechanically processed to

produce storable, transportable, and more homoge-neous fuel for combustion. Its production needs anumber of processing stages: eventually a manualseparation line, screening using trommel or vibratingscreens, shredding or hammer milling with additionalscreening steps, magnetic separation, pelletizing orbaling of combustibles. The complexity of this pro-cessing results in high operating and maintenancecosts and in a reduced reliability of RDF productionfacilities. Also, capital costs are higher for RDF incin-eration units than for other incineration options.However, good quality RDF can be burnt in industrialrotating kilns like those used for cement production.Slightly apart from the RDF, EBS (in German,Ersatzbrenn stoff) is obtained by sorting MSW in orderto separate high calorific value parts. EBS is burnt inspecialized plants to produce energy, mainly inGermany. Are also worth mentioning the Mechanicaland Biological Treatment (MBT) processes mainlydeveloped in Germany and in Austria. MBT typicallyincludes a mechanical treatment stage to separate thebiodegradable and dry fractions of waste. The bio -degradable fraction undergoes biological treatmentcomposting or anaerobic digestion). The dry fraction iseither eliminated by incineration or discarded intolandfill, or undergoes an additional treatment aimed atincreasing its calorific value, reducing its particle size,and limiting its content in terms of undesirable sub-stances (mainly chlorine and ash), with a view torecovering RDF in the cement-making process. MBTshould be considered as a pre-treatment operationsince waste is sorted, rather than eliminated.

Environmental impact of MSW incinerationprocesses

This impact comes from potential emission of conta-minants into the air through exhaust stacks and intowater through ash leaching effluent.

Concerning air emissions, the main concerns aremetals, especially mercury, lead and cadmium; organ-ics such as dioxins and furans; acid gases such assulphur dioxide and hydrogen chloride; particulatematter such as dust and grit; nitrogen oxides (ozoneprecursors); and other substances such as carbonmonoxide.

Modern incinerators all include effective flue gascleaning equipment. CO is controlled by combustionconditions, not by end-of-pipe flue gas treatment. Dustis retained either by electrostatic precipitators or (mostfrequently) by bag filters. Acid gases are neutralizedby neutralizing agents, with various kinds of process-es classified in three categories:

– dry systems featuring a dry neutralizing agent (limeor sodium bicarbonate) injected in the flue gas, col-lecting the dry residue at a filter;

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– semi-dry systems using the injection (spray-drying)of a slurry containing the neutralizing agent (limemilk) and collecting the dry residue at a filter;

– wet systems using absorption columns wherewater and/or caustic soda solution neutralises theacids, the liquid effluent being thereafter processedand/or released in a receiving water.

Heavy metals and residual organic compounds areadsorbed onto an injected sorbent substance likeactivated carbon. NOx production is first reduced bycontrolling the temperature in the combustion cham-ber and finally neutralized either by urea injection orby catalytic reduction with ammonia injection com-bined with a catalyst bed. There are two options:Selective Non-Catalytic Reduction (SNCR) in a tem-perature window of 700°C to 1,000°C, and SelectiveCatalytic Reduction (SCR) in a temperature window of300°C to 400°C.

These various processes have to be combined toobtain a layout achieving complete flue gas cleaning.There are a large number of possible combinations,especially because of extensive historical retro fitting.The choice of a flue gas treatment layout has to bemade considering at least the following criteria:

– efficiency (ability and easiness to comply with emis-sion limits, even with very variable flue gas pollutantcontents – peak management);

– effect on energetic balance of plant (impact on theenergy recovery efficiency);

– type, quantity and management of flue gas clean-ing residues (e.g. recycling possibilities);

– ease of operation (reagent handling, equipmentcontrol complexity, etc…);

– economic balance (investment cost; operationcosts: utilities, reagents, final residues);

– environmental balance: beyond the compliancewith the requirements, includes elements like waterconsumption, assessment of type and quantity offinal waste, transfer of pollution, etc…).

Residual incinerator ash can contain concentrationsof heavy metals such as lead, cadmium, mercury,arsenic, copper, and zinc, which originate from plas-tics, coloured printing inks, batteries, certain rubberproducts, and hazardous waste from households andsmall industrial generators. Bottom ashes are gener-ally less contaminated than fly ash recovered frombag filters, electrostatic precipitators or scrubbers. Itis possible to reduce drastically the concentrationof contaminants by removing at source from themainstream household batteries, thermostats, fluo-rescent lamps, plastics and solder-bearing items (e.g.consumer electronics, light bulb sockets, and platedmetals). Some incinerators (mainly in Japan) areequipped with small additional furnaces to meltbottom ashes, recovering separately a molten slagand a metallic phase containing most of the contami-

nants. It is also possible to treat separately fly ash bysuch a process, after fixation by cement.

Biodegradables in MSW

Biodegradables in MSW are easily processed inmass-burn incinerators, modular incinerators andfluidized-bed incinerators without selective col-lection. Kitchen and garden waste like vegetables,potatoes, fruits, gardening residues except woodenparts too large in diameter are very quicklydried/dehydrated and undergo combustion within theresidence time. Meat, fish, bones, sausages, eggs,bread, any kind of cooked items can also be treatedeven if they are packed in plastics and/or cardboard.The only kind of organics that should be avoided isthat contained in aerosol cans that were not emptied:they could result in small explosions in the combustionchamber. The same holds for closed cans containingliquids like milk, beer, soda, etc… As alreadymentioned above, waste must anyhow undergo apreparation step before introduction in a fluidized-bedincinerator. RDF has one more advantage: it can beco-fired in existing cement, lime, or power plant.However, in some countries such as Italy, RDF ismade from mixed MSW, i.e. containing biodegradablewaste.

4.2.B. Gasification and pyrolysis

While incineration aims at burning any carbonaceousmaterial contained in the MSW completely to carbondioxide, gasification and pyrolysis tend instead to gen-erate combustible gases and/or solids to be convertedfurther into power in a separate plant.

Pyrolysis

Pyrolysis processes are based on thermochemicaldecomposition by endothermic reactions under reduc-ing, generally self-generated atmospheres. Heat isprovided either by indirect heating from an externalsource or internally by partial combustion of the load.External heating generally proceeds through a wall(temperatures < 500°C). At higher temperatures a cir-culating heat carrier is preferred, as in double fluidizedbed systems (cf. catalytic cracking in the oil industry).Pyrolysis products resulting at the same time from thisdecomposition are gases, condensable vapours, andsolids (pyrolysis coke), their relative proportionsdepending on feedstock, temperature and retentiontime: fast pyrolysis at medium temperature (a few sec-onds at 600°C-900°C for MSW) favours liquids, whileslow pyrolysis at low temperature (10 minutes at400°C-500°C for MSW) favours coke formation.Usually, part of the produced gases is burnt to providethe heat needed for the process, characterised mainlyby drying and endothermic reactions: the heat neededfor drying biodegradable waste is substantial.

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Pyrolysis is by no means a new process: charcoalfrom wood is still being produced worldwide alongprehistoric methods, and metallurgical coke is alsoproduced since more than a century by heating coalexternally in large furnaces. When MSW is the load,although coke produced by the process is consideredas a secondary solid fuel, it is of very poor quality: lowcarbon content and rich in ash, sulphur, chlorine,heavy metals, etc… initially present in the MSW.Extensive purification is necessary to allow its use assecondary fuel, but this is very difficult to achieve,which reduces the significance of the process. Also,besides light gases, heavy condensable gases areproduced (tar) and special care must be taken toavoid any clogging in the system.

For one Mg of dry carbonaceous material contained inMSW, slow pyrolysis generates typically 570 kg/Mgcombustible gases (a complex mixture of non con-densable gases like hydrogen, carbon monoxide,methane, etc… for about 380 kg/Mg; plus heavyhydrocarbons or tar for about 190 kg/Mg); and 430kg/Mg coke. In the case of fast pyrolysis, the gas frac-tion is increased to 840 kg/Mg (730 kg/Mg heavyhydrocarbons and 110 kg/Mg non condensablegases), while coke production decreases to 160kg/Mg.

Heating of the system can be provided – from an external source (indirect heating) by com-bustion of part of the gases or of the coke producedby the process; heating with a double envelopegives low heat transfer coefficients and is restrictedto slow pyrolysis

– by internal heating, burning part of the load even -tually in a counter-current system or by heatingreducing gases by a plasma (see a separate para-graph on the subject of plasma)

– by means of an intermediate heat transfer medium(sand, ceramic balls, molten salts, etc…) heated inan external loop.

Special pyrolysis operations are sometimes operatedunder vacuum (rubber tyres), or under hydrogen pres-sure (plastics, not for MSW).

A number of pyrolysis processes were tried, mainly forsolid industrial residues, sludge from water purifica-tion, paper and cardboard residues, plastics, biomassand wood residue, tyres, etc… Only a few were devot-ed to MSW: as an example, a rotary kiln processbased on French technology was implemented inJapan with a capacity of up to 70,000 Mg MSW peryear. It is still in operation, but no more offeredbecause of a lot of problems linked to kiln sealing, tarcondensation, duct clogging, heat transfer and corro-sion. Also, the heavy hydrocarbons must be crackedto lighter fractions: distillation only gives a lot of sepa-rate products, many of them without possible market.

Gasification

Gasification processes essentially proceed in twosteps: pyrolysis followed by gasification of the solidresidue (coke) and of the light and heavy hydrocar-bons, resulting in so called synthesis gas by means ofa gasification agent: either air/water vapour or oxy-gen/water vapour. Heating in the second step the car-bonaceous solid residue produced by the first step,under an atmosphere containing water vapour andcarbon dioxide instead of oxygen will avoid combus-tion of the gases evolved from the residue. Combinedcarbon in the residue will react with water vapour andcarbon dioxide at 850°C-900°C in endothermic reac-tions generating carbon monoxide, hydrogen andmethane. Working at low pressure (usually near 1 bar)and high temperature (850°C-900°C) generates agas rich in carbon monoxide and hydrogen calledsynthesis gas. On the contrary, working at high pres-sure (10 bars -20 bars) with hydrogen injection, and atlower temperature (700°C) generates a gas rich inhydrocarbons, mainly methane (hydro-gasification).The heat necessary for the process is generated byburning a small part of the load either with air or withoxygen: the process is thermally self-sustaining.Depending on the amount of nitrogen introducedin the system, the final gas produced will be lean(< 8 MJ/Nm3) or semi-rich (8-18 MJ/Nm3) comparedto natural gas (35 MJ/Nm3). The final gas can berecovered, eventually thermally cracked to suppressany heavy hydrocarbons (tar), purified and cooled (ina boiler) to feed a piston or a turbine engine, providedthat the characteristics of the purified gas are satis-factory. By this process, any combined carbon in thecoke disappears entirely and the final solid residue isinert.

The process is not new: gas generators were used ata large scale for more than a century to convert coalto carbon monoxide and hydrogen for public lightingand home heating. During World War II, the processwas also used for cars, trucks, and even (German)tanks.

Today, various reactors were developed: rotating kiln,fixed bed and fluidized bed.Differences arise from means used to support thesolids in the reactor (grates), the respective flow direc-tions of load and oxidant (counter-, co-, and cross-current ), and the heat source to the reactor.

Commercially developed processes pertain to fourclasses:

– fixed bed gasification with extraction of final ashes,either “dry” or molten

– fluidized bed gasification (dense, atmosphericcirculating , pressurized, rotating)

– entrained flow gasification – two steps pyro-gasification.

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Plasma processes will be dealt with in a separateparagraph.

Most processes and reactors were developed to treatcoal eventually bituminous, brown coal, petroleumcoke, solid or liquid industrial residues, wood, woodresidues, RDF, incineration residues, hospital resi dues,sludge, plastics residues, biomass, agriculture resi -dues, crushed car residues, tyres, etc… For MSWtreatment there remain a few commercially oper-ated processes:

– a vertical shaft atmospheric pressure gasifier withco-current fixed bed developed by a big Japanesesteel company, derived from blast furnace technol-ogy. After pre-processing, MSW is introduced withsome lime (50 kg/Mg MSW) at the middle of thereactor acting as the pyrolyser, while coke (also50 kg/Mg MSW) is introduced with combustion airat the lower part of the shaft, where temperature ishigh enough to obtain a molten slag and a moltenmetallic phase (bad iron). The upper part of theshaft works as a cocurrent gasifier. Besides gases,the outputs are granulated slag (90 kg/Mg MSW),iron (10 kg/Mg MSW) and fly ash (30 kg/Mg MSW),the latter being sent to landfill. This process ismarket leader for large scale MSW conversion inJapan. Plants capable of 110,000 tons MSW peryear are in operation.

– another vertical shaft gasifier with fixed counter cur-rent bed working under pressure (24 bars) devel-oped by a German engineering company, initiallydevoted to the treatment of coal or coke in connec-tion with a British gas company, was further devel-oped to treat various solid waste, including MSW atthe scale of 250,000 Mg per year to produce syn-thesis gas (for the production of methanol). Bottomashes are molten and recovered in water

– a pressurized (5 bar to 16 bar) rotating fluidized bedgasifier developed by a Japanese engineering com-pany. The rotating bed allows separation of notcombustible solids and of metal fractions to recycle.Temperature in the fluidized bed is between 600°Cand 800°C. At the bottom of the shaft furnace, acyclonic combustion chamber operated at hightemperature (1,300°C to 1,500°C) allows sinteringof fine ashes as granules. Furnaces capable of50,000 Mg MSW per year are in operation in Japan.The process is offered for commercialisation inEurope.

– a two stages pyro-gasifier developed by a Britishengineering company. Two horizontal tubularpyrolysers making use of endless screws feed theintermediate coke to a vertical shaft gasifier. Onefurnace was installed in the United Kingdom in2003 at the scale of 60,000 Mg MSW per year.

Of course, if the scope encompasses biomassand wood, the number of commercially operatedprocesses is considerably enlarged.

As already mentioned, pyrolysis of MSW is notsuccessful so far.

Considering only gasification (including pyro-gasification), the expected advantages comparedto mass-burn incineration are

– a reduced volumetric flow (4,000 Nm3/Mg MSW forgasification instead of 6,000 Nm3/Mg MSW forincineration). This reduces the draft fan powerrequirements and, if a boiler is used to generateelectricity, enhances its efficiency

– a wide flexibility with respect to their input (highervalues of the lower heating value are acceptable)

– cleanliness of the residues, especially if they aremolten

– excellent recovery of the metal values – maximum diversion from landfill

Disadvantages are as follows

– MSW must be pre-processed, adding to the plantcomplexity and to power expenditure. Rotatingfluidized bed are advantageous with this respect:they need only a rough preparation of the load

– the heat content of the molten slag is normally lost – fouling and corrosion problems are numerous – investment and operating costs are higher – counter current vertical shaft reactors show someinternal sintering /melting of the load, leading topreferential gas paths and eventually blockage ofthe downward move of the load. Accordingly, theload must be carefully prepared

– humid waste is not adequate as feedstock in termsof energy production

– although some processes aim at feeding gas tur-bines, the required purification level of the gases isin that case very difficult to achieve. A better choicewould be burning the gases in a boiler to generateelectricity via a steam turbine, gas purificationoccurring via a flue gas cleaning system similar tothat in use for mass-burn incineration.

There is a lack of quantitative data for MSWtreatment . Total costs, including investment areoften cited between 100 Euros/Mg and 150 Euros/MgMSW.

Environmental impact of MSW gasificationprocesses

It is similar to that already mentioned for incineration.Air emissions could be worse than in incinerationbecause of eventual leakage between the pyrolysisand the gasification steps of the process and alsobecause a larger amount of metals is evaporated.Residual solids as bottom ashes are usually meltedand granulated in water. In that case, they are inert.Fly ash must be further treated because it containsmost of the hazardous materials.

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Biodegradables in MSW could be treated in someof the above-mentioned gasifiers without selec-tive collection. However, they must be compatiblewith the load preparation system and with the condi-tions prevailing in the pyrolysis part of the equipment.Other remarks are very similar to those presentedabove for incinerators.

4.2.C. Plasma processes

At first sight, the idea to make use of an expensiveenergy like electricity to provide heat for the treatmentof MSW seems surprising. However, plasma process-ing shows advantages like reducing gas flow andseparating heating requirements from synthesis gasproduction that give sense to examining its capabili-ties further.

Plasma is considered as the fourth state of matterbesides solid, liquid and gas. In the universe, almost99% of matter is in that state. In a laboratory, it is gen-erally produced by electric discharge. Plasmas aregases containing atoms, molecules and ions in a fun-damental or in an excited state, electrons and pho-tons. Electrons are very light particles against ionsand neutral atoms and molecules. They are stronglyaccelerated by electromagnetic fields and play a spe-cial role. There are various criteria allowing distinctionbetween different types of plasma, but this is beyondthe scope of this report. All processes described inthis section make use of thermal plasmas. Theyare generated at pressures near atmospheric pres-sure (0.1 to 20 bars) mainly by electric arcs or by radiofrequency discharges. At that pressure, particlescollide very frequently and ionisation results mainlyfrom a thermal effect, with elastic type collisions. Attemperatures between 6,000 and 25,000 K, electrondensities are between 1020 and 1024 m-3. In the bulk,plasmas are electrically neutral. Thermal plasmas areat thermodynamic equilibrium provided they includeno reactant other than the plasma generating gases:electrons, ions and atoms are at the same tempera-ture. Plasmas are electrical conductors due to thepresence of electrically charged free particles.Electrical conductivity is in the range of one thou-sandth that of metallic copper.

There are many industrial applications of thermalplasmas at moderate power (below 200-300 kW): cut-ting and welding metallic pieces, surface treatmentand deposition, spheroidizing and purification of parti-cles, chemical analysis, synthesis of nanopowders,etc… At higher power (between 0.5 and 100 MW)there are some applications in chemical synthesis andin extractive metallurgy. During the fourth part of thetwentieth century, interest for their use in waste treat-ment started to grow.

Recovery and purification of machining residues ofhigh value added metals (titanium, zirconium and

super-alloys) as well as the treatment of steelmakingdust were first considered. Later, processes weredeveloped for the treatment of toxic liquids like PCB,CFC and HCFC; of solid waste like contaminatedsoils, weakly radioactive nuclear residues, variousarmy residues including destruction of conventionalammunition, asbestos fibres, industrial chlorinatedresidues, metallurgical slags, medical residues, allkind of waste on board of ships, etc…

Many improvements were introduced to plasma gen-erators. However, there are power limits for each type.Radio frequency plasma torches remain under 400kW.

Metallic direct current plasmas torches are built likewater-cooled hollow cylinders and the electric dis-charge occurs either between two inside successiveelectrodes (cathode and anode), the discharge beingblown out as a plasma “flame” protruding from thetorch (blown arc), or between only a cathode built inthe torch and the load of the furnace (transferred arc).Cathodes may be made of either thoriated tungsten(hot cathode) or of copper (water cooled cold cath-ode). In the latter case, a rotating magnetic fieldensures that the hot spot on the cathode is alwaysmoving.

Superimposing a high frequency component on thedirect current for a short while helps to shorten thetime necessary to initiate the arcing (a few seconds).A maximum power of 3 MW per plasma torch can beachieved in both cases. Both radio frequency andmetallic direct current torches can be operated withalmost any kind of gas: argon, helium, hydrogen, nitro-gen, methane, carbon monoxide, carbon dioxide, oxy-gen, air and mixtures. Much higher power values (100MW) can be obtained with direct current graphite hol-low electrodes working with axial injection of a non-oxidant gas like nitrogen. It should also be mentionedthat some cold cathode metallic plasma torches work-ing with alternating current are also available, up to 3MW.

Concerning MSW, a few processes were devel-oped at commercial scale:

– in Japan, an American electric engineering compa-ny installed an 8 MW MSW treatment plant capableof 300 Mg per day, working as a counter currentvertical shaft gasifier, fed from the top. Coke is nec-essary to obtain a stable bed above the plasmatorches, ensuring a relatively constant temperature.Below the torches, temperature reaches 1500-1700°C to obtain a metallic phase and a slag phasein the molten state. Above the coke bed, air isinjected for a secondary combustion to generatehydrogen and carbon monoxide. After dust separa-tion by a cyclone, the flue gases are sent to a boil-er to generate steam to produce electricity

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– another process makes use of a furnace inspiredfrom an arc steelmaking furnace where the graphiteelectrodes are replaced by arc blown plasma torch-es. The load is introduced via an endless screwfeeder. Water vapour is injected in the reactor thatworks like a gasifier. Secondary water vapour isinjected to cool down the gases at their exit fromthe furnace

– another process makes use of a cylindrical arc fur-nace with two graphite electrodes fed with directcurrent and generating the electric arc betweenthem above a molten slag. Three other graphiteelectrodes dip in the molten slag for further heatingwith alternating current by Joule effect in the slag

– in France, a plasma torch gasifier aiming to treat50,000 Mg MSW per year is under construction inMorcenx. It should produce enough synthesis gasto generate 12 MW electrical power.

Vitrification of fly ash coming from MSW incinera-tors in plasma torch furnaces is more common.They contain alkaline salts, heavy metals and chlori-nated or fluorinated organic compounds. Accordingly,they must be dumped in hazardous waste landfillsafter stabilization by means of hydraulic binders. A firstplasma plant capable of 2,500 Mg per year was builtat Cenon (near Bordeaux - France) in 1995 and wassuccessful. At least ten plants are presently in use inJapan, based either on the French technology or onan American one.

Advantages claimed by plasma torch processespromoters are

– the heating energy delivered by the torch is inde-pendent of the nature of the wastes: any change inMSW composition can be faced easily

– the heating energy available at temperatures aboveorganic compounds dissociation is much largerthan with combustion processes

– there is no need for an excess of air like in normalcombustion

– the total flow of gases is minimum. However, forcopper cathodes, the minimum air flow is in therange of 0.1 kg/MW, and plasma temperatures arebetween 6,000 and 10,000 °K

– decomposition of organic materials occurs at highspeed and it is easy to generate synthesis gas bycontrolled injection of air or of oxygen in the reactor

– the bottom glass obtained from the mineral part ofthe waste could be recycled

– reactors are relatively small, allowing compact plantdesign, eventually mobile

– plasma torches can be switched on or off in a fewseconds only

Disadvantages are

– short life time of copper electrodes (500 to3,000 hours)

– high power density results in high load evaporationlosses

– large amount of dust can be carried along, due tostrong turbulence of the gas flow

– possible generation of nano-particles if the veryhigh temperature zone of the plasma flame hits theload

– flue gas cleaning necessitates special considera-tion: very quick cooling at the exit of the furnace inorder to avoid generating new complex hazardousorganic molecules; special care for NOx and forheavy metals

– high investment and operating costs – in the case of MSW containing 30% humidity, theplasma reactor working as a gasifier and deliveringthe synthesis gas to a conventional coal-firedpower plant, taking into account the global efficien-cy for converting heat of combustion to electricity,the energetic ratio between electric power deliveredby the power plant and electric power consumed bythe plasma furnace is only around two.

There is a lack of reliable quantitative data for MSWtreatment. However, here are some estimated dataappearing in literature:

– investment between 120 Euros and 280 Euros perannual Mg MSW at the scale of 1,000 Mg MSW perday; the most favourable case is when the synthe-sis gas is fed in a coal-fired power plant to replacepart of the coal, assuming that the flue gas clean-ing of the power plant does not need any change toaccommodate the new feed

– electric power needed to generate synthesis gasfrom MSW at 30% humidity: 600 kWh/Mg

– operating costs, besides investment, are citedbetween 25 Euros and 80 Euros per Mg MSW,depending largely on the cost of electricity.

The environmental impact of plasma treatment ofMSW should be similar to that mentioned for incinera-tion. Air emissions could be worst because of thevery high temperature generated by the plasma torch-es, resulting in heavy metals evaporation and possiblyin nano-particles generation, very difficult to recoverby the flue gas cleaning system. Residual solidsrecovered at the bottom of the furnace as vitrifiedglass could be recycled, as well as the eventual metal-lic phase, reducing landfill concern. However, flyingashes remain a problem as hazardous materials.

Biodegradables in MSW could be considered forplasma treatment, but this does not look very attrac-tive due to high investment and operating costs.

4.3. Low temperature processes.

These processes are not able to treat MSW assuch. They can only cope with biodegradablescontained in MSW, and necessitate either separate

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collection of these biodegradables at the sourceor adequate selective sorting of MSW before load-ing in the process.

There are two possibilities: aerobic treatment (com-posting) or anaerobic digestion (biomethanisation).The two processes depend on microorganisms fortheir correct functioning.

4.3.A. Composting

According to the above-mentioned Solid WasteManagement Sourcebook issued by the UnitedNations Environment Programme (see: incineration),composting solid waste for use as a soil amendment,fertilizer or growth medium is important in many coun-tries. Some Asian countries have a long tradition,while Western Europe avail of a range of moderntechnologies. However, composting is the waste man-agement option with the highest rate of failed facilitiesworldwide, due to high operation and managementcosts, high transportation costs, lack of marketsand/or poor quality product as a result of poor pre-sorting (especially plastic and glass fragments),poor understanding of the composting process andcompetition from chemical fertilizers. It is accordinglynecessary to build enough knowledge to avoid mis-takes.

To start with, it must be clearly stated that compost-ing does not allow the recovery of a substantialamount of energy from the partial oxidation of thebiodegradables in MSW.

Scientific considerations

From a scientific standpoint, composting is part of theglobal biogeochemical cycles of our planet. Duringeach cycle, mineral elements from soil and air arecaptured by plants and later by animals during theirgrowth to constitute part of their organic materials.After their death, they go back to the environment tobe used again by other plants and animals after goingthrough complex processes. The main biogeochemi-cal cycles in the biosphere deal with carbon, water,nitrogen, phosphorus and sulphur. The two maincycles for composting are carbon and nitrogen.

The carbon dioxide contained in the atmosphere ordissolved in water delivers the carbon necessary toconstitute organic materials contained in plant mater-ial and other living bodies. It is first captured by chloro-phyll containing plants to be transformed by photo-synthesis into carbohydrates (sugars) and further inprotides (proteins) and lipids (fats). These substancesfeed animals and other organisms containing nochlorophyll, such as mushrooms. All living organismsrespire and reject carbon dioxide in the atmosphere.After their death, the remains undergo decomposition

and are mineralized by other living organisms. Finally,carbon is recycled as carbon dioxide. This processoccurs also for all residues coming from living organ-isms like dejections and secretions.

For nitrogen, the major reservoir is the atmosphere,almost 80% of which is molecular nitrogen. To be usedby living organisms, it must first be mineralized.Microorganisms, living freely in the soil or in symbio-sis with legumes are able to break the molecularbond, fix nitrogen in an organic form and provide it totheir host plants or release it in the soil. Other microor-ganisms transform it, successively, into solubleammonium-, nitrite- and nitrate- nitrogen. Plants gen-erally absorb it in the latter form, even if the nitrogencomes from decaying organic matter in the soil, suchas manure or compost. The absorbed nitrogen is usedto synthesize amino-acids and proteins. The organicnitrogen in dead plants and animals is mineralised bybacterial consortia and converted in a series of stepsto nitrates that are taken up again by plant roots.Under anaerobic conditions, part of it may be trans-formed to molecular nitrogen or nitrous oxide andreleased to the atmosphere (denitrification).

Whereas most organic residues feed microorganismswith all nutrients necessary for their growth, there isstill an optimum value of the carbon to nitrogen(C/N) ratio.The ideal value to start the composting process is C/N= 30. If the ratio is higher than 50, the time necessaryfor composting will be too long because somemicroorganisms will start consuming other deadmicroorganisms instead of decomposing the waste. Atthe end of the composting process, the ratio C/N inthe compost is around 15: compost is an intermedi-ate state between death of living organisms andtheir final mineralization. It contains not only miner-als but also enough organic materials and a numberof living microorganisms and even insects necessaryto feed plants. It should be stated that these livingorganisms are no pathogens: they are only able todecompose plants and animal wastes.

During the process of composting, it is necessarythat both water and oxygen (air) reach easily theorganic materials undergoing decomposition, other-wise anaerobic conditions will occur with generation ofmethane, 30 times more efficient than carbon dioxideas greenhouse gas. Also, carbon dioxide evolutionand venting must be easy. Accordingly, a compostingbed must be highly porous (not too much water,between 40% and 60%) and must be regularly turnedup. It is advantageous to alternate layers of variouskinds: grass (high nitrogen content), finely groundwooden parts (high carbon content), vegetables andweed (more equilibrated C/N ratio), manure (if avail-able, well equilibrated C/N ratio provided it containsenough straw), etc…

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During composting, there is an initial steep increase intemperature: easily degradable materials undergodecomposition through the action of bacteria duringthe first days raising the temperature up to 60-65°Cafter approximately 10 days. It is essential to reachhigh enough a temperature to kill all pathogenmicroorganisms and destroy all undesirable seeds.After this maximum, the decomposition process goesfurther under the action of the remaining microorgan-isms and insects, involving progressively parts thatare not so finely divided in the waste, and temperaturedecreases slowly. After each turning of the bed, thereis an increase in temperature.After a period of 3 months, the temperature remainsunder 30°C. The process is not completed at thatstage and must go on (for instance in a static pile) forseveral months.

Unpleasant odours occur only during the first days ofthe composting process when the protein content(from meat, diary products and certain vegetables) ofthe load is too high. This is a major objection againstthe process, even if smells can be attenuated, e.g.using compost or bark filters or operating in a confinedbuilding. It is accordingly recommended to avoid thesetypes of organic waste while collecting selectively forcomposting.

Practical considerations

In practice, a certain number of conditions are nec-essary for composting to be successful. Somedepend on the scale at which the process occurs.Usually, composting processes are divided in two cat-egories: decentralized and centralized composting.

Decentralized compostingHome composting, also called backyard com-posting covers already different ways to proceed.

– When the amount of biodegradable matter issmall and contains only kitchen residues, wormcomposting (also called vermicomposting or ver-miculture) is a favoured option. Redworms andearthworms break down compostables by eatingthem. Biochemical decomposition occurs via bac -teria and chemicals in the worm’s digestive system.The worms take care of aeration by digginggalleries . The only remaining care is to ensureappropriate humidity. Special containers are com-mercially available. Some of them allow recovery ofthe liquid aqueous residue: after diluting 10 timeswith water, this is a valuable fertilizer for greenplants.Vermiculture does not kill all pathogens: someviruses and parasites can survive the process. Theinput materials must accordingly be selected toavoid any risk: no meat or dairy products.

– When the garden is very small (less than 30 m2),or if worm composting is not favoured, barrel

composting should be used. Suitable barrels arecommercially available. They must allow goodaeration from the bottom and have a ventilated lidto protect the load from rain. The barrel shouldreceive heat from the sun. It is necessary to addsome wooden chips if only kitchen residues areprocessed, to maintain a favourable C/N ratio.

– When the garden has a medium size (30 to100 m2), box or silo composting prevails. At leasttwo boxes are easily built one against the othere.g. with wooden boards, leaving enough aperturesfor air to circulate easily. The two cubes have openbottom to allow access to the compost for livingorganisms coming from the earth beneath. Turningof the bed is needed. After 3 months, part of thecompost of one box should be transferred tothe other to start another compost. Finally, after6 months the compost is ready for screening anduse as a soil amendment or fertilizer.

– When the garden is larger than 100 m2, pilecomposting should be considered.It is operated like for larger facilities (see below).

– Collective composting (neighbourhood-, block-,or business-scale composting) provides a wastemanagement opportunity to a small group ofpeople living or working in the same area: house-holds, shops, institutions, etc… Composting isdone on unused land beside community gardens,or in parks. These sites usually process less than5 Mg of waste per day. The site must be accessibleto all who want to use them, be clearly designatedwith signs easy to interpret, be sited with the agree-ment of the surrounding land users, have adequatefencing or control to prevent their becoming anopen dump, and have appropriate soil to absorb theleachate. A compost monitor or supervisor shouldbe responsible for maintaining order and cleanli-ness. Back up from the municipal government isneeded to remove undesired items and for turningthe piles.Decentralized composting at the village andcommunity scale usually process between 2 Mgand 50 Mg of waste a day. The guidelines are thesame as above, but the site must accommodatemore turning, processing, and storage than atsmaller scales.

Centralized composting – Centralized composting at the municipal scalerefers to composting animal and plant wastes frommultiple sources, where the wastes are transportedfrom several points to a facility that can oftenreceive between 10 Mg to 200 Mg per day. Sitingthe compost facility is a formal process includingtechnical and environmental assessment of thepotential sites; a formal evaluation and selectioninvolving all stakeholders; a formal remediationor compensation programme to minimize and/orcompensate for nuisance effects of traffic, odour,

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leachate and noise; a separate collection and/orpre-processing system; and a formal system forusing and/or marketing the finished compost.

– Centralized composting at the regional scaledeals with facilities capable of more than 50 Mg andas much as 1,000 Mg per day. In addition to thesiting and design requirements cited above, agree-ments between the various participating municipal-ities or jurisdictions must be reached includingwaste delivery and – very important – the uses andoutlets of the finished compost.

All large scale composting facilities shouldinclude – a pre-processing stage, necessary to createthe conditions for bacterial action. It includes threeseparate types of operations: separation or removalof oversize, non-compostable, or dangerous mate-rials; size reduction (chipping, grinding, or shred-ding) to create many small particles suitable to sus-tain bacterial action; blending and compounding toadjust the C/N ratio, moisture content, or structureof the materials to be composted. Mechanical pre-processing is often the most costly part of thecomposting system and also the most vulnerable tobreakdown. Source separation and separate collec-tion is necessary to minimize preprocessing

– one of the available composting systems: – windrow and active pile systems, simple andeasy to manage. The size of the windrows mustbe large enough to allow adequate heat build-up.Their shape is related to the type of aeration andthe type of equipment used to aerate. They canbe either open or covered depending on climateand moisture content of the waste. The spacingof the windrows depends on the site and on theequipment used for turning: crews with shoveland rakes, or more often bulldozer, tractor orwindrow turning machine

– static pile systems do not turn the windrows:they are aerated continuously or periodicallyusing blown air fed through channels built intothe pad on which the piles sit

– in-vessel systems represent a higher technolo-gy approach: much of the composting process iscarried out inside a large, enclosed chamber inwhich mechanical mixing and/or forced aerationare performed where moisture, air and tempera-ture can be controlled. The residence time in thevessel is between 3 days and 30 days, followedby a period of 21 days to 180 days of active com-posting in an active or a static pile. Once theactive composting is completed, the material isstored in piles or windrows for curing for up to2 years.

– appropriate leachate recovery and treatmentto avoid problems linked to high levels of biolog-ical oxygen demand and phenols, plus eventualheavy metals and undesired chemical organic

compounds present when inputs other thanselected biodegradables from MSW are accept-ed ( sewage sludge, manure, residues from thefood industry, domestic or industrial wastewater,…).

For the composting process to succeed, it isabsolutely necessary to sort out the input stream,avoiding plants with seeds (the seeds only start todecompose above 60°C and could remain in the finalcompost, resulting in undesired plants growing wherethe compost is spread), any plant treated with chemi-cals, plants residue showing any kind of disease(should be burnt), large pieces of wood (more than afew millimetres in diameter: would take too much timeto compost); cat litter except if fully biodegradable,coal cinder (too rich in mineral salts and acts in thecompost as a weed killer), plastics and textiles, glass,metals, paper and cardboard (could be composted,but can be recycled instead), sand and soil. Shouldalso be avoided meat, diary products and cookeditems (source of bad odours, generate pathogens,and attract undesirable flies and rodents). Of course,unopened cans or boxes must be rejected.

The advantages of composting are– generation of a solid soil amendment and/orfertilizer of high quality when fed only with wellselected biodegradables; in that case, the leachatehas also rich fertilizer properties

– in the case of home composting and collectivecomposting, the process occurs at the source of thewaste, reducing the waste stream volume andthe associated costs; in that case, total costs areminimum

– reduction of transport needs and the associatedCO2 emissions in the case of decentralized com-posting

– the weight of waste that is suitable for compostingis reduced by the process: 100 kg of waste give30 kg to 40 kg of compost

The disadvantages of composting are – there is no energy recovery: there is in fact energyconsumption

– for medium or large scale composting, pre-processing is needed and is a source of malfunc-tions

– people must be trained for the process to succeed:compost monitors are needed

– for small scale composting, leachate must beabsorbed by the soil underneath; if it is not, itshould be recovered and treated (if left running as asurface stream, it could be a vector of pathogens):this is compulsory at large scale

– bad odours are generated by the incoming streamor during the first stages of the process (can be par-tially tackled by using bio-filters if these activitiesoccur in a closed building)

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– there does not seem to be a real commercial mar-ket for the final compost, due to mistrust by farmersand the public in general because of uncertainquality level; this is reinforced by the occasionalpractice of adding metalcontaining sewage sludgeto the material used for composting

There is a lack of reliable cost data for composting.This is not surprising because the ground belongsusually to a local public authority, handling equipmentfor feeding the windrows and for turning them alsobelongs to that authority and is used for other activi-ties (except for large composting plants). This is alsothe case for the manpower. Despite that, operatingcosts are evaluated between 35 Euros/Mg and75 Euros/Mg of feed. When leachate treatment isnecessary , the costs are substantially higher. As theprocess is slow, large ground surfaces are needed.Energy consumption is around 50 kWh/Mg of waste.The price of good quality compost could reach14 Euros/Mg.

The environmental impact of composting is mainlylinked to bad odours generated by the incomingstream and during the first stages of processing. Gasemissions and noise generated by the combustionengines used to power windrow turning machines andgrinders add to the discomfort. If turning the windrowsis not correctly done, anaerobic bacteria develop andmethane evolves. Rich leachate production is impor-tant. It has high levels of biological oxygen demand(BOD) and phenols (by-product of the decompositionof the lignin in leaves). It poses few problems ifabsorbed into the ground or passed through a sandfilter . High concentrations of BOD in runoff to surfacewater is on the contrary a problem because this canreduce the amount of dissolved oxygen in lakes andstreams that is available for aquatic life. High organicscontent and highly mineralized nitrogen concentrationcould also be a problem (eutrophication). An environ-mental problem due to compost use is its possiblerole in transferring heavy metals and undesired chem-icals to the soil (see above at appropriate leachaterecovery and treatment).Problems could also be linked to poor sorting ofkitchen and garden waste, introducing contaminantsto the final product (mainly metals, plastics and glassused for packaging).

In summary, composting biodegradables con-tained in MSW is possible. However, they must becarefully sorted out, preferably at the source (byhouseholders). Home composting reduces the wastestream as well as costs (collection and treatment) formunicipalities and should be encouraged. Large scaleprocessing requires severe control. Marketing the finalproduct remains a not so easy task.

4.2.B. Anaerobic digestion (biomethanisation)

The first substances that were digested anaerobicallywere human and animal waste.

Such operations already appeared in the antiquity andcountless small units have been operated in India andChina, as well as in western farms. However, indus trialoperations need sufficient know-how.

During more than three decades, national andEuropean Union subsidies helped financing R&D anddemonstration plants, resulting sometimes in suc-cessful processes. Both technical difficulties and costshave hampered a more widespread application. Thepromise of green subsidies will probably give a posi-tive impulse.

Scientific considerations

To start with, it must be clearly stated that anaerobicdigestion allows recovery of only part of thepotential oxidation energy of biodegradables inMSW.

From a scientific standpoint anaerobic digestion, likecomposting, is also part of the global biochemicalcycles of our planet, as already described above, andcarbon and nitrogen cycles are again the two mostimportant ones.The term digestion refers to the process by whichfood is dissolved and chemically converted so that thecells of a living body can absorb the food to maintainits vital functions. Thereby, complex carbohydrates,fats, fibres and proteins are converted into simplercompounds before being assimilated into cells. Duringdigestion, these organic compounds are reduced tomonomer units by hydrolytic and other enzymessecreted by bacteria and glands.

The process of anaerobic digestion (AD) employsspecialised bacteria to break down organic waste,converting it into biogas (a mixture of carbon dioxideand methane), and a stable semi-solid (digestate). Inmost cases, complex populations develop that areable to conduct consecutive processes capable tobreak down biodegradable waste in a sequence ofhydrolysis, acidogenesis (formation of fatty acids),acetogenesis and eventually methanogenesis, in abalanced, steady and controllable fashion.

These four consecutive steps normally proceedside by side, but the first and the last ones are some-times singled out because they may require special-ized conditions, controls and auxiliaries. Hydrolysismay be conducted with separate aerobic, thermal,chemical or enzymatic means. Acidogenic bacteria

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then turn the products of hydrolysis into simple organ-ic compounds, mostly short chain (volatile) acids oralcohols and acetone. The specific concentrations ofproducts formed at this stage vary with the type ofbacteria as well as with feed and conditions such astemperature and pH. Acetogenesis occurs throughcarbohydrate fermentation and other metabolicprocesses with acetate as the main product. Longchain fatty acids, formed from the hydrolysis of lipids,are oxidised to acetate or propionate and hydrogengas is formed. Under standard conditions, the pres-ence of hydrogen in the solution inhibits oxidation, sothat bacteria consuming hydrogen are required toensure the conversion of all acids. The transition fromorganic material to organic acids causes the pH of thesystem to drop. This is beneficial for acidogenic andacetogenic bacteria, but problematic for methano -gens, which prefer neutral or slightly alkaline condi-tions and are very sensitive to abrupt changes: if thepH falls below 6, they cannot survive. Since they areslow to develop, they may fail to adapt to changes, e.g.in inlet temperature, concentration, or other condi-tions. Therefore, for the digester to remain stable anequilibrium based on complex interactions of severalvarieties of bacteria is required.

Practical considerations

Anaerobic digestion is conducted in a variety ofmodes and processes: batch or continuous, single,double or multiple steps (staged digesters), vertical orhorizontal treatment units, static units or others, usingvarious mixing methods, and “dry” (high solids) or“wet” (low solids concentration) digestion. A simpledigester consists of a single, suitably shaped, static ormixed digester, in which the most desirable operatingconditions are carefully maintained, yet in a robustmanner.

Batch versus continuous processes– in the batch process, the substrate is sealed in thedigester for the complete retention time. Whenunmixed, the content of the digester stratifies intolayers of gas, scum, supernatant, an active layer,and stabilized solids at the bottom. Retention timesrange from 30 days to 60 days, with typically abiodegradable loading rate between 0.5 kg and 1.6kg total volatile solids per cubic meter reactor vol-ume per day. Long retention times, low biodegrad-ables loading rates and the formation of a scumlayer are obvious disadvantages. Also, the produc-tion of biogas follows a bell curve with time.

– in the continuous process, fresh material eithercontinuously or periodically (e.g. daily) enters thetank and an equal amount of digested material isremoved. Ideally, all processes occur at a fairlysteady rate, resulting in a constant biogas produc-tion. Because of flow, there is some movement,material is somewhat more mixed and does not

become stratified so easily inside the tank. Theremoved effluent is, however, a mix of completelyand partially digested material. Some of the moresuccessful designs dictate the path of the digestateinside the chamber, or use either plug-flow or acascade of consecutive units. Some designs takeadvantage of the successive phases of digestion,optimising each one under distinct conditions.

Single stage versus multiple stage digester– in a single stage digester, all bacteria inhabit thesame volume and their relative growth rates arekept in balance. The operating conditions are notoptimal for any bacteria, but are acceptable to all.The most crucial parameter is pH, kept close toneutral to ensure survival of methanogens.

– in a multiple stage digester, the substrate passesprogressively through sequential chambers, whereAD occurs in a staged approach. If two tanksare used, the first tank features hydrolysis, acido-genesis and acetogenesis, while the secondoptimizes methanogenesis conditions from volatileacids.The first tank is heated to a uniform temperatureand mixed and fed continuously. The pH is allowedto fall. The residence time in this chamber is 10days to15 days. The second tank must maintain ahigher pH and provide capacity for gas collection orstorage. Two-stage digesters can be more efficientbecause the microorganisms have specific nutrientneeds, growth capacities, and abilities to cope withenvironmental stress. In more complex, multiplestage digesters, each tank has a unique purposeand living environment, but this advantage is com-pensated by higher investment and operating costs.

– there are also multiple stage systems with dif-ferent criteria for solids and liquids. Incomingwaste is pulped, and the liquid, which containssoluble biodegradables, is sent immediately to amethane-producing tank. The remaining solid ishydrolized under more drastic operating conditionsin a different tank, dewatered, and the liquid fromthat tank is also sent to the methane productiontank. This system can take advantage of the sig -nificantly lower retention time required of liquidscompared to solids.

Dry versus wet digestion– digestion is subdivided into two categories of solidscontent: dry digestion, with a typical dry solids(DS) content of 25-30% and wet digestion, with aDS value of less than 15%. When the feedstock isderived from MSW, both systems require addingwater to the feedstock to lower the total solids (TS)content. A higher TS content leads to smaller, lesscostly digesters, but requires more expensivepumps and more maintenance. Systems will lowerTS have better mixing and are amenable to co-digestion with dilute feedstock like sewage sludge

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or manure. They allow also easier settling of denseparticles like sand and glass to the bottom.

– for many waste streams, large amounts of watermust be added to reduce the solids content, there-by adding to the cost of dewatering the digestate toreuse the process water.

Global flow and mixing– in a gravity driven system, the material is fed fromthe top into a vertical chamber and effluent isremoved at the bottom, with gravity being the onlydriving force to bring the waste through the bacter-ial population living in the chamber. For this system,the ideal solids content is 2-10%. A plug-flowdigester is suitable for higher solids content,because the more viscous material may move as aplug through the tank.

– mixing can take place as a result of the pathwaythe waste must travel before its removal. Some sys-tems have interior walls that increase (static) mix-ing. More intense mixing results through the use ofmechanical, hydraulic, or gas mixers to keep solidsin suspension. Mechanical mixing is less commonbecause of a difficult maintenance. Mixers also getwrapped with solids or entangled. Recirculatingheated digested waste inoculates the fresh waste,improves mixing and ensures temperature control.Biogas is bubbled through the digester for mixing.

Control parameters– control parameters are of two types: physical(temperature, mixing, space loading rate, food tomicro-organisms ratio) and chemical (Redox poten-tial, pH, carbon to nitrogen ratio, nutrient balanceand alkalinity). The following parameters are typi-cally monitored: physical (temperature, pressure,residence times, flow rates, biogas production) andchemical (pH, volatile fatty acids, alkalinity andhydrogen).

– temperature is the most critical process para-meter. Anaerobic bacteria survive from freezing to70°C, but thrive best in either a mesophilic (25°Cto 40°C, preferably 35°C) or a thermophilic range(50°C to 65°C, preferably <55°C). Thermophilicdigestion allows higher loading rates and achievesa more complete pathogen destruction and degra-dation efficiency of the substrate, yet it is moresensitive to toxins and changes in the environmentand less attractive from an energetic point ofview. Furthermore, a month or more is required toestablish a population. Mesophilic bacteria tolerategreater changes in their environment, includingtemperature. The stability of the mesophilic processmakes it more popular, albeit at the expense oflonger retention times.

– pH is a major variable to be monitored and con-trolled. The range of acceptable pH is theoreticallyfrom 5.5 to 8.5. However, most methanogens func-tion only in a pH range between 6.7 and 7.4. A

falling pH can point toward acid accumulation,which typically occurs if there is an overload ofvolatile solids in the digester. The acidogenic bacte-ria then thrive, producing more organic acids andlowering the pH to a level lethal to methanogens. Adeclining methanogen population leads to furtheracid accumulation and action to restore processstability is required, such as recycling more water.Conversely, prolific methanogenesis may result in ahigher concentration of ammonia, increasing thepH above 8.0, which will impede acidogenesis. Thisis opposed by adding fresh feedstock, spurringacidogenesis and acid formation. Maintaining pH isespecially delicate at start-up because fresh watermust undergo acid forming stages before anymethane forming can begin.

– as with composting, the optimum C/N ratio isbetween 20 and 30.

– a practical difficulty is that the substrate in somecases is available only on a cyclic basis (e.g. incase of an annual harvest). In most plants feedingoccurs once a day whereas residence times varyfrom 10 days to rather long time periods (one ormore months). AD is a slow process and the reac-tor concept and operating conditions must becarefully adjusted towards the feed or the mix offeedstocks employed.

Feedstocks, pre-and post-treatments, major out-puts– potential feedstocks are from different origins:agricultural origin (animal waste, crop waste,dedicated energy crops), industrial origin (wastewater, sludge, by-products) and municipal origin(sewage sludge, MSW).

– bio-waste contains components that are notreadily available as substrates for anaerobic diges-tion. Consequently, a substantial portion ofpotentially available carbon is not convertedinto methane and the incompletely digestedresidues require additional processing prior to theirreturn to the environment.

– some pre-treatment and post-treatment are oftenrequired: waste storage and pre-treatment (sorting,chopping, warming, acidification, mixing, recycling);biogas storage, treatment and upgrading (storageat low, medium, or high pressure, elimination ofwater and sulphur and nitrogen compounds,removal of carbon dioxide). The treatment ofbiodegradable fractions from MSW necessi-tates selective collection of garden waste andkitchen waste, and/or mechanical sorting ofresidual MSW. The most common treatment isseparation (remove metals, glass, plastic, etc…)and shredding (to reduce size of the solids).

– major output flows are: – the gas phase (biogas), with as quality criteriathe biogas formation rate and the relativeamount of the main compounds (methane and

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carbon dioxide), the level of fatty acids and ofacetic and formic acids, and the level of impuri-ties (sulphur and nitrogen compounds, mainly ashydrogen sulphide and ammonia). Options ofusage of the gas phase are heat generation,substitute natural gas (upgrading to pipelinequality is needed), power generation (spark ignit-ed and Diesel engines), motion of motor vehi-cles, and combined heat and power generation.The gas phase strictly requires separation ofentrained droplets and of condensable watervapour (using cooling, condensers and demis-ters), and also of corrosive gases. Beforeupgrading to pipeline quality, the pressure will beraised, generally to 10 to 30 bars, rendering thetreatment operations more efficient and com-pact.

– the solid and liquid effluents (digestate). Partof it is recycled with process water, with the aimof reducing the volume of effluent liquids; howev-er, there are limits to recycling, since toxic orundesirable compounds, such as salts andheavy metals, should be sluiced out continuous-ly or periodically. Success stories on a prosper-ous application of the resulting solids as a soilstructure improving substrate are scarce andrelatively less documented in AD literature. Thedigestate after aerobic post-treatment is a stable,organic humus-like material, the subsequent useof which depends on market conditions for com-post and on the feedstock characteristics of theAD process. In the case of MSW the digestatewill be contaminated with sand, plastics, heavymetals and eventually stable undesired organicmolecules, severely limiting the scope of itseventual application.

Advantages of anaerobic digestion are– production of biogas that can be used for heatand/or power generation

– reduction of weight between the loaded biodegrad-able MSW and the digestate cake (for 100 kg MSWat 30% humidity, there remains 67 kg digestatecake at 45% humidity; or for 70 kg dry MSW, thereremains 37 kg dry digestate cake). If there is nomarket for the digestate cake, there is anyhowsome weight reduction of the residue to be land-filled (or incinerated)

– allows treating humid biodegradable waste

Disadvantages of anaerobic digestion are – although the process does not make use ofpathogens, if bacteria or viruses of that type areintroduced in the process, they will not be killed andwill remain in the digestate; a post-treatment at70°C during an hour is needed for sanitary reasons

– for the process to succeed, it is absolutely neces-sary to sort out the input stream (almost like forcomposting; the process is capable to treat meat,

but this should be avoided due to the odours duringsorting out and preprocessing)

– bad odours are generated by the incoming wasteand during sorting out and pre-processing (couldbe partially tackled by using bio-filters if these activ-ities occur in a closed building)

– being a slow process, relatively large plants arenecessary, to be maintained under moist and cor-rosive conditions

– there does not seem to be any true commercialmarket for the final compost (see above at com-posting) or digestate; there are however specialagreements with farmers who provide manure andstraw to the plant and are obliged to recover thefinal digestate

– the process concentrates heavy metals and unde-sirable chemical compounds eventually present inthe input in the final digestate or compost.

Following are some approximate data for anaerobicdigestion (which should be defined for each specificcase; plants are capable to treat between 20,000 Mg/yto 100,000 Mg/y): – biogas productiondepends largely on the type of feedstock (from25 m3/Mg for bovines manure to 800 m3/Mg forwaste grease; source: IRCO); however, productionspeeds are very different. For biodegradables inMSW, values fall between 80 m3/Mg to 150 m3/Mgof waste feedstock

– biogas compositionafter purification, contains from 50 to 80%methane, usually 65%; remainder mainly carbondioxide with trace elements of other gases likehydrogen sulphide, siloxanes, ammonia, watervapour and organochlorines

– biogas net calorific valueat 65% methane, approximately 24 MJ/m3N

– electrical power to sellas approximately 30% of the electrical power isused for the plant itself, there remains between70 kWh/Mg to 200 kWh/Mg of waste treated.For biodegradables in MSW, values are above100 kWh/Mg (source: ACR+).

– digestate and/or compostthey are very difficult to sell, due to poor quality:in Flanders, prices vary from 1 Euro/Mg to2.5 Euros/Mg

– investment costs350 to 500 Euros per annual Mg of feedstock

– operating costs after separate collection, 70 Euros/Mg to150 Euros/Mg of feedstock; the additional cost forsource sorting and separate collection rangeswidely between 15 Euros/Mg and 135 Euros/Mg

Without subsidies, including so called “green certifi-cates” for the electricity produced, the process is noteconomically viable.

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The environmental impact of anaerobic digestionis mainly linked to bad odours generated by theincoming stream and during the first stages of pro-cessing. Biofilters in the roof of buildings can some-what reduce the emissions. Despite of that, all plantsare built at a certain distance from cities, though nottoo far to enable the use of the cooling water of powergenerators for urban heating (loss of one degree tem-perature for each km).

Environmental problems due to compost or liquiddigestate use are linked to the potential convey ofheavy metals and of undesired chemicals to the soiland also to poor properties as soil amendment.

In summary, the anaerobic digestion of biodegrad-ables in MSW is possible. However, they must becarefully sorted out, preferably at the source, andthe technique is economically dependent on sub-sidies.

Recently, biochar has received increased interest.The idea is to remove water from the digestate and toperform pyrolysis on the remaining solids. Theprocess is in fact similar to the production of charcoalfrom wood. Biochar sequestrates carbon very effi-ciently under an almost inert form, and can be usedfor soil amendment.

However, its production requires additional funds tocover investments and operational costs, consumesenergy, produces CO2, and concentrates furtherheavy metals present in the digestate in the biochar,while the destruction of undesirable organic mole-cules depends on the temperature conditions prevail-ing during pyrolysis.

Biochar production applied to other biodegradablesthan those found in MSW is mentioned in the litera-ture, but falls beyond the scope of this report.

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5.1. Preliminary remarks

It should be clearly stated that all processes consid-ered in this report deal with chemistry and biochem-istry and necessarily follow the principles of massconservation. Accordingly, any chemical elementpresent in the incoming stream will anyhow bepresent in the outputs in the same quantity. Itcould be present though, in different chemical com-pounds and/or under another physical form.

Whatever the process considered in this report,any carbon present in the biodegradables in MSW willend as carbon dioxide, eventually after combustionof biogas (some methane could be released in theatmosphere e.g. in poorly aerated composting opera-tions) and despite some short term carbon sequestra-tion in the case of composting or biomethanisation.None of these processes can claim any ’ab initio’“green house” effect advantage. Detailed LifeCycle Assessment (LCA) should be carried out tocompare specific cases.

From a thermodynamic standpoint, as enthalpy is afunction of state, for any reaction starting with thesame agents and ending with the same products,the total enthalpy change (heat of reaction)remains the same, whatever the path followed.Accordingly, consider a combustion reaction startingwith biodegradables in MSW and ending with CO2

and H2O and showing a net calorific effect called Q.If this process is split into two partial processes, withthe first of the two exothermic with a net calorific effectq, then the net calorific effect for the second partialwill be reduced to Q-q. This is the case for biogascombustion.

Most flawed waste policies forget and leave outthermo dynamics.

Whatever the process, people tend to have “not in mybackyard” (NIMBY) emotional reactions against it.In this report, these reactions are ruled out if thetechnology does not justify them.

The following discussion is divided into two parts: thefirst considers only scientific, technological, economi-cal and environmental factors; the second legal or reg-ulatory considerations.

5.2. Scientific, technological, economical andenvironmental considerations

Economic comparison between the different process-es is difficult because the net results of the process(the boundaries of the system) are not identical. Moreover, various subsidies or local conditions influencethe investment and operating costs.Accordingly, a general but sound discussion should bebased mainly on the merits of the processes withrespect to reliability, possible material and/or energyrecovery, decrease of volume and/or weight ofresidues eventually to be dumped in landfills, environ-mental impacts, and capability to treat correctly thebiodegradables contained in MSW after or withoutpreliminary sorting out.

Starting with high temperature processes, inciner-ation appears as a fully mature technology. Theprocess is reliable; it leads to satisfactory energyrecovery (economic especially if the steam producedcan be delivered to a nearby electric power plant);there is a strong decrease in volume and/or weight ofresidues to be dumped in landfills; European direc-tives guarantee now a very low level of environmentalimpact, especially due to efficient flue gas cleaningsystems; and it is capable to treat correctly thebiodegradables in MSW without any need for a pre-liminary sorting out neither at the source nor beforefeeding the furnace at the plant. Of course, vegetablematter having a high water content, it is at the lowerend of the lower heating value in the feed of the fur-nace; but this is an advantage because the meanlower heating value of the load introduced in the incin-erators has steadily increased with time andapproaches values at which it would be necessary todecrease the rate of combustion. Improvements coulddeal with further processing of the bottom residuesand of the fly ash: the advantage is that all heavymetals could be recovered in a separate molten phaseand/or condensate suitable for eventual recyclingthrough adequate metallurgical treatment. It shouldalso be emphasized that incineration allows destruc-tion of most undesirable organic chemicals.

Other high temperature processes are more question-able: pyrolysis and gasification cannot beassessed as being reliable and proven in Europe,even though the Japanese experience appearsmore positive. More time is needed for the largest

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Chapter 5

DISCUSSION


scale European plants to demonstrate their capabili-ties. Plasma processes suffer from their need forelectric power consumption: this is a high cost energy,and in most cases generates carbon dioxide for itsproduction. These processes could be of interest forsmall quantities of special types of waste.

Low temperature processes are also of interest andhave been operated for a long time. Composting aswell as anaerobic digestion will concentrate anyheavy metal or undesirable organic chemicals presentin the incoming stream in the compost and itsleachate for the first one, and in the liquid digestate orin the dried compost for anaerobic digestion. As aresult, although high concentrations would kill theactive microorganism and block the production, peo-ple who should use it, especially farmers, are veryreluctant to do so. Accordingly, there is no real marketfor these products. If nobody wants them, they shouldgo either to landfills or, if the remaining lower heatingvalue is high enough, to incineration. Anyway, bothprocesses need efficient sorting out and pre-process-ing of the incoming stream. Moreover, a number offailures have been reported due to various malfunc-tions either of the process or of the equipment. Bothprocesses are able to treat biodegradables containedin MSW.However, preliminary sorting out is needed, preferablyat the source. Backyard or locally decentralized com-posting should be encouraged. Anaerobic digestion isinteresting because of energy recovery for heatingand/or for electric power generation. It operates atbest with feedstock coming from agriculture: in thatcase, the quality of the incoming stream is undercontrol and farmers are less reluctant to spread thedigestate on their fields. However, the digestate has alower agricultural value than the feedstock because ofa reduced C/N ratio, mineralization of nitrogen andloss of soil amendment properties. Extra costs forselective collection of biodegradables sorted atsource in MSW are very high. Carbon dioxide gener-ated during handling and transportation must also betaken into consideration.

Without subsidies, a further comparison can bemade between incineration and anaerobic diges-tion, considering both processes under their bestoperational conditions: a mass-burn incinerator pro-ducing steam fed to a nearby electric power station,and an anaerobic digestion plant with a feedstockcoming mainly from agriculture and producing elec-tricity by means of spark ignition engines.

For the incinerator capable to treat between150,000 Mg/y to 450,000 Mg/y MSW, investmentcosts are between 300 and 360 Euros per annual Mgof waste; operating costs including capital charges areapproximately of 100 Euros per Mg of waste without

supplement for feeding biodegradables contained inthe MSW; 450 kWh/Mg to 500 kWh/Mg of waste aresold to the distribution network.

For the anaerobic digestion plant capable to treatbetween 20,000 Mg/y to 100,000 Mg/y biodegradablefeedstock, investment costs are between 350 and500 Euros per annual Mg of waste; operating costsin the case of selective collection are between70 Euros/Mg and 150 Euros/Mg of waste, butsource sorting and selective collection add between15 Euros/Mg and 135 Euros/Mg of waste, so that ifonly 20% of the feed comes from biodegradables inMSW, the total operating costs should be between73 Euros/Mg and 177 Euros/Mg of waste; 70 kWh/Mgto 200 kWh/Mg of waste (more than 100 kWh/Mg forbiodegradables in MSW) are sold to the distributionnetwork.

Accordingly, an incinerator shows an economicaladvantage versus anaerobic digestion and alsoproduces more electrical energy. This is notsurprising : the incinerator needs neither a selectivecollection nor sorting out of biodegradables containedin MSW; concerning energy, in an incinerator all theorganics are burnt to CO2 whereas in anaerobic diges-tion, only part of the carbon is finally burnt throughbiogas combustion and moreover, as the digestion isexothermic, the thermal energy to be recovered frommethane is also reduced for a given amount of carboncontained in the feedstock and finally converted toCO2. The difference increases with the proportion ofbiodegradables in MSW.

5.3. Legal considerations

Directive 2008/98/EC introduces in its Article 4, point1, a hierarchy in five steps to apply as a priorityorder in waste prevention and management legislationand policy

(a) prevention (b) preparing for re-use (c) recycling (d) other recovery, e.g. energy recovery (e) disposal.

However, in the same Article, point 2, it is stated that“Member States shall take measures to encouragethe options that deliver the best overall environmentaloutcome. This may require specific waste streamsdeparting from the hierarchy…” Further in the sameArticle, point 2, “Member States shall take intoaccount the general environmental protection princi-ples of precaution and sustainability, technical feasi-bility and economic viability, protection of resourcesas well as the overall environmental, human health,economic and social impacts, …”

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Accordingly, the hierarchy is not mandatory andfor instance economical or technical considera-tions could justify departing from it.

Everybody would agree with processes dealing withprevention. The first efficient way to reduce thebiodegradables fraction in MSW consists in leavingfreshly cut grass on the lawn, and leaves and branch-es of bushes and trees on the spot after grinding. Thesecond is home-, collective-, and possibly small scaledecentralized composting provided the feedstock isnot collected by municipal services and the compostis used on the spot. However, some training is neces-sary (compost monitors). Of course, decreasingkitchen waste by improving the behaviour of peopledealing with food is also very efficient. All these meth-ods reduce the extent of the waste stream.

In Chapter I, Article 3 Definitions “13. “Re-use” meansany operation by which products or components thatare not waste are used again for the same purpose forwhich they were conceived”. This is not applicable tobiodegradables in MSW. In the same Article 3Definitions “14. “Preparing for re-use” means check-ing, cleaning or repairing recovery operations, bywhich products or components of products thathave become waste are prepared so that they can bere-used without any other preprocessing”.Again, this is not applicable to biodegradables inMSW.

In the same Chapter I, Article 3 Definitions “17.“Recycling” means any recovery operations by whichwaste materials are reprocessed into products, mate-rials or substances whether for the original or otherpurposes. It includes the reprocessing of organicmaterials but does not include energy recovery andthe reprocessing into materials that are to be used asfuels or for backfilling operations”. This is applicableto the biodegradables collected selectively fromMSW. According to the hierarchy, this would give thepriority to large scale composting facilities,provided that the quality of the compost is such thatpeople agree to make use of it.That does not seem to be the case. Up to now thereare no European standards for compost quality.However, a survey by ACR+ (Municipal Waste inEurope – Collection Environnement – VictoiresEditions – Paris – 2009 – p. 185) shows that concen-trations of heavy metals in compost from MemberStates are high: for instance for lead between100 ppm and 180 ppm. The “Arrêté du Gouvernementwallon” of 18 June 2009 authorizes to incorporate inthe feedstock for composting, biomaterials containing

up to 500 ppm of lead. The same concentrations areaccepted for final digestates from biomethanisation(see Table 3 in the “Arrêté du Gouvernement wallon”in preparation as introduced in Chapter 2 of thisreport).

In the same Chapter I, Article 3 Definitions “15“Recovery” means any operation the principal resultof which is waste serving a useful purpose by replac-ing other materials which would otherwise have beenused to fulfil a particular function, or waste being pre-pared to fulfil that function, in the plant or in the widereconomy.Annex II sets out a non-exhaustive list of recoveryoperations.” This is applicable to the biodegrad-ables in MSW. In this particular case, as mentioned inR1 of Annex II “Use principally as a fuel or othermeans to generate energy”, biomethanisation andincineration are included. There are many con-straints on incineration (Directive 2000/76/EC;BREF documents linked to Directive 2008/1/EC; and aformula in Directive 2008/98/EC Annex II R1 imposingminimum values for the efficiency for incinerators tobe considered as “energy recovery” plants). No simi-lar constraints appear for anaerobic digestion orfor composting.

In the same Directive, Chapter III Waste ManagementArticle 22 “Bio-waste” “Member States shall takemeasures , as appropriate, and in accordance withArticles 4 (waste hierarchy) and 13 (protection ofhuman health and the environment), to encourage: (a) the separate collection of bio-waste with a viewto the composting and digestion of bio-waste;

(b) the treatment of bio-waste in a way that fulfils ahigh level of environmental protection;

(c) the use of environmentally safe materials pro-duced from bio-waste.”

The same article mentions that an assessment willbe performed, leading to eventual “setting of min-imum requirements for bio-waste managementand quality criteria for compost and digestate …”.

Taking into account all legal constraints, it can be con-cluded that separate collection of bio-waste is notmandatory, and that large scale composting appearsin the hierarchy in preferred position compared to bio-methanisation and incineration considered as energyrecovery systems. Owing to the facts that com-posts and digestates do not show so far satis -factory properties, and that biomethanisationdelivers less energy than incineration (essentiallymassburn), the latter should be preferred.

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The initial question was “MSW: What to do withbiodegradables”? The final answer is as follows,keeping in mind that:

– none of the available processes can claim anygreenhouse effect advantage as the final productsare anyway CO2 and H2O

– maximum energy recovery is obtained by incinera-tion, followed by biomethanisation; composting con-sumes energy.

– biomethanisation has the advantage of producinggas that could be used for automotive vehicles;technical and economical constraints remain to bestudied

– incineration is the only available process satisfyingall the “best available techniques” criteria

– although Directive 2008/98/EC introduces a hierar-chy in 5 steps, step 2 (preparing for re-use) is notapplicable to biodegradables in MSW; the hierarchyis not mandatory: technical and economical feasi-bility and environmental factors must be taken intoaccount; if there were markets for composts anddigestates, the hierarchy would give an advantageto composting and place at the same level bio-methanisation and incineration.The directive also encourages selective collectionof bio-waste; again, this is not mandatory.

Accordingly, priority should be given in all circum-stances to reducing the amount of biodegradablescontained in MSW: decrease kitchen waste byimproving the behaviour of people dealing with food(buy no more than you can eat; if there are remainsfrom a meal, keep them at low temperature and cookthem again for another meal; buy items without unnec-essary packing); leave freshly short cut grass on thelawn, and leaves and branches of bushes and treeson the spot after grinding; encourage home-, collec-tive-, and possibly small scale decentralized compost-ing provided the feedstock is not collected by munici-pal services and that the compost is used on the spot.For efficient composting, some training is needed(compost monitors).

For garden and park waste directly collected andbrought by citizens or by the park owners to speciallarge containers park, they can be used either forlarge scale composting or for biomethanisation.

For what is left as biodegradables in MSW, a deci-sion has to be taken whether or not to proceed to

their separate collection at the source. The follow-ing points should be considered:

(a) the costs of separate management of this specialwaste stream from the collection point to the plantwhere it will be used as a feedstock, not only interms of expenditure (investment in modifiedtrucks; eventual intermediate storage; land area;man power) but also in terms of impact on theenvironment (CO2 emissions; odours; recovery andeventual treatment of liquids)

(b) the interest of feeding that waste to a large scalecomposting plant (no energy recovery: energy con-sumption instead; lack of control on the contentof the collected bags could result in problems withthe composting process and products; need toestablish a contract with the composting plantowner concerning processing fees and possibleuses of the final compost)

(c) the interest of feeding that waste to a biomethani-sation plant (partial energy recovery; lack of controlon the content of the collected bags could resultin problems with the process and products; needto establish a contract with the plant owner con-cerning processing fees and possible uses of thefinal liquid digestate or solid compost)

(d) the interest of not proceeding to the separate col-lection at the source and instead feed the MSWcontaining the biodegradables as such in an incin-erator, preferably mass-burn (high energy recoverymainly as electric power and/or for urban heating;minimum amount of residues to landfill; low envi-ronmental impact due to EU directives; concentra-tion of heavy metals in separate residues anddestruction of most undesirable organic chemicals)

Further points of interest are:

– citizens have already to cope with a large numberof source separations. Sorting out is not so easy. Itis not evident that it will be done properly, evenwhen another difficult source separation and theassociated costs are accepted.

– large scale composting is not that easy to operatesuccessfully. The final compost is usually contami-nated with heavy metals and undesirable organicchemicals. Accordingly, people, and especiallyfarmers, are reluctant to use compost as soilamendment or fertilizer.

– anaerobic digestion (biomethanisation) is not eco-nomically viable without substantial subsidies. Itallows partial energy recovery, but requires expen-

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Chapter 6

CONCLUSIONS.


sive sorting out and pre-processing of the incomingstream. The final liquid digestate and/or solid com-post show the same problems of contamination asalready mentioned. On top of that, soil amendmentproperties are lost (except perhaps if biochar is pro-duced from the digestate, but as already explained,this would be expensive and lead to further con-centration of heavy metals)A true market does not seem to exist for compostand digestate. If they must go to landfill, the volumeand weight reduction is too small. Their incinerationwould be more interesting, but in that case unsort-ed incineration would be more advantageous.

– if no subsidies are involved, incineration is eco-nomically more advantageous than anaerobicdigestion, and the advantage grows when the per-centage of biodegradables coming from MSW inthe feed increases. Also, the amount of electricalenergy produced is in favour of incineration.

Final recommendations are as follows:

– the bio-waste suitable for anaerobic digestion islargely over evaluated in the literature; it should beestimated again taking into account technical, eco-nomical and environmental factors. Feeding thegases from biomethanisation to automotive vehi-cles should be further investigated

– new EU directives and BREF documents are need-ed for composting and anaerobic digestion. Theyshould define the minimum quality requirements forcomposts and/or digestates. Current practice ofintroducing various contaminated waste (sewagesludge, industrial residues, etc…) in the feedstockshould be drastically reduced or even suppressed.

Water effluents should be treated. Any new legisla-tion should aim at protecting not only the upper lay-ers of soil in the short term, but also the deepunderground soil and water in the long term. Itshould be remembered that on the long term soilcontamination is more dangerous than air contami-nation, although less visible

– incineration (especially mass-burn) is the onlyMSW treatment process allowing to recover heavymetals and to destroy most of the undesirableorganic chemicals with a very small amount of haz-ardous waste to landfill. It is also the only processsatisfying to all conditions required for a best avail-able technique. European Directives and BREFdocuments introduce already many constraints onthe process. Instead of looking for additional con-straints like efficiency limits imposed to incineratorsqualified as energy recovery centres, it would bebetter to encourage further treatment of bottomresidues and of fly ashes aiming at zero landfill andrecovery of metals by chemical or metallurgical pro-cessing. Detailed flying ash analysis would proba-bly identify costly and less common chemical ele-ments of industrial interest. Research dealing withthe last two points should be encouraged

– some more time is needed for correctly evaluatingpyrolysis and gasification processes

– plasma processes should be further evaluated forthe treatment of specific waste available in smallquantities

– many industrial processes should be equipped withproven flue gas cleaning systems. The same shouldalso be envisaged for wood burning systems.��

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BIBLIOGRAPHY

1. Sixth Environment Action Programme of the EU 2002-2012 (http://ec.europa.eu/environment/newprg/strate-gies_en.htm)

2. European Union’s approach to waste management (http://ec.europa.eu/environment/waste/index.htm) 3. EU Waste Policy – The story behind the strategy (http://ec.europa.eu/environment/waste) 4. EU Legislation (http://europa.eu/legislation_summaries/environment/waste_management/index_en.htm) 5. Waste Disposal Directive 2006/12/EC 6. EC Communication: A strategy on the Prevention and Recycling of Waste 2005 COM (2005) 666 7. Directive 2008/1/EC on Integrated Pollution Prevention and Control (the IPPC Directive) 8. New Directive 2008/98/EC on waste and repealing certain Directives 9. EC Regulation n° 2150/2002 on Waste Management Statistics 10. COM(98)463 on Competitiveness of the recycling industries 11. Directive 1999/31/EC on Landfill of waste 12. Directive 2000/76/EC on Waste Incineration 13. Directive 91/689/EEC on Controlled Management of Hazardous Waste 14. Vlaams Reglement inzake Afvalvoorkoming en Afvalbeheer (VLAREA) 1997 and + 15. Plans wallons des déchets 1996 and + 16. Arrêté du Gouvernement wallon du 18 juin 2009 déterminant les conditions sectorielles relatives aux

installations de compostage...17. Droit bruxellois de l’environnement http://www.bruxellesenvironnement.be/Templates/DroitContent.aspx?lang-

type=2060 18. Eurostat : http://epp.eurostat.ec.europa.eu/portal/page/portal/waste/data/ 19. OECD/Eurostat Joined Questionnaire 20. EPEC "Support in the Drafting of an ExIA on the Thematic Strategy on the Prevention and Recycling of Waste

(TSPRW)" prepared for the IPPC – Directive 2008/1/EC 21. Glossary of statistical terms – OECD (http://stats.oecd.org/glossary/download.asp.) 22. UN Solid Waste Management Sourcebook (http://www.unep.or.jp/ietc/estdir/pub/msw/) 23. UE COM(2008)811 final, "Livre vert sur la gestion des biodéchets dans l’UE" 24. Directive 86/278/EEC on the protection of the environment, and in particular of the soil, when sewage sludge

is used in agriculture.

NB : As mentioned at the end of the Introduction, many technical and scientific references were provided by theexperts. No attempt was made to list them.

FURTHER READINGS

BLIEFERT and PERRAUD – Chimie de l’environnement - 2d edition – De Boeck Université – Brussels 2009 –ISBN 978-2-8041-5945-0

Municipal Waste in Europe – ACR+ - Collection Environnement – Victoires Editions -Paris – 2009 – ISBN 978-2-35113-049-0

Les Guides de l’Écocitoyen – Région wallonne – Direction générale des ressources naturelles et de l’environne-ment – Jambes (Namur): Gérer les déchets ménagers Composter les déchets organiques

SULZBERGER – Compostage et fertilisation – Editions Chantecler – Belgique -ISBN-13 978-2-8034-4645-2 FAUCHAIS – Plasmas thermiques: aspects fondamentaux – dans Techniques de l’Ingénieur D 2810 FAUCHAIS – Technologies plasma: applications au traitement des déchets dans Techniques de l’Ingénieur G 2055

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