2008 february treatment of municipal solid waste anaerobic digestion technologies

7/30/2019 2008 February Treatment of Municipal Solid Waste Anaerobic Digestion Technologies

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www.pytheas.net

Treatment of Municipal Solid Waste

Anaerobic Digestion Technologies

February 2008
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Copyright 2008 Pytheas Limited 21 February 2008 2

Anaerobic Digestion (AD) technologies have theotential to immensely reduce the environmental

mpact of waste disposal while capturing biogas energy;hey also complement other organic waste diversionechnologies such as composting


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Index

Glossary of Terms 4

1.0 Introduction 7

2.0 Background 9

2.1 Digestion Process Description 9

2.2 State of MSW disposal in the U.S. and Europe 11

3.0 Categories of Engineered Anaerobic Digestion Systems 13

3.1 Material Handling Systems A typical sorting line 15

4.0 Review of Commercial AD Technologies for MSW treatment 18

4.1 Single-stage Wet Systems 18

4.1.1 The Wabio Process 184.1.2 The Waasa Process 20

4.1.3 The BIMA Digester 21

4.2 Single-stage Dry Systems 21

4.2.1 The Dranco Process 21

4.2.2 The Valorga Process 22

4.2.3 The Kompogas Process 23

5.0 Multi-stage Digesters 25

5.1 The BTA Process 25

5.2 The Linde-KCA Process 27

5.3 The ArrowBio Process 28

5.4 The Biopercolat Process 30

6.0 Batch Digesters 31

6.1 The Biocel System 31

6.2 The SEBAC System 32

6.3 The Anaerobic Phased Solids (APS) Digester 33

7.0 Digester Performance 35

7.1 Biogas Yield 35

7.2 Life Cycle Analysis 37

8.0 Environmental Impacts 398.1 Municipal Solid Waste Anaerobic Digestion Emissions 39

8.2 Anaerobic Digestion Emissions from use of Biogas 40

8.3 VOC Emissions from Composting and Digestate 40

8.4 Landfill Gas Emissions 41

8.5 Solid Residues 41

8.5.1 Compost Quality Heavy Metals 41

8.5.2 Dioxin in Trash and Compost 42

8.5.3 Pesticides and Herbicides in Compost and Composted Digestate 43

9.0 Economics 44

9.1 Costs 44

Sources 46


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Glossary of Terms

Aerobic With or in the presence of oxygen.

Alternative daily cover Material other than soil used to cover the surface of active landfills at the endof each day to control diseases, fires, odors, etc.

Anaerobic Without oxygen.

Anaerobic digester A dedicated unit process for controlling the anaerobic decomposition of organicmaterial. Typically consists of one or more enclosed, temperature controlledtanks with material handling equipment designed to prevent the introduction ofoxygen from the atmosphere.

Biogasification Decomposition of biomass into methane by anaerobic bacteria. Also calledbiomethanization.

Biomixer A rotating drum often with a trommel screen used for size reduction andpretreatment of the organic fraction in mixed MSW for sorting. Can be aerated

to encourage biological breakdown. Can be operated at retention times fromseveral hours to several days.

Bioreactor-landfill A landfill operated as a bioreactor using leachate recycling (or othermanagement schemes) to increase the rate of organic decomposition andbiogas production. Not to be confused with anaerobic digester.

Biochemical oxygendemand (BOD)

Biochemical oxygen demand is the amount of oxygen required for complete(aerobic) biological decomposition of a material. The standard laboratorymethod (BOD5) tests the amount of dissolved oxygen consumed in a closedaqueous system over a five-day period. It is a fairly direct but time-consumingmeasure of biodegradability of liquid streams.

Biothermal energy Biothermal Energy is the concept of utilizing composting material to generateheat and biogas for use as an alternative energy source. Organisms which

assist in the breaking down waste material also create heat, methane, carbondioxide and other gases. These byproducts can be used in a variety ofapplications. Most prominently, greenhouses can use the biothermal energyand carbon dioxide to promote the growth of plants.

Co-digestion The simultaneous digestion of a mixture of two or more feedstocks. Co-digestion is sometimes employed to balance nutrient requirements of theprocess, improve gas production rate, as well as to process wastes frommultiple sources.

Combustion A rapid conversion of chemical energy into thermal energy. The reaction isexothermic. Organic matter is oxidized with sufficient air (or oxygen) forreactions to go to completion. The carbon and hydrogen are oxidized to carbondioxide and water, respectively.

Compost Compost here refers to stabilized and screened organic material ready for

horticultural or agricultural use. If anaerobically digested material is used ascompost, it must be biologically stabilized, typically through aeration andmaturation.

Continuously stirred tank

reactor

A digester configuration in which the entire digester contents are mixed to

create homogeneous slurry.

Digestion Either in the presence of oxygen (aerobic) or in an oxygen-depletedatmosphere (anaerobic), the process in which microbes digest biogeniccarbonaceous materials and emit any number of energetic, inert gases andliquids.

Fermentation The use of microorganisms such as yeast, bacteria, and fungi to convertsubstrates such as sugar into products. In the absence of oxygen, theseproducts can include ethanol, methane, and carbon dioxide plus some increase

in cell mass. When oxygen is present, the increase in cell mass is generallymuch greater with water and carbon dioxide usually the primary products.


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Gasification Production of energetic gases from solid or liquid organic feedstocks usually bypartial oxidation. Primary energetic gases produced are hydrogen, carbonmonoxide, and methane, along with an inorganic ash residue.

Grey waste The material left over after separation of recyclables and putrescible materialfrom the mixed waste stream. Composed mostly of inorganic material, greywaste usually contains a significant amount of organic material. Depending onits composition, grey waste and can be treated biologically or burned prior tofinal disposal.

Hydraulic retention time(HRT)

The average time a volume elementof fluid resides in a reactor. It iscomputed from liquid-filled volume of an anaerobic digester divided by thevolumetric flow rate of liquid medium. Increasing the HRT allows more contacttime between substrate and bacteria but requires slower feeding and/or largerreactor volume.

Hydrogasification Gasification using hydrogen gas to react with the carbon in organic materials toproduce a methane rich gas effluent, and provide heat for the process. Anypyrolytic products present are usually converted into methane. Steam pyrolysisis often used as a precursor process that can enhance the hydrogen reactionkinetics, despite the presence of water in the feed. Since oxygen is notintentionally introduced, carbon oxides are reduced and methane increased asthe hydrogen pressure is increased. Toxic hydrocarbons, like furans anddioxins, are chemically reduced by hydrogasification to less hazardous chemicalcompounds.

Hydrolysis A chemical or biological process in which water is added to other molecules (theconditions are wide ranging and many molecules can be hydrolyzed).Hydrolysis is a pre-treatment or preliminary step in fermentation processes thatultimately yield biogas or ethanol. For cellulose and hemicellulose, a variety ofhydrolysis methods can be used to break down the long chain polymer intosimple glucose molecules, Efficiencies of hydrolysis vary among methods andfeedstocks.

Mechanical biological

treatment

A waste processing system that combines a sorting facility for materials

recovery (the mechanical portion) with biological treatment, either aerobic oranaerobic, for stabilizing the organic fraction before landfilling.

Materials recovery facility A facility where mixed MSW is sorted in order to recover material for reuse orrecycling. In California, the post MRF fraction is typically landfilled.

Mechanically separatedOFMSW

Organic material separated from the mixed waste stream by mechanical means(i.e., trommel screens, shredders, magnets, density dependent mechanisms).

Mesophilic One of two optimal temperature ranges for microorganisms involved inanaerobic digestion. Mesophilic temperature is around 35 C (95 F).

Municipal solid waste(MSW)

MSW includes all of the solid wastes that are generated from residentialsources, commercial and business establishments, institutional facilities,construction and demolition activities, municipal services, and treatment plant

sites. Hazardous wastes are generally not considered MSW. Note that someregions or countries consider only residential solid waste as MSW.

Organic Material containing carbon and hydrogen. Organic material in MSW includes thebiomass components of the waste stream as well as hydrocarbons usuallyderived from fossil sources (e.g., most plastics, polymers, the majority of wastetire components, and petroleum residues).

Organic fraction ofmunicipal solid waste(OFMSW)

The biogenic fraction of MSW. OFMSW can be either removed from the wastestream at the source (source-separation) or downstream by mechanicalseparation, picking lines a combination of the two.

Plug flow digester A digester in which materials enter at one end and push older materials towardthe opposite end. Plug flow digesters do not usually have internal mixers, andthe breakdown of organic matter naturally segregates itself along the length of

the digester.


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Pre-treatment In reference to municipal solid waste, pre-treatment can refer to any processused to treat the raw MSW stream before disposal or management. Thisincludes separation, drying, comminuting, hydrolysis, biological treatment,heating, pyrolysis, and others.

Pyrolysis A thermochemical decomposition of organic material at elevated temperatureswithout the participation of oxygen. It involves the simultaneous change ofchemical composition and physical phase, and is irreversible. Pyrolytic oils, fuelgas, chars and ash are produced in quantities that are highly dependent ontemperature, residence time and the amount of heat applied.

Solids retention time The average length of time solid material remains in a reactor. SRT and HRTare equal for complete mix and plug flow reactors. Some two-stage reactorconcepts and UASB reactors decouple HRT from the SRT allowing the solids tohave longer contact time with microbes while maintaining smaller reactorvolume and higher throughput.

Source-separated OFMSW(SS-OFMSW)

Organic solid waste separated at the source (i.e., not mixed in with the othersolid wastes). Often comes from municipal curbside recycling programs inwhich yard waste and sometimes kitchen scraps are collected separately fromthe rest of the MSW stream.

Thermophilic One of two optimal temperature ranges for microorganisms (bacteria) involvedin anaerobic digestion. Thermophilic optimum temperature range is between 45and 70 C (104 and 160 F).

Total solids (TS) The amount of solid material (or dry matter) remaining after removingmoisture from a sample. Usually expressed as a percentage of the as-receivedor wet weight. Moisture content plus TS (both expressed as percentage of wetweight) equals 100%.

Trommel A trommelis a screened cylinder used to separate materials by size - forexample, separating the biodegradable fraction of mixed municipal waste orseparating different sizes of crushed stone.

Ultimate methanepotential

This is a standard laboratory technique used to measure the anaerobicbiodegradability and associated methane yield from a given substrate. The testis run until no further gas production is detected and can last up to 100 days.The results can be influenced by the substrate concentration and particle size,the inoculum source, the food to microorganism ratio, and the presence orbuild-up of inhibitory compounds among others (also known as ultimatebiomethane potential, BMP, and Bo).

Upflow anaerobic sludgebasket (UASB)

UASB digestion is a generic, mature technology specifically designed for thetreatment of high strength wastewaters, such as in dairy and candymanufacture and other industries.

Volatile organiccompounds (VOCs)

VOCs are organic chemicals that have a high vapor pressure at ordinary, room-temperature conditions. Their high vapor pressure results from a low boilingpoint, which causes large numbers of molecules to evaporate or sublimate from

the liquid or solid form of the compound and enter the surrounding air.

Volatile solids (VS) The amount of combustible material in a sample (the remainder is ash). Thevalue is usually reported as a percentage of the TS, but may occasionally begiven as a fraction of the wet weight.VSis used as an indicator or proxy forthe biodegradability of a material, though recalcitrant biomass (i.e., lignin)which is part of the VS is less digestible.
http://en.wikipedia.org/wiki/Thermochemicalhttp://en.wikipedia.org/wiki/Thermochemical


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1.0 Introduction

Biodegradation of organic material occurs in nature through the action of both aerobic and

anaerobic microorganisms. In aerobic systems, partial oxidation of the ingested organicmaterial is the result yielding carbon dioxide and water and undigested residue. Anaerobicbacteria will also degrade organic matter, including some abiogenic forms, in the absence ofoxygen with ultimate products being nonreactive residues, carbon dioxide, and methane.

These bacteria naturally occur in the environment in anaerobic niches such as marshes,sediments, wetlands, and the digestive tracts of ruminants and certain species of insects.Facultative organisms are capable of persisting in either environment.

Anaerobic digestion (AD) is a bacterial fermentation process that operates without freeoxygen and results in a biogas containing mostly methane and carbon dioxide. It occursnaturally in anaerobic niches such as marshes, sediments, wetlands, and the digestive tractsof ruminants and certain species of insects. AD is also the principal decomposition processoccurring in landfills.

AD systems are employed in many wastewater treatment facilities for sludge degradation andstabilization, and are used in engineered anaerobic digesters to treat high-strength industrialand food processing wastewaters prior to discharge. There are also many instances of ADapplied at animal feeding operations and dairies to mitigate some of the impacts of manureand for energy production.

AD of municipal solid waste (MSW) is used in different regions worldwide to:

Reduce the amount of material being landfilled;Stabilize organic material before disposal in order to reduce future environmentalimpacts from air and water emissions; andRecover energy.

Over the past 30 years, AD of MSW technology has advanced inEurope because of waste management policies enacted to reducethe long-term health and environmental impacts of landfill disposal.

This has led to relatively high landfill tipping fees, which, incombination with generous prices paid for renewable energy, hascreated an active commercial market for AD and other MSWtreatment technologies in Europe. Installed AD capacity for thetreatment of MSW in Europe is estimated to be more than 6 milliontons per year.

In some parts of Europe, source separation of the organic fraction ofmunicipal solid waste (OFMSW) is common and even mandatory,

which contributes to the growth of biological treatment industries.Regions outside of Europe are also enacting more stringent wastedisposal regulations, leading to the development of new AD andother MSW conversion plants.

Generally in the U.S. landfills continue to be the lowest-cost option for managing MSW, sinceunlike Europe and Japan, space for new landfills is not as scarce, waste management policiesare less rigorous, and full life-cycle costs and impacts are not accounted for. Furthermore, theenergy market and regulatory mechanisms for licensing MSW AD and other conversionfacilities in most of the U.S. have not been developed to easily accommodate commercialsystems.

Composting of the OFMSW has increased significantly over the past 20 years, particularly for

source-separated wastes, but by far the majority of the yard and food waste generated stillgoes to landfills. AD facilities are capable of producing energy and reducing the biodegradablecontent of the organic waste prior to composting, which reduces emissions of pollutants andgreenhouse gases. However, in many parts of the world these environmental and public

Investors and cityplanners will bemore likely to adoptAD of MSW if

additional revenuesare providedinitially; theserevenues can come

from supports for

the energy produced,(i.e., tax credits andguaranteedmarkets), increasedtipping fees and,

potentially, green orcarbon credits


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health benefits have not been adequately internalized economically, especially considering thelack of familiarity with the technology. Investors and city planners will be more likely to adoptAD of MSW if additional revenues are provided initially. These revenues can come from

supports for the energy produced (i.e. tax credits and guaranteed markets), increased tippingfees and, potentially, green or carbon credits.

Many European countries have passed laws mandating that utilitycompanies purchase green energy, whereas in the U.S. few of thefarms or wastewater treatment facilities that produce excesselectricity from biogas have secured contracts with the utilities.Additionally, while European Union directives have called formandatory pre-treatment and decreased disposal of biodegradablematerial in landfills, no equivalent regulations exist in U.S. federal orstate codes. However, waste diversion requirements or targets existin many states in the U.S., and reducing OFMSW disposal has beena focus of waste managers and municipalities attempting to achieve

the targets. Nonetheless, interest in AD of MSW is growing, andseveral U.S. jurisdictions are investigating landfill alternatives thatinclude AD. The technologies have been used successfully for overtwenty five years in Europe where the industry continues to expand. Facilities were also builtrecently in Canada, Japan, Australia and several other countries.

The European market has shown a large preference for single-stage over two-stage digestersand a slight preference for dry digestion systems over wet systems. However, the choice ofAD technology depends on the composition of the waste stream, co-product markets, andother site-specific requirements. The design of any new digester facility should be based on athorough feasibility study, and special attention should be paid to all aspects of the treatment

process, including waste collection and transportation, pre-treatment processing (i.e. pulping,grinding, and sieving), material handling, post-treatment processing (i.e. aeration and

wastewater treatment), public education, and strategic location of the system to be sited.

Landfill bioreactors may merit further consideration in their own right, but special attentionshould be paid to their performance and air/water emissions. In addition to electricity, othervalue-added product streams from AD systems could provide revenue to help improve theeconomic viability of organic waste treatment technologies. For example, technologies forupgrading biogas to natural-gas quality biomethane are available, as are technologies thatutilize lignocellulosic materials which include residues from digesters. However, regulatoryand definitional barriers need to be minimized in order to fully capitalize on these technologiesand product streams.

The public desire for change in waste management practices will lead to a reduction in landfillavailability. AD and other conversion technologies have the potential to minimize the

environmental impact of waste disposal by reducing the amount of biodegradable materials inlandfills. Public policies that encourage organic solid waste disposal reduction will help tofacilitate the adoption of such technologies. In addition, as the technologies advance, theirinstallation costs should decrease. Equally important, AD technology developers need to workclosely with waste collection and management companies in order to develop and implementappropriate digester system designs and material handling strategies and achieve successfulenterprises.

More states have toadopt laws (that are

enforced),mandating, thatutility companiespurchase green

energy; requiring

also the pre-treatment anddecreased disposalof biodegradablematerial in landfills


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2.0 Background

AD is a biological process typically employed in many wastewater treatment facilities for

sludge degradation and stabilization, and it is the principal biological process occurring inlandfills.

Internationally, AD has been used for decades, primarily in rural areas, for the production ofbiogas for use as a cooking and lighting fuel. Many household-scale digesters are employed inrural China and India for waste treatment and gas production. Over the past twenty plusyears, Europe has developed large-scale centralized systems for municipal solid wastetreatment with electricity generation as a co-product. Other industrialized countries havefollowed the European model.

Biodegradation of organic material occurs in nature principally through the action of aerobicmicroorganisms. Ultimately, complete oxidation of the carbonaceous organic materials resultsin the production of carbon dioxide (CO2) and water (H2O). Anaerobic microorganisms

degrade the organic matter in the absence of oxygen with ultimate products being CO2 andmethane (CH4), although lignin and lignin-encased biomass degrade very slowly. Anaerobicmicroorganisms occur naturally in low-oxygen niches such as marshes, sediments, wetlands,and in the digestive tract of ruminant animals and certain species of insects.

2.1 Digestion Process Description

The anaerobic digestion of organic material is accomplished by a consortium ofmicroorganisms working synergistically. Digestion occurs in a four-step process: Hydrolysis,Acidogenesis, Acetogenesis, and Methanogenesis (Figure 1):

1. Step 1. Large proteinmacromolecules, fats and

carbohydrate polymers(such as cellulose and

starch) are broken downthrough hydrolysis toamino acids, long-chainfatty acids, and sugars.

2. Step 2. These products arethen fermented duringacidogenesis to form three,four, and five-carbonvolatile fatty acids, such aslactic, butyric, propionic,

and valeric acid.

3. Step 3. In acetogenesis,bacteria consume thesefermentation products andgenerate acetic acid,carbon dioxide, andhydrogen.

4. Step 4. Finally,methanogenic organismsconsume the acetate,hydrogen, and some of the

carbon dioxide to produce methane. Three biochemical pathways are used bymethanogens to produce methane gas. The pathways along with the stoichiometries ofthe overall chemical reactions are:

Figure 1. Anaerobic Digestion Four-step ProcessSource: Soil Water & Life Solutions


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a. Acetotrophic methanogenesis: 4 CH3COOH 4 CO2 + 4 CH4b. Hydrogenotrophic methanogenesis: CO2 + 4 H2 CH4 + 2 H2Oc. Methylotrophic methanogenesis: 4 CH3OH + 6 H2 3 CH4 + 2 H2O

Methanol is shown as the substrate for the methylotrophic pathway, although othermethylated substrates can be converted. Sugars and sugar-containing polymers such asstarch and cellulose yield one mole of acetate per mole of sugar degraded. Since acetotrophicmethanogenesis is the primary pathway used, theoretical yield calculations are often madeusing this pathway alone.

From the stoichiometry above, it can be seen that the biogasproduced would theoretically contain 50% methane and 50% carbondioxide. However, acetogenesis typically produces some hydrogen,and for every four moles of hydrogen consumed byhydrogenotrophic methanogens a mole of carbon dioxide isconverted to methane. Substrates other than sugar, such as fats

and proteins, can yield larger amounts of hydrogen leading to highertypical methane content for these substrates. Furthermore,hydrogen and acetate can be biochemical substrates for a number ofother products as well. Therefore, the overall biogas yield and

methane content will vary for different substrates, biologicalconsortia and digester conditions. Typically, the methane content ofbiogas ranges from 40-70% (by volume).

Anaerobic conditions are required for healthy methanogenesis to occur. This means that thereactors used must be well sealed which allows the biogas to be collected for energyconversion and eliminates methane emissions during the anaerobic digestion process. Inaddition to methane and carbon dioxide, semi-harmful contaminants such as hydrogen sulfideand ammonia are produced, albeit in much smaller amounts (


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kinetics which, in practice, translates to smaller reactors needed to process a given wastestream. However, the micro-organisms themselves are adapted to relatively narrowtemperature ranges. Mesophilic and thermophilic microbes are adapted to roughly 30-40 C

(86-104 F) and 50-60 C (122-140 F) respectively.

2.2 State of Municipal Solid Waste disposal in the U.S. and Europe

In 2007, the U.S. generated about 255 million tons of MSW; about, 63 million tons (24.7%)were recycled, 22 million tons composted (8.5%) and 32 million tons combusted with energyrecovery (12.5%). The remaining about 138 million tons (54.3%) were discarded in landfills.

Handling and treatment of OFMSW is more difficult than treating wastewater or manure. Assuch, the AD of OFMSW requires a larger amount of investment and technological experience.Furthermore, capital and operating costs are higher for AD than for composting or landfilling.The low tipping fees charged by landfills in the U.S. and relatively low energy prices comparedto those in Europe make it difficult for AD and other conversion technologies to be cost-

competitive. However, life-cycle analyses (LCA) have shown that AD of MSW reduces theenvironmental impact and is more cost-effective (in Europe) on a whole-system basis thanlandfilling or composting over the life of the project.

The European Union (EU) passed a regulation in 2002 to standardize reporting and timing ofdata collection for waste disposal, and the first year of data was included in the most recentEurostat Yearbook (2006-2007); for comparison with past years, only average per capitastatistics were reported. As of 2004, Europeans disposed of an average of 0.6 MT (0.7 tons)of MSW; however, unlike U.S. statistics on MSW production, this waste did not includeconstruction and demolition debris which makes up 30 % of the reported MSW in the U.S.Nonetheless, the Western European average per capita disposal was almost half of the U.S.

average.

Figure 2. U.S. Total MSW generation Figure 3. U.S. Per Capita MSW generationSource: United States Environmental Protection Agency Source: United States Environmental Protection Agency

In Europe, the per capita MSW production increased over the past ten years, but landfilldisposal declined slightly. Per capita combustion with energy recovery has remained relativelyconstant while composting, recycling, and other treatments almost doubled since 1995.OFMSW typically comprises 50-60% by weight of the solid waste stream collected bymunicipalities in Europe that do not practice source separation. In 2004, this totaled some200 million MT (220 million tons).

In 1999, the EU adopted the Landfill Directive (Council Directive 99/31/EC) which becameenforceable in 2001. It required the biodegradable portion of MSW to be reduced by 25% ofthat disposed in 1995 within five years, 50% within eight years, and 65% within 15 years.

Furthermore, Article 6(a) required that all waste that gets landfilled must be treated, with theexception of inert materials for which treatment is not technically feasible. Each country in

the EU is held to this standard as a minimum requirement, but in practice Germany, Austria,Denmark, Luxembourg, the Netherlands, and Belgium had already imposed such restrictionsand now have even stricter requirements while France, Italy, Sweden, England and Finland


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converted their facilities subsequent to adopting the law. Greece, Ireland, Portugal and Spainwere in the process of converting their facilities as of 2004.

As a consequence, installed AD capacity in Europe has increased sharply and now stands atmore than 4 million tons annual capacity (Figures 4 and 5). Most notably, Spain recentlyinstalled several large-scale AD facilities and now processes over 1 million tons OFMSW peryear which accounts for over 50% of the organic waste produced there.

In Germany the Recycling and Waste Law, the Directive on Residential Waste Disposal, theFederal Ordinance on Handling of Biowaste, and the Ordinance for Environmentally SoundLandfilling set stringent limits for the composition of treated MSW prior to disposal in landfills.For example, in Germany the upper limits on total organic carbon and energy content ofmaterial going to landfill were set at 18% and 6,000 kJ/kg (2,580 BTU/lb).

Furthermore, energy prices in Europe are generally higher than in the U.S. and manyEuropean countries provide financial incentives to renewable energy producers. For example,

in Germany the Renewable Energy Act guaranteed renewable electricity producers a highpercentage of the retail electricity price (75-90%) with biomass earning 0.15-0.25 $/kWh(converted from Euros to PPP-adjusted U.S. dollars). Tariffs with prescribed annual reductionswere guaranteed for up to 20 years. The regulation also required utilities to connectrenewable producers to the grid. In addition many AD of MSW facilities in the EU also sellgreen certificates and carbon credits. Direct subsidies and soft loans are also used to supportnew renewable energy producers.


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3.0 Categories of Engineered Anaerobic Digestion Systems

The most common MSW Anaerobic Digestion technologies are:

One-stage Continuous Systems- Low-solids or Wet- High-solids or DryTwo-stage Continuous Systems- Dry-Wet- Wet-WetBatch Systems- One Stage- Two Stage

Single-stage digesters aresimple to design, build, andoperate and are generally less

expensive. The organic loadingrate (OLR) of single-stagedigesters is limited by the abilityof methanogenic organisms to

tolerate the sudden decline in pHthat results from rapid acidproduction during hydrolysis.Two-stage digesters separatethe initial hydrolysis and acid-producing fermentation from

methanogenesis, which allowsfor higher loading rates butrequires additional reactors andhandling systems. In Europe,about 90% of the installed ADcapacity is from single-stagesystems and about 10% is fromtwo-stage systems (Figure 4).

Another important design parameter is the total solids (TS) concentration in the reactor,expressed as a fraction of the wet mass of the prepared feedstock. The remainder of the wetmass is water by definition. The classification scheme for solids content is usually described asbeing either high-solids or low-solids. High-solids systems are also called dry systems and

low-solids systems may be referred to as wet systems. A prepared feedstock stream with lessthan 15% TS is considered wet and feedstock with TS greater than 15-20% are considereddry (although there is no established standard for the cutoff point). Feedstock is typicallydiluted with process water to achieve the desirable solids content during the preparationstages.

Before AD became an accepted technology for treating MSW, single-stage wet digesters wereused for treating agricultural and municipal wastewater. However, MSW slurry behavesdifferently than wastewater sludge. Because of the heterogeneous nature of MSW, the slurrytends to separate and form a scum layer which prevents the bacteria from degrading theseorganics. The scum layer tends to evade the pump outlets and can clog pumps and pipeswhen it is removed from the reactors. To prevent this, pretreatment to remove inert solidsand homogenize the waste is required. Solids can also short circuit to the effluent pipe before

they have broken down completely, therefore design modifications were made to allow longercontact time between bacteria and dense, recalcitrant material. Furthermore, MSW tends tocontain a higher percentage of toxic and inhibitory compounds than wastewater. In dilutedslurry, these compounds diffuse quickly and evenly throughout the reactor. In high enough

Figure 4. Growth of MSW AD technology by number of stages in EuropeSource: Soil Water & Life Solutions


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concentrations, this can shock the microorganisms, whereas in a dry system the lowerdiffusion rate protects the microbes.

Because of these constraints,dry systems have becomeprevalent in Europe (Figure 5),making up 60% of the single-stage digester capacity installedto date. Dry digesters treatwaste streams with 20-40%total solids without addingdilution water. However, thesesystems may retain someprocess water or add somewater either as liquid or in theform of steam used to heat the

incoming feedstock.Furthermore, as organic matterbreaks down, the internal MC ofthe digester will increase (from64-72%).

Nonetheless, heavy duty pumps,conveyors, and augers arerequired for handling the waste,which adds to the systems capital costs. Some of this additional cost is offset by the

reduction in pretreatment equipment required. Most dry digesters operate as plug flow

digesters, but due to the viscosity of the feed, the incoming waste does not mix with thecontents of the digester. This prevents inoculation of the incoming waste which can lead to

local overloading. Therefore, most of the digester designs include an inoculation loop in whichthe incoming OFMSW is mixed with some of the exiting digestate paste prior to loading.

Multi-stage systems are designed to take advantage of the fact that different portions of theoverall biochemical process have different optimal conditions. By optimizing each stageseparately, the overall rate can be increased. Typically, two-stage processes attempt tooptimize the hydrolysis and fermentative acidification reactions in the first stage where therate is limited by hydrolysis of complex carbohydrates. The second stage is optimized formethanogenesis where the rate in this stage is limited by microbial growth kinetics. Sincemethanogenic archaea prefer pH in the range of 78.5 while acidogenic bacteria prefer lowerpH, the organic acids are diluted into the second stage at a controlled rate. Often a closedrecirculation loop is provided to allow greater contact time for the unhydrolyzed organicmatter.

Some multi-stage systems apply a microaerophilic process in an attempt to increase theoxidation of lignin and make more cellulose available for hydrolysis. Although adding oxygento an anaerobic environment seems counterintuitive, sludge granules can shield the obligateanaerobes from oxygen poisoning and the practice has been shown to increase biogas yield insome situations. In two-stage systems, because methanogens are more sensitive to oxygenexposure than fermentative bacteria, the air may preferentially inhibit methanogens, whichcould help maintain a low pH in the hydrolysis stage. However, if the oxygen is not completely

consumed and the biogas contains a mixture of oxygen and hydrogen and/or methane,hazardous conditions could be created.

Process flexibility is one of the advantages of multi-stage systems. However, this flexibilityalso increases cost and complexity by requiring additional reactors, material handling and

process control systems. On the opposite end of the spectrum, batch or sequential batchsystems aim to reduce complexity and material handling requirements. As opposed tocontinuous wet and dry systems, the feedstock does not need to be carefully metered into abatch reactor, thereby eliminating the need for complex material handling equipment. The

Figure 5. Growth of MSW anaerobic digester technology by solids content (20% TS = Dry) in EuropeSource: Soil Water & Life Solutions


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primary disadvantage of batch digesters is uneven gas production and lack of stability in themicrobial population. To surmount these issues, batch systems can also be combined withmulti-stage configurations.

3.1 Material Handling Systems A typical sorting line

Figure 6. A Municipal Waste Treatment Plant Diagram Material Handling SystemsSource: Ecoenergy Gesellschaft fur Energie- und Umwelttechnik mbh

Extensive pre- and post-digestion processing units, regardless of the waste source or digestertype are used. Pre-sorting is necessary to prevent clogging of the pumps and to reduce theamount of reactor volume occupied by inert material. Even source-separated waste inevitably

contains metal and plastic contaminants and must be pre-sorted. A typical sorting lineincludes the following components (also seen in Figure 6 above and Table 1 below):

Receiving- Can include some visual (manual or robotic) sorting and removal of bulky or

potentially harmful items;- Provides a buffer for inflow rate fluctuations.Particle size reduction- Can be mechanical and/or biological;- Relies on the relative ease of reducing the particle size of the organic fraction.Separation

- Can be based on magnetism, density, and size.The receiving area allows for unloading of raw MSW and isolation of MSW from differentsources. Some receiving areas use robotics to minimize human contact with the waste. Others


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incorporate a sorting line for workers to manually remove the most obvious inorganicmaterials. Once the MSW has been loaded into the mechanical separation system, humancontact is minimal as biological and mechanical processes prepare the MSW for density and/or

size separation.

Density separation requireswetting the MSW; therefore it ismore commonly applied whenusing low-solids digesters.Organic material breaks intosmaller particles more easilythan inorganic material,therefore a mechanicalmacerator or agitator is oftenemployed prior to screening. Inaddition, some aerobic treatment

can help break down the organicmatter. This may also beaccompanied by a loss ofdigestible organic matter;therefore short retention timesare used. Between several hoursand one or two days is typical forrotating drums, or biomixers,

which combine agitation with aerobic treatment.

Organic materials have high biogas and methane yields even when the MSW had spent only

24 hours in the rotating drum.

In a rotating drum system, a sieve may line the sides of the drum allowing undersizedparticles to pass to the dosing unit while expelling oversized, primarily inorganic, particles.

Alternatively, the waste may pass through one or more trommel screens after the drum forsieving. Dosing units store mixed waste to even out fluctuations in the content and volume ofMSW going to the digester. They can also be used for heating and inoculating the digesterfeed. Heat may be added as steam, which can be produced using waste heat from enginegenerators. Some systems have a separate feed mixer which combines the sorted MSW withdigester paste in order to inoculate the new feed and bring it to the appropriate MC.

In Bassano, Italy a Valorga digester accepts source-separated waste and grey waste. As canbe seen from the diagram below (Figure 8), even source-separated waste passes through aprimary sieve and a magnetic metals removal unit.

Figure 8. MSW Treatment Plant at Bassano, Italy Pre-processing DiagramSource: Bolzonella, D., P. Pavan, S. Mace, and F. Cecchi. Dry Anaerobic digestion of differently sorted organic municipal waste: A full scaleexperience. Water Science & Technology. 2006. 53(8): p. 23-32 Modified by SWLS

The grey waste which is the inorganic fraction of the source-separated waste consistsprimarily of inorganic materials. (In fact, organics make up only 10-16% of this material, andpaper makes up an additional 34-50%).

Figure 7. By-products and extracted inert material of an MSW Treatment PlantSource: Ecoenergy Gesellschaft fur Energie- und Umwelttechnik mbh


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The grey waste passes through an additional drum screen and densimetric separator whichsuspends the waste in water, removing the floating layer as well as the heavy particles thatsink to the bottom.

The Treviso wastewater treatment facility found its anaerobic digesters to be too large forprocessing waste activated sludge (WAS) only, so they built a separation unit to remove theorganic fraction of MSW for co-digestion with the sludge. As can be seen in Figure 9, thewaste passes through a shredder and magnetic separator, then a second shredder andtrommel screeners, and finally a density separator. The emerging waste is 96% organics andpaper as compared with 76% for the incoming waste and 24% of the incoming organic andpaper materials are lost during the sorting process. Metals are reduced by 100%, plastics arereduced by 93%, and glass is reduced by 98%.

The digestate that exits ananaerobic digester containsundigested organics that will

continue to break down if nottreated further. This can lead tomethane emissions typically notaccounted for when analyzingthe environmental impact of AD.In the EU and particularly inGermany, where the compositionof OFMSW entering a landfill istightly regulated, extensive post-treatment processing isincorporated into the AD facility.

This eliminates transportationcosts which could be quite high

considering the relatively highMC (40-50%) of the exiting

digestate.

It should be noted, however, that the inorganic materials separated from the incoming MSWstream still have to be transported to a processing facility, typically a material recoveryfacility or landfill. Dewatering units allow for the re-capture of process water which canprovide inoculants and reduce the cost of adding water to the digester. Novel digesters cansubject digester paste to a steam treatment step followed by a second digester in order toproduce a high quality peat for use as a planting medium.

# Separation Technique Separation property Materials targeted Key concerns

1 Trommels and screens Size

Oversize (paper, plastic);

Small (organics, glass, fines)

Air containment and

cleaning

2 Manual separation Visual examination Plastics, contaminants, oversizeEthic of role, health and

safety issues

3 Magnetic separation Magnetic properties Ferrous metals Proven technique

4 Eddy current separation Electrical conductivity Non ferrous metals Proven technique

5 Wet separation technology Differential densitiesFloats (plastics, organics);

Sinks (Stones, glass)

Produces wet waste

streams

6 Air classification WeightLight (plastics, paper);Heavy (stones, glass)

Air cleaning

7 Ballistic separation Density and elasticityLight (plastics, paper);Heavy (stones, glass)

Rates of throughput

8 Optical separation Diffraction Specific plastic polymers Rates of throughput

Table 1. Municipal waste separation techniquesSource: Soil Water & Life Solutions

Figure 9. Mass Balance of the Treviso Wastewater Treatment DigesterSource: Soil Water & Life Solutions


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4.0 Review of Commercial Anaerobic Digestion Technologies for MSW Treatment

A number of commercial vendors have designed a variety of digesters for the global market

(Table 2). These commercial systems span the full range of categories of engineered ADsystems. The following review attempts to summarize the research reported in the literaturefor a number of the existing and emerging systems, with special attention paid to the mostcommercially successful and innovative systems.

Number Capacity Number Total Operating

Of range of stages solids content Temperature

Process System Name Plants (tons/annum) 1 2 20% 35C 55C

AAT 8 3,000 55,000 X x x

ArrowBio 4 90,000 180,000 x x x

BTA 23 1,000 150,000 X x x x x

Biocel 1 35,000 X x x

Biopercolat 1 100,000 x x x

Biostab 13 10,000 90,000 x x x

DBA-Wabio 4 6,000 60,000 x x x

Dranco 17 3,000 120,000 x x x

Entec 2 40,000 150,000 x x x

Haase 4 50,000 200,000 x x x x

Kompogas 38 1,000 110,000 x x x

Linde-KCA/BRV 8 15,000 150,000 x x x x x x

Preseco 2 24,000 30,000

Schwarting-Uhde 3 25,000 87,6000 x x x

Valorga 22 10,000 270,000 x x x x

Waasa 10 3,000 230,000 x x x x

Table 2. Commercial anaerobic digester technologies with large scale reference plans in EuropeSource: Nichols, C.E., Overview of anaerobic digestion technologies in Europe. BioCycle. 2004. 45(1): p. 47-53 and SWLS

4.1 Single-stage Wet Systems

Single-stage wet systems have been built by a number of different companies throughoutEurope. Since this was the most familiar configuration from wastewater treatment, it was oneof the first systems tested on OFMSW.

Below, the Wabio and Waasa systems are described in more detail, but other companies havealso provided components and full scale systems to many wet OFMSW digesters, most notablyBiotechnische Abfallverwertung GmbH & Co. KG (BTA), Ecoenergy Gesellschaft fr Energie-und Umwelttechnik mbh (Schubio), and Linde-KCA-Dresden GmbH.

4.1.1 The Wabio Process

For the first time fermentation of municipal solid waste on industrial scale has beenimplemented in 1989 in Vaasa, Finland with the Wabio-Process. The Wabio-Process wasdeveloped by Outokumpu EcoEnergy Oy and the first plant has been in continuous operationsince then. Outokumpu EcoEnergy also licensed its Wabio to Deutsche Babcock Anlagen(DBA) which erected in 1992 in Bottrop, Germanys first fermentation plant for biowaste. After

DBA merged with the company Steinmller in 1999, the DBA-Wabio-Process was abandoned.

The Wabio-Process is an anaerobic digestion process that operates at higher feed rates thanother digestion systems. In the processors, inert material and both lighter and heavierfractions can be separated from the reactor feed, thus improving the quality of the humus.

In the biothermal method waste material is pretreated so that the biologically good-

decomposable material, the so called compost fraction, and the material suitable forcombustion, the refuse-derived fuel, are separated. In the pretreatment also magnetic metalsare removed from the waste material. The compost fraction is decomposed biologically. Theraw material is the compost fraction of municipal solid waste material for the compost and the


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sludge from the sewage disposal works and the final product is humic earth and aconsiderable amount of biogas.

The refuse-derived fuel (RDF) of the biothermal energy method is acombustion material which is homogenized in the pretreatment and ofwhich heat content is about 20% higher than that of wooden chips.The refuse-derived fuel can advantageously be burned for example inthe combination of a fluidized bed boiler and an after-burning chamber(Figure 10).

In order that the combustion of the municipal solid waste materialsshould be safe in regard to the environment, on one hand a sufficientlow combustion temperature is required so that nitrogen oxides arenot simultaneously created when the concentration of carbonmonoxide is small, and on the other hand the temperature must besufficiently high that the organic compounds are fully decomposed.

In the Wabio-Process (see Figures 11 and 12),when using biogas as a burning material in theafter-burning chamber of the combustion gases ofthe refuse-derived fuel, the temperature can beadjusted advantageous for the essentiallycomplete burning and the decomposing of theharmful components, such as the polyaromatichydrocarbons (PAH) and the polychlorateddibenzo-p-dioxines (PCDD) and the dibenzo-furans(PCDF), which are possible in the municipal solid

waste material.

The after-burning chamber used is shaped so thatit forms an essentially closed system with the

fluidized bed boiler used in the treatment of therefuse-derived fuel and that after the connectionbetween the fluidized bed boiler and the after-burning chamber the gas flowing is essentiallygoing downwards. The biogas used as a burningmaterial in the after-burning chamber consists 60-65% methane and the balance carbon dioxide andis fed into the after-burning chamber through burners positioned perpendicular to thedirection of the gas flow in the after-burning chamber.

The highest point at which

biogas used as a burningmaterial is fed into the after-burning chamber isadvantageously sufficiently lowdown that the temperature atthe connection between thefluidized bed chamber and theafter-burning chamber is not

increased so that ash will not befused at the level of theconnection between the fluidizedbed chamber and the after-burning chamber.

When the gas flow is essentiallydownwards in the after-burning chamber, the ash that is at least partly fused under thetreatment of the combustion gases can be removed from the lower part of the chamber while

Figure 11. Wabio - Fluidized bed boiler and after-

burning chamberSource: United States Patent US4934285

Figure 10. RDF pelletsSource: SWLS

Figure 12. The Wabio Process Basic Flow DiagramSource: Outokumpu EcoEnergy Oy Modified by SWLS


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the gases that are essentially free of the components harmful for the environment can beconducted to a further treatment.

4.1.2 The Waasa Process (by Deutsche Babcock Anlagen)

The Waasa system, built in early 1990s was oneof the original MSW digesters. Today there are atleast ten operational Waasa plants in Europe (seeTable 2).

The Waasa system consists of a vertical pulperthat homogenizes the incoming MSW and removesfloating debris from the surface and sunken gritfrom the bottom of the pulper. Density-fractionated MSW is then pumped to the pre-chamber of a continuously stirred tank reactor

(see Figures 13 and 14). The pre-chamber helpsalleviate short circuiting and an inoculation loopensures that incoming waste is exposed tomicroorganisms in order to minimize acid buildup.

For the organic fraction of MSW to be used in thissystem, it must undergo pretreatment in a pulper which shreds, homogenizes, and dilutes thematerial to the desired concentration of total solids (10 to 15% TS). Recycled process water

and some fresh makeup water are used in the dilution. The slurry is then digested in large

complete mix (completely stirred) reactors. The pretreatment required to obtain adequate

slurry quality while removing coarse or heavy contaminants is complex and inevitably incur a15 to 25% loss of volatile solids from the portion reaching the digester.

Mechanical impellers andinjection of a portion of thebiogas into the bottom of thereactor tank are used to keepthe material continuously stirredand as homogenous as possible.

To reduce shortcircuiting of the

feed (i.e., passage of a portionof the feed through the reactorwith a shorter retention timethan that for the average bulk

material), a prechamber within

the main reactor tank is used.

Fresh material from the pulper

enters the prechamber along

with some of the biomass from

the main tank for inoculation. The prechamber operates in plug flow taking a day or two

before the material makes its way into the main reactor, thus ensuring all material entering

the process has a guaranteed few days retention time. Even with the prechamber

arrangement, enough shortcircuiting occurs that all pathogens are not eliminated requiring a

pasteurization step in the pretreatment. Steam is injected in the pulper to maintain feed at

70C for one hour.

The process can be operated at both thermophilic and mesophilic temperatures and both

types can run in parallel (the thermophilic process has a retention time of 10 days while 20days is required in the mesophilic design). The operational performance indicates that gas

production is in the range 100 to 150 m/ton of biowaste added, volume reduction of 60%,

weight reduction 50 to 60%, and a 20 to 30% internal consumption (heat) of biogas. The

Figure 13. The Waasa System Flow of methanefermentation processSource: Deutsche Babcock Anlagen

Figure 14. The Waasa Process Basic Flow DiagramSource: Soil Water & Life Solutions


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digestate can be further treated by aerobic composting, but this depends on the wastequality.

The Waasa plant located in Groningen, Netherlands, has four 2,740 m (725,000 gal) tankstreat 92,000 MT/y (101,000 tons/y) of OFMSW out of an initial 250,000 MT/y (275,000tons/y) of raw MSW. This system produces 0.10-0.15 m/kg (3.2-4.8 scf/lb) biogas from wetsource-separated waste, with a weight reduction of 50-60%. This is a relatively high biogasyield, indicating high digestibility of the feedstock and good conversion efficiency in thedigester.

The typical OLR for a single-stage wet system is 4-8 kg VS/m p.a. (0.033-0.066 lbsVS/gal/y). Assuming 15% of the reactor volume is gas head space, the working volume wouldbe 9,350m (2,470,000 gal), thus the wet loading rate would be 27 kg/d (59 lbs/d) and theresulting VS content would be 20-40%. Assuming 30% VS content and a biogas yield per wetton of 125 m (4,410 scf), the average specific biogas yield would be 0.417 m/kg VS (13.4scf/lb).

4.1.3 The BIMA Digester (by Entec Biogas GmbH)

Entec Biogas GmbH of Austria builds digesters that treat primarily agricultural, industrial, andmunicipal wastewater. One system designed for Schaalsee Biogas & Recycling GmbH in Kogel,Germany treats food and restaurant waste from Hamburg and Mecklenburg Vorpommern intwo 2,600 m (690,000 gal) constantly stirred tank reactors. The operation of the systemmirrors that of the Waasa digester.

The company also designed a self-mixing system known as the BIMA digester whicheliminates mechanical mixing by utilizing the pressure differential between two chambers

within the reactor. The two-chamber system uses the produced biogas to create a leveldifference in the chambers and in this way builds up a mixing pressure of up to 500 mbar.

The turbulent mixing occurs against the biogas production in intervals of 4-10 times a day.Ideal applications of this system are high solid sludge and waste, such as in the sewage

sludge treatment, treatment of organic solid wastewater, manure, organic household andindustrial waste, etc.

4.2 Single-stage Dry Systems

In dry, or high-solids, systems, the digester contents are kept at a solids content of 20-40%TS (equivalent to 60-80% MC). Handling material at high solids concentration requiresdifferent pre-treatment and transfer equipment (i.e., conveyor belts, screws, and specialpumps for the highly viscous streams). Research in the 1980s indicated that biogas yields andproduction rates for single-stage dry systems were as high as or greater than that of wetsystems. The challenge of dry systems is handling, mixing, and pumping the high-solids

streams rather than maintaining the biochemical reactions.

Although some of the handling equipment (such as pumps capable of handling high-solidsslurries) may be more expensive than those for wet systems, the dry systems are morerobust and flexible regarding acceptance of rocks, glass, metals, plastics, and wood pieces inthe reactor. These materials are not biodegradable and will not contribute to biogasproduction but they generally can pass through the reactor without affecting conversion of thebiomass components.

The only pretreatment required is removal of the larger pieces (greater than 5 cm [2 in]), andminimal dilution with water to keep the solids content in the desired range. This allows forreduced sorting equipment costs which can offset some of the additional material handlingexpenses.

Because of their high viscosity, loading rate, and rapid hydrolysis, materials in dry reactorsmove via plug flow (materials added on one end of the digester push older materials towardthe opposite end), and the incoming feedstock needs to be inoculated or mixed to avoid


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localized acid buildup. Two of the most commonly used commercial-scale designs inoculatethe feedstock by mixing it with a portion of the digested material, while another incorporatesmixing via high-pressure biogas injection (see Figures 15, 16, 17). All three systems operate

as plug-flow digesters.

4.2.1 The Dranco Process (by Organic Waste Systems nv)

The Dranco process wasdeveloped in the late 1980s. It isa high-solids, single-stageanaerobic digestion system thatoperates at thermophilictemperatures (Figure 15).

Feed is introduced into the top ofthe reactor and moves

downward to the conical bottomwhere an auger removesdigestate.

A fraction of the digestate istransferred to the mixing pumpwhere it is blended with freshfeed to inoculate the materialand steam to bring the feed tothe working temperature. Therest of the digestate is

dewatered to produce processwater and press cake.

There is no mixing within the reactor, other than that brought about by the downward, plug-

flow movement of the waste and some biogenic gas that bubbles upwards. The press cakecontains active bacteria, some ammonia and undigested solids and must be aerobicallystabilized for use as agricultural compost. Source separated household and industrial wastesare preferred in order to maintain the quality of the compost.

In the Dranco digester of a plant in Brecht, Belgium a high average loading rate of 15 kgVS/m/d (0.13 lbs/gal/d) was maintained over a one year period. The conditions inside the

reactor were 35% TS and 14 day hydraulic retention time (HRT). The performance of theBrecht plant was reported as 65% VS destruction with a 0.103 m/kg (1.65 scf/lb) wet weightbiogas yield. The TS content in the feedstock was reported at 40% and the VS content (as apercentage of TS) was 55%. By inference, the specific biogas yield for the system was 0.468

m/kg VS (7.50 scf/lb VS). This relatively low yield along with the relatively low VSdestruction may indicate that a large portion of the VS loaded was recalcitrant which explainshow such a high loading rate was achieved.

To support this theory, it was reported that the waste composition was 15% kitchen waste,75% garden waste, and 10% paper, whereas a Dranco system in Salzburg, Austria treating80% kitchen waste and 20% garden waste achieved a biogas yield of 0.622 m/kg VS (9.96scf/lb). The VS destruction and OLR were not reported, but elsewhere it has been stated thatthe typical Dranco system is designed for 12 Kg/VS/m/d (0.1 lbs/gal/d).

As of February 2008, 18 commercial Dranco system plants exist in Belgium, Spain, Germany,Italy, Switzerland and Japan.

4.2.2 The Valorga Process (by Valorga International S.A.S.)

The Valorga process was developed in 1981 to treat organic solid waste and accepts MSWafter appropriate separation of the recalcitrant fraction. A high-solids digester is fed with

Figure 15. The Dranco Process Schematic & Mass BalanceSource: Organic Waste Systems nv Modified by SWLS


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OFMSW that has 25-30% TS content adjusted using steam for heating and process water fordiluting the incoming feed as needed. Mesophilic or thermophilic systems are used dependingon feedstock and economics (Figure 16).

The reactor is a continuoussingle-stage modified plug-flowreactor. Typical plug-flowreactors involve only naturalmixing, but the Valorga digesteruses pressurized biogas formixing. This eliminates the needfor an inoculation loop. Thereactor consists of a verticalouter cylinder with an inner wallextending to about 2/3 of thediameter of the tank (see Figure

16). Material enters at thebottom on one side of the innerwall and must flow around thewall before it can exit. Theretention time is on the order ofthree weeks. Biogas is injectedin the base of the reactor andthe bubbles serve as a means formixing and keeping solidssuspended. The digestate isdewatered and typically

composted. Feedstock, however,with less than 20% percent TS

do not always perform well in the Valorga system because dense grit particles settle out tooquickly and clog the gas recirculation vents. Biogas yields have been reported in the range of

0.220.27 m/kg VS (7.058.65 scf/lb VS) which corresponds to 0.800.16 m/wet kg (2.65.1 scf/wet lb), indicating a VS content in the OFMSW of 35-60%. The solid retention time is18-23 days and post digestion solids composting takes about two weeks.

In the Varennes-Jarcy Valorga facility, the plant treats a total of 100 MT/y (110 tons/y) ofMSW: 70 MT/y (77 tons/y) mechanically sorted OFMSW (MS-OFMSW) and 30 MT/y (33tons/y) source sorted OFMSW (SS-OFMSW). A remotely operated mechanical claw loads thewaste into a 50 m (160 ft) rotating drum with a 2-3 day retention time. Although the drum isnot aerated, the temperature of the MSW increases indicating the possibility of biologicalactivity to help break down the organic fraction into smaller particles which are screened out.These fines are sent to a dosing unit for storage and steam heating prior to being pumped

into one of the three 4,000 m (1 million gal) reactors. Under typical operating conditions theSS-OFMSW and MS-OFMSW are loaded into separate tanks and the digestate is treatedseparately, allowing plant operators to control the quality of the compost produced.Dewatering occurs in three steps resulting in process water containing less than 3% TS. Thesolids are transferred to an enclosed aeration bed where air heated by waste generator heatis blown through the curing piles and sucked through vents in the roof to a scrubber andbiofilter. Large automated mechanisms turn the compost and transfer it to a maturation bedafter 2-3 weeks.

As of February 2008 21 Valorga plants are operating in Spain, Germany, Italy, Switzerland,and the Netherlands.

4.2.3 The Kompogas Process (by Kompogas AG)

Unlike the Dranco and Valorga single-stage dry digesters, the Kompogas system utilizes ahorizontal plug flow digester with internal rotors to assist in degassing and homogenizing thewaste (see Figure 17 below).

Figure 16. The Valorga Process - Basic Flow DiagramSource: Valorga International S.A.S. Modified by SWLS


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The system is prefabricated intwo sizes: 15,000 or 25,000

MT/y (16,500 or 27,600 tons/y).Larger capacities can be acquiredby combining the units inparallel.

The internal MC has to becarefully maintained at 7277%in order for the system to flowproperly; therefore some of theprocess water and/or digestate ismixed with incoming OFMSW.This also ensures that incomingfeed is inoculated in order to

prevent excessive acid buildupnear the front end of thedigester.

In more detail, in order to produce energy from garden and kitchen waste, organic waste iscollected separately and freed of foreign matter by screening and manual sorting. The wasteis then conditioned (shredding, etc.) and transported to an intermediate storage facility.

The storage capacity of the intermediate facility guarantees continued and fully automatedoperation of the fermentation process. During this period of storage, hydrolytic and self-heating processes take place, which favor the ensuing fermentation.

Transportation to the fermenter is done by a powerful hydraulic pump. In the enclosed

reactor, micro-organisms transform the organic substance in the material into compost andbiogas. The process takes place at a temperature of 55 to 60 C. The horizontal design of the

Kompogas digester with the plug-flow forward-motion principle ensures the desired retentiontime of about 20 days is achieved.

The green waste is transformed into compost and gas in the fermenter. Depending on thespecific composition of the organic waste, between 100 and 140 m (Nm) or at about 0.11

0.14 m/kg (3.44.2 scf/tons) wet weight of biogas are produced per ton with a methanecontent of approximately 60%. This corresponds to about 70 liters of petrol.

Another hydraulic pump extracts the fermentation residue and feeds it to the dewateringsystem. Presses then separate the fermentation residue into fresh compost and press water.In order to neutralize any odors, the fresh compost is aerated in a compost maturing hall

thereby passing from an anaerobic to an aerobic state, using odor control systems withbiofilters.

As of February 2008 30 systems are operating in Europe, 2 in the U.S., 1 in Africa, 2 inRussia, 11 in Asia, and 1 in Australia.

Figure 17. The Kompogas Process - Basic Process DiagramSource: Kompogas AG Modified by SWLS


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5.0 Multi-stage Digesters

Digesters that are operated in parallel are not multi-stage digesters; when each reactor is a

separate single-stage digester. This may be done because of tank size limitations, to simplifymanagement, or to expand capacity of an existing plant. A true multi-stage digester appliesdifferent conditions to the reactors in each stage. The difference can be in the organic loadingrate (OLR) of each stage, the presence or absence of oxygen, the introduction of anintermediate treatment, or the overall reactor configuration. Many different combinations orfactors are possible.

There are relatively few commercial, operational multi-stage AD units. Although it wasexpected that more of the multi-stage systems would be in operation by now due to theirhigher loading rates, improved process stability, and flexibility, the added complexity andpresumed expense of building and operating commercial multi-stage systems have so farnegated the yield and rate enhancements.

Nonetheless, the potential of multi-stage digesters to improve performance has promptedmuch research, and a few notable commercial multi-stage digesters have been successful.Some of these use multiple stages for reasons other than separating acidogenesis frommethanogenesis.

5.1 The BTA Process (by Biotechnische Abfallverwertung GmbH & Co.)

Developed in Germany and applied (via several licensing companies) throughout WesternEurope and in select locations in Canada and Japan, the BTA system is one of the oldest andmost successful in terms of the number of existing operational digesters. Although small unitsare single-stage, the majority of the BTA digesters are large (>100,000 MT/y [110,000

tons/y]) multi-stage, wet-wet units.

The multistage BTA digester (Figure 18) utilizes a pulper and hydrocyclone much like thoseemployed by the Wabio and Waasa single-stage digesters.

Figure 18. The BTA Process Basic Flow DiagramSource: Biotechnische Abfallverwertung GmbH & Co. Modified by SWLS

Pulped and density-fractionated MSW passes through a solid/liquid separation unit and

leachate is passed directly to a methanogenesis reactor. Solid extract is mixed with processwater to bring the MC to 75% and then pumped into a hydrolysis reactor with a residencetime of 4 days. Hydrolysis leachate is then transferred into the methanogenesis reactor whichhas a 2d HRT. Dewatered digestate is then either treated aerobically or disposed. Installations


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with a designed capacity of less than 100,000 MT/y (110,000 tons/y) often utilize the pulperas the hydrolysis tank, eliminating one step in the process.

In more detail, the BTA Process comprises two central steps: (a) the hydromechanical pre-treatment, and (b) the subsequent biological step towards anaerobic digestion.

Figure 19. Process scheme of the BTA Multi-stage wet fermentationSource: Biotechnische Abfallverwertung GmbH & Co.

The hydromechanical pre-treatment facilitates efficient removal of impurities as well ascomplete separation of digestible organic components into an organic suspension.

This happens within two core components, the Waste Pulper and the Grit Removal System.Within the Waste Pulper the feedstock is added to pre-filled process water in order to separate

the waste mixture into fractions by taking advantage of natural buoyancy and sedimentationforces. Moreover, non-soluble organic components are reduced to fibers by shearing forcesand brought into suspension. Thus, heavy materials are fed aside and light materials areskimmed off. In total, three fractions are effectively separated into (a) organic materials, (b)light materials (plastics, foil, textile, wood, etc.), and (c) heavy materials (stone, bones,batteries, etc.).

Having passed the Waste Pulper, the organic suspension still contains sand and fineimpurities, which are removed by the Grit Removal System. This reliably ensures protection ofdownstream plant components from wear, silting up, sediments, and obstruction. The clearedbio-suspension is temporarily stored in a suspension tank. This way, the processing stage isdecoupled from the digestion itself, keeping the latter independent from the working cycle ofthe waste reception unit. The organic fraction is digested within the fermenter, generally

under mesophilic conditions between 35C and 38C.

Fermenter layout and operation mode depend on the kind of feedstock as well as on project-specific requirements concerning microbiological breakdown and biogas yield. In practice,


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stirring of the fermenter is often hampered with great problems for many systems. The WastePre-Treatment ensures a virtually impurity-free organic suspension at high homogeneity.Therefore, by pressing in biogas, it is possible to keep the fermenter load homogenous, i.e.,

Safe agitation of the entire fermenter volume, high utilisation of the energy input, lowsusceptibility to faults due to the lack of moving parts within the fermenter, no maintenancework required inside the fermenter, no loss of fermenter volume by sedimentation.

Further treatment of the digested substrate can be adjusted according to the respectiveproject. Generally, a decanter centrifuge for continuous separation of solids from liquids isemployed. The solid material (ca. 30 % dry matter) is perfectly suitable for the stabilisationand production of quality compost. In most cases the supernatant is utilised as process waterand is retained within the cycle. This results in decreased freshwater consumption.

5.2 The Linde-KCA Process (by Linde-KCA-Dresden GmbH)

Linde-KCA has built low and high

solids (wet and dry) digestionsystems and mechanical-biological treatment systems(MBT) for separated MSW since1985 and currently has eightdigesters operating in Germany,Portugal, Spain, andLuxembourg, mesophilic andthermophilic.

MBT systems include aerobic

composting systems withmechanical manipulation of the

feedstock and intensive aeration.Some systems include intensive aerobic digestion as a pre-process for a feedstock that is later

anaerobically digested.

The typical dry digester is operated in two stages. The first stage is aerobic and the hydrolysisproduct is transported via conveyor to a horizontal plug-flow digester with internal rotors formixing (see Figure 21) and transporting solids to the dewatering unit (although this is a two-stage system, the first stage could also be considered an aerobic pretreatment stage apartfrom the anaerobic digester). Nevertheless, the digester is capable of handling 15-45% TSand generates roughly 0.10 m/wet kg (3.2 scf/wet lb) of biogas.

Linde-KCA has designed, built,and operated a facility in

Radeberg, Germany, which co-digests source separatedbiogenic wastes from householdand industrial sources along withsewage sludge from waste watertreatment. The company reportsthat this codigestion concept

enhances degradation of thesewage sludge component of thefeedstock (increases biogasproduction from the sewagesludge) and decreased capital and operating costs compared to those for two separatefacilities.

Another codigestion facility designed and built by Linde-KCA is located on a dairy farm in

Behringen, Germany. The plant takes the low solids manure and co-digests, with grease fromrestaurant grease traps, solids from pig manure, and a range of other food processing waste

Figure 20. Linde-KCA Two-stage Wet Digestion SystemSource: Linde-KCA-Dresden GmbH

Figure 21. Linde-KCA Dry Digestion Plug Flow SystemSource: Linde-KCA-Dresden GmbH Modified by SWLS


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including brewery sludge, rape seed press cake, and low grade seed potatoes. The facilityprocesses about 100 t/day (wet) of which 75% is dairy cow manure. The facility also producesabout 650 kW from two Jenbacher 450 kW gensets; 30% of the power production is used on

site for operating the plant and the dairy with the balance sold to the grid.

5.3 The ArrowBio Process (by ArrowBio, ArrowEcology & Engineering Overseas Ltd.)

The ArrowBio process integratesseparation and preparation(preprocessing) and advancedanaerobic digestion (UpflowAnaerobic Sludge Blanketdigestion or UASB) via themedium of water. The water isderived from the moisturecontent of the waste. Clean

recyclables are recovered frommixed waste via gravitationalseparation in water, andbiodegradable organics areextensively converted tomethane-rich biogas.

It achieves system integration byexploiting the moisture contentof MSW; moisture content levels are typically around 30% by weight. Thus a ton of MSWconsists of approximately 1,400 pounds of material and 600 pounds of water (or, 70 gallons

of water per ton). In the course of anaerobic digestion, moisture in MSW is liberated as liquidwater. The mechanism is that, as organics are converted to methane and carbon dioxide, and

the gases bubble out of solution, water originally imparting moistness is left behind in liquidform (Figures 22 and 23).

A portion of the water liberated at the back-end biological stage (UASB) is exchanged with vatwater at the front-end physical separation/preparation stage. As such, the bioreactors receivefresh substrate, and the vat receives makeup water. Among UASBs benefits are extensive

conversion of organics to methane-rich biogas, production of a relatively small amount ofclean, stabilized, digestate, and a modest facility footprint. These characteristics stem fromintegration of the two stages. The front-end physical stage (a) removes grit and otherlandfillables to recover traditional non-biodegradable recyclables (e.g., plastic bottles andjugs), (b) other secondary material, and (c) to isolate and prepare the biodegradables forUASB digestion at the back-end stage. These functions are performed in unison and areinextricable.

The non- biodegradable and biodegradable fractions are separated gravitationally in water asabetted by filter screens and size-reduction devices. Separation in water is far more efficientthan in air, owing to the comparative densities (relative buoyancies) of the two fluids. Thus,depending on their solubility or specific gravity and tendency to absorb water, items dissolve,sink, float, or become suspended in the water.

Incidental benefits of tipping into water include the suppression of dust, and the

neutralization of odors. Neutralization is immediate because odorous compounds are solublein water. Their biodegradation soon follows as vat water is continuously pumped to the back-end enclosed digesters. Also, because the system is watery throughout, any input surges areevened out, contributing to the systems overall resiliency.

The load of mixed waste is tipped onto a walking floor, from which it falls into the water vatimmediately upstream of a partially submerged rotating paddle. The paddle urges floaters andbuoyancy-neutral items forward into the main body of water. Sinkers are diverted to the leftand passed sequentially to a bag breaker, magnetic pickup, eddy current device, and a

Figure 22. The ArrowBio Process logicSource: ArrowBio, ArrowEcology & Engineering Overseas Ltd.


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pneumatic (vacuum/forced draft) station from which film plastic is swept into ductwork. Ductsfrom several such stations converge on the cyclone. Thereby, metals and film plastic areremoved. Items that escape this processing train the first time around reenter the water vat

for another chance to dissolve, float or sink or, if buoyancy-neutral, be suspended in theforward-moving water column.

Overflow from the water vat,screened to exclude large items,passes though smaller enclosedtrommel screens and thence,according to partitioning criteria,to large and small settlers. Inthe settlers grit is separatedfrom organics and removed fromthe system.

Meanwhile, larger floaters andbuoyancy-neutral items are liftedto a slow speed shredder andthence to the large trommelscreen. The overs from this

trommel consist mostly of filmplastic and are removed at apneumatic station. The unders

(material that passed throughscreen) are washed into a non-mechanical device for furthersolubilizationand sizereduction. Non-soluble

substances are thus reduced to asuspension of fine particles

whose surfaces are roughened tofavor microbial colonization.

Thus non-biodegradables arerecovered for recycling assecondary material commodities, and soluble and particulate organics come into solution orfine suspension, including food sticking to containers and the contents of unopened diapers.The latter are disrupted in the processing train, freeing the feces and urine-soaked cottonyabsorbent. Insoluble biodegradable organics (e.g., non-source-separated food-tainted paperproducts, tough fruit rinds) get increasingly soggy and fragmented, ultimately to the point ofpassing screens of selected sizes. The organics, now in watery isolation, are pumped to the

biological stage. Reciprocally, return-water from the digester system refreshes theseparation/preparation water vat. Within half an hour after tipping the last load of the day,the work of the physical separation/preparation element is complete. This part of the plant isthen shut down until deliveries resume the next working day.

The organic flow (Figure 23) first enters acidogenic bioreactors for several hours ofpreliminary treatment. There, readily metabolized substances already in solution arefermented (e.g., sugars fermented to alcohols), while certain complex molecules are

biologically hydrolyzed to their simpler components (cellulose to sugar, fats to acetic acid).The overflow, rich in such intermediate metabolites, then enters the UASB digestionbioreactor. Operationally, excess biological granules suspended in similarly excess water (bothexcesses represent growth at the expense of the waste) are transferred to a settling tank.Supernatant is pumped to the physical separation/preparation element as needed for makeup

water, or to an aerobic tank for polishing if necessary. Water may be stored or usedimmediately as in irrigation. The solids are dewatered for use as a stabilized organic soilamendment. When needed to maintain UASB digestion at its optimum temperature of ~35C(95F), some of the biogas is used to fire boilers. Most of the gas is used to fuel a generator,

Figure 23. Waste digestion in the ArrowBio Process Basic Flow DiagramSource: ArrowBio, ArrowEcology & Engineering Overseas Ltd. Modified by SWLS


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via storage in a reservoir. Waste heat from the generator usually suffices to maintaindigestion temperature.

5.4 The Biopercolat Process (by WEHRLE Umwelt GmbH)

The Biopercolat process by WEHRLE Umwelt GmbH is a dry-wet, two-stage process. The firsthydrolysis stage is carried out under partial aerobic conditions with high solids content (seeFigure 24).

Figure 24. Schematic waste-flow diagram of the two-stage Biopercolat ProcessSource: WEHRLE Umwelt GmbH

Process water is continually percolated through the hydrolysis reactor a horizontal tunnelthat slowly rotates in order to slightly aerate the mixture and prevent clogging andchanneling.

The leachate passes on to the second-stage fermentation reactor an anaerobic plug flowfilter filled with support material operating at mesophilic temperature. After two to three days

in the percolator, the solids are separated and transferred to an enclosed tunnel composter.The liquid fraction is transferred to the fermentation reactor, and displaced liquid is partly re-circulated back through the percolator and partly aerated for disposal as wastewater.


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6.0 Batch Digesters

Some of the first dry digesters were envisioned as modified landfills. This resulted in the

creation of batch systems that recycled leachate in a manner similar to landfill bioreactors.However, unlike landfill bioreactors the batch digester conditions were more carefullycontrolled and as a result biogas production rates were higher and retention times werelower.

In batch systems, digesters are filled once with fresh wastes, with or without addition of seedmaterial, and allowed to go through all degradation steps sequentially in the drymode, i.e.at 30-40 % TS.

Though batch systems may appear as nothing more than a landfill-in-a-box, they in factachieve 50- to 100-fold higher biogas production rates than those observed in landfillsbecause of two basic features: (a) Leachate is continuously re-circulated, which allows thedispersion of inoculants, nutrients, and acids, and in fact is the equivalent of partial mixing

and (b) batch systems are run at higher temperatures than that normally observed inlandfills.

Batch systems have up to now not succeeded in taking a substantial market share. Howeverthe specific features of batch processes, such as a simple design and process control,robustness towards coarse and heavy contaminants, and lower investment cost (ca. 40%)make them particularly attractive for developing countries. Note, however, that the land arearequired by batch processes is considerably larger than that for continuously-fed dry systems,since the height of batch reactors is about five-fold less and their OLR two-fold less, resultingin a ten-fold larger required footprint per Ton treated wastes. Operational costs arecomparable to those of other systems.

The hallmark of batch systems is the clear separation between a first phase where

acidification proceeds much faster than methanogenesis and a second phase where acids aretransformed into biogas. Three basic batch designs may be recognized, which differ in the

respective locations of the acidification and methanogenesis phases.

The primary disadvantage of batch digesters is uneven gas production and lack of stability inthe microbial population. Sequential and phased batch digesters attempt to surmount thesedisadvantages, and preliminary lab experiments have revealed complex population dynamicsin these systems resulting in the ability to separate useful fermentation products such ashydrogen and organic acids.

6.1 The Biocel System (by Orgaworld bv)

The Biocel system was developed in the 1980s and 1990s in Holland at the Wageningen

University as a part of the early research on high-solids digestion of MSW. The initial goal ofthe system was to reduce cost by simplifying material handling and eliminating the need formixing while simultaneously achieving relatively high loading and conversion rates. Successwith the lab-scale system led to construction of a pilot 5 m (1,000 gal) reactor by the early1990s which was used for more extensive testing of start-up, heating, and leachate recycling.By 1997 a full-scale 50,000

2008 february treatment of municipal solid waste anaerobic digestion technologies

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