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ANAEROBIC DIGESTION - COMPARISON OF DESIGNCONSIDERATIONS BETWEEN A BIOSOLIDS AND ORGANIC SOLID

WASTES DIGESTION FACILITY

P. Coleman, PhD PEng, AECOM

J. Blischke, MS, AECOM

ABSTRACT

Anaerobic digestion is a well understood biosolids treatment technology.The primary benefits are the production of renewable energy and the reduction ofmaterial to be disposed of. However, biosolids are a minor source of digestiblematerial when compared with organic solid wastes, industrial wastes, agriculturalwastes (e.g. manure) and energy crops. This paper will present the design of thenew Disco Source Separated Organics (SSO) Anaerobic Digestion ProcessingFacility in the City of Toronto and contrast it with the design of a typicalBiosolids Digestion Facility. Issues such as pre-processing options, digesterdesign, end product quality, process water balance and biogas production arediscussed

1. INTRODUCTION

On vacation, in 1176, Allesandro Volta collected gas released from LakeMaggiore (Italy) and showed that it was flammable. Volta experimented with thismarsh gas building a pistola which was an early precursor of the internalcombustion engine. A century later Bechamp demonstrated a microbial origin ofthis gas using an ethanol-based media inoculated with rabbit feces (Speece 2008).

In 1860, Louis Mouras built a closed container with a water seal in whichorganic matter (excrement) disappeared. This system, later referred to as theMouras Automatic Scavenger, was considered an elegant solution to the treatmentof sewage solids. However, evidence suggests that many people already relied onthis method of treatment. It is believed that the first purpose-built anaerobicdigester was built in 1859 for an Indian leper colony (Speece 2008).

Thirty one years later, W. D. Scott-Moncrieff constructed a tank with anempty space at the bottom and a submerged bed of stones on the upper partcreating probably one of the first anaerobic filters.

The first noteworthy linking of liquefaction of sewage solids and gasproduction/utilization did not occur until 1895. Cameron constructed a septic tankin Exeter (UK) that produced gas that powered nearby gas lights.

Today, anaerobic systems are used to produce renewable energy from highstrength industrial effluents, agricultural material, municipal wastes, organic

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industrial byproducts and biosolids. Anaerobic digestion provides a means toreduce pollution from organic wastes while producing a renewable fuel – biogas.Biogas can used to produce heat, steam, electricity and replace fossil-basedvehicle fuels.

2. MICROBIOLOGY

Anaerobic digestion (see Figure 1) can be divided into four steps (Henze,van Loosdrecht et al. 2008):

1. Hydrolysis: Enzymes excreted by fermentative bacteria breakdown complex undissolved material into less complex, dissolvedcompounds that can pass through cells walls and membranes offermentative bacteria. When digestion source separated organics,approximately 15% of ammonia is released into solution at thisstep (Zhang, Walker et al. 2010).

2. Acidogenesis: Dissolved compounds within fermentative cells areconverted to simpler compounds and excreted. These includevolatile fatty acids, ammonia, alcohols (e.g. ethanol), lactic acid,carbon dioxide and hydrogen sulfide.

3. Acetogenesis: (intermediary acid production) digestion productsare converted into acetate, hydrogen, and carbon dioxide.

4. Methanogenesis: Acetate, hydrogen, carbon dioxide pluscarbonate, formate or methanol are converted into methane andcarbon dioxide (i.e. biogas). Typically, 70% of the methane isproduced from acetate (by aceticlastic methanogens) and 30%from carbon dioxide and hydrogen (by hydrogenotrophicmethanogens).

3. FEEDSTOCK

There are a number of organic feedstocks suitable for anaerobic digestion(Figure 2). In a typical North American city, the responsibility for thesefeedstocks is spread across city departments (e.g. solid waste vs. water andsewage). Therefore, the management of these feedstocks is fragmented andopportunity to cooperate to obtain the best solution for a city is more often thannot missed.

A typical SSO digester produces about 110 m3 of biogas per wet tonne ofmaterial processed which is about 80% of the biogas potential of the SSO. This isabout 0.8 to 1.0 m3/kg volatile matter destroyed which is similar to the biogasyield for sewage sludge. The yield will change depending on what organics arepresent in the SSO and how the SSO is prepared for digestion (e.g. hydrolysed).

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Unlike SSO, the organic make up of sewage will only vary significantly from onesite to another if there is a significant industrial discharge to the sewer system.

Apart from the variability in the feed characteristics, the other differencesbetween SSO and sewage sludges are (1) water content and (2) contaminants inthe feedstock.

Sewage sludge consists of three types of materials: raw volatile solids(VS), biomass solids (e.g. waste activated sludge), and inert material. The inertmaterial consists of material that arrives with the sewage and material created inthe process (e.g. iron phosphates).

Screenings and grit, if not removed from the raw sewage, create problemsin the digester. Screenings form rafts on the water surface and interfere withrotating equipment (e.g. pumps, mixers). Plastics, if they make it into the finaldigested product, can limit the use of the end product. Grit increases equipmentwear and settles out in the digester reducing the volumetric capacity.

The primary processing challenge for sewage treatment sludges is water.Sewage sludge is dilute. In North America digesters are fed a sludge mix at about4% solids (Krause 2010):

A typical design sustained-peak loading rate for mesophilic digesters is1.9 to 2.5 kg volatile solids/m3/d (0.12 to 0.16 lb volatile solids/d/cu ft).The upper limit of the volatile solids loading rate typically is determinedby the rate at which toxic materials— particularly ammonia— accumulateor methane formers wash out. A limiting value of 3.2 kg volatilesolids/m3/d (0.20 lb volatile solids/d/cu ft) is often used.

The more dilute the sewage sludge, the more energy is required to heat thefeed and to maintain the temperature in the digester. Thickening the sludge toabout 6% decreases the heat demand. In some circumstances, it is possible to heata properly insulated digester fed at 6% solids using only the waste heat from aproperly sized biogas co-generation engine.

In Europe, where mechanical thickening of primary sludge is morecommon, digesters are fed with sludge at 5% to 8% solids. This is because athickening belt or rotary drum can reliably achieve 6% and 9% dried solids whenthickening waste activated sludge and primary sludge respectively. The averagedigester VS loading is between 3 and 4 kg VS/m3/d.

The experience in the wastewater industry with “high solids digestion”started with the first thermal hydrolysis plant installed at Hias, Norway in 1995(Fjærgård and Sander 1999). Attempts to load conventional digesters at higherrates without pasteurizing or hydrolyzing the sludge failed because of foaming orsouring (Brown and Sale 2002).

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A number of thermal hydrolysis installations have followed including oneto be built for District of Columbia Water and Sewer Authority (Washington,DC). The feed concentration is about 10% solids. The volatile solids load to thedigester is between 5 and 6 kg VS/m3/d.

The constituents of the collected SSO (e.g. each household has a separatebin) and the organic fraction of municipal solid wastes (OFMSW) (e.g. what isleft after the material is mechanically processed at a material recovery facility)varies from jurisdiction to jurisdiction.

Even the SSO collected in neighboring communities can be different dueto different rules as to what can be put into a green bin. For example, the Regionof Peel and the City of Toronto both have green bin programs. The City ofToronto allows the use of plastic bags and accepts diapers in the green bin. TheRegion of Peel only allows certified compostable plastic bags in the green bin andinsists that diapers are disposed with the regular garbage.

4. CITY OF TORONTO SSO ANAEROBIC DIGESTION FACILITIES

The City of Toronto has one operating anaerobic digestion processingfacility (Dufferin) and is currently building a second one at the Disco RoadTransfer Station site.

In the City of Toronto’s Dufferin Facility, the waste arrives as ‘raw’ SSOon the tip floor with plastic bags, heavy debris (e.g. rocks) and grit (Figure 3). Themoisture content varies by season. The moisture content is typically around 68%.

The material is pulped using a mix of fresh and recycled water as a pre-conditioning step for ‘wet’ digestion (with a total solids [TS] content of typicallyless than 15%). At the Toronto facilities, the pulp is digested and the digestate isdewatered. The maximum loading rate is about 5 kg VS/m3/d. At other non-BTAfacilities, the pulp is dewatered and only the liquid is digested.

5. USE OF WASTEWATER SIMULATION SOFTWARE TO DESIGNSSO PROCESSING FACILITIES

Wastewater treatment plant simulation software (e.g. BioWin, GPS-X)was developed to model the processing of liquid wastes (see Figure 4 as anexample). Because the Disco Road Facility includes a wastewater treatment plantand anaerobic digesters, AECOM modeled the facility using both BioWin andGPS-X. Both these packages include sophisticated pH and digester models thatinvolve complex water chemistry calculations.

AECOM also recently modeled a facility that received, digested anddewatered raw sludge cake delivered by truck to the facility. In this case, in orderto set the inputs necessary for the anaerobic digestion model, AECOM modeled

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the wastewater treatment plant and then imported the output from the WWTPplant model to the biosolids processing facility model. For the most part, thisapproach worked.

However, this approach cannot be used for an SSO facility. The designertherefore must start with solids, volatile solids and moisture content information.The nitrogen and phosphorus content is estimated from literature values (e.g.%N/wet solids) and from historic sewage and cake discharge data from anexisting facility. Measuring the characteristics of the feedstock directly isexpensive and labor intensive (Jansen, Spliid et al. 2004)

The two most difficult decisions to be made by an SSO facility modelerare (1) how to set the dissolved solids so that the anaerobic digester modelfunctions and (2) how to characterize COD, N and P waste fractions whenentering the feedstock into the model.

The knowledge of SSO and OFMSW characteristics is limited comparedto what is known about domestic sewage. Commercial simulation softwarepackages need to allow the modeler to “dumb down” the model to reflect thelimited information on the model inputs. For example, it may not be necessary tomodel the digester pH. What is more important is modeling the ammoniaconcentration in the digester, centrate and wastewater treatment plant as well asthe varying levels in the process water buffer tanks.

6. PREPROCESSING

Most wastewater treatment plants screen and de-grit the raw sewage at thehead of the treatment process. However, in jurisdictions where there is a strict ruleabout plastics in biosolids used on agricultural land, the sludge is screened asecond time through a 5mm to 10mm screen prior to thickening.

SSO also contain containments that need to be removed before thematerial is digested. These contaminants are more difficult to manage thanscreenings in sewage sludge.

Consequently, there are a number of proprietary ‘wet’ anaerobic digestionprocesses on the market (Figure 5) – each with its own approach to managingcontaminants. This paper will focus on the BTA process which is installed at oneToronto facility (Dufferin) and will be soon installed at a second Toronto facility(Disco Road).

The Dufferin SSO Facility, commissioned in 2002, was originallydesigned to process 25,000 wet tonnes per year (TPY). The facility is currentlyprocessing close to 40,000 wet TPY. The facility is to be expanded to process upto 55,000 TPY. The new Disco Road facility is designed to process 75,000 wetTPY. Both sites use the ‘wet’ BTA Process (BTA 2011). Figure 6 illustrates in asimplified flow diagram the BTA Process.

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Other preprocessing options include pasteurization (to meet strictdisinfection standards) and/or physical/chemical treatment of the sludge to makeit more digestable. There are comparable options for other organics feedstocks(e.g. SSO).

The ‘wet’ BTA pre-treatment step generates a waste suspension in thePulper (Figure 7) by adding primarily recycled water to the ‘raw’ SSO.

During the pulping process the plastic bags are broken open and removedalong with other light/floating material using a rake. The so-called Light Fractionthat includes the plastic bags are washed and pressed/de-watered. Heavy material(e.g. stones, glass, batteries, cutlery) also called Heavy Fraction sinks to thebottom of the pulper where it is removed through a heavy fraction trap. The pulp,at about 8-10% solids, is then passed to a de-gritting step. The pulp is passedthrough hydrocyclones as a key component of the grit removal system to removefiner particles. The pulp is then fed into the digester directly or temporarily storedin a suspension buffer tank before being fed to the digester.

7. DIGESTER DESIGN

Sewage digesters come in one of three shapes (Figure 8): cylindrical(height < diameter), double cone (height >diameter) and egg shaped.

Most digesters for ‘wet’ SSO digestion are cylindrical in shape. This is themost economical shape for the size of these types of digesters. The digesters arenormally constructed from coated steel or concrete. This is because solid wastefacilities differ from wastewater treatment plants in two ways: (1) expected assetlife is shorter and (2) the feedstock can be diverted during facility shutdownbecause the feedstock can be diverted to another site or stockpiled. This is not thecase for raw sewage.

Unconfined gas, confined gas, draft tube, pump, linear motion and bigblade are used in sewage digesters. This said, as the thickness of the feedincreases, mixing moves away from turbulent towards laminar systems. In thelatter case, the mixer tends to “fold” the material much like a cook uses a spatulawhen mixing cake batter.

SSO digester designers tend to shy away from mixers involving rotatingequipment when the SSO contains contaminants that will wrap around rotatingshafts. For example, Dufferin and Disco Road both use gas lances to mix thedigester.

8. PROCESS DESIGN AND INHIBITION

The destruction of organic feedstsocks containing nitrogen releasesammonia into solution. The larger the mass fraction of nitrogen in the feedstock,

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the greater the release of nitrogen per mass of volatile solids destroyed. Ammoniaexists in the digester in both its ionic form (NH4

+) and its free form (NH3). Freeammonia is toxic primarily to the hydrogentrophic methanogens. The degree ofthis toxicity is dependent partly on the availability of certain micronutrients(Chen, Cheng et al. 2008).

Conventional sewage digesters operate with ammonia concentrationsbetween 800 to 1,500 mg/L as N. A recent survey of digesters downstream ofthermal hydrolysis plants conducted for DC WASA report ammoniaconcentrations between 2,000 to 3,000 mg/L. This is typical of ammoniaconcentrations observed in SSO digesters.

The hypothesis is that digesters acclimatize to these high ammoniaconcentrations by (a) reducing the pH of the digester by accumulating highervolatile acid concentrations and (b) by growing different bacteria.

9. DISCO ROAD FACILTY

The Disco Road SSO Facility is designed to process 75,000 wet tonnes ofSSO per year (Figure 9).

The material is received 5 days per week, 16 hours per day. The digestersand the wastewater treatment plant Sequencing Batch Reactors (SBRs) are fed 24hours per day, 7 days per week. The transition from 16/5 to 24/7 operation ismediated by the suspension buffer tank, the process water 1 tank and the SBRfeed buffer tank. The suspension buffer tank holds enough SSO to feed thedigesters over the weekend. The process water 1 tank and the SBR feed buffertank store centrate to be used for pulping and to feed the SBR during the week.

The material is fed by a front end loader into one of two hoppers whichfeed three BTA pulpers..

The SSO is pulped and the Light Fraction and Heavy Fraction areremoved.

The pulped material is de-gritted by passing it through a total of threehydrocyclones as part of the grit removal system. The de-gritted pulp is thenpumped to an air mixed suspension buffer tank. The suspension buffer tank issized to store enough pulp to feed two digesters through the weekend.

The three residues (Light Fraction, Heavy Fraction, Classified Grit) arecompressed and loaded into residue trailers.

The homogeneous and contaminant-free pulp is pumped to one of twocylindrical confined gas mixed digesters (5,300 m3 each). The digesters aremixed using gas lances. The hydraulic retention in the digesters is greater than 15days.

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The digestate is dewatered by two of three centrifuges. The centrifugesonly use polymer when centrate (PW1+) is being produced for the wastewatertreatment plant. Between 65% to 70% of the centrate is recycled and used to pulpthe incoming SSO. The balance is sent to one of two 750 m3 SBRs.Approximately 2/3 of the SBR effluent is then re-used in the process. The balanceis discharged to the sanitary sewer.

The dewatered digestate is loaded in trailers and taken to another site forfurther processing (composting).

9.1 Water Balance

Most wastewater treatment plants (WWTPs) have two process waters:potable water and effluent. This is because there is an abundance of good qualitywater available at a WWTP.

This is not the case at a solid organic waste anaerobic treatment facilitywhere there is a water shortage in process water of higher quality. For this reason,the Disco Road SSO facility has five different quality process waters driven bythe site-specific requirement to reduce the amount of potable water to the greatestextend feasible (Table 1) and only to treat the required volume of water to therequired quality.

Water enters the process in one of three ways: with the SSO, rainwater(NPW) harvested from the roof of the facility (non-potable water) and nearbytransfer station and potable water (PW).

Potable water is used to prepare polymers, rinse instruments, and irrigatebiofilters. Nonpotable water is used to humidify odorous air and washfloors/equipment.

SBR effluent (Process Water 2) is used to wash the Light Fraction and theGrit as wella s to top up condensate and overflow traps. The target effluent qualityis 350 total suspended solids (TSS) and 100 mg/L TKN. Centrate when thecentrifuges are not using polymer (Process Water 1) is recycled and used to pulpthe SSO. Process Water1 is typically 1.5% to 2% solids. The dirtiest watercollected via floor and pulper drains (Process Water 0) is used to pulp the SSO.

Water leaves the process in one of three ways: with dewatered digestateand residues, as SBR effluent discharged to sewer and the biofilter drainage tosewer.

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Table 1: Process Water

Grade Source UsePROCESS WATER 0 (PW0)From Pulper Sump

This is the “dirtiest” or lowest grade ofProcess Water, and is comprised ofuntreated floor drainage and liquors passedthrough the pulper sump sieve screw forgross particle removal.

Primary Source:Floor drainsDelivery truck sumpsResiduals liquors

Secondary Source:None

Used in pulper(s) for pulping SSO sothat the solids and liquors are capturedand digested; this is the only use

PROCESS WATER 1 (PW1)Centrate with no polymer addition thathas been passed through a 2mm BowSieve

The Process Water 1 is mainly centratethat has passed through the Bow Sieve*for particle removal.

Primary Source:Condensate drains & screenedcentrate

Secondary Source:WTP Effluent (PW2)

Used for making pulp once the pulpersump has been drawn down

SBR FEED (PW1+)Centrate with polymer addition that hasbeen passed through a 2mm Bow Sieve

The solids content of PW1+ is lower thanPW1. PW1+ is produced exclusively tofeed the WTP

Primary Source:Screened centrate from acentrifuge that is dosed withpolymer for solids capture

Secondary Source:Settled PW1

Fed to WTP to produce PW2

PROCESS WATER 2 (PW2)WTP effluent to meet Sewer Bylaw

Parameter Not to exceedBOD5 300 mg/LTSS 350 mg/LTotal Phosphorus 10 mg/LTKN 100 mg/L

The BOD5 should be inhibited fornitrification to obtain carbonaceousdemand

Primary Source:WTP effluent

Secondary Source:topped up using Non-potablewater if the need arises (notnormally anticipated)

Sprays and washing, top up traps

NON-POTABLE WATER (NPW)Rainwater

Primary Source:Rain water from roof

Secondary Source:Topped up using City water inprolonged dry periods

Floor / Truck / Facility WashingPolymer dilution

POTABLE WATER (PW) City Water Odour control unit (Irrigation sprays)and instrument rinsing.

10. CONCLUSIONS

The primary differences between digestion of sewage solids and sourceseparated organics are:

1. Types and mass of contaminants: SSO contains more debris and gritthan sewage sludge which must be removed prior to digestion. Sewagesludge has less grit because grit is removed from the raw sewage in theheadworks.

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2. Ammonia concentration in digester: The ammonia concentrations inSSO digesters is comparable to sewage digesters fed with thermallyhydrolyzed solids.

3. Mixing systems: Mixing equipment using rotating equipment (e.g. jetmixing) are not used in SSO digesters when the SSO containscontaminants that will wrap around rotating equipment.

4. Water balance: Water must be added to the SSO (pulping) prior todigestion while water is removed sewage solids prior to digestion.

11. REFERENCES

Brown, S. and R. Sale (2002). "Operating a High-Rate Digester: SouthernWater Experience." Journal CIWEM 16: 116-120.

BTA (2011). "Welcome to BTA International GmbH!". Retrieved July 31,2011, from http://bta-international.de/.

Chen, Y., J. J. Cheng, et al. (2008). "Inihibtion of anaerobic digestionprocess: A review." Bioresource technology 99: 4044-4064.

Fjærgård, T. and O. Sander (1999). Five Years' Experience with theCAMBI Process at HIAS. 4th European Biosolids and Organic ResidualsConference, November 1999

Henze, M., M. C. M. van Loosdrecht, et al. (2008). Biological WastewaterTreatment. Principles, Modelling and Design London, UK, IWA Publishing.

Jansen, J. l. C., H. Spliid, et al. (2004). "Assessment of sampling andchemical analysis of source-separated organic household waste." WasteManagement 24: 541-549.

Krause, T. L., Ed. (2010). Design of Municipal Wastewater TreatmentPlants, WEF Manual of Practice No. 8, ASCE Manuals and Reports onEngineering Practice No. 76, Fifth Edition. Alexandria, Virginia, WaterEnvironment Federation.

Speece, R. E. (2008). Anaerobic Biotechnology and Odor/CorrossionControl for Municipalities and Industries Nashville, TN, Archae Press.

Zhang, Y., M. Walker, et al. (2010). Technical Report. OptimizingProcesses for the Stable Opertaion of Food Waste Digestion. Defra Porject Code:WR1208.

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FIGURE 1 ANAEROBIC DIGESTION PROCESS

Proteine.g. Keratin Carbohydrates

e.g. SucroseLipids

e.g. Fats

Amino Acids, Sugars Fatty Acids, Alcohols

Intermediary Products(Propionate, Butyrate etc)

Acetate HydrogenCarbon Dioxide

MethaneCarbon Dioxide

Fermentation

Homoacetogenesis

AceticlasticMethanogensis

HydrogenotrophicMethanogensis

AnaerobicOxidation

Substrate

1 1 1

2 2

3

2 2

4b4a

Hydrolysis

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FIGURE 2 FEEDSTOCK

RAW SSO PULPED SSO

FIGURE 3 SOURCE SEPARATED ORGANICS (SSO)

-

Organic Wastes(manure, commercial,

residential)- Paper/Cardboard

- Energy Crops-Yard Waste

-(leaves, grass,trimmings)

- WoodWaste

Criteria:Feedstock Total Solids

(TS)

WetDigestion<15% TS

DryDigestion>15% TS

AerobicTreatment

AnaerobicTreatment

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FIGURE 4 BIOWIN™ MODEL LAYOUT FOR DISCO SSO PROCESSING FACILITY

FIGURE 5 EVOLUTION OF ‘WET’ ANAEROBIC DIGESTION OFSOLIDS WASTES

Digester

Cake

SSO

Cake+

Centrate+SBR#1

SBR#2

1986 - 1995 Garching, DPilot Plant

1999 Fürstenwalde, D1999 Radeberg, D

(1993Baden-Baden, D)

1996 Wels, A1996 Rügen, D

1995 Dietrichsdorf, D1996 Karlsruhe, D1996 Schwabach, D1997 Münster1997 Erkheim, D(1997 München, D)1998 Wadern-L, D

1999 Boden, D

2005 Hita, JPN2005 Lissabon, P2005 Burgos, E2005 Camposampiero, I2005 Salto des Negro, E

2000 Newmarket, CAN2001 Mertingen, D2001 Pulawy, Pl2002 Parramatta, AUS2002 Toronto, CAN2002 Verona, I2002 Villacidro, I

2003 Ieper, B2003 Ko-Sung, Korea2003 Mülheim (Ruhr), D2003 Pamplona, E

2003 Palma, E2003 Avila, E2004 Lanzarote,E

2005 Gescher, D2005 Västeras,S2005 Deißlingen, D2005 Volkenschw.,D

2006 Ecoparque III, E2006 Krosno, Pl2006 Tuleda,E 2008 Wien, A

2008 Jaén, E2008 Voghera, I2008 Gran Canaria, E2008 Alicante, E

2002 Barcelona, EEcoparque I

2003 Madrid, E

2005 Schaumburg, D

2006 Wiefels, DHorstmann

Biostab (Ros Roca)

mann)

BTABTA BTA (MAT)

Lohse (Linde KCA)

1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008

1992 Kaufbeuren, D

1991 Helsingør, DK

2007 Ecoparque I, EBTA (MAT)

1999 Nordhausen, D2006 Lübeck, D2006 León, E2006 Schw. Elster, D

2006 Deiderode, DLohse (AMB)

Haase

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FIGURE 6 BTA PROCESS

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FIGURE 7 BTA PULPER AND GRIT REMOVAL SYSTEM

FIGURE 8 DIGESTER SHAPES AND MIXING SYSTEMS

Cylindrical DoubleCone

Egg

D > HD < H D < H

Unconfined GasConfined Gas

Draft TubePump

Big BladeRev Draft TubeConfined Gas

Big BladeRev Draft TubeConfined Gas

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FIGURE 9 NEW DISCO ROAD SSO PROCESSING FACILITY