Module 3: Types of Anaerobic Digesters - eXtension Publish · Module 3: Types of Anaerobic Digesters ... Plug Flow Digester ... Describe the advantages and disadvantages of each digester.

Module 3: Types of Anaerobic Digesters

Author: Douglas W. Hamilton, Associate Professor, Department of Biosystems & Agricultural Engineering, Oklahoma State University

Peer Reviewer: M. Charles Gould, Extension Educator-Agricultural Bioenergy and Energy Conservation, Agriculture and Agribusiness Institute, Michigan State University

This guide is also available as an online learning module at:http://blogs.extension.org/andig1/modules/andig-3-types-of-anaerobic-digesters/

Additional modules on Energy independence, Bioenergy Generation, and Environmental Sustainability are available online at: http://fyi.uwex.edu/biotrainingcenter/

This material is based upon work supported by the National Institute of Food and Agriculture, U.S. Department of Agricul-ture, under Agreement No. 2007-51130-03909. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

Types of Anaerobic Digesters

Table of ContentsSummary: ....................................................................................................................... 2Learning Objectives: ....................................................................................................... 2Unit 3.1: Introduction ...................................................................................................... 3 Background and Guiding Questions .......................................................................... 3Unit 3.2: Passive Systems .............................................................................................. 3 Background and Guiding Questions .......................................................................... 3 Covered Lagoon ....................................................................................................... 5Unit 3.3: Low Rate Systems ........................................................................................... 5 Background and Guiding Questions .......................................................................... 5 Complete Mix Digester .............................................................................................. 5 Plug Flow Digester .................................................................................................... 6Unit 3.4: High Rate Systems ........................................................................................... 7 Background and Guiding Questions .......................................................................... 7 Low Rate Digesters with Solids Recycle .................................................................... 7 Fixed Film Digester .................................................................................................... 8 Suspended Media Digesters ..................................................................................... 9 Sequencing Batch Digester ..................................................................................... 10Unit 3.5 High Solids Anaerobic Digestion ...................................................................... 11 Background and Guiding Questions ........................................................................ 11 High Solids Digesters .............................................................................................. 12Unit 3.6: Choosing a Digester ....................................................................................... 13 Background and Guiding Questions ........................................................................ 13 Choosing the Right Digester .................................................................................... 14 Conclusions ............................................................................................................ 14Self Quiz ....................................................................................................................... 15References ................................................................................................................... 17

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Summary This module provides information on the types of digesters, including suggestions on how to identify the appropriate digester for various farm conditions, uses and feedstock mixes.

Learning Objectives1. Describe the process of producing biogas for each digester.2. Describe the conditions best suited for biogas production for each digester.3. Describe the advantages and disadvantages of each digester.4. Understand how to use tools that enable farmers to determine what digester is most appropriate for their

particular farm operation, and detail the pros and cons of potential options.

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Types of Anaerobic Digesters 3

Unit 3.1: Introduction

Unit 3.2: Passive Systems

All anaerobic digesters perform the same basic func-tion. They hold organic matter in the absence of oxygen and maintain the proper conditions for methane forming microorganisms to grow. There are a wide variety of anaerobic digesters, each performing this basic function in a subtly different way. Eight of the most common digesters are described in this module. Construction and material handling techniques can vary greatly within the

main categories. For clarity, digesters are divided into three categories — passive systems, low rate systems, and high rate systems. We will also look at the special case of high solids digestion.

Background A passive system is defined as a system where biogas recovery is added to an existing manure treatment compo-nent. A covered lagoon is an example of a passive system.

Guiding Questions• What is an example of a passive digester?• What are the advantages and disadvantages of a

passive digester?

Covered Lagoon This system captures biogas under an imperme-able cover (Fig. 1), while taking advantage of the low maintenance requirement of a lagoon. Two lagoon cells are needed for the system to operate efficiently. The first cell is covered, and the second cell is uncovered (Fig. 2). A lagoon is storage, as well as, a treatment system. The liquid level on the second cell must rise and fall to create storage, while the level on the first cell remains constant to promote manure breakdown. Since they are not heated, the temperature of covered lagoons follows seasonal patterns. Methane production drops when lagoon tem-peratures dip below 68ºF (20ºC). A covered lagoon located in the tropics will produce biogas year-round, but gas production drops considerably during the winter as you move further north. Since sludge is stored in lagoons for up to 20 years, methane-forming microorganisms also remain in the covered lagoon for up to 20 years. This means that much of the fertilizer nutrients, particularly phosphorus, also remain trapped in the covered lagoon for a long

time. If lagoon effluent is recycled to remove manure from buildings, liquid retention time is generally 30 to 60 days — depending on the size and age of the lagoon.

Advantages• Inexpensive• Easily adapted to hydraulic flushing• Simple construction and management

Disadvantages• Poor mixing• Poor energy yield• Solids settling reduces useable volume• Bacteria can wash out if short circuiting occurs• Limited to warmer weather or warm climates since

digestion depends on temperature


Figure 1: First covered cell of a lagoon located on the Oklahoma State University Swine Research and Education Center. Source: Hamilton

Figure 2: Schematic drawing of covered lagoon digestion system. Source: Hamilton


Complete Mix Digester A complete mix digester is basically a tank in which manure is heated and mixed with an active mass of microorganisms (Fig. 3). Incoming liquid displaces volume in the digester, and an equal amount of liquid flows out. Methane forming microorganisms flow out of the digester with the displaced liquid. Biogas production is maintained by adjusting volume so that liquids remain in the digester for 20 to 30 days. Retention times can be shorter for thermophilic systems. The digester can be continuously mixed, or intermittently mixed. Intermittent mixing means the tank is stirred during feeding and only occasionally between feedings. Sometimes the process takes place in more than one tank. For instance, fermenting bacteria can break down manure in one tank, and then methanogens convert organic acids to biogas in a second tank. Com-plete mix digesters work best when manure contains 3-6

percent solids. Digester size can be an issue at lower solids concentrations. Lower solids mean greater volume, which means you need a larger digester to retain the microbes in the digester for 20 to 30 days (Fig. 4).

Advantages• Efficient• Can digest different levels of dry matter content• May take energy crops• Good mixing• Good solids degradation

Disadvantages• No guarantee on how much time the material spends

in the tank• Bacteria wash out when short circuiting occurs.• Relatively expensive

Unit 3.3: Low Rate Systems

Background A low rate system is defined as a system where feedstock stays in a digester for an extended period of time. Feedstocks in low rate systems typically spend more time in the digester to maximize biogas output. Examples of these types of systems include complete mix and plug flow digesters.

Guiding Questions• What is an example of a low rate digester?• What are the advantages and disadvantages of a low

rate digester?

Figure 3: Schematic drawing of a complete mix digester. Source: Hamilton


Figure 4: Complete mix digester located on the Crave Brothers Farm in Waterloo, Wisconsin. Source: Gould

Plug Flow Digester The idea behind a plug flow digester (Fig. 5) is the same as a complete mix digester — manure flowing into the digester displaces digester volume, and an equal amount of material flows out (Fig. 6). However, the contents of plug flow digester manure are thick enough to keep particles from settling to the bottom. Very little mixing occurs, so manure moves through the digester as a plug — hence the name “plug flow.” Plug flow digesters do not require me-chanical mixing. Total solids (TS) content of manure should be at least 10 or 15 percent, and some operators recom-mend feeding manure with solids as high as 20 percent. Plug flow digesters are usually 5 times longer than they are wide. Recommended retention time is 15 to 20 days.

Advantages• Inexpensive• Simple to operate and fix• Can take energy crops

Disadvantages• Poor mixing• Poor energy yield• Hard top difficult to open to remove settled solids• Membrane top subject to weather (wind and snow)

Figure 5: Schematic drawing of plug flow digester. Source: Hamilton


Figure 6: Plug flow digester located on the Emerling Farm in Perry, NY. Source: Cornell University/USEPA

Unit 3.4: High Rate Systems

Background A high rate system is defined as a system where liquids stay in the digester for a short period of time; where-as, solids are held longer. This allows for a smaller reactor size, while maintain high gas production.

Guiding Questions• What is an example of a high rate digester?• What are the advantages and disadvantages of a high

rate digester?

Low Rate Digesters with Solids Recycle Returning some of the active organisms to the digester decreases digestion time. This is done in plug flow sys-tems by pumping some of the effluent leaving the digester to the front of the digester. In complete mix systems, solids are settled in an external clarifier, and the microbe-rich slurry is recycled back to the digester. When used with plug flow, these types of systems are called Advanced Plug Flow. Complete mix digesters with solids recycle are called Contact Stabilization Digesters or Anaerobic Contact Digesters (Fig. 7).

Advantages• Efficient• Useful microbes returned to reactor• Smaller reactor size

Disadvantages• Increased complexity• More mechanical components


Figure 7: Schematic drawing of contact stabilization digester. Source Hamilton

Fixed Film Digester A fixed film digester (Fig. 8) is essentially a column packed with media, such as wood chips or small plastic rings. Methane forming microorganisms grow on the media. Manure liquids pass through the media (Fig. 9). These digesters are also called Attached Growth Digesters or Anaerobic Filters. The slimy growth coating the media is called a biofilm. Retention times of fixed film digesters can be less than 5 days, making for relatively small digesters. Usually, effluent is recycled to maintain a constant upward flow. One drawback to fixed film digesters is that manure solids can plug the media. A solid separator is needed to remove particles from the manure before feeding the digester. Efficiency of the system depends on the efficiency of the solid separator; therefore, influent manure concentration should be adjusted to maximize separator performance (usually 1-5 percent TS). Some potential biogas is lost due to removing manure solids.

Figure 8: Fixed film digester located at the University of Florida Dairy Research Farm. Source: Ann Wilkie, University of Florida


Figure 9: Schematic drawing of a fixed film digestion system. Source: Hamilton

Figure 10: Schematic diagram of upflow anaerobic sludge blanket (UASB) digester. Source: Hamilton

Advantages• Efficient• Low bacteria wash out• High gas production per volume

Disadvantages• Suspended solids must be removed• Expensive• Plugging of bacterial growth media• Lower gas production due to removed solids

Suspended Media Digesters In these types of digesters, microbes are suspended in a constant upward flow of liquid. Flow is adjusted to allow smaller particles to wash out, while allowing larger ones to remain in the digester. Microorganisms form biofilms around the larger particles, and methanogens stay in the digester. Effluent is sometimes recycled to provide steady upward flow. Some designs incorporate an artificial media such as sand for microbes to form a biofilm; these are called Fluidized Bed Digesters. Sus-

pended media digesters that rely on manure particles to provide attachment surfaces come in many variations. Two common types of suspended media digesters are the Upflow Anaerobic Sludge Blanket Digester or UASB Digester (Fig. 10), and the Induced Blanket Reactor or IBR Digester (Fig. 11). The main difference between these two systems is that UASB digesters are better suited for dilute waste streams (< 3 percent Total Suspended Solids [TSS]); whereas, the IBR digester works best with highly concentrated wastes (6-12 percent TS).

Advantages• Very efficient• Can treat heavy loaded wastewater• Good bacteria retention

Disadvantages• Expensive• Complex operation• Do not accept fat


Figure 11: Schematic drawing of induced bed reactor (IBR) digester. Source: Conly Hansen, Utah State University

Sequencing Batch Digester An Anaerobic Sequencing Batch Reactor (Fig. 12), or ASBR Digester, is a variation on an intermittently mixed digester. Methane forming microorganisms are kept in the digester by settling solids and decanting liquid. An ASBR operates in a cycle of four phases (Fig. 13). The digester is fed during the fill stage, manure and microbes are mixed during the react phase, solids are settled during the settle stage, and effluent is drawn off during the decant stage. The cycle is repeated up to 4 times a day for nearly constant gas production. Liquid retention times can be as short as 5 days. Although ASBR digesters work well with manure in a wide range of solids concentrations, they are particularly well suited for very dilute manures (< 1 percent TS). If filled with active microbes during start-up, they can even produce biogas with completely soluble organic liquids. Sludge must be removed from the ASBR digester

periodically. Concentrated nutrients are harvested during sludge removal.

Advantages• High gas yield per substrate load• Works well with dilute manure• Small reactor size• Can accept high energy liquid co-digestion products

Disadvantages• Complex operation• Relatively expensive• Low gas yield per reactor volume• Works best with low solids substrate


Figure 12: ASBR digester located on the Oklahoma State University Swine Research and Education Center. Source: Hamilton

Figure 13: Four phases of an ASBR digester cycle. Source: Hamilton

Unit 3.5: High Solids Anaerobic Digestion

Background High solids digestion is an emerging field. Every day new processes are being created to anaerobically digest stackable, high solids content organic matter. These processes are similar to anaerobic composters; however a high solids anaerobic digester (HSAD) is specifically designed to capture methane and energy during the process.

Guiding Question• What are the most common types of HSAD?


High Solids Digesters Also known as “dry fermentation”, high solids digestion is used to treat stackable materials such as food scraps, yard debris, and municipal solids wastes. This process is also called HSAD which stands for High Solids Anaerobic Digestion. As the name implies, high solids digesters work with materials that have a solids content of 25% – 50%. Manure may be mixed with the solids to add fermentative microbes. High solids substrates are loaded into digestion chambers. Chambers can either be vertical (silo) (Fig. 14) or horizontal (tunnel) (Fig. 15). Many patented systems have chambers that are operated in either anaerobic or aerobic phases. The chamber is kept airtight during the anaerobic phase. Air is forced into the chamber during the aerobic phase. Leachate is collected at the bottom of chamber during both phases. During the anaerobic phase leachate is high in organic acids. Leachate can either be sprayed on the top of the solids pile within the digestion chamber, and the organic acids are converted to methane by microbes forming a biofilm on the solids. In other systems, the

leachate is pumped to a separate liquid digestion reactor where methane production takes place. With either system, liquids are returned to the solid digestion chamber to aid in solids decomposition. After about two weeks of anaerobic treatment, the chamber is opened to the atmosphere and the partially degraded material is aerobically composted in place, or transferred to windrows for further composting.

Advantages• Can use solid materials• Treats under both anaerobic and aerobic conditions

Disadvantages• Needs Stackable material with at least 25% solids• Complicated process• Expensive

Figure 14: Schematic drawing of silo type high solids digestion system with external leachate digester. Source: ?


Figure 15: Tunnel style high solids anaerobic digestion chamber. Source: www.hotrot.co.uk

Unit 3.6: Choosing a DigesterBackground A well-designed and operated digester requires modest daily attention and maintenance. According to Ag-STAR, the choice of which digester to use is driven by the existing (or planned) manure handling system at the farm (U.S. Environmental Protection Agency, 2004). The digester must be designed to operate as a component of the farm’s operations.

Guiding Question• What digester is most appropriate for a particular farm

operation?

This module has presented information on many types of digesters. Table 1 provides a summary of the character-istics of these digesters.


Character-istic

Covered Lagoon 2

Plug Flow Digester 3

Mixed Plug Flow Digester1

Complete Mix Digester 3

Fixed Film Digester 2,3

Induced Blanket Digester 2

ASBR Digester

High Solids Digester

Digestion vessel

In-ground clay or syntheti-cally lined storage

Rectangular in-ground tank

Rectangular in-ground tank

In/above ground tank

In/above-ground tank

In/above ground tank

In/above- ground tanks

Above ground chambers

Ease of Operation/Maintenance

Low Low Medium Medium Medium High High High

Supplemen-tal heat

No Yes Yes Yes Yes Yes Yes Yes

Total solids 0-6% 11-13% 3-13% 3-10% 2-4% 6-12% 0-3% 25-50%

Solids characteris-tics

Coarse Coarse Medium-Coarse

Coarse Fine - Fine to coarse

Fine to coarse

Hydraulic retention time (days)

60+ 18-20 20-30 5-20 <4 3-5 <6 ≈14

Farm type Dairy, Swine

Dairy Dairy, Swine

Dairy, Swine

Dairy, Swine

Dairy Dairy, Swine Dairy

Optimum location

Temperate / warm

All climates All climates All climates All climates All climates All climates All climates

Table 1: Summary of liquid digester technologies. Sources: 1. VanOrnum, M. 2007. Personal conversation with Charles Gould, October 2007. GHD, Inc., Chilton, Wis. 2. U.S. Department of Agriculture (2007) 3. U.S. Environmental Protection Agency (2002)

Choosing the right digester The decision to install an anaerobic digester is a complex one. Meyer and Power (2011) recommend that farmers identify the management and environmental needs of their farm and then determine whether a digester instal-lation and operation will achieve those objectives. They lay out the benefits and challenges (unintended or undesirable consequences) of digesters and provide detailed informa-tion on evaluating anaerobic digester technology. If it ap-pears that a digester is a viable option, conducting a more in-depth study is the next step. This can be accomplished by using one of two free computer programs referenced in ANDIG 5. The results of either of these programs will determine if a full feasibility study is warranted. Choosing a consultant is also discussed in ANDIG 5. Published digester economic assessments tend to

show that the most successful digesters are those that have generated added value from separated manure fiber, charged tipping fees for accepting off-farm food processing wastes and had a nearby high-value use for the biogas or electricity. Even an unprofitable digester may be regarded as successful if it provides nonmonetary benefits such as odor control (Lazarus, 2012).

Conclusion There are a wide variety of anaerobic digesters. Digest-ers can be divided into three systems — passive, low rate and high rate. Digesters within each system generate biogas in a subtly different ways. Choosing a digester is an important decision and asking the right questions up front before any money is spent is critical in choosing the right digester for a farm.

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Self-quiz questions

1. A passive system is defined as a system where biogas recovery is added to an existing manure treatment component. a. True b. False2. A __________ is an example of a passive system. a. High solids digester b. Covered lagoon c. Complete mix digester d. All of the above3. A low rate system is defined as a system where feedstock stays in a digester for an extended period of time. Feed-stocks in low rate systems typically spend more time in the digester to maximize bio-gas output. a. True b. False4.Examples of low rate systems include: a. Complete mix digester b. Anaerobic Sequencing Batch Reactor c. Plug flow digester d. Both a and c5. A high rate system is defined as a system where liquids stay in the digester for a short period of time; whereas, solids are held longer. This allows for a smaller reactor size, while maintain high gas production. a. True b. False6.Examples of high rate digesters include: a. Fixed Film digester b. Induced Blanket Reactor digester c. Anaerobic Sequencing Batch Reactor digester d. All of the above7. Also known as “dry fermentation”, high solids digestion is used to treat stackable materials such as food scraps, yard debris, and municipal solids wastes. a. True b. False 8.High solids digesters work best with materials that have a solids content of : a. Less than 10% b. 10% - 15% c. 25% - 50% d. 80% - 90%9.The type of digester a particular farm needs depends on: a. The type of manure management system currently in use. b. The amount of water added to manure from milking parlors and flush systems. c. Climate.

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d. All of the above10. Published digester economic assessments tend to show that the most successful digesters are those that: a. Have generated added value from separated manure fiber b. Charged tipping fees for accepting off-farm food processing wastes c. Had a nearby high-value use for the biogas or electricity d. All of the above

Answers1. A2. B3. A4. D5. A6. D7.A8.C9.D10.D

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Unit 3.1Primary source of information: Types of Digesters, Douglas W. Hamilton, Waste Management Specialist, Oklahoma Coop-erative Extension Service. Available at:http://www.extension.org/pages/Types_of_Anaerobic_Digesters verified 10/8/12

Unit 3.2Anon. 2007. Feasibility Study-Anaerobic Digester and Gas Processing Facility in the Fraser Valley, British Columbia (Appendix C). Prepared by Electrigaz Technologies Inc, 7-10301 Ch. Sainte-Marguerite Trois-Rivières (Quebec) G9B 6M6 Canada. Appendix C available at:http://www.llbc.leg.bc.ca/public/pubdocs/bcdocs/443243/feasibility_study_anaerobic_appendices.pdf verified 10/8/12 The full feasibility study available at:http://www.llbc.leg.bc.ca/public/pubdocs/bcdocs/443243/feasibility_study_anaerobic.pdf verified 10/8/12

D.W. Hamilton. 2011. Lagoons for Livestock Waste Treatment. BAE 1736. Stillwater OK: Oklahoma Cooperative Extension Service.

D. Jones. 2006. PowerPoint presentation entitled Anaerobic Digestion – The Basics. Available at:www.in.gov/oed/files/Presentation_Don_Jones.ppt verified 10/8/12

Unit 3.3Anon. 2007. Feasibility Study-Anaerobic Digester and Gas Processing Facility in the Fraser Valley, British Columbia (Appendix C). Prepared by Electrigaz Technologies Inc, 7-10301 Ch. Sainte-Marguerite Trois-Rivières (Quebec) G9B 6M6 Canada. Appendix C available at:http://www.llbc.leg.bc.ca/public/pubdocs/bcdocs/443243/feasibility_study_anaerobic_appendices.pdf verified 10/8/12 The full feasibility study available at:http://www.llbc.leg.bc.ca/public/pubdocs/bcdocs/443243/feasibility_study_anaerobic.pdf verified 10/8/12

D.W. Hamilton. 2010. Anaerobic Digestion of Animal Manures: Types of Digesters. BAE 1750. Stillwater, OK: Oklahoma Cooperative Extension Service.

D. Jones. 2006. PowerPoint presentation entitled Anaerobic Digestion – The Basics. Available at: www.in.gov/oed/files/Presentation_Don_Jones.ppt verified 10/8/12 Unit 3.4Anon. 2007. Feasibility Study-Anaerobic Digester and Gas Processing Facility in the Fraser Valley, British Columbia (Appendix C). Prepared by Electrigaz Technologies Inc, 7-10301 Ch. Sainte-Marguerite Trois-Rivières (Quebec) G9B 6M6 Canada.

References

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http://www.extension.org/pages/Types_of_Anaerobic_Digesters

http://www.llbc.leg.bc.ca/public/pubdocs/bcdocs/443243/feasibility_study_anaerobic_appendices.pdf

http://www.llbc.leg.bc.ca/public/pubdocs/bcdocs/443243/feasibility_study_anaerobic.pdf

www.in.gov/oed/files/Presentation_Don_Jones.ppt



www.in.gov/oed/files/Presentation_Don_Jones.ppt


Learning for life

Appendix C available at:http://www.llbc.leg.bc.ca/public/pubdocs/bcdocs/443243/feasibility_study_anaerobic_appendices.pdf verified 10/8/12 The full feasibility study is available at: http://www.llbc.leg.bc.ca/public/pubdocs/bcdocs/443243/feasibility_study_anaerobic.pdf verified 10/8/12

Unit 3.5Anon. Undated. PowerPoint presentation entitled Swine Digesters: commercial, scalable, operating. Available at:http://www.chpcentermw.org/pdfs/070531-Indiana/8%20Speaker%208%20Larsen.pdf verified 10/8/12

M.C. Gould, Crook, M. 2010. Michigan On-farm Anaerobic Digester Operator Handbook. Michigan State University Extension, East Lansing, MI.

U.S. Department of Agriculture. 2007. An Analysis of Energy Production Costs from Anaerobic Digestion Systems on U.S. Livestock Production Facilities. Technical Note 1. Washington, D.C.: Natural Resources Conservation Service.

U.S. Environmental Protection Agency. 2002. Managing Manure with Biogas Recovery Systems Improved Performance at Competitive Costs. EPA-430-F-02-004. Washington, D.C.: AgSTAR Program.

Moser. Washington D.C.: AgSTAR Program. Available at:http://www.epa.gov/agstar/documents/chapter1.pdf verified 10/8/12

M. VanOrnum. 2007. Personal conversation with M. Charles Gould, October 2007.

U.S. Environmental Protection Agency. 2004. AgSTAR Handbook: A Manual for Developing Biogas Systems at Commer-cial Farms in the United States. Chapter 1. Overview of Biogas Technology. Editors: K.F. Roos, J.B. Martin, Jr., and M.A. Moser. Washington D.C.: AgSTAR Program. Available at: http://www.epa.gov/agstar/documents/chapter1.pdf verified 10/8/12

D. Meyer, Power, T. 2011. Manure Treatment Technologies: Anaerobic Digesters. Publication 8409. University of California Agriculture and Natural Resources. University of California, Davis, CA. Available at: http://anrcatalog.ucdavis.edu/pdf/8409.pdf verified 10/8/12

W.F. Lazarus. 2012. Economics of Anaerobic Digesters for Processing Animal Manure. Available at: http://www.extension.org/pages/Economics_of_Anaerobic_Digesters_for_Processing_Animal_Manure verified 10/8/12

D. Hamilton. 2012. Types of Anaerobic Digesters. Module 3 in S. Lezberg, C. Gould and M. Jungswirth (eds.). Introduction to Anaerobic Digestion Course. On-line Curriculum. Bioenergy Training Center. http://fyi.uwex.edu/biotrainingcenter

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http://www.chpcentermw.org/pdfs/070531-Indiana/8%20Speaker%208%20Larsen.pdf

http://www.epa.gov/agstar/documents/chapter1.pdf

http://www.epa.gov/agstar/documents/chapter1.pdf

http://anrcatalog.ucdavis.edu/pdf/8409.pdf

http://www.extension.org/pages/Economics_of_Anaerobic_Digesters_for_Processing_Animal_Manure

Module 3: Types of Anaerobic Digesters - eXtension Publish · Module 3: Types of Anaerobic Digesters ... Plug Flow Digester ... Describe the advantages and disadvantages of each digester.

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