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Anaerobic Digestion – the Basics

Anaerobic Digestion – the Basics. Don Jones Ag and Biological Engineering, Purdue Summer 2006. What is Anaerobic Digestion?. Anaerobic digestion occurs when organic material decomposes biologically in the absence of oxygen. Benefits of Anaerobic Digestion.

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Anaerobic Digestion – the Basics

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  1. Anaerobic Digestion – the Basics Don JonesAg and Biological Engineering, PurdueSummer 2006

  2. What is Anaerobic Digestion? • Anaerobic digestion occurs when organic material decomposes biologically in the absence of oxygen

  3. Benefits of Anaerobic Digestion • Odor reduction (digestion stabilizes degradable organic matter) • Digested manure retains most of its nutrient/fertilizer value • Digestion minimizes release of methane to atmosphere during later storage

  4. History of Biogas • 10th century BC - Used to heat water in Assyria • 16th century – Used to heat water in Persia • 17th century - Flammable gases found to be emitted from decaying organic matter • 1776 - Relationship between the amount of decaying organic matter and the amount of flammable gas produced demonstrated (www.biogasworks.com)

  5. History, cont. • 1808 - Methane produced via controlled anaerobic digestion of cattle manure • 1859 - First digestion plant built in Bombay • 1895 - Biogas recovered from a sewage treatment plant in England fueled street lamps • 1930s - Developments in microbiology identified the anaerobic bacteria and conditions needed to promote methane production (www.biogasworks.com)

  6. History, cont. • 1970s - Energy crisis renewed interest in AD • 1970s - 80s - Lack of understanding and overconfidence resulted in numerous failures • China, India and Thailand reported 50% failure rates • Failures of farm digesters in U.S. approached 80% Shefali Verma, Fu Foundation Engineering and Applied Science, Columbia University

  7. Reasons for Failures • Inadequate operator training • Benefits oversold • Operations too small to justify digester • Excessive operating costs • Unreliable market for biogas • Management failures

  8. What’s Different Now: • Improved designs • Better understanding of O&M requirements and options to accomplish • High prices for liquid fuel & natural gas • Market evolving for “carbon credits” as well as biogas energy

  9. Currently - • Hundreds of farm-based digesters operating in Europe plus several centralized AD systems • Danish systems co-digest manure, organic industrial wastes and municipal solid waste • Large numbers of family-sized, low technology digesters in developing world provide biogas for cooking and lighting • Renewed interest in U.S. Shefali Verma, Fu Foundation Engineering and Applied Science, Columbia University

  10. Environmental Benefits • Reduces odor from land application • Protects water resources • Reduces pathogens • Weed seed reduction • Fly control after digestion • Greenhouse gas reduction

  11. Biogas Production Liquefaction Acid Production End Products Liquefying Bacteria Acid-Forming Bacteria Methane-Forming Bacteria Biogas (Methane, CO2, misc.) Liquefied soluble organic compounds Simple organic acids Effluent Insoluble Compounds(organic, inorganic, water) Manure Anaerobic Digestion Process

  12. Temperature • Two distinct temperature ranges in which anaerobic bacteria produce significant amounts of methane gas – • mesophillic range (90-110F) • thermophillic range (120-140F).

  13. Gas Production Potential - Manure Daily BioGas Production Per Mature Animal* Animal Type 50-80 ft3 (~40,000 Btu) 4-6 ft3 (~3000 Btu) Dairy cow Hog Source: EPA 2005 * Optimistic for most farms

  14. BioGas cleanup • Corrosion can be a problem in contact with metal. May need treatment to remove. hydrogen sulfide and water vapor from gas. • Use condenser to remove water vapor alone. • Use of molecular sieve to remove carbon dioxide in future? – Space Station technology.

  15. Types of Digesters • Covered Lagoon • Complete Mix • Plug Flow • Fixed Media

  16. Medium/High Low Medium Deep lagoon Round, square in/above ground Rectangular in ground No Yes Yes Typical Digesters in Midwest Covered Lagoon Complete Mix Plug Flow Level of Technology Digestion Vessel Supplemental Heat

  17. 0.5-2% 3-8% 6-11% 45+ 15+ 15+ Warm climates All climates All climates Typical Digesters, cont. Covered Lagoon Complete Mix Plug Flow Solids Concentration HRT 1 (days) Optimum Location 1 HRT = Hydraulic Retention Time

  18. GAS UTILIZATION LAND APPLICATION Cover COLLECTION PRE- TREATMENT DIGESTER STORAGE Lagoon typically ½-2% solids Covered Lagoon Source: EPA 2005

  19. Covered Lagoon • Advantages: • Relatively inexpensive to build • Adaptable to hydraulic flushing • Construction and management is simple • Limitations: • Limited to warm weather areas since digestion depends on temperature • Manure not completely digested, so odor released when land applied Source: David Downing, Iowa Department of Natural Resources, 2002

  20. GAS UTILIZATION LAND APPLICATION COLLECTION PRE- TREATMENT STORAGE DIGESTER Slurry typically 3-10% solids Complete Mix Digester Source: EPA 2005

  21. Complete Mix System • Advantages: • Mechanical mixers prevents crust and keeps solids from collecting on the bottom of the digester tank, delaying cleaning • Limitations: • Requires more maintenance Source: David Downing, Iowa Department of Natural Resources, 2002

  22. GAS UTILIZATION LAND APPLICATION PRE- TREATMENT COLLECTION DIGESTER STORAGE Slurry typically 6 - 11% solids Plug Flow Digester Source: EPA 2005

  23. Plug Flow Digester • Advantages: • Medium cost range • Simpler design • Less labor/management intensive to operate • Limitations: • Most compatible with daily scrape dairy systems • Requires pre-mix tank Source: David Downing, Iowa Department of Natural Resources, 2002

  24. Fixed-film Anaerobic Filter • Upflow digester with porous media • Bacteria grows on media and feeds on organics passing by • Wastewater passes thru in 4-6 days but slow-growing bacteria remain in digester • Low Food:Mass ration makes system stable

  25. Carbon Credit Programs • Greenhouse gases - Methane is over 20 times more damaging than carbon dioxide • Agricultural carbon credits traded on exchange to industries who have trouble meeting environmental requirements • Several producers already enrolled in carbon credit programs worth millions of dollars (www.envcc.com) Source: EPA 2005

  26. Safety Considerations • Methane is extremely explosive when mixed with air at the proportions of 6-15 percent methane • Digester gas is heavier than air and settles to the ground, displacing oxygen • If hydrogen sulfide is present, digester gas can be a deadly poison

  27. 51 Plug Flow 26 Complete Mix Source: EPA 2005 1 2-Stage Mix 5 MesophilicCoveredlagoon 1FixedMedia 13AmbientTemperatureCovered Lagoons Agricultural Digesters in US Source: EPA 2005

  28. Methane Energy Recovery Incentives AgSTAR • Directed to show dairy and pork producers "how to manage manure profitably while protecting the environment. • For potential funding information, contact AgSTAR: 1-800-95-AgSTAR http://www.epa.gov/outreach/agstar/iondex.htm

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