Presentation on theme: "Anaerobic Digestion – the Basics"— Presentation transcript:
1 Anaerobic Digestion – the Basics Don Jones Ag and Biological Engineering, Purdue Summer 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 valueDigestion 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 Persia17th century - Flammable gases found to be emitted from decaying organic matterRelationship between the amount of decaying organic matter and the amount of flammable gas produced demonstrated(www.biogasworks.com)
5 History, cont.Methane produced via controlled anaerobic digestion of cattle manureFirst digestion plant built in BombayBiogas recovered from a sewage treatment plant in England fueled street lamps1930s - 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 failuresChina, India and Thailand reported 50% failure ratesFailures 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 digesterExcessive operating costsUnreliable market for biogasManagement failures
8 What’s Different Now: Improved designs Better understanding of O&M requirements and options to accomplishHigh prices for liquid fuel & natural gasMarket evolving for “carbon credits” as well as biogas energy
9 Currently -Hundreds of farm-based digesters operating in Europe plus several centralized AD systemsDanish systems co-digest manure, organic industrial wastes and municipal solid wasteLarge numbers of family-sized, low technology digesters in developing world provide biogas for cooking and lightingRenewed interest in U.S.Shefali Verma, Fu Foundation Engineering and Applied Science, Columbia University
10 Environmental Benefits Reduces odor from land applicationProtects water resourcesReduces pathogensWeed seed reductionFly control after digestionGreenhouse gas reduction
11 Anaerobic Digestion Process LiquefactionLiquefyingBacteriaAcid ProductionLiquefiedsoluble organiccompoundsInsolubleCompounds (organic,inorganic,water)Acid-FormingBacteriaBiogasProductionSimpleorganicacidsMethane-FormingBacteriaBiogas(Methane,CO2, misc.)EffluentEnd ProductsManureFigure77. Schematic of the biological processes involved with methane production from manure.
12 TemperatureTwo distinct temperature ranges in which anaerobic bacteria produce significant amounts of methane gas –mesophillic range (90-110F)thermophillic range ( F).
13 Gas Production Potential - Manure Daily BioGasProduction PerMature Animal*Animal Type50-80 ft3 (~40,000 Btu)4-6 ft3 (~3000 Btu)Dairy cowHogSource: EPA 2005* Optimistic for most farms
14 BioGas cleanupCorrosion 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 DigestersCovered LagoonComplete MixPlug FlowFixed Media
16 Typical Digesters in Midwest CoveredLagoonCompleteMixPlugFlowLevel ofTechnologyLowDeeplagoonNoMedium/HighRound, squarein/above groundYesMediumRectangularin groundYesDigestionVesselSupplementalHeat
18 Covered Lagoon Cover Lagoon typically ½ -2% solids Source: EPA 2005 DIGESTERGASUTILIZATIONLagoon typically ½ -2% solidsCoverSTORAGELANDAPPLICATIONCOLLECTIONPRE-TREATMENTSource: EPA 2005
19 Covered Lagoon Advantages: Limitations: Relatively inexpensive to buildAdaptable to hydraulic flushingConstruction and management is simpleLimitations:Limited to warm weather areas since digestion depends on temperatureManure not completely digested, so odor released when land appliedSource: David Downing, Iowa Department of Natural Resources, 2002
20 Complete Mix Digester Slurry typically 3-10% solids Source: EPA 2005 GASUTILIZATIONSlurry typically 3-10% solidsSTORAGELANDAPPLICATIONCOLLECTIONPRE-TREATMENTSource: EPA 2005
21 Complete Mix System Advantages: Limitations: Mechanical mixers prevents crust and keeps solids from collecting on the bottom of the digester tank, delaying cleaningLimitations:Requires more maintenanceSource: David Downing, Iowa Department of Natural Resources, 2002
22 Plug Flow Digester Slurry typically 6 - 11% solids Source: EPA 2005 GASUTILIZATIONSlurry typically % solidsSTORAGELANDAPPLICATIONCOLLECTIONPRE-TREATMENTSource: EPA 2005
23 Plug Flow Digester Advantages: Limitations: Medium cost range Simpler designLess labor/management intensive to operateLimitations:Most compatible with daily scrape dairy systemsRequires pre-mix tankSource: David Downing, Iowa Department of Natural Resources, 2002
24 Fixed-film Anaerobic Filter Upflow digester with porous mediaBacteria grows on media and feeds on organics passing byWastewater passes thru in 4-6 days but slow-growing bacteria remain in digesterLow Food:Mass ration makes system stable
25 Carbon Credit Programs Greenhouse gases - Methane is over 20 times more damaging than carbon dioxideAgricultural carbon credits traded on exchange to industries who have trouble meeting environmental requirementsSeveral 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 methaneDigester gas is heavier than air and settles to the ground, displacing oxygenIf hydrogen sulfide is present, digester gas can be a deadly poison
27 Agricultural Digesters in US Source: EPA 200512-Stage Mix51Plug Flow26Complete Mix5Mesophilic Covered lagoon13 Ambient Temperature Covered Lagoons1 Fixed MediaAgricultural Digesters in USSource: EPA 2005
28 Methane Energy Recovery Incentives AgSTARDirected to show dairy and pork producers "how to manage manure profitably while protecting the environment.For potential funding information, contact AgSTAR: AgSTAR
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