Anaerobic Digestion: Co-Digestion and Operational Issues


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A study was conducted to assess the performance of various mixing regimes on methanogen biomass content in anaerobic digesters. Methane production in anaerobic digesters is directly related to the methanogens within the system. Current systems involve mixing to increase biogas production and system efficiency, however little is known about the underlying mechanisms of this relationship. In this study three pilot scale anaerobic digestion systems with three different mixing regimes were run with replication to examine the impacts to methanogen biomass content and biogas production. The results will provide insight for operational recommendations as well as the basic microbial processes with digestion systems which are critical for optimization.

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  • Anaerobic Digestion: Co-Digestion and Operational Issues

    1. 1. DIGESTER OPTIMIZATIONBecky LarsonApril 5, 2013
    2. 2. Why install a digester?• Energy independence• Reductions in green house gases• Flexibility in manure management• Reduced odors & pathogens• Reduction in other environmental impacts• Potential for additional asset streams (?)• Bedding• Tipping fees• Environmental credits• Do I dare say profits?• What can’t digesters do?
    3. 3. Anaerobic Digestion in the U.S.USEPA AgSTAR, 2011
    4. 4. U.S. System Types
    5. 5. Why Optimize?
    6. 6. Anaerobic Digestion Process FlowSlurryBiogasSeparatorDigestateFiltrate (liquid)Fiber (solid)Feedstocks
    7. 7. Characterized byrapid bacterialgrowth“Weak link” inthe process ofproducingbiogasGoal?...Avoidinhibition,maintainstabilization!Stage Three:AcetogenesisStage One:HydrolysisStage Two:AcidogenesisStage Four:MethanogenesisCO2 reducingmethanogensaceticiasticmethanogensacetogens(H2producing)fermentativebacteriafermentativebacteriaPROPIONATE,BUTYRATE, etc.(Short-chain volatileorganic acids)fermentativebacteriafermentativebacteriaCOMPLEX POLYMERS(Proteins, polysaccharides, etc.)MONOMERS AND OLIGOMERS(Sugars, amino acids, peptides)acetogens(H2 consuming)H2 + CO2 ACETATECH4, CO2Source: Syed Hashsham, PhD, lecture notes, Michigan State UniversityThe Biological Process of Producing Biogas
    8. 8. Key Parameters• Temperature – sensors, controls (~100°F,~135°F)• pH (methanogens 6.4-8.2; manure = good buffer)• Microorganism populations• Feedstocks/Loading• Mixing• Limit Toxicity• Micro/Macro Nutrients• HRT – engineering design
    9. 9. Different Digester Organisms Result in DifferentBiogas YieldsControl = Seed biomass microbial communityActive = Different microbial community (bioaugmented)Source: Kaushik Venkiteshwaran, Ph.D. student, Marquette University15% more methane
    10. 10. Anaerobic Digestion FeedstocksCheese WheySwine ManureDairy ManureCattle ManureCucumberWasteYard ClippingsMunicipal OrganicsHuman WasteFood ProcessingWasteVegetablesGrasses
    11. 11. Gas Yields of Different FeedstocksKestutis Navickas. 2007. Bioplin Tehnologija in Okolje,
    12. 12. Additional Substrates• What to use?• Avoid toxicity – trust yoursupplier (now have brokers)• Producers range – smart onesare cautious• Evaluate additional substrates• What will substrates do to thedigestate?• Rule of thumb – must be worthmore than the cost of spreading(some use the idea that tippingfees must be ≥ application costs)• Load slowlyElectrical $ 420,000Bedding $ 275,000Fertilizer $ 440,000Heating $ 30,000Carbon C $ 125,000Tipping $ 75,000Tax Credit $ 40,000Total $1,384,000Revenues from Digester
    13. 13. Examining Additional Feedstocks
    14. 14. Examining Additional Feedstocks
    15. 15. Cheese Whey and Cheese WheyPermeate• Evaluated in triplicates• 0% (control – all manure), 20%, 40%, 80%, and 100%substrate additions• Varying performance• Generally < 40% reached a maximum biogas production
    16. 16. Mixing• Microorganisms must comeinto contact with the food• Benefits of mixing• Speeding up the breakdown ofthe volatile solid• Increase in biogas production• There is a cost associatedwith mixing!• Still done on a trail and errorbasis
    17. 17. Case Study: Optimizing a Small ScaleDigester
    18. 18. Case Study• Don’t assume anything!• Make operators record EVERYTHING1.Verify temperature throughout tank2.Take measurements (pH, VS, COD, etc.) over time3.Don’t change things quickly, allow time to stabilize• Lower gas production than predicted• Made lots of changes to feedstocks and feed operations• Contemplated design changes• Retention time was too short, found through lowdestruction• Increased temperature resulted in 2-3x the biogasproduction
    19. 19. Destruction
    20. 20. Evaluating Toxicity – BMP’s
    21. 21. Take Away• Many things affect digester performance, optimize one byone, may need to go back• Feedstocks need to be evaluated on an individual basis• Mixing is still a trail and error process• Digesters require operators and farm employees tounderstand the system• Make the simple and low cost adjustments first, typicallyhave the highest pay back• Future:• Mixing impacts to methanogens and modeling to reducebuild-up (we will finish eventually)
    22. 22. Becky 890-3171Thank you!