Anaerobic Digestion: Co-Digestion and Operational Issues
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Proceedings available at: http://www.extension.org/67744 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. DIGESTER OPTIMIZATION Becky Larson April 5, 2013
- 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. Anaerobic Digestion in the U.S. USEPA AgSTAR, 2011
- 6. Anaerobic Digestion Process Flow Slurry Biogas SeparatorDigestate Filtrate (liquid) Fiber (solid) Feedstock's
- 7. Characterized by rapid bacterial growth “Weak link” in the process of producing biogas Goal?...Avoid inhibition, maintain stabilization! Stage Three: Acetogenesis Stage One: Hydrolysis Stage Two: Acidogenesis Stage Four: Methanogenesis CO2 reducing methanogens aceticiastic methanogens acetogens (H2 producing) fermentative bacteria fermentative bacteria PROPIONATE, BUTYRATE, etc. (Short-chain volatile organic acids) fermentative bacteria fermentative bacteria COMPLEX POLYMERS (Proteins, polysaccharides, etc.) MONOMERS AND OLIGOMERS (Sugars, amino acids, peptides) acetogens (H2 consuming) H2 + CO2 ACETATE CH4, CO2 Source: Syed Hashsham, PhD, lecture notes, Michigan State University The Biological Process of Producing Biogas
- 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. Different Digester Organisms Result in Different Biogas Yields Control = Seed biomass microbial community Active = Different microbial community (bioaugmented) Source: Kaushik Venkiteshwaran, Ph.D. student, Marquette University 15% more methane
- 10. Anaerobic Digestion Feedstocks Cheese Whey Swine Manure Dairy Manure Cattle Manure Cucumber Waste Yard Clippings Municipal Organics Human Waste Food Processing Waste Vegetables Grasses
- 11. Gas Yields of Different Feedstocks Kestutis Navickas. 2007. Bioplin Tehnologija in Okolje,
- 12. Additional Substrates • What to use? • Avoid toxicity – trust your supplier (now have brokers) • Producers range – smart ones are cautious • Evaluate additional substrates • What will substrates do to the digestate? • Rule of thumb – must be worth more than the cost of spreading (some use the idea that tipping fees must be ≥ application costs) • Load slowly Electrical $ 420,000 Bedding $ 275,000 Fertilizer $ 440,000 Heating $ 30,000 Carbon C $ 125,000 Tipping $ 75,000 Tax Credit $ 40,000 Total $1,384,000 Revenues from Digester
- 15. Cheese Whey and Cheese Whey Permeate • Evaluated in triplicates • 0% (control – all manure), 20%, 40%, 80%, and 100% substrate additions • Varying performance • Generally < 40% reached a maximum biogas production
- 16. Mixing • Microorganisms must come into contact with the food • Benefits of mixing • Speeding up the breakdown of the volatile solid • Increase in biogas production • There is a cost associated with mixing! • Still done on a trail and error basis
- 17. Case Study: Optimizing a Small Scale Digester
- 18. Case Study • Don’t assume anything! • Make operators record EVERYTHING 1.Verify temperature throughout tank 2.Take measurements (pH, VS, COD, etc.) over time 3.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 low destruction • Increased temperature resulted in 2-3x the biogas production
- 19. Destruction
- 20. Evaluating Toxicity – BMP’s
- 21. Take Away • Many things affect digester performance, optimize one by one, 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 to understand the system • Make the simple and low cost adjustments first, typically have the highest pay back • Future: • Mixing impacts to methanogens and modeling to reduce build-up (we will finish eventually)
Editor's Notes
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