DIGESTER OPTIMIZATIONBecky Larson
April 5, 2013
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?
Anaerobic Digestion in the U.S.
USEPA AgSTAR, 2011
U.S. System Types
Why Optimize?
Anaerobic Digestion Process Flow
Slurry
Biogas
SeparatorDigestate
Filtrate (liquid)
Fiber (solid)
Feedstock's
Characterized by rapid bacterial growth
“Weak link” in the process of producing biogas
Goal?...Avoid inhibition, maintainstabilization!
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
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
Different Digester Organisms Result in Different Biogas Yields
Control = Seed biomass microbial communityActive = Different microbial community (bioaugmented)Source: Kaushik Venkiteshwaran, Ph.D. student, Marquette University
15% more methane
Anaerobic Digestion Feedstocks
Cheese Whey
Swine Manure
Dairy Manure
Cattle Manure
Cucumber Waste
Yard ClippingsMunicipal
Organics
Human Waste
Food Processing Waste
Vegetables
Grasses
Gas Yields of Different Feedstocks
Kestutis Navickas. 2007. Bioplin Tehnologija in Okolje,
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 $ 420,000Bedding $ 275,000Fertilizer $ 440,000Heating $ 30,000Carbon C $ 125,000Tipping $ 75,000Tax Credit $ 40,000Total $1,384,000
Revenues from Digester
Examining Additional Feedstocks
Examining Additional Feedstocks
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
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
Case Study: Optimizing a Small Scale Digester
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 low
destruction • Increased temperature resulted in 2-3x the biogas
production
Destruction
Evaluating Toxicity – BMP’s
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)