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)

Becky Larson

ralarson2@wisc.edu

(608) 890-3171

Thank you!