Anaerobic Digestion

Overall Summary

 Primary Application

• Dairy farms with over 500 cows or farms with meaningful organics for co-digestion.
• Vacuumed/scraped manure, manure slurries, bedded pack that is diluted with digester effluent.

 Economic/Return on Investment Considerations

• Economics are almost always a challenge; on a value of renewable energy basis, AD is hard to justify, at present received prices for electricity and gas.
• AD does provide several non-monetary benefits to a farm (see below).

 Industry Uptake

• 200 dairy-based U.S. installations and thousands worldwide.

Technology Maturity

• Refined, standard designs available from multiple technology providers.

Primary Benefits

• Odor reduction – 70 to 95% reduction of indicator acids.
• Manure organic matter reduction – 35%.
• Renewable electrical energy production – 2,000 kWh/cow possible each year.
• Pathogen reduction – 90%+ elimination of fecal coliform organisms as a typical indicator pathogen.
• Greenhouse gas emission reduction – amount varies by location and farm-specific, but reductions can be large, on the order of 67%+.
• Nutrient preservation/transformation – key crop nutrients in manure are not consumed by AD and the nutrient form is more plant available than when not digested.
• Contributes to society’s goal for organic landfill diversion – co-digestion easily achieved enhancing above benefits.

Secondary Benefits

• Pre-treatment for tertiary treatments like ammonia stripping.
• Post treatment of waste separation can produce adequate recycled manure solids for bedding livestock.
• Renewable thermal heat production – 13,500 Btu’s/cow or more possible each year.
• Nutrients converted for a more plant available form.
• Increased crop yields possible.
• Possible reduction of impact on water quality.

How it Works

• Raw or pre-treated manure is conveyed into a gas tight vessel on a regular basis (daily or more often) that operates at a set temperature (38 ⁰C in most cases).
• Naturally occurring microbes in manure break solids down into energy-rich biogas.
• Biogas is used to fuel engine-generators to make electricity or is cleaned to make a natural gas replacement.
• Some of the produced gas, or heat produced by an engine-generator set is used to heat the digester making it a net energy production system.

 Pre-treatment and/or Post-treatment Required

• Pre-treatment not required when organic material is used to bed stalls and/or when manure is not substantially diluted. Pre-treatment to remove bedding sand is required with sand-bedded stalls.
• Pre-treatment may be used to remove excess moisture from influent from barns where hydraulic flushing is used.
• Post-treatment not required but may be employed based on overall goals of the manure treatment system.

Limitations

• Does not reduce volume.
• Does not work well with raw manure containing bedding sand.
• Does not work with highly diluted manure due to cost and heat demands for a large vessel.

Other Considerations

• Currently, most systems are farmer managed, more consistent results may be achieved by dedicated operators.
• Adding co-digestible material will increase the nutrient content of the digestate and will need to be addressed in the nutrient management plan.
• A portion of nutrients are converted from organic to inorganic. More nutrients are available for immediate update by crops, so nutrient management plans should be updated to reduce the potential for water quality concerns.