Technology Type - Anaerobic Digestion

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.
• Renewable thermal heat production – 13,500 Btu’s/cow or more possible each year.
• Less impact on water quality.
• Increased crop yields possible.

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. Pretreatment to remove bedding sand is required with sand-bedded stalls.
• Pre-treatment may be used to remove excess moisture from influent from barns were 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.