Technology Type - Anaerobic Digestion
Technology Strengths,Weaknesses and Critical Indicators
Anaerobic Digester technology:
- Long usable life and can be run reliably
- Creates energy and generates environmental credits
- Requires proper preparation of the feedstock
- Requires other technologies for energy utilization
- Requires other technologies for digestate handling
- Proper feeding & system monitoring is required to avoid system downtime
- Proven technology for odor control, GHG reduction and pathogen reduction
- 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).
- 200 dairy-based U.S. installations and thousands worldwide.
- Refined, standard designs available from multiple technology providers.
- 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.
- 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.
- 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.
- Currently, most systems are farmer managed, more consistent results may be achieved by dedicated operators.
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Massé, D. I., Talbot, G., & Gilbert, Y. (2011). On farm biogas production: A method to reduce GHG emissions and develop more sustainable livestock operations. Animal feed science and technology, 166, 436-445.
Möller, K., & Müller, T. (2012). Effects of anaerobic digestion on digestate nutrient availability and crop growth: a review. Engineering in Life Sciences, 12(3), 242-257.
Owen, J. J., & Silver, W. L. (2015). Greenhouse gas emissions from dairy manure management: a review of field‐based studies. Global change biology, 21(2), 550-565.
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Summers, M., Williams D. (2013). Energy and environmental performance of six dairy digester systems in California. A final report for the Energy, Economic, and Environmental Performance of Dairy Bio-power and Bio-methane Systems project (contract number PIR-08-041) conducted by Summers Consulting, LLC. CED-500-2014-001-VI, March 2013.
Topper, P. A., Graves, R. E., & Richard, T. (2006). The fate of nutrients and pathogens during anaerobic digestion of dairy manure. Lehman (PA): Penn State University. College of Agricultural Science, Cooperative Extension Bulletin G, 71.
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Wright, P. E., Inglis, S. F., Stehman, S. M., & Bonhotal, J. (2003). Reduction of selected pathogens in anaerobic digestion. In Animal, Agricultural and Food Processing Wastes-IX (p. 1). American Society of Agricultural and Biological Engineers.
Technology Providers in order of 9 - Point Scoring System