Technology Type - Anaerobic Digestion
Technology Strengths,Weaknesses and Critical Indicators
Anaerobic Digester Associated Technology:
NOTE: There are several technology types that are used as part of an integrated manure management system that includes an anaerobic digester and are not applicable to manure management in other cases. The impact of these technologies on the critical indicators are represented as those of an entire anaerobic digester system.
- 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. 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 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.
Technology Providers in order of 9 - Point Scoring System