With over 200 dairy-based U.S. installations and thousands worldwide, anaerobic digestion (AD), is a proven biological approach to stabilize organic matter, including dairy manure, with simultaneous production of renewable energy and reduction of GHG emissions, while also mitigating several important environmental concerns.

 With various manure management approaches across different scales and climate conditions, a variety of anaerobic digestion technologies, with different yields and performance efficiencies, are employed. Analysis of U.S.-based on-farm digesters show concerns related to consistent and effective operations, long-term economic viability, equipment runtimes, efficiencies and capacity utilization—all pointing to a need for improved project development and operations moving forward as an industry.

 Despite the varying designs and operational parameters for different digesters, studies indicate that AD of dairy manure, when effectively operated, typically reduces organic matter content indicators such as Total Solids (TS, 20-35%), Volatile Solids (VS, 30-45%),  Chemical Oxygen Demand (COD, 40-50%), and odor-causing Volatile Fatty Acids (VFAs, 70-95%), while producing biogas at a rate of 4 to 7 ft³ per pound of VS added daily (0.24 +/- 0.06 m³ CH₄/kg VS; 90-115 ft³ biogas/Holstein wet cow/day when all manure treated). Methane content for biogas produced from the digestion of dairy manure is usually between 50 and 58%.  With respect to the reduction of VFAs, AD reduces odor indicating compounds by 70-95% as compared to untreated manure. Long retention time and high temperature conditions during digestion lead to significant reductions in various indicator pathogens, on the approximate order of 2-3 log (90%+) for the most common indicator (fecal coliforms). Whole farm analyses show that AD significantly reduces emissions of greenhouse gas, especially methane, that is otherwise released during baseline non-treatment, playing a significant mitigating role. Produced effluent is composed of initial nutrient inputs (less atmospheric and in-vessel accumulation losses) with increased pH, reduced carbon content as well as C to N ratio, while increasing ammonia-N to total-N ratio, partially mineralizing phosphorus, and overall decreasing viscosity.