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
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.
Holly, M. A., Larson, R. A., Powell, J. M., Ruark, M. D., & Aguirre-Villegas, H. (2017). Greenhouse gas and ammonia emissions from digested and separated dairy manure during storage and after land application. Agriculture, Ecosystems & Environment, 239, 410-419.
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.
Page, L. H., Ni, J. Q., Heber, A. J., Mosier, N. S., Liu, X., Joo, H. S., ... & Harrison, J. H. (2014). Characteristics of volatile fatty acids in stored dairy manure before and after anaerobic digestion. biosystems engineering, 118, 16-28.
Pain, B.F., Misselbrook, T.H., Clarkson, C.R. & Rees, Y.J. (1990). Odour and ammonia emissions following the spreading of anaerobically-digested pig slurry on grassland. Biol. Wastes 34:259-267.
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.
Welsh, F.W., Schulte, D.D., Kroeker, E.J. & Lapp, H.M. (1977). The effect of anaerobic digestion upon swine manure odors. Can. Agric. Eng. 19:122-126. Wilkie, A. C. (2000). Anaerobic digestion: holistic bioprocessing of animal manures. Gainesville, FL, University of Florida
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.