Anaerobic digestion has been used in wastewater treatment processes for decades. Recent innovations, however, are making the technology more viable for commercial applications. Anaerobic digestion occurs when microbes degrade organic matter in the absence of oxygen gas. It can be used to treat sewage effluent, agricultural byproducts, and solid municipal wastes. The microbes utilize oxygen from organic matter and produce sludge, water, and gases, including methane, carbon dioxide, and smaller amounts of hydrogen, carbon monoxide, and nitrogen. When captured, the gaseous products, often referred to as biogas, are potentially valuable as a source of energy for electricity, heating, and vehicle fuels1.
Typical anaerobic digestion processes use different types of microorganisms to break down organic matter through several stages. Complex organic inputs of waste or sludge are degraded into organic acids, which are then used by other bacteria to produce biogas and residual solids. The resultant biogas typically contains 50-80% methane, but must be refined and purified2. Typical digesters include both plug-flow and tank designs3.
Recent innovations in anaerobic digestion seek to reduce operational costs and increase potential revenue from products. New digester designs can co-digest multiple waste inputs (solid and liquid wastes), achieve better mixing for efficient decomposition, improve energy efficiency and recovery, and reduce treatment times4,5. The first commercial-scale anaerobic digester to use high-solid (~50%) inputs and high-rate digestion opened in 2012 at the American River Packing Company in Sacramento, CA. The plant technology, which processes corrugated paper and food waste inputs to deliver 1300 kWh of electricity per day, was primarily developed by UC Davis professor Ruihong Zhang and subsequently commercialized by Clean World Partners6,7. In another example, Gills Onion, a large onion processor in southern California, developed a digester that uses established anaerobic technology (Upflow Sludge Blanket Reactor) for an onion-waste to methane gas system that feeds two 300-kW fuel cells and reduces wastes8.
Anaerobic digestion processes have advantages and disadvantages compared to aerobic digestion. Anaerobic processes allow for energy recovery through methane production, but typically decompose solids much slower. Operating costs for anaerobic processes are lower due to lower energy requirements of oxygenation, but initial capital costs are higher since anaerobic digesters require more infrastructure. Anaerobic digestion can remove more water from sludge, which improves its usability as fertilizer, but it also removes fewer organics.
References
1. DOE. Alternative Fuels Data Center: Biogas. (U.S. Department of Energy, Energy Efficiency and Renewable Energy (EERE): Washington, D.C., 2012).at <http://www.afdc.energy.gov/fuels/emerging_biogas.html>
2. Rutledge, B. California Biogas Industry Assessment. (WestStart-CALSTART: Pasadena, CA, 2005).
3. ODOE. Bioenergy in Oregon Biogas Technology. (Oregon Department of Energy: 2010).at <http://www.oregon.gov/ENERGY/RENEW/Biomass/Pages/Biogas.aspx>
4. UC Davis News and Information New Technology Turns Food Leftovers Into Electricity, Vehicle Fuels. (2006).at <http://www.news.ucdavis.edu/search/news_detail.lasso?id=7915>
5. Parry, D. High Performance Anaerobic Digestion. (Water Environment Federation, Residuals and Biosolids Committee, Bioenergy Technology Subcommittee: 2004).
6. Bang, D. Waste Not, Want Not! UC Davis College of Engineering: Engineering Progress 2–3 (2012).
7. Zhang, R. UC Davis Biogas Energy Project. (2012).
8. Gerber, K. & Johnston, D. Onion Processor Uses AD, Fuel Cells To Convert Waste Into Energy. Biomass Magazine (2010).at <http://biomassmagazine.com/articles/5036/onion-processor-uses-ad-fuel-cells-to-convert-waste-into-energy>