List of Abbreviations
|ATA||Anaerobic Toxicity Assay|
|BMP||Biochemical Methane Potential|
|C/N||Carbon to Nitrogen Ratio|
|CAFO||Concentrated Animal Feeding Operation|
|DAF||Dissolved Air Flotation|
|FOG||Fats, Oils, and Greases|
|HRT||Hydraulic Retention Time|
|NMP||Nutrient Management Plan|
|REC||Renewable Energy Credits|
|RNG||Renewable Natural Gas|
|SDSS||Spatial Decision Support System|
|TAN||Total Ammonia Nitrogen|
|TKN||Total Kjeldahl Nitrogen|
Anaerobic digestion (AD) is a biochemical waste treatment process that occurs in an anaerobic environment (without oxygen), in which microorganisms convert complex organic materials to biogas, a source of renewable energy. In addition to producing energy, the process mitigates many environmental concerns associated with organic residuals and wastewaters, including odors, pathogens, greenhouse gas emissions (GHG), and air and water quality issues. At dairies and other livestock operations, co-digestion, the practice of purposefully supplementing the manure feedstock with multiple forms of organic material (known as substrates), is often used to improve biogas yield, economics, and the AD process. Figure 1 illustrates the dairy AD process under a co-digestion scenario. The main products include fiber (often used as animal bedding), biogas (combusted to produce electricity and heat), and effluent (land applied as a fertilizer).
The number of digesters on dairies in the United States has steadily increased, but the technology is still not common. As of September 2014, there were an estimated 193 dairy based digesters operating on commercial dairy farms—serving an estimated 4% of dairy cows (AgStar 2014; USDA-ERS 2013). Unfortunately, in many regions of the U.S., including the Pacific Northwest, new and existing manure-based AD projects have been hindered by decreasing revenue (Novak 2012). Most manure-only AD projects receive revenue from up to four sources: electrical sales, fiber sales, carbon credits, and renewable energy credits (RECs)—with electrical sales the largest contributor to revenue. Yet, only revenue from fiber has remained steady in recent years (Coppedge et al. 2012). Carbon credits and RECs have been highly volatile due to market fluctuations and regulatory uncertainty. Meanwhile, received electrical prices have decreased across the nation, due in part to breakthroughs in domestic natural gas extraction, which have driven down received electrical prices for renewable energy projects across the U.S.
In the Pacific Northwest, these forces have dropped traditionally low, hydroelectric-driven electrical rates even lower (Coppedge et al. 2012). For example, the largest dairy AD project in eastern Washington was paid $0.07 per kWh in 2012. In 2013, when a new power purchase agreement was formed, this rate fell to $0.04 (Coppedge et al. 2012). Similarly, in Oregon, received electrical rates dropped to around $0.05 per kWh in 2012, slowing the development of projects in the state (Sullivan 2012).
Challenges to AD project profitability have led to increased interest in methods for generating additional revenue at dairy manure-based anaerobic digesters by farmers, third party project developers, and regulatory agencies. This publication provides information on co-digestion with high-energy organic substrates—one business model that is being used to benefit AD project profitability. The emphasis is on dairy manure digesters, though lessons learned can be adapted for other organic residual and wastewater digesters. A companion publication entitled Considerations for Building, Operating, and Maintaining Anaerobic Co-Digestion Facilities on Dairies (Kennedy et al. forthcoming) provides stakeholders with an understanding of the impacts of co-digestion on AD project operation. In addition, Anaerobic Digestion Effluents and Processes: The Basics (Mitchell et al. 2015) offers basic information on the AD process.