On-Farm Co-Digestion of Dairy Manure with High Energy Organics

On-Farm Co-Digestion of Dairy Manure with High Energy Organics

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Nicholas Kennedy, Associate in Research, Center for Sustaining Agriculture and Natural Resources, Washington State University, Georgine Yorgey, Associate in Research, Center for Sustaining Agriculture and Natural Resources, Washington State University, Dr. Craig Frear, Assistant Professor, Center for Sustaining Agriculture and Natural Resources, Washington State University, Chad Kruger, Director, Center for Sustaining Agriculture and Natural Resources, Washington State University
The Anaerobic Digestion Systems Series provides research-based information to improve decision-making for incorporating, augmenting, and maintaining anaerobic digestion systems for manures and food by-products. This publication focuses on pre-consumer food wastes that can sustainably be used as substrates for co-digestion with dairy manure and increase the value of co-products. Topics covered include complementary and problematic substrates, the substrate procurement process, regulations, and solutions for co-digestion processing issues.
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List of Abbreviations

AD Anaerobic Digestion
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
GHG Greenhouse Gas
HRT Hydraulic Retention Time
K Potassium
N Nitrogen
NMP Nutrient Management Plan
P Phosphorus
REC Renewable Energy Credits
RNG Renewable Natural Gas
SDSS Spatial Decision Support System
TAN Total Ammonia Nitrogen
TKN Total Kjeldahl Nitrogen
TP Total Phosphorus
U.S. United States

Introduction

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).

Each of these products can be further processed using add-on technologies to produce higher value co-products, such as horticultural fiber, renewable natural gas (RNG), and fertilizer products.

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.

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