Considerations for Incorporating Co-Digestion on Dairy Farms

Considerations for Incorporating Co-Digestion on Dairy Farms

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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

This publication discusses the impacts of incorporating co-digestion at dairy-based anaerobic digesters. That is, mixing manure with non-manure waste in anaerobic digesters. This information is based on stakeholder perspectives and a literature review of infrastructure, operational upgrades, and related costs and revenues when non-manure wastes are added to dairy digesters.

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 byproducts.

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List of Abbreviations

AD anaerobic digestion
ATA anaerobic toxicity assay
BMP biochemical methane potential
CH4 methane
CHP combined heat and power
CSTR continuous stirred tank reactors
DAF dissolved air flotation
FOG fats, oils, and grease
K potassium
N nitrogen
NMP nutrient management plan
P phosphorus
REC renewable energy credits
RIN renewable identification numbers
RNG renewable natural gas


Many dairy digesters currently operating in the United States practice co-digestion. During co-digestion, off-farm organics (also called substrates), are added to anaerobic digesters along with dairy manure to produce renewable energy and other products. The primary motivation for co-digestion is usually increased biogas production, though tipping fees, tax credits, and fiber sales are also generated in many cases (Atandi and Rahman 2012). Because of the potential for increased revenues (Innovation Center for U.S. Dairy 2013), many anaerobic digestion (AD) facilities are considering utilizing co‑digestion. These benefits are especially important when received electricity rates for AD are notably low across the nation.

Along with potential revenue increases, costs associated with practicing co-digestion must also be considered by farmers, project developers, regulatory agencies, and others who are evaluating whether co-digestion is a viable option for dairy digesters. The main objective of this manual is to aid decision-making by providing an overview of the engineering design and management issues associated with co-digestion at dairy AD facilities.

To provide an insider’s look at design and management considerations, five individuals with extensive experience in co-digestion at dairy digesters were interviewed. The sample size is relatively small because few individuals have technical expertise with co-digestion in the United States, and some of those candidates were not willing to be interviewed. Several of the sources work primarily in the Pacific Northwest where the authors are located; however, to the extent possible, individuals with broader experience throughout the United States were included.

Interviewees included:

[A] a scientist with in-depth knowledge of AD and co-digestion;

[B], [C] two systems engineers who have designed numerous digesters that incorporate substrates;

[D] a dairy farmer who owns and operates a co-digestion facility; and

[E] a project developer who has successfully implemented co-digestion at a number of dairy digesters.

To preserve anonymity, interviewees are labeled as A, B, C, D, and E. Tips from these industry experts are included throughout the manual, cited by the letters denoted above.


This manual assumes a working knowledge of AD, co-digestion, and nutrient recovery. Readers interested in an overview of AD can refer to Anaerobic Digestion Effluents and Processes: The Basics (Mitchell et al. 2015). Readers will also likely be interested in a companion publication that discusses considerations relating to choice of substrate, On-Farm Co-Digestion of Dairy Manure with High Energy Organics (Kennedy et al. 2015a). More detailed information on permitting and nutrient considerations are covered in Anaerobic Co-Digestion on Dairies in Washington State: The Solid Waste Handling Permit Exemption (Yorgey et al. 2011) and The Rationale for Recovery of Phosphorus and Nitrogen from Dairy Manure (Yorgey et al. 2014).



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