An overwhelming percentage of farm-based, anaerobic digestion projects practice co-digestion for improved business models that result from revenues enhanced by tipping fees and extra biogas production. This presentation utilizes over a decade of research and practical experience available within the Pacific Northwest regarding co-digestion, highlighting its benefits, potential pitfalls, and project considerations. Throughout, specific industry examples, made available through a scientific survey of experts, are used to relay information.
To provide an insider’s look at design and management considerations, five individuals with extensive experience in co-digestion at dairy digesters were interviewed. Interviewees included a project developer who has successfully implemented co-digestion at a number of dairy digesters, a dairy farmer who owns and operates a co-digestion project, a scientist with in-depth knowledge of AD and co-digestion, and two system engineers who have designed numerous digesters. The sample size was relatively small, but few individuals have technical expertise in co-digestion in the US, and not all individuals with expertise were willing to be interviewed. Several of these individuals work primarily in the Pacific Northwest where authors are located; however, to the extent possible, individuals with broader experience throughout the US were included.
Co-digestion can provide a significant economic boost to AD operations at dairies. However, after talking with numerous experts in the field of co-digestion, it is clear that careful consideration and planning is required to successfully incorporate substrates. Substrates should be chosen to complement existing waste streams, and should be carefully screened to avoid inhibition. In most cases, the selection of a substrate will be limited by location and volume attainable, and project developers may need to invest considerable time and effort into developing and maintaining the necessary relationships for acquiring substrates. Regulatory restrictions and nutrient management implications are also important. A solid understanding of these issues can contribute to successful implementation of co-digestion.
Successful co-digestion depends on multiple factors including but not limited to type of substrate, hauling costs, location of digester compared to substrate, local substrate competition, tipping fees, and nutrients. Before beginning co-digestion, developers need to first determine whether co-digestion makes economic sense at a particular dairy operation. Otherwise, co-digestion may turn into an economic burden for project developers that are already economically strained by high AD capital costs and low received electrical rates. If a sound business plan is developed and implemented, co-digestion can provide additional profit to project owners.
In the US, most post-consumer food scrap recycling is currently achieved via composting. For example, in western Washington State, many residents of Seattle and King County have their food scraps recycled along with yard waste into saleable compost. While this effectively diverts food scraps from landfilling, AD could capture the energy within food scraps and use it to replace fossil-derived energy, providing additional benefits. When linked with nutrient recovery, the process could also produce saleable fertilizers. If dairy farmers are located near post-consumer food scrap sources, they may be able to position themselves well as an environmentally conscious (lower odor production) and less expensive (shorter hauling distances and lower tipping fees) recycling option.
Existing barriers to co-digestion of post-consumer food wastes include current regulations excluding these wastes from AD, and the extensive pretreatment required so that these wastes could be viably fed to digesters. However, if solutions to these issues could be found, it could be a win-win scenario for food waste diversion and AD projects looking to remain viable.
Jingwei Ma, Research Associate at Washington State University firstname.lastname@example.org
Nick Kennedy, Associate in Research at Washington State University, Georgine Yorgey, Research Associate at Washington State University, Chad Kruger, Director of CSANR, Craig Frear, Assistant Professor at Washington State University
This research was supported by funding from USDA National Institute of Food and Agriculture, Contract #2012-6800219814; National Resources Conservation Service, Conservation Innovation Grants #69-3A75-10-152; Biomass Research Funds from the WSU Agricultural Research Center; and the Washington State Department of Ecology, Waste 2 Resources Program.
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