BioCycle September 2011, Vol. 52, No. 9, p. 44
Animal waste, food processing residuals, yard trimmings and other feedstocks could be available for anaerobic digestion.
Dan Lemke

AN initiative sponsored by the Agricultural Utilization Research Institute’s (AURI) office in Crookston, Minnesota is giving northwest Minnesota digester projects a better idea of available substrates. Becky Johnson, a student at the University of Minnesota-Crookston (UMC), conducted an inventory of potential digester feedstocks in northwest Minnesota as part of the AURI-sponsored study.
Animal waste, food processing residuals, yard trimmings and other feedstocks could be available for anaerobic digestion. “Companies and communities are evaluating digesters for waste handling and methane gas production,” says Jennifer Wagner-Lahr, AURI project director. “We are looking to see how we can boost gas production by incorporating something else that is a problematic waste.”
In summer 2010, Johnson evaluated feedstocks available on the UMC campus. Manure was the most abundant feedstock (about 620 tons/year, including bedding). Dining services estimated around 50 lbs/day of food waste was generated during the 9-month school year. Data from Polk County Environmental Services estimated less than 1,000 tons/year of yard trimmings are generated in Crookston.
Johnson also surveyed regional food processors, including American Crystal Sugar Company. Based in Moorhead, Crystal Sugar has a pilot-sized digester powered by beet waste tailings – sugarbeet chips that are too small to process, and vegetation that mixes in with beets during harvest.
The company has been considering an additional full-scale unit if it proves economical. However, Crystal Sugar only processes beets about nine months out of the year. The digester would essentially be idle the other three months unless other feedstocks could be found to fuel it. “Having other available feedstocks may be key to viability,” says Dave Malmskog, American Crystal Sugar economic analysis director. “If we are able to identify two or three significant items we could add, it could be a game changer in terms of economic feasibility.”
After several months taking inventory of potential fuel streams, Johnson determined there is enough feedstock supply to meet potential demand. “It would be a matter of getting the right contacts and finding synergies, but there are enough resources,” she says.

BIOGAS POTENTIAL
The feedstock inventory is the first phase of AURI’s initiative to evaluate biogas production potential. Subsequent research includes testing some of the available feedstocks in a small-scale digester at AURI’s coproduct utilization lab in Waseca, Minnesota. The continuous flow reactors function primarily on dairy manure, and AURI Coproducts Scientist Kevin Hennessy is hoping to establish a baseline level of gas production. Introducing additional substrates will help him determine if they increase the amount of gas the digester produces. “The key to testing these codigestion substrates is not only to determine if they will help a digester produce more gas, but will it keep the digester healthy,” Hennessy says.
Hennessy’s work in the AURI lab could help determine an optimal inclusion rate for adding biomass to a digester, as well as the proper ratio for blending available fuels. “We’re taking a stab at a really big question by focusing on a smaller region,” Wagner-Lahr says. “By the time we are done with this project, hopefully we’ll be able to answer questions about what the methane production is from combinations of these feedstocks, then suggest best practices for how they could be codigested.”

Den Lemke is Director of Communications at the Agricultural Utilization Research Institute. The Digester Feedstock Inventory report is available at www.auri.org.