Anaerobic co-digestion of forage radish and dairy manure in complete mix digesters

Authors: BELLE, ASHLEY - University Of Maryland; LANSING, STEPHANIE - University Of Maryland; Mulbry, Walter; WEIL, RAY - University Of Maryland

Submitted to: Bioresource Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/6/2014
Publication Date: 9/15/2014
Publication URL: http://handle.nal.usda.gov/10113/61562
Citation: Belle, A., Lansing, S., Mulbry III, W.W., Weil, R. 2015. Anaerobic co-digestion of forage radish and dairy manure in complete mix digesters. Bioresource Technology. 178:230-237.

Interpretive Summary: Anaerobic digestion technology is not considered an economically viable practice for small to mid-sized US dairy farmers due to the relatively small amounts of manure being treated and large capital costs. In addition, since dairy manure has a low energy density in comparison to other digestion feedstocks, the economic returns from dairy manure digestion are low to negative. However, biogas production from dairy digesters can become a more economically favorable option by using additional biodegradable feedstocks located in close proximity to the dairy facility. Co-digesting the dairy manure with other substrates, such as fats, oils, and grease, slaughterhouse waste, or energy crops with higher biogas potential have been shown to increase biogas production by 100 to 500%. As part of a larger effort to evaluate the environmental benefits of the cover crop winter radish, this research sought to determine if additional benefits can be obtained by harvesting the above-ground radish material prior to winter kill and utilizing it as a co-substrate in dairy digesters to increase methane production. The overall objectives of this research were: 1) to determine potential methane and hydrogen sulfide production when co-digesting forage radish cover crops with dairy manure in batch pilot-scale complete mix digesters, 2) to determine how the forage radish organic loading rate (OLR) affects methane production, and 3) to quantity the radish crop acreage required for co-digestion at the farm-scale level and how inclusion of radish cover crops affects on-farm energy production potential. Results showed that co-digestion of radish and dairy manure increased methane production by nearly 40% compared to digestion of manure alone. The high sulfur content of radish did not have an inhibitory effect on methane production. These results could be useful for those interested in additional benefits of cover crops, and to dairy farmers interested in increasing methane production in digesters during the winter months when the demand for supplemental heating is the greatest.

Technical Abstract: Farmers are increasingly using forage radish as a winter cover crop to achieve multiple soil and environmental benefits. In this study, pilot-scale mixed digesters were used to quantify methane (CH4) and hydrogen sulfide (H2S) production when using forage radish, a sulfur-rich cover crop, as a co-digestion feedstock in dairy manure-based digesters. During two trials, triplicate digesters (850 L) were operated in batch mode for 33 days with manure-only or radish+manure at two organic loading rates (OLR) (134 and 266g per kg radish by wet weight). Co-digestion increased CH4 production by 39%. Overall, the high sulfur content of radish did not have an inhibitory effect on CH4 production. H2S concentration rapidly declined during incubation with radish co-digestion actually lowering the biogas H2S beyond that of manure-only digestion. Corn-silage based dairy farmers limited by the low biogas potential of dairy manure may benefit from co-digesting with forage radish in the wintertime.