Methane and hydrogen sulfide production during co-digestion of forage radish and dairy manure

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

Submitted to: Biomass and Bioenergy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/25/2015
Publication Date: 5/15/2015
Publication URL: http://handle.nal.usda.gov/10113/61548
Citation: Belle, A., Lansing, S., Mulbry Iii, W.W., Weil, R. 2015. Methane and hydrogen sulfide production during co-digestion of forage radish and dairy manure. Biomass and Bioenergy. 80:44-51.

Interpretive Summary: The economics of dairy manure-based digesters are not always favorable due to the relatively low biogas yield of dairy manure compared to other organic wastes. However, co-digesting dairy manure with other substrates, such as fats, oils, and grease, food waste, or energy crops, have been shown to increase biogas production up to 5-fold compared to digestion of manure without amendment. As part of a larger effort to evaluate the environmental benefits of the cover crop winter radish, this research sought to determine optimal mixtures of dairy manure and chopped forage radish for anaerobic digestion. The specific research objectives were to: (1) determine the optimal co-digestion ratio of dairy manure and forage radish above-ground biomass based on methane (CH4) production and hydrogen sulfide (H2S) concentration and (2) determine the effect of forage radish harvest date on CH4 production. Results showed that methane production increased as the radish content increased from 0% to 100%, revealing that co-digestion with radish does increase CH4 production from dairy manure-based digesters. Results also showed that the forage radish harvest date did not affect CH4 production during co-digestion with dairy manure. 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: Forage radish cover crops were investigated as a co-substrate to increase biogas production from dairy manure-based anaerobic digestion. Lab-scale batch digesters (300 mL) were operated under mesophilic conditions during two experiments. In the first experiment, the optimal co-digestion ratio for radish and dairy manure based on methane (CH4) production and hydrogen sulfide (H2S) concentration was determined by increasing the radish content (above-ground biomass) from 0–100% (wet weight). Results showed that forage radish had 1.5-fold higher CH4 potential than dairy manure on a volatile solids basis. While no synergistic effect on CH4 production resulted from co-digestion, increasing the radish content of the co-digestion mixture significantly increased CH4 production. Initial H2S production increased as the radish content increased, but the sulfur-containing compounds were rapidly utilized, resulting in all treatments having similar H2S concentrations (0.10-0.14%) and higher CH4 content in the biogas (48-70% CH4) over time. The 100% radish digester had the highest specific CH4 yield (372 ± 12 L CH4 per kg VS). The co-digestion mixture containing 40% radish had a lower specific CH4 yield (345 ± 2 L CH4 per kg VS) but also showed significantly less H2S production at start-up and high quality biogas (58% CH4). Results from the second experiment showed that the radish harvest date (October versus December), did not significantly influence radish C:N ratios or CH4 production during co-digestion with dairy manure. These results suggest that dairy farmers could utilize forage radish, a readily available substrate that does not compete with food supply, to increase CH4 production of manure digesters in the fall/winter.