Anaerobic digestion technology was coupled with a new forage radish cover cropping system in order to increase biogas production of a dairy manure digester. Specifically, this research investigated forage radish as a renewable source of energy in terms of methane (CH4) production, the effect of radish co-digestion on hydrogen sulfide (H2S) production, and the relationship between H2S production and methanogenesis limitations. Optimal substrate co-digestion ratios and inoculum to substrate ratios (ISR) were determined in the laboratory with biochemical methane potential assays (300 mL) and pilot-scale complete mix batch digesters (850 L) were constructed and operated to determine energy production potential at the farm-scale level.

Laboratory results showed that forage radish had 1.5-fold higher CH4 potential than dairy manure on a volatile solids basis, with increasing the radish content of the co-digestion mixture significantly increasing CH4 production. Initial H2S production also 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/kg VS). The co-digestion mixture containing 40% radish had a lower specific CH4 yield (345 ± 2 L CH4/kg VS), but also showed significantly less H2S production at start-up and high quality biogas (58% CH4). Utilizing 40% radish as substrate, decreasing the ISR below 50% (wet weight) resulted in unstable digestion conditions with decreased CH4 production and an accumulation of butyric and valeric acids.

Pilot-scale experiments revealed that radish co-digestion increased CH4 production by 39% and lowered the H2S concentration in the biogas (0.20%) beyond that of manure-only digestion (0.34% - 0.40%), although cumulative H2S production in the radish + manure digesters was higher than manure-only. Extrapolated to a farm-scale (200 cows) continuous mixed digester, co-digesting with a 13% radish mixture could generate 3150 m3 CH4/month, providing a farmer additional revenue up to $3125/month in electricity sales. These results suggest that dairy farmers could utilize forage radish, a substrate that does not compete with food production, to increase CH4 production of manure digesters.