Biogas production from anaerobic co-digestion of pig manure with grass or maize silage
Xie, S.1, Lawlor, P.2, Frost, P.3, Wu, G.1, Gilkinson, S.3, Lynch, B.2, Rodgers, M.1 and Zhan, X.1, 1National University of Ireland, Ireland, 2Teagasc Moorepark Research Centre, Ireland, 3Agri-Food and Biosciences Institute, Northern Ireland, UK
(free)
Landspreading of pig manure (PM) can cause serious environmental problems and suitable pig manure management strategies are required. Anaerobic co-digestion of PM and grass/maize silage is one of the sustainable technologies. Anaerobic digestion can reduce odours, pathogens and greenhouse gas emissions, produce digestate that can be used as a fertilizer and biofuel (methane). In this study, co-digestion of PM and grass/maize silage was investigated.
Hydrolysis is the rate-limiting step in anaerobic digestion of grass. Therefore, hydrolysis of grass was specially examined in batch experiments under conditions (i) various PM to silage ratios and (ii) different types of inoculums at the silage volatile solid (VS) concentration of 3.37 g/l. During hydrolysis, the soluble chemical oxygen demand (COD) production rates ranged 230 – 269 mg/l·d for grass silage and 194 – 337 mg/l·d for maize silage. Acidification occurred, with volatile fatty acids (VFAs) production rates ranging 216 – 408 mg/l·d for grass silage and 168 – 312 mg/l·d for maize silage. VS removal rates were 36% – 38% for maize silage and 39% – 40% for grass silage within the 11-day hydrolysis period.
Methane production was investigated in both batch reactors and constantly mixed reactors. In the batch reactors, methanogens could tolerate the organic concentration of up to 28.3 g VS/l grass silage at the silage to inoculum ratio (VS/VS) of 6.47, and the specific methane yield was 367 ml/g VS. In the constantly mixed reactors, under a low organic loading rate of 0.39 g VS/l·d and the silage to PM ratio (VS/VS) of 20%, the specific methane yield was 183 ml/g VS with grass silage co-digested, and 238 ml/g VS with maize silage co-digested; at the loading rate of 0.31 g VS/l·d with only PM added, the specific methane yield was 114 ml/g VS. The results show that anaerobic co-digestion of PM with maize/grass silage is a promising technology for PM management.
