Anaerobic co-digestion of pig manure and food waste; effects on digestate biosafety, dewaterability, and microbial community dynamics

C. Dennehy, P.G. Lawlor, M.S. McCabe, P. Cormican, J. Sheahan, Y. Jiang, X. Zhan, G.E. Gardiner

This study assessed the effect of varying pig manure (PM)/food waste (FW) mixing ratio and hydraulic retention time (HRT) on methane yields, digestate dewaterability, enteric indicator bacteria and microbial communities during anaerobic co-digestion. Three 10 L digesters were operated at 39 °C, each with a PM/FW feedstock composition of 85%/15%, 63%/37% and 40%/60% (volatile solids basis). While the PM/FW ratio was different among reactors, the organic loading rate applied was equal, and increased stepwise with reducing HRT. The effects of three different HRTs were studied: 41, 29, and 21 days. Increasing the proportion of FW in the feedstock significantly increased methane yields, but had no significant effect on counts of enteric indicator bacteria in the digestate or specific resistance to filtration, suggesting that varying the PM/FW feedstock composition at the mixing ratios studied should not have major consequences for digestate disposal. Decreasing HRT significantly increased volumetric methane yields, increased digestate volatile solids concentrations and increased the proportion of particles >500 µm in the digestate, indicating that decreasing HRT to 21 days reduced methane conversion efficiency High throughput 16S rRNA sequencing data revealed that microbial communities were just slightly affected by changes in digester operating conditions. These results would provide information useful when optimizing the start-up and operation of biogas plants treating these substrates.