Methane yields during anaerobic co-digestion of animal manure with other feedstocks: A meta-analysis

Anaerobic co-digestion of animal manure with other feedstocks (aka co-digestion) is increasingly being used to enhance methane yield and organic waste management. The benefits accruing from co-digestions compared to mono-digestions, however, vary greatly in the literature. The goal of this research was to use meta-analysis to critically compare methane yields between mono- and co-digestions and identify relevant factors (co-substrate type, substrate dose, carbon to nitrogen (C/N) ratio, volatile solids (VS), substrate pH, organic loading rate (OLR), and hydraulic retention time (HRT)) contributing to methane yield. Published studies (n = 64 representing 384 case-studies) with sufficient detail on methane yield were identified for the meta-analysis. Analysis indicated that co-digestion of animal manure with other feedstocks significantly increased methane yield (249 L kg^-1[VS]), compared with anaerobic mono-digestion of animal manure (171 L kg^-1[VS]). Similar methane yields increases (47-57 L kg^-1[VS]) were obtained from co-digestions in batch reactors of swine (238-287 L kg^-1[VS]), poultry (213-260 L kg^-1[VS]), and cattle manure (147-204 L kg^-1[VS]). In continuous digesters of cattle manure (175-299 L kg^-1[VS]) co-digestion had the greatest methane yield improvement of 124 L kg^-1[VS], swine manure (212-322 L kg^-1[VS]) co-digestion ranked second with 110 L kg^-1[VS], and poultry manure ranked third with 70 L kg^-1[VS]. Improved methane yield were obtained at optimum C/N ratio ranging from 26 to 34. The respective optimum OLR for co-digestion of swine, poultry, and cattle manure were 1.2, 1.4 and 3.4 kg VS m^-3 d^-1, while the recommended HRT was 30-40 d. Taken together, anaerobic co-digestion of animal manure with other feedstock significantly improved anaerobic digestion. Factors contributing to methane yields included: substrate-type and dose, VS, C/N, OLR, and HRT.