Optimization of methane production from anaerobic co-digestion of municipal solid waste and sewage sludge

Anaerobic digestion (AD) of sieved and shredded organic fraction of municipal solid waste (OFMSW), and source-separated biowaste (BW) and sewage sludge (SS), and co-digestion of BW and SS were studied in laboratory scale semi-continuously fed continuous stirring tank reactors (CSTRs) at 35 °C with organic loading rate (OLR) from 1 to 2 kg volatile solid (VS) per m3 d. The aim of this work was to study effect of OLR on reactor performance, comparing the effect of co-digestion of BW and SS with mono-digestion of SS, and comparison between source-separation of BW and mechanical treatment of OFMSW regarding the resulted methane yield. Average methane yield of 386, 385, 198, and 318 L CH4/kg VS were obtained for OFMSW, BW, SS, and co-digestion of BW and SS respectively in reactor experiment. Process was stable and there was not volatile fatty acids (VFA) accumulation and pH fluctuation. Average methane yield of SS was increased by 61% as a result of co-digestion with BW. VS removal of SS could increase from 36% to 85% as a result of co-digestion with BW. Methane yield of 603, 534, and 369 L CH4/kg VS were obtained for BW, OFMSW and SS respectively in batch experiments at 35 °C. Methane yield of source-separated BW was 12% higher than methane yield observed for mechanically treated OFMSW, which can be interpreted as a positive effect of source-separation on methane yield. In conclusion, AD of OFMSW, BW, and SS and co-digestion of BW and SS were shown to be feasible with OLR of 1 to 2 kg VS/m3 d in mesophilic conditions. OFMSW and BW were found to have higher methane yield (around 2 times) than SS. Co-digestion of BW and SS could increase methane yield and improve reactor performance in comparison to mono-digestion of SS. Source-separated BW could result in higher methane yield than mechanically treated OFMSW in batch experiment