Biogas Production from Food Residues—The Role of Trace Metals and Co-Digestion with Primary Sludge
Moshe Habagil, Alexander Keucken, Ilona Sárvári Horváth
Abstract
The majority of municipal Wastewater Treatment Plants (WWTPs) in Sweden produce biogas from sewage sludge. In order to increase the methane production, co-digestion of internal sludge with Organic Fraction of Municipal Solid Waste (OFMSW) might be feasible in the future. The objective of this study was therefore to find a beneficial solution for the utilization of OFMSW at the WWTP in Varberg, Sweden. The effects of co-digesting primary sludge (PS) and OFMSW collected in the municipality, in different mixing ratios, were investigated by semi-continuous anaerobic digestion assays. Furthermore, the effects of the addition of a commercial trace elements mixture solution (CTES), available on the market in Sweden, were also examined. Co-digestion of OFMSW and PS resulted in specific methane yields of 404, 392, and 375 NmL CH4/g volatile solids (VS), obtained during semi-continuous operations of 301, 357 and 385 days, for the reactors fed with OMFSW:PS ratio of 4:1, 3:1, and 1:1, and at maximum organic loading rates (OLRs) achieved of 4.0, 4.0 and 5.0 gVS/L/d, respectively. Furthermore, mono-digestion of OFMSW failed already at OLR of 1.0 gVS/L/d, however, an OLR of 4.0 gVS/L/d could be achieved with addition of 14 µL/g VS Commercial Trace Element Solutions (CTES) leading to 363 mL CH4/g VS methane production. These experiments were running during 411 days. Hence, higher process efficiency was obtained when using co-digestion of OFMSW and PS compared to that of OFMSW in mono-digestion. Co-digestion is a more feasible option where a balanced Carbon/Nitrogen (C/N) ratio and nutrient supply can be maintained.