Characteristics of biogas production and synergistic effect of primary sludge and food waste co-digestion

Abstract Co-digestion implementation in wastewater treatment plants reduces waste output and enhances biogas yield. In this regard, the objective of this research was to determine the optimal ratio of biodegradable waste and sewage sludge for co-digestion process in a city plant in central Serbia. The increase in biogas production was investigated through batch tests while synergistic effects were evaluated by chemical oxygen demand (COD) balance. Analyses were performed in four volume basis ratios (3/1, 1/1, 1/3, 1/0) of primary sludge and food waste with added low food waste: 3.375%, 4.675%, and 5.35%, respectively. The best proportion was found to be 1/3 with the highest biogas production (618.7 ml/g VS added) and the organic removal of 52.8% COD elimination. COD reductions further supported the synergistic impact; specifically, an additional 7.1%, 12.8%, and 17% of COD were converted into biogas during the co-digestions 1, 2, and 3, respectively. The rise in co-substrate concentrations was accompanied by a uniform hydrolysis rate constant, the absence of any lag phase, and a greater biogas generation rate. The amount of biogas produced within co-digestions proves to be equivalent to the volume of biogas acquired from individual substrates and the mass of added COD in co-digestions but inversely proportional to the mass of COD utilized in mono-digestion. Further on, specific biogas productions and COD values in digesters show linear dependence. Finally, the study points out that COD method can be used for developing relatively accurate model for biogas potential estimation in wastewater treatment plants.