No abstract

For small-scale wastewater treatment plants in rural areas (WWTPRs) of Japan, reducing sludge disposal costs is an important issue. Co-digestion of sludge from WWTPRs and food waste is a promising method of reducing maintenance costs and producing energy resources such as methane. In this study, co-digestion experiments using sludge and kitchen garbage were conducted to investigate the stability of the co-digestion from the perspective of trace metal requirements. The gas production rate and pH were stable in the digester with HRTs of 140–70 days. With a HRT of 30 days, the pH gradually dropped from 7.0 to 6.5, and VFA accumulated up to a concentration of 2,200 mg/L, indicating process failure. Cobalt (Co) concentration was approximately one-tenth of the required amount reported by a previous study. When Co was added to the digester, the pH and gas production rate became stable even with a higher VS loading rate. Therefore, the unstable situation was attributed to a lack of Co in the sludge. The Co concentration of sludge from WWTPRs in Japan is generally low, similar to the sludge used in this study. The addition of Co is required for stable co-digestion of sludge from WWTPRs and food waste.