The use of co-digested solid fraction as feedstock for biogas plants

A comparative study was set up in order to assess the technical feasibility of the long-term reuse of the mechanically separated co-digested solid fraction as a feedstock for anaerobic digestion plants (ADP). The biogas yields of two feedstock mixtures (A and B) were assessed in mesophilic conditions (40°C±2°C) using 6 lab-scale continuous stirred tank reactors. Feedstock mixture A (control) consisted of pig slurry (70%), farmyard manure (4%), sorghum silage (12%) and maize silage (14%). Feedstock mixture B was the same as the control plus the solid fraction derived from the mechanical separation of the output raw codigestate collected from the reactors. All reactors were fed simultaneously, three times a week, over a period of nine month. According to the study results, the reuse of the co-digested solid fraction as feedstock for ADP could increase the methane yield by approximately 4%. However, ADP efficiency evaluation (e.g., daily yield of methane per m3 of digester) suggests limiting this practice to a maximum time period of 120 days.