Mesophilic and thermophilic anaerobic co-digestion of rendering plant and slaughterhouse wastes

Co-digestion of rendering and slaughterhouse wastes was studied in laboratory scale semi-continuously fed continuously stirred tank reactors (CSTRs) at 35 and 55 °C. All in all, 10 different rendering plant and slaughterhouse waste fractions were characterised showing high contents of lipids and proteins, and methane potentials of 262–572 dm3 CH4/kg volatile solids (VS)added. In mesophilic CSTR methane yields of ca 720 dm3 CH4/kg VSfed were obtained with organic loading rates (OLR) of 1.0 and 1.5 kg VS/m3 d, and hydraulic retention time (HRT) of 50 d. For thermophilic process, the lowest studied OLR of 1.5 kg VS/m3 d, turned to be unstable after operation of 1.5 HRT, due to accumulating ammonia, volatile fatty acids (VFAs) and probably also long chain fatty acids (LCFAs). In conclusion, mesophilic process was found to be more feasible for co-digestion than thermophilic process, methane yields being higher and process more stable in mesophilic conditions.