Thermophilic anaerobic digestion of pulp and paper mill primary sludge and co-digestion of primary and secondary sludge

Anaerobic digestion of pulp and paper mill primary sludge and co-digestion of primary and secondary sludge were studied for the first time in semi-continuously fed continuously stirred tank reactors (CSTR) in thermophilic conditions. Additionally, in batch experiments, methane potentials of 210 and 230 m ³CH ₄/t volatile solids (VS) _added were obtained for primary, and 50 and 100 m ³CH ₄/tVS _added for secondary sludge at 35 °C and 55 °C, respectively. Anaerobic digestion of primary sludge was shown to be feasible with organic loading rates (OLR) of 1-1.4 kgVS/m ³d and hydraulic retention times (HRT) of 16-32 d resulting in methane yields of 190-240 m ³CH ₄/tVS _fed. Also the highest tested OLR of 2 kgVS/m ³d and the shortest HRT of 14-16 d could be feasible, if pH stability is confirmed. Co-digestion of primary and secondary sludge with an OLR of 1 kgVS/m ³d and HRTs of 25-31 d resulted in methane yields of 150-170 m ³CH ₄/tVS _fed. In the digestion processes, cellulose and hemicellulose degraded while lignin did not. pH adjustment and nitrogen deficiency needs to be considered when planning anaerobic digestion of pulp and paper mill wastewater sludges. © 2012 Elsevier Ltd.