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 m3CH4/t volatile solids (VS)added were obtained for primary, and 50 and 100 m3CH4/tVSadded 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/m3d and hydraulic retention times (HRT) of 16–32 d resulting in methane yields of 190–240 m3CH4/tVSfed. Also the highest tested OLR of 2 kgVS/m3d 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/m3d and HRTs of 25–31 d resulted in methane yields of 150–170 m3CH4/tVSfed. 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.

► Pulp and paper mill primary (PS) and secondary sludge (SS) were characterized.
► Methane potentials of PS and SS were determined in two temperatures.
► In reactor studies, digestion of PS and PS + SS were shown to be feasible.
► pH adjustment and nitrogen deficiency of primary sludge needs to be considered.
► Cellulose and hemicellulose degraded in the process while lignin did not.