Synergistic effect from anaerobic co-digestion of sewage sludge and organic wastes
Anaerobic mono-digestion and co-digestion of primary sludge and two organic wastes (namely food waste or paper pulp reject) were evaluated by biomethane potential assessment and kinetics modelling to elucidate the synergistic effect. The specific methane yields were 159, 652 and 157 mL/g VS added during mono-digestion of primary sludge, food waste and paper pulp reject, respectively. Co-digestion of primary sludge with either food waste or paper pulp reject resulted in much higher specific methane yields of 799 and 368 mL/g VS, respectively. pH and intermediate inhibitions (e.g. volatile fatty acids and ammonium-N) were not observed. The synergistic effect was also confirmed by examining the VS and COD removals. COD balance also identified and validated the enhanced specific methane yields from both primary sludge and organic waste (i.e. additional 32 and 19% of COD was converted to biogas during co-digestion of primary sludge with food waste or paper pulp reject, respectively). The apparent first order rate constant derived from kinetics modelling increased from 0.18 to 0.63 d−1 during mono-digestion of paper pulp reject and co-digestion of primary sludge with paper pulp reject, which can be attributed to the initial high soluble biodegradable fraction in primary sludge.