Mesophilic anaerobic digestion of waste activated sludge: influence of the solid retention time in the wastewater treatment process

The performance of mesophilic anaerobic digesters of four large Italian wastewater treatment plants without primary sedimentation were studied. Only the waste activated sludge is stabilised by means of the mesophilic (35-37 °C) anaerobic digestion process. The anaerobic digesters generally worked with a hydraulic retention time in a range of 20-40 days and an organic loading rate of some 1 kg VS/mreactor3 day. The solids content of the sludge fed to the digesters was in the range 2.6-3.9% and the gas produced per kilogram of volatile solids added was in the range 0.07-0.18 m3/kg VSfed. The specific gas production per kilogram of volatile solids destroyed was in the range 0.5-0.9 m3/kg VSdestroyed and the reduction of the volatile solids concentration was in the range 13-27% (average 18%). These figures are particularly significant when designing anaerobic digesters for the treatment of waste activated sludge as single substrate. Moreover, it was observed that the higher the applied solid retention time in the activated sludge process for wastewater treatment, the lower the gas production. In particular, the specific gas production decreased from 0.18 to 0.07 m3/kg VSfed when increasing the solid retention time in the wastewater treatment line from 8 to 35 days. Finally, a mathematical model for the prediction of biogas production on the solid retention time applied in the wastewater treatment process was developed.