Anaerobic co-digestion of sewage sludge and primary clarifier skimmings for increased biogas production

The objective of the study was to identify the impact of co-digesting clarifier skimmings on the overall methane generation from the treatment plant and additional energy value of the increased methane production. Biogas production from co-digesting clarifier skimmings and sewage sludge in pilot-scale fed-batch mesophilic anaerobic digesters has been evaluated. The digester was fed with increasing quantities of clarifier skimmings loads: 1.5, 2.6, 3.5 and 7.0 g COD equivalent/(L.d) (COD: chemical oxygen demand). Average volatile solids reduction of 65% was achieved in the scum-fed digester, compared with 51% in the control digester. Average 69% COD removal was achieved at highest scum loading (7 g COD eq/(L.d)) with approximate methane yield of 250 L CH4/kg COD fed (4 ft(3)/lb COD fed). The results show that scum as co-substrate in anaerobic digestion systems improves biogas yields while a 29% increase in specific CH4 yield could be achieved when scum load is 7 g COD eq/(L.d). Based on the pilot-scale study results and full-scale data from South East Water Pollution Control Plant and Northeast Water Pollution Control Plant the expected annual energy recovery would be approximately 1.7 billion BTUs or nearly 0.5 million kWh.