Performance of the Bio-electrochemical Anaerobic Digestion of Sewage Sludge at Different Hydraulic Retention Times

The performance of bio-electrochemical anaerobic digester, applied with 0.3 V between anode and cathode, for sewage sludge was explored at different hydraulic retention times (HRTs) ranging from 20 to 5 days. The state variables including pH, alkalinity, VFA, and SCOD are quite stable and in suitable ranges for anaerobic digestion. At 20 days of HRT, the performance is considerably high in volatile solids (VS) reduction (70.5%), the specific methane production rate (407 mL L-1 d(-1)), and the methane content (76.9%) in biogas. The VS reduction and the methane content in biogas slightly deteriorated as the HRT decreased from 20 to 5 days, while the methane production rate increased to 1,339 mL L-1 d(-1). The overall energy efficiencies for methane recovery are in the range of 69.1-98.7%, and the maximum energy efficiency appears at 10 days of HRT. These results showed that the bio-electrochemical anaerobic digestion system could obtain much higher VS reduction and energy recovery with shorter HRTs than the conventional anaerobic digestion technologies.