Feasible Power Production from Municipal Sludge Using an Improved Anaerobic Digestion System

Abstract

Anaerobic digestion with ozonation is a promising process to enhance the digestion efficiency and reduce the sludge quantity for disposal. In this study, new process schemes by incorporating thermophilic digestion were studied for further improvements. Pilot tests were performed with three schemes having mesophilic, thermophilic or mesophilic-thermophilic hybrid reactors. In the process scheme with thermophilic digestion, the degradation ratio of VSS components was observed to improve by 5.5% over mesophilic digestion. The amount of ozone consumption could also be reduced by 18%. However, biogas conversion ratio was not improved due to considerable non-degradable organic fraction remaining in soluble form. In batch tests, this soluble fraction was found to readily degrade by mesophilic microorganism. Based on this observation, a mesophilic-thermophilic hybrid flow scheme was developed. In this flow scheme, thermophilic microbes rapidly degraded ozonated sludge and remaining soluble organic components were converted to biogas by mesophilic microbes. This flow scheme reduced ozone consumption as well as improved the biogas conversion of municipal sludge to 78.6%. The cost performance analysis of a municipal WWTP (population equivalent 150,000) considering electricity production resulted in electricity production cost of 5.0 JPY/kWh, lower than the current market price of 9.3 JPY/kWh.