Designing Anaerobic Digestion for Energy Recovery and Pathogen Inactivation

A combined analysis of inactivation potency, operational costs and energy generation is necessary to evaluate current and alterative designs for anaerobic sewage sludge digesters. Firstorder inactivation rate constants have been determined for Escherichia coli, Eneterococcus faecalis and bacteriophage MS-2 at temperatures of 25, 37, 50, 55 and 60°C in simulated anaerobic sludge digester conditions. Inactivation rates were found to increase dramatically with a jump in digester temperature from 55 to 60°C in E. coli and E. faecalis and less dramatically in MS-2. Net energy production was modeled for four digester configurations of interest, including mesophilic treatment at 37°C, thermophilic treatment at 55°C, temperature-phased treatment combining 55 and 37°C and finally, mesophilic treatment at 37°C paired with 60°C pretreatment. The net energy production per volume of treated sludge for mesophilic treatment, temperature-phased treatment and 60°C pretreatment configurations was positive and virtually the same, with a standard deviation of about 1%. Thermophilic treatment produced about 15 % less net energy per volume of treated sludge. Pathogen inactivation performance was estimated for E. coli, E. faecalis and bacteriophage MS-2 for each treatment configuration using the batchderived inactivation rate constants. The 60°C pretreatment configuration displayed dramatically greater inactivation potency, 14+ log at 60°C versus 1-5 log in other cases. These results demonstrate that incorporating a 60°C pretreatment phase can increase pathogen inactivation performance without decreasing energy capture from digestion.