Modeling of pathogen inactivation in thermal septic tanks

Thermal application has been widely used for pathogen inactivation in various fields. The purpose of this research was to develop a model of pathogen inactivation in septic tanks operating at various temperatures. Four laboratory-scale septic tanks fed with septage were operated at temperatures of 30, 40, 50 and 60 °C and Escherichia coli (E. coli) was selected as the pathogenic indicator. The efficiencies of E. coli inactivation were found to increase with increasing temperatures, while the opposites were observed for chemical oxygen demand (COD) reduction. At 60 °C, the E. coli concentrations were reduced from 9.6 × 106 to about 10 most probable number (MPN)/100 mL or 6 log reduction. The kinetics of E. coli reduction followed a modified Weibull model which could be applied to septic tank design and operation. The percentage COD removal was found to be 93, 94, 89 and 84 at temperatures of 32, 40, 50 and 60 °C, respectively. The results of this study suggested that pathogenic microorganisms in septic tanks could be inactivated to be at a safe level with thermal application.