Application of solarization for sanitization of sewage sludge compost

Abstract

Performance of the solarization process on inactivation of pathogenic bacteria was explored to provide the sanitization standards set for the sewage sludge compost. For evaluation of the microbiological quality of sludge compost and the sanitation efficiency of solar application, Escherichia coli, thermotolerant coliforms, Clostridium, and Enterococci were selected as the indicator microorganisms. Unsolarized (control) treatment was conducted at room temperature (25 ± 2 °C) in laboratory conditions. Solarization treatment was performed under heating conditions, where the sufficient ultraviolet radiation was directly provided by sun. The results indicated that solarization remarkably increased the temperature of sludge compost to maximum 65 °C at 5 cm of compost depth. The inactivation rates of indicator microorganisms exposed to the solarization treatment were recorded significantly higher than the unsolarized treatment. Among the indicator microbial agents, E. coli was found as the most susceptible microorganism and lowered from 4 log CFU g−1 to undetectable levels after 6 days of solarization process. However, the reduction rate of the unsolarized group of this bacteria was determined to be less than the 1-log within 15 days of the entire experimental period. For the solarized compost, the viability of Enterococci ranged from 2.31 log CFU g−1 to 1.80 log CFU g−1 within 6 days and reduced to below the detection limit (2 × 101 cells as CFU) after 12 days of solar application. The reduction rates of thermotolerant coliforms and Enterococci were slow (kmax = 3.22 and 3.27 day−1, respectively), but they were reduced to below the detection limit within 12–15 days. The inactivation curves demonstrated that Clostridium showed more resistance to heat provided by solarization compared with other indicator microorganism species. The Clostridium reduction in solarized treatment was determined as 3-log, while the unsolarization treatment provided 0.7-log reduction. Findings of this study clearly corroborated that the temperature profile generated by the solarization process was adequate for elimination and/or inactivation of various microbial pathogens to achieve the desired standards within two weeks.