Performance of non-catalytic thermal hydrolysis and wet oxidation for sewage sludge degradation under moderate operating conditions

Land disposal of waste activated sludge pose environmental risks due to the presence of heavy metals, pathogens and organic pollutants. Anaerobic digestion (AD) is one of the preferred treatment methods for sludge treatment. However, sludge hydrolysis is often found the rate-limiting step thereby reducing the biogas generation potential. Therefore, an effort was made to determine the optimum conditions for sludge solubilization by means of hydrothermal pretreatment. In this study, response surface methodology using Box Behnken design approach was used to optimize four hydrothermal reaction variables (i.e., initial pH, time, temperature, and oxidation coefficient) for sludge solubilization and total chemical oxygen demand (tCOD) reduction. Temperature and pH were found to be the most significant parameters. The maximum tCOD reduction and volatile suspended solids solubilization of 58% and 52%, respectively, were obtained at the following optimum conditions: temperature = 180 degrees C, time = 5 h, pH = 3.3 and oxidation coefficient = 0.5. Under similar conditions and alkaline pH, Maillard's reaction occurs which may have adverse impact on the performance of downstream AD process. The highest NH4+-N and volatile fatty acids (VFAs) concentrations were detected in the treated sludge at the optimum conditions. The future studies should be aimed at the recovery of proteins, VFAs and biogas using appropriate methods.