Enhancing sludge solubilisation and altering the structure of sludge flocs using a low-temperature thermal hydrolysis process

Abstract

This study evaluated the effects of thermal treatment at 50-90°C for 0.5-24 hours on the physicochemical properties, microstructure, and solubilisation of waste activated sludge (WAS) from a municipal wastewater treatment plant. Thermal treatment significantly altered biomass and settleability: mixed liquor suspended solids (MLSS) decreased by approximately 11%, whereas the sludge volume index (SVI₃₀) rose from 112 to 189 ml/g, indicating weakening of the sludge floc structure and disruption of the extracellular polymeric substances (EPS) network. Scanning electron microscopy (SEM) images showed a shift from dense, cohesive sludge flocs to fragmented, porous structures following treatment at 80°C, confirming the thermal vulnerability of EPS. Organic matter solubilisation increases markedly with temperature and treatment time. Solubilisation remained limited at 50-60°C but surged at 80-90°C where soluble chemical oxygen demand (sCOD) increased by 2.5-3.3 fold (1450-1504 mg/l), with the sCOD/total chemical oxygen demand (tCOD) ratio rising from 4.6 to 15.7%, mainly within the first 5-10 hours due to the hydrolysis of readily degradable EPS and intracellular components. These results confirm low-temperature thermal hydrolysis process (LT-THP) as a promising and cost-effective solution for enhancing anaerobic digestion and energy recovery from municipal sewage sludge.