Enhancing the ability to dissolve sludge and altering the structure of sludge flocs from municipal wastewater treatment plants using low-temperature thermal hydrolysis process

Abstract

Low-temperature Thermal Hydrolysis Process (LT-THP) is an energy-saving sludge pretreatment method with high application potential. This study evaluates 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 domestic wastewater treatment plant. Thermal treatment significantly alters biomass and settleability: Mixed Liquor Suspended Solids (MLSS) decreases by approximately 11%, while Sludge Volume Index (SVI₃₀) rises from 112 to 189 ml/g, indicating the weakening of the floc structure and the disruption of the extracellular polymeric substances (EPS) network. Scanning electron microscopy (SEM) images show a shift from dense, cohesive flocs to fragmented, porous structures after 80°C treatment, confirming EPS vulnerability to heat. Organic matter solubilisation markedly increases with temperature and time. Limited at 50- 60°C, it surges at 80-90°C, boosting Soluble Chemical Oxygen Demand (sCOD) 2.5-3.3 times (1450-1504 mg/l), with sCOD/Total Chemical Oxygen Demand (tCOD) ratio rising from 4.6 to 15.7%, mainly within the first 5-10 hours due to hydrolysis of readily degradable EPS and intracellular components. Results confirm LT-THP as an efficient and cost-effective solution for enhancing anaerobic digestion and energy recovery from municipal wastewater treatment systems.