Fractionated volatile solids for understanding thermophilic pretreatment of waste activated sludge at 55, 65, and 75°C

Autohydrolysis or enzyme hydrolysis pretreatment under thermophilic conditions significantly accelerates organic solubilization of waste activated sludge (WAS), allowing enhanced methanogenesis in subsequent mesophilic anaerobic digestion. Solubilization mechanisms can hardly be explained and clarified using only conventional analytical measurements, such as soluble chemical oxygen demand (COD) and volatile suspended solids (VSS). Here, we proposed a new but readily available analytical method where volatile solids (VS) are fractionized into high volatile solids (VS₂₀₅ ), moderate volatile solids (VS₃₅₀ ), and low volatile solids (VS₅₀₅ ). In a laboratory-scale experiment, anaerobic digesters were operated at 55, 65, and 75°C with thickened WAS. The high volatile solids (VS₂₀₅ ) sensitively reflected the temperature effect while the low volatility solids (VS₅₀₅ ) showed relatively insensitive results to the examined temperature conditions. This finding indicates that hydrolysis of high volatile solids (VS₂₀₅ ) was accelerated more effectively with the increased temperature. Also, based on the experimental results with the fractionized volatile solids, we recommend that autohydrolysis pretreatment should be operated at 75°C for 5 hr to achieve both rapid hydrolysis and reduced energy consumption. PRACTITIONER POINTS: The volatile solids (VS) were divided into high volatile, moderate volatile, and low volatile fractions. The fractionated VS showed how organic solids were hydrolyzed in thermophilic pretreatment of thickened waste activated sludge. At the higher temperature (75°C), the high volatile fraction increased substantially compared to 55 or 65°C. The fractionated VS responded more sensitively to the thermophilic temperatures compared to common analysis parameters (COD, VSS). We recommend thermophilic pretreatment at 75°C for 5 hr for thickened waste activated sludge.