Comparative analysis of anaerobic sludge digestion in conventional and high-rate activated sludge systems

Anaerobic sludge digestion is widely applied in municipal wastewater treatment plants for sludge stabilisation and renewable energy recovery through biogas production. However, digestion performance is governed by upstream biological treatment configuration, which determines sludge origin and biodegradability. This study presents a comparative assessment based on experimentally generated from a full-scale high-rate activated sludge (HRAS) system and literature-based datasets for conventional activated sludge (CAS) systems. Long-term anaerobic batch digestion tests were conducted under mesophilic conditions using plant-representative mixed sludge prepared from primary and secondary sludges collected from HRAS treatment line of a full-scale municipal wastewater treatment plant in Türkiye. Methane production and solids reduction were evaluated. The experimental results were further interpreted using plant-wide modelling implemented in the SUMO simulation platform. Methane yields normalised to volatile solids fed varied substantially with sludge origin and sludge age. The HRAS plant operated at the lowest sludge age exhibited the highest methane yield (530 L CH₄/kg VS_fed), whereas literature reported CAS systems yielded between 193 and 375 L CH₄/kg VS_fed. When normalised to treated wastewater, biogas production from CAS systems in Türkiye was significantly lower than that from HRAS configurations and reported CAS systems in Europe and North America, reflecting differences in carbon capture and upstream processes. Good agreement between experimental observations and plant-wide modelling confirms that anaerobic digestion performance cannot be optimised independently of upstream treatment design. Overall, the results highlight the importance of high-rate carbon capture strategies and unit wastewater-based performance indicators for improving plant-wide energy recovery in wastewater treatment systems.