A critical analysis of mass and energy balances for external sludge hydrolysis process configurations in mesophilic anaerobic digesters intensification

ABSTRACT This article examines the application of external sludge hydrolysis processes (ESHP), such as thermal hydrolysis process (THP), biological hydrolysis process (BHP), and thermal alkaline hydrolysis (TAH), in the treatment of solids at wastewater treatment plants. The aim of this study was to evaluate the benefits and challenges associated with ESHP, focusing on its impact on the mass and energy balances of the solid phase. This evaluation is carried out suing a simplified mathematical model fed with data from the literature. This study uses simplified modeling of the solid phase, considering ESHP, anaerobic digestion, sludge dewatering, and thermal drying. The design parameters are based on conservative estimates from the literature. The findings indicate that enhanced process performance can significantly reduce energy demand, especially by lowering the water content in solid-phase processes, which is a major contributor to thermal energy consumption. The results show that ESHP offers significant advantages, including increased infrastructure capacity and energy optimization. This process allows plants to thermally dry all sludge without relying on external energy sources such as natural gas. Moreover, it facilitates greater energy extraction from dried sludge using cleaner thermal technologies like pyrolysis. The study also emphasizes the importance of adjusting the solids content in sludge or, alternatively, directing the energy flow first to the waste activated sludge (WAS) to perform hydrolysis. This adjustment aims to improve the overall mass and energy balances. In conclusion, ESHP enhances sludge treatment performance, leading to more than a 20% reduction in sludge cake production and a 7–25% decrease in thermally dried sludge.