Systematic review on the residual chemicals in wastewater treatment sludge: Specifically focusing on the occurrence state and anaerobic bioprocess

Anaerobic digestion (AD) is an effective method for the secure and resourceful disposal of waste activated sludge (WAS). Although the extensive utilization of wastewater treatment chemicals has resulted in their accumulation in WAS, their potential impacts have persistently been overlooked in engineering practice. Based on typical chemical usage scenarios in wastewater treatment plants, this systematic review focuses on typical WTCs, including coagulants (ferric chloride, polymeric ferric sulfate, polymeric aluminum chloride, polyacrylamide), defoamers (linear alkylbenzene sulfonates), and algaecides (quaternary ammonium compounds). A comprehensive summary of their occurrence, behavior, and effects on AD properties, including short-chain fatty acids, methane production, and microbial communities was provided. The above-mentioned WTCs disrupt AD performance by modifying sludge morphology, competing for electrons, weakening enzymatic activity, and stressing microorganisms. Although the impact of some of these WTCs, such as ferric chloride, on AD remains controversial, there is still a need to keep observing them. Future research should delve into the microscopic behavior and compounding effects of WTCs in AD. Additionally, long-term observation of microbial changes and exploration of corresponding detoxification strategies should be conducted based on these findings. This review provides a theoretical guidance for further quality enhancement of AD in engineering practice.