Impacts of chlorine disinfection of municipal sewage effluent on receiving rivers: changes in organic matter and microbial communities

Effluents from wastewater treatment plants (WWTPs) can impact various aspects of receiving aquatic ecosystems, yet the specific effects of chlorine disinfection of effluents on these ecosystems remain poorly understood. In this study, a simulated flow-through channel system was employed to evaluate the changes in water quality and microbial community in receiving rivers resulting from the discharge of WWTP effluent, with or without chlorination. Results showed that dissolved organic matter (DOM) in secondary effluent from WWTPs exhibited higher fluorescence intensity and elevated levels of biopolymers, humic acids, and low molecular weight compounds compared to river water. Microbial analysis revealed that the input of secondary effluent promoted the proliferation of diverse microbial communities in periphyton of the receiving water, while the chlorinated effluents selectively inhibited chlorine-sensitive taxa in periphyton and favored chlorine-tolerant ones. Chlorine disinfection effectively reduced most pathogens in effluents; however, certain genera, such as Neisseriaceae and Escherichia-Shigella, persisted. Moreover, exposure to chlorinated effluent significantly elevated the relative abundance of Pseudomonas in periphyton compared to other conditions, raising concerns about the persistence of chlorine-tolerant pathogens in aquatic environments. These findings highlight the critical need to further evaluate the impact of the disinfection process in WWTPs on the long-term health and stability of riverine ecosystems.