Disinfection byproducts inhibit methane production and emissions in WWTP effluents and receiving rivers.

Wastewater treatment plant (WWTP) effluents are an important source of disinfection by-products (DBPs) in urban waters; however, their potential influence on riverine methane (CH4) dynamics remains unclear. By combining field investigations with microcosm experiments, this study examined the associations between typical DBPs and dissolved CH4 concentrations and diffusive fluxes in WWTP-receiving rivers and evaluated the effects of haloacetic acids (HAAs) on sediment CH4 production. Field monitoring showed that the annual average concentrations of haloacetic acids (HAAs), trihalomethanes (THMs), and haloacetonitriles (HANs) at WWTP outfalls were 13.62 ± 3.17, 15.62 ± 4.27, and 2.09 ± 0.62 μg L-1, respectively. Dissolved CH4 concentrations (annual average 0.14 ± 0.05 μmol L-1) and diffusive fluxes (annual average 4.72 ± 1.36 μmol m-2 h-1) exhibited negative spatial correlations with DBP concentrations, particularly along upstream-downstream transects near the outfalls. The 28-day microcosm experiments showed an overall decline in sediment CH4 production with increasing HAA concentrations. Across the A1-A4 treatments, corresponding to 10, 50, 100, and 200 μg L-1 HAAs, cumulative CH4 concentrations decreased from 77.90 to 43.35 μmol L-1, while the methane production rate (MPR) decreased from 0.39 to 0.26 μg CH4 g-1 wet sediment d-1. CH4 production at the lowest HAA concentration of 10 μg L-1 was also lower than that in the control. The abundance of the methanogenic functional gene mcrA also declined with increasing HAA concentrations. Mantel tests and partial least squares structural equation modeling (PLS-SEM) further supported negative associations of HAA concentration with mcrA abundance and CH4 production. Overall, the microcosm results provide preliminary evidence that HAAs may inhibit sediment CH4 production potential, whereas the field observations indicate that DBPs are associated with lower dissolved CH4 concentrations and diffusive CH4 fluxes near WWTP outfalls. These findings highlight the potential relevance of DBPs to riverine CH4 dynamics and the suggest that DBPs may represent an overlooked factor associated with CH4 cycling in wastewater-impacted river systems.