Trihalomethanes as chlorination by-product: insights from an extended two decades study across six full-scale water treatment plants

Examinations based on long-term monitoring that investigate sustainable chlorination strategies are essential to solve the long-standing concern regarding harmful disinfection by-products (DBPs) formation. This study examined the trends in total trihalomethanes (THMs), a representative DBP class, in Seoul City tap water from six full-scale water treatment plants over the last two decades. Statistical correlation analyses of long-term monitoring data over this period revealed an evident relationship between total THMs, raw water quality, and chlorination operational factors. A disconcerting increase in total THMs was observed over the last 10 years (2012–2023), which was conspicuous during the warmer seasons (summer and fall). Moreover, the residual chlorine in treated water exhibited a strong correlation with total THMs during this period, suggesting that the pre- and post-chlorination input rates were the primary contributors. These findings provide valuable evidence that chlorination influences DBP formation and underscore the need for disinfection strategies balancing the benefits of chlorination and the associated risks such as DBP formation.