The role of biofilms on the formation and decay of disinfection by-products in chlor(am)inated water distribution systems

This study investigated the role of biofilms on the formation and decay of disinfection by-products (DBPs) in chlorine (Cl2) or monochloramine (NH2Cl) disinfected reactors under the conditions related to drinking water distribution systems (DWDSs). Biofilm analysis results revealed that at 0.5 mg/L of disinfectant residual, both Cl2 and NH2Cl were not effective to remove biofilms. As the disinfectant residual increased, biofilms could be eradicated by Cl2, while remaining biofilms were still present even under the highest allowable NH2Cl dose (4 mg/L) for 25 days. Low DBP formation was observed under the recommended minimum Cl2 residual (0.5 mg/L), which could be attributed to limited Cl2 reactions with biofilms, as well as a combination of the volatilization and biodegradation of DBPs. However, when Cl2 residuals reached 2 mg/L, DBP concentrations in bulk water increased sharply beyond the DBP formation of the feed solution, with trihalomethanes and haloacetic acids being the most prevalent DBP species. The sharp increase was temporary for 15 days because of the removal of biofilms. For unregulated DBPs, high levels of haloacetonitriles were observed as attached biofilms reacted with the increased Cl2 dose and provided an additional organic nitrogen source for nitrogenous DBP formation. When maximum Cl2 residual (4 mg/L) was applied, no further increase of DBPs was observed because of biofilm eradication. For NH2Cl disinfection, the DBP levels were much lower than those of Cl2 disinfection, with small differences in DBP formation for different NH2Cl residuals. Overall, this study provides insights into optimizing disinfection protocols for water utilities by balancing the benefits of disinfection application for biofilm control with minimized toxic DBP formation in DWDSs.