Chlorine conversion impact on activated carbon block filters: Water quality and nitrification risk in chloraminated water

The effectiveness of a periodic free chlorine conversion period (FClP) as a nitrification mitigation measure has been scrutinized in chloraminated distribution systems, and knowledge about the response of activated carbon block (ACB) point‐of‐use (POU) filters to this practice is unknown. In this study, three similar filters were monitored bracketing three successive FClPs in the laboratory, aimed at studying the response of ACB POU filters to FClP and the occurrence of nitrification. Results indicate that stagnating filters fostered biofilm growth and increased nitrite concentration (2.5 mg/L‐N) above the EPA maximum contaminant level (MCL). Despite FClP being effective at mitigating nitrification and reducing nitrite concentration far below EPA MCL, this was only temporary. Filters that were microbially seeded before an FClP event re‐nitrified immediately after chloramination resumed. Successive FClPs induced a microbial community change and failed at controlling nitrification, resulting in elevated nitrite and microbial exposure.