Short-term effects of disinfectant switching on disinfection byproduct formation in drinking water distribution systems

Line chart summarizing phased responses after chloramine-to-chlorine switching, with turbidity peaking at D2 and ATP and DBPs peaking around D8. Disinfectant switching can disturb drinking water distribution systems (DWDSs), yet its short-term impacts on disinfection byproduct (DBP) formation remain insufficiently characterized due to limited temporal resolution in previous studies. Here, we monitored 14 DBPs across four classes, trihalomethanes (THMs), nitrosamines (NAs), haloacetonitriles (HANs), and haloacetamides (HAMs), together with physicochemical parameters and biomass indicators (ATP), during a 36-day disinfectant switching experiment in pipe loops. Switching from chloramine to free chlorine induced phased disturbances. Turbidity peaked on Day 2 (122% increase), whereas bulk-water ATP reached its maximum on Day 8 (46% increase). THMs and HAMs increased markedly (by 67–135% and 279%, respectively) and peaked on Day 8 relative to the chloramine baseline. Although overall NA concentrations declined after the switch, certain nitrogenous DBPs exhibited transient rebounds, with N-nitrosodiethylamine increasing by 95% and even exceeding the chloramine control. Among HANs, species-specific behavior was observed, with DCAN increasing by 162%. By Day 36, most DBPs and ATP declined but remained elevated for several species, indicating gradual re-equilibration under free chlorine. Strong positive correlations between ATP and DBPs suggest that DBP amplification is likely driven by biological destabilization. Overall, the first week after disinfectant switching is a critical period requiring enhanced monitoring.