Impacts of wastewater intrusion on disinfection byproduct formation: An experimental study

Characteristics of intermittent piped water supply, in which the distribution system is not continuously pressurized, can allow contamination to enter pipes as intrusion. Two source waters were chlorinated and dosed with synthetic and real wastewater to simulate intrusion into a pipe with stagnant water to measure its impact on DBP formation potential. The formation and speciation of DBPs, including trihalomethanes, haloacetic acids, and haloacetonitriles, were measured throughout 7-day long experiments. Concentrations of regulated DBPs were greatest in experiments with either no intrusion or in experiments simulating intrusion of synthetic wastewater that lacked known chloramine-forming compounds. Intrusion from real and synthetic wastewater with chloramine-forming compounds quickly quenched existing free chlorine residual, limiting reactions with chlorine throughout the rest of the experiment. However, we observed up to 3.8 times greater dichloroacetonitrile concentrations 30 minutes following modified synthetic intrusion than pre-intrusion concentrations, compared to only a slight increase with no intrusion. After intrusion, regulated DBPs accounted for 77.9–92.6% of the total mass concentrations of DBPs measured in this study yet only accounted for 8.9–71.4% of the calculated additive cytotoxicity. In this study, we demonstrate that wastewater intrusion into a drinking water distribution system during stagnation likely leads to the formation of chloramine compounds if free chlorine is available. Certain types of intrusion, particularly with waters that do not contain chloramine-forming compounds, may increase overall DBP formation and is particularly pronounced for nonregulated haloacetonitriles, which contribute a greater effect on overall toxicity than currently regulated DBPs. These findings justify future work regarding the water safety impacts of wastewater intrusion, particularly increased DBP-attributed toxicity and haloacetonitrile formation, highlighting the need for strategic interventions to protect chemical drinking water quality in piped water systems with intermittent supplies or those that are vulnerable to outages.