Haloacetonitriles (HANs): Factors Affecting Their Formation and Mitigation

Haloacetonitriles (HANs) are nitrogenous disinfection byproducts (DBPs) detected frequently in chlorinated and chloraminated water. HANs are considered a serious concern in drinking water and water reuse, as some unregulated HANs have been found to be more cytotoxic and genotoxic compared to regulated DBPs. This research focused on two issues regarding HAN formation during chlorination and chloramination of source waters. The first issue investigated the effects of various water quality and operation parameters, including disinfection mode, chlorine (or chloramine) dose, chlorine (or chloramine) contact time, pH, and bromide concentration, on the formation of HANs during the chlorination and chloramination of a tertiary filtered wastewater effluent and surface water used as a drinking water source. HAN formation potentials in two sources were determined under varying levels of the aforementioned factors, and statistical analyses were conducted to assess their significance. Tertiary wastewater effluent exhibited a higher potential for HAN formation compared to surface water samples. HAN formation was greater during free chlorination than during in-situ chloramination with prechlorination and preformed monochloramination. Dihalogenated acetonitriles (DHANs) were the predominant HAN species across all the disinfection schemes for both source waters. The results indicated that disinfectant dosage and contact time consistently and positively correlated with HAN formation across all disinfection schemes. Conversely, pH demonstrated an inverse relationship with HAN formation, which was significant only during preformed monochloramination of both water sources. The net DHAN formation was higher at lower pH levels and decreased as pH increased from 7 to 9. Bromide concentration significantly impacted net DHAN concentrations across all disinfection schemes for both samples except during preformed monochloramination of wastewater effluent, with bromide having the greatest influence on HAN formation among all considered factors. The HAN speciation shifted toward brominated HANs from their chlorinated analogs in presence of bromide in source water. Therefore, water treatment authorities should monitor bromide concentrations of source water whenever HAN formation is a concern. The second issue examined the effects of prechlorination time on the formation of chlorinated and brominated HANs during in-situ chloramination of two surface waters and aimed to identify an optimal prechlorination strategy that minimized DBP-associated toxicity while achieving the oxidant exposure necessary for pathogen inactivation. Chlorination was performed at different doses until achieving a free chlorine exposure sufficient for the three-log inactivation of Giardia cysts at pH 7 and 20 °C. DBP analysis indicated that DHANs and total trihalomethanes (TTHMs) were the predominant HAN and THM species, while haloacetic acids (HAAs) were detected below method reporting levels. Both THM and HAN concentrations as well as toxicity associated with them decreased with increasing chlorine dose and shorter prechlorination time. DBP-associated toxicity was higher in Virgin River samples compared to blended surface water samples under all conditions of initial chlorine concentration and reaction time used to achieve the targeted Ct. DBP analyses and their associated toxicity revealed that employing a high chlorine dosage and a brief prechlorination time effectively minimized both DBP formation and associated toxicity during in-situ chloramination of source waters. This research identifies optimizing prechlorination time as a viable strategy to reduce DBP formation and subsequent toxicity during water treatment.