Reduction of haloacetonitrile-associated risk by adjustment of distribution system pH

Abstract

Haloacetonitriles (HANs) and haloacetamides (HAMs) have been scrutinized recently due to their relative toxicity compared to regulated carbonaceous disinfection byproducts (DBPs). Rates of HAN hydrolysis to HAMs and HAMs to haloacetic acids (HAAs) increase with pH and the ranked toxicity of these groups is HAN > HAM > HAA. Thus, intentional hydrolysis during distribution of drinking water would result in reduced exposure to HANs and HAMs, and subsequently lower toxicological risk. To evaluate the potential for such a hydrolysis scheme to reduce the toxicological burden of delivered water, we examined the formation and degradation of HANs and HAMs in conventionally treated surface water and in Milli-Q water spiked with two natural organic matter isolates and free chlorine at varying pH. HAN concentrations in finished drinking water at pH 6, 7.5, and 9 at the end of the 5-day experimental period were 12.5, 7.1, and 1.9 μg L⁻¹, respectively, equivalent to a 77% (pH 6 to 9) and 45% (pH 7.5 to 9) decrease in summed calculated toxicity from the disinfection by-products measured. Similar conclusions followed from experiments with natural organic matter isolates. In samples from the full-scale treatment plant and experiments conducted with isolates, the potential to exceed THM and HAA₅ regulatory limits was increased while driving HANs, an unregulated but highly toxic class of DBPs, to lower toxicity hydrolysis products. We find that increasing distribution system pH is an effective method to reduce HAN-associated toxicity, although potential tradeoffs in distribution system corrosion, scale stability, and the potential to violate THM and HAA regulations should be carefully evaluated prior to implementation.