Enhanced formation of brominated and nitrogenous disinfection byproducts in drinking water disinfection with chlorocyanurates

Chlorocyanurates are the stabilised chlorine most people know as swimming-pool tablets. They are cheap, they keep for years and they are easy to dose. Small rural water systems and emergency responders use them to disinfect drinking water. Until this study, almost all that was known was that they produce less of the two byproduct families regulations measure, the trihalomethanes and haloacetic acids.

We compared several chlorocyanurate formulations against ordinary chlorine, measuring regulated and unregulated byproducts side by side. The regulated byproducts did come down, by 10 to 50 percent. But bromine incorporation went up by 50 to 200 percent across most byproduct classes, and the nitrogen-containing byproducts went up too. The calculated toxicity of the water ended up 1.5 to 2 times higher than with plain chlorine under most conditions. Part of the reason is that chlorocyanurates break down to trichloramine, which feeds the formation of dichloroacetonitrile, one of the most toxic byproducts we know of.

By the regulated measures chlorocyanurates look safer, but by calculated toxicity they are worse. A disinfectant can meet the regulated byproduct limits and still raise the calculated toxicity of the water, because those limits cover a different set of compounds. The paper shows that chlorocyanurates take part in byproduct formation rather than simply producing less of the regulated ones. It also gives practical guidance to the systems that depend on them, such as avoiding source waters with elevated bromide and preferring the higher chlorine-to-cyanurate formulations. Those systems are overwhelmingly small, rural and low-income, in the United States and elsewhere.