The impact of disinfection Ct values on cytotoxicity of agricultural wastewaters: Ozonation vs. chlorination

Toxicity arising from toxic disinfection byproducts is an unintended result of disinfection during water reclamation. To ensure safe water reclamation treatment, it is important to develop a disinfection strategy with minimal formation of overall toxicity in the reclaimed water. The cumulative disinfectant concentration over time (Ct) is a useful concept for pathogen control during reuse water disinfection. We evaluated the toxicity impact of Ct values and different methods to achieve identical Ct values by ozonation or chlorination of wastewaters from four agricultural sources on mammalian cells. N-acetylcysteine (NAC) reactivity of the wastewater organic extracts was determined to reveal their impact on the thiol-specific biological detoxification mechanism. The results demonstrated that for two sources and for both ozonation and chlorination, higher Ct values enhanced cytotoxicity. The ozonated waters were at least 10% less toxic and as much as 22.4 times less toxic than either the non-disinfected controls or the chlorinated waters. Chlorination consistently induced higher cytotoxicity than ozonation by between 2.2 and 22.4 fold, respectively, and induced similar or higher cytotoxicity than the non-disinfected controls, by at most 4.4 fold. Given the same Ct values, the combination of high disinfectant concentration and short contact time produced finished wastewaters with higher toxicity, than the combination of low disinfectant concentration and long contact time. NAC thiol reactivity was positively and significantly correlated with mammalian cell cytotoxicity, and agreed with 80% of the cytotoxicity rank order. This suggests that the induction of cytotoxicity involved reactions with agents that acted as thiol pool quenchers. The overall results indicate that the cytotoxicity of wastewaters may increase when higher Ct values are applied to inactivate recalcitrant pathogens. To counteract the potential increase in cytotoxicity at high Ct values, for both ozonation and chlorination, lower disinfectant dose and longer contact time may be adopted.