In vitro toxicity of surface water disinfected by different sequential treatments

The in vitro toxicity of extracts of Hanjiang water disinfected by different sequential treatments was evaluated. Hanjiang water was disinfected using ozone, chloride dioxide or chlorine as the primary disinfectant followed by chlorine as the secondary disinfectant. HepG₂ cells were exposed to extracts corresponding to concentrations of 0.2, 1, 5, 25 and 125 mL water/mL medium. Compared with control, HepG₂ cells exposed to extracts of raw water and all disinfected water for 24 h increased oxidative stress level, DNA damage and micronuclei frequency, and decreased cell viability. Water disinfected by Cl₂ + Cl₂ had the highest DNA double-strand breaks. All disinfected water and raw water increased micronuclei frequency via clastogenic and aneugenic effects. Oxidative stress induced DNA strand breaks and micronuclei frequency and therefore reduced cell viability either in disinfected water or raw water. Compared with raw water, water after disinfection increased DNA strand breaks, decreased cell viability and changed oxidative stress potential. Compared with chlorination, sequential treatment using O₃ or ClO₂ as primary disinfectant followed by chlorine disinfection reduced chlorinated by-products, DNA double-strand breaks and cell viability, but did not decrease micronuclei frequency and other DNA damage such as DNA single-strand break, alkali liable sites and incomplete excision sites. Sequential treatments did not significantly reduce in vivo toxicity of disinfected Hanjiang water.