Toxicity effects of γ-irradiated wastewater effluents

Conventional wastewater disinfection is typically accomplished via chlorination, ozonation or ultraviolet (UV) irradiation. For each of these disinfectants, changes in the composition of the disinfected waters are known to occur and have been linked to changes in the toxicity responses of the disinfected effluent. The toxicity responses of wastewater effluent samples exposed to an alternative disinfectant, gamma (γ) radiation, have been studied and compared with the responses to chlorinated (with dechlorination) and undisinfected municipal wastewater effluent samples. Predisinfection samples were taken from six wastewater treatment plants. Multiple samples from each facility were split and subsamples were exposed to γ-irradiation, as well as chlorination/dechlorination in batch reactors. The disinfected and undisinfected subsamples were then assayed for toxicity using the Ceriodaphnia dubia chronic toxicity test. Wastewater disinfectant doses were sufficient to meet coliform limitations for effluent quality in all cases; furthermore, the batch disinfectant exposure protocols accomplished extents of coliform inactivation that were comparable to those observed in the disinfection unit operations at the full-scale facilities from which the samples were collected. Analysis of data from these assays showed that γ-irradiation induced significantly less toxicity in terms of C. dubia survival and reproduction than chlorination/dechlorination. Toxicity in the γ-irradiated effluent either decreased or was unchanged as compared to an undisinfected sample; effluent samples subjected to chlorination/dechlorination often showed a statistically significant increase in toxicity as compared to the undisinfected samples and the γ-irradiated samples.