Effects of Wastewater Disinfection on Human Health

Wastewater disinfection is practiced with the goal of reducing risks of human exposure to pathogenic microorganisms. Ideally, this goal is to be met without introducing other risks, such as those that could be associated with disinfection by-products. The purpose of this research was to assess the effects of wastewater disinfection on human health. This assessment was conducted by examining two fundamental questions regarding the application of disinfection:Should municipal wastewater effluents be disinfected prior to discharge?, andUnder circumstances where disinfection is necessary, how should it be accomplished?Undisinfected effluent samples from a several municipal wastewater treatment facilities were collected for analysis. Facilities were selected to provide a broad spectrum of effluent quality, particularly as related to nitrogenous compounds. Samples were subjected to bench-scale chlorination/dechlorination and UV irradiation under conditions that allowed compliance with relevant discharge regulations, and such that disinfectant exposures could be accurately quantified. Disinfected samples were subjected to a battery of microbiological and toxicological assays to assess the immediate and long-term effects of wastewater disinfection.In general, (viable) bacterial populations showed an immediate decline as a result of disinfectant exposure; however, incubation of disinfected samples under conditions that were designed to mimic the conditions in a receiving stream resulted in substantial recovery of the total bacterial community. The bacterial groups that are commonly used as indicators do not provide an accurate representation of the response of the bacterial community to disinfectant exposure and subsequent recovery in the environment. UV irradiation and chlorination/dechlorination both accomplished measurable inactivation of indigenous phage; however, the extent of inactivation was fairly limited under the conditions of disinfection used in this study. UV irradiation was consistently more effective as a virucide than chlorination/dechlorination under the conditions of application, based on measurements of virus (phage) diversity and concentration.Experiments were also conducted to characterize the behavior of organic amines in chlorine-based disinfection processes. In general, these compounds were found to undergo substitution reactions to yield mono- or dichlorosubstituted organic chloramines, which tend to be fairly stable. Toxicological assays based on Microtox and O2 uptake indicated that the organic chloramines were more toxic than their non-chlorinated parent compounds.Taken together, and when considered in conjunction with previously published research, the results of these experiments illustrate several important limitations of common disinfection processes as applied in the treatment of municipal wastewaters. In general, it is not clear that disinfection processes, as commonly implemented, are effective for control of the risks of disease transmission, particularly as associated with viral pathogens. Microbial and chemical quality in receiving streams may not be substantially improved by the application of these methods; under some circumstances, it could be argued that disinfection may actually yield a decrease in effluent and receiving water quality. Decisions regarding the need for effluent disinfection must account for site-specific characteristics, but it is not clear that disinfection of municipal wastewater effluents is necessary or beneficial for all facilities. When direct human contact or ingestion of municipal wastewater effluents is likely, disinfection appears to be necessary. Under these circumstances, UV irradiation appears to be superior to chlorination in terms of microbial quality, as well as chemistry and toxicology. This advantage is particularly evident in effluents that contain appreciable quantities of ammonia-N or organic-N.