Disinfection by-products (DBPs) have been a water treatment concern since their discovery in the 1970's. Today, only eleven drinking water DBPs are regulated by the Environmental Protection Agency (EPA). However, over 500 DBPs have been identified in drinking waters with advances in analytical methods within the last two decades. Halonitromethanes (HNMs) were among the fifty-plus unregulated high-priority DBPs that were monitored for and detected in drinking waters in a recent nationwide occurrence study funded by EPA. The toxicology studies conducted in recent years showed that HNMs are one of the most cytotoxic and genotoxic classes among the emerging DBPs, and they have orders of magnitude higher toxicity levels (i.e., cytotoxicity and genotoxicity) than any of the regulated organic DBPs. Although DBP formation is typically studied in the confines of the drinking water industry, there is growing concern about the effects of wastewater treatment plant discharges on DBP formation. As drinking water demands increase and drought conditions become more frequent, indirect potable reuse of wastewater is gaining more attention and interest around the world. Finally, in recent years, some DBP species have been included in discharge permits of wastewater treatment effluents. Therefore, it is also important to examine and understand the formation of DBPs and the presence of DBP precursors in wastewater effluents.
The objectives of this project were to investigate the HNM formation potentials of wastewater treatment plant effluents under different disinfection conditions, to study the effect of ultraviolet light (UV) disinfection in wastewater treatment facilities on HNM formation potentials, and to quantify the actual occurrence of HNMs at the effluents of selected wastewater treatment plants. The wastewater treatment plants that participated in this project include facilities with and without nitrification capabilities. The five disinfection scenarios investigated in this study were ozonation, ozone-chlorination, chlorination, ozone-chloramination, and chloramination.
The results showed that among the five disinfection scenarios tested, HNM yields were in the order of ozonation-chlorination >>> ozonation-chloramination > chlorination > chloramination > ozonation. Pre-ozonation prior to chlorination and chloramination significantly increased HNM formation, while ozonation alone did not produce any HNMs. Therefore, HNM formation is expected to be problematic mainly in treatment plants using ozonation followed by chlorination. The use of chloramines after ozonation significantly decreases HNM formation compared to using chlorine after ozonation. Chloramination produced the least amount of HNMs of all the disinfection scenarios tested. Despite some of its disadvantages (e.g. weak disinfectant, nitrification problems in distribution systems), it appears that chloramination produces significantly lower amounts of HNMs as well as regulated trihalomethanes and haloacetic acids than chlorination.
The typical UV disinfection conditions in full-scale municipal wastewater treatment plants (WWTPs) using low pressure lamps did not change the HNM formation potential (compared to the same water prior to UV disinfection) in wastewater effluents. Actual HNM concentrations at the effluents of full-scale WWTPs using chlorine disinfection were below 2 ug/L, while no HNM formation was observed at the effluents
of WWTPs using UV disinfection. It appears that at the typical disinfectant doses and detention time of wastewater disinfection, trace amounts of HNMs will form. However, WWTPs still release HNM (and other DBP) precursors, as observed with the formation potential tests, to downstream users. Comparison of HNM formation potential yields of the Greenville drinking water treatment plant (DWTP) effluent with the effluents of the WWTPs located in the service area of the Greenville DWTP showed that the HNM yields were higher in the wastewater effluents than in the treated drinking water, except for the case of chlorination. Significantly higher HNM yields in the wastewater effluents were attributed to significant amounts of HNM precursors produced during biological treatment and/or present in the influent wastewater, particularly from industrial discharges.