Impacts of Bromide and Nitrogen Wastewater Discharges on Downstream Drinking Water Treatment Plant Disinfection Byproducts
Bromide and nitrogen are wastewater effluent constituents that have raised concern for their potential to affect disinfection byproduct (DBP) formation at drinking water treatment plants (DWTPs) downstream of wastewater discharges. Despite the toxicity of brominated and nitrogenous DBPs, wastewater treatment does not usually remove bromide and nitrogen prior to discharge. Bromide is a conservative chemical and thus is not removed from surface waters via natural transformations after being released, although it may be diluted. Nitrogen is biologically reactive; it can be transformed and removed by bacteria in the environment and in wastewater treatment plants (WWTPs). While this nitrogen cycling may reduce impacts on downstream DWTPs, it also produces nitrous oxide (N2O), a greenhouse gas, as a byproduct. Thus, interaction with the environment alters the potential impact of bromide and nitrogen on drinking water systems. The objective of this work was to examine the impacts of nitrogen and bromide present in wastewater effluent on DBP formation at downstream DWTPs, within the context of current and possible future DBP regulations. Three major conclusions were reached. First, within Southwestern Pennsylvania, an area where surface water bromide concentrations have increased due to fossil fuel extraction-based wastewater discharge, no statistically significant improvement in species-specific risk was observed over the past 20 years, despite decreasing TTHM levels. Second, nitrogen treatment decisions at WWTPs influence the formation of N-nitrosodimethylamine (NDMA), an unregulated but toxic DBP, at downstream DWTPs in areas of high de facto reuse (DFR). More plants using nitrifying wastewater treatment resulted in significantly lower NDMA detection rates and concentrations observed at chloraminating DWTPs located downstream. Third, the current approach to estimating national emissions of N2O related to wastewater treatment may underestimate emissions by an order of magnitude. Further, nitrogen removal at wastewater treatment plants reduces total N2O emissions attributable to wastewater treatment by reducing the N2O that would be generated in receiving surface waters. Thus, in both cases, impacts of WWTPs on downstream DWTP DBPs were observable. While natural cycling may remove nitrogen from surface water, in-plant removal of nitrogen prevents large amounts of N2O emissions.