Impact of Wastewater Treatment Processes on Organic Carbon, Organic Nitrogen, and DBP Precursors in Effluent Organic Matter

Abstract

Unintentional, indirect wastewater reuse often occurs as wastewater treatment plant (WWTP) discharges contaminate receiving waters serving as drinking-water supplies. A survey was conducted at 23 WWTPs that utilized a range of treatment technologies. Samples were analyzed for typical wastewater and drinking-water constituents, chemical characteristics of the dissolved organic matter (DOM), and disinfection byproduct (DBP) precursors present in the effluent organic matter (EfOM). This was the first large-scale assessment of the critical water quality parameters that affect the formation of potential carcinogens during drinking water treatment relative to the discharge of upstream WWTPs. This study considered a large and wide range of variables, including emerging contaminants rarely studied at WWTPs and never before in one study. This paper emphasizesthe profound impact of nitrification on many measures of effluent water quality, from the obvious wastewater parameters (e.g., ammonia, biochemical oxygen demand) to the ones specific to downstream drinking water treatment plants (e.g., formation potentialsfor a diverse group of DBPs of health concern). Complete nitrification reduced the concentration of biodegradable dissolved organic carbon (BDOC) and changed the ratio of BDOC/DOC. Although nitrification reduced ultraviolet absorbance (UVA) at 254 nm, it resulted in an increase in specific UVA (UVA/DOC). This is attributed to preferential removal of the less UV-absorbing (nonhumic) fraction of the DOC during biological treatment. EfOM is composed of hydrophilic and biodegradable DOM, as well as hydrophobic and recalcitrant DOM, whose proportions change with advanced biological treatment. The onset of nitrification yielded lower precursor levels for haloacetic acids and nitrogenous DBPs (haloacetonitriles, N-nitrosodimethylamine). However, trihalomethane precursors were relatively unaffected by the level of wastewater treatment Thus, one design/operations parameter in wastewater treatment, the decision to have a long enough solids retention time to get reliable nitrification, affected much beyond its immediate goal of ammonium oxidation.