Constructed wetlands and soil-aquifer treatment systems: Effects on the character of effluent organic matter

Abstract

Within the context of potable reuse, there is a need for a more comprehensive examination of the quality of dissolved organic matter (DOM) in treated wastewater and the efficacy of different treatment schemes in removing or transforming DOM. In particular, there are significant information gaps regarding the character, fate, and health risks associated with effluent organic matter (EfOM). Two research goals guided this research. The first goal was to evaluate the efficacy of constructed wetlands for wastewater polishing in a hot, arid environment, from the perspective of season-dependent effects on DOM. To this end, behavior of organics was evaluated over a 22-month period during treatment in a local constructed wetlands facility. The second goal was to examine changes in character of EfOM that accompany passage through natural treatment systems (either constructed wetlands or soil aquifer treatment, SAT). This was accomplished via isolation and characterization of organics collected along flowpaths of these treatment systems. Wetland effluent concentrations of dissolved organic carbon (DOC) and nonbiodegradable DOC were positively correlated with temperature. That is, the highest concentrations occurred in summer and were attributed to the combined effects of evapotranspiration (ET) by wetland vegetation along with production of wetland-derived natural organic matter (NOM). There was little if any change in the hydrophobic-hydrophilic character of DOM attending wetland treatment. Biodegradation of labile EfOM combined with contribution of wetland-derived NOM resulted in modest (at best) changes in distribution of carbon moieties in hydrophobic (HPO) and hydrophilic (HPI) acid isolates. Aliphatic carbon decreased during wetland treatment. Elemental analysis suggested that microbial activity is the dominant process controlling the character of wetland-derived NOM. Reactivity of isolates in forming trihalomethanes (THMs) during chlorination increased as consequence of wetland treatment. Wetland-derived NOM was more reactive than EfOM in forming THMs. Uniform trends occurred among isolates of EfOM and wetland-derived NOM between biodegradability and THM production upon chlorination. Ultrahydrophilic EfOM was preferentially removed during vadose zone percolation of secondary effluent. The chemical character of EfOM (HPO- and HPI-acids) became more similar to NOM as a consequence of SAT. Genotoxicity of HPO-acids, on a per mass basis, increased after SAT.