Elsevier

Water Research

Volume 44, Issue 2, January 2010, Pages 449-460
Water Research

The role of organic matter in the removal of emerging trace organic chemicals during managed aquifer recharge

https://doi.org/10.1016/j.watres.2009.08.027Get rights and content

Abstract

This study explored the effect of different bulk organic carbon matrices on the fate of trace organic chemicals (TOrC) during managed aquifer recharge (MAR). Infiltration through porous media was simulated in biologically active column experiments under aerobic and anoxic recharge conditions. Wastewater effluent derived organic carbon types, differing in hydrophobicity and biodegradability (i. e., hydrophobic acids, hydrophilic carbon, organic colloids), were used as feed substrates in the column experiments. These carbon substrates while fed at the same concentration differed in their ability to support soil biomass growth during porous media infiltration. Removal of degradable TOrC (with the exception of diclofenac and propyphenazone) was equal or better under aerobic versus anoxic porous media infiltration conditions. During the initial phase of infiltration, the presence of biodegradable organic carbon (BDOC) enhanced the decay of degradable TOrC by promoting soil biomass growth, suggesting that BDOC served as a co-substrate in a co-metabolic transformation of these contaminants. However, unexpected high removal efficiencies were observed for all degradable TOrC in the presence of low BDOC concentrations under well adopted oligotrophic conditions. It is hypothesized that removal under these conditions is caused by a specialized microbial community growing on refractory carbon substrates such as hydrophobic acids. Findings of this study reveal that the concentration and character of bulk organic carbon present in effluents affect the degradation efficiency for TOrC during recharge operation. Specifically aerobic, oligotrophic microbiological soil environments present favorable conditions for the transformation of TOrC, including rather recalcitrant compounds such as chlorinated flame retardants.

Introduction

Managed aquifer recharge (MAR) systems, such as riverbank filtration (RBF) and soil aquifer treatment (SAT), are widely used natural processes for drinking water augmentation projects using source water that might be impaired by wastewater discharge. Previous studies have demonstrated that MAR systems are effective in dampening and reducing the concentrations of dissolved organic carbon (DOC) as well as various trace organic contaminants (TOrC) that might be present in impaired source waters (Drewes and Fox, 1999, Brauch et al., 2000, Grünheid et al., 2005). The presence of TOrC has become a key concern for drinking water augmentation projects during the past decade (Kolpin et al., 2002, Heberer, 2002, Focazio et al., 2008). Although adverse human health effects caused by these compounds at concentrations commonly observed in impaired water resources are very unlikely (Schwab et al., 2005), minimizing exposure of wastewater derived contaminants in these projects is desired. Previous research on the fate of TOrC during MAR has primarily focused on collecting anecdotal and site specific information on their occurrence and removal (Drewes et al., 2002, Montgomery-Brown et al., 2003, Grünheid et al., 2005, Dillon et al., 2008). Studies delineating the mechanisms and boundary conditions for the transformation of wastewater derived TOrC during MAR are lacking.

Previous studies demonstrated that the type and bioavailability of effluent organic matter (EfOM) controls the extent of soil biomass growth in MAR systems (Rauch and Drewes, 2004, Rauch and Drewes, 2005). EfOM may consequently effect the degradation of TOrC by serving as a co-substrate in microbiologically facilitated transformations (Stratton et al., 1983). The diversity and expression of the soil microbial community also depends on composition and concentration of the organic carbon substrate controlling trophic cycles in the subsurface (Preuß and Nehrkorn, 1996, Szewzyk et al., 1998). The composition of EfOM (i.e. in terms of its bioavailability) is primarily determined by the degree of wastewater treatment employed (Drewes and Fox, 1999), which can vary widely from primary to conventional to advanced wastewater treatment. As a result, effluent qualities fed to MAR systems can vary in biodegradable dissolved organic carbon (BDOC) concentrations from less than 1 up to 15 mg/L or more. As a consequence, soil microbial communities growing on different levels of BDOC can differ widely in total biomass and diversity.

The objectives of this research were to investigate the role of 1) abiotic vs. biotic conditions, 2) BDOC and 3) the type of organic carbon matrices on the removal of select TOrC, such as pharmaceutical residues, personal care products, and household chemicals, during MAR.

Section snippets

Target organic contaminants

Compounds selected for this study represent small molecular weight organic chemicals (180 to 360 Dalton) that are hydrophilic at neutral pH regimes as indicated by an octanol/water partition coefficient at pH 7 (log DpH=7 of less than 2.6). TOrC with these properties have a high potential to migrate into groundwater and are not expected to be adsorbed onto porous media. The molecular structures and physicochemical properties of the target compounds chosen for this study are presented in Table 1.

Adsorption behavior of target TOrC

In order to assess whether physical retardation would be a contributing factor in the removal of select TOrC in the column experiments, an abiotic column experiment was conducted to study sorption of the compounds onto the virgin porous media used in the columns. None of the six TOrC included in this experiment exhibited significant retardation during flow through porous media as compared to the conservative tracer bromide (indicated by Rd values very close to 1, Table 3). These findings reveal

Conclusions

Hydrophilic, small-molecular size TOrC can survive conventional and advanced wastewater treatment processes and are one of the key concerns in drinking water augmentation projects using impaired source waters. More research is needed to better understand the trophodynamic role of organic carbon on the removal of TOrC in MAR applications. In this study we conducted column experiments to illuminate key factors for the metabolic removal of twelve TOrC during soil infiltration. Experiments were

Acknowledgments

Partial funding for this study was provided by the Gwangju Institute of Technology in Korea. We are thankful for analytical support during this study provided by Dr. Thomas Heberer at the Technical University in Berlin, Germany, Matt Oedekoven, and Stephan Wagner.

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