Article pubs.acs.org/est
Bioanalytical Assessment of the Formation of Disinfection Byproducts in a Drinking Water Treatment Plant Peta A. Neale,† Alice Antony,‡ Michael E. Bartkow,†,§ Maria José Farré,∥ Anna Heitz,⊥ Ina Kristiana,⊥ Janet Y. M. Tang,† and Beate I. Escher*,† †
The University of Queensland, National Research Centre for Environmental Toxicology (Entox), Brisbane, Queensland 4108, Australia ‡ UNESCO Centre for Membrane Science and Technology, The University of New South Wales, Sydney, New South Wales 2033, Australia § Queensland Bulk Water Supply Authority (Seqwater), Brisbane, Queensland 4000, Australia ∥ The University of Queensland, Advanced Water Management Centre (AWMC), Brisbane, Queensland 4072, Australia ⊥ Curtin Water Quality Research Centre (CWQRC), Curtin University, Perth, Western Australia 6102, Australia S Supporting Information *
ABSTRACT: Disinfection of drinking water is the most successful measure to reduce water-borne diseases and protect health. However, disinfection byproducts (DBPs) formed from the reaction of disinfectants such as chlorine and monochloramine with organic matter may cause bladder cancer and other adverse health effects. In this study the formation of DBPs through a full-scale water treatment plant serving a metropolitan area in Australia was assessed using in vitro bioanalytical tools, as well as through quantification of halogen-specific adsorbable organic halogens (AOXs), characterization of organic matter, and analytical quantification of selected regulated and emerging DBPs. The water treatment train consisted of coagulation, sand filtration, chlorination, addition of lime and fluoride, storage, and chloramination. Nonspecific toxicity peaked midway through the treatment train after the chlorination and storage steps. The dissolved organic matter concentration decreased after the coagulation step and then essentially remained constant during the treatment train. Concentrations of AOXs increased upon initial chlorination and continued to increase through the plant, probably due to increased chlorine contact time. Most of the quantified DBPs followed a trend similar to that of AOXs, with maximum concentrations observed in the final treated water after chloramination. The mostly chlorinated and brominated DBPs formed during treatment also caused reactive toxicity to increase after chlorination. Both genotoxicity with and without metabolic activation and the induction of the oxidative stress response pathway showed the same pattern as the nonspecific toxicity, with a maximum activity midway through the treatment train. Although measured effects cannot be directly translated to adverse health outcomes, this study demonstrates the applicability of bioanalytical tools to investigate DBP formation in a drinking water treatment plant, despite bioassays and sample preparation not yet being optimized for volatile DBPs. As such, the bioassays are useful as monitoring tools as they provide sensitive responses even at low DBP levels.
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INTRODUCTION Drinking water disinfection is critical to ensure public health. However, reactions of commonly used disinfectants, such as chlorine, monochloramine, chlorine dioxide, and ozone, with natural organic matter (NOM) can result in the formation of disinfection byproducts (DBPs).1 There is considerable concern regarding chronic exposure to DBPs in drinking water, with some epidemiological studies indicating a causal relationship between exposure to chlorinated water and bladder cancer in humans.2−4 Due to stricter regulations, many utilities are changing to alternative disinfectants, such as monochloramine, since this forms lower levels of regulated DBPs compared with chlorination.5 However, this does not mean © 2012 American Chemical Society
that toxic DBPs are not formed from these alternative disinfectants. For example, nonhalogenated N-nitrosodimethylamine (NDMA), a probable human carcinogen, can be formed during disinfection with monochloramine, particularly in waters with elevated nitrogen levels.6 Despite considerable research on identification and formation of DBPs, there are still many unknown DBPs. During chlorination, only 50% of the halogenated DBPs (measured as total organic halogens Received: Revised: Accepted: Published: 10317
May 28, 2012 July 31, 2012 August 8, 2012 August 8, 2012 dx.doi.org/10.1021/es302126t | Environ. Sci. Technol. 2012, 46, 10317−10325
Environmental Science & Technology
Article
and chemical analysis. While previous studies have explored DBP formation in pilot plants or focused singularly on bioanalytical tools or chemical analysis, the current study represents the first comprehensive examination of DBP formation and toxicity using an extensive suite of bioassays, along with analysis of individual DBPs and halogen-specific AOXs, through key treatment steps in a conventional WTP serving a major metropolitan area.
(TOXs) or adsorbable organic halogens (AOXs)) can be attributed to the known DBPs, while the corresponding figure is only 20% after chloramination.7 AOXs should be considered surrogates for halogenated DBPs, rather than an absolute measure, since complete recovery of halogenated organic compounds is seldom achieved, particularly in complex mixtures such as chlorinated drinking water.8 Bioassays complement chemical analysis of DBPs as they respond to mixture effects, including contributions from unidentified DBPs, and can take into account interactive effects potentially leading to antagonism and synergy of DBPs in a complex mixture. Furthermore, bioassay responses are riskscaled; i.e., more potent compounds will contribute more to the bioassay response than less potent chemicals.9 Another advantage of applying bioassays in a drinking water treatment setting is that the micropollutants that would potentially interfere with the bioassay results typically occur at very low levels in source water so that bioassays are fairly specific to DBP formation. Therefore, many studies have applied bacterial or cell-based in vitro bioassays to assess the toxicity of DBPs formed during drinking water treatment,10,11 as well as in recreational waters,12 with a particular focus on genotoxic and mutagenic end points given the cancer concerns. Increases in cytotoxicity and nonspecific toxicity have also been observed during drinking water treatment.13,14 If in vitro bioassays are to be used as a monitoring tool, effects that are based on the ultimate manifestation of an effect (such as DNA damage or cell death) can be complemented with more sensitive early warning signs, such as the onset of stress responses and defense mechanisms in cells. Cellular stress responses are activated before actual harm occurs, but they indicate the presence of associated stressors.15 The DNA damage repair mechanisms, detoxification by glutathione conjugation, and the oxidative stress response are especially relevant for DBPs because many of them are electrophilic or, in the case of nitrosamines, become electrophilic after metabolic activation.16 From chemical considerations, the electrophilic properties of DBPs will not be limited to reactivity toward DNA. Hard electrophiles will react with hard biological nucleophiles such as DNA, while soft electrophiles will attack cysteine groups in proteins and peptides.17−19 Therefore, we propose to expand the bioassay battery to include different aspects of reactive modes of toxic action and cellular stress responses for the assessment of DBP formation and toxicity during drinking water treatment. Specifically, to represent effects of hard electrophiles, we apply the umuC assay20 as an indicator of the induction of DNA repair. The reactivity of soft electrophiles toward biomolecules will be assessed by an assay based on the sensitivity differences of an Escherichia coli strain pair, one of which can produce glutathione and therefore detoxify electrophilic chemicals, while the other one cannot produce glutathione and is therefore more sensitive in the presence of electrophiles.21,22 An additional, more generic indicator of oxidative stress response is the AREc32 assay that is based on the induction of the Nrf2-ARE signaling pathway23 and has recently been adopted for water quality testing.24 The aim of this study was to assess DBP formation and associated mixture effects at each step in the train of a full-scale drinking water treatment plant (WTP) using in vitro bioassays for general cytotoxicity and reactive modes of toxic action, halogen-specific AOX analysis, organic matter characterization,
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EXPERIMENTAL SECTION Sample Site. Samples were collected on the Dec 8, 2010, and March 2 and May 11, 2011 from a full-scale WTP in South East Queensland, Australia. The plant has two parallel treatment trains, the first comprising coagulation, sand filtration, chlorination, and chloramination (East Bank), with a second train of coagulation, dissolved air flotation, sand filtration, chlorination, and chloramination (West Bank). There were several disinfection points along the treatment train: ∼3.5 mg/L hypochlorite was added before rapid sand filtration as an oxidant, a residual of 2−2.5 mg/L free chlorine was maintained at the primary disinfection step after filtration, sodium fluorosilicate and lime were added before the storage tank, where the water was maintained for several hours, and finally chloramination was achieved by addition of aqueous ammonia (obtained by purging gaseous ammonia into water) followed by hypochlorite addition to achieve a Cl/N ratio of ∼5 and a 2.5 mg/L total chlorine residual in the form of monochloramine. Samples were collected from the WTP inlet and at sampling points after coagulation, after sand filtration, postchlorination (primary disinfection), after chemical addition (lime and fluoride), at WTP storage, and after chloramination (secondary disinfection), i.e., at the outlet of the WTP (Figure S1, Supporting Information). Field and laboratory blanks (Milli-Q water) were also collected during sampling. Details of the processes on the plant, sample collection, and preservation procedures are provided in the Supporting Information, section SI-1. Organic Matter Characterization. Dissolved organic carbon (DOC) was analyzed using liquid chromatography organic carbon detection (LC-OCD) (DOC-Labor, Karlsruhe, Germany). For each sample 1000 μL was injected into a size exclusion chromatography (SEC) column (HW-50S, Tosoh, Stuttgart, Germany) with a 28 mM phosphate mobile phase (pH 6.58). The sample was separated into biopolymers, humic substances, building blocks (degraded humic substances), and low molecular weight (LMW) neutral fractions, and the DOC concentration, specific UV absorbance (SUVA), and molecular weight were measured.25 Since the first two sampling campaigns provided very similar LC-OCD results, indicating similar NOM characteristics, only nonpurgeable organic carbon (NPOC) was measured in the May 2011 sampling campaign using an Analytik Jena Multi N/C 3100 (Jena, Germany). AOX Analysis. Samples were acidified to pH 2 using concentrated HNO3 and passed through two activated carbon columns for adsorption of the organic halogens using a Mitsubishi TX-3AA adsorption module (Chigasaki, Japan). The adsorbed sample was combusted in the presence of oxygen for 10 min at 1000 °C using a Mitsubishi AQF-100 automated quick furnace unit, and the produced hydrogen halide gas was collected in Milli-Q water in the Mitsubishi GA-100 absorption unit. The aqueous sample was then analyzed for Cl−, Br−, and I− using a Dionex ICS-3000 dual-channel ion chromatograph system (Sunnyvale, CA). The concentrations of the measured 10318
dx.doi.org/10.1021/es302126t | Environ. Sci. Technol. 2012, 46, 10317−10325
Environmental Science & Technology
Article
Table 1. Summary of Results Obtained from Analyses of Halogen-Specific AOXs and Selected DBPs and Assessment of Nonspecific and Reactive Toxicity in Samples Collected from the East Bank Treatment Train in May 2011 inlet
coagulation
[DOC] (mg/L) [AOCl] (μg/L) [AOBr] (μg/L) [AOI] (μg/L) ∑[AOX] (μg/L)
4.52 ± 0.08 13.5 ± 0.7 9.0 ± 0.0 9.3 ± 0.4 31.8 ± 1.1
2.72 ± 0.09 17.5 ± 2.1 15.0 ± 1.4 3.4 ± 0.5 35.9 ± 4.0
[CHCl3] (μg/L) [CHBrCl2] (μg/L) [CHBr2Cl] (μg/L) [CHBr3] (μg/L) [Cl3AN] (μg/L) [Cl2AN] (μg/L) [BrClAN] (μg/L) [Br2AN] (μg/L)
7.5 ± 1.6 0.2 ± 0.0 0.3 ± 0.0
