Improved (and Singular) Disinfectant Protocol for Indirectly Assessing

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Environmental Science & Technology

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An improved (and singular) disinfectant protocol for indirectly assessing organic

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precursor concentrations of trihalomethanes and dihaloacetonitriles

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AUTHORS:

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Thien D. Do a, Justin R. Chimka b, and Julian L. Fairey a,*

6 AUTHOR AFFILIATIONS:

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a

Department of Civil Engineering, University of Arkansas, Fayetteville, AR 72701

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b

Department of Industrial Engineering, University of Arkansas, Fayetteville, AR 72701

*

Corresponding author:

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Julian L. Fairey, Ph.D., P.E.

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Associate Professor, Department of Civil Engineering

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Address: 4190 Bell Engineering Center, Fayetteville, AR 72701

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Phone: (479) 575-4023

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Fax: (479) 575-7168

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Email: [email protected]

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SUBMITTED TO:

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1 ACS Paragon Plus Environment


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Abstract

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Measurements of disinfection byproduct (DBP) organic precursor concentrations (OPCs)

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are crucial to assess and improve DBP control processes. Typically, formation potential

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tests – specified in Standard Methods (SM) 5710-B/D – are used to measure OPCs. Here,

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we highlight several limitations of this protocol for dihaloacetonitriles and

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trihalomethanes and validate a novel Alternative Method (AM). The effects of pH,

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disinfectant type (free chlorine and monochloramine), and chlor(am)ine residual (CR)

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were examined on DBP formation in a suite of waters. Using the SM, DHAN decreased

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43-47% as the CR increased from 3- to 5 mg L-1 as Cl2, compromising OPC assessments.

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In contrast, a high monochloramine dose (250 mg L-1 as Cl2) at pH 7.0 (the AM)

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accurately reflected OPCs. The two methods were compared for assessing DBP precursor

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removal through three granular activated carbon (GAC) columns in series. Breakthrough

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profiles assessed using the AM only showed DBP precursor sorption occurred in each

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column that decreased over time (p=0.0001). Similarly, the AM facilitated ranking of

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three types of GAC compared in parallel columns, whereas the SM produced ambiguous

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results. Fluorescence intensity of a humic-like fluorophore (i.e., I345/425) correlated

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strongly to precursor removal in the GAC columns. The practical implications of the

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results are discussed.

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Keywords: disinfection byproduct precursor removal; disinfectant regime; fluorescence;

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Standard Methods 5710; natural organic matter isolates

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Introduction

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Removal of organic disinfection byproduct (DBP) precursors in processes at

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drinking water treatment plants (DWTPs) is a critical aspect of DBP control. Accurate

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organic precursor concentration (OPC) measurements are crucial to assess and improve

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DBP control processes. However, such measurements are complicated because they are

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indirect, inferred by calculating differences in formation of individual species or groups

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of DBPs before and after treatment. As proposed by Steven and Symons,1 the DBP

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formation potential (DBPFP) test is commonly applied to indirectly measure OPCs

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and/or assess the effectiveness of DBP-precursor removal processes. Standard Methods

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5710-B and D (SM 5710-B/D) specifies the conditions for a DBPFP test2 for the four

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regulated trihalomethanes (THM4) and other DBPs, respectively, which include: water

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buffered at pH 7.0 with phosphate, incubation at room temperature in the dark, and a 7-

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day disinfectant residual between 3-5 mg L-1 as Cl2 to be quenched by a suitable salt,

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which depends on the target DBPs.

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In previous studies by this research group, SM 5710-B was used to assess OPCs

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of THM4 in studies aimed at developing fluorescence-based precursor surrogates.3-5 One

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nuisance of the DBPFP tests stems from the difficulty in achieving the 3-5 mg L-1 as Cl2

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residual after 7-days. Often, multiple bottles containing the same water are dosed at

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different chlorine concentrations, usually informed by preliminary chlorine demand tests.

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This can be problematic when sample water volumes are limited (i.e., waters from a lab-

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scale treatment process). More importantly, the target disinfectant residual of 3-5 mg L-1

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Cl2 may be problematic for the assessment of dihaloacetonitrile (DHAN) OPCs using

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either free chlorine (FC) or monochloramine (MC). DHANs are a group of non-regulated



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DBPs but have been shown to be more toxic than some regulated DBPs.6 If FC is used,

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the required residual may be too high, which promotes base-catalyzed DHAN

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decomposition7 and may cause erroneous OPC assessment. Further, samples with higher

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chlorine demands have higher average chlorine concentrations8 – calculated as ([initial

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Cl+] + [final Cl+])/2 – and therefore have higher DHAN formation and decomposition.

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This leads to ambiguity regarding DHAN precursor assessment with FC, even if the

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residuals are the same. Alternatively, if MC is used, the required residual may be too low

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to produce measurable DHAN concentrations due to the comparatively low reactivity of

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this oxidant.9-12 Another weakness of the DBPFP test is the different quenching salts

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required, depending on the target group of DBPs. SM 5710-B specifies sodium sulfite to

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quench the chlorine residual prior to THM measurement, whereas SM 5710-D specifies

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ammonium chloride prior to DHAN measurement. The need for multiple quenching salts

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adds to the analytical workload, perhaps unnecessarily.

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The objective of this study was to develop a single disinfectant protocol to

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accurately measure OPCs of THM4 and DHAN. We analyzed DBP formation as a

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function of the 7-day chlor(am)ine residuals (CR) under four disinfectant regimes: free

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chlorine at pH 7.0 (FC7) and pH 6.0 (FC6), and monochloramine at pH 7.0 (MC7) and

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pH 8.3 (MC8). Experiments were performed using natural lake waters, reconstituted

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natural organic matter (NOM) extracts, and wastewater effluents, which were selected

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because they were upstream of the lake water, a situation that is becoming increasingly

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common in drinking water treatment.13 We compared the SM 5710-B/D protocol with the

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Alternative Method (AM), which was formulated to address critical weaknesses in the

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currently accepted methodology. The AM was validated for a common DBP precursor


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removal process – sorption in granular activated carbon (GAC) columns. Results from

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this study could transform assessment of THM4 and DHAN precursors and guide the

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development of similar tests for other priority DBPs, such as N-nitrosamines.

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Materials and Methods

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This section describes the three phases of the experimental design (Phases I, II

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and III, Fig. S1) and the associated experimental procedures.

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Phase I: Assessing DBP formation: Effects of pH, disinfectant type, and CR

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Phase I was designed to assess THM4 and DHAN formation as a function of CR

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for the FC7, MC7, and MC8 regimes. Natural waters for the Phase I DBP tests were

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collected from two sites – one wastewater treatment plant (WWTP) effluent and one

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drinking water source. Effluent samples were collected from the Noland WWTP

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(Fayetteville, AR) on 7/24/13 (WW1) and 9/19/13 (WW2). The drinking water source

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was Beaver Lake,3,

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(Lowell, AR) on 8/28/13 (LW). The sample waters were collected in high-density

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polyethylene (HDPE) carboys, stored at 4°C in the dark, and warmed to room

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temperature before use. The DOC was 11.4- and 12.2 mg L-1 as C for WW1 and WW2,

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respectively, and 3.0 mg L-1 as C for LW. However, the SUVA254 were similar for all

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three waters (1.40-1.54 L mg-1 m-1, Table S2), indicating the fraction of aromatic carbon

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in each water was proportional to the DOC.

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sampled at the intake structure of the Beaver Water District

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An analogous set of DBP experiments were performed with a reconstituted Ohio

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River NOM isolate prepared following a lyophilization method, which was shown

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previously to preserve the NOM without substantially altering the DBP precursor

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concentrations.15 A stock NOM solution was prepared by combining 45.9 g L-1 of freeze-



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dried NOM with Milli-Q water, mixed for 24 hours, and filtered through pre-rinsed 0.45

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µm polyethersulfone (PES) filters. This stock solution was used to prepare three NOM

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extracts in Milli-Q water at dilution factors of 250 (N×1), 62.5 (N×4), and 25 (N×10).

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The natural waters and NOM extracts were used in experiments for the FC7, MC7, and

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MC8 disinfectant regimes. To attempt to curb DHAN destruction by OCl– and OH–, an

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additional regime was assessed (FC6) for the NOM extracts only.

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Phase II: Assessing promising disinfectant protocols

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The goal of the Phase II experiments was to compare the SM 5710-B/D protocol

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with promising disinfectant regimes identified in Phase I for assessing OPCs of THM4

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and DHAN in GAC-treated waters. The packed-bed GAC column setup consisted of a

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50-L source tank, a multi-channel peristaltic pump, and three 1.0-cm inner diameter glass

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columns in series. Filtrasorb 400 (F400, Calgon Carbon) GAC was ground and sieved to

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generate the 40×60 US Mesh fraction, washed with Milli-Q water to remove fines, dried

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to constant mass at 105°C, and loaded wet into the columns. Settled water from the A.B.

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Jewell DWTP (Tulsa, OK), collected between 04/11/14 and 7/17/14, was used as the

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influent source for the GAC columns. Water was supplied at a surface-loading rate of

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10.2 cm min-1 (9.6 mL min-1). A high capacity inline filter (Whatman POLYCAP PES)

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preceded the columns to prevent pressure buildup within the column media. The GAC

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bed was held in place using glass beads (1 mm beads upstream and 0.2-0.3 mm beads

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downstream) to distribute flow evenly across the column cross-section. Two GAC

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column experiments were performed to assess DBP precursor removal: (1) 1.2 g dry

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weight of GAC per column (Empty Bed Contact Time, EBCT = 0.22 min) with samples

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collected on Day 10 of operation (Sample Set A) and (2) 3.0 g dry weight of GAC per


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column (EBCT = 0.56 min) with samples collected on Days 10, 50, and 80 (Sample Sets

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B, C, and D, respectively). Influent (C0) and effluent samples from the three columns in

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series (C1, C2, and C3, respectively) were collected for each Sample Set and assessed for

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OPCs of THM4 and DHAN. An influent-normalized concentration statistical model was

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used to compare SM 5710-B/D and the most promising disinfectant regime identified in

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Phase II (the AM).

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Phase III: Practical Application

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SM 5710-B/D and the AM were compared for assessing sorption of THM4 and

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DHAN precursors with three types of GAC using A.B. Jewell settled water collected

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from 10/8/14 to 12/18/14. The GACs used were: Calgon F400 (GAC A), Nuchar WV-B

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30 (GAC B), and Evoqua AC Series (GAC C). The three GACs were assessed in parallel

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columns (3.0 g of GAC per column) using a similar experimental setup as in Phase II, but

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with a lower flowrate (3.2 mL min-1). Due to differences in GAC densities, the EBCTs

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for GACs A, B, and C were 1.67, 3.90, and 2.11 min, respectively. Tukey’s paired

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comparison method was used to analyze triplicate data produced using the SM and AM

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regimes.

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Experimental Procedures

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All sample waters were filtered with pre-rinsed 0.45 µm PES membranes, stored

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at 4°C in the dark, and warmed to room temperature before use. Prior to beginning each

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DBP formation experiment, each sample water was amended with a 20 mM bicarbonate

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buffer16 and adjusted to a desired pH using HCl or NaOH. Phosphate buffer was not used

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because preliminary experiments showed this buffer caused precipitates to form in WW1,

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likely caused by reactions between phosphate species and metals.



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The FC6 and FC7 experiments were designed to assess the effect of CR and pH

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on THM4 and DHAN formation using SM 5710-B/D,2 with modifications to the buffer

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and quenching salt as detailed later. Filtered source waters were adjusted to the desired

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pH and chlorinated using a standardized NaOCl stock solution. Similarly, the MC7 and

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MC8 experiments were designed to examine the effect of CR and pH on THM4 and

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DHAN formation with preformed monochloramine. After the 7-day hold time,

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chlorinated and chloraminated samples were quenched with ascorbic acid, extracted

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immediately, and analyzed for THM4 and DHAN following EPA Method 551.1.

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Ascorbic acid has been shown by others to be a suitable quenching salt for THMs and

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HANs.17 The remaining chlorinated and chloraminated samples were measured for pH,

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total ammonia, dissolved organic carbon (DOC), total chlorine and monochloramine

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residuals, and fluorescence excitation-emission matrices (EEMs).

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Results and Discussion

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Phase I: DHAN

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Fig. 1a shows DHAN vs. CR for the FC7 regime. There were similar trends

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amongst the source waters with maximum concentrations between 25-31 µg L-1 for WW1

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and WW2 and 4 µg L-1 for LW. All DHAN profiles peaked at a CR between 1.0-1.5 mg

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L-1 as Cl2 and decreased thereafter. This pattern is consistent with previous research that

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showed DCAN decomposition was catalyzed by hypochlorite and hydroxide ions.7,

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Logically, more DHAN formed in the reconstituted NOM extracts at pH 6.0 (Fig. S2a)

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than at pH 7.0 (Fig. 1a). For CRs between 3-5 mg L-1 as Cl2 (the recommended range for

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SM 5710-B/D), the DHAN concentration decreased 43-47% as the CR increased from 3-

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to 5 mg L-1 as Cl2. For example, DHAN concentrations at 7-day residuals of 3- and 5 mg

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L-1 as Cl2, respectively, were estimated by interpolation to be 20.7- and 11.8- for WW1

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(43% change), 2.8- and 1.5- for LW (46% change), and 16.2- and 8.5 µg L-1 for WW2

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(47% change). This change in concentration within the recommended CR range

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illustrates a potential drawback of SM 5710B/D for assessing organic DHAN precursors

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with FC, a contention that is explored further in Phases II and III.

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Figs. 1c and 1e show DHAN vs. CR for the MC7 and MC8 regimes, respectively.

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Higher DHAN formation in the wastewater effluents compared to the lake water indicates

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these precursors were enriched in WW1 and WW2. While DHAN concentrations for

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MC8 were low (Fig. 1e) or below the method detection limits (MDLs, Table S1), MC7

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had higher DHAN concentrations and larger differences between sample waters tested.

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For example, at a CR of 4.5 mg L-1 as Cl2, the DHAN concentrations in WW1 for MC7

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and MC8 (Sample No. 19 and 22, respectively, Table S4 and S6) were 11.7- and

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0.93, Table S9) but were higher for DHAN with the AM (R2
0.85 for the AM). Taken together, these results indicate that

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oxidation effects for the AM and SM were similar.

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Phase III – Fluorescence as a precursor surrogate for DHAN and THM4

However, in this study,

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As any DBPFP protocol is time-consuming and analytically intensive, it was of

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interest to develop spectrophotometric surrogates to monitor DBP precursor removal.

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Influent-normalized DHAN and THM4 concentrations (i.e., Ci/C0) were regressed against

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each influent-normalized fluorescence intensity excitation-emission wavelength pair,

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(IEx/Em)i/(IEx/Em)0, for Phase III samples using SM 5710-B/D and the AM. This regression

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produces correlations between the fraction of DBP surrogates removed in the GAC

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treatment and the fractional decrease in DBPs formed. Using SM 5710-B/D, the linear

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correlations were weak for DHAN (Fig. S7, R2MAX ≈ 0.35) and moderate for THM4


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(R2MAX ≈ 0.73). Using the AM, these correlations were strong for both DHAN (R2MAX ≈

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0.98) and THM4 (R2MAX ≈ 0.84). The correlation coefficients for the AM were the

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highest at I230/480 and a region around I345/425. These IEx/Em pairs have been reported by

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others respectively as Peak A and C31 or Peak α’ and α,32 both humic-like fluorophore

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groups. The strong correlations imply that humic-like NOM moieties were the primary

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organic precursors of DHAN and THM4 and were removed in the GAC columns. These

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results indicate that influent-normalized fluorescence data could be used to reliably track

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organic DBP precursor removal in the GAC columns, which could permit rapid screening

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of DBP-precursor removal processes.

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Implications

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The results presented have important implications that can be used to improve

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assessment and removal of organic precursors of DHAN and THM4 in drinking water

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systems. The weaknesses associated with the SM 5710-B/D protocol were overcome with

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the AM, which utilizes a high dose of monochloramine (i.e., 250 mg L-1 as Cl2), sample

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waters at pH 7.0 buffered with 20 mM bicarbonate, and quenching of residuals with

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ascorbic acid after a 7-day hold time. The differences between the two methods and the

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rationale behind each change are summarized in Table 2. The AM can be applied to

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assess OPCs of DHAN and THM4 in a single protocol, reducing the required analytical

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workload compared to SM 5710-B/D, while facilitating improved assessment of DBP

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precursor removal. Due to differences in DBP formation pathways with FC compared to

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MC (see DHAN discussion of Phase I), water utilities that use the AM but employ free

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chlorine only may inadvertently optimize removal of organic precursors that are relevant

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to chloramination (i.e., aldehyde removal for DHAN control). However, presuming DBP



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precursor removal processes do not target certain groups preferentially (i.e., assuming

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aldehydes and amino groups are removed to similar extents in a given process), we

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conclude that the AM is a valuable tool for free chlorine utilities seeking to improve DBP

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precursor removal. Certainly, the AM is a vast improvement over SM 5710-B/D for

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utilities that use chloramines. As a point of caution, however, care must be used in

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formulating the concentrated monochloramine stock solutions required for dosing in the

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AM. Future work includes assessing the AM protocol for other groups of DBPs, such as

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haloacetic acids and N-nitrosamines, and applying it to develop and optimize DBP

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precursor removal processes.

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ASSOCIATED CONTENT

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Supporting Information

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Chemical reagents and washing procedures; methods for generating preformed

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monochloramine; water quality tests; bromine-substitution calculations; results and

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discussion related to the water quality parameters; DBP formation and speciation;

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fluorescence EEMs; and the statistical models in Phase II and III experiments. This

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information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

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Corresponding Author

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*Tel: (479) 575-4023; Fax: (479) 575-7068; Email: [email protected]

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Notes

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The authors declare no competing financial interest.

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Acknowledgments

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Financial support from the Tulsa Metropolitan Utility Authority, Beaver Water District,

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Arkansas Water Resources Center, and the National Science Foundation (CBET Award

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Number 1254350 to JLF) is gratefully acknowledged. The authors thank Jonathan

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Pressman (USEPA) for providing the Ohio River NOM extract and David G. Wahman

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(USEPA) for his thorough review of this manuscript.

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508

5901-5910.

509

(31) Coble, P. G., Characterization of marine and terrestrial DOM in seawater using

510 511

excitation emission matrix spectroscopy. Mar. Chem. 1996, 51, (4), 325-346. (32)

Parlanti, E.; Worz, K.; Geoffroy, L.; Lamotte, M., Dissolved organic matter

512

fluorescence spectroscopy as a tool to estimate biological activity in a coastal

513

zone submitted to anthropogenic inputs. Org. Geochem. 2000, 31, (12), 1765-

514

1781.

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Table 1. Factor p-values from models of normalized concentration for Standard Methods (SM) 5710-B/D and the Alternative Method (AM) Factor

SM 5710-B/D

AM

Sampling Day

0.4459

0.0001

Disinfectant Dose

N/A

0.1992

Column Number

0.3772

0.0001

N/A – not applicable

518



519 520 521

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Table 2. Differences between Standard Methods 5710-B/D (SM) and the Alternative Method (AM) for assessing organic precursor concentrations of trihalomethanes (THM4) and dihaloacetonitriles (DHAN) Specification SM

AM

Rationale

Buffer

10 mM phosphate

20 mM bicarbonate

Avoid precipitation of solids in waters containing reduced metals

Disinfectant

Free chlorine

Monochloramine Avoid decomposition of DHAN

Residual vs. Dosing

Target 7-day residual between 3-5 mg L-1 as Cl2

Applied NH2Cl dose at a discrete value between 200300 mg L-1 as Cl2)

Quenching Agent

Na2SO3 for THMs and NH4Cl for DHAN

Dissimilar chlorine demands amongst samples produce differing amounts of base-catalyzed destruction of DHAN NH2Cl dose must be large enough to exceed method detection limits for DBPs, but not too high such as to produce monochloramine instability (by dropping the pH) and/or oxidation effects larger than those observed with the SM.

Ascorbic acid for Decreases the analytical workload THMs and and cuts the number of samples for DHAN the gas chromatograph-electron capture detector (GC-ECD) in half

522 523


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524 525 526 527 528 529 530 531 532 533 534

Fig. 1 – Dihaloacetonitrile (DHAN) and trihalomethane (THM4) concentrations in µg L-1 as a function of disinfectant residual following a 7-day free chlorination period at pH 7.0 (Panels a and b) and a 7-day chloramination period at pH 7.0 (Panels c and d) and pH 8.3 (Panels e and f). Sample waters included: Noland wastewater effluent collected on 7/24/13 (WW1) and 9/19/13 (WW2), Beaver Lake raw water collected on 8/28/13 (LW), and reconstituted Ohio River natural organic matter (NOM) extracts adjusted for dilution factors of 250 (N×1), 62.5 (N×4), and 25 (N×10). Grey-shaded regions are between a residual of 3-5 mg L-1 as Cl2. DHAN is the sum of dichloroacetonitrile, dibromoacetonitrile, and bromochloroacetonitrile; THM4 is the sum of trichloromethane, dichlorobromomethane, dibromochloromethane, and tribromomethane.



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535 536 537 538 539 540 541 542 543 544 545 546

Fig. 2 – Influent-normalized effluent concentrations from the three granular activated carbon (GAC) columns in series sampled on Days 10, 50, and 80 assessed using Standard Methods (SM) 5710-B/D (Panels a and b) and the Alternative Method (AM, Panels c and d). The values presented were calculated by dividing the molar disinfection byproduct (DBP) concentrations in the GAC column effluents for column 1 (C1), column 2 (C2), and column 3 (C3) by their corresponding influent concentration (C0). The empty bed contact time (EBCT) for each column was 0.56 min. DHAN is the sum of dichloroacetonitrile, dibromoacetonitrile, and bromochloroacetonitrile; THM4 is the sum of trichloromethane, dichlorobromomethane, dibromochloromethane, and tribromomethane.


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547

548 549 550 551 552 553 554 555 556 557 558

Fig. 3 – Dihaloacetonitrile (DHAN) breakthrough profiles in Phase III for three types of granular activated carbon (GAC) in parallel columns. DHAN precursors were measured using Standard Methods (SM) 5710-B/D (upper panel) and the Alternative Method (AM, lower panel). The values were calculated by dividing the molar DHAN concentrations in the GAC column effluents by their corresponding influent concentration for each Day. The empty bed contact time (EBCT) for GACs A, B, and C were 1.67, 3.90, and 2.11 min, respectively. DHAN is the sum of dichloroacetonitrile, dibromoacetonitrile, and bromochloroacetonitrile. The red, grey, and blue shaded areas are the 95% confidence intervals of GACs A, B, and C, respectively.

559



ACS Paragon Plus Environment

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Improved (and Singular) Disinfectant Protocol for Indirectly Assessing

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