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Monochloramine Cometabolism by MixedCulture Nitrifiers under Drinking Water Conditions Juan Pedro Maestre, David G. Wahman, and Gerald E. Speitel Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b05641 • Publication Date (Web): 19 May 2016 Downloaded from http://pubs.acs.org on June 4, 2016

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Environmental Science & Technology

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Monochloramine Cometabolism by Mixed-Culture Nitrifiers under Drinking Water

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Conditions

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Authors: Juan P. Maestre†, David G. Wahman‡, and Gerald E. Speitel Jr. †*

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ABSTRACT: Chloramines are the second most used secondary disinfectant by United States

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water utilities. However, chloramination may promote nitrifying bacteria. Recently,

†

University of Texas at Austin, Department of Civil, Architectural and Environmental Engineering, Austin, TX 78712 ‡ United States Environmental Protection Agency, Office of Research and Development, Cincinnati, OH 45268

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monochloramine cometabolism by the pure culture ammonia-oxidizing bacteria, Nitrosomonas

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europaea, was shown to increase monochloramine demand. The current research investigated

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monochloramine cometabolism by nitrifying mixed cultures grown under more relevant drinking

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water conditions and harvested from sand-packed reactors before conducting suspended growth

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batch kinetic experiments. Four types of batch kinetic experiments were conducted: (1) positive

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controls to estimate ammonia kinetic parameters, (2) negative controls to account for biomass

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reactivity, (3) utilization associated product (UAP) controls to account for UAP reactivity, and

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(4) cometabolism experiments to estimate cometabolism kinetic parameters. Kinetic parameters

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were estimated in AQUASIM with a simultaneous fit to the experimental data. Cometabolism

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kinetics were best described by a first-order model. Monochloramine cometabolism kinetics

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were similar to those of ammonia metabolism, and monochloramine cometabolism accounted for

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30% of the observed monochloramine loss. These results demonstrated that monochloramine

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cometabolism occurred in mixed cultures similar to those found in drinking water distribution

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systems; therefore, monochloramine cometabolism may be a significant contribution to

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monochloramine loss during nitrification episodes in drinking water distribution systems.

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Keywords: Monochloramine, nitrification, cometabolism, ammonia-oxidizing bacteria, drinking

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water, AQUASIM 1 ACS Paragon Plus Environment


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INTRODUCTION

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Chloramines remain a popular choice for a secondary disinfectant by United States (US) water

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utilities. For example, a 2008 water utility survey found that 30% were using chloramines to

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maintain distribution system residual.1 Moreover, US chloramine use is predicted to increase

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due to implementation of the Stage 2 Disinfectant and Disinfection Byproducts Rule.2

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Chloramines may promote nitrifying bacteria.3 Specifically, ammonia-oxidizing bacteria (AOB)

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and possibly the recently discovered comammox bacteria.4-6 Indeed, utility surveys report 30–

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63% of chloraminating systems suffer nitrification episodes.7 Source water ammonia, residual

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ammonia from initial chloramine formation, and chloramine released ammonia provide AOB’s

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growth substrate (i.e., ammonia).3, 8 Additional factors impacting nitrification include chlorine to

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ammonia–nitrogen (Cl2:N) mass ratio, temperature, and water age.3 Overall, nitrification control

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is a growing issue as more water utilities convert to chloramination.

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Using the non-specific enzyme ammonia monooxygenase (AMO), AOB cometabolize

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many chemicals. As Hooper, et al.9 summarized, N. europaea’s AMO enzyme cometabolizes

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over 35 halogenated chemicals. In more recent studies, N. europaea, as well as nitrifier mixed

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cultures present in natural waters, treatment plants, and distribution systems, have been observed

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to cometabolize four regulated trihalomethanes (THMs) at rates relevant in drinking water

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treatment applications.10, 11 More recently, Maestre, et al.12 observed monochloramine

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cometabolism by N. europaea, likely based on monochloramine’s structural similarity to

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ammonia and other chemicals that AOB can cometabolize. Woolschlager13 hypothesized that

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AMO converts monochloramine to chlorohydroxylamine. Subsequently and in an analogous

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fashion to ammonia metabolism, chlorohydroxylamine would be converted to nitrite and chloride

2 ACS Paragon Plus Environment

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by hydroxylamine oxidoreductase (HAO), or because chlorohydroxylamine is unstable in water,

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chlorohydroxylamine may abiotically decay.14

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Monochloramine decay (autodecomposition)/demand and nitrification in chloraminated

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drinking water systems has received considerable attention, but monochloramine cometabolism’s

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significance in monochloramine demand has only been quantified recently with the AOB pure

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culture, N. europaea.12 Maestre, et al.12 demonstrated monochloramine cometabolism’s possible

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importance during nitrification and provided an approach for including the relevant reactions in

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water quality models.

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Although an important first step, further study with microbial assemblages similar to

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those found in drinking water distribution systems was needed. Therefore, we conducted

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experiments with nitrifying mixed cultures grown under more relevant drinking water conditions

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to (i) examine monochloramine cometabolism, (ii) characterize important reactions and

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associated kinetics, and (iii) quantify monochloramine cometabolism’s significance to

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chloramine loss. To our knowledge, the current research is the first to demonstrate that

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monochloramine cometabolism may occur in nitrifying mixed cultures and monochloramine

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cometabolism may be a significant contribution to monochloramine loss during nitrification

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episodes in drinking water distribution systems. Furthermore, the current work established

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kinetic parameters required for efforts with mixed culture biofilm grown under drinking water

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representative conditions.15

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MATERIALS AND METHODS

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Nitrifier mixed culture grow conditions. An attached-growth approach was

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implemented for obtaining and growing a nitrifier mixed culture. The experimental setup

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consisted of two parallel 0.25 m long, 0.05 m diameter glass columns packed with sand (2 mm



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diameter). Columns were inoculated by recirculating water obtained from Lake Austin (Austin,

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Texas) for 12 hours and subsequently fed City of Austin, Texas tap water. Even though the

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experimental conditions were selected to mimic water quality found in chloraminated

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distribution systems, some modifications were introduced to facilitate nitrifying biofilm

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establishment at quantities required for experiments. Influent monochloramine concentrations

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were kept below 0.1 mg Cl2/L by installing columns filled with Centaur® granular activated

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carbon.16 Also, tap water pH (approximately pH 9.6) was adjusted down to pH 8 by sulfuric acid

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addition. To avoid oxygen limitation, dissolved oxygen (DO) was increased up to 16 mg O2/L

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using pure oxygen. Moreover, an organic carbon cocktail (0.1 mg C/L) composed of acetate,

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pyruvic acid, propionaldehyde, ethyl alcohol, and oxalate17 was added to the influent to facilitate

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the formation of a thin heterotrophic biofilm on which the nitrifying bacteria could more easily

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establish.17, 18 The reactors were covered in aluminum foil to prevent inactivation by light.9

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Between 2 and 4 mg TOTNH3/L (0.26 L/min flow rate) was fed to each column. TOTNH3 is

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total free ammonia, which is the sum of ammonia (NH3–N) and ammonium (NH4+–N). The

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TOTNH3 concentrations were selected as a balance between (1) growing sufficient biomass to

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conduct the various suspended growth experiments while preventing oxygen limitations and (2)

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keeping the TOTNH3 on the order of what may be added in actual chloraminated drinking water

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distribution systems. Nitrification was observed (70-90% conversion of ammonia, according to

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ammonia, nitrite, and nitrate measurements) and maintained for a month before conducting batch

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experiments. The variability in the observed nitrification efficiency resulted from backwashing

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of the columns to harvest bacteria and fluctuations in influent TOTNH3.

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Bacteria harvesting for batch kinetic experiments. To conduct suspended batch

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kinetic experiments with the attached-growth mixed culture, filter backwash water was collected.


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Backwashing consisted of buffer addition and reactor inversion (10 times). Organisms were

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subsequently washed and centrifuged three times before being resuspended in fresh buffer (4

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mM sodium bicarbonate, pH 8.3). The buffer was aerated to increase the DO concentration to

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levels that would not be fully consumed by ammonia degradation during the experiment (e.g., 7

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to 9 mg DO/L).

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Biomass concentrations were measured on an Agilent 8453 UV-visible spectroscopy

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system at 600 nm wavelength (OD600). Total suspended solids (TSS) were measured using

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Whatman cellulose nitrate 0.2-µm filter, according to Standard Methods.19 A standard curve

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(TSS vs. OD600) was developed for biomass concentrations ranging from 40 to 240 mg TSS/L.

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Batch kinetic experiment types. Four batch kinetic experiment types (Table 1) were

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conducted to determine the kinetic parameters discussed in greater detail in Modeling approach

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and determination of kinetic parameters (see schematic provided in Supporting Information

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[SI], Figure S1):

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1. Five positive control experiments contained biomass and ammonia and evaluated

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ammonia biodegradation in the absence of monochloramine (Table 2, Ammonia

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Monod).

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2. Six negative control experiments contained inactive biomass, monochloramine,

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and ammonia and evaluated the direct reaction of monochloramine and biomass

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(Table 2, Biomass Reactivity). Prior to beginning negative control experiments,

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chlorobenzene was added to the solution (50 mg/L) to inactive the biomass and

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halt ammonia metabolism.20 Chlorobenzene was selected based on its ability to

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inhibit ammonia oxidation without reacting with monochloramine.



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3. Five utilization associated product (UAP) control experiments contained inactive

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biomass, monochloramine, and ammonia. During ammonia metabolism, nitrifiers

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generate soluble metabolic products (SMPs) that may react with monochloramine.

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Rittmann and co-workers21-23 developed a framework for SMP formation that

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considers two SMP classes: (i) biomass associated products (BAPs) and (ii)

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UAPs. Monochloramine likely reacts with both BAPs and UAPs. The reactivity

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with BAP was evaluated through negative control experiments. Therefore, UAP

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control experiments were designed and carried out to account for the UAP

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reaction with monochloramine. Essentially, a positive control was run to generate

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UAPs. Once the ammonia was depleted, chlorobenzene was added to inactivate

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the biomass as in the negative control experiments. Subsequently,

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monochloramine and ammonia were added to the solution to conduct the UAP

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control experiment to evaluate UAP reactivity with monochloramine (Table 2,

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UAP Reactivity).

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4. Seven cometabolism experiments contained biomass, monochloramine, and

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ammonia and evaluated the biomass’ ability to biologically remove

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monochloramine (Table 2, First-Order Cometabolism) and monochloramine

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biomass inactivation (Table 2, Biomass Inactivation). The seven cometabolism

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experiments were also used to assess the significance of enzyme competition.

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For five experiments (A through E), all four experiment types were conducted. For two

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experiments (F and G), only a subset of the four experiment types was conducted as these

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experiments were designed to study the requirement for ammonia to be present to provide

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reducing power for monochloramine cometabolism. For Experiment F, only a negative control


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and cometabolism experiment were conducted as Experiment E served as its positive and UAP

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control experiments. For Experiment G, only a cometabolism experiment was conducted as

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Experiments E served as the positive and UAP controls while Experiment F served as the

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negative control.

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Batch kinetic experiment methodology. Batch kinetic experiments were carried out in

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500-mL Erlenmeyer glass flasks (250-mL experiment volume) wrapped in aluminum foil to

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prevent light inactivation and containing a small Teflon-coated stir bar so that the contents were

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well mixed. To being a batch kinetic experiment, the monochloramine and/or ammonia were

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added. Samples for measuring chemical and biomass concentrations, pH, and DO were then

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temporally collected. The batch kinetic experiments were run rapidly (20–70 min) to avoid

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significant changes in the biomass’ metabolic state during experiments.20 Replicate analyses of

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chemical concentrations were not performed because of limitations imposed by time and sample

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volume. Batch kinetic experiments were performed at room temperature (22.5±0.5°C). Initial

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monochloramine concentrations between 0.6 and 1.4 mg Cl2/L were used (Table 1). Initial

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monochloramine concentrations were selected to balance using too great of a monochloramine

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concentration so as to completely inactivate the biomass with providing a high enough

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monochloramine concentration to allow collection of enough data points for kinetic parameter

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estimation. Biomass concentrations (64–72 mg TSS/L, Table 1) were selected to allow for

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reasonable experimental durations, and pH was set to typical mid-range pHs (8.2–8.4, Table 1)

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found in chloraminated systems. Even though the TSS was greater than would be expected in

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actual drinking water distribution systems, the originating biomass was grown under relatively

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low TOTNH3 concentrations and provided a reasonable extension from previous pure culture



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experiments grown under significantly larger concentrations (i.e., > 100 mg TOTNH3/L) in

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batch.12

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Analytical methods. An ion selective electrode, Thermo Orion 9512, connected to an

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Orion Model 920A pH/ISE electrode meter, was used to measure ammonia. An ammonia

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standard curve was developed to span the anticipated range of ammonia concentrations in the

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experiments. DO was measured with a YSI 5905 oxygen probe on a YSI Model 54ARC oxygen

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meter calibrated per the manufacturer’s recommendation. pH was measured using an Orion

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ROSS™ combination pH electrode on an Orion model 920A pH/ISE meter calibrated with pH

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standards of 4, 7, and 10.

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Monochloramine was prepared as described by Wahman, et al.24 Monochloramine,

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nitrite, and nitrate were measured by HACH colorimetric methods 10171, 5807, and 10023,

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respectively.

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Modeling approach and determination of kinetic parameters. A similar approach to

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that applied in our previous pure culture studies12 was followed. The widely-accepted

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chloramine formation and decay model25, 26, as well as the reactions between nitrite and

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monochloramine and hypochlorous acid 27, formed the starting basis for describing abiotic

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monochloramine chemistry. These kinetic and equilibrium expressions were implemented into

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the computer program AQUASIM.28 Rate expressions for the biotic portion of the model (Table

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2) were then added to consider ammonia metabolism (Table 2, Ammonia Monod),

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monochloramine cometabolism (Table 2, First-Order Cometabolism), monochloramine biomass

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inactivation (Table 2, Biomass Inactivation), and monochloramine reactions with biomass (Table

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2, Biomass Reactivity) and UAPs (Table 2, UAP Reactivity).


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Initial data analysis of the negative control experiments indicated that biomass was a

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limiting reactant (i.e., biomass could not be assumed in great excess and constant with regards to

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its reaction with monochloramine). In contrast, biomass was not a limiting reactant in the pure

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culture negative control studies.12 The method developed in Wahman, et al.29 was implemented

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to account for the decline in biomass reactivity over time. As in the pure-culture experiments,

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the initial value of active biomass (Xa) was set equal to the measured initial biomass

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concentration (X). Unlike the pure culture experiments, Xa likely included some small portion of

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nitrite-oxidizing bacteria (NOB) and heterotrophic bacteria; therefore, the value of Xa overstated

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the AOB concentration to some extent. The impact of overestimating Xa is that some of the rate

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constants may be underestimated.

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Kinetic parameters were estimated using the secant routine in AQUASIM which was

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configured to minimize the weighted residual sum of squares (WRSS) between measurements

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and calculated model results in a similar manner previously described for N. europaea (Eq. 1).12

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y , − y y , − y WRSS = = (1) W y , In Eq. 1, ymeas,i is the i–th chemical measurement, W is the weighting factor, and yi is the

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model calculated chemical concentration corresponding to the i–th chemical measurement. To

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prevent greater concentrations from biasing the fitting procedure, ymeas,i was implemented for W,

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resulting in a dimensionless WRSS.30, 31

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Using the experimental data (TOTNH3 and monochloramine concentrations), rate

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constants and initial concentrations were estimated in three steps. First, kinetic parameter and

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initial TOTNH3 and monochloramine concentration estimates were generated by analyzing 9 ACS Paragon Plus Environment


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negative and positive control experiments separately: (i) positive control experiments were used

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to estimate the ammonia kinetic coefficients (kTOTNH3 & KSNH3–N) and initial TOTNH3

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concentrations and (ii) negative control experiments estimated the monochloramine reaction rate

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constant with biomass (kBiomass), reaction yield (YXr), and initial monochloramine concentrations.

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Second, the UAP control experiments and the kinetic parameters determined from the negative

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control experiments estimated the UAP formation coefficient (fUAP), the monochloramine

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reaction rate constant with UAPs (kUAP), and initial TOTNH3 and monochloramine

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concentrations. Third, the cometabolism experiments and kinetic parameters determined from

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the positive, negative, and UAP control experiments were used to estimate the monochloramine

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cometabolism rate constant (k1NH2Cl), monochloramine biomass inactivation rate constant (kInact),

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and initial TOTNH3 and monochloramine concentrations. The initial values for active biomass

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(Xa) and reactive biomass (Xr) were set equal to the measured initial biomass (X) shown in Table

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1 except for the negative and UAP control experiments where Xa was set to 0 mg TSS/L as the

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biomass had been inactivated by chlorobenzene treatment.

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RESULTS AND DISCUSSION

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Ammonia kinetics. The ammonia Monod kinetic model (Table 2, Ammonia Monod)

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was simultaneously fit to five positive control experiments (Experiments A through E). Unique

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values of the ammonia half saturation coefficient (KsNH

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utilization rate (kTOTNH3) were determined (Table 3). Model fits are shown in Figure 1A for the

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mixed culture positive control experiments. The estimated KsNH

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0.083±0.038 mg NH3-N/L and 1.7±0.55 mg TOTNH3/mg TSS-day, respectively. The fit to the

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experimental data indicates the high reproducibility of the ammonia metabolism by the mixed

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culture from experiment to experiment, even though culture population abundances and diversity

) and ammonia maximum specific

3-N

3-N


and kTOTNH3 were

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might have temporally changed. Compared to the AOB pure culture N. europaea, the value of

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KsNH

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comparable to those measured with other nitrifying mixed cultures originating from Lake Austin

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and grown in suspension (0.08 to 0.19 mg NH3-N/L)32 or a packed bed (0.029 to 0.21 mg NH3-

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N/L)33 which was backwashed to provide biomass for batch kinetic experiments as in the current

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research.

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3-N

was slightly lower (0.083 vs. 0.13 mg NH3-N/L). The KsNH

3-N

value was also

Monochloramine reaction with biomass and utilization associated products. During

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metabolism and mentioned previously, bacterial populations in the mixed culture, including

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nitrifiers among others, can generate two SMP types that may react with monochloramine: (1)

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BAPs, reaction accounted for in negative control experiments; and (2) UAPs, reaction evaluated

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in UAP control experiments. Because monochloramine reacts with reactive biomass (Xr; Table

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2, Biomass Reactivity) and UAPs (Table 2, UAP Reactivity), these reactions must be considered

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to prevent overestimating monochloramine cometabolism.

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To first account for the biomass reactivity with monochloramine, six negative control

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experiments (Experiments A through F) were used to estimate the monochloramine reaction with

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biomass rate constant (kBiomass) and associated reaction yield (YXr) in the biomass reactivity

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kinetic expression (Table 2, Biomass Reactivity). Figure 1B shows model simulations for the

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negative control experiments, providing a suitable description of observed monochloramine loss

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in the presence of inactivated biomass. For the negative controls, no TOTNH3 consumption was

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observed. Rather, a slight increase in TOTNH3 was observed which would be expected as

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monochloramine reacts with cellular material, releasing TOTNH3. The estimated kBiomass was

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1.5±0.15 L/mg TSS-day, and YXr was 57±5.1 mg TSS/mg Cl2 (Table 3). Wahman, et al.29

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estimated these two rate constants for N. europaea. Their estimate for YXr (95% confidence



252

limit, 59±11 mg TSS/mg Cl2) was not statistically different than for the current mixed culture

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(95% confidence limit, 57±10 mg TSS/mg Cl2), but their estimate of kBiomass (95% confidence

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limit, 0.66±0.17 L/ mg TSS-day) was significantly smaller than the value for the current mixed

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culture (95% confidence limit, 1.5±0.30 L/ mg TSS-day). The significantly different kBiomass

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values between the mixed and pure cultures (1.5 vs. 0.66 L/mg TSS-day) suggests a

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fundamentally greater reactivity with monochloramine for the mixed culture, which might be

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inherent to the mixed culture (e.g., presence of heterotrophs) or a result of biofilm growth.

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In UAP control experiments, biomass was inactivated with chlorobenzene after

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metabolizing different amounts of ammonia: 1.4 (Experiment A), 1.0 (Experiment B), 0.64

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(Experiment C), 0.75 (Experiment D), or 0.48 (Experiment E) mg TOTNH3/L. Therefore, if

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UAP was produced and reacted with monochloramine, the monochloramine concentration

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should decrease faster than in a corresponding negative control experiment without UAPs. The

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UAP formation fraction from TOTNH3 metabolism (fUAP) and the rate constant for

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monochloramine reaction with UAPs (kUAP) were estimated by fitting the model simultaneously

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to five UAP control experiments (Experiments A through E; Figure 1C). Accelerated

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monochloramine loss in UAP control experiments was observed compared to negative controls

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(Figure 1D, Experiment C for example), suggesting UAP formation and subsequent reaction with

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monochloramine. Thus, UAP formation and reactivity with monochloramine was included in

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data analysis and modeling efforts. Data analysis included the UAP reaction to avoid

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overestimating the monochloramine cometabolism rate constant. Compared to estimated values

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for N. europaea, the mixed culture fUAP was approximately double (Table 3), indicating that the

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mixed culture produced substantially more UAP per unit mass of ammonia degraded. In a mixed

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culture, other bacteria (e.g., NOB and heterotrophs) would be producing UAP along with any


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AOB present; therefore, this may explain the higher fUAP value. For kUAP, the mixed culture

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value was slightly greater than that determined from N. europaea (approximately 25%),

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indicating that UAPs produced in the mixed culture may have reacted faster than those produced

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by N. europaea.

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Cometabolism Kinetic Model Selection. Following the approach of Maestre, et al.12

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with the AOB pure culture N. europaea, two questions were considered: the need to account for

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(1) competition between ammonia and monochloramine for AMO’s active site and (2) reducing

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power to drive cometabolism. Two lines of evidence support that neither competition nor

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reducing power needed to be accounted for in the current experiments.

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First, the seven cometabolism experiments (Experiments A through G) were used to

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evaluate four first-order kinetic models for monochloramine cometabolism: (1) first-order

286

model, (2) first-order competition model, (3) first-order reductant model, and (4) first-order

287

competition and reductant model. The four evaluated models are summarized in SI, Table S1

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along with the weighted residual sum of squares (WRSS) resulting from each model analysis.

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Because the analysis of the various models resulted in the estimation of the same, single kinetic

290

parameter (k1NH2Cl), the WRSS for each model was directly compared to assess the appropriate

291

model. The simplest model (i.e., first-order model) resulted in the lowest WRRS, supporting its

292

selection.

293

Second, regarding the need for reducing power, previous studies10, 12 demonstrated that

294

cometabolism did not occur in ammonia’s absence, presumably because of a shortage of

295

reducing power for AMO. To further evaluate the appropriate rate expression, first-order

296

cometabolism or first-order-reductant cometabolism, two cometabolism experiments (Table 1,

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Experiments F and G) were run at a 5:1 Cl2:N mass ratio, which is near the stoichiometric mass



298

ratio of chlorine to ammonia (5.07) for monochloramine formation. Consequently, the ammonia

299

concentration was very low (Figure 2). Monochloramine loss in the cometabolism experiments

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was higher than in the negative control, indicating that reducing power derived from the presence

301

of ammonia was not needed to drive monochloramine cometabolism. The first-order

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cometabolism expression (Table 2, First-Order Cometabolism) provided a good description of

303

the experimental data; and consequently, further supported its selection.

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Monochloramine cometabolism. The mixed-culture cometabolism experiments were

305

characterized by more rapid monochloramine loss relative to the negative controls (Figure 3).

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As was observed in pure culture experiments, the ammonia concentration initially decreased

307

rapidly and then slowed, indicating monochloramine inactivation of biomass. Therefore, as with

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N. europaea,12 a biomass inactivation term (kInact) was used in the modeling to account for a

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reduction in active biomass (Xa). Figure 3 shows representative experiments for trends seen at

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relatively high (Experiment B) and low (Experiment E) monochloramine concentrations. For

311

experiments conducted at elevated initial monochloramine concentrations (e.g., Experiments A,

312

B, and C), ammonia oxidation shut down faster than the model predicted (Figure 3A). At lower

313

initial monochloramine concentrations (e.g., Experiments D through G), the model performed

314

well in describing both the monochloramine and ammonia concentration profiles (Figure 3B).

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Experimental results (symbols) and model simulations (lines) are shown in Figure 4 for

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all seven cometabolism experiments; the model satisfactorily explained the experimental results

317

over the entire test duration in the majority of experiments. However, as discussed above, the

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model tended to describe ammonia concentrations better in those experiments at lower initial

319

monochloramine concentrations (Experiments D, E, F, and G, Figure 4). These results suggest a

320

greater complexity in the interaction between monochloramine and ammonia metabolism than


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was captured in the kinetic expressions used in this research. For example, the current model

322

does not account for the direct reaction of AOB produced hydroxylamine (the intermediate

323

chemical as AOB oxidize ammonia to nitrite) with monochloramine,29, 34 which may inhibit

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ammonia oxidation as monochloramine concentration increases.

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Summary of estimated kinetic parameters. A summary of kinetic parameter estimates

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is presented in Table 3. k1NH2Cl was on the same order of magnitude but larger than those

327

measured for two THMs: chloroform (k1TCM) and bromoform (k1TBM)32 but less than that

328

estimated for the AOB pure culture N. europaea.12 As these were mixed cultures, it is

329

reasonable to assume that a portion of the biomass was not AOB; therefore, the lower estimate of

330

k1NH2Cl for the mixed cultures compared to the pure culture may be partly explained by an

331

overestimation of biomass responsible for cometabolism (Xa) in these experiments. Overall, the

332

monochloramine cometabolism rate constant was substantially larger than the corresponding rate

333

constants for THM cometabolism, paralleling the observation with N. europaea.12 Also, k1NH2Cl

334

can be compared to k1TOTNH3. At the nominal pH of these experiments (pH 8.3), k1NH2Cl was

335

40% of k1TOTNH3 (0.80±0.17 vs. 2.0±1.1 L/mg TSS-day), indicating that the mixed culture readily

336

catalyzed monochloramine oxidation. Because of the simplifying assumption that the entire

337

biomass is AOB, estimated values for kTOTNH3 (positive control experiments) and kInact and

338

k1NH2Cl (cometabolism experiments) should be considered lower-bound estimates as they would

339

likely increase if AOB, NOB, and heterotrophic biomass were accounted for separately in the

340

biomass.

341

Monochloramine cometabolism significance. A summary of model simulated relative

342

contributions of each monochloramine loss pathway for each experiment is shown in Figure 5.

343

The relative contributions were determined by tracking each possible monochloramine loss 15 ACS Paragon Plus Environment


344

pathway in AQUASIM. Under the experimental conditions, autodecomposition and nitrite

345

reaction contributions were negligible. Reactions with UAP generally accounted for

346

approximately 10% of the monochloramine loss, except for experiments where very little

347

ammonia metabolism was observed and no UAP formation predicted (Figure 5, Experiments F

348

and G). Across all experiments, monochloramine cometabolism and biomass reactions were

349

more significant, accounting for approximately 30 and 60% of the monochloramine loss,

350

respectively.

351

al.12 for the AOB pure culture N. europaea: monochloramine cometabolism may represent a

352

relevant mechanism for monochloramine loss during nitrification episodes in water distribution

353

systems.

Therefore, the mixed culture experiments confirm the observations of Maestre, et

354

The first-order model was sufficient to describe monochloramine cometabolism with the

355

mixed culture; whereas, the more complex reductant model was needed with N. europaea. One

356

possibility for this discrepancy is the lower ammonia half saturation coefficient for the mixed

357

culture. At very low ammonia half saturation coefficient values, the reductant model simplifies

358

to the first order model. In general, monochloramine reactivity with the biomass accounted for a

359

larger percentage of the monochloramine loss in the mixed culture experiments and perhaps this

360

reactivity masked in some way the need for reductant (e.g., ammonia release from the biomass

361

reactions). Although ammonia may or may not be needed to observe monochloramine

362

cometabolism in short-term kinetic experiments, ammonia’s absence over extended periods

363

would most certainly adversely affect AOB activity and growth, thereby decreasing or

364

eliminating monochloramine cometabolism.


Page 16 of 29

Page 17 of 29


365

The current research evaluated monochloramine cometabolism at a relatively fixed pH

366

(pH 8.2 to 8.4) and in suspended kinetic experiments, future research should evaluate the extent

367

of monochloramine cometabolism over a wider pH range and with biofilm.

368

ASSOCIATED CONTENT

369

Supporting information. Table S1 provides a summary of the cometabolism kinetic

370

models evaluated, and Figure S1 shows a schematic of bacteria growth and batch kinetic

371

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

372

AUTHOR INFORMATION

373

Corresponding Author

374

* Phone: (512) 471-4996. Fax: (512) 475-6743. E-mail: [email protected].

375

* Corresponding author, mailing address: 301 E. Dean Keeton St. Stop C2100, Austin, TX

376

78712-0284.

377

Notes

378

The authors declare no competing financial interest.

379

ACKNOWLEDGMENT

380

This research was funded by the Water Research Foundation, Denver, CO, USA. The

381

USEPA collaborated in the research described herein. It has been subjected to the Agency’s peer

382

and administrative review and has been approved for external publication. Any opinions

383

expressed are those of the authors and do not necessarily reflect the views of the Agency;

384

therefore, no official endorsement should be inferred. Any mention of trade names or

385

commercial products does not constitute endorsement or recommendation for use.



386

REFERENCES

387 388 389

(1)

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(2)

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American Water Works Association, Nitrification Prevention and Control in Drinking Water. In 2nd ed.; M56, Ed. American Water Works Association: Denver, 2013.

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(4)

Daims, H.; Lebedeva, E. V.; Pjevac, P.; Han, P.; Herbold, C.; Albertsen, M.; Jehmlich, N.; Palatinszky, M.; Vierheilig, J.; Bulaev, A.; Kirkegaard, R. H.; von Bergen, M.; Rattei, T.; Bendinger, B.; Nielsen, P. H.; Wagner, M., Complete nitrification by Nitrospira bacteria. Nature 2015, 528, (7583), 504-509.

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van Kessel, M.; Speth, D. R.; Albertsen, M.; Nielsen, P. H.; Op den Camp, H. J. M.; Kartal, B.; Jetten, M. S. M.; Lucker, S., Complete nitrification by a single microorganism. Nature 2015, 528, (7583), 555-559.

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Pinto, A. J.; Marcus, D. N.; Ijaz, U. Z.; Bautista-de lose Santos, Q. M.; Dick, G. J.; Raskin, L., Metagenomic Evidence for the Presence of Comammox Nitrospira-Like Bacteria in a Drinking Water System. mSphere 2016, 1, (1).

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Seidel, C. J.; McGuire, M. J.; Summers, R. S.; Via, S., Have utilities switched to chloramines? J. Am. Water Works Ass. 2005, 97, (10), 87-97.

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(8)

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Hooper, A. B.; Vannelli, T.; Bergmann, D. J.; Arciero, D. M., Enzymology of the Oxidation of Ammonia to Nitrite by Bacteria. Antonie Van Leeuwenhoek International Journal of General and Molecular Microbiology 1997, 71, (1-2), 59-67.

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Wahman, D. G.; Katz, L. E.; Speitel, G. E., Jr, Cometabolism of trihalomethanes by Nitrosomonas europaea. Appl. Environ. Microbiol. 2005, 71, (12), 7980-7986.

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Wahman, D. G.; Katz, L. E.; Speitel, G. E., Jr, Trihalomethane cometabolism by a mixed-culture nitrifying biofilter. J. Am. Water Works Ass. 2006, 98, (12), 2-20.


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Maestre, J. P.; Wahman, D. G.; Speitel, G. E., Jr, Monochloramine cometabolism by Nitrosomonas europaea under drinking water conditions. Water Res. 2013, 47, (13), 4701-4709.

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Woolschlager, J. E. A Comprehensive Disinfection and Water Quality Model for Drinking Water Distribution Systems. Dissertation, Northwestern University, Evanston, 2000.

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Woolschlager, J.; Rittmann, B.; Piriou, P.; Kiene, L.; Schwartz, B., Using a Comprehensive Model to Identify the Major Mechanisms of Chloramine Decay in Distribution Systems. Water Science and Technology: Water Supply 2001, 1, (4), 103110.

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Wahman, D. G.; Maestre, J. P.; Speitel Jr, G. E., Monochloramine cometabolism by drinking water relevant nitrifying biofilm. J. Am. Water Works Ass. In Press.

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Fairey, J. L.; Speitel, G. E., Jr; Katz, L. E., Monochloramine destruction by GAC: effect of activated carbon type and source water characteristics. J. Am. Water Works Ass. 2007, 99, (7), 110-120.

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Gagnon, G. A.; O'Leary, K. C.; Volk, C. J.; Chauret, C.; Stover, L.; Andrews, R. C., Comparative analysis of chlorine dioxide, free chlorine and chloramines on bacterial water quality in model distribution systems. Journal of Environmental Engineering-Asce 2004, 130, (11), 1269-1279.

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1 2 3 4


Table 1. Summary of experimental conditions for mixed culture batch kinetic experiments. Unless otherwise noted, each experiment set consisted of four types of batch kinetic experiments: (1) positive control, (2) negative control, (3) utilization associated product control, and (4) cometabolism experiment. Exp.

Measured Initial Total Free Ammonia (mg N/L)

Measured Initial Monochloramine (mg Cl2/L)

Calculated Initial Total Ammonia (mg N/L)

Calculated Initial Chlorine to Ammonia-Nitrogen Mass Ratio (Cl2:N)

Biomass pH (mg TSS/L)*

Negative Control Experiments A

1.3

1.3

1.5

0.88

70

8.3

B

1.2

1.2

1.4

0.84

72

8.4

C

0.79

0.99

1.0

1.0

68

8.3

D

0.84

0.92

1.0

0.9

69

8.4

E

0.56

0.64

0.69

0.93

64

8.4

F†

0.068

0.60

0.19

3.2

64

8.4

Positive Control Experiments A

1.4

N/A

1.4

0

70

8.3

B

1.2

N/A

1.2

0

72

8.3

C

0.81

N/A

0.81

0

68

8.3

D

0.78

N/A

0.78

0

69

8.3

E

0.57

N/A

0.57

0

64

8.3

Utilization Associated Product (UAP) Control

Experiments§

A

1.3

1.4

1.5

0.89

70

8.3

B

1.4

1.5

1.7

0.87

72

8.4

C

0.95

0.94

1.1

0.83

68

8.3

D

1.0

0.99

1.2

0.80

69

8.4

E

0.50

0.60

0.62

0.97

64

8.3

Cometabolism Experiments A

1.3

1.4

1.6

0.91

70

8.3

B

1.1

1.4

1.4

0.98

72

8.2

C

0.99

0.98

1.2

0.82

68

8.3

D

0.85

0.86

1.0

0.84

69

8.2

E

0.58

0.60

0.70

0.86

64

8.3

F†

0.044

0.65

0.17

3.8

64

8.3

G‡

0.058

0.71

0.20

3.6

69

8.3

*Biomass concentration was used to set initial concentrations of active biomass (X ) and reactive biomass (X ) in all a r modeling except for the negative and UAP control experiments where Xa was set to 0 mg TSS/L as the biomass had been inactivated by chlorobenzene treatment. †For Experiment F, only a negative control and cometabolism experiment were conducted ‡For Experiment G, only a cometabolism experiment was conducted N/A – Chemical not present during experiment

5

ACS Paragon Plus Environment


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Page 22 of 29

Table 2 Biotic process matrix for mixed culture batch kinetic experiments Reaction Stoichiometry Reaction

Ammonia Monod First‐Order Cometabolism Biomass Reactivity Biomass Inactivation UAP Reactivity

Reaction Rate Expression

k K

X S S

STOTNH3

SNH2Cl

SNO2‐

–1

1

fUAP

α α

SUAP Xa

k

X S

–1

1

k

X S

1

–1

–YXr

–1

1

–1

–1

k

k

X S

S

S

α1 = ammonia‐nitrogen (NH3‐N) fraction of TOTNH3 fUAP = UAP formation fraction from TOTNH3 degradation, moles UAP formed/moles TOTNH3 degraded k1NH2Cl = monochloramine first‐order cometabolism rate constant, L/mg TSS–day kBiomass = monochloramine reaction with biomass rate constant, L/mg TSS–day kInact = active biomass inactivation rate constant, L/moles Cl2–day KsNH

Xr

= ammonia half‐saturation constant, moles NH3‐N/L

3‐N

kTOTNH3 = ammonia maximum specific rate of degradation, moles TOTNH3/mg TSS–day kUAP = monochloramine reaction rate constant with UAP, L/moles UAP‐day SNH2Cl = monochloramine concentration, moles Cl2/L SNO2‐ = nitrite concentration, moles N/L STOTNH3 = TOTNH3 concentration, moles TOTNH3/L SUAP = UAP concentration, moles UAP/L Xa = active biomass concentration, mg TSS/L Xr = reactive biomass concentration, mg TSS/L YXr = stoichiometric link between reactive biomass lost per monochloramine reacted, mg TSS/moles Cl2

2


Page 23 of 29


Table 3 Summary of estimated kinetic parameters for the mixed culture and comparison with the pure culture N. europaea Parameter

Description

Units

N. europaea Pure Culture

Current research unless otherwise noted Estimate

Standard Error

ammonia maximum specific rate of degradation

mg TOTNH3/mg TSS–day

2.9*

1.7

0.55

ammonia half-saturation constant

mg NH3-N/L

0.13†

0.083

0.038

k1TOTNH3

ammonia first-order rate constant

mg TOTNH L mg NH N mg TSS day

22.3†

20

11

k1TOTNH3 * α1pH 8.3

ammonia first-order rate constant at pH 8.3

L/mg TSS-day

2.3†

2.0

1.1

kBiomass

monochloramine reaction with biomass rate constant

L/mg TSS–day

0.66‡

1.5

0.15

YXr

monochloramine reaction with biomass reaction yield

mg TSS/mg Cl2

59‡

57

5.1

kInact

active biomass inactivation rate constant

L/mg Cl2–day

224†

139

26

fUAP

UAP formation fraction from TOTNH3 metabolism

mole UAP/mole TOTNH3

0.029†

0.062

0.013

kUAP

monochloramine reaction rate constant with UAP

1/M-day= L/moles UAP-day

1.9x107†

2.3x107

1.2x107

k1NH2Cl

monochloramine first-order cometabolism rate constant

L/mg TSS–day

2.1†

0.80

0.17

k1TCM

chloroform first-order cometabolism rate constant

L/mg TSS–day

0.10*

0.072-0.12#

N/A

k1TBM

bromoform first-order cometabolism rate constant

L/mg TSS–day

0.23*

0.16-0.30#

N/A

kTOTNH3 KsNH

3-N

Certain kinetic parameter values obtained from the literature as follows: *Wahman, et al.10; †Maestre, et al.12; ‡Wahman and Speitel28; and #Wahman, et al.31



A

Exp A Data Exp B Data Exp C Data Exp D Data Exp E Data

Exp A Model Exp B Model Exp C Model Exp D Model Exp E Model

B

Exp A Data Exp B Data Exp C Data Exp D Data Exp E Data Exp F Data

Exp A Model Exp B Model Exp C Model Exp D Model Exp E Model Exp F Model

1.0

1.0

0.5

0.5

0.0

0.0 0

10

C

20 30 Time (min)

50

Exp A Model Exp B Model Exp C Model Exp D Model Exp E Model

0

1.2

10

D

20 30 Time (min)

40

50

Exp C Negative Control Data Exp C UAP Control Data Exp C Negative Control Model Exp C UAP Control Model

1.0 NH2ClmgCl2 L

Exp A Data Exp B Data Exp C Data Exp D Data Exp E Data

40

NH2ClmgCl2 L

1.5

1.5

NH 2ClmgCl2 L

TOTNH3mgN L 

1.5

Page 24 of 29

1.0

0.8 0.6

0.5

0.4 0.2

0.0

0.0 0

10

20 30 Time (min)

40

50

0

10

20 30 Time (min)

40

50

Figure 1 Comparison of model simulations and experimental data for the various mixed culture control experiments (Initial conditions: pH 8.3-8.4 and 4 mM bicarbonate buffer, refer to Table 1 for additional experimental conditions): (A) Five positive control experiments measuring the total free ammonia (TOTNH3) oxidation rate in the absence of monochloramine (NH2Cl); (B) Six negative control experiments measuring the abiotic loss of monochloramine in the presence of chlorobenzene inactivated biomass; (C) Five utilization associated product (UAP) control experiments measuring the abiotic loss of monochloramine in the presence of chlorobenzene inactivated biomass where before the start of each UAP control experiment and prior to chlorobenzene biomass inactivation 1.4 (Exp A), 1.0 (Exp B), 0.64 (Exp C), 0.75 (Exp D), or 0.48 (Exp E) mg TOTNH3/L had been degraded to produce UAPs; and (D) Experiment C’s UAP and negative control experiments detailing the increased loss of monochloramine in the UAP control experiment versus the negative control.


Page 25 of 29


Figure 2 Comparison of monochloramine (NH2Cl) and total free ammonia (TOTNH3) model simulations and experimental data in mixed culture Experiment F (Comet. = Cometabolism and Neg. Cont. = Negative Control. Initial conditions: pH 8.3-8.4, 4 mM bicarbonate buffer, and 5:1 Cl2:N mass ratio)



Figure 3 Comparison of monochloramine (NH2Cl) and total free ammonia (TOTNH3) model simulations and experimental data in mixed culture Experiment B (Panel A, top) and Experiment E (Panel B, bottom). Comet. = Cometabolism and Neg. Cont. = Negative Control. Initial conditions: pH 8.2-8.4 and 4 mM bicarbonate buffer


Page 26 of 29

Page 27 of 29


1.5

Exp A Data Exp B Data Exp C Data Exp D Data Exp E Data Exp F Data Exp G Data

Exp A Model Exp B Model Exp C Model Exp D Model Exp E Model Exp F Model Exp G Model

NH2ClmgCl2 L

1.0

0.5

A

0.0 1.5

TOTNH3mgN L

1.0

0.5

B

0.0 0

10

20

30

40

Time (min)

50

60

Figure 4 Comparison of monochloramine (NH2Cl, Panel A, top) and total free ammonia (TOTNH3, Panel B, bottom) model simulations and experimental data for the seven mixed culture cometabolism experiments (Initial conditions: pH 8.2-8.3 and 4 mM bicarbonate buffer)


Reaction with Biomass Cometabolism of Monochloramine Reaction with Utilization Associated Products (UAPs) Abiotic Autodecomposition and Reaction with Nitrite

20

40

60

80

100

Page 28 of 29

0

Percent of total monochloramine loss associated with given reaction pathway


A

B

C

D

E

F

G

Cometabolism Experiment

Figure 5 Relative contributions of reaction mechanisms to monochloramine loss for all cometabolism experiments


UAPs PageEnvironmental 29 of 29 Science & Technology

NH3

BAPs AMO

NH2Cl

AOB ACS Paragon Plus Environment Nitrite

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