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. 2023 Jul 19;57(47):18499–18508. doi: 10.1021/acs.est.3c00802

Optimizing Ozone Disinfection in Water Reuse: Controlling Bromate Formation and Enhancing Trace Organic Contaminant Oxidation

Samantha Hogard †,‡,*, Robert Pearce †,‡, Raul Gonzalez ‡, Kathleen Yetka ‡, Charles Bott ‡
PMCID: PMC10690711  PMID: 37467303

Abstract

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The use of ozone/biofiltration advanced treatment has become more prevalent in recent years, with many utilities seeking an alternative to membrane/RO based treatment for water reuse. Ensuring efficient pathogen reduction while controlling disinfection byproducts and maximizing oxidation of trace organic contaminants remains a major barrier to implementing ozone in reuse applications. Navigating these challenges is imperative in order to allow for the more widespread application of ozonation. Here, we demonstrate the effectiveness of ozone for virus, coliform bacteria, and spore forming bacteria inactivation in unfiltered secondary effluent, all the while controlling the disinfection byproduct bromate. A greater than 6-log reduction of both male specific and somatic coliphages was seen at specific ozone doses as low as 0.75 O3:TOC. This study compared monochloramine and hydrogen peroxide as chemical bromate control measures in high bromide water (Br– = 0.35 ± 0.07 mg/L). On average, monochloramine and hydrogen peroxide resulted in an 80% and 36% decrease of bromate formation, respectively. Neither bromate control method had any appreciable impact on virus or coliform bacteria disinfection by ozone; however, the use of hydrogen peroxide would require a non-Ct disinfection framework. Maintaining ozone residual was shown to be critical for achieving disinfection of more resilient microorganisms, such as spore forming bacteria. While extremely effective at controlling bromate, monochloramine was shown to inhibit TrOC oxidation, whereas hydrogen peroxide enhanced TrOC oxidation.

Keywords: Ozone; Water reuse; Virus; Disinfection; Bromate; 1,4-dioxane

Short abstract

The results of this study will inform readers of how to optimize ozone treatment processes in water reuse thus improving the microbial safety of treated wastewater for potable reuse and allowing for more widespread application of ozone.

1. Introduction

Ensuring pathogen removal and microbial safety of finished water remains the greatest acute health concern in water reuse applications. Full advanced treatment trains commonly employed for water reuse in the United States achieve pathogen removal via membrane filtration and high dose UV.1,2 In recent years, ozone/biofiltration based treatment has emerged as an alternative that can meet nearly every treatment goal for indirect potable reuse.3−6 The lack of regulations surrounding pathogen removal in water reuse has in some cases led to the extension of drinking water treatment regulations to these applications.5 Applying conservative drinking water frameworks presents a challenge when high log-removal values are required. For example, the use of Ct (concentration*time) to quantify ozone inactivation of viruses has been shown to be relatively conservative and difficult to manage in high bromide secondary effluent.7 Other approaches can be considered and validated to ensure that sufficient pathogen removal is achieved in water reuse. For example, one study showed that parameters such as change in UV254 absorbance can correlate with virus log reduction even when no measurable Ct is achieved.8

Ozone is a well-known disinfectant that has been applied in water treatment applications for decades,9,10 and recent studies demonstrate similar efficacy in water reuse.11 Ozone reacts relatively fast with microorganisms with reaction rates of 105–106 M–1 s–1 and 1.04 × 105 M–1 s–1 reported for enteric viruses and E. coli, respectively.12,13 Ozone is also effective at inactivating chlorine-resistant microorganisms such as Cryptosporidium parvum (k = 6.7 × 102 M–1 s–1).9 Pilot validation studies performed previously have shown a greater than 6.5 log removal of MS2 in filtered secondary wastewater effluent.3 Ozone also provides the benefit of efficiently oxidizing trace organic contaminants (TrOCs).14,15 The primary drawback to ozone application is the formation of disinfection byproducts (DBPs) such as bromate, aldehydes, and nitrosamines such as NDMA.16−18 This is a challenge in water reuse applications, where the concentration of precursor compounds is elevated. Many DBPs formed during ozonation are biodegradable in downstream biofiltration (e.g., NDMA19); however, bromate is not. Therefore, bromate is of particular concern due to the 10 μg/L maximum contaminant level (MCL) put in place by the United States EPA and the World Health Organization.20,21 Common bromate control methods used in water treatment include pH suppression and ammonia addition;22 however, these are often insufficient in wastewater treatment applications where bromide is elevated and hydroxyl radical (*OH) bromate formation pathways dominate.16 Alternative chemical control measures including chlorine/ammonia-based strategies and the use of hydrogen peroxide have been shown to suppress bromate formation below the MCL.23,24 Multiple studies have been completed demonstrating the efficacy of preformed monochloramine for bromate control in a high bromide reuse water.5,7

Each of these bromate control methods has other impacts on the ozonation process that are not well documented, especially in potable reuse. Preformed monochloramine is a well-known *OH scavenger; in fact, this is hypothesized to be one of the primary mechanisms for bromate control5,7,25 and could be a detriment to *OH mediated TrOC oxidation.7 Additionally, monochloramine itself may contribute to further DBP formation.26,27 Hydrogen peroxide limits bromate formation by accelerating ozone decomposition and minimizing ozone exposure as well as reducing hypobromous acid which forms during bromate formation.23,28,29 The addition of hydrogen peroxide during ozonation has also been shown to enhance TrOC oxidation in some cases.29,30 Currently, the requirement to maintain a dissolved ozone residual for Ct credit would preclude a treatment facility from using hydrogen peroxide for bromate control.

As the application of ozone in water reuse becomes more prevalent, it is important to determine the optimal strategy to balance disinfection and TrOC oxidation with bromate formation. The objectives of the study described herein were to (1) investigate the performance of ozone disinfection in water reuse at the pilot scale, (2) compare monochloramine and hydrogen peroxide as chemical bromate control methods in high bromide secondary effluent, and (3) determine the ancillary impacts of these bromate control strategies on disinfection and oxidation of TrOCs during ozonation.

2. Materials and Methods

2.1. Ozone Pilot Description

Experiments done as a part of this study were performed on a 3.78 Lpm (1 gpm) ozone pilot located at the Hampton Roads Sanitation District’s (HRSD) demonstration scale indirect potable reuse plant and research center in southeast Virginia. Unfiltered secondary clarifier effluent from HRSD’s five-stage Bardenpho Nansemond Wastewater Treatment Plant was the feedwater for these experiments. The ozone pilot was operated in fine bubble diffusion with the first column being a counter flow bubble diffusion chamber (detention time = 1.5 min) and the remaining five columns functioning as the contact chamber. The ozone pilot included an oxygen concentrator and ozone generator. The ozone dose was entered manually and controlled automatically by varying the ozone gas concentration and gas flow rate. Ozone feed gas and off-gas concentrations were measured using ozone gas analyzers, and transfer efficiency typically remained >95%. The residence time of the entire ozone pilot was approximately 10 min total. For both the monochloramine and hydrogen peroxide testing conditions, chemicals were added upstream of the ozone contactor. Preformed monochloramine was obtained on site from the chemical feed system at the demonstration scale indirect potable reuse plant. This system includes a GAC contactor and ion exchange unit that are used to dechlorinate and soften tap water which serves as the carrier water prior to sequential addition of sodium hypochlorite and ammonium sulfate. This chemical stock solution was collected on the day of testing to ensure the concentration was >1500 mg/L-Cl2. A fixed monochloramine dose of 3 mg/L-Cl2 was used for the testing described herein. Monochloramine was quenched at the end of the ozone contactor before sampling by using sodium bisulfite. Hydrogen peroxide (30%) was diluted 100-fold for use on the pilot scale. A molar ratio of 1:1 H2O2:O3 was used for these experiments. A control test was also completed for each of these chemicals, where ozone was turned off in order to observe the effects of these chemicals alone.

2.2. Microbial Surrogates

Challenge experiments were performed by spiking model viruses including male specific coliphage, MS2 and somatic coliphage, T1. Escherichia coli was used as a vegetative bacteria surrogate, and the spore forming bacteria Bacillus subtillis and Clostridium perfringens were used as surrogates for protozoan pathogens. Pepper mild mottle virus (PMMoV), crAssphage, and the general Bacteroides assay (GenBac) were identified as useful molecular indicators, as they are frequently detected at elevated concentrations in the secondary clarifier effluent. PMMoV and the bacteriophage, crAssphage, are commonly used as markers for fecal contamination and can be representative of pathogenic virus removal in wastewater treatment processes.31E. coli and total coliform were assayed using the 24-h IDEXX Colilert-18 method. Male-specific and somatic coliphages were quantified using the single agar layer procedure on undiluted or diluted samples.32 Although MS2 and T1 were used as representative viruses, this culture based analysis also includes background concentrations of indigenous male specific and somatic coliphages. Therefore, results will indicate the log reduction of total male specific and somatic coliphages. Clostridium perfringens and aerobic spore forming bacteria (SFB) were quantified by BCS Laboratories in Gainesville, FL using ASTM D5916 and Standard Method 9218, respectively. In addition to these added surrogates, the removal of indigenous viruses and bacterial indicators was also examined by the droplet digital polymerase chain reaction (ddPCR) in this study. The molecular sampling methods and workflow for sample processing are included in the Supporting Information (Section SI-2).

The added culture concentrations were approximately 106–107 PFU/100 mL of both MS2 and T1 coliphages, 105 CFU/100 mL E. coli, and 105 CFU/100 mL Bacillus subtilis spores. This concentration of the coliphage was selected in an effort to limit the organic carbon that was introduced to the feedwater via the added culture. Stock solutions were prepared by adding the viral/bacterial cultures to five liters of phosphate buffered saline. These solutions were then added to the feedwater via Masterflex peristaltic pumps. The addition points were followed by static mixers to ensure sufficient mixing and even distribution of the added cultures. A diagram of the pilot test setup can be found in Figure 1.

Figure 1.

Figure 1

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Nansemond SCE water quality and the HRSD ozone pilot testing setup including sampling locations indicated with star symbols.

2.3. Water Quality Analytical Methods

Field parameters, such as pH, temperature, and UV absorbance, were measured in a timely manner on the day of testing. A Thermo Scientific pH probe was used to periodically check that the pH remained constant in ozone influent samples. Temperature was controlled using a chiller system that recirculated water to the pilot feed tank, and it was verified on every sample using a HANNA Instruments thermometer. UV 254 absorbance was measured on 0.45 μm filtered influent and ozone effluent samples using a Thermo Scientific Genesys 150 UV–vis Spectrophotometer. Dissolved ozone residual was measured according to the Standard Method 4500 Indigo colorimetric method. Samples for water quality parameters, including alkalinity, total organic carbon (TOC), bromate, and bromide, were analyzed by HRSD’s Central Environmental Laboratory. TOC was analyzed according to Standard Method 5310 using a Shimadzu TOC 4200. Bromate was analyzed according to EPA method 300.1 with a Dionex 5000 plus ion chromatograph, and bromide was analyzed by EPA method 300 using a Dionex Integrion high-pressure ion chromatograph. NDMA and 1,4-dioxane were analyzed according to EPA methods 521 and 522, respectively, using an Agilent 7010B GC/MS Triple Quadrupole (Santa Clara, CA). A suite of 106 TrOCs was analyzed by Eurofins Eaton Analytical (Monrovia, CA) using liquid chromatography with tandem mass spectrometry (LC-MS-MS).

3. Results and Discussion

3.1. Ozone and *OH Exposure

The secondary clarifier effluent water quality characteristics are summarized in Figure 1. The temperature of the pilot feed was controlled to 20 ± 0.87 °C, while TOC and nitrite only varied slightly with an average of 8.4 and 0.05 mg/L-N, respectively. As a result, the ozone decay characteristics were similar between testing days when evaluating the control condition with no chemical addition (Table SI-1). For every test, the 0.25 O3:TOC condition resulted in minimal measurable ozone residual, and therefore, ozone decay could not be characterized. The average first order ozone decay rate in the control condition decreased in magnitude from 3 to 0.62 min–1 when increasing the ozone dose 0.5–1 O3:TOC as expected.33 The initial ozone demand (IOD) is defined as the applied ozone dose minus the dissolved ozone concentration measured in the effluent of the diffusion column. In the absence of any chemicals added for bromate control, the IOD was between 56 and 68% of the applied ozone dose at 0.75–1 O3:TOC for both tests 1 and 2, and this proportion increased to ∼80% at 0.5 O3:TOC. This demonstrates the potential for bulk organic matter alone to impose a significant ozone demand. The addition of 3 mg/L-Cl2 monochloramine resulted in a minor increase in the ozone decay rate and increased IOD at each applied O3:TOC ratio. This was primarily attributed to the reaction between ozone and monochloramine (O3 + NH2Cl → NO3– + Cl– + H+, k = 26 M–1 s–1).34 A previous study performed on the same source water found that monochloramine had little impact on the ozone decay rate and IOD.7 In general, the impact of monochloramine may vary depending on the concentration of other ozone demand inducing constituents, and it is likely that ozone decay will be governed by the bulk organic concentration and reactivity in wastewater. The addition of hydrogen peroxide dramatically increased the ozone decay rate which is consistent with previous studies in drinking water.29 Example ozone decay curves for an ozone dose of 0.75 O3:TOC under all bromate control conditions are shown in Figure 2. In addition, complete ozone residual curves are shown for each test condition in the Supporting Information (Figures SI-1–6). For this work, the maximum instantaneous single point Ct was used to evaluate disinfection results, and a summary of maximum single point Ct, total integrated ozone exposure, and the average decay rate for each testing condition is presented in Table SI-1 and Table SI-2. While there are multiple methods used for calculating ozone exposure, the single point Ct approach was used to represent full-scale operational data. This method is limited in several ways as it underestimates the total ozone exposure, it does not include the ozone exposure that occurs during dissolution, and it can vary widely depending on where the residual is measured. These data can be compared to the total ozone exposure in Tables SI-1 and SI-2 to emphasize the conservative nature of single point Ct. Total ozone exposure is calculated by integrating the ozone residual curve beginning immediately after the ozone diffuser.

Figure 2.

Figure 2

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Representative ozone decay characteristics for each bromate control scenario at a specific ozone dose of 0.75 O3:TOC. The ozone dose for each scenario was 6.93 mg/L (control), 7.7 mg/L (3 mg/L-Cl2 NH2Cl), and 6.88 mg/L (1:1 O3:H2O2).

The *OH exposure can be calculated using the second order reaction rate constant and measured removal of 1,4-dioxane using eq 135 shown in the SI, where the reaction rate of 1,4-dioxane and *OH is 3 × 109 M–1 s–1.10 The total ozone exposure and *OH exposure for test 1 are summarized in Tables SI-1 and 3, respectively. When increasing the ozone dose from 0.25 to 1 O3:TOC, the average *OH exposure for two tests increased linearly from 0.65 to 3.04 × 10–10 M*s. At an O3:TOC of 1, the addition of 3 mg/L-Cl2 NH2Cl resulted in an average 48% reduction of *OH exposure, while a 1:1 molar ratio of H2O2 resulted in a 40% increase.

3.2. Microorganism Inactivation by Ozone

3.2.1. Coliphage

Male specific and somatic coliphage inactivation is shown for each treatment condition in Figure 3. These data include the indigenous coliphage as well as spiked MS2 and T1 coliphages. Even at the lowest ozone dose of 0.25 O3:TOC, greater than 2-log reduction of male specific and somatic coliphages was observed for all conditions. Given the relatively fast reaction rate reported for ozone with the male specific coliphage, MS2 (k = 1.9 × 106 M–1 s–1), and a morphologically similar somatic coliphage T4 (1.3 × 106 M–1 s–1), this result is expected.12 At the higher ozone doses (0.75–1 O3:TOC), a greater than 6-log reduction of both male specific and somatic coliphages was demonstrated. These elevated ozone doses resulted in nearly nondetect values, producing the maximum LRV possible (>6 log) under these conditions. One study found that a 6.5-log reduction of MS2 is equivalent to a 5 log-reduction of poliovirus, which is required by California’s Title 22 water reuse regulations.36 The observed log-reduction appears to level off after 0.75 O3:TOC due to the limitation associated with spiking additional coliphages discussed previously. These results are aligned with previous bench-scale ozonation studies performed on filtered wastewater effluent which showed a greater than 5-log reduction of MS2 at ozone doses greater than 0.25 O3:TOC.8 The results of the present study highlight the efficacy of ozone for virus disinfection in highly treated wastewater, regardless of tertiary filtration. A recent review on ozone disinfection indicated that somatic coliphages (DNA viruses) may follow more similar inactivation kinetics to mammalian viruses in pure water, whereas MS2 displays the greatest susceptibility to inactivation by ozone.37 However, the results presented herein suggest that both male specific and somatic coliphages experience similar inactivation in this wastewater matrix for a given ozone dose among all tested conditions.

Figure 3.

Figure 3

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Log-10 reduction of male specific and somatic coliphages by ozone. Solid bars represent the control condition, checkered bars are the hydrogen peroxide condition, and dotted bars are the monochloramine condition. Bars represent average log-removal data for two independent tests with error bars showing the range.

It is generally believed that molecular ozone is primarily responsible for virus inactivation, whereas *OH is not expected to play a significant role in disinfection.15 However, the results of this experiment demonstrate that marginally elevated coliphage inactivation was achieved with the addition of hydrogen peroxide when compared with the control condition and the monochloramine condition. This disparity between the bromate control conditions is more apparent at lower ozone doses where minimal measurable ozone exposure is achieved. A control test was completed for both monochloramine and hydrogen peroxide to determine if each oxidant was responsible for virus inactivation independent from ozone. There was negligible removal achieved by hydrogen peroxide and monochloramine alone (<0.5 log removal of male specific and somatic coliphages in all cases).

Given that each bromate control method did not have any impact on the inactivation of coliphage, these data were correlated with three independent variables including change in UV254 absorbance, applied specific ozone dose (O3:TOC corrected for nitrite), and single point ozone Ct. These correlations are presented in Figure 4a-c. The correlation of male specific and somatic coliphage inactivation with change in UV absorbance and O3:TOC display relatively high correlation coefficients (R2 = 0.78 and 0.74, respectively, and p < 0.05) suggesting these would be useful parameters for process monitoring and disinfection verification. This is well aligned with previous bench scale studies that have shown that MS2 inactivation correlates well with change in UV254 absorbance, change in fluorescence, and applied ozone dose.8,38 Further, the correlation of coliphage inactivation with ozone exposure is relatively weak, with the data reaching an asymptote beyond a single point Ct of 0.5 mg/L-min as a result of the coliphage detection limit being nearly reached. This is especially true for scenarios where hydrogen peroxide is used for bromate control and minimal ozone exposure is achieved. In Figure 4c, the EPA model for virus inactivation by ozone at 20 °C is overlaid on the data collected in the present study. This highlights the conservative nature of drinking water frameworks when compared with the actual observed coliphage inactivation at lower ozone doses.

Figure 4.

Figure 4

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Log reduction of male specific (circles) and somatic (triangles) coliphages correlated with (a) % change in UV absorbance (R2 = 0.78), (b) O3:TOC corrected for nitrite (R2 = 0.74), and (c) single point ozone exposure (C*t) with the US EPA model @ 20 C overlaid.

3.2.2. Molecular Markers

In addition to the culturable viruses, a group of viral and bacterial indicators was analyzed by molecular methods. As discussed previously, PMMoV, crAssphage, and the general Bacteroides assay (GenBac) were identified as useful indicators due to the elevated concentration present in the secondary effluent. These data are presented in Figure SI-7 for each of the treatment conditions tested. In the control condition, there is a clear trend between the applied O3:TOC ratio and log-removal of each indicator. However, with the introduction of monochloramine and hydrogen peroxide, the trends become less distinct with some indicators having little relationship with applied O3:TOC at all. The appearance of negative removal of PMMoV is likely explained by the difference in the influent sample volume and variations in recovery during the ddPCR process. These details are outlined in Table SI-4. There are several well-documented shortcomings associated with using molecular methods to demonstrate inactivation by oxidative disinfection methods.39,40 These analytical methods are inherently conservative in that they cannot distinguish between viable and nonviable viruses, and they only target a relatively small segment of the genome of interest.39,41 Therefore, molecular results will show inactivation only if the oxidant damages the specific segment of the genome that is targeted by the molecular assay. The addition of monochloramine and hydrogen peroxide may inhibit the attack of molecular ozone on the specific gene segment of interest for some indicators. The present study highlights the difficulty associated with using molecular methods to demonstrate log-reduction of viral and bacterial indicators by ozonation.

3.2.3. Coliform Bacteria

Both E. coli and total coliform log-removal are presented in Figure 5. It is apparent from these results that coliform bacteria cannot be effectively used as an indicator of ozone treatment when using monochloramine for bromate control due to the high log reduction achieved by monochloramine alone. Samples were quenched with sodium bisulfite upon collection; therefore, the contact time with monochloramine was equal to the residence time of the ozone contactor (∼10 min). The results from the control condition and hydrogen peroxide condition were very similar, suggesting that the majority of inactivation happens rapidly during the initial phase of ozone exposure and decomposition. Negligible log-reduction was observed with the use of hydrogen peroxide alone. This is also well aligned with data from previous studies that estimated a relatively high reaction rate of ozone with E. coli of 1.04 × 105 M–1 s–1.13 Other studies have shown similar results with the addition of hydrogen peroxide or a radical scavenger, such as tertiary butanol, not having a significant impact on total coliform or E. coli disinfection by ozone.8,13

Figure 5.

Figure 5

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(a) E. coli and (b) total coliform removal by ozone for each bromate control condition.

3.2.4. Spore Forming Bacteria

The removal of spore-forming bacteria by ozone is presented in Figure 6. The average influent concentration of C. perfringens was 790 CFU/100 mL. The trend of C. perfringens removal with an increasing ozone dose is consistent with each treatment condition. However, the addition of monochloramine alone results in approximately an additional 1 log-reduction of the bacteria. Therefore, the bars for the monochloramine condition are approximately 1 log unit above the others for every ozone dose. It also appears that these bacteria are more susceptible to molecular ozone than *OH due to the suppressed removal when hydrogen peroxide is added. This is in direct contrast with results presented in a previous study, showing enhanced removal of C. perfringens during ozonation with the addition of hydrogen peroxide in surface water (TOC = 2–4 mg/L).42 Another study performed in surface water (DOC = 2.1–5.5 mg/L) found that elevated TOC concentration resulted in enhanced removal of C. perfringens, and the authors hypothesized that this could be a result of organics promoting *OH formation.43 The present study shows that the conclusions of these previous studies are not necessarily valid for ozonation of a wastewater matrix.

Figure 6.

Figure 6

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(a) C. perfringens and (b) aerobic SFB removal by ozone for each bromate control condition.

Aerobic SFB inactivation was also quantified under each treatment condition. Bacillus subtilis spores are commonly used as surrogates for ozone disinfection of heartier microorganisms such as C. parvum.9,44 Both chemicals used for bromate control were apparently responsible for nearly a 0.5 log-reduction of aerobic SFB alone. It is clear from the trend in removal of these bacteria that they are much more resistant to ozonation compared to other pathogens and indicators discussed previously. One study in drinking water reported a Ct-lag of 2.9 mg/L-min,44 and another study performed in wastewater found a Ct-lag of 9 mg/L-min.8 In the present study, a >0.7 log reduction of aerobic SFB was not observed until an ozone exposure of approximately 5 mg/L-min was achieved in the control and monochloramine conditions. Negligible log removal was achieved at any ozone dose with the use of hydrogen peroxide due to the lack of measurable ozone residual.

3.3. Bromate and 1,4-Dioxane

The background bromide concentration averaged 0.35 ± 0.07 mg/L throughout the duration of testing. In the control condition, the average molar conversion of bromide to bromate (mol Br–/mol BrO3–) increased from <0.1% to ∼5% when increasing O3:TOC from 0.25 to 1. This can be seen in Figure 7a. At each O3:TOC, the addition of hydrogen peroxide and monochloramine resulted in 36 ± 8% and 80 ± 14% suppression in bromide conversion, respectively, when compared with the control condition. Understanding the relative contributions of molecular ozone and *OH is imperative in order to sufficiently limit bromate formation in wastewater ozonation. It has been hypothesized that bromate formation in treated wastewater is dominated by the “indirect” reaction with *OH as opposed to “direct” reactions with molecular ozone.16 The primary mechanism of bromate control by monochloramine is elucidated by Figure 7b that displays the difference in 1,4-dioxane oxidation. As mentioned previously, this compound is primarily oxidized by *OH (k*OH = 3 × 109 M–1 s–1, kO3 = <1 M–1s–1),10 and by using this compound as a surrogate to calculate *OH exposure, we can conclude that the addition of monochloramine results in up to 60% lower hydroxyl radical exposure. Monochloramine has also been shown to react to form intermediate compounds that act as reservoirs of bromine and prevent bromate formation.45 Additionally, a recent study showed the rapid reaction rate between monochloramine and the bromine radical during bromate formation which provides another important mechanism for limiting bromate formation.46 Hydrogen peroxide primarily limits bromate formation by both reacting relatively rapidly with ozone (kpH=7.1 = 1.92 × 102 M–1 s–1)10,47 resulting in minimal measurable ozone exposure, as well as reducing hypobromous acid to bromide.29,48 The significant impact of hydrogen peroxide on ozone decay is shown by the example ozone decay curve shown in Figure 2. Additionally, hydrogen peroxide moderately enhances *OH exposure, as shown by the increased removal of 1,4-dioxane in Figure 7b. These data support the hypothesis that *OH bromate formation pathways dominate during wastewater ozonation seeing as the reduction in *OH exposure by monochloramine was far more effective in controlling bromate compared with the elimination of molecular ozone exposure coupled with enhanced *OH exposure by hydrogen peroxide. Ultimately, these results may vary for wastewater matrices with variable organics composition and reactivity. The impact of hydrogen peroxide and monochloramine on NDMA formation was also investigated; however, these data were not conclusive and are presented in Figure SI-8 and Section SI-3.3.

Figure 7.

Figure 7

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(a) Molar conversion of bromide to bromate. Data points represent average values for three independent tests with error bars showing standard deviation. (b) Percent removal of 1,4-dioxane. Data points represent average values for two independent tests, with error bars showing the range.

3.5. Trace Organic Contaminants

As a part of this study, a full suite of 106 TrOCs was monitored at each treatment condition in a separate test conducted on a single day. Duplicate influent samples were collected, and values were averaged for redundancy. Of the 106 TrOCs, only 50 were detected in both influent samples. Additionally, out of the 50 compounds detected in both influent samples, ten were removed below the detection limit in every ozone treatment condition. This is due to the very high reaction rate of ozone with these compounds. These raw data are summarized in the Supporting Information (Table SI-5).

The compounds, which are more resistant to ozonation or have a higher affinity for reacting with *OH, reveal more information about the relative ozone and *OH exposure. The removal of these compounds is summarized in Table 1. Sucralose, iohexol, primidone, and meprobamate can be used as indicator compounds that have well-defined, relatively high reaction rates with *OH. Low-moderate removal is observed for each of these compounds with ozone alone, and removal is enhanced marginally with the addition of hydrogen peroxide. The addition of monochloramine for bromate control results in decreased removal of almost every compound for both 0.25 and 0.75 O3:TOC when compared with the control condition. As discussed previously, any of these compounds can be utilized to effectively measure *OH exposure, and for the purposes of this study, 1,4-dioxane was used as a probe compound. The resulting hydroxyl radical exposure is shown in Table SI-3. The addition of 1 mol H2O2/mol O3 resulted in approximately 23% greater *OH exposure on average at 0.5 O3:TOC, and this increased to 40% greater *OH exposure at a specific ozone dose of 1 O3:TOC. This is supported by previous lab scale experiments that show the benefit of hydrogen peroxide is greater at higher applied ozone doses.49 These results are also aligned with those of previous lab-scale studies showing that the addition of just 0.5 mol H2O2/mol O3 can enhance *OH production by as much as 30–40% in a source water with similar DOC concentrations (8.6–12.2 mg/L).30 In contrast, monochloramine resulted in a 40–48% decrease in *OH exposure for every ozone dose tested. Sulfamethoxazole is a compound that reacts rapidly with both molecular ozone and *OH. Regardless of the complete elimination of a dissolved ozone residual with the addition of hydrogen peroxide, a similar removal of this compound is observed for every treatment scenario. It is clear from the results presented that utilizing hydrogen peroxide provides a benefit with regard to enhancing *OH exposure, while monochloramine will diminish the oxidative capacity of ozonation.

Table 1. Trace Organic Contaminant Removal by Ozonea.

3.5.

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a

% removal values denoted with an asterisk (*) were calculated using the detection limit as the raw value was below the method detection limit. Gradient color scale represents the relative removal of each compound.

b

Reaction rate sources53−57

4. Engineering Implications

Ozone provides a robust barrier for pathogen inactivation and TrOC oxidation when applied in water reuse applications. This study elucidates several important mechanisms related to disinfection, TrOC oxidation, and DBP formation, which occur during the ozonation of treated wastewater. When ozone is applied in these treatment scenarios, it is critical to optimize the relative ozone and *OH exposure in order to balance treatment goals with DBP formation. In this study, two chemical bromate control strategies were evaluated, including hydrogen peroxide and monochloramine, for the ozonation of a high bromide secondary clarifier effluent. The results of this study show that three primary considerations must be made before selecting a bromate control method including (1) ozone disinfection objectives, (2) ambient bromide concentration, and (3) TrOC oxidation objectives. If the treatment goal is to achieve protozoa inactivation with ozone, considering these surrogate results, hydrogen peroxide cannot be used for bromate control, as this eliminates ozone exposure necessary for the inactivation of these microorganisms. If virus/E. coli inactivation is the goal, either monochloramine or hydrogen peroxide can be used for bromate control as a greater than 6-log reduction of male specific and somatic coliphages and a 4-log reduction of E. coli was observed at ozone doses of 0.75–1 O3:TOC for every bromate control condition. The inactivation of model viruses correlated well with operational parameters other than ozone exposure, which supports the move away from Ct based disinfection frameworks for virus inactivation in water reuse. While monochloramine presents a significant benefit with regard to bromate control, it also quenches *OH thus preventing complete oxidation of ozone-resistant TrOCs. If protozoa disinfection and ozone resistant TrOC oxidation are both treatment goals to be addressed with ozone, then it may be necessary to implement alternate treatment technologies or remove point sources of bromide to the source water.

This study provides a continuous flow pilot-scale validation of ozone disinfection for water reuse in high bromide source water. Continuous flow pilot scale studies allow for more confident extrapolation to full-scale applications to aid in the process design and control. Further, this study provides guidance to utilities that will implement ozonation for water reuse and wastewater disinfection applications. The guidance provided regarding alternative ozone monitoring frameworks will ultimately allow for lower overall ozone doses to achieve virus disinfection, representing capital and operational cost savings. Recent trends have suggested the use of ozone is on the rise as it is a more efficient alternative to other advanced oxidation processes.50 In fact, the California State Waterboard has created guidelines for direct potable reuse that require ozone/biofiltration as a pretreatment step for reverse osmosis.51 Additionally, many utilities in Europe have planned to implement ozone to treat TrOCs in wastewater.52 This study shows that even when applied at relatively low doses, TrOC oxidation and significant disinfection can be achieved in wastewater ozonation. Future research should include the development of kinetic models that can more accurately determine the mechanisms responsible for pathogen inactivation, bromate control, and TrOC oxidation in various water matrices. Additionally, further studies should be completed to validate the alternative (non-Ct) ozone disinfection frameworks under a range of water quality parameters (e.g., pH, temperature, and TOC concentrations).

Acknowledgments

Funding for this project was provided by the Hampton Roads Sanitation District. The authors would like to thank the staff of the HRSD Pathogen Laboratory and Central Environmental Laboratory for their hard work performing analyses to support this project. Specifically, the efforts of Hannah Thompson, Hila Stephens, and Errin Carter are very appreciated.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.3c00802.

  • Experimental methods, additional tables, figures, and text (PDF)

The authors declare no competing financial interest.

Supplementary Material

es3c00802_si_001.pdf (180.9KB, pdf)

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es3c00802_si_001.pdf (180.9KB, pdf)

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