N-nitrosodimethylamine (NDMA) formation during ozonation of N,N-dimethylhydrazine compounds: Reaction kinetics, mechanisms, and implications for NDMA formation control

Feasibility of NDEA formation control from DEDTC in chlorination/chloramination by pre-ozonation: Mechanisms and influencing factors

N-nitrosodiethylamine (NDEA), which is the most toxic nitrosamine among the 9 detected species, has been widely detected in drinking water. Amines containing diethylamine (DEA) groups in the structure would generate NDEA during the disinfection processes. The aim of this study was to evaluate the feasibility of reducing NDEA formation from a commonly used dithiocarbamate pesticide sodium diethyldithiocarbamate (DEDTC) in subsequent chlorination and chloramination by pre-ozonation. The results demonstrated that NDEA could be generated directly during ozonation, its amounts increased from 0 to 14.34 μg/L with increasing ozone dosages (0-4 mg/L), which was higher than that chlorination (2.68 μg/L) and chloramination (4.91 μg/L) when the initial concentration of DEDTC was 20 μM. Pre-ozonation significantly raised NDEA formation from 2.68 to15.32 μg/L in subsequent chlorination; and that from 4.91 to 9.54 μg/L during subsequent chloramination processes. The addition of •OH scavenger tert-butanol (tBA) increased the production of NDEA from 8.14 to 20.80 μg/L during ozonation, and that from 6.76 to17.98 μg/L in O3/HClO process, 8.74 to 17.33 μg/L in O3/NH2Cl process. Except for NO3- and CO32-, most of the co-existing substances promoted NDEA generation from DEDTC under disinfection conditions. Based on the results of Gaussian theory calculations, GC/MS and UPLC-Q-TOFMS analysis, the influencing mechanisms of pre-ozonation on NDEA generation in the subsequent disinfection process were proposed. In addition, not only acute/chronic toxicity calculation but also luminescent bacteria test was performed to assess the possibility of pre-ozonation on the risk control of DEDTC. The research results fill a gap in the control of NDEA pollution and help to develop a safer ozone oxidation technology.

Oxidation of amine-based pharmaceuticals with unactivated peroxymonosulfate: Kinetics, mechanisms, and elimination efficiency of NDMA formation

Amine-based pharmaceuticals are a significant class of N-nitrosodimethylamine (NDMA) precursors. This study investigated the use of unactivated peroxymonosulfate (PMS) to control amine-based pharmaceuticals and their NDMA formation potential. Kinetic analysis and product identification revealed that sumatriptan and doxylamine primarily underwent reactions at their tertiary amine group, while ranitidine and nizatidine had both tertiary amine and thioether group as reaction sites. The NDMA formation from sumatriptan and doxylamine during post-chloramination was significantly reduced with the abatement of the parent contaminants, while the formation of NDMA remained high even if full abatement of ranitidine and nizatidine was achieved. Product formation kinetics and reference standard tests revealed the great contribution of transformation products to NDMA formation. Ranitidine could be oxidized to sulfoxide-type product ranitidine-SO and N-oxide type product ranitidine-NO. Ranitidine-SO exhibited a high NDMA yield comparable to that of ranitidine (>90%), while ranitidine-NO showed a low NDMA yield (2%). With further oxidation of ranitidine-SO at the tertiary amine group, NDMA formation was reduced by more than 90%. The underlying mechanism for the importance of the tertiary amine group in NDMA formation was demonstrated by quantum chemical calculation. These findings underscore the potential of PMS pre-oxidation on NDMA control.

Mitigating risks and maximizing sustainability of treated wastewater reuse for irrigation

Scarcity of freshwater for agriculture has led to increased utilization of treated wastewater (TWW), establishing it as a significant and reliable source of irrigation water. However, years of research indicate that if not managed adequately, TWW may deleteriously affect soil functioning and plant productivity, and pose a hazard to human and environmental health. This review leverages the experience of researchers, stakeholders, and policymakers from Israel, the United-States, and Europe to present a holistic, multidisciplinary perspective on maximizing the benefits from municipal TWW use for irrigation. We specifically draw on the extensive knowledge gained in Israel, a world leader in agricultural TWW implementation. The first two sections of the work set the foundation for understanding current challenges involved with the use of TWW, detailing known and emerging agronomic and environmental issues (such as salinity and phytotoxicity) and public health risks (such as contaminants of emerging concern and pathogens). The work then presents solutions to address these challenges, including technological and agronomic management-based solutions as well as source control policies. The concluding section presents suggestions for the path forward, emphasizing the importance of improving links between research and policy, and better outreach to the public and agricultural practitioners. We use this platform as a call for action, to form a global harmonized data system that will centralize scientific findings on agronomic, environmental and public health effects of TWW irrigation. Insights from such global collaboration will help to mitigate risks, and facilitate more sustainable use of TWW for food production in the future.

Control of N-nitrosodimethylamine (NDMA) formation from N,N-dimethylhydrazine compounds by ozone-based advanced oxidation processes

N-nitrosodimethylamine (NDMA) is formed during ozonation of model compounds with dimethylhydrazine groups, such as daminozide (DMZ) and 2-furaldehyde 2,2-dimethylhydrazone (2-F-DMH) at pH 7 with yields of 100 % and 87 %, respectively. In this study, ozone/hydrogen peroxide (O₃/H₂O₂) and ozone/peroxymonosulfate (O₃/PMS) were investigated to control NDMA formation, and O₃/PMS (50-65 %) was more effective than O₃/H₂O₂ (10-25 %) with a ratio of H₂O₂ or PMS to O₃ of 8:1. The reaction of PMS or H₂O₂ to decompose ozone could not compete with the ozonation of model compound because of the high second-order rate constants of the ozonation of DMZ (5 ×10⁵ M^-1 s^-1) or 2-F-DMH (1.6 ×10⁷ M^-1 s^-1). The R_ct value of the sulfate radical (SO₄^•-) showed a linear relationship with NDMA formation, indicating that SO₄^•- significantly contributed to its control. NDMA formation could be further controlled by injecting small quantities of ozone numerous times to minimize the dissolved ozone concentration. The effects of tannic acid, bromide and bicarbonate on NDMA formation were also investigated during ozonation, O₃/H₂O₂, and O₃/PMS processes. Bromate formation was more pronounced in the O₃/PMS process than in the O₃/H₂O₂ process. Therefore, in practical applications of O₃/H₂O₂ or O₃/PMS processes, the generation of NDMA and bromate should be detected.

Influencing pathways and toxicity changes of pre-ozonation on carcinogenic NDEA formation from greenhouse gas adsorbent DEAPA in subsequent disinfection processes

This study was to evaluate the feasibility of controlling carcinogenic nitrosodiethylamine (NDEA) formation from greenhouse gas adsorbent 3-diethylaminopropylamine (DEAPA) by pre-O₃ in subsequent chlorination/chloramination processes. The result indicated that the NDEA yields (0.4 %) during chlorination was 1.3 times of that during chloramination (0.3 %); pre-oxidation with 4 mg/L O₃ significantly cut down its formation; the reduction rates were up to 67.5 and 48.5 %, respectively. OH scavenger greatly augmented the final NDEA amount from 1.86 to 5.05 μg/L during ozonation, while its roles on subsequent processes differed with disinfection methods as well as O₃(g) dosages. Most of co-existed substances inhibited NDEA generation, except NO₂^-, CO₃^2- and SO₄^2-, which slightly promoted during ozonation. Basing on Gaussian calculation, GC/MS and UPLC-Q-TOF-MS analysis, the influencing mechanisms of pre-O₃ on NDEA formation in subsequent disinfection processes were proposed. In addition, the calculated toxicity analysis as well as the whole toxicity was applied to evaluate the possibility of pre-O₃ on risk control.

Efficiency of ozonation and O3/H2O2 as enhanced wastewater treatment processes for micropollutant abatement and disinfection with minimized byproduct formation

Ozonation, a viable option for improving wastewater effluent quality, requires process optimization to ensure the organic micropollutants (OMPs) elimination and disinfection under minimized byproduct formation. This study assessed and compared the efficiencies of ozonation (O₃) and ozone with hydrogen peroxide (O₃/H₂O₂) for 70 OMPs elimination, inactivation of three bacteria and three viruses, and formation of bromate and biodegradable organics during the bench-scale O₃ and O₃/H₂O₂ treatment of municipal wastewater effluent. 39 OMPs were fully eliminated, and 22 OMPs were considerably eliminated (54 ± 14%) at an ozone dosage of 0.5 gO₃/gDOC for their high reactivity to ozone or ^•OH. The chemical kinetics approach accurately predicted the OMP elimination levels based on the rate constants and exposures of ozone and ^•OH, where the quantum chemical calculation and group contribution method successfully predicted the ozone and ^•OH rate constants, respectively. Microbial inactivation levels increased with increasing ozone dosage up to ∼3.1 (bacteria) and ∼2.6 (virus) log₁₀ reductions at 0.7 gO₃/gDOC. O₃/H₂O₂ minimized bromate formation but significantly decreased bacteria/virus inactivation, whereas its impact on OMP elimination was insignificant. Ozonation produced biodegradable organics that were removed by a post-biodegradation treatment, achieving up to 24% DOM mineralization. These results can be useful for optimizing O₃ and O₃/H₂O₂ processes for enhanced wastewater treatment.

Comprehensive computational study on reaction mechanism of N-Nitroso dimethyl amine formation from substituted hydrazine derivatives during ozonation

N- Nitrosodimethyl amine, the simplest member of the N-Nitrosamine family, is a carcinogenic and mutagenic agent that has gained considerable research interest owing to its toxic nature. Ozonation of industrially important hydrazines, such as unsymmetrical dimethylhydrazine (UDMH) or monomethylhydrazine (MMH), has been associated with NDMA formation and accumulation in the environment. UDMH/MMH - ozonation also leads to several other transformation products such as acetaldehyde dimethyl hydrazine (ADMH), tetramethyl tetra azene (TMT), diazomethane, methyl diazene, etc, which can be either precursors or competitors for NDMA formation. However, the relevant chemistry detailing the formation of these transformation products from UDMH/MMH -ozone reaction and their subsequent conversion to NDMA is not well understood. In this work, we explored the formation mechanism of ADMH and TMT from UDMH-ozonation and their further oxidation to NDMA using the second-order Moller Plesset perturbation theory employing the 6-311G(d) basis set. We have also investigated how MMH selectively forms methyl diazene and diazomethane under normal conditions and NDMA in the presence of excess ozone. Our calculations indicate that the reactions proceed via an initial H abstraction from the hydrazine -NH₂ group, followed by the oxidation of the generated N-radical species. The formation of ADMH from the UDMH-ozone reaction involves an acetaldehyde intermediate, which then reacts with a second UDMH molecule to generate ADMH. The preferable attack of ozone molecule on N=C bond of ADMH generates DMAN intermediate, which subsequently undergoes oxidation to form NDMA. Unlike other transformation products, TMT formation occurs via the dimerization of DMAN. ¹Though there exists an N=N bond in the TMT, which are preferable attacking sites for ozone, experimental studies show the lower yields of NDMA formation, which corroborates with the high activation barrier required for the process (42 kcal/mol). Overall, our calculated results agree well with the experimental observations and rate constants. Computational calculations bring new insights into the electronic nature and kinetics of the elementary reactions of this pathway, enabled by computed energies of structures that are not possible to access experimentally.

Exploring Br-'s roles on non-brominated NDMA formation during ozonation: Reactive oxygen species contribution and brominated intermediate path validation

Bromide ions (Br^-) affected non-brominated nitroso-dimethylamine (NDMA) formation during ozonation, but the mechanism is still unclear. 1,1,1',1'-tetramethyl-4,4'-(methylene-di-p-phenylene) di-semicarbazide (TMDS) was chosen to further probe this problem. The results indicated that low levels of Br^- (≤20 μM) enhanced NDMA from 3.27 to 7.56 μg/L, while its amount slightly dropped to 6.22 μg/L raising Br^- to 100 μM. It was experimentally verified that intermediates 1,1-dimethylsemicarbazide (DMSC) and 1,1-dimethylhydrazine (UDMH) played important roles on promoting NDMA generation, whose contribution rates were 40.2% and 32.2%, respectively. The brominated substances with higher NDMA molar yields were detected. ∙OH reduced NDMA formation without Br^-, while it played promotion role with Br^-; the corresponding contribution rates were - 26.9% and 29.2%, respectively. No matter with or without Br^-, both ∙O₂^- and ^lO₂ brought a boost to NDMA formation, their contribution ratios were 34.9% and 58.1% without Br^-, while raised significantly to 64.6% and 81.5% when Br^- existed. Br^- not only facilitated NDMA formation, but also benefited the degradation of TMDS. Based on the calculation results and intermediates detected, the influence mechanisms of Br^- were proposed. The results would provide theoretical basis and technical guarantee for treating NDMA precursors and bromide co-existing water in the future.

NDMA formation during ozonation of DMAPA: Influencing factors, mechanisms, and new pathway exploration

Aliphatic amines, common constituents that contribute to dissolved organic nitrogen (DON), can quickly react with ozone due to the lone electron pair on the nitrogen atom and this may produce carcinogen N-Nitrosodimethylamine (NDMA). 3-(Dimethylamino)-1-propylamine (DMAPA) was chosen as a representative to elucidate the NDMA formation characteristics, kinetic rates, reaction pathways, and influencing factors during ozonation in this study. The results demonstrated that NDMA generated directly from DMAPA during ozonation. Moreover, the NDMA yields increased with ozone dosages. The NDMA molar yield increased and then decreased when the pH raised from 5 to 9, achieving the maximum value at pH 8. Hydroxyl radical (∙OH) played a promotional role in NDMA formation, and its scavenger dramatically cut down its yields. Low levels of Br^- facilitated NDMA formation, while the value significantly reduced when Br^- was up to 1 mM. The NDMA amount was slightly raised by NO₂^-, but it was inhibited by NH₄⁺ and NO₃^-. Moreover, it was also depressed by co-existing components in actual lake water. Based on the result of the Gaussian calculation, the LC-MS/MS and GC-MS analysis, four possible transformation pathways were proposed. The radical recombination was verified to be the primary pathway for ozone promoting NDMA formation from DMAPA.

Formation mechanism and control strategies of N-nitrosodimethylamine (NDMA) formation during ozonation

This review summarizes major findings over the last decade related to N-nitrosodimethylamine (NDMA) formed upon ozonation, which was regarded as highly toxic and carcinogenic disinfection by-products. The reaction kinetics, chemical yields and mechanisms were assessed for the ozonation of potential precursors including dimethylamine (DMA), N,N-dimethylsulfamide, hydrazines, N-containing water and wastewater polymers, dyes containing a dimethylamino function, N-functionalized carbon nanotubes, guanidine, and phenylurea. The effects of bromide on the NDMA formation during ozonation of different types of precursors were also discussed. The mechanism for NDMA formation during ozonation of DMA was re-summarized and new perspectives were proposed to assess on this mechanism. Effect of hydroxyl radicals (•OH) on NDMA formation during ozonation was also discussed due to the noticeable oxidation of NDMA by •OH. Surrogate parameters including nitrate formation and UV₂₅₄ after ozonation may be useful parameters to estimate NDMA formation for practical application. The strategies for NDMA formation control were proposed through improving the ozonation process such as ozone/hydrogen peroxide, ozone/peroxymonosulfate and catalytic ozonation process based on membrane pores aeration (MEMBRO₃X).

Ozonation of organic compounds in water and wastewater: A critical review

Ozonation has been applied in water treatment for more than a century, first for disinfection, later for oxidation of inorganic and organic pollutants. In recent years, ozone has been increasingly applied for enhanced municipal wastewater treatment for ecosystem protection and for potable water reuse. These applications triggered significant research efforts on the abatement efficiency of organic contaminants and the ensuing formation of transformation products. This endeavor was accompanied by developments in analytical and computational chemistry, which allowed to improve the mechanistic understanding of ozone reactions. This critical review assesses the challenges of ozonation of impaired water qualities such as wastewaters and provides an up-to-date compilation of the recent kinetic and mechanistic findings of ozone reactions with dissolved organic matter, various functional groups (olefins, aromatic compounds, heterocyclic compounds, aliphatic nitrogen-containing compounds, sulfur-containing compounds, hydrocarbons, carbanions, β-diketones) and antibiotic resistance genes.

N-nitrosodimethylamine formation during oxidation of N,N-dimethylhydrazine compounds by peroxymonosulfate: Kinetics, reactive species, mechanism and influencing factors

This study found that peroxymonosulfate (PMS) oxidation without activation has the potential to generate a suspected human carcinogen, N-nitrosodimethylamine (NDMA), in water containing N,N-dimethylhydrazine compounds. Considerable amounts of NDMA formed from three compounds by PMS oxidation were observed. 1,1,1',1'-Tetramethyl-4,4'-(methylene-di-p-phenylene) disemicarbazide (TMDS), which is an industrial antiyellowing agent and light stabilizer, was used as a representative to elucidate the kinetics, transformation products, mechanism and NDMA formation pathways of PMS oxidation. TMDS degradation and NDMA formation involved direct PMS oxidation and singlet oxygen (¹O₂) oxidation. The oxidation by PMS/¹O₂ was pH-dependent, which was related to the pH-dependent characteristics of the reactive oxygen species and intermediates. The degradation mechanism of TMDS mainly included the side chain cleavage, dealkylation, and O-addition. NDMA was generated from TMDS mainly via O-addition and 1,1-dimethylhydrazine (UDMH) generation. The cleavage of amide nitrogen in O-addition products and primary amine nitrogen in UDMH are likely the key steps in NDMA generation. The results emphasized that the formation of harmful by-products should be taken into account when assessing the feasibility of PMS oxidation.

Changes in levels of N-nitrosamine formed from amine-containing compounds during chloramination via photocatalytic pretreatment with immobilized TiO2: Effect of source water and pH

We investigated the effectiveness of photocatalytic pretreatment (PCP) of precursors in minimizing the formation potentials (FPs) of carcinogenic nitrosamines, including N-nitrosodimethylamine (NDMA), N-nitrosodiethylamine (NDEA), and N-nitrosodiethanolamine (NDELA), during water chloramination. A steel mesh substrate with immobilized TiO₂ was highly efficient at mitigating nitrosamine formation and removing targeted precursors such as ranitidine, nizatidine, trimebutine, triethanolamine, and metoclopramide. Compared to UVC/H₂O₂, PCP under UVA irradiation (intensity of 0.67 mW cm^-2) was more effective for reducing nitrosamine-FPs during post-chloramination. However, the PCP efficacies varied with the water source, pretreatment pH, and irradiation time. For example, PCP of eutrophic water increased the NDMA-FPs, but produced notable reductions (up to 99%) for NDELA- and NDEA-FPs. Shorter irradiation times, up to 15 min, increased the NDELA-FP in triethanolamine, and the NDMA-FP in nizatidine and trimebutine. However, the nitrosamine-FP decreased by > 50% after PCP at a pH > 5.6, following irradiation for 120 min. Oxygen addition, N-de(m)ethylation, and N-dealkylation were responsible for decreasing nitrosamine-FPs via the destruction of key moieties; this has been elucidated by mass spectroscopy. This study suggests that PCP could be used as an alternative strategy for minimizing nitrosamine-FPs during water treatment.

Predictable Liquid Chromatography Quadrupole Time-of-Flight Mass Spectrometry Fragmentation of Ozone-Reactive N-Nitrosodimethylamine Precursors Coupled with In Silico Fragmentation and Ion Mobility-Quadrupole Time-of-Flight Facilitates Their Identification in Sewage

This study investigated the liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF/MS) fragmentation of 10 potent model ozone (O₃)-reactive N-nitrosodimethylamine (NDMA) precursors bearing (CH₃)₂N-N or (CH₃)₂N-(SO₂)-N. Fragments (m/z 61.0766, 60.0688 Da loss, and 72.0688 Da loss) were discovered as pertinent diagnostic fragments for precursors bearing (CH₃)₂N-N, whereas a loss of 108.0119 Da was consistent for precursors bearing (CH₃)₂N-S(O₂)-N. Using the fragments as structural hints on a sewage fraction with a high concentration of O₃-reactive precursors, peaks of precursors sharing m/z 61.0766, a 60.0688 Da loss, or both were flagged. Then, using in silico fragmenters and (CH₃)₂N-N as a substructure filter on online-chemical structure databases, we identified PubChem's compound identifier (PCCID) 141210417 and 1,1,1',1'-tetramethyl-4,4'-(methylene-di-p-phenylene)disemicarbazide (TMDS). TMDS was confirmed using an authentic standard, and ion mobility (IM)-QTOF/MS confirmed its rider peak as PCCID 141210417. PCCID 141210417 is an isomer of TMDS, and its environmental occurrence is associated with technical-grade TMDS and industrial effluents. The estimated contribution of TMDS to the total NDMA formation potential of the sewage fraction was 20-24%, which was suggestive of the significance of PCCID 141210417 and other precursors.

NDMA reduction mechanism of UDMH by O3/PMS technology

Carcinogenic N, N-Dimethylnitrosamine (NDMA) has been reported to generate significantly during ozonation of fuel additive unsymmetrical dimethylhydrazine (UDMH), the combined ozone/Peroxy-Monosulfate (O₃/PMS) technology was tried for reducing its formation in this study. The influence of PMS dosages, ozone concentrations, pH, Br^- and humic acid (HA) on NDMA formation from UDMH were investigated. In addition, the reduction mechanisms were explored by intermediates identification and Gaussian calculation. The results demonstrated that O₃/PMS technology was effective on NDMA reduction, reaching an efficiency of 81% with 80 μM PMS. Higher NDMA reduction rates were achieved by O₃/PMS with increasing pH within the scope of research (from 5 to 9), achieving a maximum of 69.9% at pH 9. The presence of bromide ion facilitated NDMA generation during ozonation, but the reduction efficiency by O₃/PMS slightly improved from 66.3% to 70.6%. The presence of HA reduced NDMA formation in O₃/PMS system. The contribution of SO₄^•- on NDMA reduction accounted for ~64%, which was higher than that of •OH (41.4%); however, its promotion role on conversing UDMH to NDMA was lower than O₃. Therefore, the technology could reduce NDMA formation effectively. In addition, the results of Gaussian calculation manifested that the N atom in -NH₂ group of UDMH was easily attacked not only by •OH but also by O₃, so it is the key path that determines final NDMA formation. This study would provide reference for reducing NDMA formation during ozonation of UDMH-containing water matrixes.

Oxidation of 51 micropollutants during drinking water ozonation: Formation of transformation products and their fate during biological post-filtration

Micropollutants (MP) with varying ozone-reactive moieties were spiked to lake water in the influent of a drinking water pilot plant consisting of an ozonation followed by a biological sand filtration. During ozonation, 227 transformation products (OTPs) from 39 of the spiked 51 MPs were detected after solid phase extraction by liquid chromatography high-resolution mass spectrometry (LC-HRMS/MS). Based on the MS/MS data, tentative molecular structures are proposed. Reaction mechanisms for the formation of a large number of OTPs are suggested by combination of the kinetics of formation and abatement and state-of-the-art knowledge on ozone and hydroxyl radical chemistry. OTPs forming as primary or higher generation products from the oxidation of MPs could be differentiated. However, some expected products from the reactions of ozone with activated aromatic compounds and olefins were not detected with the applied analytical procedure. 187 OTPs were present in the sand filtration in sufficiently high concentrations to elucidate their fate in this treatment step. 35 of these OTPs (19%) were abated in the sand filtration step, most likely due to biodegradation. Only 24 (13%) of the OTPs were abated more efficiently than the parent compounds, with a dependency on the functional group of the parent MPs and OTPs. Overall, this study provides evidence, that the common assumption that OTPs are easily abated in biological post-treatment is not generally valid. Nevertheless, it is unknown how the OTPs, which escaped detection, would have behaved in the biological post-treatment.

NDMA formation during ozonation of metformin: Roles of ozone and hydroxyl radicals

Metformin, a high-consumed pharmaceutical for diabetes, has been reported to generate carcinogenic nitroso-dimethylamine (NDMA) during treatment of its containing wastewater. However, whether it would produce NDMA during ozonation or not is unclear, let alone discriminate roles of ozone (O₃) and hydroxyl radicals (OH). In this paper, effects of ozonation on NDMA formation from metformin were investigated, roles of O₃ and OH were also distinguished by adding tert-butyl alcohol (tBA) as OH scavenger. Moreover, various influencing factors and reaction mechanisms were demonstrated. The results indicated that NDMA could be directly formed from metformin during ozonation, the addition of OH scavenger significantly enhanced its formation (0-46.2 ng/L vs 0-139.1 ng/L). The formation of NDMA by O₃ and OH was more affected by bromide and HCO₃^- than those with only O₃; while the impacts of pH and sulphate on the latter were more notable. No matter without/with tBA in the solution, the formed NDMA during ozonation of metformin increased with raising pH (from 5 to 9) and achieved the maximum 69.6 ng/L and 235.9 ng/L at pH 9, respectively; small amount of bromide (0.1 μM) promoted NDMA production, high levels of bromide (10 μM) inhibited its formation; the existence of HCO₃^- enhanced the amounts of NDMA from 44.5 to 73.5 ng/L (raised by 65.2%) by O₃ and OH and from 102.9 to 130 ng/L with only O₃ (raised by 26.3%); with the addition of sulphate, NDMA concentration raised by 43.8% by O₃ and OH, while the value was high up to 134.6% with only O₃. Based on the result of UPLC-Q-TOF and density functional theory, the oxidation intermediates were identified and possible transformation pathways of metformin during ozonation were proposed. The findings in this paper would provide reference when treating metformin-containing water in future.

Validation of the promotion mechanism between bromide and UDMH to form NDMA during ozonation

Unsymmetrical dimethylhydrazine (UDMH) is found to generate substantial carcinogenic nitroso-dimethylamine (NDMA) during ozonation, moreover, its formation is promoted by ubiquitous bromide ions (Br^-) in water matrixes, but the mechanism is still unclear. In this study, effects of Br^- on NDMA formation during ozonation of UDMH were studied, meanwhile, its promotion pathways were also determined. The results demonstrated that Br^- promoted NDMA formation from UDMH during ozonation, the formation rate constant with Br^- is over 7 times of that without Br^-. NDMA amount raised from 142.5 to 327.5 μg/L when Br^- dosages increased from 0 to 100 μM. No matter with or without Br^-, the augment of O₃ dosages facilitated NDMA formation; the maximum value was achieved at pH 8. NDMA decreased dramatically from 173.8 to 123.5 μg/L with HCO₃^- raising from 0 to 160 μM, while increasing remarkably to 222.5 μg/L with SO₄^2- dosing. In addition, NOM inhibited NDMA formation from UDMH during ozonation. The mass spectrum of LC-MS/MS verified that the generation of Br-UDMH was main cause for promoting NDMA formation. Moreover, hypobromous acid (HBrO) was confirmed to be responsible for Br-UDMH formation. In addition, the position that oxidants and Br^- attacked was demonstrated based on Gaussian calculation. The results of this study could provide theoretical basis for the application of ozonation in bromine-containing water matrixes.

The Occurrence of N-nitrosodimethylamine (NDMA) in Swimming Pools: An Overview

The occurrence of several disinfectant byproducts has been investigated in swimming pools. Until now, there are only a few studies on nitrosamine, particularly N-nitrosodimethylamine in swimming pool water. This could be due to the lack of a suitable method that is sensitive enough for the measurement of N-nitrosodimethylamine in pool waters. Other disinfectant byproducts formed in pool water widely documented are trihalomethanes, haloacetic acids, halonitromethanes, and chloramines but inadequate information on N-nitrosodimethylamine. This paper provides a review of the nitrogenous disinfectant byproduct in swimming pools and its health implications. Anthropogenic substances introduced by swimmers such as sweat, lotions, and urine contribute to the formation of N-nitrosodimethylamine. The reaction of secondary amines such as dimethylamine with mono/dichloroamines produced dimethyl hydrazine and further undergo oxidation to form N-nitrosodimethylamine. The reaction of chlorine and other disinfectants with these anthropogenic sources in swimming pools cause cancer and asthma in human tissues. Thus, the assessment of N-nitrosodimethylamine in the swimming pool is less well documented. Therefore, the health consequences, mutagenic, and genotoxic potentials of N-nitrosodimethylamine should be the focus of more research studies.

Contribution of N,N-dimethylformamide to formation of N-nitrosodimethylamine by chloramination in sewage treatment plants and receiving rivers

The contribution of specific precursors to N-nitrosodimethylamine formation potential (NDMA FP) upon chloramination depends not only on their NDMA molar yields but also on their concentrations in the actual environment. We investigated the seasonal and diurnal patterns of the NDMA precursor N,N-dimethylformamide (DMF) and NDMA FP in the Yodo River basin, Japan, by examining water samples taken from inside the basin's largest sewage treatment plant (STP) as well as samples from five final effluents from four STPs, two main stream sites, and two tributary sites in the same basin. DMF and NDMA FP were found to be high in influent (raw sewage), and were found to be mostly removed during the STP treatment processes (especially with biological treatment). Nevertheless, DMF was found in concentrations of 0.06 to 31.7 µg/L in chlorinated effluents and in receiving rivers, while NDMA FP was detected in concentrations of 3.57 to 306 ng/L. Thus, STPs were shown to be an important source of DMF and NDMA FP to rivers. A strong positive correlation between NDMA FP and DMF was confirmed in the receiving river (K-M), indicating that DMF was an important NDMA precursor in the Yodo River basin. The contribution of DMF to NDMA FP was 15.8±11.2% (n = 4) in summer and 82.1±10.2% (n = 4) in winter in the main stream (site K-M) of the river due to insufficient dilution of chlorinated effluents from the largest STP. From the viewpoint of NDMA and NDMA FP control at downstream sites, monitoring and control of DMF at upstream sites are important.

Degradation Kinetics of Antibiotic Resistance Gene mecA of Methicillin-Resistant Staphylococcus aureus (MRSA) during Water Disinfection with Chlorine, Ozone, and Ultraviolet Light

Degradation kinetics of antibiotic resistance genes (ARGs) by free available chlorine (FAC), ozone (O₃), and UV₂₅₄ light (UV) were investigated in phosphate buffered solutions at pH 7 using a chromosomal ARG (mecA) of methicillin-resistant Staphylococcus aureus (MRSA). For FAC, the degradation rates of extracellular mecA (extra-mecA) were accelerated with increasing FAC exposure, which could be explained by a two-step FAC reaction model. The degradation of extra-mecA by O₃ followed second-order reaction kinetics. The degradation of extra-mecA by UV exhibited tailing kinetics, which could be described by a newly proposed kinetic model considering cyclobutane pyrimidine dimer (CPD) formation, its photoreversal, and irreversible (6-4) photoproduct formation. Measured rate constants for extra-mecA increased linearly with amplicon length for FAC and O₃, or with number of intrastrand pyrimidine doublets for UV, which enabled prediction of degradation rate constants of extra-mecA amplicons based on sequence length and/or composition. In comparison to those of extra-mecA, the observed degradation rates of intracellular mecA (intra-mecA) were faster for FAC and O₃ at low oxidant exposures but significantly slower at high exposures for FAC and UV. Differences in observed extra- and intracellular kinetics could be due to decreased DNA recovery efficiency and/or the presence of MRSA aggregates protected from disinfectants.

An Organic Chemist's Guide to N-Nitrosamines: Their Structure, Reactivity, and Role as Contaminants

N-Nitrosamines are a class of compounds notorious both for the potent carcinogenicity of many of its members and for their widespread occurrence throughout the human environment, from air and water to our diets and drugs. Considerable effort has been dedicated to understanding N-nitrosamines as contaminants, and methods for their prevention, remediation, and detection are ongoing challenges. Understanding the chemistry of N-nitrosamines will be key to addressing these challenges. To facilitate such understanding, we focus in this Perspective on the structure, reactivity, and synthetic applications of N-nitrosamines with an emphasis on alkyl N-nitrosamines. The role of N-nitrosamines as water contaminants and the methods for their detection are also discussed.

Synergistic effects of combining ozonation, ceramic membrane filtration and biologically active carbon filtration for wastewater reclamation

In this study, we proposed to apply an integrated process which is comprised of in situ ozonation, ceramic membrane filtration (CMF) and biologically active carbon (BAC) filtration to wastewater reclamation for indirect potable reuse purpose. A pilot-scale (20 m³/d) experiment had been run for ten months to validate the prospect of the process in terms of treatment performance and operational stability. Results showed that the in situ O₃ + CMF + BAC process performed well in pollutant removal, with chemical oxygen demand, ammonia, nitrate nitrogen, total phosphorus and turbidity levels in the treated water being 5.1 ± 0.9, 0.05 ± 0.01, 10.5 ± 0.8, <0.06 mg/L, and <0.10 NTU, respectively. Most detected trace organic compounds were degraded by>96%. This study demonstrated that synergistic effects existed in the in situ O₃ + CMF + BAC process. Compared to pre-ozonation, in situ ozonation in the membrane tank was more effective in controlling membrane fouling (maintaining operational stability) and in degrading organic pollutants, which could be attributed to the higher residual ozone concentration in the tank. Because of the removal of particulate matter by CMF, water head loss of the BAC filter increased slowly and prolonged the backwashing interval to 30 days. BAC filtration was also effective in removing ammonia and N-nitrosodimethylamine from the ozonated water.

Reversible photochromic photocatalyst Bi2O3/TiO2/Al2O3 with enhanced visible photoactivity: application toward UDMH degradation in wastewater

1,1-Dimethylhydrazine (UDMH) and its by-products were considered carcinogenic toxins and represent a serious health hazard to the population once present in water under natural conditions without treatment. The conventional degradation method suffers from incomplete removal of intermediate products (especially N-nitrosodimethylamine (NDMA)), the powdery catalysis being difficult to recover and results in high energy consumption. In this study, a series of Bi₂O₃/TiO₂/Al₂O₃ (BTA) photocatalysts have been successfully synthesized by a simple dry mixing method with powder material followed by their immobilization. It was evaluated by the photocatalytic degradation of UDMH present in wastewater, which can be recovered by rapid filtration and utilizes only solar energy. The catalyst exhibited markedly enhanced photocatalytic activity for the degradation of UDMH wastewater compared with conventional TiO₂/Al₂O₃ (TA) catalysts under UV, visible and solar irradiation. Besides, the intermediate NDMA was gradually completely degraded. The photocatalysts were extensively characterized using scanning electron microscopy, energy dispersive spectrometry, specific surface area (BET), X-ray diffraction, X-ray photoelectron spectroscopy, UV-visible diffuse reflectance spectroscopy and photo-electrochemical I-t curves evaluation. The results revealed that all the BTA composites exhibited high stability and stronger absorbance in visible light. In addition, the BTA exhibited a reversible photochromic property that can effectively expand the range of light absorption and enhance the photocatalytic activity. The reversible photochromic properties of BTA explained in the proposed mechanism model are expected to be useful for detecting and sensing UDMH or other organic contaminants.

Elimination efficiency of organic UV filters during ozonation and UV/H2O2 treatment of drinking water and wastewater effluent

The efficiency of elimination of organic UV filters by ozonation and UV_254nm/H₂O₂ processes was assessed and predicted in simulated treatments of sewage-impaired drinking water and wastewater effluent in bench-scale experiments. Second-order rate constants (k) for the reactions of the eight UV filters with ozone and OH were determined by quantum chemical calculations and competition kinetics methods, respectively. The UV filters containing phenolic (ethylhexyl-salicylate, homosalate, and benzophenone-3) and olefinic moieties (4-methylbenzylidene-camphor, benzyl-cinnamate, and 2-ethylhexyl-4-methoxycinnamate) showed high ozone reactivity (k ≥ 8 × 10⁴ M^-1s^-1 at pH 7), while those without such electron-rich moieties (isoamyl-benzoate and benzophenone) were ozone-refractory. All the UV filters showed high OH reactivity (k ≥ 6.2 × 10⁹ M^-1s^-1). In concordance with the rate constant information, the phenolic and olefinic UV filters were efficiently eliminated by ozone treatment, requiring specific ozone doses of <0.5 mgO₃/mgDOC for ∼100% elimination. The UV filters were eliminated by ≤ 38% at a UV fluence of 1500 mJ/cm² in the UV_254nm-only treatment. Rapid photoisomerisation between the E and Z geometric isomers was observed for the olefinic UV filter, benzyl-cinnamate. The addition of H₂O₂ (10 mg/L) greatly enhanced the elimination of all UV filters, indicating that OH was the main contributor to their elimination in the UV_254nm/H₂O₂ treatment. A chemical kinetics approach developed previously for ozonation and UV/H₂O₂ processes was shown to predict the elimination of the UV filters in the tested water matrices reasonably well, demonstrating that the chemical kinetics method can be used for a priori prediction of micropollutant elimination in oxidative treatment processes for potable reuse of municipal wastewater effluents.

Investigation on the compositions of unsymmetrical dimethylhydrazine treatment with different oxidants using solid-phase micro-extraction-gas chromatography-mass spectrometer

The majority of unsymmetrical dimethylhydrazine (UDMH) treatments produce lots of toxic by-products, among which N-Nitrosodimethylamine (NDMA) is a strong carcinogen. The compositions of the by-products are important for evaluating the treatment efficiency and understanding the UDMH degradation mechanism to achieve UDMH mineralization. The intermediate and end products of UDMH treatment with different oxidants were investigated by using a simple and fast method, solid-phase micro-extraction (SPME) in combination with gas chromatography-mass spectrometry (GC-MS). The effects of several parameters (coating fibre, salt addition, pH, sampling time and desorption time) were studied to optimize analyte recovery. The best response can be attained by the 65 µm PDMS/DVB fibre at pH 7 during 10 min after desorption of 1 min in the GC inlet. The intermediate and final oxidative products of UDMH wastewater treatment with different oxidants (O₃, Mn²⁺/O₃, Fe²⁺/H₂O₂) were investigated. The results showed that the UDMH treatment with O₃ could lead to high yields of NDMA. Metal catalytic ozonation could largely minimize the formation of NDMA. No NDMA was observed in the final decontaminated samples after treatment with Fe²⁺/H₂O₂. The NDMA formation and degradation mechanism were discussed based on the intermediates. This study is expected to provide useful information for controlling NDMA formation during UDMH wastewater treatment.

NDMA formation mechanisms from typical hydrazines and hydrazones during ozonation: A computational study

N-nitrosodimethylamine (NDMA) as the most frequently detected disinfection by-product has aroused widespread concern due to its unusually high carcinogenicity, however, there is still limited understanding of its formation mechanisms. In this study, the formation mechanisms of NDMA from some typical hydrazines and hydrazones with high NDMA conversion yields (60%∼90%) during ozonation, i.e., unsymmetrical dimethylhydrazine (UDMH), 1-formyl-2,2-dimethylhydrazine (FDMH), formaldehyde dimethylhydrazone (FDH) and acetone dimethylhydrazone (ADMH), were investigated by using DFT method with the M05 functional. A new NDMA formation mechanism from hydrazines during ozonation was proposed, in which the initial step is hydrogen abstraction rather than previously reported oxygen addition. For hydrazones, the C atom of the -N = C moiety in hydrazones is preferred to be attacked by ozone to generate N,N-dimethylaminonitrene (DMAN), which is an important intermediate in NDMA formation during ozonation. Moreover, the reactivity order of the following H atoms in hydrogen/hydride ion abstraction (HA) by ozone is -NH₂ > -N(CH₃)₂ > -CO-NH ∼ =C(CH₃)₂ > =CH-. Additionally, formation pathways of some experimentally detected compounds, i.e., HOOOH, HOOH and HCOH, in the ozonation of hydrazine were elucidated in this study. The results are expected to expand our understanding of NDMA formation mechanisms and ozone reaction characteristics.

NDMA formation from 4,4'-hexamethylenebis (HDMS) during ozonation: influencing factors and mechanisms

N-nitrosodimethylamine (NDMA), a toxic disinfection byproduct commonly associated with chloramination, has recently been found to form from an anti-yellowing agent (4,4'-hexamethylenebis (1,1-dimethylsemicarbazide) (HDMS)) during ozonation but the mechanisms are unclear. In this paper, the potential roles of molecular ozone (O₃) and hydroxyl radical (∙OH) on NDMA formation from HDMS were investigated under various oxidation conditions (ozone dosages, pH) and different components in water (bromide ion (Br^-), bicarbonate ion (HCO₃^-), sulfate ion (SO₄^2-), and humic acid (HA), as well as natural organic matter (NOM) from a lake). Moreover, HDMS transformation pathways by ozonation were determined. The results indicated that the formation of NDMA was enhanced through the combined effect of O₃ and ∙OH compared to that by O₃ alone (addition of tert-butyl alcohol (tBA) as ∙OH scavenger). ∙OH itself cannot generate NDMA directly; however, it can transform HDMS to intermediates with higher NDMA yield than parent compound. The NDMA generation was affected (small dosages promoted but high dosages inhibited) by HA or Br^- no matter with or without tBA. The presence of SO₄^2- and HCO₃^- ions lowered NDMA formation through ∙OH scavenging effect. Increasing pH not only increased degradation rate constant by enhancing ∙OH generation but also affected HDMS dissociation ratio, reaching the maximum NDMA formation at pH 7-8. Natural constituents in selected water matrix inhibited NDMA formation. Impacts of these influencing factors on NDMA formation by only O₃ however were significantly less pronounced over that by the joint roles of O₃ and ∙OH. Based on the result of Q-TOF, LC/MS/MS, and GC/MS, the possible transformation pathways of HDMS by ozonation were proposed. The NDMA enhancement mechanism by the combined effect of O₃ and ∙OH can be attributed to greater amounts of intermediates with higher NDMA yield (such as unsymmetrical dimethylhydrazine (UDMH)) produced. These findings provide new understanding of NDMA formation upon ozonation of typical amine-based compounds.

Products and Mechanistic Investigations on the Reactions of Hydrazines with Ozone in Gas-Phase

The toxic transformation products of hydrazines are of great concern. These products’ properties combined with their formation mechanisms are needed to assess their potential environmental and human impacts. In this study, the gas-phase reaction of hydrazine (N2H4), monomethyldrazine (MMH) and unsymmetrical dimethyhydrazine (UDMH) with O3 have been studied at varying reactant ratios, both in the presence and absence of a radical trap. Gas chromatography-mass spectroscopy (GC-MS) has been implied to follow reactant consumption and product formation. Apart from the reported products detected by Fourier transform infrared spectroscopy (FT-IR), the newly found compounds (hydrazones, formamides, dimethylamine, 1,1,4,4-tetramethyl-1,2-tetrazene,dimethylamino-acetonitrile, N2, H2O, et al.) are identified by GC-MS. The relative yields of the organic products vary considerably at different O3/MMH or UDMH ratios. UDMH and MMH are confirmed as high potential precursors of N-nitrosodimethylamine (NDMA). The presence of hydroxyl radicals (HO·) hinders NDMA formation in MMH-O3 system. Meanwhile, it increases NDMA formation in UDMH-O3 system. The suggested reaction mechanisms which account for the observed products are discussed.

Evaluation of a full-scale wastewater treatment plant upgraded with ozonation and biological post-treatments: Abatement of micropollutants, formation of transformation products and oxidation by-products

To protect the ecosystem and drinking water resources in Switzerland and in the countries of the downstream catchments, a new Swiss water protection act entered into force in 2016 aiming to reduce the discharge of micropollutants from wastewater treatment plants (WWTPs). As a consequence, selected WWTPs must be upgraded by an advanced treatment for micropollutant abatement with suitable and economic options such as (powdered) activated carbon treatment or ozonation. WWTP Neugut (105'000 people equivalent) was the first WWTP in Switzerland to implement a long-term full-scale ozonation. Differing specific ozone doses in the range of 0.35-0.97 g O₃/g DOC were applied to determine the adequate ozone dose to fulfill the requirements of the Swiss water protection act. Based on this assessment, a specific ozone dose of 0.55 g O₃/g DOC is recommended at this plant to ensure an average abatement of the twelve selected indicator substances by ≥80% over the whole treatment. A monitoring of 550 substances confirmed that this dose was very efficient to abate a broad range of micropollutants by >79% on average. After ozonation, an additional biological post-treatment is required to eliminate possible negative ecotoxicological effects generated during ozonation caused by biodegradable ozonation transformation products (OTPs) and oxidation by-products (OBPs). Three biological treatments (sand filtration, moving bed, fixed bed) and granular activated carbon (GAC, fresh and pre-loaded) filtration were evaluated as post-treatments after ozonation. In parallel, a fresh GAC filter directly connected to the effluent of the secondary clarifier was assessed. Among the three purely biological post-treatments, the sand filtration performed best in terms of removal of dissolved organic carbon (DOC), assimilable organic carbon (AOC) and total suspended solids (TSS). The fresh activated carbon filtration ensured a significant additional micropollutants abatement after ozonation due to sorption. The relative abatement of the indicator substances ranged between 20 and 89% after 27'000 bed volumes (BV) and was still substantial at 50'000 BV. In an identical GAC filter running in parallel and being fed with the effluent of the secondary clarifier, the elimination was less efficient. Seven primary OTPs (chlorothiazide and six N-oxides) formed during ozonation could be quantified thanks to available reference standards. Their concentration decreased with increasing specific ozone doses with the concomitant formation of other OTPs. The seven OTPs were found to be stable compounds and were not abated in the biological post-treatments. They were sorbed in the fresh GAC filter, but less efficiently than the corresponding parent compounds. Two OBPs, bromate (BrO₃^-) and N-nitrosodimethylamine (NDMA), were formed during ozonation but did not exceeded 5 μg/L for bromate and 30 ng/L for NDMA at the recommended specific ozone dose of 0.55 g O₃/g DOC. NDMA was well abated in all post-treatments (minimum 41% during fixed bed filtration, maximum 83% during fresh GAC filtration), while bromate was very stable as expected.

N-Nitrosodimethylamine (NDMA) and its precursors in water and wastewater: A review on formation and removal

This review summarizes major findings over the last decade related to N-Nitrosodimethylamine (NDMA) in water and wastewater. In particular, the review is focused on the removal of NDMA and of its precursors by conventional and advanced water and wastewater treatment processes. New information regarding formation mechanisms and precursors are discussed as well. NDMA precursors are generally of anthropogenic origin and their main source in water have been recognized to be wastewater discharges. Chloramination is the most common process that results in formation of NDMA during water and wastewater treatment. However, ozonation of wastewater or highly contaminated surface water can also generate significant levels of NDMA. Thus, NDMA formation control and remediation has become of increasing interest, particularly during treatment of wastewater-impacted water and during potable reuse application. NDMA formation has also been associated with the use of quaternary amine-based coagulants and anion exchange resins. UV photolysis with UV fluence far higher than typical disinfection doses is generally considered the most efficient technology for NDMA mitigation. However, recent studies on the optimization of biological processes offer a potentially lower-energy solution. Options for NDMA control include attenuation of precursor materials through physical removal, biological treatment, and/or deactivation by application of oxidants. Nevertheless, NDMA precursor identification and removal can be challenging and additional research and optimization is needed. As municipal wastewater becomes increasingly used as a source water for drinking, NDMA formation and mitigation strategies will become increasingly more important. The following review provides a summary of the most recent information available.

Surrogate-Based Correlation Models in View of Real-Time Control of Ozonation of Secondary Treated Municipal Wastewater-Model Development and Dynamic Validation

New robust correlation models for real-time monitoring and control of trace organic contaminant (TrOC) removal by ozonation are presented, based on UVA₂₅₄ and fluorescence surrogates, and developed considering kinetic information. The abatement patterns of TrOCs had inflected shapes, controlled by the reactivity of TrOCs toward ozone and HO^• radicals. These novel and generic correlation models will be of importance for WRRF operators to reduce operational costs and minimize byproduct formation. Both UVA₂₅₄ and fluorescence surrogates could be used to control ΔTrOC, although fluorescence measurements indicated a slightly better reproducibility and an enlarged control range. The generic framework was validated for several WRRFs and correlations for any compound with known kinetic information could be developed solely using the second order reaction rate constant with ozone (k_O3). Two distinct reaction phases were defined for which separate linear correlations were obtained. The first was mainly ozone controlled, while the second phase was more related to HO^• reactions. Furthermore, parallel factor analysis of the fluorescence spectra enabled monitoring of multiple types of organic matter with different O₃ and HO^• reactivity. This knowledge is of value for kinetic modeling frameworks and for achieving a better understanding of the occurring changes of organic matter during ozonation.

Enhanced treatment of secondary municipal wastewater effluent: comparing (biological) filtration and ozonation in view of micropollutant removal, unselective effluent toxicity, and the potential for real-time control

Ozonation and three (biological) filtration techniques (trickling filtration (TF), slow sand filtration (SSF) and biological activated carbon (BAC) filtration) have been evaluated in different combinations as tertiary treatment for municipal wastewater effluent. The removal of 18 multi-class pharmaceuticals, as model trace organic contaminants (TrOCs), has been studied. (Biological) activated carbon filtration could reduce the amount of TrOCs significantly (>99%) but is cost-intensive for full-scale applications. Filtration techniques mainly depending on biodegradation mechanisms (TF and SSF) are found to be inefficient for TrOCs removal as a stand alone technique. Ozonation resulted in 90% removal of the total amount of quantified TrOCs, but a post-ozonation step is needed to cope with an increased unselective toxicity. SSF following ozonation showed to be the only technique able to reduce the unselective toxicity to the same level as before ozonation. In view of process control, innovative correlation models developed for the monitoring and control of TrOC removal during ozonation, are verified for their applicability during ozonation in combination with TF, SSF or BAC. Particularly for the poorly ozone reactive TrOCs, statistically significant models were obtained that correlate TrOC removal and reduction in UVA₂₅₄ as an online measured surrogate parameter.