Influence of the order of reagent addition on NDMA formation during chloramination

Adsorption of typical NDMA precursors by superfine powdered activated carbon: Critical role of particle size reduction

Control of N-nitrosodimethylamine (NDMA) in drinking water could be achieved by removing its precursors as one practical way. Herein, superfine powdered activated carbons with a diameter of about 1 µm (SPACs) were successfully prepared by grinding powdered activated carbon (PAC, D50=24.3 µm) and applied to remove model NDMA precursors, i.e. ranitidine (RAN) and nizatidine (NIZ). Results from grain diameter experiments demonstrated that the absorption velocity increased dramatically with decreasing particle size, and the maximum increase in k2 was 26.8-folds for RAN and 33.4-folds for NIZ. Moreover, kinetic experiments explained that rapid absorption could be attributed to the acceleration of intraparticle diffusion due to the shortening of the diffusion path. Furthermore, performance comparison experiments suggested that the removal of RAN and NIZ (C0=0.5 mg/L) could reach 61.3% and 60%, respectively, within 5 min, when the dosage of SAPC-1.1 (D50=1.1 µm) was merely 5 mg/L, while PAC-24.3 could only eliminate 17.5% and 18.6%. The adsorption isotherm was well defined by Langmuir isotherm model, indicating that the adsorption of RAN/NIZ was a monolayer coverage process. The adsorption of RAN or NIZ by SAPC-1.1 and PAC-24.3 was strongly pH dependent, and high adsorption capacity could be observed under the condition of pH > pka+1. The coexistence of humic acid (HA) had no significant effect on the adsorption performance because RAN/NIZ may be coupled with HA and removed simultaneously. The coexistence of anions had little effect on the adsorption also. This study is expected to provide an alternative strategy for drinking water safety triggered by NDMA.

Continuous Monitoring of Monochloramine in Water, and Its Distinction from Free Chlorine and Dichloramine Using a Functionalized Graphene-Based Array of Chemiresistors

Monochloramine (MCA) is commonly added to drinking water as a disinfectant to prevent pathogen growth. The generation of MCA at the treatment plant requires tight control over both pH and the ratio of free chlorine (FC) to ammonia to avoid forming undesirable byproducts such as dichloramine (DCA) and trichloramine (TCA), which can impart odor and toxicity to the water. Therefore, continuous monitoring of MCA is essential to ensuring drinking water quality. Currently, standard colorimetric methods to measure MCA rely on the use of reagents and are unsuitable for online monitoring. In addition, other oxidants can interfere with MCA measurement. Here, we present a solid-state, reagent-free MCA sensing method using an array of few-layer graphene (FLG) chemiresistors. The array consists of exfoliated FLG chemiresistors functionalized with specific redox-active molecules that have differential responses to MCA, FC, and DCA over a range of concentrations. Chemometric methods were employed to separate the analytes' responses and to generate multivariate calibration for quantification. A minimum of three sensors are required in the array to maintain full functionality. The array has been demonstrated to quantify MCA in buffered and tap water as a low-cost, reagent-free approach to continuous monitoring.

Complex impact of metals on the fate of disinfection by-products in drinking water pipelines: A systematic review

Metals in the drinking water distribution system (DWDS) play an important role on the fate of disinfection by-products (DBPs). They can increase the formation of DBPs through several mechanisms, such as enhancing the proportion of reactive halogen species (RHS), catalysing the reaction between natural organic matter (NOM) and RHS through complexation, or by increasing the conversion of NOM into DBP precursors. This review comprehensively summarizes these complex processes, focusing on the most important metals (copper, iron, manganese) in DWDS and their impact on various DBPs. It organizes the dispersed 'metals-DBPs' experimental results into an easily accessible content structure and presents their underlying common or unique mechanisms. Furthermore, the practically valuable application directions of these research findings were analysed, including the toxicity changes of DBPs in DWDS under the influence of metals and the potential enhancement of generalization in DBP model research by the introduction of metals. Overall, this review revealed that the metal environment within DWDS is a crucial factor influencing DBP levels in tap water.

Revisiting the chloramination of phenolic compounds: Formation of novel high-molecular-weight nitrogenous disinfection byproducts

Disinfection is critical for ensuring water safety; however, the potential risks posed by disinfection byproducts (DBPs) have raised public concern. Previous studies have largely focused on low-molecular-weight DBPs with one or two carbon atoms, leaving the formation of high-molecular-weight DBPs (HMW DBPs, with more than two carbon atoms) less understood. This study explores the formation of HMW DBPs during the chloramination of phenolic compounds using a novel approach that combines high-resolution mass spectrometry with density functional theory (DFT) calculations. For the first time, we identified nearly 100 previously unreported HMW nitrogenous DBPs (N-DBPs), with nearly half of those being halogenated N-DBPs. These N-DBPs were tentatively identified as heterocyclic (e.g., pyrrole and pyridine analogs) and coupling heterocyclic N-DBPs. Through detailed structure analysis and DFT calculations, the key formation steps of heterocyclic N-DBPs (monochloramine-mediated ring-opening reactions of halobenzoquinones) and new bonding mechanisms (C-N, C-O, and C-C bonding) of the coupling heterocyclic N-DBPs were elucidated. The selective formation of these novel N-DBPs was significantly influenced by factors such as contact time, monochloramine dosage, pH, and bromide concentration. Our findings emphasize the occurrence of diverse HMW heterocyclic N-DBPs, which are likely toxicologically significant, underscoring the need for further research to evaluate and mitigate their potential health risks in water disinfection.

N-Nitrosodimethylamine formation from anthropogenic nitrogenous compounds during preozonation and post-chloramination with characteristic low treatment dose

One effective option to minimize N-nitrosodimethylamine (NDMA) in finished drinking water is to identify and control its precursors. However, previous works to identify significant precursors use formation potential (FP) tests using high doses to assure the maximum NDMA formation rather than the NDMA formation in finished waters. In this study, we applied characteristic low treatment doses of ozone (O3)-to-dissolved organic carbon (DOC) of target compounds of 0.8 mg/mg and NH2Cl of 2.5 ± 0.2 mg Cl2/L to evaluate the NDMAFP yields of organic compounds bearing N,N-dimethylamine (DMA) and N,N-dimethylhydrazine (DMH) during preozonation and post-chloramination. The results in pH-buffered Milli-Q water showed a significant decrease from ≤ 52% to non-detectable levels in the O3-NDMAFP yields of O3-reactive precursors (i.e., DMH-like compounds) after preozonation and post-chloramination. Similarly, a significant decrease from 0.5 to 12% to nonquantifiable levels was observed for the NH2Cl-NDMAFP yields of NH2Cl-reactive precursors; however, the NH2Cl-NDMAFP yields of N,N-dimethylbenzylamine (DMBzA)-like compounds only decreased from ~ 110 to ≤ 43%, suggesting that these compounds could contribute to NH2Cl-NDMAFPs even after preozonation. The effect of the matrix in sewage-effluent and lake water samples varied and was specific for precursors; for example, the O3-NDMAFP yield of 1,1,1',1'-tetramethyl-4,4'-(methylene-di-p-phenylene) disemicarbazide (TMDS), an important O3-reactive NDMA precursor, did not significantly decrease when tested in sewage-effluent samples. Based on the previous occurrence concentration of TMDS in sewage samples, we estimated an NDMAFP of ~ 315 ng/L. This estimate exceeds the guidance concentrations of NDMA (3-100 ng/L), highlighting the importance of TMDS and its related compounds for NDMA formation.

Kinetics of chlorine and chloramine reactions in reverse osmosis permeate and their impact on radical formation during UV/chlorine advanced oxidation for potable reuse

Knowledge of the speciation of chlorine and chloramines in reverse osmosis (RO) permeate is needed to estimate the performance (i.e., pollutant log reduction) of subsequent UV/chlorine advanced oxidation processes (AOPs). To accurately predict the speciation, a previously reported breakpoint chlorination kinetic model was experimentally validated for pH 5.5 and reaction times < 3 min and used to predict the kinetics of breakpoint chlorination in RO permeate. The predictions showed that eliminating chloramines by adding chlorine at a dose beyond the chlorine-to-nitrogen (Cl/N) breakpoint ratio is not practical due to the high breakpoint Cl/N ratio for RO permeate (∼3.0 molar ratio) and an estimated > 40 min reaction time. The conversion from monochloramine (NH2Cl) to dichloramine (NHCl2) is the major process involved, and either or both free chlorine and chloramines may be the major species present, depending on the Cl/N ratio. Model simulations showed that increasing the oxidant dose may not always enhance the performance of UV/chlor(am)ine in RO permeate, due to the need for a low free chlorine dose for optimal •OH exposure in RO permeate. Further UV/AOPs modelling showed that it is important to control the NH2Cl concentration to improve the UV/AOP performance in RO permeate, which may be achieved by extending the reaction time after chlorine is added or increasing the applied Cl/N ratio (e.g., increasing chlorine dose). However, these measures only enhance the pollutant percentage removal by about 5 % under the conditions modelled. A simulation tool was developed and is provided to predict the speciation of chlor(am)ine in RO permeate.

Minimizing N-Nitrosodimethylamine Formation During Disinfection of Blended Seawater and Wastewater Effluent

Augmenting seawater with wastewater has the potential to reduce the energy demand and environmental impacts associated with seawater desalination. Alternatively, as wastewater reuse becomes more widespread, augmenting wastewater with seawater can increase the available water supply. However, the chemistry of disinfecting a blended stream has not been explored. Toxic byproducts, including N-nitrosodimethylamine (NDMA), are expected to form during disinfection, and the extent of formation will likely be a function of which stream is chlorinated and whether disinfection happens before or after blending. In this work, three blending-disinfection scenarios were modeled and experimentally evaluated in bench-scale systems treating synthetic and authentic waters. Modeling results suggested that chlorinating preblended wastewater and seawater would produce the most NDMA because it yielded the highest concentrations of bromochloramine, which was previously found to promote NDMA formation. However, chlorinating wastewater prior to blending with seawater, which modeling indicated would form the most dichloramine, produced the most NDMA in experiments. When seawater was disinfected prior to blending with wastewater, bromide likely converted most chlorine to free bromine. Bromamines formed after blending, however, did not lead to an elevated level of NDMA formation. Therefore, to minimize NDMA formation when disinfecting blended wastewater-seawater, seawater should be disinfected prior to introducing wastewater.

Closing Dichloramine Decomposition Nitrogen and Oxygen Mass Balances: Relative Importance of End-Products from the Reactive Nitrogen Species Pathway

In drinking water chloramination, monochloramine autodecomposition occurs in the presence of excess free ammonia through dichloramine, the decay of which was implicated in N-nitrosodimethylamine (NDMA) formation by (i) dichloramine hydrolysis to nitroxyl which reacts with itself to nitrous oxide (N2O), (ii) nitroxyl reaction with dissolved oxygen (DO) to peroxynitrite or mono/dichloramine to nitrogen gas (N2), and (iii) peroxynitrite reaction with total dimethylamine (TOTDMA) to NDMA or decomposition to nitrite/nitrate. Here, the yields of nitrogen and oxygen-containing end-products were quantified at pH 9 from NHCl2 decomposition at 200, 400, or 800 μeq Cl2·L-1 with and without 10 μM-N TOTDMA under ambient DO (∼500 μM-O) and, to limit peroxynitrite formation, low DO (≤40 μM-O). Without TOTDMA, the sum of free ammonia, monochloramine, dichloramine, N2, N2O, nitrite, and nitrate indicated nitrogen recoveries ±95% confidence intervals were not significantly different under ambient (90 ± 6%) and low (93 ± 7%) DO. With TOTDMA, nitrogen recoveries were less under ambient (82 ± 5%) than low (97 ± 7%) DO. Oxygen recoveries under ambient DO were 88-97%, and the so-called unidentified product of dichloramine decomposition formed at about three-fold greater concentration under ambient compared to low DO, like NDMA, consistent with a DO limitation. Unidentified product formation stemmed from peroxynitrite decomposition products reacting with mono/dichloramine. For a 2:2:1 nitrogen/oxygen/chlorine atom ratio and its estimated molar absorptivity, unidentified product inclusion with uncertainty may close oxygen recoveries and increase nitrogen recoveries to 98% (ambient DO) and 100% (low DO).

Chloramination of Nitromethane: Incomplete Chlorination and Unexpected Substitution Reaction

Ozone is commonly used as a predisinfectant in potable water reuse treatment trains. Nitromethane was recently found as a ubiquitous ozone byproduct in wastewater, and the key intermediate toward chloropicrin during subsequent secondary disinfection of ozonated wastewater effluent with chlorine. However, many utilities have switched from free chlorine to chloramines as a secondary disinfectant. The reaction mechanism and kinetics of nitromethane transformation by chloramines, unlike those for free chlorine, are unknown. In this work, the kinetics, mechanism, and products of nitromethane chloramination were studied. The expected principal product was chloropicrin, because chloramines are commonly assumed to react similarly to, although more slowly than, free chlorine. Different molar yields of chloropicrin were observed under acidic, neutral, and basic conditions, and surprisingly, transformation products other than chloropicrin were found. Monochloronitromethane and dichloronitromethane were detected at basic pH, and the mass balance was initially poor at neutral pH. Much of the missing mass was later attributed to nitrate formation, from a newly identified pathway involving monochloramine reacting as a nucleophile rather than a halogenating agent, through a presumed SN2 mechanism. The study indicates that nitromethane chloramination, unlike chlorination, is likely to produce a range of products, whose speciation is a function of pH and reaction time.

Assess the formation of disinfection by-products from pyrogenic dissolved organic matter (pyDOM): impact of wildfire on the water quality of forest watershed

Wildfires can release pyrogenic dissolved organic matter (pyDOM) into the forest watershed, which may pose challenges for water treatment operations downstream due to the formation of disinfection by-products (DBPs). In this study, we systematically assessed the physio-chemical properties of pyDOM (e.g., electron-donating and -accepting capacities; EDC and EAC) and their contributions to DBP formation under different disinfection scenarios using (1) ten lab samples produced from various feedstocks and pyrolysis temperatures, and (2) pre- and post-fire field samples with different burning severities. A comprehensive suite of DBPs-four trihalomethanes (THMs), nine haloacetic acids (HAAs), and seven N-nitrosamines-were included. The formations of THM and HAA showed an up to 5.7- and 8.9-fold decrease as the pyrolysis temperature increased, while the formation of N-nitrosamines exhibited an up to 6.6-fold increase for the laboratory-derived pyDOM. These results were supported by field pyDOM samples, where the post-fire samples consistently showed a higher level of N-nitrosamine formation (i.e., up to 5.3-fold), but lower THMs and HAAs compared to the pre-fire samples. To mimic environmental reducing conditions, two field samples were further reduced electrochemically and compared with Suwannee River natural organic matter (SRNOM) to evaluate their DBP formation. We found increased DBP formation in pyDOM samples following electrochemical reduction but not for SRNOM, which showed increased N-nitrosamines but decreased THMs and HAAs post-electrochemical reduction. Furthermore, this study reported for the first time the formation of two previously overlooked N-nitrosamines (i.e., nitrosodiethylamine (NDEA), N-nitrosodi-n-propylamine (NDPA)) in both laboratory and field pyDOM samples, raising concerns for drinking water safety given their higher toxicity as compared to the regulated counterparts. Results from this study provide new insights for DBP mitigation during post-fire recovery, which are particularly relevant to communities that rely on forest watersheds as their drinking water sources.

Critical Role of Trichloramine Interaction with Dichloramine for N-Nitrosamine Formation during Breakpoint Chlorination

Breakpoint chlorination is prevalent in drinking water and potable reuse water treatment. Breakpoint chlorination enhances the formation of N-nitrosamines through reactions that form nitrosating agents. The most recent study suggests that nitroxyl (HNO) can react with free chlorine (HOCl) to form the nitrosyl chloride (ClNO) nitrosating agent but has not experimentally verified its importance in breakpoint chlorination. This study first assessed the formation of N-nitrosamines from model N-chloro-alkylamine precursors when they were added to a mixture of HOCl and HNO-derived nitrosating agents generated by chlorinating hydroxyurea. Results demonstrated negligible N-nitrosamine formation. Instead, we observed that the interaction of NCl3 with NHCl2 (total Cl2/total N molar ratio = 2.4-3:1) produced an intermediate capable of nitrosating N-chloro-alkylamines to N-nitrosamines at yields 8-fold higher to those observed in NHCl2 treatment alone, within a very short timescale (<3 min). We examined the stoichiometry of the reaction of NCl3 with NHCl2 using a UV-spectrum-based approach. Nitrosyl chloride was proposed as the key intermediate, likely formed alongside the reformation of NHCl2. Further isotopic experiments, byproduct measurements, and kinetic modeling supported the hypotheses. Modeling indicated that the reaction of NCl3 with NHCl2 explained ∼75% of NDMA formation during breakpoint chlorination. Because NCl3 is mainly derived from the reaction of HOCl with NHCl2, controlling NHCl2 (e.g., with additional treatment) is critical for minimizing nitrosamine formation in waters where breakpoint chlorination occurs.

Formation, toxicity, and mechanisms of halonitromethanes from poly(diallyl dimethyl ammonium chloride) during UV/monochloramine disinfection in the absence and presence of bromide ion

Bromide ion (Br^-) is known as a prevalent component in water environments, which exhibits significant impacts on halonitromethanes (HNMs) formation. This study was performed to explore and compare the formation, toxicity, and mechanisms of HNMs from poly(diallyl dimethyl ammonium chloride) (PDDACl) in the absence and presence of Br^- in the UV/monochloramine (UV/NH₂Cl) disinfection process. The results showed that chlorinated HNMs were found in the absence of Br^-, while brominated (chlorinated) HNMs and brominated HNMs were found in the presence of Br^-. Furthermore, the peaks of total HNMs were promoted by 2.0 and 2.4 times, respectively when 1.0 and 2.0 mg L^-1 Br^- were added. Also, the peaks of total HNMs were enhanced with the increase of the NH₂Cl dosage, which were reduced with the increase of pH. It should be noted that Br^- induced higher toxicity of HNMs, and the cytotoxicity and genotoxicity of HNMs with the addition of 2.0 mg L^-1 Br^- were 78.0 and 3.7 times those without the addition of Br^-, respectively. Meanwhile, both the reaction mechanisms of HNMs produced from PDDACl were speculated in the absence and presence of Br^-. Finally, different HNMs species and yields were discovered in these two real water samples compared to those in simulated waters. These findings of this work will be conducive to understanding the significance of Br^- affecting HNMs formation and toxicity in the disinfection process.

Photolysis of chloramines in vacuum-UV and vacuum-UV/chlorine advanced oxidation processes for removal of 1,4-dioxane: Effect of water matrix, kinetic modeling, and implications for potable reuse

Advanced oxidation processes (AOPs) are a key step in eliminating persistent micropollutants in potable reuse trains. Under such conditions, chloramines are an inevitable component in the AOP feed water given their application as an antifouling agent for the upstream membrane processes. In cases when other oxidants, such as free chlorine, are to be used in the AOP treatment, the effect of background chloramines and any potential interplays between the oxidants should be considered. In this study, vacuum-UV (VUV) and VUV/Cl₂ have been proposed as promising AOP alternatives for potable reuse and the effect of chloramine photolysis has been considered on the removal of 1,4-dioxane. Results indicated that while presence of chloramine reduces the treatment efficiency in the VUV AOP, coexistence of free chlorine and chloramine oxidants significantly improves 1,4-dioxane degradation rates. Experimental data and kinetic modeling both confirmed the roles of OH^• and Cl₂^•- in 1,4-dioxane removal with 62.5% and 32.5% contribution in the VUV/Cl₂/chloramines, respectively. Among the other water matrix conditions, Cl^- was shown to improve the degradation rates while HCO₃^- suppressed the reactions by scavenging radical species. Overall, the findings of this research are informative for the design and development of VUV AOPs at small scale potable reuse facilities.

Electrochemical Dechlorination of Municipal Wastewater Effluent

The use of sodium bisulfite as an electron donor to quench chloramine disinfectant residuals in municipal wastewater effluents prior to discharge incurs the cost of purchasing and transporting bisulfite to the utility and increases the loading of salts to the receiving water. In this study, degradation of chloramine residuals within authentic municipal wastewater effluents was achieved within a 30 min timescale using a reductive electrochemical reactor, which supplied electrons via a stainless-steel cathode under galvanostatic conditions without an ion exchange membrane separating the cathode and anode. Application of a 0.26 mA/cm2 cathodic current density reduced chloramines to ammonia and avoided oxidation at the IrO2-coated titanium anode of chloride to chlorine or chlorate and of ammonia to nitrite or nitrate. Net chloramine production was observed at a higher current density (2 mA/cm2). Chloramine degradation rates and Coulombic efficiencies were highest and electrical energy per order (EEO) values were lowest for the 304-grade stainless-steel cathode, which contains the highest nickel content, and for a stainless-steel cathode with a high surface area. Differences in ionic strength and pH were less important. For chloraminated municipal wastewater samples, the highest Coulombic efficiency was 4.1% and the lowest EEO value was 0.08 kWh/m3. An initial comparison indicated that the electricity cost associated with this EEO value would be comparable to the cost of sodium bisulfite for areas with low electricity costs.

A Critical Review on Chemical Speciation of Chlorine-Produced Oxidants (CPOs) in Seawater. Part 2: Sampling, Sample Preparation and Non-Chromatographic and Mass Spectrometric-Based Methods

Chlorination of seawater forms a range of secondary oxidative species, collectively termed "chlorine-produced oxidants" (CPOs). These compounds do not have the same biocidal efficacy, the same fate and behavior in the marine environment, the same potential formation of chlorination by-products (CBPs), nor the same effects on marine organisms. Their chemical speciation is an important step toward an accurate assessment of the effectiveness of chlorination and the potential impacts of its releases, among others. The aim of this paper - which is the second of a trilogy dedicated to the chemical speciation of CPOs in seawater - is to cover all aspects related to CPOs analysis in seawater, from sampling to instrumental determination. First, it discusses the procedures involved in synthesis, storage, and standardization of analytical standards. Second, it deals with sampling and sample preparation, addressing all relevant issues related to these two key steps. Third, it provides a comprehensive and up-to-date overview of the colorimetric, titrimetric, and electrochemical methods used for CPOs determination and thoroughly discusses their advantages and limitations. Finally, this review ends with some recommendations for progress in the field of CPO analysis with the three aforementioned approaches. Chromatographic and mass spectrometric-based methods will be covered in the third and final article (Part III).

Insight into the degradation of ciprofloxacin by medium-pressure UV-activated monochloramine process

The degradation efficiency and mechanisms of ciprofloxacin (CIP), a typical antibiotic, by a medium-pressure ultraviolet/chloramine (MPUV/NH₂Cl) treatment were investigated. The results showed that CIP degradation by MPUV/NH₂Cl was significantly higher than that by NH₂Cl oxidation and MPUV photolysis, and that this degradation processes were consistent with pseudo-first-order kinetics. The initial CIP concentration (7.5-30.2 μM) and the presence of HCO₃^- (0.5-10 mM) significantly inhibited CIP degradation with k_obs,CIP 0.0090-0.0069 and 0.0078-0.0048 cm²/mJ. In contrast, NO₃^- (50-500 μM) and Br^- (0.5-10 mM) significantly promoted the degradation with k_obs,CIP 0.0078-0.0102 and 0.0078-0.0124 cm²/mJ. The effect of Cl^- (0.5-10 mM) and natural organic matter (1-5 mg/L) were negligible. The NH₂Cl dosage (30-60 μM) presented a dual effect, in which its increase within the optimal concentration range (30-40 μM) accelerated CIP degradation due to the formation of reactive radicals, whereas an excessive increase (40-60 μM) quenched the free radicals, ultimately quenching the free radicals and inhibiting the degradation. The optimum pH for CIP degradation under MPUV/NH₂Cl treatment was 7.0. The contribution of reactive halogen species (i.e., reactive chlorine species and reactive nitrogen species) to CIP degradation was substantially greater than that of hydroxyl radicals under acidic or neutral conditions. We identified the degradation products of CIP and proposed degradation pathways, which included defluorination and cracking of the piperazine ring, with the latter being dominant. Compared to haloacetic acid (HAA) and nitrogenous disinfection byproducts (N-DBPs), MPUV/NH₂Cl significantly reduced trihalomethane (THM) production and theoretical cytotoxicity by 80.1% and 78.4% respectively, compared to the background experiment in natural water at a UV dose of 300 mJ/cm².

Effect of cetyltrimethylammonium chloride on various Escherichia coli strains and their inactivation kinetics by ozone and monochloramine

Cethyltrimethylammonium chloride (CTMA) is one of the most used quaternary ammonium compounds (QACs) in consumer products. CTMA and other QACs are only partially eliminated in municipal wastewater treatment and they can interact with bacteria in biological processes. Currently, there is only limited information on the antimicrobial efficiency of CTMA in matrices other than standard growth media and if and how CTMA influences conventional chemical disinfection. The results obtained in this study showed that the susceptibility of E. coli to CTMA was significantly enhanced in phosphate-buffered saline, lake water and wastewater compared to broth. In broth, a minimum inhibitory concentration (MIC) of CTMA of 20 mgL^-1 was observed for E. coli, whereas a 4-log inactivation occurred for CTMA concentrations of about 4 mgL^-1 in buffered ultra-purified water, a lake water and wastewater effluent. The impacts of the pre-exposure and the presence of CTMA on inactivation by ozone and monochloramine were tested with three different E. coli strains: AG100 with the efflux pump acrAB intact, AG100A with it deleted and AG100tet with it overexpressed. Pre-exposure of E. coli AG100 to CTMA led to an increased susceptibility for ozone with second-order inactivation rate constants (∼ 10⁶ M^-1s^-1) increasing by a factor of about 1.5. An opposite trend was observed for monochloramine with second-order inactivation rate constants (∼ 10³ M^-1s^-1) decreasing by a factor of about 2. For E. coli AG100tet, the second-order inactivation rate constant decreased by a factor of almost 2 and increased by a factor of about 1.5 for ozone and monochloramine, respectively, relative to the strain AG100. The simultaneous presence of CTMA and ozone enhanced the second-order inactivation rate constants for CTMA concentrations of 2.5 mgL^-1 by a factor of about 3. For monochloramine also an enhancement of the inactivation was observed, which was at least additive but might also be synergistic. Enhancement by factors from about 2 to 4.5 were observed for CTMA concentrations > 2.5 mgL^-1.

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).

Evaluation of preformed monochloramine for bromate control in ozonation for potable reuse

Bromate, a regulated disinfection byproduct, forms during the ozonation of bromide through reactions with both ozone and hydroxyl radical. In this study, preformed monochloramine was evaluated for use as a bromate suppression method in pilot testing of wastewater reuse with an average bromide concentration of 422±20 µg/L. A dose of 3 mg/L NH₂Cl-Cl₂ decreased bromate formation by an average of 82% and was sufficient to keep bromate below the MCL at ozone doses up to 8.6 mg/L (1.2 O₃:TOC). Removal of 1,4-dioxane through ozonation decreased with increasing NH₂Cl dose, confirming that monochloramine suppresses bromate formation, at least in part, by acting as a hydroxyl radical scavenger. This may negatively impact oxidation objectives of ozonation in reuse applications. Increasing monochloramine contact time did not improve bromate suppression, indicating that monochloramine probably did not mask bromide as NHBrCl or other haloamines prior to ozonation. However, NHBrCl and NH₂Br may be formed from reactions between HOBr and NH₂Cl and excess free ammonia during ozonation. NDMA was formed by ozonation at concentrations up to 79 ng/L and was not enhanced by NH₂Cl addition.

Formation of odorous aldehydes, nitriles and N-chloroaldimines from free and combined leucine during chloramination

Amino acids (AAs) are a major group of odorous disinfection by-product (O-DBP) precursors. O-DBPs formations during free chlorine disinfection has been previously investigated. However, knowledge regarding the O-DBP formation mechanism and kinetics under chloramination of AAs is very limited. In this study, the generation of odorous isovaleraldehyde, isovaleronitrile and N-chloroisovaleraldimine from leucine (Leu), a typical and abundant AA in many drinking water sources, in its free and combined forms during chloramination under several typical addition schemes of disinfectants was investigated. Free Leu and glycylleucine (Gly-Leu) were chosen as model compounds since they have been indicated to be O-DBP precursors during chlorination. Intermediate product analysis and kinetics studies were conducted to study the reaction mechanisms. Impacts of disinfectants dosages and pH were also investigated in experiments and simulations. The results indicated that comparing with chlorination, chloramination of Leu has its uniqueness by participating in reacting with isovaleraldehyde to form N-chloroisovaleraldimine. And all the three O-DBPs formations from free Leu and Gly-Leu during chloramination (with preformed NH₂Cl) were less than those during chlorination, indicating that using NH₂Cl for disinfection ensures control over the off-flavor problems to some degree. When chloramination was realized by adding chlorine and ammonia separately, a longer pre-chlorination time led to greater yields of the O-DBPs from both precursors, whereas adding ammonia before chlorine promoted more isovaleraldehyde formation from free Leu. Under alkaline conditions, more isovaleronitrile and N-chloroisovaleraldimine were produced, and acidic conditions led to more isovaleraldehyde formation during chloramination. Notably, O-DBPs yields from free Leu were approximately 1000 times greater than those from Gly-Leu during chloramination under all the schemes. In addition, chlor(am)ination experiments with real water from Taihu Lake (the third largest freshwater lake and water source for twenty million people in China) indicated the formation of N-chloroisovaleraldimine and isovaleraldehyde was highly likely to cause odorous problems in drinking water. This study facilitates further understanding of the causes of off-flavor issues in drinking water and can help control the odorous problems by optimizing the operating parameters of drinking water treatment plants.

Ultraviolet-Light-emitting-diode activated monochloramine for the degradation of carbamazepine: Kinetics, mechanisms, by-product formation, and toxicity

Monochloramine (NH₂Cl) oxidant combined with a Ultraviolet (UV)-Light-emitting-diode (LED) light source forms a new advanced oxidation process (AOP), which can achieve high-efficiency degradation of carbamazepine (CBZ). The degradation of CBZ displayed pseudo-first-order reaction kinetics (R² > 0.98, k_CBZ = 0.0043 cm² mJ^-1 at pH 7). The degradation of CBZ was dependent on UV-LED wavelength, with maximum degradation efficiency observed at 265 nm since it was the lowest wavelength studied among UV-LEDs. Variation in pH across the range, which might be expected under normal environmental conditions (pH 6-8), and the presence of Cl^- had no significant effect on the degradation efficiency of CBZ, while the presence of HCO₃^- and natural organic matter (NOM) inhibited degradation. Electron paramagnetic resonance (EPR) experiments detected OH in the system. Probe compounds were used to distinguish the contribution of reactive chlorine species (RCS). It was proved that OH and Cl played major roles and OH was responsible for around 50% of the observed degradation of CBZ. Eight transformative products (TPs) in the degradation process of CBZ were identified, with a generally decreasing toxicity. The concentration of disinfection by-products (DBPs) formed during CBZ degradation was all within limits of WHO and China standard for drinking water. Although the concentration of nitrogen-containing DBPs (N-DBPs) was the lowest, N-DBPs were the main contributors to toxicity, and these would require more attention in practical applications. UV-LED/NH₂Cl AOP was identified as an effective way to degrade pharmaceutically active compounds.

Factors controlling the effectiveness of rechlor(am)ination to recover chloramine from nitrification

The two most commonly adopted strategies, rechlorination (addition of chlorine) and rechloramination (addition of chlorine and ammonia), to recover and stabilise chloramine from nitrification were comprehensively evaluated in laboratory- and full-scale systems. Laboratory-scale batch experiments were conducted in a nitrifying sample (~0.05 mg-N/L). In the full-scale service reservoir, repeated rechlorination was ineffective in suppressing nitrification and microbial chloramine decay during warmer months (>20 °C), even when rechlorination was started at nitrite <0.005 mg-N/L. Measurement of decay rates through microbial chloramine decay factor method provides a deeper understanding of a water sample than traditional nitrification indicators. The method has the ability to provide an early warning (one month in advance), show the presence of microbial chloramine decay in non-nitrified water and that of chloramine decaying proteins in any samples. In the batch sample, nitrification and the production of chloramine-decaying proteins and bacterial regrowth had to be suppressed to recover chloramine. Rechloramination (~2.5 mg/L) outperformed rechlorination, as it maintained a relatively higher chloramine concentration. Microbes were killed within 30 min of dosing chlor(am)ine, likely due to shock or compounds formed during chloramine formation reactions; however, microbes regrew (or survive) to a different degree in all samples despite the prolonged presence of chloramine (large CxT), defying the CxT concept. The key to the recovery of chloramine appears to be consistently maintaining chloramine >1.7 mg/L and shocking with a high chloramine dose. The findings will assist water utilities in designing and assessing the effectiveness of nitrification remediation strategies in chloraminated water supply systems.

Sunlight-Driven Chlorate Formation during Produce Irrigation with Chlorine- or Chloramine-Disinfected Water

The increasing use of chlorine- or chloramine-containing irrigation waters to minimize foodborne pathogens is raising concerns about the formation and uptake of disinfection byproducts into irrigated produce. Chlorate has received particular attention in the European Union. While previous research demonstrated the formation of chlorate from dark disproportionation reactions of free chlorine and uptake of chlorate into produce from roots, this study evaluated chlorate formation from solar irradiation of chlorine- and chloramine-containing irrigation droplets and uptake through produce surfaces. Sunlight photolysis of 50 μM (3.6 mg/L as Cl₂) chlorine significantly enhanced the formation of chlorate, with a 7.2% molar yield relative to chlorine. Chlorate formation was much less significant in sunlit chloramine solutions. In chlorinated solutions containing 270 μg/L bromide, sunlight also induced the conversion of bromide to 280 μg/L bromate. Droplet evaporation and the resulting increase in chlorine concentrations approximately doubled sunlight-induced chlorate formation relative to that in the bulk solutions in which evaporation is negligible. When vegetables (broccoli, cabbage, chicory, lettuce, and spinach) were sprayed with chlorine-containing irrigation water in a sunlit field, sunlight promoted chlorate formation and uptake through vegetable surfaces to concentrations above maximum residue levels in the European Union. Spraying with chloramine-containing waters in the dark minimized chlorate formation and uptake into the vegetables.

Mechanistic and kinetic understanding of micropollutant degradation by the UV/NH2Cl process in simulated drinking water

The UV/monochloramine (UV/NH₂Cl) process has attracted increasing attention in water treatment, in which hydroxyl radicals (HO^•), reactive chlorine species (RCS) and reactive nitrogen species (RNS) are produced. This study investigated the effects of water matrices including halides, natural organic matter (NOM), alkalinity and pH, on the degradation kinetic of a variety of micropollutants and radical chemistry in the UV/NH₂Cl process. The presence of chloride blunted HO^• and Cl^• impacts, but enhanced Cl₂^•- effect on micropollutants reactive toward Cl₂^•-. The presence of 30 μM bromide led to an 82% decrease in the specific pseudo-first-order rate constants (k') by HO^• (k_HO•'), and significantly diminished RCS efficacy. Reactive bromine species (RBS) were formed in the presence of bromide, while the contribution could not compensate for the decrease of HO^• and RCS due to their lower reactivity toward micropollutants. Iodide rapidly transformed to HOI via reacting with NH₂Cl, which resulted in a 59% decrease of k_HO•' and 12% ∼ 100% decreases of k' by reactive halogen species (RHS) and RNS (k_RHS + RNS') for most micropollutants. Nevertheless, k' of phenolic compounds, such as paracetamol, bisphenol A and salbutamol, increased in the presence of iodide by 78%, 360% and 130%, respectively, due to the roles of HOI and reactive iodine species (RIS). Bicarbonate decreased the contributions of HO^• and RCS, but enhanced that of CO₃^•- for micropollutants reactive toward CO₃^•-. The presence of 1 mg/L NOM scavenged over half the amount of HO^•, and also consumed RCS and RNS, resulting in significantly decreased removal of micropollutants. High pH value witnessed enhanced degradation for those micropollutants reactive toward RCS and RNS through deprotonation. The degradation of most micropollutants was inhibited in real drinking water and in the coexistence of halides. This study provides a better understanding of radical chemistry in the UV/NH₂Cl process under a practical water treatment condition.

Reactions of α,β-Unsaturated Carbonyls with Free Chlorine, Free Bromine, and Combined Chlorine

Chemical disinfectants employed in water and wastewater treatment can produce a variety of transformation products, including carbonyl compounds (e.g., saturated and unsaturated aldehydes and ketones). Experiments conducted under conditions relevant to chlorination at drinking water treatment plants and residual chlorine application in distribution systems indicate that α,β-unsaturated carbonyl compounds readily react with free chlorine and free bromine over a wide pH range but react slowly with combined chlorine (i.e., NH₂Cl). For nearly all of the 11 α,β-unsaturated carbonyl compounds studied, the apparent second-order rate constants for the reaction with free chlorine increased in a linear manner with hypochlorite (OCl^-) concentrations, yielding species-specific second-order rate constants for the reaction with OCl^- ranging from 0.21 to 12 M^-1 s^-1. Predictions based on the second-order rate constants indicate that a substantial fraction (i.e., >60%) of several of the more prominent α,β-unsaturated carbonyls (e.g., acrolein, crotonaldehyde) will be transformed to an appreciable extent in distribution systems by free chlorine. Products from the reaction of chlorine with acrolein, crotonaldehyde, and methyl vinyl ketone were tentatively identified using nuclear magnetic resonance (NMR) and gas chromatography coupled to high-resolution time-of-flight mass spectrometry (GC-HRT-MS). These products lacked unsaturated carbons and, in some cases, contained multiple halogens.

Reaction of phenazone-type drugs and metabolites with chlorine and monochloramine

This work studies the chlorination and monochloramination reaction kinetics of two phenazone-type drugs (phenazone - Phe and propyphenazone - PrPhe) and three metabolites of phenazone-type drugs (4-formylaminoantipyrine - FAA, 4-aminoantipyrine - AA and 4-acetoamidoantipyrine - AAA). Kinetics were faster with chlorine (apparent second-order constants between 100 and 66,500 times higher) than with monochloramine. For FAA and AAA, no significant reaction was observed during monochloramination. Further, apparent rate constants decreased as the pH increased from pH 5.7 to 8.3, except during chlorination of AA. The transformation products (TPs) formed were also elucidated by liquid chromatography-high resolution mass spectrometry. The main transformation pathway for Phe and PrPhe consisted of halogenations, hydroxylations and dealkylations, while AAA and FAA were firstly transformed to AA, then followed by pyrazole ring opening and hydroxylations. The extend of the reaction was also tested in real water samples, where, in general, slower reaction kinetics were obtained during monochloramination, while the chlorination reaction showed similar half-lives to ultrapure water. Finally, acute and chronic toxicity of the TPs were estimated using two quantitative structure-activity relationship (QSAR) software (ECOSAR and TEST), showing that some TPs could be more toxic than their precursor compounds.

Updated Reaction Pathway for Dichloramine Decomposition: Formation of Reactive Nitrogen Species and N-Nitrosodimethylamine

The N-nitrosodimethylamine (NDMA) formation pathway in chloraminated drinking water remains unresolved. In pH 7-10 waters amended with 10 μM total dimethylamine and 800 μeq Cl₂·L^-1 dichloramine (NHCl₂), NDMA, nitrous oxide (N₂O), dissolved oxygen (DO), NHCl₂, and monochloramine (NH₂Cl) were kinetically quantified. NHCl₂, N₂O, and DO profiles indicated that reactive nitrogen species (RNS) formed during NHCl₂ decomposition, including nitroxyl/nitroxyl anion (HNO/NO^-) and peroxynitrous acid/peroxynitrite anion (ONOOH/ONOO^-). Experiments with uric acid (a ONOOH/ONOO^- scavenger) implicated ONOOH/ONOO^- as a central node for NDMA formation, which were further supported by the concomitant N-nitrodimethylamine formation. A kinetic model accurately simulated NHCl₂, NH₂Cl, NDMA, and DO concentrations and included (1) the unified model of chloramine chemistry revised with HNO as a direct product of NHCl₂ hydrolysis; (2) HNO/NO^- then reacting with (i) HNO to form N₂O, (ii) DO to form ONOOH/ONOO^-, or (iii) NHCl₂ or NH₂Cl to form nitrogen gas; and (3) NDMA formation via ONOOH/ONOO^- or their decomposition products reacting with (i) dimethylamine (DMA) and/or (ii) chlorinated unsymmetrical dimethylhydrazine (UDMH-Cl), the product of NHCl₂ and DMA. Overall, updated NHCl₂ decomposition pathways are proposed, yielding (1) RNS via NHCl2→HNO/NO-→O2ONOOH/ONOO- and (2) NDMA via ONOOH/ONOO-→UDMH-ClorDMANDMA.

Identification and quantification of chloramines, bromamines and bromochloramine by Membrane Introduction Mass Spectrometry (MIMS)

A Membrane Introduction Mass Spectrometry (MIMS) method was developed to differentiate and quantify the different chlorinated and brominated-amines, present in drinking water during chloramination. The representative mass to charge ratios (m/z) of 53, 85, 97, 175 and 131 corresponding to the mass of the parent compounds were selected to monitor NH₂Cl, NHCl₂, NH₂Br, NHBr₂ and NHBrCl and the detection limits were found to be 0.034, 0.034, 0.10, 0.12 and 0.36 mg/L as Cl₂, respectively. NHCl₂, NHBr₂ and NHBrCl fragments interfere with the analysis/quantification of NH₂Cl and NH₂Br via protonation reactions at hot metal surfaces inside the mass spectrometer. To accurately quantify NH₂Cl or NH₂Br in mixtures of NH₂Cl/NHCl₂ or NH₂Br/NHBr₂, the interference from NHCl₂ or NHBr₂ was subtracted to the signal of the parent compound. If NHBrCl is present, NH₂Br and NH₂Cl cannot be accurately quantified since the interference from the NHBrCl fragment cannot be distinguished from the signal of the parent compound. Under drinking water conditions, the interference from NHBrCl on NH₂Cl was negligible. The different halamines were monitored and quantified for the first time in two surface waters and one seawater that were chloraminated to mimic a realistic disinfection scenario.

Nontargeted Identification of an N-Heterocyclic Compound in Source Water and Wastewater as a Precursor of Multiple Nitrosamines

N-Nitrosamine disinfection byproducts (DBPs) are a health concern because they are probable human carcinogens. Complex organic nitrogenous compounds, nitrosamine precursors, are largely unidentified in source water. Using stable isotopic labeling-enhanced nontargeted analysis, we identified a natural product N-heterocyclic amine 1-methyl-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid (MTCCA) in source water. Interestingly, we discovered that chloramination of MTCCA-containing water could produce four nitrosamines: methylethylnitrosamine, N-nitrosopyrrolidine, N-nitrosoanatabine, and N-nitrosoanabasine. Computational modeling and experimental results helped explain potential pathways of nitrosamines generated from chloramination of MTCCA. Further investigations confirmed widespread occurrence of MTCCA in source water and wastewater. Its concentration ranged from high in upstream creeks (23.2-332.2 ng L^-1) to low in the river (5.7-37.6 ng L^-1) during the 2020 spring runoffs, indicating that sources of MTCCA came from creeks around farms. Analysis of wastewater before and after ultraviolet, as well as microfiltration with subsequent ozonation treatments, showed increased MTCCA after treatments, demonstrating a difficulty to degrade and remove MTCCA in water. This study discovered the extensive presence of MTCCA in source water and wastewater, suggesting that natural N-heterocyclic compounds may serve as a new source of nitrosamine precursors.

N-Nitrosodimethylamine Formation during UV/Hydrogen Peroxide and UV/Chlorine Advanced Oxidation Process Treatment Following Reverse Osmosis for Potable Reuse

Chloramines applied to control microfiltration and reverse osmosis (RO) membrane biofouling in potable reuse trains form the potent carcinogen, N-nitrosodimethylamine (NDMA). In addition to degrading other contaminants, UV-based advanced oxidation processes (AOPs) strive to degrade NDMA by direct photolysis. The UV/chlorine AOP is gaining attention because of its potential to degrade other contaminants at lower UV fluence than the UV/hydrogen peroxide AOP, although previous pilot studies have observed that the UV/chlorine AOP was less effective for NDMA control. Using dimethylamine (DMA) as a model precursor and secondary municipal wastewater effluent, this study evaluated NDMA formation during the AOP treatment via two pathways. First, NDMA formation by UV treatment of monochloramine (NH₂Cl) and chlorinated DMA (Cl-DMA) passing through RO membranes was maximized at 350 mJ/cm² UV fluence, declining at higher fluence, where NDMA photolysis outweighed NDMA formation. Second, this study demonstrated that chlorine addition to the chloramine-containing RO permeate during the UV/chlorine AOP treatment initiated rapid NDMA formation by dark breakpoint reactions associated with reactive intermediates from the hydrolysis of dichloramine. At pH 5.7, this formation was maximized at a chlorine/ammonia molar ratio of 3 (out of 0-10), conditions typical for UV/chlorine AOPs. At 700 mJ/cm² UV fluence, which is applicable to current practice, NDMA photolysis degraded a portion of the NDMA formed by breakpoint reactions. Lowering UV fluence to ∼350 mJ/cm² when switching to the UV/chlorine AOP exacerbates effluent NDMA concentrations because of concurrent NDMA formation via the UV/NH₂Cl/Cl-DMA and breakpoint chlorination pathways. Fluence >700 mJ/cm² or chlorine doses greater than the 3:1 chlorine/ammonia molar ratios under consideration for the UV/HOCl AOP treatment are needed to achieve NDMA control.

Challenges and opportunities for on-line monitoring of chlorine-produced oxidants in seawater using portable membrane-introduction Fourier transform-ion cyclotron resonance mass spectrometry

The present study reports the first evaluation of a MIMS device equipped with a high-resolution Fourier transform-ion cyclotron resonance mass spectrometer (FT-ICR MS) for comprehensive speciation of chlorine-produced oxidants (CPO) in seawater. A total of 40 model compounds were studied: 4 inorganic haloamines (mono-, di-, and trichloramine and monobromamine), 22 organic N-haloamines, 12 N-haloamino acids, and 2 free oxidants (HOCl/ClO^- and HOBr/BrO^-). The main key factors influencing the analytes' introduction and their detection were optimized. Under optimized conditions, the rise and fall times of the MIMS signal ranged from 8 to 79 min and from 7 to 73 min, respectively, depending on the compound. Free oxidants and N-haloamino acids, which are ionic or too polar at seawater pH, hardly crossed the membrane, and MIMS analysis was thus unsuitable. Nevertheless, better enrichment and therefore better sensitivity were achieved with organic N-haloamines than with inorganic haloamines. The observed detection limits ranged from tens of μM to sub-μM levels. Oxidant decomposition occurred inside the MIMS device, at a higher rate for N-bromamines than for chlorinated analogues.Graphical abstract.

ClO2 pre-oxidation impacts the formation and nitrogen origins of dichloroacetonitrile and dichloroacetamide during subsequent chloramination

Chlorine dioxide (ClO₂) can be used as a pre-oxidant when chloramination is performed in water treatment plants. However, the effects of ClO₂ pre-oxidation on the formation of nitrogenous disinfection by-products, such as dichloroacetonitrile (DCAN) and dichloroacetamide (DCAcAm), during chloramination are not well understood. In this study, the effects of ClO₂ pre-oxidation on the formation of DCAN and DCAcAm during chloramination of 28 model compounds and seven real water samples were investigated. The sources of nitrogen for DCAN and DCAcAm formation were investigated using ¹⁵N-labeled monochloramine. ClO₂ pre-oxidation affected DCAN and DCAcAm formation during chloramination of model compounds in different ways. ClO₂ pre-oxidation increased unlabeled and ¹⁵N-labeled DCAN and DCAcAm formation during chloramination of six amino acids and peptides and five indoles and tertiary amines. ClO₂ pre-oxidation decreased DCAN formation but increased DCAcAm formation during chloramination of three hydroxybenzamide compounds, but had the opposite effects for four tetracyclines. ClO₂ pre-oxidation generally decreased DCAN and DCAcAm formation during chloramination of the phenolic compounds that are precursors not containing nitrogen. 2-Aminoacetophenone, formamid-trans-muconic acid, and unsaturated ketones were found to be transformation products of ClO₂ oxidation of 3-methylindole, salicylamide, and resorcinol, respectively. Possible DCAN and DCAcAm formation pathways during chloramination after ClO₂ oxidation were identified. For most of the water samples, ClO₂ pre-oxidation decreased the amounts of DCAN and DCAcAm formed during chloramination by 36%-70% and 11%-59%, respectively. This may have been caused by ClO₂ oxidation destroying phenolic precursors and macromolecular proteins rather than amino acids in the water samples.

Alternative application of preformed monochloramine as a drinking water disinfectant for redosing in long drinking water distribution system servicing remote locations

Chloramine is the disinfectant of choice for long drinking water distribution systems since it is more stable than chlorine, therefore providing a longer lasting disinfectant residual and producing less disinfection byproducts. However, to deliver safe drinking water in remote areas, redosing stations are used where (mostly) chlorine is added to the water to re-establish a disinfectant residual. In this study, the use of a highly concentrated preformed monochloramine solution instead of chlorine for redosing in tanks or reservoirs along extensive distribution systems was investigated. This will avoid unwanted reactions due to chlorine reactivity (formation of disinfection byproducts) and increase the disinfectant stability. Highly concentrated monochloramine solutions (15 mM, 1 gCl₂ L^-1) are most stable at high pH (10) and low temperature. It was found that the current kinetic model used for drinking water application was not accurate for highly concentrated monochloramine. A simplified kinetic model was developed and successfully used to predict monochloramine stability in our conditions. This will assist water utilities for application of preformed concentrated monochloramine allowing monochloramine redosing in long drinking water distribution systems. Hydrazine, a compound of health concern, might form at high monochloramine concentrations and high pH and needs to be carefully controlled.

Formation of N-nitrosodimethylamine precursors through the microbiological metabolism of nitrogenous substrates in water

N-nitrosodimethylamine (NDMA) as one emerging disinfection by-product has been investigated globally since 1990s. However, its main precursors are still unclear. We found that NDMA formation potential (NDMAFP) of various water samples increased firstly and then decreased gradually during incubation with microorganism. We hypothesized that NDMA precursors could be produced through metabolism of nitrogenous components and then gradually be biodegraded. To verify this hypothesis, six amino acids (AAs), peptone and ammonium were separately incubated with microorganism and NDMAFP was measured regularly. The average molar yield of the substrates to NDMAFP were 60-200 × 10^-6 for the AAs, 350 × 10^-6 for peptone under aerobic condition. The extracellular fraction with molecular weight (MW) less than 1 k Dalton contributed the majority to NDMAFP in the peptone experiment, followed by that with MW between 10 k and 0.22 μm and the intracellular materials. Dimethylamine and methylamine were detected during the experiments but their contribution to NDMAFP is quite limited. The results indicate that the nitrosamine precursors may not be the direct metabolite of AAs or peptones but the excretion of living bacteria or the components in dead bacteria body. Our results inferred that AA metabolism may give an NDMAFP of 0.12 nmol/L (maximum) or 0.09 nmol/L (average) in water under aerobic condition. This estimation of NDMAFP from AA metabolism can account for 38% (maximum) or 27% (average) of the median NDMAFP in waters of China (0.32 nmol/L) reported before.

DEET degradation in UV/monochloramine process: Kinetics, degradation pathway, toxicity and energy consumption analysis

The degradation of N,N-diethyl-meta-toluamide (DEET) in aqueous solution by the UV/monochloramine (UV/NH₂Cl) process was examined systematically in this study. DEET was resistant to UV photolysis and chloramination, while the synchronous combination of UV irradiation and NH₂Cl can effectively eliminate DEET, which was caused by the generation of hydroxyl radicals and reactive chlorine species. The former played the critical role in DEET degradation, while the contribution of the latter can be ignored. Under all investigated experimental conditions, DEET degradation in the UV/NH₂Cl process followed the pseudo-first-order kinetic model. The water quality parameters exerted the complicated impact. Reducing solution pH and raising water temperature both favored the DEET removal. The presence of sulfate, humic acid and fulvic acid accelerated the degradation, while the introduction of bicarbonate and high-concentration chloride retarded the removal. The plausible degradation pathways of DEET in the UV/NH₂Cl process were proposed through the combination of QTOF/MS analysis and DFT calculation, and mainly involved in the cleavage of C-N bond, dealkylation, mono- and polyhydroxylation. The acute toxicity of reacted solution underwent a trend of first increasing and then decreasing with the prolonged irradiation time, which can be well illustrated by quantitative structure-activity relationship analysis. Electrical energy per order was employed to determine the energy consumption and the optimal conditions were determined as UV fluence of 369.9-493.2 mJ cm^-2 and NH₂Cl dosage of 5-20 mg L^-1.

Insight into PPCP degradation by UV/NH2Cl and comparison with UV/NaClO: Kinetics, reaction mechanism, and DBP formation

The UV/NH₂Cl process is an emerging advanced oxidation process (AOP) that is greatly effective in degrading pharmaceuticals and personal care products (PPCPs). However, detailed information regarding the process is lacking. The degradation of ibuprofen (IBP, an electron-withdrawing PPCP) and naproxen (NPX, an electron-donating PPCP) in UV/NH₂Cl and UV/NaClO processes was performed to investigate the applicability and security of the UV/NH₂Cl process and compare with those of UV/NaClO. UV/NH₂Cl was effective in degrading both IBP and NPX and the degradation followed pseudo-first order kinetics (k_IBP = 0.0037 cm²/mJ and k_NPX = 0.0044 cm²/mJ). This indicated the broad applicability of UV/NH₂Cl to different kinds of PPCPs. Ranges of values of UV intensity (0.3-1.0 mW/cm²) and pH (6.0-8.0) showed little effect on the degradation of PPCPs by UV/NH₂Cl based on UV Dose but HCO₃^- (2-8 mM), natural organic matter (NOM, 2-8 mg/L), and the natural water matrixes were inhibitory. Increasing the dosage of NH₂Cl from 0.15 mM to 0.75 mM, resulted in an even increase of k_IBP; however, k_NPX increased slowly after 0.3 mM NH₂Cl. Mechanism experiments involving nitrobenzene showed that •OH was the major radical involved in degrading IBP and NPX via UV/NH₂Cl. The electron spin resonance spectroscopy and kinetic modeling results also indicated the larger amount of •OH and weaker reactive chlorine species (mainly ClO• and ClO₂•) in UV/NH₂Cl compared with UV/NaClO. Compared to UV/NaClO in synthetic and natural water, UV/NH₂Cl was a more stable degrader with little pH- and substrate-dependence, while UV/NaClO preferred degrading the electron-donating PPCP and at low pH. The UV/NH₂Cl produced less halogenated disinfection byproducts (DBPs) (even nitrogenous DBPs) and was less cytotoxic theoretically than UV/NaClO based on the DBPs included in this study. Thus UV/NH₂Cl process may be an effective AOP for water treatment.

A comprehensive review of mathematical models developed for the estimation of organic disinfection byproducts

In this review, we present comparative and comprehensive views on the foundations, potentials and limitations of the previously reported mathematical models for the estimation of the concentration of disinfection byproducts (DBPs) generated during the chlor(am)ination of water. To this end, DBPs models were divided into two major categories: static variable (SV) and dynamic variable (DV) or differential models. In SV models, variables remain in their original form throughout a chlor(am)ination modelling period while DV models consider the changes driven by a chlor(am)ination treatment as the variables. This classification and the comparative study of the two types of models led to a better understanding of the assumptions, potentials, and limitations of the existing DBP models. In opposition to several claims in the literature, certain DV models based on UV absorbance/fluorescence failed to selectively track the chromophores responsible for DBP formation. In this critical review, a conceptual model for the photophysics of dissolved organic matter (DOM) based on the theory of electron delocalization was proposed to explain some inconsistent spectroscopic properties of DOM following chlor(am)ination and several unique photophysical properties of DOM. New insights for the development and deployment of mathematical models were also provided to estimate DBPs in various settings.

Mechanisms of Advanced Oxidation Processes, the Principle of Detailed Balancing, and Specifics of the UV/Chloramine Process

Advanced oxidation processes tend to have very complex reaction mechanisms, and models containing over 150 steps have been developed to describe the chemistry. Without the aid of automation, it is extremely difficult to avoid the development of kinetic mechanisms that violate the principle of detailed balancing. Here, we apply DETBAL, a computer application, to systematically identify many violations of the principle of detailed balancing in a model proposed for the UV/chloramine process. We then show that these violations can also be found in dozens of other proposed models for advanced oxidation processes. Suggested repairs to these violations are provided. These repairs lead to no significant changes in the model predictions because the illegal loops include steps that are unnecessary under the conditions modeled. The model omits certain steps that do have significant effects on the model predictions.

+Chlorination by-products in indoor swimming pools: Development of a pilot pool unit and impact of operating parameters

Chlorine addition in swimming pools ensures the microbiological quality of the water and the bathers' safety. However, water chlorination is associated with disinfection byproducts (DBP) formation and adverse health effects. The impact of operating parameters and innovative water treatment systems on DBPs levels has been reported in several studies, but sampling campaign in real pools remain difficult to carry out, mainly due to unexpected attendance variations. This study presents the development of a pilot pool plant allowing to perform experiments under controlled and reproducible conditions. Bathers inputs were simulated both for the organic load and for the mechanical agitation of water. Two sampling campaigns were performed during the building of the pilot, before and after the hall was closed. Key operating parameters such as chlorine dose, water temperature and attendance were controlled and monitored. DBP levels in the pilot plant were representative of French indoor swimming pools and the impact of bathers' activity was visible on volatile DBPs. Furthermore, correlations could be stated between operating parameters and DBP levels. Stripping effectively reduced volatile DBP concentrations in water. Moreover, energy consumption data, which are usually very scarce in experimental studies, showed the influence of heat pump consumption on the global energy consumption.

Effect of bromide on NDMA formation during chloramination of model precursor compounds and natural waters

In this work, we investigated the effect of bromide ion (Br^-) on NDMA formation using model precursor compounds, wastewater effluents and surface waters. Previous studies showed that Br^- reacts with chloramines and forms bromochloramine, a reactive compound responsible for NDMA formation enhancement. Some limitations of those studies were the highest Br^- concentrations used, and the limited number of precursors considered. Here, we observed enhancement of NDMA formation from most of the model precursor compounds within the Br^- range (0-1000 μg/L) but this effect was suppressed in the presence of NOM. Also, NDMA formation was favored at pH 8 in the presence of Br^- compared to pH 6. Nevertheless, Br^- suppressed NDMA formation in wastewater effluent samples at low monochloramine doses while no effects were observed in surface waters.

Application of stabilized hypobromite for controlling membrane fouling and N-nitrosodimethylamine formation

Chloramination is a conventional and successful pre-disinfection approach to control biological fouling for reverse osmosis (RO) treatment in water reuse. This study aimed to evaluate the possibility of using a new disinfectant-stabilized hypobromite-in controlling membrane fouling and the formation of a particular carcinogenic disinfection byproduct (DBP)-N-nitrosodimethylamine (NDMA). Our accelerated chemical exposure tests showed that the new disinfectant reduced the permeability of a polyamide RO membrane permeability from 6.7 to 4.1 L/m²hbar; however, its treatment impact was equivalent to that of chloramine. The disinfection efficacy of stabilized hypobromite was greater than that of chloramine when evaluated with intact bacterial counts, which suggests its potential for mitigating membrane biofouling. Additional pilot-scale tests using synthetic wastewater demonstrated that pre-disinfection with the use of stabilized hypobromite inhibits membrane fouling. Among 13 halogenated DBPs evaluated, the formation of bromoform by stabilized hypobromite was higher than that by chloramine at a high dose of 10 mg/L, thus suggesting the need for optimizing chemical doses for achieving sufficient biofouling mitigation. NDMA formation upon stabilized hypobromite treatment in two different types of actual treated wastewaters was found to be negligible and considerably lower than that by chloramine treatment. In addition, NDMA formation potential by stabilized hypobromite was 2-5 orders of magnitude lower than that by chloramine. Our findings suggest the potential of using stabilized hypobromite for controlling NDMA formation and biofouling, which are the keys to successful potable water reuse.

The mechanism of monochloramine disproportionation under acidic conditions

Monochloramine is a widely employed agent in water treatment technologies. However, its utilization has some drawbacks like the transformation of the active species into the undesired dichloramine. Although it is more pronounced in acidic solutions, the features of this reaction have still remained largely unexplored in the pH < 4 region. In this study the decomposition of monochloramine is examined under such conditions by using kinetic and computational methods. Fast kinetics measurements have convincingly showed that the disproportion into dicloramine is relatively fast and can be studied without any interference from side reactions. By varying the pH, the deprotonation constant of monochloramine has been determined by UV spectroscopy (K_a = 0.023 ± 0.005 M for I = 1.0 M NaClO₄, and T = 25.0 °C). Dichloramine formation via monochloramine disproportion was found to follow second-order kinetics. The computations have provided the reaction mechanism and its free energy profile in accord with the proposed kinetic model. This involves the reaction between the protonated and unprotonated forms of monochloramine, with a rate constant k = 335.3 ± 11.8 M^-1 s^-1, corresponding to an activation free energy barrier of 14.1 kcal mol^-1. The simulations predicted a barrier of 14.9 kcal mol^-1 and revealed a key short-lived chlorine-bridged intermediate which yields dichloroamine and ammonium ion through a deprotonation-coupled chlorine shift.

Pilot-scale evaluation of oxidant speciation, 1,4-dioxane degradation and disinfection byproduct formation during UV/hydrogen peroxide, UV/free chlorine and UV/chloramines advanced oxidation process treatment for potable reuse

Advanced oxidation using UV/free chlorine and UV/chloramines are being considered as alternatives to UV/H₂O₂ for treatment of reverse osmosis (RO) permeate in treatment trains for the potable reuse of municipal wastewater. This pilot-scale comparison of the three advanced oxidation processes (AOPs) evaluated three factors important for selecting among these alternatives. First, the study characterized the speciation of oxidants serving as the source of radicals within the AOPs to facilitate process modeling. Kinetic modeling that included consideration of the chloramines occurring in RO permeate accurately predicted oxidant speciation. Modeling of the UV/free chlorine AOP indicated that free chlorine is scavenged by reactions with ammonia and monochloramine in RO permeate, such that oxidant speciation can shift in favor of dichloramine over the short (∼30 s) timescale of AOP treatment. Second, the order of efficacy for degrading the target contaminant, 1,4-dioxane, in terms of minimizing UV fluence was UV/free chlorine > UV/H₂O₂ ≫ UV/chloramines. However, estimates indicated that the UV/chloramines and UV/H₂O₂ AOPs could be similar on a cost-effectiveness basis due to savings in reagent costs by the UV/chloramines AOP, provided the RO permeate featured >3 mg/L as Cl₂ chloramines. Third, the study evaluated whether the use of chlorine-based oxidants within the UV/free chlorine and UV/chloramines AOPs enhanced disinfection byproduct (DBP) formation. Even after AOP treatment and chloramination, total halogenated DBP formation remained low at <15 μg/L for all three AOPs. DBP formation was similar between the AOPs, except that the UV/free chlorine AOP promoted haloacetaldehyde formation, while the UV/H₂O₂ and UV/chloramines AOPs followed by chloramination increased chloropicrin formation. However, total DBP formation on a toxic potency-weighted basis was similar among the AOPs, since haloacetonitriles and haloacetamides were the dominant contributors and did not differ significantly among the AOPs.

Disinfection byproducts and their toxicity in wastewater effluents treated by the mixing oxidant of ClO2/Cl2

Mixing oxidant of chlorine dioxide (ClO₂) and chlorine (Cl₂) often applied in water disinfection. Two secondary wastewater effluents at different ammonium-N levels (0.1 and 1.6 mg N L^-1) were treated with the mixing oxidant (ClO₂/Cl2) to evaluate the formation of disinfection byproducts (DBPs) and the associated cytotoxicity of treated wastewaters. The total chlorine concentrations of ClO₂ and Cl₂ were maintained at 10 mg L^-1 as Cl₂ with varied mixing ratios of ClO₂ to Cl₂. The formation of 37 halogenated DBPs, including nitrogenous, brominated and iodinated analogues, and 2 inorganic DBPs (chlorite and chlorate) was examined. The sum concentrations of the halogenated DBPs were reduced remarkably with decreasing Cl₂ percentages, but each individual DBP group had distinct features. The regulated trihalomethanes reduced the most when ClO₂ was present in chlorination, but decreasing Cl₂ percentage from 70% to 30% was not quite effective to reduce the formation of iodinated trihalomethanes, haloacetic acids and haloacetontriles in low ammonium-N wastewater. The bromine and iodine substitution factors tend to increase with decreasing Cl₂ percentages, indicating that destruction of DBP precursors by ClO₂ favored bromine and iodine incorporation. Additionally, decreasing Cl₂ percentages in the mixing oxidant (ClO₂/Cl₂) was often accompanied with lower chlorate formation but higher chlorite formation. The toxicity of treated wastewaters was evaluated through two approaches: the calculated cytotoxicity based on the concentrations of detected DBPs and the experimental cytotoxicity using the Chinese hamster ovary (CHO) cells. The calculated cytotoxicity decreased with decreasing Cl₂ percentages, with haloacetonitriles and haloacetaldehydes as predominate contributors. However, the experimental cytotoxicity tests showed that treatment of high ammonium-N wastewater with ClO₂/Cl₂ exhibited considerable higher (> 3 times) cytotoxicity potency than using single disinfectant.

Predicting the Contribution of Chloramines to Contaminant Decay during Ultraviolet/Hydrogen Peroxide Advanced Oxidation Process Treatment for Potable Reuse

Chloramines applied to control membrane biofouling in potable reuse trains pass through reverse osmosis membranes, such that downstream ultraviolet (UV)/H₂O₂ advanced oxidation processes (AOPs) are de facto UV/H₂O₂-chloramine AOPs. Current models for UV/chloramine AOPs, which use inaccurate chloramine quantum yields and ignore the fate of ^•NH₂, are unable to simultaneously predict the loss of chloramines and contaminants, such as 1,4-dioxane. This study determined quantum yields for NH₂Cl (0.35) and NHCl₂ (0.75). Incorporating these quantum yields and the formation from ^•NH₂ of the radical scavengers, ^•NO and NO₂^-, was important for simultaneously modeling the loss of chloramines, H₂O₂, and 1,4-dioxane in the UV/H₂O₂-chloramine AOP. Although the level of radical production was higher for the UV/H₂O₂-chloramine AOP than for the UV/H₂O₂ AOP, the UV/H₂O₂ AOP was at least 2-fold more efficient with respect to 1,4-dioxane degradation, because chloramines efficiently scavenged radicals. At low chloramine concentrations, the UV/chloramine AOP efficiency increased with an increase in chloramine concentration, as the level of radical production increased relative to that of radical scavenging by the dissolved organic carbon in RO permeate. However, the efficiency leveled out at higher chloramine concentrations as radical scavenging by chloramines offset the increased level of radical production. The level of 1,4-dioxane degradation was ∼30-50% lower for the UV/chloramine AOP than for the UV/H₂O₂-chloramine AOP when the concentration of residual chloramines in RO permeate was ∼50 μM (3.3 mg/L as Cl₂). Initial cost estimates indicate that the UV/chloramine AOP using the residual chloramines in RO permeate could be a cost-effective alternative to the current UV/H₂O₂-chloramine AOP in some cases, because the savings in reagent costs offset the ∼30-50% reduction in 1,4-dioxane degradation efficiency.

Comparison of Toxicity-Weighted Disinfection Byproduct Concentrations in Potable Reuse Waters and Conventional Drinking Waters as a New Approach to Assessing the Quality of Advanced Treatment Train Waters

Advanced treatment trains based on oxidation, biofiltration, and/or granular activated carbon (Ox/BAF/GAC) are an attractive alternative to those based on microfiltration, reverse osmosis, and advanced oxidation (MF/RO/AOP) for the potable reuse of municipal wastewater effluents, but their effluent quality is difficult to validate with respect to chemical contaminants. This study evaluated the sum of the concentrations of 46 disinfection byproducts (DBPs) after treatment by chlorine or chloramines weighted by metrics of toxic potency in 10 full- or pilot-scale reuse trains to estimate the DBP-associated toxicity of their effluents. These total toxicity-weighted DBP concentrations were compared to those measured in their local, conventional drinking waters as a benchmark for water quality receiving regulatory and widespread public acceptance. The results indicated that while the DBP-associated quality of MF/RO/AOP-based reuse waters can readily exceed that of drinking waters, that of Ox/BAF/GAC-based reuse waters can approach or exceed that of drinking waters, particularly when they are chloraminated. Unregulated, halogenated DBPs were the dominant contributors to the estimated DBP-associated toxicity. While RO/AOP treatment preferentially reduced the concentrations of the more toxic brominated DBP species, BAC and GAC treatment favored brominated DBP species by removing DOC but not bromide. Comparing the total toxicity-weighted DBP concentration between reuse and drinking waters provides drinking water as a rational benchmark for water quality comparison, explicitly recognizes that contaminants occur as mixtures, provides utilities flexibility in selecting the most efficient treatment trains to reduce estimated toxicity, and can be expanded to encompass new contaminants as toxic potency data become available.