CHO cell cytotoxicity and genotoxicity analyses of disinfection by-products: An updated review

Surrogates for the removal by ozonation of the cytotoxicity and DNA double-strand break effects of wastewater on mammalian cells

Effluents from wastewater treatment plants (WWTPs) may contain various pollutants with potential toxic effects. Ozonation is widely applied to purify wastewater, which may influence the toxicity and water quality indices simultaneously. The main goal of this study was to reveal influence of ozonation on toxicity of WWTP effluents and to find the surrogates for toxicity changes. Cytotoxicity and DNA double-strand break (DSB) effect of WWTP effluents were measured based on Chinese hamster ovary (CHO) cells. Changes of water quality parameters and molecular weight distribution of WWTP effluents were also measured. The organic extracts in WWTP effluents were shown to decrease the cell viability. Besides, an increased level of DNA DSBs was found in cells when exposed to the organic extracts. Ozonation significantly eliminated cytotoxicity and DNA DSB-based genotoxicity of WWTP effluents, with removal rates of 53-66% and 51-76% for cytotoxicity and genotoxicity, respectively, with 10 mg/L ozone dose. Although the DOC contents in WWTP effluents were hardly removed by ozonation, the chromophores and fluorophores were significantly eliminated. Organic matter in WWTP effluents mainly consists of fractions with molecular weight (MW) < 500 Da. Ozonation generally decreased the fluorescence intensity and UV₂₅₄ values of all the MW fractions, but increased the DOC contents of the 100-500 Da fraction. During ozonation, the removal rates of UV₂₅₄ and SUVA₂₅₄ were significantly correlated to the removal rates of both cytotoxicity and genotoxicity. UV₂₅₄ might be an ideal surrogate for cytotoxicity and genotoxicity reduction during wastewater ozonation.

Chlorinated effluent organic matter causes higher toxicity than chlorinated natural organic matter by inducing more intracellular reactive oxygen species

During unplanned indirect potable reuse, treated wastewater that contains effluent organic matter (EOM) enters the drinking water source, resulting in different toxicity from natural organic matter (NOM) in surface water during chlorination. This study found that, during chlorination, EOM formed more total organic halogen (TOX) and highly toxic nitrogenous disinfection byproducts (DBPs) like dichloroacetonitrile and trichloronitromethane than NOM did. Oxidative stress including both reactive oxygen species (ROS) and reactive nitrogen species (RNS) in Chinese hamster ovary (CHO) cells substantially increased when exposed to chlorinated EOM and chlorinated NOM. The excessive ROS damaged biological macromolecules including DNA, RNA to form 8-hydroxy-(deoxy)guanosine and proteins to form protein carbonyls. Impaired macromolecule further triggered cell cycle arrest at the S and G₂ phases, led to cell apoptosis and eventual necrosis. Cytotoxicity and genotoxicity of chlorinated EOM were both higher than those of chlorinated NOM. Adding the blocker L-buthionine-sulfoximine of intracellular antioxidant glutathione demonstrating that oxidative stress might be responsible for toxicity. ROS was further identified to be the main cause of toxicity induction. These findings highlight the risk from chlorinated EOM in the case of unplanned indirect potable reuse, because it showed higher level of cytotoxicity and genotoxicity than chlorinated NOM via inducing more ROS in mammalian cells.

Human exposure and risk of trihalomethanes during continuous showering events

Exposure to disinfection byproducts (DBPs) in municipal water mainly occurs through ingestion, inhalation during showering, house cleaning and dermal permeation. While showering, the air-phase DBPs [e.g., trihalomethanes (THMs)] can pose risk to humans through inhalation pathway. In assessing inhalation risk during showering, current approaches assume negligible initial concentrations of air-phase THMs in shower stalls, which may not be realistic in places with common shower stalls (e.g., gymnasiums). The time difference between successive showering events in these places is likely to be minimal, resulting in significant initial concentrations of THMs due to prior showering events. In this study, exposure to air-phase THMs during the successive showering events were predicted. In the 2nd showering event, averages of chronic daily intake (CDI), cancer risk (CR) and hazard index (HI) were 1.82, 1.83 and 1.85 times the CDI, CR and HI in the first event, respectively. In the 3rd event, these were 2.50, 2.54 and 2.58 times, respectively. The increasing trends of CDI, CR and HI were observed for up to the 9th event (5.06, 4.98 and 5.60 times, respectively). By widening the time-gap between the successive showering events, reducing showering duration and enhancing ventilation, human exposure and risk can be controlled.

Formation potential of emerging disinfection by-products during ozonation and chlorination of sewage effluents

This study investigates the formation potential of emerging DBPs (haloacetonitriles, halonitromethanes and halopropanones) during ozonation and ozonation/hydrogen peroxide treatment and subsequent chlorination of sewage effluent under various experimental conditions. Estimation of possible risk due to DBPs by calculation of cytotoxicity and genotoxicity was attempted. The studied DBPs showed different formation behavior during chlorination, with maximum yields within 0.5-48 h. Maximum cytotoxicity and genotoxicity was observed after 4 h of chlorination with dibromoacetonitrile being the major contributor. Ozonation and O₃/H₂O₂ treatment resulted in increase of trichloronitromethane followed by a decline at higher doses, and reduction of haloacetonitriles. High ozone doses reduced cytotoxicity and genotoxicity of treated effluents. The presence of bromide shifted to bromo-DBPs formation and enhanced both cytotoxicity and genotoxicity. Particulate fraction in effluents significantly contributed to the formation of DBPs and consequently to the their toxicity.

Effects of residual carbon materials on the disinfection byproduct formation in artificial and natural waters

As the effective adsorbents, carbon materials (CMs) are typically used in the removal of disinfection byproduct (DBP) precursors during the water treatment by adding CMs before disinfection procedure. However, after the separation of CMs from the treated water by flocculation, sedimentation, and filtration, a small amount of loaded activated carbon could be released into the water treatment system and affect the DBP formation in the following disinfection. In this study, three CMs, including coal-made activated carbon (CAC), sawdust charcoal (SCC), and hydroxylated multiwall carbon nanotubes (OH-MWCNT), were used to explore the effects of residual CMs in the formation of DBPs. The results indicated that some DBP precursors could be irreversibly adsorbed into the pore structure of CMs and hardly to be extracted and determined, then affected the DBP formation in the water system. In the chlorination process of surface water samples, CMs have similar effects on the formation of DBPs. However, given that water samples contain a variety of complex substances, the effects of residual CMs on the formation of DBPs were also slightly changed.

Anion-exchange resin adsorption followed by electrolysis: A new disinfection approach to control halogenated disinfection byproducts in drinking water

Bromide and natural organic matter (NOM) are both precursors of halogenated disinfection byproducts (DBPs) in drinking water. During drinking water treatment process, chloride-form anion-exchange resin adsorption is expected to be capable of removing these DBP precursors and in the meantime releasing chloride ions. The released chloride as well as the chloride initially present in source water could be oxidized through electrolysis to generate free chlorine for disinfection. Based on the above assumptions, we developed a new disinfection approach using chloride-form anion-exchange resin adsorption followed by electrolysis to control halogenated DBPs. Parameter setup and optimization were performed for resin adsorption and electrolysis processes. Results showed that 93.7% of NOM and 90% of bromide could be removed at a resin dose of 20 mL per 2 L of simulated source water sample with a contact time of 1 h. Meanwhile, 49.5 mg/L of chloride was released from the resin to the water sample via anion-exchange, and the released chloride was further oxidized by electrolysis (Ti/RuO₂-IrO₂ anode and graphite cathode, current intensity of 0.4 A) to generate free chlorine (5 mg/L as Cl₂) within 192 s. With this new approach, formation of total organic halogen, four trihalomethanes, and five haloacetic acids was reduced by 86.4%, 98.5%, and 93.2%, respectively, compared with chemical chlorination alone. Although the new approach might enhance the formation of some phenolic DBPs by decreasing bromide levels in source water, the overall cytotoxicity of the water samples treated with the new approach was significantly decreased by 68.8% according to a human hepatoma cell cytotoxicity assay. Notably, disinfection ability evaluation showed that the new approach achieved 3.36-log₁₀ reductions of three seeded bacteria (Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus) in 19 s, suggesting that it was not only effective to E. coli but also effective to the chlorine-resistant bacteria (P. aeruginosa and S. aureus).

Formation of metastable disinfection byproducts during free and combined aspartic acid chlorination: Effect of peptide bonds and impact on toxicity

The formation and occurrence of haloacetonitriles (HANs) in drinking water is of increasing concern because recent data have shown that they are the major contributors to DBP-associated toxicity of disinfected waters. Earlier research on HAN formation had established free amino acids as important HAN precursors due to their high reactivity with chlorine. However, free amino acids are unlikely to be the primary precursors for HANs in natural waters, mainly because the actual concentrations of these compounds are too low to sufficiently account for observed HAN formation. On the other hand, combined amino acids (i.e., peptides and proteins) are of much higher abundance even though it is unclear if they can contribute to HAN formation given that nearly all the amino nitrogen is tied up in peptide linkages. In order to clarify the reactivity of combined amino acids with chlorine to form HANs, dichloroacetonitrile (DCAN) formation kinetics was compared between free aspartic acid and two aspartyl-containing tetrapeptides (i.e., Asp-Asp-Asp-Asp and Arg-Gly-Asp-Ser). Results indicated that aspartyl residue could also lead to DCAN formation upon chlorination, whereas the rate of DCAN formation was much slower compared to that from free aspartic acid chlorination. Moreover, DCAN formation from the two model peptides was catalyzed by high pH. This is because chlorine-induced peptide backbone degradation is the key to DCAN formation from the chlorination of combined amino acids and this slow stepwise process is base-catalyzed. Perhaps most importantly, regardless of the precursors, DCAN was continuously formed but simultaneously degraded especially at alkaline pHs, leaving the corresponding N-chloro-2,2-dichloroacetamide (N-Cl-DCAM) and dichloroacetic acid (DCAA) as major end products. It was observed that over increasing chlorine exposure, there exists an important transition from initial organic precursors through metastable chlorination intermediates (e.g., DCAN and N-Cl-DCAM) and finally to stable end products (e.g., DCAA). By weighting DBP concentrations by their respective cytotoxic potencies, it is estimated that the aggregate cytotoxicity of chlorinated water would reach its maximum at relatively short chlorine contact times. In general, shorter water age and lower pH both resulted in higher levels of metastable intermediates (i.e., DCAN) and thus higher levels of aggregate calculated cytotoxicity. The resulting toxicity profile is different from the prevailing notion that supports current DBP regulations. Therefore, there is a risk that by placing regulatory limits and control strategies exclusively on regulated end products (e.g., HAAs), the overall toxicity of drinking water might be inadvertently elevated.

Removal of bromide from natural waters: Bromide-selective vs. conventional ion exchange resins

The presence of bromide (Br^-) in water results in the formation of brominated disinfection byproducts (DBPs) after chlorination, which are much more cytotoxic and genotoxic than their chlorinated analogs. Given that conventional water treatment processes (e.g., coagulation, flocculation, and sedimentation) fail to remove Br^- effectively, in this study, we systematically tested and compared the performance of different anion exchange resins, particularly two novel Br-selective resins, for the removal of Br^-. The resins performance was evaluated under both typical and challenging background water conditions by varying the concentrations of anions and organic matter. The overall Br^- removal results followed the trend of Purolite-Br ≥ MIEX-Br > IRA910 ≥ IRA900 > MIEX-Gold > MIEX-DOC. Further evaluation of Purolite-Br resin showed Br^- removal efficiencies of 93.5 ± 4.5% for the initial Br^- concentration of 0.25 mg/L in the presence of competing anions (i.e., Cl^-, NO₃^-, NO₂^-, SO₄^2-, PO₄^3-, and a mixture of all five), alkalinity and organic matter. In addition, experiments under challenging background water conditions confirmed the selectivity of the resins (i.e. Purolite-Br and MIEX-Br) in removing Br^-, with SO₄^2- and Cl^- exhibiting the greatest influence upon the resin performance followed by NOM concentration, regardless of the NOM characteristic. After Br^- removal, both the subsequent formation of brominated DBPs (trihalomethanes, haloacetic acids, and haloacetonitriles), and the total organic halogens (TOX), decreased by ∼90% under the uniform formation conditions. Overall, Br-selective resins represent a promising alternative for the efficient control of Br-DBPs in water treatment plants.

Precursors for brominated haloacetic acids during chlorination and a new useful indicator for bromine substitution factor

Brominated haloacetic acids (HAAs) are much more cytotoxic and genotoxic than chlorinated one, yet little information is available for their organic precursors. In the present study, 8 water samples were collected in East China: 2 from lakes, 2 from rivers, 2 from reservoirs, a well and a mountain spring. Dissolved organic carbon (DOC), UV absorbance at 254 (UVA), specific UVA (SUVA) and chlorophyll a (Chl-a) were determined in raw water samples; formation of 9 HAA species as well as bromine substitution factor (BSF) were measured in chlorinated water samples. Results showed that water samples located in city generally contained higher levels of DOC (6.4-12.2mg/L) and UVA (0.124-0.194/cm), while those in the country side, low DOC (2.4-5.9mg/L) and UVA (0.061-0.109/cm) levels were observed. Negative relationship (p<0.01) was found between SUVA values and Chl-a levels. Among 9 HAA species, 4 brominated HAA were detected. As for BAA and DBAA (i.e. Br-HAAs), their yields (μg/L) were significant related (p<0.05) with DOC; In terms of BCAA and BDCAA (i.e. ClBr-HAAs), they were not only related with DOC, but also with UVA. These two results were quite different from DCAA and TCAA (Cl-HAAs), whose yields (μg/mg C) were only correlated with SUVA values, suggesting that precursors of Cl-HAA, Br-HAA and ClBr-HAA were different from each other, and their aromaticity/hydrophobicity may be in the order of Br-HAA<ClBr-HAA<Cl-HAA. Interrelationship between Br/DOC, SUVA and BSF revealed that BSF can be influenced by SUVA and Br/DOC, but in comparison, Br/UVA was the best indicator to describe BSF. This pattern is not only true in di-HAAs and tri-HAAs in this study, but also valid in other water samples and other species of disinfection by-product (e.g. trihalomethanes, dihaloacetonitriles, trihalonitromethanes).

Chloramination of iodide-containing waters: Formation of iodinated disinfection byproducts and toxicity correlation with total organic halides of treated waters

The formation of iodinated disinfection byproducts (I-DBPs) in drinking waters is of a concern due to their higher cyto- and genotoxicity than their chlorinated and brominated analogues. This study investigated the formation of I-DBPs under chloramination conditions using preformed chloramine and associated cyto- and geno-toxicities obtained with Chinese Hamster Ovary (CHO) cell assay. Cyto- and geno-toxicity of the samples were also calculated using DBP toxicity index values and correlated with total organic halide (TOX) formation. In low iodide (I^-) (0.32 μM, 40 μg L^-1) water, increasing dissolved organic carbon (DOC) concentration of selected waters from 0.1 to 0.25 mg L^-1 increased the formation of iodinated trihalomethanes (I-THMs), while further increases from 0.25 to 4 mg L^-1 produced an opposite trend. In high iodide water (3.2 μM, 400 μg L^-1), increasing DOC from 0.5 to 4 mg L^-1 gradually increased the I-THM formation, while a decrease was observed at 5.4 mg L^-1 DOC. Iodoform was the most influenced species from the changes in DOC concentration. While increasing the initial iodide concentration from 0 to 5 μM increased the formation of iodoform, it did not make any considerable impact on the formation of other I-THMs. The measured cytotoxicity of samples was significantly correlated with increasing DOC concentration. Unknown TOCl and TOI showed a high correlation with measured cytotoxicity, while calculated total organic chlorine (TOCl) and total organic iodine (TOI) did not correlate. The comparison of measured and calculated cytotoxicity values showed that the calculated values do not always represent the overall cytotoxicity, since the formation of unknown DBPs are not taken into consideration during the toxicity calculations.

Using UV/H2O2 pre-oxidation combined with an optimised disinfection scenario to control CX3R-type disinfection by-product formation

The effects of UV/H₂O₂ pre-oxidation or disinfection methods on the formation of partial disinfection by-products (DBPs) have been studied previously. This study assessed the effect of UV/H₂O₂ pre-oxidation combined with optimisation of the disinfection method on the formation of six classes of CX₃R-type DBPs, including trihalomethanes (THMs), haloacetic acids (HAAs), haloacetaldehydes (HALs), haloacetonitriles (HANs), halonitromethanes (HNMs), and haloacetamides (HAMs). Experimental results showed that a simulated distribution system (SDS) in-situ chloramination or pre-chlorination followed by chloramination effectively decreased total CX₃R-type DBP formation by 51.1-63.5% compared to SDS chlorination, but little reduction in DBP-associated toxicity was observed. The dominant contributors to the calculated toxicity were HANs and HALs. UV/H₂O₂ pre-oxidation was able to destroy the aromatic and dissolved organic nitrogen components of natural organic matter. As a consequence, THM, HAA, and HAL formations increased by 49.5-55.0%, 47.8-61.9%, and 42.0-67.1%, respectively, whereas HAN, HNM, and HAM formations significantly decreased by 52.1-83.6%, 42.9-87.3%, and 74.1-100.0%. UV/H₂O₂ pre-oxidation increased total CX₃R-type DBP formation, during SDS chlorination, whereas SDS in-situ chloramination or pre-chlorination followed by chloramination of UV/H₂O₂-treated water produced lower total CX₃R-type DBPs than water without UV/H₂O₂ pre-oxidation. Nevertheless, the DBP-associated toxicity of water with UV/H₂O₂ pre-oxidation was substantially lower than the toxicity for water without UV/H₂O₂ pre-oxidation, decreased by 24.1-82.7%. HALs followed by HANs contribute to major toxic potencies in UV/H₂O₂ treated water. The best DBP concentration and DBP-associated toxicity abatement results were achieved for water treated by UV/H₂O₂ coupled with in-situ chloramination treatment.

Effects of dechlorination conditions on the developmental toxicity of a chlorinated saline primary sewage effluent: Excessive dechlorination is better than not enough

Chlorine-disinfected sewage effluents are typically dechlorinated by using NaHSO₃, Na₂SO₃, or Na₂S₂O₃, as chlorine residual could be harmful to aquatic organisms upon discharge of sewage effluents into receiving marine water. In this study, we systematically investigated the effects of dechlorination-related factors on the developmental toxicity of a chlorinated saline primary sewage effluent, via direct exposure of the embryos of a marine polychaete to the effluent. The results showed that dechlorination ratio (i.e., the ratio of the dosed amount to the requisite stoichiometric amount of a dechlorination agent) and mixing condition were critical factors affecting the toxicity of the effluent. The toxicity of the effluent under insufficient dechlorination conditions was mainly caused by residual chlorine, especially monochloramine. Although the three dechlorination agents generally performed similarly, dechlorination with Na₂S₂O₃ required a more vigorous mixing condition than that with NaHSO₃ or Na₂SO₃, as the relatively high density of Na₂S₂O₃ might affect the mixing efficiency. Under insufficient mixing conditions, a prolonged dechlorination time was beneficial to achieving complete dechlorination and thus lowered the toxicity of the effluent. Moreover, because disinfection byproducts (DBPs) may have chronic effects on aquatic organisms, the developmental toxicity of the DBP mixtures in the chlorinated effluent in different dechlorination scenarios was also evaluated. The results indicated that increasing the dechlorination ratio reduced the developmental toxicity of the DBP mixture in the chlorinated saline sewage effluent, which might be ascribed to the decrease of the levels of overall brominated and iodinated DBPs; the dechlorination agent (NaHSO₃ or Na₂S₂O₃) might act as a nucleophile in the nucleophilic substitution and cause the substitution of bromine or iodine atoms in brominated and iodinated DBPs. The results from this study might aid in the design and operation of dechlorination facilities in sewage treatment plants.

Identification, Formation, and Predicted Toxicity of Halogenated DBPs Derived from Tannic Acid and Its Biodegradation Products

Humic substances are commonly known disinfection byproduct (DBP) precursors. Tannic acid is one precursor of humic substances in organic degradation, and it occurs ubiquitously in both source water and wastewater. In this study, the biological degradation process was simulated under laboratory conditions, and the characteristics of DBP formation generated from the chlorination of tannic acid samples with different biodegradation times were explored. Twenty-six emerging halogenated DBPs were identified, and the formation pathways of the tannic acid-derived DBPs were tentatively proposed. Moreover, results demonstrated that the profile of the chlorinated DBP formation was significantly different from its brominated counterpart during biodegradation, and a general increasing trend of the ratio of TOBr/TOX or TII_PIS79/(TII_PIS79+TII_PIS35) as biodegradation time increasing was noticeable. The observed trend could be mainly ascribed to the reactive sites of tannic acid shifting from relatively fast to slow sites during biodegradation. In addition, the comparative toxicity of the detected DBPs derived from tannic acid was predicted by using two quantitative structure-activity relationship models established previously. On the basis of both the two toxicity metrics (involving developmental toxicity and growth inhibition potency), the predicted toxicity data indicated that the emerging DBP group trihalo-(di)hydroxycyclopentane-1,3-diones may possess extremely high toxic potencies.

Influence of UV irradiation on the toxicity of chlorinated water to mammalian cells: Toxicity drivers, toxicity changes and toxicity surrogates

UV irradiation was reported to be able to degrade some kinds of DBPs, yet its influence on the toxicity of chlorinated water to mammalian cells remains unknown. This study systematically investigated the influence of low-pressure UV irradiation on the DBPs and toxicity of chlorinated drinking water (DW) and reclaimed water (RW). The apparent first-order rate constant (k_obs) of degradation kinetics of known DBPs increased with the increased Br substitutions. Haloacetonitriles were identified as toxicity drivers among the detected DBPs, which even contributed more to the toxicity after UV irradiation, mainly due to the refractory bromochloroacetonitrile (BCAN) and dichloroacetonitrile (dCAN). Both total organic halogen, cytotoxicity and genotoxicity were significantly removed under UV irradiation, with the removal rate of 22.9%-41.7% for cytotoxicity and a higher rate of 33.1%-55.5% for genotoxicity under 2400 mJ/cm² irradiation. UV irradiation significantly decreased the UV₂₅₄, SUVA₂₅₄ and fluorescence intensity (FLU) of chlorinated water. Results from high performance size exclusion chromatography revealed that chlorinated DW mainly contained high molecular weight (MW) compounds (>1000 Da) while chlorinated RW mainly contained lower MW compounds (100-500 Da). Chromophores and fluorophores in compounds of 100-500 Da increased in chlorinated DW while decreased in chlorinated RW under UV irradiation. Both the removal of UV₂₅₄, SUVA₂₅₄, FLU, MW-based UV₂₅₄ (>1000 Da) and MW-based FLU (each fractions) were significantly correlated (p < 0.05) with the removal of toxicity under UV irradiation. The UV₂₅₄ of chlorinated water was recommended as the optimal surrogate for toxicity removal.

Evidence-based analysis on the toxicity of disinfection byproducts in vivo and in vitro for disinfection selection

Disinfection is a key step in drinking water treatment process to prevent water-borne infections. However, reactions between chlorine, one of the most common disinfectants, and natural organic matter (NOM) often lead to the formation of hazardous disinfection byproducts (DBPs). However, the cytotoxicity of some DBPs is still poorly understood. Such knowledge is critical for proper selection of disinfection processes. We investigated the effects of DBPs on mouse acute liver injury. The exacerbation of liver damage increased with the DBPs concentrations, likely due to the increased hepatic macrophages. Haloacetonitriles (HANs) and haloketones (HKs) are more toxic to Human Hepatocellular (Hep3B) cells than trihalomethanes (THMs). Cytotoxicity of DBPs were governed by the halogen type (brominated DBPs > chlorinated DBPs) and the numbers of halogen atoms per molecule. Then, we used the pilot-scale WDS to study the best conditions for reducing the formation of DBPs. The result showed that the formation of DBPs followed the order: stainless-steel (SS) > ductile iron (DI) > polyethylene (PE) pipe. Higher flowrate promoted the formation of DBPs in all three pipes. The results suggest that the formation of DBPs in chlorine disinfection can be reduced by using PE pipes and low flow rate in water distribution systems (WDS).

Reduction of DBPs in synthetic water by indoor techniques and its implications on exposure and health risk

Disinfection byproducts (DBPs) in municipal supply water have been a concern. Many DBPs have been characterized as possible and probable human carcinogens, which can pose elevated cancer risks through lifetime exposure to municipal supply water. Few DBPs are regulated in many countries to control human exposure and risk from DBPs. In risk assessment studies, concentration of DBPs in water distribution systems is often used, whereas populations are typically exposed to indoor tap water. Through employing several techniques, DBPs can be reduced prior to water consumption, which is likely to reduce human exposure and risk of DBPs. This study investigated six indoor techniques in reducing trihalomethanes (THMs) and haloacetic acids (HAAs) in synthetic water and the effects of these techniques on exposure and risk. The techniques are: S1, S2: storing water in a refrigerator with and without lids respectively; S3, S4: boiling water for 1 min followed by storing in a refrigerator with and without lids respectively; S5, S6: filtering water using new and used granular activated carbon (GAC) filters and storing in a refrigerator without lids. Storing of water (S1, S2) reduced THMs in the range of 14.8-47.2% while boiling (S3, S4) and filtration (S5, S6) reduced THMs in the range of 77.3-92.8%. In S1-S4 techniques, HAAs were not reduced significantly while in S5 - S6 techniques, HAAs were reduced in the range of 64.7-69.8%. In S3-S6 techniques, overall cancer and non-cancer risks were reduced by 45.5-82.6% and 26.3-80.0% respectively. The findings might prove useful in understanding DBPs exposure, associated risks, strategies to minimize exposure to these contaminants and updating regulatory guidelines for better protection of health risks from DBPs.

Coagulation of Iodide-Containing Resorcinol Solution or Natural Waters with Ferric Chloride Can Produce Iodinated Coagulation Byproducts

Iodinated disinfection byproducts (I-DBPs) are of particular concern in drinking water due to the more cytotoxic and genotoxic properties than their chlorinated and brominated analogs. Formation of I-DBP primarily results from the oxidation of iodide-containing waters with various oxidants and the chlor(am)ination of iodinated organic compounds in drinking water. This study first reports that ferric chloride (FeCl₃) can lead to the formation of iodinated coagulation byproducts (I-CBPs) from iodide-containing resorcinol solution or natural waters. The unwanted I-CBP formation involved the oxidation of iodide by ferric ions to generate various reactive iodine species, which further oxidize organic compounds. Although the oxidation rate of iodide by FeCl₃ was several orders of magnitude slower than that by chlorine or chloramine, most of the converted iodide under the ferric/iodide system was transformed into iodine and iodinated organic compounds rather than iodate. Formation of four aliphatic I-CBPs was observed, and four aromatic I-CBPs were identified by gas chromatography mass-spectrometry and theoretical calculation. Coagulation of iodide-containing waters with FeCl₃ also produced I-CBPs ranging from 12.5 ± 0.8 to 32.5 ± 0.2 μg/L as I. These findings call for careful consideration of the formation of I-CBPs from coagulation of iodide-containing waters with ferric salts.

Global Transcriptional Analysis of Nontransformed Human Intestinal Epithelial Cells (FHs 74 Int) after Exposure to Selected Drinking Water Disinfection By-Products

BACKGROUND

Drinking water disinfection inadvertently leads to the formation of numerous disinfection by-products (DBPs), some of which are cytotoxic, mutagenic, genotoxic, teratogenic, and potential carcinogens both in vitro and in vivo.

OBJECTIVES

We investigated alterations to global gene expression (GE) in nontransformed human small intestine epithelial cells (FHs 74 Int) after exposure to six brominated and two chlorinated DBPs: bromoacetic acid (BAA), bromoacetonitrile (BAN), 2,6-dibromo-p-benzoquinone (DBBQ), bromoacetamide (BAM), tribromoacetaldehyde (TBAL), bromate (BrO3-), trichloroacetic acid (TCAA), and trichloroacetaldehyde (TCAL).

METHODS

Using whole-genome cDNA microarray technology (Illumina), we examined GE in nontransformed human cells after 4h exposure to DBPs at predetermined equipotent concentrations, identified significant changes in gene expression (p≤0.01), and investigated the relevance of these genes to specific toxicity pathways via gene and pathway enrichment analysis.

RESULTS

Genes related to activation of oxidative stress-responsive pathways exhibited fewer alterations than expected based on prior work, whereas all DBPs induced notable effects on transcription of genes related to immunity and inflammation.

DISCUSSION

Our results suggest that alterations to genes associated with immune and inflammatory pathways play an important role in the potential adverse health effects of exposure to DBPs. The interrelationship between these pathways and the production of reactive oxygen species (ROS) may explain the common occurrence of oxidative stress in other studies exploring DBP toxicity. Finally, transcriptional changes and shared induction of toxicity pathways observed for all DBPs caution of additive effects of mixtures and suggest further assessment of adverse health effects of mixtures is warranted. https://doi.org/10.1289/EHP4945.

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.

Degradation kinetics, byproducts formation and estimated toxicity of metronidazole (MNZ) during chlor(am)ination

The residues of pharmaceuticals and personal care products (PPCPs) in environmental waters have been widespread concerned. Metronidazole (MNZ), normally employed to treat inflammation and infection, was chosen as one model PPCP. The degradation of MNZ by chlorination could be fitted by pseudo-first-order kinetics as the observed pseudo-first-order rate constants increasing from 0.0302 min^-1 to 0.2872 min^-1. However, the kinetics during chloramination of MNZ followed pseudo-second-order reaction, whose estimated half-live was approximately 6-8 times longer than chlorination. The chlor(am)ination of MNZ especially formed chloroform (CF), dicholoacetamide (DCAcAm), tricholoacetamide (TCAcAm) and dichloroacetonitrile (DCAN), and their yields were overall lower under chloramination than chlorination. During chlorination, the yield of CF was increased from 0.35 ± 0.02% to 2.06 ± 0.12% with 1-20 chlorine/MNZ molar ratio, whereas the formations of DCAcAm, TCAcAm and DCAN increased firstly and then decreased. Increasing chloramine dosage promoted the concentrations of scheduled disinfection byproducts (DBPs). CF and TCAcAm kept continuous generation in chlor(am)ination versus reaction time. Compared with the chlorination, the chloramination of MNZ was more dependent on pH value due to the self-degradation of chloramine. Faintly acidic condition favored N-DBPs' formation in MNZ when it was subjected to chlor(am)ination. The chloramination of MNZ produced cytotoxicity and genotoxicity by 10-15 folds lower than chlorination, and DCAN formed during chloramination dominated both DBPs' yields and toxicity contribution. Opposite to chlorination, the integrated toxicity of MNZ during chloramination varied linearly versus N-DBPs' yields.

Iodination of Dimethenamid in Chloraminated Water: Active Iodinating Agents and Distinctions between Chlorination, Bromination, and Iodination

Few studies have elucidated the agent(s) that generate iodinated disinfection byproducts during drinking water treatment. We present a kinetic investigation of iodination of dimethenamid (DM), a model compound lacking acid-base speciation. Water chemistry parameters (pH, [Cl^-], [Br^-], [I^-], and [pH buffer]) were systematically varied. As pH increased (4-9), DM iodination rate decreased. Conventional wisdom considers hypoiodous acid (HOI) as the predominant iodinating agent; nevertheless, HOI (pK_HOI = 10.4) could not have produced this result, as its concentration is essentially invariant from pH 4-9. In contrast, [H₂OI⁺] and [ICl] both decrease as pH increases. To distinguish their contributions to DM iodination, [Cl^-] was added at constant pH and ionic strength. Although chloride addition did increase the iodination rate, the reaction order in [Cl^-] was fractional (≤0.36). The contribution of ICl to DM iodination remained below 47% under typical drinking water conditions ([Cl^-] ≤ 250 mg/L), implicating H₂OI⁺ as the predominant iodinating agent. Distinctions between DM iodination versus chlorination or bromination include a more pronounced role for the hypohalous acidium ion (H₂OX⁺), negligible contributions by hypohalous acid and molecular halogen (X₂), and a more muted influence of XCl, leading to lesser susceptibility to catalysis by chloride.

Disinfection Byproducts in Rajasthan, India: Are Trihalomethanes a Sufficient Indicator of Disinfection Byproduct Exposure in Low-Income Countries?

The implementation of chlorine disinfection in low-income countries reduces the risk of waterborne illness but initiates exposure to disinfection byproducts (DBPs). Like high-income countries, low-income countries typically are adopting regulations focusing on trihalomethanes (THM4) as an indicator of overall DBP exposure. However, the use of impaired water sources can decouple the formation of THM4 from other DBP classes that are more potent toxins. The documentation of DBP species other than THM4 is rare in low-income countries, where water sources may be degraded by inadequate sanitation infrastructure and other uncontrolled wastewater discharges. We measured THM4 and 21 unregulated DBPs in tap waters and laboratory-treated source waters from two cities in northwestern India. The contribution of each DBP class to the cumulative toxicity was estimated by weighting each species by metrics of toxic potency; haloacetonitriles typically were the dominant contributor, while the contribution of THM4 was negligible. THM4 concentrations did not correlate with the total toxic potency-weighted DBP concentrations. Although THM4 rarely exceeded international guidelines, DBPs of greater toxicological concern were observed in high concentrations. The total toxic potency-weighted DBP concentrations in some waters were elevated compared to conventional drinking waters in high-income countries and more closely resembled chlorine-disinfected wastewater effluents. Artificial sweeteners confirmed widespread contamination of both surface and groundwaters by domestic sewage. The results suggest that THM4 may not be an adequate indicator of overall DBP exposure in impaired water supplies prevalent in some low-income nations.

500 days of swimmers: the chemical water quality of swimming pool waters from the beginning

Many studies of disinfection by-products (DBPs) in pools have focused on haloacetic acids, trihalomethanes, and chloramines, with less studies investigating the occurrence of other DBPs, such as haloketones, haloacetaldehydes, haloacetonitriles, halonitromethanes, and haloacetamides. Furthermore, while many studies have achieved a broadscreen analysis across several pools, fewer studies have followed the water quality of pools over time, with information regarding the production and fate of DBPs in pools over extended periods (e.g. > 1 year) being limited. This study reports the occurrence of 39 DBPs and several general water quality parameters in two newly built and filled swimming pools over 15 months, where investigations began prior to opening. DBP concentrations measured in this study were generally similar to or higher than those previously reported in chlorinated pools, with concentrations of chloroacetic acid, dichloroacetic acid, trichloroacetic acid, and chloral hydrate (trichloroacetaldehyde) in some samples being higher than previously reported maximum concentrations. Considering both pools, lower concentrations of DBPs were measured in the pool where a steady state non-purgeable organic carbon concentration was achieved, highlighting the importance of the establishment of a steady state balance of mineralisation versus addition of organic carbon to reduce precursors for DBP formation in pools. Pools were found to exhibit significantly higher estimated cytotoxicity than their filling water, which reflects the significantly higher concentrations of DBPs measured in the pools in comparison to the filling water. Chloral hydrate accounted for up to 99% the total estimated cytotoxicity and was found to be correlated to the number of pool entries, suggesting that swimmers may be a potential source of chloral hydrate precursors in pools. The presence and subsequent peak of non-purgeable organic carbon and DBPs prior to, and soon after, opening suggest that the building process and/or new pool infrastructure may have had a significant impact on the chemical water quality, particularly on DBP formation. This study includes the first quantification of bromochloroacetaldehyde, bromodichloroacetaldehyde, bromochloronitromethane, and dichloronitromethane in chlorinated swimming pools, and provides important new knowledge on the long-term trends of DBPs in pools.

Formation of iodinated trihalomethanes and noniodinated disinfection byproducts during chloramination of algal organic matter extracted from Microcystis aeruginosa

The increasing occurrence of harmful algal blooms in surface waters may increase the input of algal organic matter (AOM) to the dissolved organic matter pool. The formation of iodinated trihalomethanes (I-THMs) and noniodinated disinfection byproducts (DBPs) in synthetic waters containing AOM extracted from Microcystis aeruginosa was investigated in chloramination (preformed and in-situ formed chloramine, NH₂Cl and Cl₂-NH₂Cl, respectively) and chlorination (Cl₂) processes. AOM is much more favorable for iodine incorporation than natural organic matter (NOM). For example, the formation of I-THM from AOM is much higher than NOM isolate extracted from treated water (e.g., 3.5 times higher in the NH₂Cl process), and thus higher iodine utilization and substitution factors from AOM were observed. Short contact time (2 min) chlorination in Cl₂-NH₂Cl process leading to the formation of halogenated intermediates favored I-THM formation, compared with NH₂Cl process. However, further increasing chlorine contact time from 5 min to 24 h facilitated the conversion from iodide to iodate and thus I-THM formation decreased. Meanwhile, the formation of noniodinated THM4, haloacetonitriles (HANs), and haloacetaldehydes (HALs) increased. Factors including concentrations of AOM and bromide, pH, and chlorine/nitrogen ratios influenced the formation of I-THMs and noniodinated DBPs. To evaluate the benefit of mitigating I-THM formation over the risk of noniodinated DBP formation, measured DBPs were weighed against their mammalian cell toxicity indexes. Increasing the chlorine exposure increased the calculated cytotoxicity based on concentrations of measured I-THMs and noniodinated DBPs since unregulated HANs and HALs were the controlling agents.

Disinfection byproducts potentially responsible for the association between chlorinated drinking water and bladder cancer: A review

Epidemiological studies have consistently associated the consumption of chlorinated drinking water with an enhanced risk of bladder cancer. While this suggests that some disinfection byproducts (DBPs) are bladder carcinogens, causal agents are unknown. This study aims to highlight likely candidates. To achieve this, structures of known and hypothesised DBPs were compared with 76 known bladder carcinogens. The latter are dominated by nitrogenous and aromatic compounds; only 10 are halogenated. Under 10% of the chlorine applied during drinking water treatment is converted into identified halogenated byproducts; most of the chlorine is likely to be consumed during the generation of unidentified non-halogenated oxidation products. Six nitrosamines are among the nine most potent bladder carcinogens, and two of them are known to be DBPs: N-nitrosodiphenylamine and nitrosodibutylamine. However, these and other nitrosamines are formed in insufficiently low concentrations in chlorinated drinking water to account for the observed bladder cancer risk. Furthermore, although not proven bladder carcinogens, certain amines, haloamides, halocyclopentenoic acids, furans and haloquinones are potential candidates. At present, most identified bladder carcinogens are nitrogenous, whereas >90% of natural organic matter is not. Therefore, non-nitrogenous DBPs are likely to contribute to the bladder cancer risk. Given the high proportion of DBPs that remains uncharacterised, it is important that future research prioritises compounds believed to be potent toxicants.

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.

Comparative toxicity of organic mixture attached to road deposited sediments: Inadequacy of conventionally using individual pollutants to assess comprehensive hazard effects

Organic pollutants attached on road deposited sediments (RDS) during dry days can be washed-off into stormwater runoff during rainfall events, undermining stormwater reuse safety. Previous research studies commonly utilized individual pollutant groups and their quantity to evaluate the hazard effect of pollutants attached to RDS in terms of stormwater reuse. Since many types of organic pollutants are present together rather than individually, conventional approaches might not permit a comprehensive understanding of how appropriately the RDS polluted stormwater can be reused. This study undertook a toxicity test of organic pollutants attached to RDS using Chinese hamster ovary cells (CHO), testing a hypothesis that solely focusing on individual pollutant groups are not adequate to represent hazard effects of resulting stormwater and hence their adequacy for reuse. It is noted that comparative toxicity of RDS is not strongly related to total solids (commonly seen as the key carrier of pollutants) and chemical oxygen demand (COD, representing organic matters). Additionally, the comparison results of spatial distributions of toxicity (in this study) and individual pollutants in previous studies did not show a similar trend. These results imply that toxicity should be also used to indicate how stormwater can be safely reused while solely investigating individual pollutants can not adequately show a comprehensive hazard effect in terms of ensuring stormwater reuse safety. Based on study outcomes, a new assessment approach considering both pollutant and toxicity were proposed. This will assist on effective stormwater reuse and ensuring their reuse safety.

Disinfection byproduct formation during drinking water treatment and distribution: A review of unintended effects of engineering agents and materials

Unintended effects of engineering agents and materials on the formation of undesirable disinfection byproducts (DBPs) during drinking water treatment and distribution were comprehensively reviewed. Specially, coagulants, biologically active filtration biofilms, activated carbons, nanomaterials, ion-exchange resins, membrane materials in drinking water treatment and piping materials, deposits and biofilms within drinking water distribution systems were discussed, which may serve as DBP precursors, transform DBPs into more toxic species, and/or catalyze the formation of DBPs. Speciation and quantity of DBPs generated rely heavily on the material characteristics, solution chemistry conditions, and operating factors. For example, quaternary ammonium polymer coagulants can increase concentrations of N-nitrosodimethylamine (NDMA) to above the California notification level (10 ng/L). Meanwhile, the application of strong base ion-exchange resins has been associated with the formation of N-nitrosamines and trichloronitromethane up to concentrations of 400 ng/L and 9.0 μg/L, respectively. Organic compounds leaching from membranes and plastic and rubber pipes can generate high NDMA (180-450 ng/L) and chloral hydrate (∼12.4 μg/L) upon downstream disinfection. Activated carbon and membranes preferentially remove organic precursors over bromide, resulting in a higher proportion of brominated DBPs. Copper corrosion products (CCPs) accelerate the decay of disinfectants and increase the formation of halogenated DBPs. Chlorination of high bromide waters containing CCPs can form bromate at concentrations exceeding regulatory limits. Owing to the aforementioned concern for the drinking water quality, the application of these materials and reagents during drinking water treatment and distribution should be based on the removal of pollutants with consideration for balancing DBP formation during disinfection scenarios. Overall, this review highlights situations in which the use of engineering agents and materials in drinking water treatment and distribution needs balance against deleterious impacts on DBP formation.

Hazard and mode of action of disinfection by-products (DBPs) in water for human consumption: Evidences and research priorities

Disinfection of water system is an essential strategy to protect human health from pathogens and prevent their regrowth during water distribution, but the reaction of disinfectant agents with organic matter can lead to the formation of disinfection by-products (DBPs). Given their widespread occurrence, potential human health impacts and (eco)toxicity associated with exposure to DBPs are of particular interest due to their potential carcinogenicity and vary non-carcinogenic effects, such as endocrine disruption. Understanding the public health implications of this emerging issue is crucial for societies and decision-makers, supporting more effective water safety plans. Here, we review the recent literature on the effects of DBPs presented in drinking water and treated swimming pools water, focusing particularly in unregulated compounds and the putative underlying mode of action, linking the available data with adverse health outcomes. Overall, the majority of studies highlight the limited knowledge in the understanding of the underlying mode of action of DBPs. Yet, available evidences indicate that different signaling pathways seem to be involved in the adverse outcomes associated with distinct DBPs classes. The main knowledge gaps in this field are also identified, and future research priorities discussed.

Integrated control of CX3R-type DBP formation by coupling thermally activated persulfate pre-oxidation and chloramination

The alternative disinfectant chloramine can lower the formation of carbonaceous DBPs (C-DBPs) but promote the formation of nitrogenous DBPs (N-DBPs), which are more cytotoxic and genotoxic. In this study, the combination of thermally activated persulfate pre-oxidation and post-chloramination (TA/PS-NH₂Cl) was proposed to control the formation and reduce the toxicity of both C-DBPs and N-DBPs. The formation, speciation and toxicity of trihalomethanes, haloacetic acids, haloaldehydes, haloacetonitriles, halonitromethanes and haloacetamides, collectively defined as CX₃R-type DBPs, under TA/PS-NH₂Cl process were compared with processes of chlorination alone (Cl₂), chloramination alone (NH₂Cl) and coupled thermally activated persulfate pre-oxidation with post-chlorination (TA/PS-Cl₂). Results showed that chloramination could reduce formation of C-DBPs and total organic halogen (TOX) while increase N-DBP formation, and the introduction of TA/PS pretreatment process slightly increased the formation of C-DBPs and TOX but sharply reduced the formation of N-DBPs with higher toxicity as well as brominated CX₃R-type DBPs that are more toxic than their chlorinated analogues. By comprehensive toxicity calculation, an outright decline of both cytotoxicity and genotoxicity risk of CX₃R-type DBPs was observed during TA/PS-NH₂Cl process compared with Cl₂, NH₂Cl, and TA/PS-Cl₂ processes. In summary, TA/PS-NH₂Cl process was a potential effective method for integrally controlling the formation of CX₃R-type DBPs and their toxicity and is suggested to be used to treat raw waters containing no bromide or low levels of bromide considering bromate caused by TA/PS pre-oxidation. The study may provide a feasible and economical method for DBP control on the background of global warming.

Conversion of chlorine/nitrogen species and formation of nitrogenous disinfection by-products in the pre-chlorination/post-UV treatment of sulfamethoxazole

Pre-chlorination and UV disinfection are two common processes in drinking water treatment plants. Sulfamethoxazole (SMX), an antibiotic widely detected in source water, was selected as a precursor to study the conversion of chlorine/nitrogen species and DBP formation in pre-chlorination/post-UV process. The combined chlorine (mainly organic chloramines) produced in pre-chlorination of SMX can self-degrade and release free chlorine back again as pre-chlorination time goes on. With free chlorine dose increasing, the self-degradation rate of combined chlorine increased obviously. But the combined chlorine stopped self-degrading and remained stable around 1 mg-Cl₂/L after adding 0.30 mM chlorine for 30 min. Post-UV treatment after pre-chlorination can enhance the degradation and achieve a complete removal of combined chlorine (including organic chloramines). Deamination occurred during pre-chlorination/post-UV process and deamination amount (-NH₂) per SMX concentration was 0.19 M/M. Radicals in this process had no obvious influence on chlorine/nitrogen species conversion. Direct chlorination of SMX had the lowest DBP formation potentials while the application of pre-chlorination and UV enhanced them. Compared with UV treatment only, dichloroacetonitrile formation potential of SMX reduced by 1.58 × 10^-3 mol/mol-SMX (17.37 μg/l) after pre-chlorination/post-UV treatment. During pre-chlorination/post-UV/final-chlorination treatment of SMX, Br^- and natural organic matter can enhance DBP formation and toxicity-weighted values. Acid conditions showed a very high DBP risk, while alkaline conditions could cut this risk obviously, especially for the toxicity-weighted values of these DBPs.

Antibacterial performance of polymer quaternary ammonium salt-capped silver nanoparticles on Bacillus subtilis in water

In this study, we prepared polymer quaternary ammonium salt–capped silver nanoparticles (PQAS–AgNPs) and investigated their antimicrobial activities. The antimicrobial effectiveness of PQAS–AgNPs on Bacillus subtilis (B. subtilis), and the effect of dose, pH, chloride ion and humic acid (HA) were studied. It was found that PQAS–AgNPs revealed excellent antimicrobial activity to B. subtilis, compared with polyvinylpyrrolidone-capped silver nanoparticles (PVP-AgNPs), which was the reference antimicrobial material. The positive surface, the antimicrobial activity of PQAS, and the synergistic antibacterial effect between PQAS and AgNPs contributed to the significant antibacterial superiority of PQAS–AgNPs. This study demonstrated that the impact of the dose of the material was positive and the microbiocidal efficacy of PQAS–AgNPs was stronger at lower pH. In addition, the antibacterial performance of PQAS–AgNPs decreased in the presence of Cl⁻ and HA. Finally, in combination with the results of FCM and adenosine triphosphate (ATP) content, it was found that PQAS–AgNPs destroyed the respiratory chain of bacterial cells, reduced the synthesis of ATP, and destroyed the cell wall and cell membrane function.

Polymer quaternary ammonium salt–capped silver nanoparticles (PQAS–AgNPs) were synthesized, and they exhibited significant antibacterial capacity against Bacillus subtilis.

Toxicological Comparison of Water, Wastewaters, and Processed Wastewaters

Drinking water utilities will increasingly rely on alternative water sources in the future, including wastewater reuse. Safety must be assured in the application of advanced oxidation processes (AOPs) and supporting treatments for wastewater effluent reuse. This study developed toxicological profiles for source and tap waters, wastewaters, and treated effluents by different processes from four military installation locations. The objective of this study was to evaluate the toxicity of extracted organics from diverse source waters and after reuse treatments. The toxicity analyses included thiol reactivity, mammalian cell cytotoxicity, and genotoxicity. Differences in toxicity between source or tap waters and effluents from wastewater treatment processes supported AOP treatment to reduce risks of potable reuse. An anoxic and aerobic activated sludge process followed by sand filtration controlled toxicity to levels similar to a municipal drinking water. An anaerobic membrane bioreactor process exceeded the toxicity levels of a typical drinking water. Two AOP processes (ultraviolet (UV) + reverse osmosis (RO) + chlorination (NaOCl) or RO + UV-H₂O₂ + NaOCl) significantly reduced toxicity. The integration of the wastewater systems with ultrafiltration, AOP, and RO was effective to reduce the toxicity to levels comparable to, or better than, tap water samples.

A one-year long survey of temporal disinfection byproducts variations in a consumer's tap and their removals by a point-of-use facility

In order to better understand the occurrence of disinfection byproducts (DBPs) in tap water and their real impacts on consumers, this study made a one-year long survey of the temporal variations of a series of DBPs before and after a point-of-use (POU) treatment facility installed in a building serving for ∼300 people. Water samples were collected every week at a fixed location and time for 1 year, and frequent samplings were carried out every 6 h a day for 1 month at selected seasons, which ultimately amounted to 322 samples. The results show that the concentrations of DBPs were higher in the summer than other seasons, with the lowest DBP levels being observed in spring. Within one week, higher levels of haloacetic acids (HAAs) were identified on weekdays than those on weekends. Diurnally, trihalomethanes, HAAs, and haloacetaldehydes were found to be higher at noon but lower in the evening. Consistent with other studies, the variations of most DBPs were somewhat positively related to the changes of temperature and organic matter, but negatively related to the quantity of free chlorine. With the use of a POU facility, which equips with two activated carbon cartridges and a boiler in sequence, most of DBPs were dramatically reduced, leading to 62-100% lower cytotoxicity for the measured DBPs. The study hence provides a real-water evidence about the DBP occurrences in a typical distribution system endpoint and the efficiency of a typical POU on mitigating DBP risks.

The occurrence and transformation behaviors of disinfection byproducts in drinking water distribution systems in rural areas of eastern China

The occurrence and transformation behaviors of disinfection byproducts (DBPs) were investigated in the finished water and tap water of 14 water treatment plants in rural areas of eastern China. Mammalian cell toxicity data from previous studies were used to evaluate the cytotoxicity of trihalomethanes (THMs), haloacetic acids (HAAs), haloacetonitriles (HANs), trichloronitromethane (TCNM) and the genotoxicity of HAAs, HANs and TCNM. Correlation analysis was conducted to identify the factors that might influence the variability of DBPs. The measured median values were 29.76 μg/L for THMs, 20.47 μg/L for HAAs, 3.98 μg/L for HANs, 0.76 μg/L for haloketones (HKs) and 0.03 μg/L for TCNM. The spatial variability analysis showed that the total concentrations of HAAs and HANs decreased during long hydraulic residence time (HRT) in seven drinking water distribution systems, which could result in reduced mammalian cell cytotoxicity and genotoxicity at consumers' taps. The concentrations of trihalogenated-DBPs were more stable than those of dihalogenated-DBPs and monohalogenated-DBPs during long HRT. Bromine acted as a more efficient substituting agent than chlorine for THMs and dihaloacetonitriles (DHANs) in actual drinking water. The dominant chlorinated-THMs and chlorinated-DHANs would transfer to brominated -THMs and brominated-DHANs when the concentration of bromide ion exceeds 450.67 and 610.25 μg/L, respectively. Correlation analysis indicated that particulate and soluble manganese (Mn) might play critical roles in promoting the production of DBPs in bulk water. Hydraulic disturbance could also result in secondary release of DBPs from loose deposits accumulated on distribution pipe walls.

Formation of Brominated Organic Compounds and Molecular Transformations in Dissolved Organic Matter (DOM) after Ballast Water Treatment with Sodium Dichloroisocyanurate Dihydrate (DICD)

Estuarine water treated with a ballast water management system (BWMS) using a solution of dissolved dichloroisocyanurate dihydrate (DICD) resulted in the formation of newly described brominated disinfection byproducts (Br-DBPs). Analysis of dissolved organic matter (DOM) in untreated water with ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) identified 3897 m/z ions and their exact molecular formulas. After DICD treatment, a total of 213 halogenated molecular ions with relative abundance of at least 1% were assigned and confirmed using isotope simulation. Halogenated ions were assigned in four DBP elemental groups including CHOBr (180), CHONBr (13), CHOCl (16), and CHOBrCl (4). Forty-nine of the 197 brominated formulas have not been previously reported. We also were able to tentatively assign possible structures to the formula C₃HBr₃N₂ due to very limited isomeric possibilities. The tentatively assigned compound found at 6.4% relative abundance was identified as either tribromoimidazole or tribromopyrazole. Our results show the formation of complex halogenated DBPs that are formed in the treatment of water with a novel BWMS that employs granular DICD as a biocide. The toxicological and mutagenic properties as well as the fate of these newly identified brominated DBPs are unknown.

Re-assessing ICR GAC Treatment Study Database: Effect of Bromide on DBP Formation

While granular active carbon (GAC) can effectively remove disinfection byproduct (DBP) precursors, its use has raised concerns over increased formation of some brominated DBP species in treated water following postchlorination, especially for waters with high bromide concentrations. The Information Collection Rule Treatment Study Database contains results of the most extensive GAC studies ever conducted nationwide. Data were analyzed to assess the extent of DBP speciation changes and overall reduction of brominated DBPs by GAC to gain new insights of the bromide effect. Results showed that formation of three brominated trihalomethanes (collectively, Br-THM3) varied greatly depending on TOC removal and bromide concentrations. Low TOC concentrations in GAC effluents resulted in greatly reduced Br-THM3 formation, except for a few cases where Br-THM3 formation increased. GAC followed by chloramination were likely to better control Br-THM3 formation for waters with high TOC and high bromide. Lastly, the chlorine demand reduction by GAC was quantified.

Chronic exposure to dichloroacetamide induces biochemical and histopathological changes in the gills of zebrafish

To evaluate the impact of DCAcAm on zebrafish gill, we measure the responses of antioxidant enzyme (superoxide dismutase, SOD), lipid peroxidation (malondialdehyde, MDA), ATPase (Na⁺ /K⁺ -ATPase and Ca²⁺ /Mg²⁺ -ATP) and histopathological changes of gill in adult zebrafish, after exposed to different concentrations of DCAcAm (0, 1, 10, 100, and 1000 μg L^-1 ) for 30 days. Results indicated that DCAcAm first increased and then decreased SOD activity, and DCAcAm also lowered the activities of Na⁺ /K⁺ -ATPase and Ca²⁺ /Mg²⁺ -ATPase. These results indicated that high affinity of DCAcAm probably be a main factor, which can damage the structures of enzymes, thereby inhibiting the SOD and ATPase activities. Besides, histopathological investigation results also manifested that chronic exposure to DCAcAm can damage the gill tissues, disrupting the normal function of gills. We conclude that chronic exposure to DCAcAm was harmful to organisms, not only influence gill function, but also further cause damage on the gill tissues.

Application of (LC/)MS/MS precursor ion scan for evaluating the occurrence, formation and control of polar halogenated DBPs in disinfected waters: A review

Water disinfection can result in the unintended formation of halogenated disinfection byproducts (DBPs), which have been the subject of intensive investigation over the past 40 years. Robust methods for evaluating and characterizing the formation of halogenated DBPs are prerequisites for ultimately controlling the formation of DBPs and ensuring quality and safe disinfected water. Only a fraction of the total organic halogen (TOX) formed during disinfection has been chemically identified or even well characterized by the classical (derivatization-)gas chromatography/mass spectrometry (GC/MS) method. Such a method may not be amenable to the detection of polar halogenated DBPs, which constitute a major portion of the TOX that is still unaccounted for. Accordingly, a novel precursor ion scan (PIS) method using (liquid chromatography/) electrospray ionization-triple quadrupole mass spectrometry was developed for the rapid selective detection of all polar halogenated DBPs-no matter whether the DBPs are known or unknown-in water. This article reviews recent literature on the application of the PIS method for evaluating the occurrence, formation and control of polar halogenated DBPs in disinfected waters. The challenges in developing the PIS method were briefly summarized. Application of the powerful method pinpointed >150 previously unknown DBPs and revealed the formation, speciation and transformation of halogenated DBPs in disinfected drinking water, wastewater effluents, and swimming pool water. For the same source water, positive correlations were found between the total ion intensity (TII) levels in the PIS spectra of m/z 35/79/126.9 and the total organic chlorine/bromine/iodine levels in the disinfected water sample, and a disinfected sample with a higher TII level generally showed a higher toxic potency. Accordingly, the TII value can be used as a surrogate to comparatively reflect the water quality and assess the efficiency of a DBP control approach. To achieve a more comprehensive and systematic understanding of the DBP compositions in different waters and thus better control the DBP formation and reduce their overall toxicity, topics for future work were discussed.

Emerging Polar Phenolic Disinfection Byproducts Are High-Affinity Human Transthyretin Disruptors: An in Vitro and in Silico Study

Phenolic disinfection byproducts (phenolic-DBPs) have been identified in recent years. However, the toxicity data for phenolic-DBPs are scarce, hampering their risk assessment and the development of regulations on the acceptable concentration of phenolic-DBPs in water. In this study, the binding potency and underlying interaction mechanism between human transthyretin (hTTR) and five groups of representative phenolic-DBPs (2,4,6-trihalo-phenols, 2,6-dihalo-4-nitrophenols, 3,5-dihalo-4-hydroxybenzaldehydes, 3,5-dihalo-4-hydroxybenzoic acids, halo-salicylic acids) were determined and probed by competitive fluorescence displacement assay integrated with in silico methods. Experimental results implied that 2,4,6-trihalo-phenols, 2,6-dihalo-4-nitrophenols, and 3,5-dihalo-4-hydroxybenzaldehydes have a high binding affinity with hTTR. The hTTR binding potency of the chemicals with electron-withdrawing groups on their molecular structures were higher than that with electron-donor groups. Molecular modeling methods were used to decipher the binding mechanism between model compounds and hTTR. The results documented that ionic pair, hydrogen bonding and hydrophobic interactions were dominant interactions. Finally, a mechanism-based model for predicting the hTTR binding affinity was developed. The determination coefficient ( R²), leave-one-out cross validation Q² ( Q_LOO²), bootstrapping coefficient ( Q_BOOT²), external validation coefficient ( Q_EXT²) and concordance correlation coefficient ( CCC) of the developed model met the acceptable criteria ( Q² > 0.600, R² > 0.700, CCC > 0.850), implying that the model had good goodness-of-fit, robustness, and external prediction performances. All the results indicated that the phenolic-DBPs have the hTTR disrupting effects, and further studies are needed to investigate their other mechanism of endocrine disruption.

Water Disinfection Byproducts Increase Natural Transformation Rates of Environmental DNA in Acinetobacter baylyi ADP1

The process of natural transformation allows for the stable uptake, integration, and functional expression of extracellular DNA. This mechanism of horizontal gene transfer has been widely linked to the acquisition of antibiotic resistance and virulence factors. Here, we demonstrate that bromoacetic acid (BAA)-a regulated drinking water disinfection byproduct (DBP)-can stimulate natural transformation rates in the model organism Acinetobacter baylyi ADP1. We demonstrate that transformation stimulation in response to BAA is concentration-dependent and is linked to the ability of this compound to generate DNA damage via oxidative stress. In presence of BAA, transcription of recA was upregulated 20-40% compared to the nontreated controls, indicating that this component of the DNA damage response could be associated with the increase in transformation. Other genes associated with DNA translocation across the cytoplasmic membrane (i.e., pilX, comA) did not exhibit increased transcription in the presence of BAA, indicating that the enhancement of transformation is not associated with increased translocation rates of environmental DNA. Overall, these results lead us to speculate that elevated recA transcription levels could lead to increased integration rates of foreign DNA within the recipient cell during DNA repair. Lastly, we show that an artificial DBP cocktail simulating the environmental concentrations of five water DBP classes stimulates natural transformation by almost 2-fold. The results of this study suggest that mutagens like DBPs may play an important role in enhancing the fixation rates of extracellular DNA in the environmental metagenome.

Fate and transport of estrogenic compounds in an integrated swine manure treatment systems combining algal-bacterial bioreactor and hydrothermal processes for improved water quality

An integrated manure treatment system, including a mixed algal-bacterial bioreactor (MABB) and hydrothermal processing of biomass solids, was found to remove 76.4-97.0% of the total estrogenic hormones (estrone (E1), 17β-estradiol (E2), and estriol (E3)) from the liquid portion of animal manure (LPAM). The mixed biomass was converted into either biocrude oil with a yield up to 40% via hydrothermal liquefaction (HTL) or syngas with a yield up to 54% yield via catalytic hydrothermal gasification (CHG). Adding granular activated carbon (GAC) in the MABB enhanced the removal of estrogenic hormones (+ 7.2%), cytotoxicity (+ 58%), and heavy metals (+ 10.5%). After the integrated system with the MABB, HTL, and CHG processes, the overall percent removal of heavy metals from the LPAM ranged from 27.1 to 40.3%. The concentrations of potentially toxic heavy metals (lead (Pb), copper (Cu), zinc (Zn), cadmium (Cd), nickel (Ni), chromium (Cr)) in the aqueous phase after HTL and CHG tests ranged from 0.01 to 25.3 mg/L.

Assessing the role of different dissolved organic carbon and bromide concentrations for disinfection by-product formation using chemical analysis and bioanalysis

Concerns regarding disinfection by-product (DBP) formation during drinking water treatment have led water utilities to apply treatment processes to reduce the concentration of DBP precursor natural organic matter (NOM). However, these processes often do not remove bromide, leading to high bromide to dissolved organic carbon (DOC) ratios after treatment, which can increase the formation of more toxic brominated DBPs. In the current study, we investigated the formation and effect of DBPs in a matrix of synthetic water samples containing different concentrations of bromide and DOC after disinfection with chlorine. Trihalomethanes and haloacetic acids were analysed by chemical analysis, while effect was evaluated using in vitro bioassays indicative of the oxidative stress response and bacterial toxicity. While the addition of increasing bromide concentrations did not alter the sum molar concentration of DBPs formed, the speciation changed, with greater bromine incorporation with an increasing Br:DOC ratio. However, the observed effect did not correlate with the Br:DOC ratio, but instead, effect increased with increasing DOC concentration. Water samples with low DOC and high bromide did not exceed the available oxidative stress response effect-based trigger value (EBT), while all samples with high DOC, irrespective of the bromide concentration, exceeded the EBT. This suggests that treatment processes that remove NOM can improve drinking water quality, even if they are unable to remove bromide. Further, iceberg modelling showed that detected DBPs only explained a small fraction of the oxidative stress response, supporting the application of both chemical analysis and bioanalysis for monitoring DBP formation.

Predicting the cytotoxicity of disinfection by-products to Chinese hamster ovary by using linear quantitative structure-activity relationship models

A suitable model to predict the toxicity of current and continuously emerging disinfection by-products (DBPs) is needed. This study aims to establish a reliable model for predicting the cytotoxicity of DBPs to Chinese hamster ovary (CHO) cells. We collected the CHO cytotoxicity data of 74 DBPs as the endpoint to build linear quantitative structure-activity relationship (QSAR) models. The linear models were developed by using multiple linear regression (MLR). The MLR models showed high performance in both internal (leave-one-out cross-validation, leave-many-out cross-validation, and bootstrapping) and external validation, indicating their satisfactory goodness of fit (R² = 0.763-0.799), robustness (Q²_LOO = 0.718-0.745), and predictive ability (CCC = 0.806-0.848). The generated QSAR models showed comparable quality on both the training and validation levels. Williams plot verified that the obtained models had wide application domains and covered the 74 structurally diverse DBPs. The molecular descriptors used in the models provided comparable information that influences the CHO cytotoxicity of DBPs. In conclusion, the linear QSAR models can be used to predict the CHO cytotoxicity of DBPs.

Conversion of haloacid disinfection byproducts to amino acids via ammonolysis

Haloacetic acids (HAAs) are the major disinfection byproducts (DBPs) that are formed during chlorination of drinking water. In this paper, the conversion of HAAs to amino acids (e.g., glycine) via ammonolysis was studied. First, a new and sensitive method for detecting glycine was developed by setting selected ion recording m/z 76 in positive electrospray ionization mass spectrometry coupled with ultra performance liquid chromatography. Second, among the mono-HAAs under the same test conditions, iodoacetic acid (49.3%) showed a considerably higher conversion to glycine during ammonolysis than chloroacetic acid (4.2%) and bromoacetic acid (27.7%). The conversion of iodoacetic acid to glycine increased with increasing temperature, increasing reaction time, or decreasing the ratio of (NH₄)₂CO₃ to NH₃·H₂O in the aminating agent. Hydrolysis of iodoacetic acid to glycolic acid was also observed during ammonolysis, and it accounted for at most 50% of the iodoacetic acid conversion. The conversion to amino acids and the hydrolysis were the two major pathways during ammonolysis of HAAs. Third, compared with the iodoacetic acid sample and the simulated tap water sample without ammonolysis, the developmental toxicity of the corresponding samples with ammonolysis decreased by up to 10.4% and 32.1%, respectively. The ammonolysis was thus demonstrated to be a detoxification process for both individual HAAs and DBP mixture in chlorinated tap water. In practice, the ammonolysis of haloacid DBPs in tap water may be realized by simply adding an appropriate amount of an aminating agent during cooking.

Does Granular Activated Carbon with Chlorination Produce Safer Drinking Water? From Disinfection Byproducts and Total Organic Halogen to Calculated Toxicity

Granular activated carbon (GAC) adsorption is well-established for controlling regulated disinfection byproducts (DBPs), but its effectiveness for unregulated DBPs and DBP-associated toxicity is unclear. In this study, GAC treatment was evaluated at three full-scale chlorination drinking water treatment plants over different GAC service lives for controlling 61 unregulated DBPs, 9 regulated DBPs, and speciated total organic halogen (total organic chlorine, bromine, and iodine). The plants represented a range of impacts, including algal, agricultural, and industrial wastewater. This study represents the most extensive full-scale study of its kind and seeks to address the question of whether GAC can make drinking water safer from a DBP perspective. Overall, GAC was effective for removing DBP precursors and reducing DBP formation and total organic halogen, even after >22 000 bed volumes of treated water. GAC also effectively removed preformed DBPs at plants using prechlorination, including highly toxic iodoacetic acids and haloacetonitriles. However, 7 DBPs (mostly brominated and nitrogenous) increased in formation after GAC treatment. In one plant, an increase in tribromonitromethane had significant impacts on calculated cytotoxicity, which only had 7-17% reduction following GAC. While these DBPs are highly toxic, the total calculated cytotoxicity and genotoxicity for the GAC treated waters for the other two plants was reduced 32-83% (across young-middle-old GAC). Overall, calculated toxicity was reduced post-GAC, with preoxidation allowing further reductions.

Occurrence of disinfection by-products in swimming pools and the estimated resulting cytotoxicity

Swimming pools are disinfected to protect against the risk of microbial disease, however, the formation of disinfection by-products (DBPs) is an unwanted consequence. While many studies have reported the occurrence of commonly investigated DBPs (trihalomethanes and haloacetic acids) in pools, few studies have investigated emerging DBP classes, such as the haloketones or haloacetaldehydes, and the nitrogenous haloacetamides, halonitromethanes, haloacetonitriles and N-nitrosamines. This study investigated the occurrence of sixty four DBPs from the eight aforementioned DBP classes in pools employing different treatment methods. Approximately 70% of the DBPs were detected in at least one of the pools, with most concentrations being equal to or greater than those previously reported. Chloral hydrate (trichloroacetaldehyde) was one of many DBPs detected in all chlorinated waters (202 to 1313 μg/L), and, on a molar basis, was the predominant DBP. Several other DBPs, namely chloroacetic acid, dichloroacetic acid, trichloroacetic acid, dichloroacetamide, dibromoacetamide, dibromochloroacetamide and trichloroacetamide, and many of the N-nitrosamines, were measured at concentrations greater than previously reported: up to 200 to 479 μg/L for the haloacetic acids, 56 to 736 μg/L for the haloacetamides and up to 1093 ng/L for some N-nitrosamines. The higher disinfectant residuals required to be employed in Australian pools, and poor pool management (e.g. of chlorine residual and pH) are likely factors contributing to these relatively high DBP concentrations. Where possible, the cytotoxicity values of the investigated DBPs were evaluated, with chloral hydrate representing over 90% of the total chronic cytotoxicity despite only representing up to 64% of the total molar DBP concentration. This study is the first report of bromodichloroacetaldehyde and bromochloroacetaldehyde in pools and is the first investigation of N-nitrosamines in a brominated pool. Furthermore, this work aids in understanding DBPs in both chlorine and bromine treated pools, the latter being the subject of only limited previous studies.

The toxic potentials and focus of disinfection byproducts based on the human embryonic kidney (HEK293) cell model

Disinfection byproducts (DBPs) are inevitably generated during drinking water disinfection processes, and their hazards have not been well characterized. Because they plausibly cause toxicological and pathological damage to human kidney, we selected the human embryonic kidney (HEK293) cell, instead of the commonly used CHO cell, as a model to investigate the toxic potential and target of 10 DBPs, including 3 haloacetamides, 2 trihaloacetaldehydes and 5 iodomethanes. Based on the chronic toxicity parameter EC₁₀ of the cell viability test, we obtained a toxic rank of the tested DBPs different from previous studies and calculated their risk quotients by combining their actual concentrations in drinking water systems. Then, dichloroacetamide (DCAM), trichloroacetaldehyde (TCAL), and bromochloroiodomethane (BCIM) were selected to conduct multiple mechanistic bioassays, including cellular lactate dehydrogenase (LDH) assay, ATP metabolism, ROS production, mitochondria-derived apoptosis and qRT-PCR assay. All bioassays revealed the effects of interrupting the molecular, physiological and biochemical processes relevant to mitochondrial functions, such as oxidative respiration, apoptosis, and energy metabolism. Our study improved the human risk assessment of DBPs with the help of a convenient model and parameter and revealed that mitochondrion is a potential toxic focus of DBPs exposure at the cellular level.