Nitrogenous Disinfection By-products Research Articles

Contribution of filtration and photocatalysis to DOM removal and fouling mechanism during in-situ UV-LED photocatalytic ceramic membrane process

The use of ceramic membranes and ultraviolet light-emitting diodes (UV-LEDs) has advanced the application of photocatalytic membrane for water treatment. We systematically evaluated the contribution of filtration and photocatalysis to dissolved organic matter (DOM) removal and fouling mechanism during in-situ UV-LED photocatalytic ceramic membrane filtration. The results showed that physical rejection primarily led to removal of 4–15 kDa molecules and photocatalysis further increased the removal of 1-4 kDa molecules, causing small sized microbial humic-like or protein-like materials in the permeate. In-situ UV-LED photocatalysis had an excellent effect on membrane fouling mitigation regardless of DOM sources. The dominant fouling mechanism changed from partial blockage to gel layer formation with increasing Ca2+ concentration but did not change with UV treatment. Correlation analysis revealed that the removal of 1-4 kDa molecules contributed to the mitigation of both reversible and irreversible fouling resistance, and the small molecules were the major cause of irreversible fouling resistance. Removal of 1–4 kDa terrestrial humic acid-like contributed to the pore blockage mechanism for synthetic water. Removal of 4-15 kDa protein-like materials was closely correlated to the pore blockage mechanism for real water. Trihalomethanes (THMs) and haloacetic acids (HAAs) formation potential (FP) were both significantly reduced after photocatalytic ceramic membrane process, but precursors of nitrogenous disinfection by-products (N-DBPs) with high toxicity were not removed by filtration or by photocatalysis, which deserves attention. Membrane rejection made higher contribution to better DBPFP control than photocatalysis. This study provides novel insights into the impact of UV-LED on DOM removal, DBPFP control and fouling mitigation, promoting the development of photocatalytic ceramic membrane filtration.

Cytotoxicity of nitrogenous disinfection byproducts: A combined experimental and computational study

Nitrogenous disinfection byproducts (N-DBPs), such as halocetamides (HAcAms), haloacetonitriles (HANs) and halonitromethanes (HNMs), are emerging DBPs in drinking water. They are more toxic than currently regulated DBPs, attracting more attention to their toxic effects and mechanism. In this study, human embryonic kidney (HEK) 293T cells were employed to explore the cytotoxicity of 29 N-DBPs. The influence of molecular structures and different halogenations on cytotoxicity has been comparatively analyzed. As toxicity is the downstream of chemico-biological interactions, the thiol reactivity of 29 N-DBPs has thus been evaluated by using glutathione (GSH) as a model nucleophile, which is the most prevalent cellular thiol and acts as an antioxidant to protect cells by detoxifying electrophilic compounds. Results show that the cytotoxicity of N-DBPs follows by the order of HAcAms > HANs > HNMs, which is different from their reactivity with GSH (the median of kGSH ranks as HNMs > HAcAms > HANs). However, a significant correlation (p < 0.001) between log kGSH and log IC50 (concentration causing 50% inhibition) has been respectively observed for HAcAms and HANs subset and HNMs subset, indicating such chemical reaction is a probable trigger for these DBPs to result in cytotoxicity. Finally, two separate quantitative structure − activity relationship (QSAR) models based on HANs & HAcAms subset and HNMs subset have been developed for estimating IC50 values. The good statistical performance makes the models possible to quickly and accurately predict IC50 values of other N-DBPs, providing basic data for their health risk assessment and greatly reducing in vivo and in vitro experiments.

Does Snowfall Introduce Disinfection By-product Precursors to Surface Water?

Snow with large specific surface area and strong adsorption capacity can effectively adsorb atmospheric pollutants, which could/might lead to the increase of disinfection by-product (DBP) precursors in surface water. In this study, the contents and characteristics of dissolved organic matter (DOM) in meltwater were investigated, and DBP formation and the DBP-associated cytotoxicity index during chlorination of meltwater was first explored. Overall, meltwater exhibited high nitrogen contents. Meltwater-derived DOM was mainly composed of organics with low molecular weights, low aromaticity, and high unsaturated degrees. DBP formation potentials and cytotoxicity indexes in chlorinated meltwater were positively correlated with air quality index and were significantly impacted by snowfall stages. The trihalomethane and haloacetic acid yields from meltwater were relatively low, while yields of highly cytotoxic DBPs, especially halonitromethanes (6.3-10.8 μg-HNMs/mg-DOC), were significantly higher than those of surface water (1.7 μg-HNMs/mg-DOC). Notably, unsaturated nonaromatic organic nitrates in meltwater were important precursors of halonitromethanes. The actual monitoring results showed that snowfall significant increased the haloacetaldehydes and nitrogenous DBP formation levels of surface water. Considering increased DBP formation and DBP-associated toxicity, it was demonstrated that DOM derived from snowfall in atmosphere-polluted areas could deteriorate surface water quality and pose potential risks to drinking water.

Treatment of bromate in UV/sulfite autoxidation process enhances formation of dibromoacetonitrile during chlorination

The integration of UV/sulfite autoxidation process (USAP, i.e., UV activation of sulfite in the presence of 5 ∼ 10 mg/L O2) into conventional water to degrade micropollutants rises extensive attention, but its impact on water quality, and especially the formation of disinfection byproducts is still unclear. Herein, the formation of dibromoacetonitrile (DBAN) from bromate (BrO3–) upon treatment with USAP followed by chlorination was evaluated, in the presence of amino acids (AAs) selected as representative organic matter in drinking water. Results revealed that hydrated electrons (eaq–) produced during USAP contribute to the reduction of BrO3– to Br–, which is then converted into HBrO/BrO– during post-chlorination. At the same time, sulfate radicals (SO4•–) and hydroxyl radicals (•OH) generated in USAP mediated AAs’ conversion via α-hydrogen abstraction and NH2-hydrogen abstraction reactions to produce HN=C(CH3)‒COOH, CH3‒CH=NH, and CH3‒CN, which are released into the post-chlorination stage and therefore, enhance the bromine utilization factor (BUF) value and DBAN formation. The effects of the USAP treatment time, BrO3– concentration, AA concentration, pH, and real waters were also evaluated. Although 63.5% of BrO3– was eliminated by USAP followed by chlorination, the toxicity index (TI) was increased by 1.5-fold due to the formation of the all brominated CX3R-type nitrogenous disinfection byproducts (N-DBPs), demonstrating the potential risk of applying USAP as a treatment process in BrO3– containing waters.

Electronic Control of Traditional Iron-Carbon Electrodes to Regulate the Oxygen Reduction Route to Scale Up Water Purification.

Shifting four-electron (4e-) oxygen reduction in fuel cell technology to a two-electron (2e-) pathway with traditional iron-carbon electrodes is a critical step for hydroxyl radical (HO•) generation. Here, we fabricated iron-carbon aerogels with desired dimensions (e.g., 40 cm × 40 cm) as working electrodes containing atomic Fe sites and Fe3C subnanoclusters. Electron-donating Fe3C provides electrons to FeN4 through long-range activation for achieving the ideal electronic configuration, thereby optimizing the binding energy of the *OOH intermediate. With an iron-carbon aerogel benefiting from finely tuned electronic density, the selectivity of 2e- oxygen reduction increased from 10 to 90%. The resultant electrode exhibited unexpectedly efficient HO• production and fast elimination of organics. Notably, the kinetic constant kM for sulfamethoxazole (SMX) removal is 60 times higher than that in a traditional iron-carbon electrode. A flow-through pilot device with the iron-carbon aerogel (SA-Fe0.4NCA) was built to scale up micropolluted water decontamination. The initial total organic carbon (TOC) value of micropolluted water was 4.02 mg L-1, and it declined and maintained at 2.14 mg L-1, meeting the standards for drinking water quality in China. Meanwhile, the generation of emerging aromatic nitrogenous disinfection byproducts (chlorophenylacetonitriles) declined by 99.2%, satisfying the public safety of domestic water. This work provides guidance for developing electrochemical technologies to satisfy the flexible and economic demand for water purification, especially in water-scarce areas.

High-performance reductive decomposition of trichloroacetamide by the vacuum-ultraviolet/sulfite process: Kinetics, mechanism and combined toxicity risk

Trichloroacetamide (TCAcAm) is among of the nitrogenous disinfection by-products (N-DBPs) with high cytotoxicity and genotoxicity, which is usually detected at low concentration (μg/L) in drinking water. In this study, advanced reduction process (ARP) based on vacuum ultraviolet (VUV) was employed to eliminate TCAcAm. Compared with VUV, VUV/sulfide, and VUV/ferrous iron processes, VUV/sulfite process demonstrated excellent performance for TCAcAm decomposition, the higher removal of TCAcAm could be achieved by VUV/sulfite process (85.6 %) than VUV direct photolysis (13.5 %) due to the production of a great number of reactive species. The degradation of TCAcAm followed the pseudo-first-order kinetics well in VUV/sulfite process, and the pseudo-first-order rate constant (kobs) increased with increasing sulfite concentration. Reactive species quenching experiments demonstrated that eaq−, SO3·− and H· were involved in the degradation of TCAcAm. The in situ generated eaq−, SO3·− and HO· via VUV/sulfite process were identified by electron paramagnetic resonance (EPR), and the eaq− was proved to be the dominated species (relative contribution: 83.5 %) for TCAcAm decomposition. The second-order rate constant of TCAcAm with eaq− was determined to be 2.41 × 1010 M−1 s−1 for the first time based on competitive kinetic method. The complete TCAcAm degradation could be achieved at pH > 8.3, while TCAcAm degradation efficiency decreased to 11.9 % at pH 5.8. TCAcAm decay could be divided into two stages: rapid growth (sulfite dosage: 0.25-1.0 mM) and slow growth (sulfite dosage: 1.0-4.0 mM). The yield of eaq− was controlled by sulfite dosage, and the predict yield of eaq− increased from 3.69 × 10−14 to 2.58 × 10−12 M with increasing the sulfite dosage from 0.25 to 4.0 mM by Kintecus 6.80, which resulted in an increase in TCAcAm removal. Meanwhile, the presence of dissolved oxygen (DO), chloride (Cl−), bicarbonate (HCO3−) and humic acid (HA) posed negative influence on TCAcAm decomposition to various degrees. Dichloroacetamide (DCAcAm), trichloroacetic acid (TCAA), dichloroacetic acid (DCAA) and Cl− were identified as intermediate products, indicated that reductive dechlorination and hydrolysis coexisted during the degradation of TCAcAm in VUV/sulfite process.

Structural and functional alterations of intestinal flora in mice induced by halonitromethanes exposure.

Halonitromethanes (HNMs) as one typical class of nitrogenous disinfection byproducts (DBPs) have been widely found in drinking water and are receiving more and more attentions because of their high cytotoxicity, genotoxicity, and developmental toxicity. However, the effects of HNMs exposure on the intestinal tract and intestinal flora remain unknown. This study comprehensively determined the effects of trichloronitromethane, bromonitromethane, and bromochloronitromethane exposure on the intestinal tract and intestinal flora. Results showed that the three HNMs induced intestinal oxidative stress and inflammatory response. Further, HNMs exposure could change the diversities and community structure of intestinal flora, thereby triggering intestinal flora dysbiosis, which might be associated with the intestinal damage such as oxidative stress and inflammation. The intestinal flora dysbiosis was accompanied with mark alterations in function of intestinal flora, such as carbohydrate, lipid, and amino acid metabolisms. This research provides a new insight into studying the toxicity of HNMs exposure based on intestinal flora, which will further improve the health risk assessment of DBPs in drinking water.

Effects of biological activated carbon filter running time on disinfection by-product precursor removal.

Biological activated carbon (BAC) filtration is usually considered to be able to decrease formation potentials (FPs) of disinfection by-products (DBPs) in drinking water treatment plant (DWTP). However, BAC filters with long running time may release microbial metabolites to effluents and therefore increase FPs of nitrogenous DBPs with high toxicity. To verify this hypothesis, this study continuously tracked BAC filters in a DWTP for one year, and assessed effects of old (running time 8-9 years) and new (running time 0-13 months) BAC filters on FPs of 15 regulated and unregulated DBPs. Results revealed that dissolved organic carbon (DOC) removal was slightly higher in the new BAC than the old one. All fluorescent components of dissolved organic matter evidently declined after new BAC filtration, but fulvic acid-like and soluble microbial product-like substances increased after old BAC filtration, which could be caused by microbial leakage. Correspondingly, new BAC filter generally removed more DBP FPs than the old one. 46.5% HAA7 FPs from chlorination and 44.3% THM4 FPs from chloramination were removed by new BAC filter. However, some DBP FPs, especially HAN FPs, were poorly removed or even increased by the old BAC filter. Proteobacteria could be a main contributor for DBP precursor removal in BAC filters. Herminiimonas, most abundant genera in new BAC filter, may explain its better DOC and UV254 removal performance and lower DBP FPs, while Bradyrhizobium, most abundant genera in old BAC filter, might produce more extracellular polymeric substances and therefore increased N-DBP FPs in old BAC effluent. This study provided insight into variations of DBP FPs and microbial communities in the new and old BAC filters, and will be helpful for the optimization of DWTP design and operation for public health.

Does Biological Activated Carbon Filtration Make Chlor(am)inated Drinking Water Safer

Biological activated carbon (BAC) filtration is an effective technology for the removal of natural organic matter. However, one potential drawback of BAC, especially old BAC, is that effluents can contain soluble microbial products released from the biofilm, which are recognized as more toxic nitrogenous DBPs (N-DBPs) precursors. So far, limited studies reported the risk of DBP formation potentials (FPs) increase caused by the microbial leakage of BAC. This study compared removal differences of DBP FPs between two BAC filters operated for 1 year and 8 years in a drinking water plant. The results showed that the total summed haloacetic acid FPs and trihalomethane FPs decreased by 34.31% from chlorination, and 55.01% of the total summed halogen acetonitrile FPs from chloramination were removed by the new BAC. However, Chlorinated haloacetonitriles FPs increased by 2.33% after old BAC filtration. To sum up, BAC filtration decreased most DBP FPs, but a potential risk regarding more toxic N-DBP FPs from old BAC should receive more attention.

Trichloramine and Hydroxyl Radical Contributions to Dichloroacetonitrile Formation Following Breakpoint Chlorination.

Breakpoint chlorination is applied to remove ammonia in water treatment. Trichloramine (NCl3) and transient reactive species can be present, but how they affect the formation of nitrogenous disinfection byproducts is unknown. In this study, the dichloroacetonitrile (DCAN) formation mechanisms and pathways involved during breakpoint chlorination (i.e., free chlorine to ammonia molar ratio ≥2.0) were investigated. DCAN formation during breakpoint chlorination of natural organic matter (NOM) isolates was 14.3-20.3 μg/L, which was 2-10 times that in chlorination without ammonia at similar free chlorine residual conditions (2.1-2.9 mg/L as Cl2). The probe tests and electron paramagnetic resonance spectra supported the presence of •OH, •NO, and NCl3 besides free chlorine in breakpoint chlorination. 15N-labeled ammonium-N tests indicated the incorporation of ammonium-N in DCAN formation though ammonia was eliminated during breakpoint chlorination. Aromatic non-nitrogenous moieties, such as phenols (i.e., none DCAN precursors in the free-chlorine-only system), became DCAN precursors during breakpoint chlorination. The reactions involved in reactive nitrogen species, such as •NO/•NO2 and NCl3, led to additional nitrogen sources in DCAN formation, accounting for 36-84% of total nitrogen sources in DCAN formation from NOM isolates and real water samples. Scavenging •OH by tert-butanol reduced DCAN formation by 40-56%, indicating an important role of •OH in transforming DCAN precursors. This study improves the understanding of breakpoint chlorination chemistry.

Dissolved organic nitrogen derived from wastewater denitrification: Composition and nitrogenous disinfection byproduct formation

Microbially derived dissolved organic nitrogen (mDON) is a major fraction of effluent total nitrogen at wastewater treatment plants with enhanced nutrient removal, which stimulates phytoplankton blooms and formation of toxic nitrogenous disinfection by-products (N-DBPs). This study identified denitrifiers as major contributors to mDON synthesis, and further revealed the molecular composition, influential factors and synthetic microorganisms of denitrification-derived mDON compounds leading to N-DBP formation. The maximum mDON accumulated during denitrification was 8.92% of converted inorganic nitrogen, higher than that of anammox (4.24%) and nitrification (2.76%). Sodium acetate addition at relatively high C/N ratio (5−7) favored mDON formation, compared with methanol and low C/N (1−3). Different from acetate, methanol-facilitated denitrification produced 13–69% more lignin-like compounds than proteins using Orbitrap LC-MS. The most abundant N-DBPs formed from denitrification-derived mDON were N-nitrosodibutylamine and dichloroacetonitrile (13.32 μg/mg mDON and 12.21 μg/mg mDON, respectively). Major amino acids, aspartate, glycine, and alanine were positively correlated with typical N-DBPs. Biosynthesis and degradation pathways of these N-DBP precursors were enriched in denitrifiers belonging to Rhodocyclaceae, Mycobacteriaceae and Hyphomicrobiaceae. As intensive disinfection is applied at worldwide wastewater treatment plants during COVID-19, carbon source facilitated denitrification should be better managed to reduce both effluent inorganic nitrogen and DON, mitigating DON and N-DBP associated ecological risks in receiving waters.

A pipeline to evaluate the discrepant interactions between typical nitrogenous disinfection byproduct haloacetonitriles and human hemoglobin

To evaluate the interaction between haloacetonitriles (HANs) and human hemoglobin (Hb), a pipeline was established based on fluorescence spectra, mass spectra and molecular docking. Fluorescence spectra analysis showed the fluorescence of Hb was statically quenched by HANs in the sequence of TCAN > DBAN > DCAN > IAN > BAN > CAN. HANs could combine to multiple surface sites of Hb accounting for “hydrogen bonds” and “van der Waals forces”. The high-resolution mass spectra analysis for Hb with and without HANs further confirmed the formation of multiple HAN-Hb complexes with different conversion rates. With the assistance of MOE molecule docking, the potential combination sites and related interactions parameters between HANs and Hb were filtrated. By analyzing the correlations between the candidate interactions parameters and fluorescence quenching constants/MS conversion rates, the combination sites of HANs were fixed at Asp126 (α1/α2), Lys127 (α1/α2) in the form of “hydrogen bonds” X → Asp126 (α1/α2), N → Lys127 (α1/α2). In this way, the potential interactions between HANs and Hb were effectively evaluated.

Synthesis and characterization of a novel magnetic resin (m-MAR resin) and its removal performance for alkaline amino acids

Alkaline amino acids as dissolved organic nitrogen (DON) have raised much concern in drinking water treatment due to poor removal in conventional treatment process and high potential for nitrogenous disinfection by-products (N-DBPs). This work was intended to devise a new magnetic adsorption resin (noted as m-MAR resin) for the efficient reduction of alkaline amino acids and explore the application potential of combined MIEX and m-MAR resins. The distribution and composition of DON and amino acids was clarified for different water sources in Lake Taihu basin, in which alkaline amino acids accounted for a higher proportion. The removal of different nitrogenous organics by MIEX resin was also examined, where the resin was effective in removing phycocyanin (65.6%) and glutamic acid (74.2%), reducing the generation of disinfection by-products (DBPs). The m-MAR resin was manufactured and characterized to cope with alkaline amino acids, and batch experiments were undertaken to investigate its adsorption behaviors on histidine and arginine under different operating conditions. The maximal adsorption capacities of arginine and histidine onto m-MAR resin were 2.84 mg/g and 1.62 mg/g, respectively, which was better than MIEX resin. The removal mechanism of the two basic amino acids by m-MAR resin was mainly due to the hydrogen bonding and the acid-base reaction. Moreover, the reusability of the m-MAR resin was elucidated after six successive adsorption-desorption cycles. Finally, the effectiveness of combined MIEX and m-MAR resin in treating DON derived from Microcystis aeruginosa reached 35.2% and the DON concentration in Lake Taihu could be reduced from 0.56 to 0.16 mg/L, which simultaneously decreased the generation potential of N-DBPs. The enhancement of coagulation by the combined process of m-MIER and m-MAR as pretreatment was estimated.

Detecting trace dissolved organic nitrogen (DON) in freshwater by converting DON rapidly into nitrate with VUV/H2O2 pretreatment

Dissolved organic nitrogen (DON) plays a key role in many biogeochemistry and engineering processes such as forming nitrogenous disinfection byproducts. However, detecting aqueous DON at trace levels is challenging currently because conventional DON conversion methods have very high method detection limits (MDL). In addition, DON is measured indirectly by subtracting dissolved inorganic nitrogen (DIN) from total dissolved nitrogen (TDN), which can propagate analytical errors of each analyte. In order to solve these issues, we isolated DON from DIN with electrodialysis before and herein tested vacuum ultraviolet (VUV) in tandem with several oxidants to convert DON completely into nitrate for subsequent N analysis. Results showed that H2O2 was more suitable than chlorine and persulfate because VUV/chlorine or VUV/persulfate is either inefficient to convert DON or subjected to nonnegligible N loss. To verify the efficiency, we evaluated the effects of typical water and operating variables on the conversions of four model DON compounds and their yields of nitrate. Under optimal conditions (pH 10.3 and 500 mg/L H2O2), the process converted DON completely into nitrate within just 60 min. Compared to conventional TDN analytical methods, the VUV/H2O2 method features not only better analytical precision but also lower MDL because the formed nitrate can be analyzed at very low MDL by ion chromatography (IC). So, this approach moves one step further to achieve a conceptually new DON analytical method by coupling electrodialysis, VUV/H2O2, and IC.

Transformation mechanisms of acetaldehyde and its substituted aldehydes into the corresponding nitriles and (N-chloro)amides during chloramination: A computational study.

As an alternative disinfectant to free chlorine, monochloramine reduces the formation of regulated disinfection byproducts (DBPs); however, it also contributes to the formation of highly toxic nitrogenous DBPs (N-DBPs), especially through the aldehyde pathway. The current understanding of aldehyde pathway mechanisms is limited. In this study, the transformation pathways of acetaldehyde and its substituted aldehydes into the corresponding nitriles and (N-chloro)amides during chloramination were investigated using quantum chemical calculations. Consistent with previous studies, 1-chloroamino alcohol first forms in the chloramination of aldehydes and then undergoes competitive dehydration and HCl elimination branch reactions to generate the nitrile and (N-chloro)amide, respectively. Iminol was found to be a key intermediate for (N-chloro)amide formation. Moreover, the results indicated that acetaldehydes substituted with electron-donating groups (EDGs) and electron-withdrawing groups (EWGs) are beneficial to the formation of the respective nitriles and N-chloro-amides, while those substituted with conjugated groups (CGs) are favourable for both. Based upon the above results, in addition to acetaldehyde, other aldehydes, such as propionaldehyde, glycolaldehyde, 3-butenal, and phenylacetaldehyde, which are the α-H of acetaldehydes substituted with -CH3, -OH, -CH=CH2, and -C6H5 groups, respectively, are potential precursors of toxic nitriles and (N-chloro)amides during chloramination. Thus, more attention should be given to these aldehydes. The findings of this work are helpful for further understanding the aldehyde pathway mechanisms and predicting potential precursors of toxic nitriles and (N-chloro)amides during chloramination.

Transformation of dissolved organic matter during biological wastewater treatment and relationships with the formation of nitrogenous disinfection byproducts

Nitrogenous disinfection byproducts (N-DBPs) can be produced from dissolved organic matter (DOM) during the disinfection of secondary wastewater effluent. This study examined the transformation of DOM and the abatement of N-DBP precursors during different types of biological wastewater treatment (e.g., anaerobic/anoxic/oxic activated sludge processes and membrane bioreactor) using high-performance size exclusion chromatography (HPSEC) with dissolved organic carbon, UV, and fluorescence detectors. DOM with molecule weight (MW) larger than 3 kDa and protein-like substances smaller than 0.3 kDa was effectively bio-transformed, whereas DOM fractions with MW in the range of 0.3–3 kDa were the most bio-refractory. Complete nitrification was beneficial to the removal of small amino sugar-like and protein-like molecules (< 0.3 kDa). Haloacetonitrile (HAN) precursors were recalcitrant to biological treatment with a median removal of 17%. Halonitromethane (HNM) and N-nitrosamine (NA) precursors tended to be effectively removed in complete nitrification conditions. The abundance of low-molecular-size protein-like substances (< 0.3 kDa) was significantly correlated with the formation potential of HNM, NA, and total N-nitrosamine (TONO) in post-chloramination (r = 0.81, 0.62, and 0.68, respectively, p < 0.01). This study improved the understanding of DOM transformation and the removal of N-DBPs precursors in wastewater treatment and pointed out the benefit of provision of complete nitrification in removing low-molecular-size protein-like substances and NA and HNM precursors.

Effects of different types of nitrogen sources in water on the formation potentials of nitrogenous disinfection by-products in chloramine disinfection process based on isotope labeling

Nitrogenous disinfection by-products (N-DBPs) raise increasing concerns because of their high genotoxicity, cytotoxicity, and carcinogenicity compared to carbonaceous disinfection by-products (C-DBPs). Nitrogen-containing disinfectants, dissolved organic nitrogen (DON), and inorganic nitrogen may all promote the formation of N-DBPs. Therefore, it is urgent to explore the dominant nitrogen source of N-DBPs under the coexistence of multiple nitrogen sources. In this study, the effects of amino acids, nitrate, ammonia, and chloramine as different types of nitrogen sources on the formation of five N-DBPs were investigated systematically, including chloroacetonitrile (CAN), dichloroacetonitrile (DCAN), bromochloroacetonitrile (BCAN), dibromoacetonitrile (DBAN) and dichloroacetamide (DCAcAm). L-Aspartic acid (L-Asp) as the organic nitrogen source showed a high potential on the formation of N-DBPs by forming acetonitrile intermediates. Ammonia as the inorganic nitrogen source consumed oxidants and changed the existing form of chloramine, thus inhibiting the formation of N-DBPs. Instead of providing nitrogen to N-DBPs, nitrate as a salt promoted the volatilization of N-DBPs, thereby reducing the detected N-DBPs. Furthermore, an isotope labeling method was applied to clearly trace the nitrogen sources of N-DBPs via GC–MS with electron ionization. 15N-chloramine, 15N-amino acid, 15N-nitrate and 15N-ammonia were selected as the corresponding isotopic nitrogen sources. The results indicated that chloramine was the major nitrogen contributor to five N-DBPs during the chloramination of L-Asp under the coexistence of multiple nitrogen sources, ranging from 61 % to 79 %. The influence of environmental factors (reaction time, pH, and bromide) on the formation of N-DBPs during chloramination was also investigated. There was competition between brominated N-DBPs and chlorinated N-DBPs in chloramination. With the increase of reaction time or bromine, the formation potentials of chlorinated N-DBPs gradually decreased, while brominated N-DBPs gradually increased. Moreover, higher pH inhibited the generation of N-DBPs.

Exploring Pathways and Mechanisms for Dichloroacetonitrile Formation from Typical Amino Compounds during UV/Chlorine Treatment

The formation of disinfection byproducts (DBPs) during UV/chlorine treatment, especially nitrogenous DBPs, is not well understood. This study investigated the formation mechanisms for dichloroacetonitrile (DCAN) from typical amino compounds during UV/chlorine treatment. Compared to chlorination, the yields of DCAN increase by 88-240% during UV/chlorine treatment from real waters, while the yields of DCAN from amino compounds increase by 3.3-5724 times. Amino compounds with electron-withdrawing side chains show much higher DCAN formation than those with electron-donating side chains. Phenylethylamine, l- phenylalanine, and l-phenylalanyl-l-phenylalanine were selected to represent amines, amino acids, and peptides, respectively, to investigate the formation pathways for DCAN during UV/chlorine treatment. First, chlorination of amines, amino acids, and peptides rapidly forms N-chloramines via chlorine substitution. Then, UV photolysis but not radicals promotes the transformation from N-chloramines to N-chloroaldimines and then to phenylacetonitrile, with yields of 5.4, 51.0, and 19.8% from chlorinated phenylethylamine, l-phenylalanine, and l-phenylalanyl-l-phenylalanine to phenylacetonitrile, respectively. Finally, phenylacetonitrile is transformed to DCAN with conversion ratios of 14.2-25.6%, which is attributed to radical oxidation, as indicated by scavenging experiments and density functional theory calculations. This study elucidates the pathways and mechanisms for DCAN formation from typical amino compounds during UV/chlorine treatment.

Feel the Burn: Disinfection Byproduct Formation and Cytotoxicity during Chlorine Burn Events

Nitrification and biofilm growth within distribution systems remain major issues for drinking water treatment plants utilizing chloramine disinfection. Many chloraminated plants periodically switch to chlorine disinfection for several weeks to mitigate these issues, known as "chlorine burns". The evaluation of disinfection byproduct (DBP) formation during chlorine burns beyond regulated DBPs is scarce. Here, we quantified an extensive suite of 80 regulated and emerging, unregulated DBPs from 10 DBP classes in drinking water from two U.S. drinking water plants during chlorine burn and chloramination treatments. Total organic halogen (TOX), including total organic chlorine, total organic bromine, and total organic iodine, was also quantified, and mammalian cell cytotoxicity of whole water mixtures was assessed in chlorine burn waters for the first time. TOX and most DBPs increased in concentration during chlorine burns, and one emerging DBP, trichloroacetaldehyde, reached 99 μg/L. THMs and HAAs reached concentrations of 249 and 271 μg/L, respectively. Two highly cytotoxic nitrogenous DBP classes, haloacetamides and haloacetonitriles, increased during chlorine burns, reaching up to 14.2 and 19.3 μg/L, respectively. Cytotoxicity did not always increase from chloramine treatment to chlorine burn, but a 100% increase in cytotoxicity was observed for one plant. These data highlight that consumer DBP exposure during chlorine burns can be substantial.

Impacts of carbon-based advanced treatment processes on disinfection byproduct formation and speciation for potable reuse

For the potable reuse of municipal wastewater effluent, carbon based advanced treatment (CBAT) using coagulation, ozonation, biofiltration and/or granular activated carbon (GAC) adsorption is a promising approach for controlling disinfection byproduct (DBP) formation. However, CBAT can also favor a shift in DBP formation to more toxic brominated DBP species. To protect public health, treatment-specific DBP formation and speciation trends need to be identified and understood. First, this study systematically evaluated the treatment of six wastewater effluents with four CBAT process trains (experimental n was 55) and measured DBP formation and speciation trends. Overall, CBAT decreased DBP formation by >90% and GAC preferentially removed highly-reactive effluent organic matter as indicated by lower yields of both highly-forming and highly-toxic classes of carbonaceous and nitrogenous DBPs. Since GAC treatment also induced systematic speciation changes by increasing the ratio of bromide to dissolved organic matter, the second part of this study focused on understanding the health impacts of DBP speciation changes on calculated additive toxicity (CAT). Based on the evaluation of 20 DBPs, measured using established methods, the CAT values from cyto- and genotoxicity metrics decreased by as much as 85% due to high levels of precursor removal by GAC. Expanding the evaluation to include 52 DBPs, measured using more extensive analytical methods, resulted in the same conclusions. This study also developed a “speciation potency” metric, that re-scales class-by-class speciation trends using toxic potency factors (e.g., cytotoxicity [LC50]). The observed shifts in DBP speciation after treatment increased the class-level toxic potency factors by up to a factor of 4; a greater amount of precursor removal is required for treatment to reduce toxicity, which was achieved with CBAT trains. This proposed approach of combining speciation potency with DBP yields enables evaluation of DBP-associated risk with easily measured surrogates (i.e., bromide and dissolved organic carbon [DOC]). By identifying and quantitatively comparing DBP formation and speciation trends over multiple wastewater effluents and treatment trains, this study demonstrates that CBAT can be a robust approach to DBP precursor removal for potable reuse.