Effects of conventional ozonation and electro-peroxone pretreatment of surface water on disinfection by-product formation during subsequent chlorination

Removal of antibiotic resistant bacteria and antibiotic resistance genes by an electrochemically driven UV/chlorine process for decentralized water treatment

The UV/chlorine (UV/Cl2) process is a developing advanced oxidation process and can efficiently remove antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs). However, the transportation and storage of chlorine solutions limit the application of the UV/Cl2 process, especially for decentralized water treatment. To overcome the limitation, an electrochemically driven UV/Cl2 process (E-UV/Cl2) where Cl2 can be electrochemically produced in situ from anodic oxidation of chloride (Cl-) ubiquitously present in various water matrices was evaluated in this study. >5-log inactivation of the ARB (E. coli) was achieved within 5 s of the E-UV/Cl2 process, and no photoreactivation of the ARB was observed after the treatment. In addition to the ARB, intracellular and extracellular ARGs (tetA, sul1, sul2, and ermB) could be effectively degraded (e.g., log(C0/C) > 4 for i-ARGs) within 5 min of the E-UV/Cl2 process. Atomic force microscopy showed that the most of the i-ARGs were interrupted into short fragments (< 30 nm) during the E-UV/Cl2 process, which can thus effectively prevent the self-repair of i-ARGs and the horizontal gene transfer. Modelling results showed that the abatement efficiencies of i-ARG correlated positively with the exposures of •OH, Cl2-•, and ClO• during the E-UV/Cl2 process. Due to the short treatment time (5 min) required for ARB and ARG removal, insignificant concentrations of trihalomethanes (THMs) were generated during of the E-UV/Cl2 process, and the energy consumption (EEO) of ARG removal was ∼0.20‒0.27 kWh/m3-log, which is generally comparable to that of the UV/Cl2 process (0.18-0.23 kWh/m3-log). These results demonstrate that the E-UV/Cl2 process can provide a feasible and attractive alternative to the UV/Cl2 process for ARB and ARG removal in decentralized water treatment system.

Impact of ozonation on disinfection byproducts formation from phenylalanine during chlorination

As a strong oxidizing agent, ozone is used in some water treatment facilities for disinfection, taste and odor control, and removal of organic micropollutants. Phenylalanine (Phe) was used as the target amino acid to comprehensively investigate variability of disinfection byproducts (DBPs) formation during chlorine disinfection and residual chlorine conditions subsequent to ozonation. The results showed that subsequent to ozonation, the typical regulated and unregulated DBPs formation potential (DBPsFP), including trichloromethane (TCM), dichloroacetonitrile (DCAN), chloral hydrate (CH), dichloroacetic acid (DCAA), trichloroacetic acid (TCAA), and trichloroacetamide (TCAcAm) increased substantially, by 2.4, 3.3, 5.6, 1.2, 2.5, and 6.0 times, respectively, compared with only chlorination. Ozonation also significantly increased the DBPs yield under a 2 day simulated residual chlorine condition that mimicked the water distribution system. DBPs formations followed pseudo first order kinetics. The formation rates of DBPs in the first 6 hr were higher for TCM (0.214 hr-1), DCAN (0.244 hr-1), CH (0.105 hr-1), TCAcAm (0.234 hr-1), DCAA (0.375 hr-1) and TCAA (0.190 hr-1) than thereafter. The peak DBPsFP of TCM, DCAN, CH, TCAcAm, DCAA, and TCAA were obtained when that ozonation time was set at 5-15 min. Ozonation times > 30 min increased the mineralization of Phe and decreased the formation of DBPs upon chlorination. Increasing bromine ion (Br-) concentration increased production of bromine- DBPs and decreased chlorine-DBPs formation by 59.3%-92.2% . Higher ozone dosages and slight alkaline favored to reduce DBP formation and cytotoxicity. The ozonation conditions should be optimized for all application purposes including DBPs reduction.

Formation of halonitromethanes from different nitrophenol compounds during UV/post-chlorination: Impact factors, DFT calculation, reaction mechanisms, and toxicity

As ubiquitous chemical substances in water bodies, nitrophenol compounds (NCs) can form chlorinated halonitromethanes (Cl-HNMs) in the chlorination process. This work chose six typical NCs to explore Cl-HNMs produced during the UV/post-chlorination process, and Cl-HNMs yields from these NCs followed the increasing order of 4-, 2-, 2-amino-3-, 2-methyl-3-, 3-, and 2-chloro-3-nitrophenol. The Cl-HNMs yields increased continually or increased firstly and declined with post-chlorination time. Increasing chlorine dosage favored Cl-HNMs formation, while excessive chlorine dosage decreased Cl-HNMs produced from 2- and 4-nitrophenol. Besides, appropriate UV radiation, acidic pH, and higher precursor concentrations facilitated Cl-HNMs formation. Then, the reaction mechanisms of Cl-HNMs generated from these different NCs were explored according to density functional theory calculation and identified transformation products (TPs), and the main reactions included chlorine substitution, benzoquinone compound formation, ring opening, and bond cleavage. Moreover, the Cl-HNMs generated from 2-chloro-3-nitrophenol were of the highest toxicity, and the six NCs and their TPs also presented ecotoxicity. Finally, two kinds of real waters were used to explore Cl-HNMs formation and toxicity, and they were significantly distinguishable compared to the phenomena observed in simulated waters. This work will give new insights into Cl-HNMs formation from different NCs in water disinfection processes and help better apply the UV/post-chlorination process to water treatments.

Reaction mechanisms of chlorinated disinfection byproducts formed from nitrophenol compounds with different structures during chlor(am)ination and UV/post-chlor(am)ination

Nitrophenol compounds (NCs) have high formation potentials of disinfection byproducts (DBPs) in water disinfection processes, however, the reaction mechanisms of DBPs formed from different NCs are not elucidated clearly. Herein, nitrobenzene, phenol, and six representative NCs were used to explore the formation mechanisms of chlorinated DBPs (Cl-DBPs) during chlor(am)ination and UV/post-chlor(am)ination. Consequently, the coexistence of nitro and hydroxy groups in NCs facilitated the electrophilic substitution to produce intermediates of Cl-DBPs, and the different positions of nitro and hydroxy groups also induced different yields and formation mechanisms of Cl-DBPs during the chlorination and UV/post-chlorination processes. Besides, the amino, chlorine, and methyl groups significantly influenced the formation mechanisms of Cl-DBPs during the chlorination and UV/post-chlorination processes. Furthermore, the total Cl-DBPs yields from the six NCs followed a decreasing order of 2-chloro-3-nitrophenol, 3-nitrophenol, 2-methyl-3-nitrophenol, 2-amino-4-nitrophenol, 2-nitrophenol, and 4-nitrophenol during chlorination and UV/post-chlorination. However, the total Cl-DBPs yields from the six NCs during chloramination and UV/post-chloramination followed a quite different order, which might be caused by additional reaction mechanisms, e.g., nucleophilic substitution or addition might occur to NCs in the presence of monochloramine (NH2Cl). This work can offer deep insights into the reaction mechanisms of Cl-DBPs from NCs during the chlor(am)ination and UV/post-chlor(am)ination processes.

Comparative analysis of chlorinated disinfection byproducts formation from 4-nitrophenol and 2-amino-4-nitrophenol during UV/post-chlorination

Nitrophenol compounds (NCs) are widely distributed in water environments and regarded as important precursors of disinfection byproducts (DBPs). Herein, 4-nitrophenol and 2-amino-4-nitrophenol were selected as representative NCs to explore chlorinated DBPs (Cl-DBPs) formation during UV/post-chlorination. Dichloronitromethane (DCNM), trichloronitromethane (TCNM), dichloroacetonitrile (DCAN), and trichloromethane (TCM) were formed from 4-nitrophenol and 2-amino-4-nitrophenol during UV/post-chlorination, and the yields of individual Cl-DBPs from 2-amino-4-nitrophenol were higher than those from 4-nitrophenol. Meantime, increasing chlorine contact time, UV fluence, and free chlorine dose could enhance Cl-DBPs formation, while much higher values of the three factors might decrease the yields of Cl-DBPs. Besides, alkaline pH could decrease the yields of halonitromethane (HNMs) and DCAN but increase the yields of TCM. Also, higher concentrations of 4-nitrophenol and 2-amino-4-nitrophenol would induce more Cl-DBPs formation. Subsequently, the possible formation pathways of DCNM, TCNM, DCAN, and TCM form 4-nitrophenol and 2-amino-4-nitrophenol during UV/post-chlorination were proposed according to transformation products (TPs) and density functional theory (DFT) calculation. Notably, Cl-DBPs formed from 2-amino-4-nitrophenol presented higher toxicity than those from 4-nitrophenol. Among these generated Cl-DBPs, DCAN and TCNM posed higher cytotoxicity and genotoxicity, respectively. Furthermore, 4-nitrophenol, 2-amino-4-nitrophenol, and their TPs exhibited ecotoxicity. Finally, 4-nitrophenol and 2-amino-4-nitrophenol presented a high potential to produce DCNM, TCNM, DCAN, and TCM in actual waters during UV/post-chlorination, but the Cl-DBPs yields were markedly different from those in simulated waters. This work can help better understand Cl-DBPs formation from different NCs during UV/post-chlorination and is conducive to controlling Cl-DBPs formation.

Impact of storm events on disinfection byproduct precursors in a drinking water source in the Northeastern United States

Storm events play a crucial role in organic matter transport within watersheds and can increase the concentration and alter the composition of NOMs and DBP formation potential. To assess the impact that storm events can have on drinking water quality, samples were collected and analyzed across four storm events in the Neversink River, Catskill region, New York in 2019 and 2022. Source water natural organic matter (NOM) was characterized, and the change of NOM quality was evaluated due to storm impacts. During storm events, a high level of NOM mobilization is initiated by heavy precipitation causing overland flow and a rise in the water table. In this way, storms result in increased access to stored NOM pools that are generated during inter-storm periods. A significant correlation was observed between several organic water quality parameters such as UV absorbance (UV254), dissolved organic carbon (DOC) and chlorine demand. Precursors for the total trihalomethanes (TTHM), dichloroacetic acid (DCAA) and trichloroacetic acid (TCAA) exhibited comparable patterns with UV254, DOC, and chlorine demand for four storms. Despite the potential for increased dilution resulting from higher discharges, all organic water quality parameters, including yields of disinfection byproducts (i.e., DBP precursors), exhibited elevated concentrations during periods of higher flows. Three of the four storms showed hysteresis patterns with higher observed concentrations of organic constituents in the falling limb of the hydrographs. Precursors for the nitrogenous DBPs (N-DBPs) were proportional to the DOC for all four storms. The coefficient of determination (R2) for TTHM, DCAA, TCAA with UV254 is higher (R2 0.92-0.98) than corresponding correlations with DOC (R2 0.89-0.92). The R2 for UV254 showed the following hierarchy: DCAA≈TCAA>TTHM. Additionally, the R2 for DOC and specific ultraviolet absorbance (SUVA) had the following hierarchy: DCAA>TCAA>TTHM and TCAA>DCAA>TTHM respectively. A significant correlation between UV254 and DOC (R = 0.99) for all storms was observed. Chlorine demand also yielded a strong correlation (R = 0.91∼0.98) with UV254 and DOC. This research indicates that a significant and disproportionate export of NOM to source waters occurs during storm events compared to baseflow conditions. Consequently, it is recommended for drinking water treatment facilities to reassess chlorine dosages during these events. Treatment plants can employ UV254 as a tool to determine appropriate chlorine dosages, aiming to mitigate DBP formation in treated waters.

Simultaneous electrocoagulation and E-peroxone coupled with ultrafiltration membrane for shale gas produced water treatment

Membrane fouling caused by the organics-coated particles was the main obstacle for the highly efficient shale gas produced water (SGPW) treatment and recycling. In this study, a novel hybrid electrocoagulation (EC) and E-peroxone process coupled with UF (ECP-UF) process was proposed to examine the efficacy and elucidate the mechanism for UF fouling mitigation in assisting SGPW reuse. Compared to the TMP (transmembrane pressure) increase of -15 kPa in the EC-UF process, TMP in ECP-UF system marginally increased to -1.4 kPa for 3 filtration cycles under the current density of 15 mA/cm2. Both the total fouling index and hydraulically irreversible fouling index of the ECP-UF process were significantly lower than those of EC-UF process. According to the extended Derjaguin-Landau-Verwey-Overbeek theory, the potential barriers was the highest for ECP-UF processes due to the substantial increase of the acid-base interaction energy in ECP-UF process, which was well consistent with the TMP and SEM results. Turbidity and TOC of ECP-UF process were 63.6% and 45.8% lower than those of EC-UF process, respectively. According to the MW distribution, the variations of compounds and their relative contents were probably due to the oxidation and decomposing products of the macromolecular organics. The number of aromatic compound decreased, while the number of open-chain compounds (i.e., alkenes, alkanes and alcohols) increased in the permeate of ECP-UF process. Notably, the substantial decrease in the relative abundance of di-phthalate compounds was attributed to the high reactivity of these compounds with ·OH. Mechanism study indicated that ECP could realize the simultaneous coagulation, H2O2 generation and activation by O3, facilitating the enhancement of ·OH and Alb production and therefore beneficial for the improved water quality and UF fouling mitigation. Therefore, the ECP-UF process emerges as a high-efficient and space-saving approach, yielding a synergistic effect in mitigating UF fouling for SGPW recycling.

Formation of dichloroacetic acid and dichloroacetamide from phenicol antibiotic abatement during ozonation and post-chlor(am)ination

This study investigated the formation of dichloroacetamide (DCAM) and dichloroacetic acid (DCAA) from the abatement of three phenicol antibiotics (PABs, chloramphenicol, thiamphenicol, and florfenicol) during ozonation and post-chlor(am)ination. Results show that the three PABs have a low ozone reactivity (kO3 = 0.11‒0.12 M-1 s-1), and therefore are mainly abated through the hydrogen abstraction mechanism by hydroxyl radicals (•OH) during ozonation. During PAB degradation, the carboxamide moiety in the parent molecules can be cleaved off by •OH attack and thus gives rise to DCAM. The formed DCAM can then be further oxidized by O3 and/or •OH to DCAA as a more stable transformation product (TP). When the three PABs were adequately abated (abatement efficiency of ∼82 %‒95 %), the molar yields of DCAM and DCAA were determined to be 2.79 %‒4.71 % and 32.9 %‒37.2 %, respectively. Furthermore, post-chloramination of the ozonation effluents increased the yields of DCAM and DCAA slightly to 4.20 %‒6.45 % and 39.0 %‒41.1 %, respectively. In comparison, post-chlorination eliminated DCAM in the solutions, but significantly increased DCAA yields to ∼100 % due to the further conversion of DCAM and other ozonation TPs to DCAA by chlorine oxidation. The results of this study indicate that high yields of DCAM and DCAA can be generated from PAB degradation during ozonation, and post-chlorination and post-chloramination will result in very different fates of DCAM and DCAA in the disinfected effluent. The formation and transformation of DCAM and DCAA during PAB degradation need to be taken into account when selecting multi-barrier treatment processes for the treatment of PAB-containing water.

Characterization of UV/chlorine process for micropollutant abatement by probe compound-based kinetic models

Micropollutant (MP) abatement efficiencies are critical information for optimizing water treatment process for cost-effective operations. Nevertheless, due to the vast number of MPs in real water matrices, it is infeasible to measure their abatement efficiencies individually in practical applications. In this study, a probe compound-based kinetic model was developed for generalized prediction of MP abatement in various water matrices by the ultraviolet (UV)/chlorine process. The results show that by measuring the depletion of three probe compounds (ibuprofen, primidone, and dimetridazole) spiked in the water matrix, the exposures of main reactive chlorine species (RCS including chlorine radicals (Cl•), dichloride radicals (Cl₂^-•) and chlorine oxide radicals (ClO•)) and hydroxyl radicals (•OH) during the UV/chlorine process could be calculated using the model. Based on the determined exposures, the abatement efficiencies of various MPs in different water matrices (e.g., surface water, groundwater, and wastewater) could generally be predicted with acceptable accuracy by the model without prior water-specific calibration. In addition, the relative contribution of UV photolysis and oxidation with active chlorine, RCS, and •OH to MP abatement could be quantitatively simulated using the model to clarify the abatement mechanism of MPs during the UV/chlorine process. The probe-based kinetic model can thus offer a useful tool to guide practical water and wastewater treatment for MP abatement and to explore the mechanism of UV/chlorine process.

Efficient Removal of Ammonia Nitrogen by an Electrochemical Process for Spent Caustic Wastewater Treatment

Spent caustic wastewater produced in a soda plant has a high concentration of ammonia nitrogen (NH4+-N). As excessive NH4+-N discharging into water bodies would cause eutrophication as well as destruction to the ecology balance, developing an efficient technology for NH4+-N removal from the spent caustic wastewater is imperative in the current society. In this study, an electrochemical process with graphene electrodes was designed for the NH4+-N removal in the spent caustic wastewater. The removal efficiency of the NH4+-N during the electrochemical process could reach 98.7% at 4 A in a short treatment time (within 120 s) with an acceptable energy consumption (6.1 kWh/m3-order). NO3− and NO2− were not detected during the electrochemical process. An insignificant amount of NH2Cl, NHCl2, and NCl3 produced in the treatment suggested that little of the NH4+-N reacted with chlorine, that is, chlorination played a negligible role in the NH4+-N removal. By electron equilibrium and nitrogen conversion analysis, we think that NH4+-N was primarily converted to NH2(ads) on the surface of a graphene electrode by one-electron transfer during the direct oxidation of the electrochemical process. Due to the high calcium ion (Ca2+) in the spent caustic wastewater, the electrode scale significantly increased to 1.4 g after treatment of 240 s at 4 A. By X-ray diffraction (XRD) analysis, the composition of the electrode scale is portlandite Ca(OH)2. Although the electrode scale was obvious during the electrochemical treatment, it could be alleviated by alternating the electrode polarity. As a result, the life and efficiency of the graphene electrode for NH4+-N removal could remain stable for a long time. These results suggest that the electrochemical process with a graphene electrode may provide a competitive technology for NH4+-N removal in spent caustic wastewater treatment.

Molecular transformation of algal organic matter during sequential ozonation-chlorination: Role of pre-ozonation and properties of chlorinated disinfection byproducts

Formation of unknown chlorinated disinfection byproducts (Cl-DBPs) during chlorination gradually raised great concern, and pre-oxidation was considered as an efficient method to minimize Cl-DBP formation. In this study, pre-ozonation of algal organic matter was investigated, to explore its impacts on Cl-DBP formation and acute toxicity during subsequent chlorination. With fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis, the conversion of algal organic matter in chlorination with/without pre-ozonation was tracked. The results show that pre-ozonation reduced the formation of trichloromethane (TCM), yet the species and intensity of unknown Cl-DBPs were significantly increased in subsequent chlorination. Meanwhile, the solution acute toxicity was higher in chlorination with pre-ozonation than in chlorination only. Besides, molecular properties of these unknown Cl-DBPs were further explored and featured. One-chlorine-containing DBPs were unsaturated high molecular-weight compounds with more CH₂ structures, while two or three-chlorine-containing DBPs were mainly oxidized or saturated compounds. Of note, large amounts of one-chlorine-containing DBPs related to polycyclic aromatics and polyphenols compositions were generated, which may contribute to the high potential toxicity. Overall, the findings of this study could provide new insights into the impacts of pre-ozonation on the formation of unknown Cl-DBPs and potential toxicity during chlorination for actual application.

Challenges and pitfalls in the investigation of the catalytic ozonation mechanism: A critical review

Catalytic ozonation is a promising technology for pollutant abatement in water and wastewater treatment. However, there are many controversies and contradictions regarding the mechanisms of catalytic ozonation in literature, which has seriously confounded the development of the technology towards industrial applications. Herein, a critical review of literature is conducted to reveal possible underlying causes of the controversies and contradictions, and several common pitfalls in the experimental design and data interpretation are identified, e.g., the fundamentally flawed quenching method popularly used for evaluating the role of reactive oxygen species for pollutant abatement in catalytic ozonation and the neglect of monitoring ozone transfer doses in lab-scale experiments. Based on the identified pitfalls, several measures are suggested to improve the experimental design and data interpretation of catalytic ozonation studies. In addition, recent advances in mechanistic understanding of catalytic ozonation by principle-based modelling approaches are described. Finally, additional works that are needed to shrink the gap between academic research and practical applications and the prospect of catalytic ozonation in future water and wastewater treatment systems are analyzed.

Enhancement of E-Peroxone process with waste-tire carbon composite cathode for tinidazole degradation

The cathode is the key component in the electro-peroxone process (E-Peroxone), which is popularly constructed with carbon materials. This study developed an innovative method to fabricate a cathode with waste-tire carbon (WTC) whose performance was evaluated for the degradation of tinidazole (TNZ), an antibiotic frequently detected in water. It was found that the addition of WTC in the cathode can significantly promote the yield of H₂O₂ and the current efficiency: around 2.7 times that of commercial carbon black at the same loading. The critical influencing factors were studied, including the current density, ozone concentration, initial pH value, chlorine ions and initial TNZ concentration. The scavenger tests demonstrated the possible involvement of •OH and O₂^•-. Some transformation products of TNZ were identified with UPLC-MS and the degradation pathway was proposed accordingly. These results demonstrated the potential of WTC for developing E-Peroxone cathodes.

Moderate pre-ozonation coupled with a post-peroxone process remove filamentous cyanobacteria and 2-MIB efficiently: From bench to pilot-scale study

The increasing frequency and intensity of taste- and odour-producing cyanobacteria in water sources is a growing global issue. Odour events caused by 2-methylisoborneol (2-MIB) mainly arising from filamentous cyanobacteria have been a very common problem in water supply. Removal rates of filamentous cyanobacteria and 2-MIB by conventional water treatment, such as coagulation, and disinfection treatment processes is low. Hence, a moderate pre-ozonation of cyanobacteria (with little cell damage) was proposed in this study as an enhanced coagulation step to remove filamentous cyanobacteria and intracellular 2-MIB effectively, while avoiding the release of intracellular 2-MIB. A post-peroxone (O₃/H₂O₂) process was applied after sand filtration to degrade the residual dissolved 2-MIB. Results show that moderate pre-ozonation (0.2 mg/L O₃ oxidation for 20 min) can substantially enhance the coagulation efficiency for algae, with low cell lysis and high cell viability. Furthermore, 2.0 mg/L O₃ combined with 2.0 mg/L H₂O₂ can degrade the residual dissolved 2-MIB nearly 100% after 20 min reaction. Based on the optimal dosages, a 0.6 m³/h pilot system, including pre-ozonation, coagulation and sedimentation, sand filtration, and post-peroxone processes, was continuously run for 14 days, and it was found that the proposed process can effectively and stably remove filamentous cyanobacteria and 2-MIB.

Transformation of antiviral ribavirin during ozone/PMS intensified disinfection amid COVID-19 pandemic

Due to the spread of coronavirus disease 2019 (COVID-19), large amounts of antivirals were consumed and released into wastewater, posing risks to the ecosystem and human health. Ozonation is commonly utilized as pre-oxidation process to enhance the disinfection of hospital wastewater during COVID-19 spread. In this study, the transformation of ribavirin, antiviral for COVID-19, during ozone/PMS‑chlorine intensified disinfection process was investigated. •OH followed by O₃ accounted for the dominant ribavirin degradation in most conditions due to higher reaction rate constant between ribavirin and •OH vs. SO₄•^- (1.9 × 10⁹ vs. 7.9 × 10⁷ M^-1 s^-1, respectively). During the O₃/PMS process, ribavirin was dehydrogenated at the hydroxyl groups first, then lost the amide or the methanol group. Chloride at low concentrations (e.g., 0.5- 2 mg/L) slightly accelerated ribavirin degradation, while bromide, iodide, bicarbonate, and dissolved organic matter all reduced the degradation efficiency. In the presence of bromide, O₃/PMS process resulted in the formation of organic brominated oxidation by-products (OBPs), the concentration of which increased with increasing bromide dosage. However, the formation of halogenated OBPs was negligible when chloride or iodide existed. Compared to the O₃/H₂O₂ process, the concentration of brominated OBPs was significantly higher after ozonation or the O₃/PMS process. This study suggests that the potential risks of the organic brominated OBPs should be taken into consideration when ozonation and ozone-based processes are used to enhance disinfection in the presence of bromide amid COVID-19 pandemic.

Degradation of tetrabromobisphenol A by ferrate(VI)-CaSO3 process: Kinetics, products, and impacts on following disinfection by-products formation

Tetrabromobisphenol A (TBBPA) is one of the most widely applied brominated flame retardants and has been widely detected in water environment, which might pose risks of brominated disinfection by-products formation in water treatment system. Ferrate(VI)-CaSO₃ (Fe(VI)-CaSO₃) system could effectively degrade TBBPA at pH 7.0-9.0 but the decomposition rate of TBBPA dropped with increasing pH. The presence of 0.5 mg C/L humic acid (HA) had negligible impact on TBBPA removal, but the removal of TBBPA decreased to ~87% and 80% at pH 7.0 and 8.0, respectively, in the presence of 5.0 mg C/L HA. The transformation products of TBBPA detected in Fe(VI)-CaSO₃ process revealed that TBBPA degradation mainly proceeded via electron abstraction, debromination, and ring-opening pathways and Br^- was released. In the presence of TBBPA, Fe(VI)-CaSO₃ pre-oxidation decreased the generation of all determined DBPs during chlorination at pH 8.0 but it lessened the generation of some DBPs and slightly increased the formation of the other DBPs at pH 7.0. The toxic risk analysis showed that Fe(VI)-CaSO₃ pre-oxidation of TBBPA could reduce the toxic risk of DBPs in both synthetic water and natural water at pH 8.0, indicating that Fe(VI)-CaSO₃ process has the potential to be applied in practical water treatment.

Variations of disinfection byproduct precursors through conventional drinking water treatment processes and a real-time monitoring method

In this investigation, raw water (RW), settled water (SW), and filtered water (FW) collected from a drinking water treatment plant were fractionated into 24 natural organic matter (NOM) fractions with varying molecular weights and hydrophobicity. The yields of disinfection byproducts (DBPs) obtained during the chlorination of the NOM fractions were explored. Results revealed that the 0-1 kDa, 5-10 kDa, and hydrophobic DBP precursors dominated RW. Hydrophobic fractions cannot be effectively removed, which contributed to the high DBP precursors remaining in the FW. The optional optical parameters, including UVA (UV₃₄₀, UV₃₆₀, and UV₃₈₀), UVB (UV₂₈₀, UV₃₀₀, and UV₃₁₀), and UVC (UV₂₅₄, UV₂₆₀, and UV₂₇₂), were analyzed to determine the DBP yields during chlorination of different NOM fractions. Results revealed that UVC could be applied to indicate the regulated DBP yields of the humified precursors. Contrary to the generally accepted view, for biologically derived precursors, their regulated DBPs and dichloroacetonitrile correlated better with UVA (e.g. UV₃₄₀). Moreover, PARAFAC analysis was applied to decompose an array of 24 EEM spectra. Good linear correlations were found between the PARAFAC components and most DBP yields. Furthermore, four fluorescence parameters were proposed via a modified fluorescence picking method, which can serve as excellent surrogates of PARAFAC components. These fluorescence parameters were found to be effective in indicating most DBP yields. Finally, the fluorescence intensity at excitation wavelength/emission wavelength = 310/416 nm was found to be a promising built-in parameter for the real-time monitoring of DBP precursors, regardless of the humification degree of the precursors.

The Role of Catalytic Ozonation Processes on the Elimination of DBPs and Their Precursors in Drinking Water Treatment

Formation of disinfection byproducts (DBPs) in drinking water treatment (DWT) as a result of pathogen removal has always been an issue of special attention in the preparation of safe water. DBPs are formed by the action of oxidant-disinfectant chemicals, mainly chlorine derivatives (chlorine, hypochlorous acid, chloramines, etc.), that react with natural organic matter (NOM), mainly humic substances. DBPs are usually refractory to oxidation, mainly due to the presence of halogen compounds so that advanced oxidation processes (AOPs) are a recommended option to deal with their removal. In this work, the application of catalytic ozonation processes (with and without the simultaneous presence of radiation), moderately recent AOPs, for the removal of humic substances (NOM), also called DBPs precursors, and DBPs themselves is reviewed. First, a short history about the use of disinfectants in DWT, DBPs formation discovery and alternative oxidants used is presented. Then, sections are dedicated to conventional AOPs applied to remove DBPs and their precursors to finalize with the description of principal research achievements found in the literature about application of catalytic ozonation processes. In this sense, aspects such as operating conditions, reactors used, radiation sources applied in their case, kinetics and mechanisms are reviewed.

Evaluation of the concentration and contribution of superoxide radical for micropollutant abatement during ozonation

Due to the fast reaction of superoxide radical (O₂^•-) with ozone (O₃), it has been suggested that O₂^•- is present at very low concentrations during ozonation. Therefore, while O₂^•- has been considered a critical chain carrier for promoting O₃ decomposition to hydroxyl radicals (•OH), the direct reactions of O₂^•- with micropollutants have been assumed to be insignificant during ozonation. In this study, we monitored the exposures of O₃, •OH, and O₂^•- by following the depletion of O₃, p-chlorobenzoic acid (pCBA, as •OH probe), and tetrachloromethane (CCl₄, as O₂^•- probe) during ozonation of various water matrices (surface water, groundwater, and secondary wastewater effluent). For a given water matrix, the ratio between •OH and O₃ exposures (R_ct), O₂^•- and O₃ exposures (R_SO), as well as O₂^•- and •OH exposures (R_SH) remained almost constant over the entire reaction time. This suggests that during ozonation, the ratios between the transient concentrations of •OH and O₃, O₂^•- and O₃, and O₂^•- and •OH were also constant and equaled to the R_ct, R_SO, and R_SH, respectively. Based on the O₃, •OH, and O₂^•- exposures observed during ozonation, a chemical kinetic model was proposed to simulate the abatement of ten ozone-resistant micropollutants in the three water matrices by ozonation. The results indicate that due to the higher concentrations of O₂^•- than •OH (R_SH = ~5-8), the reactions with O₂^•- played a non-negligible or even dominant role in the abatement of some micropollutants that have similar or higher O₂^•- reactivity than •OH reactivity (e.g., tetrachloroethylene, chloroform, and PFOA). Compared with the previous model that neglected the contribution of O₂^•- to micropollutant abatement, the proposed model more accurately simulated the abatement efficiencies of the test micropollutants during ozonation. These results indicate that the proposed model can provide a useful tool for the generalized prediction of micropollutant abatement by ozonation.

Oxygen vacancies and Lewis sites activating O3/H2O2 at wide pH range via surface electron transfer over CeOx@SiO2 for nitrobenzene mineralization

The low efficiency of peroxone (O₃/H₂O₂) at acidic and neutral pH restrained its application in water purification. To overcome this shortcoming, CeO_X@SiO₂ with large surface area, abundant surface oxygen vacancies (V_o), Lewis sites (L sites) and high Ce(III)/Ce(IV) ratio were synthesized to change the traditional electron transfer pathway between O₃ and H₂O₂. V_o was facile in absorbing H₂O₂ to form V_o-H₂O₂ and L sites were capable of absorbing O₃ to form L-O₃. The electron at V_o could be donated to V_o-H₂O₂ and generate V_o-HO₂^-, which then effectively triggered the decomposition of L-O₃ at CeO_X@SiO₂'s interface and O₃ in bulk solution. The electron transfer at the solid-liquid interface with the help of Ce³⁺/Ce⁴⁺ redox cycle and V_o was pH independent and different from the traditional electron transfer of peroxone reaction. Nitrobenzene (NB) mineralization was promoted to 92.5% in CeO_X@SiO₂-peroxone, but only 63.8% TOC was removed in tradition peroxone process. Moreover, CeO_X@SiO₂-peroxone had a wide pH application range. NB's degradation in CeO_X@SiO₂-peroxone process followed the co-oxidation mechanism of superoxide free (•O₂^-) and hydroxyl radical (•OH). The finding of this study could broaden the popularization of peroxone in water treatment and provided a strategy for catalyst design.

Effects of coagulation-sedimentation-filtration pretreatment on micropollutant abatement by the electro-peroxone process

The electro-peroxone (EP) process has been considered an attractive alternative to conventional ozonation for micropollutant abatement in water treatment. However, how to integrate the EP process into the water treatment trains in water utilities has yet to be investigated. This study compared micropollutant abatement during the EP treatment of potable source water with and without pretreatment of biological oxidation, flocculation, sedimentation, and filtration. Results show that this pretreatment train removed 39% of dissolved organic carbon (DOC) and 28% of the UV₂₅₄ absorbance of the raw water, leading to higher ozone (O₃) stability in the treated water. By electrochemically generating hydrogen peroxide to accelerate O₃ decomposition to hydroxyl radicals (•OH), the EP process considerably shortened the time required for ozone depletion and micropollutant abatement during the treatment of both the raw and pretreated water to ∼1 min, compared to ∼3 and 7.5 min during conventional ozonation of the raw and treated water, respectively. For the same specific ozone dose of 1 mg O₃ mg^-1 DOC (corresponding to 4.3 and 2.8 mg O₃ L^-1 for the raw and treated water, respectively), the abatement efficiencies of micropollutants with moderate and low ozone reactivity were increased by ∼10-15%, while the energy consumption for micropollutant abatement was decreased by ∼24-56% during the EP treatment of the treated water than the raw water. These results indicate that partial removal of DOC and ammonia from the raw water by the pretreatment train has a beneficial effect on enhancing micropollutant abatement and reducing energy consumption of the EP process. Therefore, it is more cost-effective to integrate the EP process after the pretreatment train in water utilities for micropollutant abatement.

Addition of lemon before boiling chlorinated tap water: A strategy to control halogenated disinfection byproducts

Chlorine disinfection is required to inactivate pathogens in drinking water, but it inevitably generates potentially toxic halogenated disinfection byproducts (halo-DBPs). A previous study has reported that the addition of ascorbate to tap water before boiling could significantly decrease the concentration of overall halo-DBPs in the boiled water. Since the fruit lemon is rich in vitamin C (i.e., ascorbic acid), adding it to tap water followed by heating and boiling in an effort to decrease levels of halo-DBPs was investigated in this study. We examined three approaches that produce lemon water: (i) adding lemon to tap water at room temperature, termed "Lemon"; (ii) adding lemon to boiled tap water (at 100 °C) and then cooling to room temperature, termed "Boiling + Lemon"; and (iii) adding lemon to tap water then boiling and cooling to room temperature, termed "Lemon + Boiling". The concentrations of total and individual halo-DBPs in the resultant water samples were quantified with high-performance liquid chromatography-tandem mass spectrometry and the cytotoxicity of DBP mixtures extracted from the water samples was evaluated using human epithelial colorectal adenocarcinoma Caco-2 cells and hepatoma HepG2 cells. Our results show that the "Lemon + Boiling" approach substantially decreased the concentrations of halo-DBPs and the cytotoxicity of tap water. This strategy could be applied to control halo-DBPs, as well as to lower the adverse health effects of halo-DBPs on humans through tap water ingestion.

Covalent organic frameworks as an efficient adsorbent for controlling the formation of disinfection by-products (DBPs) in chlorinated drinking water

2,5-Dimethyl-p-phenylenediamine-1,3,5-triformylphloroglucinol covalent organic frameworks (PATP COF) were prepared and used as novel adsorbent for controlling the formation potential (FP) and reducing the toxic potential of both carbonaceous disinfection by-products (C-DBPs) and nitrogenous DBPs (N-DBPs) during their subsequent chlorination. During the PATP COF adsorption pretreatment process, the FP of C-DBPs, N-DBPs and total organic halogen (TOX) were reduced by 86.5, 75.4 and 81.1%, respectively. These removal efficiencies were significantly higher when compared with those obtained using a traditional activated carbon (AC) adsorption pretreatment process (42.7, 19.4 and 28.7%, respectively). By comprehensive toxicity calculations, a significant reduction in both the acute and chronic toxic potential of C-DBPs and N-DBPs were observed during the PATP COF adsorption process (with reduction rates of ~85 and ~ 75% observed for the C-DBPs and N-DBPs, respectively), which were comparable to the removal efficiencies observed for C-DBPs FP and N-DBPs FP by weight, suggesting the simultaneous and effective control of DBPs FP and their toxic potential. Cycling tests and stability trial also showed the excellent reusability, wide pH adaptability, and high stability of PATP COF, demonstrating its great potential application to the treatment of drinking water.

Review on electrochemical system for landfill leachate treatment: Performance, mechanism, application, shortcoming, and improvement scheme

Landfill leachate is a type of complex organic wastewater, which can easily cause serious negative impacts on the human health and ecological environment if disposed improperly. Electrochemical technology provides an efficient approach to effectively reduce the pollutants in landfill leachate. In this review, the electrochemical standalone processes (electrochemical oxidation, electrochemical reduction, electro-coagulation, electro-Fenton process, three-dimensional electrode process, and ion exchange membrane electrochemical process) and the electrochemical integrated processes (electrochemical-advanced oxidation process (AOP) and biological electrochemical process) for landfill leachate treatment are summarized, which include the performance, mechanism, application, existing problems, and improvement schemes such as cost-effectiveness. The main objective of this review is to help researchers understand the characteristics of electrochemical treatment of landfill leachate and to provide a useful reference for the design of the process and reactor for the harmless treatment of landfill leachate.

Photooxidation of atrazine and its influence on disinfection byproducts formation during post-chlorination: effect of solution pH and mechanism

Partial photooxidation of micropollutants may lead to various degradation intermediates, obviously affecting disinfection byproducts (DBPs) formation during the post-chlorination process. The photooxidation of atrazine (ATZ) in aqueous solutions with low-pressure mercury UV lamps in UV, UV/H2O2 and UV/TiO2 treatment system and the formation of chlorinated disinfection byproducts (DBPs) during subsequent chlorination processes including dichloroacetic acid (DCAA), trichloroacetic acid (TCAA), 1,1,1-trichloro-2-propanone (TCP), trichloromethane (TCM) and chloropicrin (CHP) were investigated in this study. The effect of solution pH on the oxidation pathway of ATZ in three UV photooxidation treatment process and the impact of photooxidation on the DBPs formations were assessed. Based on UPLC-ESI-MS/MS analyses, identification of main oxidation intermediates was performed and the plausible degradation pathways of ATZ in photooxidation system were proposed, indicating that photooxidation of ATZ in UV/H2O2 and UV/TiO2 process system was significantly pH-dependent processes. Dichloroacetic acid (DCAA), trichloroacetic acid (TCAA), 1,1,1-trichloro-2-propanone (TCP), trichloromethane (TCM) and chloropicrin (CHP) were detected in photooxidized ATZ solutions. Compared to the other three DBPs, TCM and TCP were the main DBPs formed. The DBPs formations were greatly promoted in oxidized ATZ solutions. Solution pH and UV irradiation time exhibited obvious impact on the DBPs formation on the basis of DBP species. The variation tendency of DBPs observed relates to the combustion of ATZ in photooxidation system and the production oxidation intermediates.

Removal of micropollutants by an electrochemically driven UV/chlorine process for decentralized water treatment

The ultraviolet/chlorine (UV/Cl₂) process is an emerging advanced oxidation technology for micropollutant abatement in water and wastewater treatment. However, the application of the conventional UV/Cl₂ process in decentralized systems is limited by the transport and management of liquid chlorine. To overcome this limitation, this study evaluated an electrochemically driven UV/Cl₂ (E-UV/Cl₂) process for micropollutant abatement under conditions simulating decentralized water treatment. The E-UV/Cl₂ process combines UV irradiation with in situ electrochemical Cl₂ production from anodic oxidation of chloride (Cl^-) in source waters. The results show that with typical Cl^- concentrations present in water sources for decentralized systems (30-300 mg/L Cl^-), sufficient amounts of chlorine could be quickly electrochemically produced at the anode to enable E-UV/Cl₂ process for water treatment. Due to its multiple mechanisms for micropollutant abatement (direct photolysis, direct electrolysis, Cl₂-mediated oxidation, as well as hydroxyl radical and reactive chlorine species oxidation), the E-UV/Cl₂ process effectively eliminated all micropollutants (trimethoprim, ciprofloxacin, metoprolol, and carbamazepine) spiked in a surface water in 5 min. In contrast, at least one micropollutant with ∼20-80% residual concentrations could still be detected in the water treated by 10 min of UV irradiation, chlorination, electrolysis, and the conventional UV/Cl₂ process under similar experimental conditions. The electrical energy per order (E_EO) for micropollutant abatement ranged from 0.15 to 1.8 kWh/m³ for the E-UV/Cl₂ process, which is generally comparable to that for the conventional UV/Cl₂ process (0.14-2.7 kWh/m³). These results suggest that by in-situ generating Cl₂ from anodic oxidation of Cl^-, the E-UV/Cl₂ process can overcome the barrier of the conventional UV/Cl₂ process and thus provide a promising technology for micropollutant abatement in decentralized water treatment systems.

Effects of ascorbate and carbonate on the conversion and developmental toxicity of halogenated disinfection byproducts during boiling of tap water

Chlorine disinfection inactivates pathogens in drinking water, but meanwhile it causes the formation of halogenated disinfection byproducts (DBPs), which may induce adverse health effects. Humans are unavoidably exposed to halogenated DBPs via tap water ingestion. Boiling of tap water has been found to significantly reduce the concentrations of halogenated DBPs. In this study, we found that compared with boiling only, adding ascorbate (vitamin C) or carbonate (baking soda) to tap water and then boiling the water further reduced the level of total organic halogen (a collective parameter for all halogenated DBPs) by up to 36% or 28%, respectively. Adding ascorbate removed the chlorine residual in tap water and thus prevented the formation of more halogenated DBPs in the boiling process. Adding carbonate elevated pH of tap water and consequently enhanced the hydrolysis (dehalogenation) of halogenated DBPs or led to the formation of more trihalomethanes that might volatilize to air during the boiling process. The comparative developmental toxicity of the DBP mixtures in the water samples was also evaluated. The results showed that adding a tiny amount of sodium ascorbate or carbonate (2.5-5.0 mg/L) to tap water followed by boiling for 5 min reduced the developmental toxicity of tap water to a substantially lower level than boiling only. The addition of sodium ascorbate or carbonate to tap water in household could be realized by preparing them in tiny pills. This study suggests simple and effective methods to reduce the adverse effects of halogenated DBPs on humans through tap water ingestion.

Revealing Decay Mechanisms of H2O2-Based Electrochemical Advanced Oxidation Processes after Long-Term Operation for Phenol Degradation

Hydrogen peroxide (H₂O₂)-based electrochemical advanced oxidation processes (EAOPs) have been widely attempted for various wastewater treatments. So far, stability tests of EAOPs are rarely addressed and the decay mechanism is still unclear. Here, three H₂O₂-based EAOP systems (electro-Fenton, photoelectro-Fenton, and photo+ electro-generated H₂O₂) were built for phenol degradation. More than 97% phenol was removed in all three EAOPs in 1 h at 10 mA·cm^-2. As a key component in EAOPs, the cathodic H₂O₂ productivity is directly related to the performance of the system. We for the first time systematically investigated the decay mechanisms of the active cathode by operating the cathodes under multiple conditions over 200 h. Compared with the fresh cathode (H₂O₂ yield of 312 ± 22 mg·L^-1·h^-1 with a current efficiency of 84 ± 5% at 10 mA·cm^-2), the performance of the cathode for H₂O₂ synthesis alone decayed by only 17.8%, whereas the H₂O₂ yields of cathodes operated in photoelectro-generated H₂O₂, electro-Fenton, and photoelectro-Fenton systems decayed by 60.0, 90.1, and 89.6%, respectively, with the synergistic effect of salt precipitation, ^•OH erosion, organic contamination, and optional Fe contamination. The lower current decay of 16.1-32.3% in the electrochemical tests manifested that the cathodes did not lose activity severely. Therefore, the significant decrease of H₂O₂ yield was because the active sites were altered to catalyze the four-electron oxygen reduction reaction, which was induced by the long-term erosion of ^•OH. Our findings provided new insights into cathode performance decay, offering significant information for the improvement of cathodic longevity in the future.

Use of spectroscopic indicators for the monitoring of bromate generation in ozonated wastewater containing variable concentrations of bromide

Time-resolved monitoring of bromate and other by-products formed into effluents treated with ozone or advanced oxidation processes in wastewater treatment plants (WWTPs) is time-consuming and expensive. This study examined whether concentrations of bromate formed in wastewater after ozonation in the presence of widely varying bromide levels (from ca. 0.7-21.2 mg/L) can be quantified based on measurements of changes in optical properties (differential UV absorbance (ΔUVA), spectral slopes, total or regional fluorescence) of the ozonated samples. Batch ozonation was carried out using a secondary effluent produced at a major wastewater treatment plant located in the Metropolitan Seattle Area. The tests involved raw and bromide-spiked samples treated with ozone doses from 0.1 to 1 mg O₃/mg DOC. Measurements of the absorbance at 254 nm (UVA254), fluorescence and bromate concentrations were performed on the treated samples. In the ozonated wastewater the concentration of bromate increased approximately linearly, from <10 ppb to ca. 200 ppb, without showing the lag phase characteristic for lower ozone doses (<0.4 mg O₃/mg DOC) that was observed in previous studies carried out with concentrations of bromide in the range of 0.05-0.5 mg/L. The highest bromide concentrations used in this study (>10 mg/L) tended to inhibit the generation of bromate. Relative reduction of UVA254 and total fluorescence (TF) were found to be good predictors of bromate generation. Specifically, exponential curves could adequately fit the non-linear relationships found to exist between the concentrations of bromate and the relative reductions of the UV254 and TF, for any initial bromide concentrations used in this study. Little formation of bromate was found to occur for reduction ranges for UVA254 and TF of 30-40% and 70-80% respectively. Conversely, rapid increases in bromate generation were observed when the decrease of UVA254 or TF exceeded these threshold values.

MBR-UV/Cl2 system in treating polluted surface water with typical PPCP contamination

This study proposed the membrane bioreactor–ultraviolet/chlorine (MBR-UV/Cl₂) process for treating polluted surface water with pharmaceutical personal care product (PPCP) contamination. Results showed that MBR-UV/Cl₂ effectively removed the organic matters and ammonia at approximately 80% and 95%. MBR-UV/Cl₂ was used in the removal of sulfadiazine(SDZ), sulfamethoxazole(SMZ), tetracycline(TC), oxytetracycline(OTC), ciprofloxacin(CIP), ofloxacin(OFX), erythromycin(ERY), roxithromycin(ROX), ibuprofen(IBU) and, naproxen(NAX) at 12.18%, 95.61%, 50.50%, 52.97%, 33.56%, 47.71%, 87.57%, 93.38%, 93.80%, and 71.46% in which their UV/Cl₂ contribution was 12.18%, 95.61%, 29.04%, 38.14%, 25.94%, 7.20%, 80.28%, 33.79%, 73.08%, and 23.05%, respectively. The removal of 10 typical PPCPs using UV/Cl₂ obtained higher contributions than those of the MBR process, except OTC, ROX, and IBU. The UV/Cl₂ process with 3-min hydraulic retention time and chlorine concentration at 3 mg/L effectively removed the trace of PPCPs. MBR-UV/Cl₂ has the potential to be developed as an effective technology in treating polluted surface water with PPCP contamination.

Comparison of emerging contaminant abatement by conventional ozonation, catalytic ozonation, O3/H2O2 and electro-peroxone processes

The abatement of several emerging contaminants (ECs) in groundwater by conventional ozonation and three ozone-based advanced oxidation processes (AOPs) - catalytic ozonation with manganese dioxide (MnO₂), conventional peroxone (O₃/H₂O₂), and electro-peroxone (EP) - was compared in this study. The addition of MnO₂, H₂O₂, or electro-generation of H₂O₂ during ozonation enhanced ozone transformation to hydroxyl radicals to different extent. These changes did not considerably influence the abatement of ECs with moderate to high ozone reactivities ( [Formula: see text] ), whose abatements were similar with >90 % during all four processes. In comparison, the abatements of ozone-refractory ECs (k_O3< 15 M^-1s^-1) were lower during conventional ozonation (∼40-85 % abatement), but could be enhanced by ∼10-40 % during the three ozone-based AOPs. Besides enhancing ozone-refractory EC abatement, the three AOPs, especially the O₃/H₂O₂ and EP processes, reduced considerably bromate formation compared to conventional ozonation. These results demonstrate that the EP process performs similarly as catalytic ozonation and O₃/H₂O₂ processes in terms of EC abatement and bromate control. Considering its more convenient, flexible, and safer way of operation, the EP process may provide an attractive alternative to the two more traditional AOPs for water treatment.

Ferrate(VI) pretreatment before disinfection: An effective approach to controlling unsaturated and aromatic halo-disinfection byproducts in chlorinated and chloraminated drinking waters

Disinfection is an essential process of drinking water treatment to eliminate harmful pathogens, but it generates potentially toxic disinfection byproducts (DBPs). Ferrate (FeO₄^2-, Fe(VI)) was used to pre-oxidize natural organic matter (NOM, the precursor of DBPs) in source water to control DBP formation in subsequent chlorine or chloramine disinfection. Currently, it is unclear how Fe(VI) changes the structure of NOM, and no information details the effect of Fe(VI) pretreatment on the aromatic DBPs or the speciation of overall DBPs generated in subsequent disinfection of drinking water. In the present paper, Fe(VI) was applied to pretreat simulated source water samples at a Fe(VI) to dissolved organic carbon mole ratio of 1:1 at pH 8.0. ¹³C nuclear magnetic resonance spectroscopy was newly employed to characterize NOM in simulated source waters with and without Fe(VI) treatment, and it was demonstrated that Fe(VI) converted unsaturated aromatic C functional groups in NOM to saturated aliphatic ones. High-resolution mass spectrometry (HRMS) and high performance liquid chromatography/triple quadrupole MS were applied to analyze the DBPs generated in chlorination and chloramination of the source waters with and without Fe(VI) pretreatment. It was confirmed that Fe(VI) pretreatment followed by chlorination (or chloramination), generated DBPs containing less unsaturated, halogenated, and aromatic moieties than chlorination (or chloramination) without pretreatment by Fe(VI). Finally, the cytotoxicity of disinfected drinking water samples were assessed with the human epithelial colorectal adenocarcinoma Caco-2 cell line (a model of the intestinal barrier for ingested toxicants), and the results show that Fe(VI) pretreatment detoxified the chlorinated and chloraminated drinking waters.

An overview of bromate formation in chemical oxidation processes: Occurrence, mechanism, influencing factors, risk assessment, and control strategies

Chemical oxidation processes have been extensively utilized in disinfection and removal of emerging organic contaminants in recent decades. Some undesired byproducts, however, are produced in these processes. Of them, bromate has attracted the most intensive attention. It was previously regarded as a byproduct that typically occurred in ozone-based oxidation processes. However, for the past decade, bromate formation has been detected in other oxidation processes such as CuO-catalyzed chlorination, SO₄^--based oxidation, and ferrate oxidation processes. This review summarizes the occurrences, mechanisms, influencing factors, risk assessment, and control strategies of bromate formation in the four oxidation processes, i.e., ozone-based oxidation, chlorine-based oxidation, SO₄^--based oxidation, and ferrate oxidation. Besides, some unresolved issues for future studies are provided: (1) Clarification of the relative contributions of SO₄^- and Br to the oxidation of bromine for bromate formation in SO₄^--based oxidation processes; (2) evaluation of the role of different reactive species in the bromate formation in the process of UV/HOCl; (3) quantification of the dual role of alkalinity in bromate formation during ozonation; (4) assessment of the risks of bromate formation in SO₄^--based oxidation processes for practical applications; and (5) exploration of strategies for inhibiting bromate formation in SO₄^--based oxidation, UV/chlorine, and metal oxide-catalyzed chlorination processes.

Oxidation byproducts from the degradation of dissolved organic matter by advanced oxidation processes - A critical review

Advanced oxidation processes (AOPs) have been increasingly used for the treatment of source waters and wastewaters. AOPs characteristically produce oxidation byproducts (OBPs) from the partial degradation of dissolved organic matter (DOM) and/or the transformation of inorganic ions (especially, halides) into highly toxic substances including bromate and halogenated organic OBPs (X-OBPs). However, despite the enormous health and environmental risks posed by X-OBPs, an integral understanding of the complex OBP formation mechanisms during AOPs is lacking, which limits the development of safe and effective AOP-based water treatment schemes. The present critical and comprehensive review was intended to fill in this important knowledge gap. The study shows, contrary to the hitherto prevailing opinion, that the direct incorporation of halide atoms (X^•) into DOM makes an insignificant contribution to the formation of organic X-OBPs. The principal halogenating agent is hypohalous acid/hypohalite (HOX/XO^-), whose control is, therefore, critical to the reduction of both organic and inorganic X-OBPs. Significant generation of X-OBPs has been observed during sulfate radical AOPs (SR-AOPs), which arises principally from the oxidizing effects of the unactivated oxidant and/or the applied catalytic activator rather than the sulfate radical as is commonly held. A high organic carbon/X^- molar ratio (>5), an effective non-catalytic activator such as UV or Fe²⁺, a low oxidant concentration, and short treatment time are suggested to limit the accumulation of HOX/XO^- and, thus, the generation of X-OBPs during SR-AOPs. At present, there are no established techniques to prevent the formation of X-OBPs during UV/chlor(am)ine AOPs because the maintenance of substantial amounts of active halogen is essential to these processes. The findings and conclusions reached in this review would advance the research and application of AOPs.

Oxidation of emerging biocides and antibiotics in wastewater by ozonation and the electro-peroxone process

This study investigated the abatement of a number of antimicrobials frequently detected in municipal wastewater by conventional ozonation and a recently developed ozone-based advanced oxidation process, the electro-peroxone (E-peroxone) process. A synthetic water and a real secondary wastewater effluent were spiked with fourteen antimicrobials, including antibiotics and biocides, and then treated by the two processes. The results show that most of the antibiotics investigated (e.g., ofloxacin, trimethoprim, norfloxacin, and ciprofloxacin) readily react with ozone (O₃) and could therefore be efficiently eliminated from the water matrices by direct O₃ oxidation during both processes. In contrast, most of the biocides tested in this study (e.g., clotrimazole, pentamidine, bixafen, propiconazole, and fluconazole) were only moderately reactive, or non-reactive, with O₃. Therefore, these biocides were removed at considerably lower rate than the antibiotics during the two ozone-based processes, with hydroxyl radical (OH) oxidation playing an important role in their abatement mechanisms. When compared with conventional ozonation, the E-peroxone process is defined by the in situ electrogeneration of hydrogen peroxide, which considerably enhances the transformation of O₃ to OH. As a result, the E-peroxone process significantly accelerated the abatement of biocides and required a considerably shorter treatment time to eliminate all of the tested compounds from the water matrices than conventional ozonation. In addition, the E-peroxone process enhanced the contributions of OH fractions to the abatement of moderately ozone reactive benzotriazoles. These results demonstrate that the E-peroxone process holds promise as an effective tertiary treatment option for enhancing the abatement of ozone-resistant antimicrobials in wastewater.

Physico-chemical processes

The review scans research articles published in 2018 on physico-chemical processes for water and wastewater treatment. The paper includes eight sections, that is, membrane technology, granular filtration, flotation, adsorption, coagulation/flocculation, capacitive deionization, ion exchange, and oxidation. The membrane technology section further divides into six parts, including microfiltration, ultrafiltration, nanofiltration, reverse osmosis/forward osmosis, and membrane distillation. PRACTITIONER POINTS: Totally 266 articles on water and wastewater treatment have been scanned; The review is sectioned into 8 major parts;  Membrane technology has drawn the widest attention from the research community.

Optimization of the Electro-Peroxone Process for Micropollutant Abatement Using Chemical Kinetic Approaches

The electro-peroxone (E-peroxone) process is an emerging electrocatalytic ozonation process that is enabled by in situ producing hydrogen peroxide (H₂O₂) from cathodic oxygen reduction during ozonation. The in situ-generated H₂O₂ can then promote ozone (O₃) transformation to hydroxyl radicals (•OH), and thus enhance the abatement of ozone-refractory pollutants compared to conventional ozonation. In this study, a chemical kinetic model was employed to simulate micropollutant abatement during the E-peroxone treatment of various water matrices (surface water, secondary wastewater effluent, and groundwater). Results show that by following the O₃ and •OH exposures during the E-peroxone process, the abatement kinetics of a variety of model micropollutants could be well predicted using the model. In addition, the effect of specific ozone doses on micropollutant abatement efficiencies could be quantitatively evaluated using the model. Therefore, the chemical kinetic model can be used to reveal important information for the design and optimization of the treatment time and ozone doses of the E-peroxone process for cost-effective micropollutant abatement in water and wastewater treatment.

Research progress of disinfection and disinfection by-products in China

Disinfection is an indispensable water treatment process for killing harmful pathogens and protecting human health. However, the disinfection has caused significant public concern due to the formation of toxic disinfection by-products (DBPs). Lots of studies on disinfection and DBPs have been performed in the world since 1974. Although related studies in China started in 1980s, a great progress has been achieved during the last three decades. Therefore, this review summarized the main achievements on disinfection and DPBs studies in China, which included: (1) the occurrence of DBPs in water of China, (2) the identification and detection methods of DBPs, (3) the formation mechanisms of DBPs during disinfection process, (4) the toxicological effects and epidemiological surveys of DBPs, (5) the control and management countermeasures of DBPs in water disinfection, and (6) the challenges and chances of DBPs studies in future. It is expected that this review would provide useful information and reference for optimizing disinfection process, reducing DBPs formation and protecting human health.

The beneficial effect of cathodic hydrogen peroxide generation on mitigating chlorinated by-product formation during water treatment by an electro-peroxone process

The formation of chlorinated by-products is a major concern associated with electrochemical water treatment processes. This study investigated the formation of chlorinated by-products during surface water treatment by a newly developed electrochemical advanced oxidation process (EAOP), the electro-peroxone (E-peroxone) process, which couples ozonation with in situ electro-generation of hydrogen peroxide (H₂O₂) from cathodic oxygen reduction. Due to the enhanced ozone (O₃) conversion to hydroxyl radicals (•OH) by electro-generated H₂O₂, the E-peroxone process considerably accelerated the abatement of ozone-refractory micropollutants such as clofibric acid and chloramphenicol in the selected surface water compared to conventional ozonation. In addition, the cathodically generated H₂O₂ effectively quenched hypochlorous acid (HOCl) derived from the anodic oxidation of chloride in the surface water. Therefore, the formation of trichloromethane (TCM) and chloroacetic acids (CAAs) from the reactions of HOCl with dissolved organic matter (DOM) was insignificant during the E-peroxone process, and similar levels of TCM and CAAs were generally observed in the conventional ozonation and E-peroxone treated water. In contrast, considerable amounts of HOCl could be generated from the anodic oxidation of chloride and then accumulated in the surface water during conventional electrolysis process, which resulted in significantly higher concentrations of TCM and CAAs in the electrolysis treated water. The results of this study suggest that the E-peroxone process can overcome the major limitation of conventional electrochemical processes and provide an effective and safe EAOP alternative for micropollutant abatement during water treatment.

Hydrogen peroxide generation from O2 electroreduction for environmental remediation: A state-of-the-art review

The electrochemical production of hydrogen peroxide (H2O2) by 2-electron oxygen reduction reaction (ORR) is an attractive alternative to the present complex anthraquinone process. The objective of this paper is to provide a state-of-the-arts review of the most important aspects of this process. First, recent advances in H2O2 production are reviewed and the advantages of H2O2 electrogeneration via 2-electron ORR are highlighted. Second, the selectivity of the ORR pathway towards H2O2 formation as well as the development process of H2O2 production are presented. The cathode characteristics are the decisive factors of H2O2 production. Thus the focus is shifted to the introduction of commonly used carbon cathodes and their modification methods, including the introduction of other active carbon materials, hetero-atoms doping (i.e., O, N, F, B, and P) and decoration with metal oxides. Cathode stability is evaluated due to its significance for long-term application. Effects of various operational parameters, such as electrode potential/current density, supporting electrolyte, electrolyte pH, temperature, dissolved oxygen, and current mode on H2O2 production are then discussed. Additionally, the environmental application of electrogenerated H2O2 on aqueous and gaseous contaminants removal, including dyes, pesticides, herbicides, phenolic compounds, drugs, VOCs, SO2, NO, and Hg0, are described. Finally, a brief conclusion about the recent progress achieved in H2O2 electrogeneration via 2-electron ORR and an outlook on future research challenges are proposed.

Simultaneous removal of tetracycline and disinfection by a flow-through electro-peroxone process for reclamation from municipal secondary effluent

Pharmaceutical and personal care products as one of the micropollutants and bacteria in secondary effluent restrict the water reuse from municipal secondary effluent. Electro-peroxone (EP) process where H₂O₂ is generated in-situ by electrolysis is an emerging advanced oxidation process and an improvement of traditional peroxone method (O₃/H₂O₂). In this work, a flow-through EP process was compared with ozonation and electrolysis for simultaneous disinfection and degradation of tetracycline (TC). The disinfection effect by EP was higher than the sum of standalone ozone and electrolysis and the coupling coefficient of ozonation and electrolysis in EP process was 1.2. The flow-through EP system presented similar efficiency for separately and simultaneously treating E. coli and TC. For the actual secondary effluent treatment, trihalomethanes, haloacetonitrile and halonitromethanes, the main disinfection by-products, were much lower than the WHO's thresholds for drinking water. TOC and COD removal was 44% and 65%, respectively, at flow rate of 35 mL/min. BOD₅, bacteria, pH and other parameters in the effluent could satisfy the recreational landscape water quality standard, and the required energy consumption was 0.47 kW h/m³ at the flow rate 35 mL/min. Most of the degradation products were small-molecule organic acids, and possible degradation pathway of TC was suggested.

Pre-ozonation of source water: Assessment of efficacy against Giardia duodenalis cysts and effects on natural organic matter

The protozoan Giardia duodenalis is one of the parasites of primary concern in drinking water treatment plants, due to its resistance to chlorination. Another matter of concern regarding chlorination of drinking water is the formation of disinfection by-products in the presence of precursors such as natural organic matter (NOM). In this study, the effects of ozonation (5 mg L^-1) on G. duodenalis cysts in raw surface water from a drinking water treatment plant were evaluated, and the presence and alteration of NOM were assessed, as an indicative of the potential to prevent total organic halogen (TOX) formation during post-chlorination. Following ozone treatment, the presence of damaged cysts was observed by direct immunofluorescence assay (IFA) and confirmed by the molecular approach propidium monoazide-polymerase chain reaction (PMA-PCR). Using an animal model, analysis of the intestinal tissues revealed that 80% of the animals inoculated with ozonated water were positive for trophozoites. This study shows that analysis of intestinal fragments is imperative to accurately assess animal infection following inoculation of treated cysts. More importantly, considering the low infective dose of Giardia cysts in susceptible hosts, an ozone dosage usually applied in drinking water treatment plants did not completely inactivate G. duodenalis cysts in surface water. Nonetheless, the results suggest that competitive reactions with NOM have occurred, and the applied ozone dosage has proven useful to remove NOM reactivity, and thus prevent halogenated DBP formation during post-chlorination.

A pilot-scale investigation of disinfection by-product precursors and trace organic removal mechanisms in ozone-biologically activated carbon treatment for potable reuse

Although granular activated carbon (GAC) has been broadly applied in ozone-biologically activated carbon filtration (O₃/BAC) systems for potable reuse of municipal wastewater, the mechanisms of various pollutant removal remain largely unknown as the regenerated GAC develops microbial populations resulting in biofiltration but loses significant adsorption capacity as it becomes spent GAC. Therefore, pilot-scale parallel performance comparisons of spent and regenerated GAC, along with a range of pre-oxidant ozone doses, were used to shed light on the mechanisms responsible for the removal of various types of treatment byproduct precursors and trace organic compounds. It was confirmed from this pilot-study that ozone alone can effectively degrade chlorinated trihalomethane (THM) and haloacetic acid (HAA) precursors, chloramine-reactive N-nitrosodimethylamine (NDMA) precursors, and 29 PPCPs. In contrast, biodegradation by microbial population on spent or regenerated GAC can remove NDMA and 22 PPCPs, while the adsorption by regenerated GAC can remove chlorinated THM and HAA precursors, PFAS, flame retardants, and 27 PPCPs. The results of this pilot study are intended to provide those interested in potable reuse with an example of the simultaneous removal capabilities and mechanisms that can be anticipated for treating a complex mixture of organics present in real municipal wastewater effluent.

Simultaneous removal of dissolved organic matter and nitrate from sewage treatment plant effluents using photocatalytic membranes

The residual dissolved organic matter (DOM) and nitrate in sewage treatment plant (STP) effluent have potential negative impacts on the aqueous environment. To that end, we used formic acid (FA) to enhance the photochemical behavior of the photocatalytic membrane for the simultaneous removal of DOM and nitrate from secondary STP effluent. Effluent samples were collected from two different biological treatment processes, Anaerobic-Oxic and Anaerobic-Anoxic-Oxic-membrane bioreactor, respectively. Through Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) analysis, we found that the addition of FA resulted in a similar molecular transformation in different STP effluent samples. Besides, the radical signal of the carboxyl anion could be observed during the photocatalytic process. Based on the results, we proposed the mechanism of the process that carboxyl anion radicals generated by FA could attack DOM and result in further oxidation of the DOM transition state to CO₂ or small molecule by nitrate. Meanwhile, CHON and CHOS compounds in DOM were attacked by the carboxyl anion radicals more easily than CHO compounds. Moreover, long-term use of the membrane confirmed its durability and reusability in practical applications. At a moderate FA concentration and lower hydraulic retention time, the nitrate and DOM removal efficiencies for the sample from JX STP were 68% and 70%, respectively, whereas those of the CD STP sample were 85% and 60%. The removal of DOM and nitrate from different STP effluents using photocatalytic membranes is an advanced approach for the treatment of secondary effluent, and may be applicable to other membranes or systems.

The electro-peroxone process for the abatement of emerging contaminants: Mechanisms, recent advances, and prospects

The electro-peroxone (E-peroxone) process is an emerging electrochemical advanced oxidation process (EAOP) that combines ozonation with in situ cathodic hydrogen peroxide (H₂O₂) production to drive the peroxone reaction for water and wastewater treatment. Over the past several years, the E-peroxone process has quickly emerged as a promising EAOP for the abatement of emerging contaminants (ECs) in water. Because of the enhanced ozone (O₃) transformation to hydroxyl radicals (OH) by electro-generated H₂O₂, the E-peroxone process can considerably increase the efficiency and decrease the energy demand for the abatement of ozone-resistant ECs compared with conventional ozonation. Meanwhile, the E-peroxone process can substantially mitigate the formation of bromate during the treatment of bromide-containing water, which has been a major concern of conventional ozonation for water treatment. Hence, by simply installing electrodes in ozone contactors, the E-peroxone process can remarkably enhance the performance of water and wastewater treatment in various aspects. Compared with other ozone-based AOPs such as the conventional peroxone (O₃/H₂O₂) and UV/O₃ processes, the E-peroxone process also represents a more convenient, cost-effective, energy-efficient, and safer option for EC abatements. This paper reviews recent research of the E-peroxone process, with focus on the abatement of ECs in real water matrices. The fundamental reaction mechanisms that are essential to the understanding, design, and operation of the E-peroxone process are described. The abatement of various ECs in natural water and wastewater by the E-peroxone process are critically reviewed. The challenges in scaling-up the E-peroxone process and integrating it in water and wastewater treatment trains for practical applications are discussed.

Chemical cleaning-associated generation of dissolved organic matter and halogenated byproducts in ceramic MBR: Ozone versus hypochlorite

This study characterized the dissolved organic matter (DOM) and byproducts generated after the exposure of activated sludge to ozone and NaClO in ceramic MBR. It was found that NaClO triggered more significant release of DOM than ozone. Proteins with the molecular weight greater than 20 kDa and humic acid like-substances were the principal components of DOM generated by NaClO, while ozone was found to effectively degrade larger biopolymers to low molecular weight substances. The results showed that more than 80% of DOM generated by NaClO and ozone could pass through the 0.2-μm ceramic membrane. Furthermore, total organic chlorine (TOCl) was determined to be the principal species of halogenated byproducts in both cases, while the generation of TOCl by NaClO was much more significant than that by ozone. Only a small fraction of TOCl was removed by the 0.2-μm ceramic membrane. More importantly, the toxic bioassays further revealed that the supernatant of sludge suspension and permeate in the MBR with NaClO cleaning exhibited higher developmental toxicity to the polychaete embryos than those by ozone. The results clearly showed that on-line chemical cleaning with ozone should be a more eco-friendly and safer approach for sustaining long-term membrane permeability in ceramic MBR.