ClO2 Pre-oxidation Research Articles

Effect of chlorine dioxide pre-oxidation on algal non-protein amino acid degradation and DBPs formation during subsequent chlor(am)ination

As a neurotoxin produced by cyanobacteria, 2,4-diaminobutyric acid (DAB) posed a significant threat to human health and served as an important precursor to disinfection by-products (DBPs). In this study, ClO2 showed the highest DAB oxidation efficiency among the four oxidants, in the order of ClO2 (86%) > HOCl (80%) > KMnO4 (20%) > NH2Cl (13%). The increase of ClO2 dosage (from 0.06 to 1.4 mg L-1) not only increased the oxidation efficiency of DAB, but also decreased the formation of DBPs after ClO2 pre-oxidation during post-chlorination, but the DBPs formation varied during post-chloramination. The enhanced formation of dichloroacetic acid (DCAA) and dichloroacetonitrile (DCAN) should be of concern when ClO2 pre-oxidation was performed at pH 7.0. In the presence of Br−, the overall concentration of formed THMs and brominated species increased. ClO2 pre-oxidation of β-N-methylamino-l-alanine (BMAA), alanine, leucine, and serine followed by post-chloramination has advantages over post-chlorination in controlling the overall and individual DBPs concentration. A possible degradation pathway and reaction schemes during ClO2 pre-oxidation of DAB were proposed. ClO2 pre-oxidation followed by post-chloramination could be an effective treatment process in the abatement of both DAB and toxicity. Results of this study about DAB oxidation and DBP formation contributed to the guidance of ClO2 pre-oxidation in the water treatment plants to cope with cyanobacterial blooms.

Kinetic and mechanistic understanding of chlorite oxidation during chlorination: Optimization of ClO2 pre-oxidation for disinfection byproduct control

Chlorine dioxide (ClO2) applications to drinking water are limited by the formation of chlorite (ClO2–) which is regulated in many countries. However, when ClO2 is used as a pre-oxidant, ClO2– can be oxidized by chlorine during subsequent disinfection. In this study, a kinetic model for the reaction of chlorine with ClO2– was developed to predict the fate of ClO2– during chlorine disinfection. The reaction of ClO2– with chlorine was found to be highly pH-dependent with formation of ClO3– and ClO2 in ultrapure water. In presence of dissolved organic matter (DOM), 60-70% of the ClO2– was transformed to ClO3– during chlorination, while the in situ regenerated ClO2 was quickly consumed by reaction with DOM. The remaining 30-40% of the ClO2– first reacted to ClO2 which then formed chlorine from the DOM-ClO2 reaction. Since only part of the ClO2– was transformed to ClO3–, the sum of the molar concentrations of oxychlorine species (ClO2– + ClO3–) decreased during chlorination. By kinetic modelling, the ClO2– concentration after 24 h of chlorination was accurately predicted in synthetic waters but was largely overestimated in natural waters, possibly due to a ClO2– decay enhanced by high concentrations of chloride and in situ formed bromine from bromide. Understanding the chlorine-ClO2– reaction mechanism and the corresponding kinetics allows to potentially apply higher ClO2 doses during the pre-oxidation step, thus improving disinfection byproduct mitigation while keeping ClO2–, and if required, ClO3– below the regulatory limits. In addition, ClO2 was demonstrated to efficiently degrade haloacetonitrile precursors, either when used as pre-oxidant or when regenerated in situ during chlorination.

Dosing low-level ferrous iron in coagulation enhances the removal of micropollutants, chlorite and chlorate during advanced water treatment

Drinking water utilities are interested in upgrading their treatment facilities to enhance micropollutant removal and byproduct control. Pre-oxidation by chlorine dioxide (ClO2) followed by coagulation-flocculation-sedimentation and advanced oxidation processes (AOPs) is one of the promising solutions. However, the chlorite (ClO2–) formed from the ClO2 pre-oxidation stage cannot be removed by the conventional coagulation process using aluminum sulfate. ClO2– negatively affects the post-UV/chlorine process due to its strong radical scavenging effect, and it also enhances the formation of chlorate (ClO3–). In this study, dosing micromolar-level ferrous iron (Fe(II)) into aluminum-based coagulants was proposed to eliminate the ClO2– generated from ClO2 pre-oxidation and benefit the post-UV/chlorine process in radical production and ClO3– reduction. Results showed that the addition of 52.1-µmol/L FeSO4 effectively eliminated the ClO2– generated from the pre-oxidation using 1.0 mg/L (14.8 µmol/L) of ClO2. Reduction of ClO2– increased the degradation rate constant of a model micropollutant (carbamazepine) by 55.0% in the post-UV/chlorine process. The enhanced degradation was verified to be attributed to the increased steady-state concentrations of HO· and ClO· by Fe(II) addition. Moreover, Fe(II) addition also decreased the ClO3– formation by 53.8% in the UV/chlorine process and its impact on the formation of chloro-organic byproducts was rather minor. The findings demonstrated a promising strategy to improve the drinking water quality and safety by adding low-level Fe(II) in coagulation in an advanced drinking water treatment train.

ClO2 pre-oxidation changes dissolved organic matter at the molecular level and reduces chloro-organic byproducts and toxicity of water treated by the UV/chlorine process

The formation of undesirable chloro-organic byproducts is of great concern in the UV/chlorine process. In this study, chlorine dioxide (ClO2) pre-oxidation was applied to control the formation of chloro-organic byproducts and the toxicity in UV/chlorine-treated water. The molecular-level changes in dissolved organic matter (DOM) were tracked by using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and ClO2 pre-oxidation was found to preferentially react with DOM moieties with high aromaticity level and with a carbon number of > 18, producing compounds with a higher degree of oxidation and lower aromaticity. The ClO2-treated DOM was found to be less susceptible to attack by radicals and free chlorine in the UV/chlorine process compared to the raw DOM. ClO2 pre-oxidation resulted in a significant decrease in the number of unknown chloro-organic byproducts (i.e., –17%) and the total intensity of organic chlorine detected by FT-ICR-MS (i.e., –31%). The molecular characteristics, such as O/C, aromaticity index, and the average number of chlorine atoms, of these unknown chloro-organic byproducts generated in the scenarios with and without ClO2 pre-oxidation were also different. Additionally, ClO2 pre-oxidation reduced the genotoxicity (SOS/umu test) and cytotoxicity (Hep G2 cytotoxicity assay) of UV/chlorine-treated water by 26% and 20%, respectively. The findings in this study highlight the merits of ClO2 pre-oxidation for controlling chloro-organic byproducts and reducing the toxicity of water treated by the UV/chlorine process in actual practice.

Alleviation of Ultrafiltration Membrane Fouling by ClO2 Pre-Oxidation: Fouling Mechanism and Interface Characteristics.

In order to alleviate membrane fouling and improve removal efficiency, a series of pretreatment technologies were applied to the ultrafiltration process. In this study, ClO2 was used as a pre-oxidation strategy for the ultrafiltration (UF) process. Humic acid (HA), sodium alginate (SA), and bovine serum albumin (BSA) were used as three typical organic model foulants, and the mixture of the three substances was used as a representation of simulated natural water. The dosages of ClO2 were 0.5, 1, 2, 4, and 8 mg/L, with 90 min pre-oxidation. The results showed that ClO2 pre-oxidation at low doses (1–2 mg/L) could alleviate the membrane flux decline caused by humus, polysaccharides, and simulated natural water, but had a limited alleviating effect on the irreversible resistance of the membrane. The interfacial free energy analysis showed that the interaction force between the membrane and the simulated natural water was also repulsive after the pre-oxidation, indicating that ClO2 pre-oxidation was an effective way to alleviate cake layer fouling by reducing the interaction between the foulant and the membrane. In addition, ClO2 oxidation activated the hidden functional groups in the raw water, resulting in an increase in the fluorescence value of humic analogs, but had a good removal effect on the fluorescence intensity of BSA. Furthermore, the membrane fouling fitting model showed that ClO2, at a low dose (1 mg/L), could change the mechanism of membrane fouling induced by simulated natural water from standard blocking and cake layer blocking to critical blocking. Overall, ClO2 pre-oxidation was an efficient pretreatment strategy for UF membrane fouling alleviation, especially for the fouling control of HA and SA at low dosages.

Effect of Al hydrates on minimization of disinfection-by-products precursors by coagulation with intensified pre-oxidation towards cyanobacteria-laden water

Pre-oxidation is warranted to improve cyanobacteria removal and minimize disinfection by-products (DBPs) precursors for subsequent coagulation with polyaluminum chloride (PACl) in drinking water treatment. However, the reduction in DBP precursors strongly depends on the Al hydrates for PACl coagulation. This study aimed to investigate the effects of intensified NaOCl and ClO2 pre-oxidation on the removal of Microcystis aeruginosa (MA) and the corresponding halogenated DBP precursors by PACl coagulation with different Al hydrates. Two PACl coagulants, namely PACl-W with 51% monomeric Al and PACl-H with 71% polymeric Al, were used for FlocCAM jar test. The results have shown that the reductions in MA cell and algogenic organic matter (AOM) are more pronounced by sweep flocculation in PACl-W coagulation coupled with NaOCl pre-oxidation. In contrast, ClO2 pre-oxidation with PACl-H coagulation outperforms the floc formation and the reduction in each fluorescent DOM substance, especially for humic acid-like (HAL) substances reduction in response to charge neutralization. Regardless of pre-oxidation approach, PACl-H coagulation exhibits a superior reduction in carbonaceous DBP formation potential (C-DBPFP) comparative PACl-W coagulation, especially for intensified pre-oxidation (Cl2:DOC = 3:1). Intensified NaOCl pre-oxidation is effective to enhance DBPFP reduction in a similar way to ClO2 oxidation by coagulation with both PACl coagulants. In addition, it clearly demonstrates that the halogenated DBP precursors are well-correlated with UV254 absorbance on the basis of principal component analysis (PCA) inference. It is concluded that intensified NaOCl pre-oxidation is an alternative approach to ClO2 pre-oxidation for the minimization of DBP precursors in oxidation-coagulation processes for cyanobacteria-laden water treatment.

Tetracycline antibiotics as precursors of dichloroacetamide and other disinfection byproducts during chlorination and chloramination

Pollution of natural water and even source water with pharmaceuticals is problematic worldwide and raises concern about the possibility of disinfection byproduct (DBP) formation during subsequent water treatment. In this study, the formation of DBPs, especially dichloroacetamide (DCAcAm), was investigated during chlorination and chloramination of tetracyclines, which are a class of broad-spectrum antibiotics. DBPs including DCAcAm were formed during chlorination and chloramination of tetracycline (TC). Although the concentrations and theoretical cytotoxicity of the DBPs formed from TC were affected by the contact time, disinfectant dose, and pH, DCAcAm was the main contributor determining the yields and cytotoxicity of the measured DBPs. The DCAcAm yields from four tetracycline antibiotics ranged from 0.43% to 54.26% for chlorination. For chloramination, the DCAcAm yields reached 44.57%, and the nitrogen in DCAcAm mainly came from tetracycline antibiotics rather than chloramines. ClO2 pre-oxidation and UV photolysis decreased DCAcAm formation during chlorination and chloramination of TC. The high yields observed in this study suggest that tetracycline antibiotics are possible precursors of DCAcAm.

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

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

Ultra-sonication for controlling the formation of disinfection by-products in the ClO2 pre-oxidation of water containing high concentrations of algae.

Eutrophication has become great concern in recent years due to the fact that rivers, lakes, and reservoirs are the main drinking water source. Studies have been performed to enhance the removal of algae with ClO2 pre-oxidation, but there was high potential in the formation of chlorite and chlorate. In this study, ultra-sonication was employed to assist algae removal and control disinfection by-products formation in ClO2 pre-oxidation processes. It was found that solo ultra-sonication for 10min (algae removal 86.11 ± 2.16%) could achieve similar algae removal efficiency as that with solo ClO2 (0.5mg/L) pre-oxidation for 10min (algae removal 87.10 ± 3.50%). In addition, no formations of chlorite and chlorate were detected in solo ultra-sonication process. Five-minutes ultra-sonication followed by 5-min 0.5mg/L ClO2 treatment (total treatment time 10min; algae removal 93.55 ± 3.22%) provided a better performance on algae removal compared to the solo ClO2 (0.5mg/L) pre-oxidation for 10min. Moreover, chlorite was undetectable. It suggests that the utilization of ultra-sonication in ClO2 pretreatment for algae removal has highly prevented the formations of chlorite and chlorate.

ClO2 pre-oxidation changes the yields and formation pathways of chloroform and chloral hydrate from phenolic precursors during chlorination

Phenolic moieties in natural organic matter (NOM) are important precursors of disinfection by-products (DBPs). In this study, the formation of chloral hydrate from chlorination of seventeen phenolic compounds, including mono-, di- and tri-hydroxybenzenes, were evaluated and the role of chlorine dioxide (ClO2) pre-oxidation on its formation pathways was explored. Chloroform, was also evaluated for comparison. Chlorination of resorcinol exhibited the highest chloral hydrate yield (2.83 ± 0.13%) followed by chlorination of 2,4,6-trichlorophenol (0.61 ± 0.03%). The median of chloral hydrate yields from the tested phenolic compounds was 0.22%. ClO2 pre-oxidation reduced the yields of chloroform from phenol derivatives by 37–97%, except 4-methoxyphenol, catechol and 2,3-dihydroxyphenol. On the contrary, ClO2 pre-oxidation of di- and tri-hydroxybenzenes tended to increase chloral hydrate yields in post-chlorination. Mixed results (both increases and decreases) were observed in chloral hydrate formation from chlorination of mono-hydroxybenzenes after ClO2 pre-oxidation. The changes of their formation were dependent on ClO2 pre-oxidation time and dosages. Identification of transformation products suggested that phenolic compounds were mainly converted to unsaturated carbonyl structures by ClO2. Chlorine substituted benzoquinones and cyclopent-4-ene-1,3-diones were important transformation products after a series of ring open, decarboxylation, hydrolysis and chlorine substitution reactions. The changes in the formation yields of chloral hydrate and chloroform were governed by the difference in initial phenolic precursors and the transformation products after ClO2 pre-oxidation. ClO2 pre-oxidation in water treatment can effectively reduce chloroform formation but may have a risk of increasing chloral hydrate formation.

Impact of ClO2 pre-oxidation on the formation of CX3R-type DBPs from tyrosine-based amino acid precursors during chlorination and chloramination

ClO2 is frequently used as a pre-oxidant in water treatment plants. However, the effects of ClO2 pre-oxidation on disinfection by-product (DBP) formation, especially the highly toxic nitrogenous DBPs, during subsequent chlor (am)ination have not been studied thoroughly. There is also limited information about DBP formation from combined amino acids (AAs), which are more abundant than free AAs in source waters. Many typical DBPs (including representative N-DBPs) have a similar structure of “CX3R” (X = H, Cl, Br or I). In the study, tyrosine and forms representing its reactivity in combined AAs (tyrosine tert-butyl ester and Boc-tyrosine) were selected as model precursors. The formation of various regulated and unregulated CX3R-type DBPs from ClO2 pre-oxidation and subsequent chlor (am)ination were studied at a wide-range of ClO2 and chlor (am)ine doses (ClO2/precursors and chlor (am)ine/precursors are at the range of 0–2.5 and 1–20 [Mol/Mol], respectively). Chloroform and chloral hydrate (CH) yields increased with chlorine dose, while haloacetonitrile and haloacetamide maximized at median chlorine dose (Cl2/Precursors = 10). All DBP yields increased with chloramine dose. ClO2 pre-oxidation increased chloroform, haloacetonitrile, trichloronitromethane and CH yields during chlorination, but ClO2 increased chloroform, CH, trichloroacetamide while decreased dichloroacetonitrile and trichloronitromethane yields during chloramination. The overall toxicity of the formed DBPs was evaluated by cytotoxicity index (CTI). ClO2 pre-oxidation increased CTI from all precursors during post-chlorination while reduced it during post-chloramination. Results imply that ClO2 is probably more suitable for use in combination with chloramination disinfection, rather than chlorination, in the integrated control of CX3R-type DBPs from source waters abundant in AAs.

Production of trihalomethanes, haloacetaldehydes and haloacetonitriles during chlorination of microcystin-LR and impacts of pre-oxidation on their formation

Microcystins (MCs) in drinking water have gained much attention due to their adverse health effects. However, little is known about the impact of pre-oxidation in the formation of disinfection by-products (DBPs) during the downstream chlorination of MCs. The present study examined the formation of both carbonaceous and nitrogenous DBPs from chlorination of MC-LR (the most abundant MC species) and evaluated the impact of permanganate (PM), hydrogen peroxide (H2O2) and chlorine dioxide (ClO2) pre-oxidation on the DBP formation in chlorination. Higher yields of chloroform (CF) (maximum 43.0%) were observed from chlorination of MC-LR than free amino acids which are included in MC-LR structure. Chloral hydrate (CH) and dichloroacetonitrile (DCAN) were also produced from the chlorination of MC-LR, and the latter one was formed probably due to the chlorination of peptide bonds. A high pH favored the production of CF and CH, but inhibited the formation of DCAN. In the presence of bromide, bromo-DBPs could be produced to pose a threat. For example, 0.58μg/L of tribromoacetaldehyde was produced from the chlorination of MC-LR at Br−=200μg/L. PM and ClO2 pre-oxidation could both reduce the DBP formation from MC-LR. In contrast, H2O2 appeared not to significantly control the DBP formation.

Coupling Technique for Deep Removal of Manganese and Iron from Potable Water

Although many treatment methods have been employed to remove soluble Fe(II) and Mn(II) from water, for some smaller water treatment plants or under emergent circumstances such as sharp increase of iron and manganese concentrations in source water during summer, traditional physicochemical removal methods are more flexible and effective. In this article, a coupling technique of chlorine dioxide (ClO2) preoxidation and manganese sand filtration for iron and manganese removal from potable water have been proposed. Fe(II) and Mn(II) removal efficiencies of different filter materials (natural manganese sand, quartz sand, and fiber bundle filter (FBF) material) were compared. Influences of ClO2 dosage, liquid flow rate, particle size of filter material, and thickness of filter bed on removal efficiency were also examined. In addition, performance of ClO2 preoxidation–manganese sand filter technique was investigated under the optimum operating conditions to remove Fe(II) and Mn(II). Results indicated that natural manganese sand filter showed higher capability of removing Fe(II) and Mn(II) than quartz sand filter and FBF. It was found that manganese removal efficiencies increased with higher dosage of preoxidant, lower liquid flow rate, smaller sand particle diameter, and thicker filter bed. When initial concentrations of Mn(II) and Fe(II) in feed water were 1.5 and 1.0 mg/L, respectively, ClO2 dosage was 0.99 mg/L, liquid flow rate was 282.6 mL/min, sand particle diameter was 0.6–1.2 mm, and filter bed thickness was 60 cm, the process showed good performance of Fe(II) and Mn(II) removal with both removal rates more than 95%.

Pretreatment of algae-laden and manganese-containing waters by oxidation-assisted coagulation: Effects of oxidation on algal cell viability and manganese precipitation

Preoxidation is manipulated to improve performance of algae and soluble manganese (Mn) removal by coagulation-sedimentation for water treatment plants (WTPs) when large amount of soluble Mn presents in algae-laden waters. This study aimed to investigate the effects of preoxidation on the performance of coagulation-sedimentation for the simultaneous removal of algae and soluble Mn, including ionic and complexed Mn. NaOCl, ClO2, and KMnO4 were used to pretreat such algae-laden and Mn containing waters. The variation of algal cell viability, residual cell counts, and concentrations of Mn species prior to and after coagulation-sedimentation step were investigated. Results show that NaOCl dosing was effective in reducing the viability of algae, but precipitated little Mn. ClO2 dosing had a strongest ability to lower algae viability and oxidize ionic and complexed soluble Mn, where KMnO4 dosing oxidized ionic and complexed Mn instead of reducing the viability of cells. Preoxidation by NaOCl only improved the algae removal by sedimentation, whereas most of soluble Mn still remained. On the other hand, ClO2 preoxidation substantially improved the performance of coagulation-sedimentation for simultaneous removal of algae and soluble Mn. Furthermore, KMnO4 preoxidation did improve the removal of algae by sedimentation, but left significant residual Mn in the supernatant. Images from FlowCAM showed changes in aspect ratio (AR) and transparency of algae-Mn flocs during oxidation-assisted coagulation, and indicates that an effective oxidation can improve the removal of most compact algae-Mn flocs by sedimentation. It suggests that an effective preoxidation for reducing algal cell viability and the concentration of soluble Mn is a crucial step for upgrading the performance of coagulation-sedimentation.

Role of Chlorine Dioxide in N-Nitrosodimethylamine Formation from Oxidation of Model Amines.

N-Nitrosodimethylamine (NDMA) is an emerging disinfection byproduct, and we show that use of chlorine dioxide (ClO2) has the potential to increase NDMA formation in waters containing precursors with hydrazine moieties. NDMA formation was measured after oxidation of 13 amines by monochloramine and ClO2 and pretreatment with ClO2 followed by postmonochloramination. Daminozide, a plant growth regulator, was found to yield 5.01 ± 0.96% NDMA upon reaction with ClO2, although no NDMA was recorded during chloramination. The reaction rate was estimated to be ∼0.0085 s(-1), and on the basis of our identification by mass spectrometry of the intermediates, the reaction likely proceeds via the hydrolytic release of unsymmetrical dimethylhydrazine (UDMH), with the hydrazine structure a key intermediate in NDMA formation. The presence of UDMH was confirmed by gas chromatography-mass spectrometry analysis. For 10 of the 13 compounds, ClO2 preoxidation reduced NDMA yields compared with monochloramination alone, which is explained by our measured release of dimethylamine. This work shows potential preoxidation strategies to control NDMA formation may not impact all organic precursors uniformly, so differences might be source specific depending upon the occurrence of different precursors in source waters. For example, daminozide is a plant regulator, so drinking water that is heavily influenced by upstream agricultural runoff could be at risk.

Effect of cell integrity on algal destabilization by oxidation-assisted coagulation

A hybrid oxidation–coagulation process is commonly adopted to destabilize algae and subsequently improve the removal of algae from water through sedimentation and filtration in water treatment plants (WTPs). Preoxidation is crucial to improving algae removal by a coagulation–sedimentation process. The goal of this study was to investigate the effect of oxidation with NaOCl and ClO2 on the cell integrity of algae (i.e. diatoms) and the destabilization of algae by means of Alum coagulation. The effects of oxidation-assisted coagulation on the performance of sedimentation as well as filtration were evaluated. The results show that ClO2 reduces cell integrity more severely than NaOCl during oxidation. The degradation of chlorophyll a and humic-like substances generated by cells ruptured by ClO2 oxidation is stronger than that by NaOCl oxidation. During oxidation both NaOCl and ClO2 fail to cause significant cell lysis, while cell settleability can be improved markedly by using only ClO2. Preoxidation with ClO2 is more effective in destabilizing the particles and algae when applying Alum coagulation at low dosages. It was found that the residual algae counts in the supernatants are inversely well-correlated to its filterability instead of its residual turbidity. The reduced cell integrity resulting from ClO2 preoxidation effectively improves the performance of coagulation–sedimentation for algae (diatoms) removal and reduces the burden of filtration operation in the WTPs.

Degradation of phenylurea herbicides by chlorine dioxide and formation of disinfection by-products during subsequent chlor(am)ination

Abstract The degradation of phenylurea herbicides (chlortoluron, isoproturon and diuron) by chlorine dioxide (ClO2) and the formation of disinfection by-products (DBPs) from subsequent chlorination or chloramination were investigated in this paper. Results showed that the reactivity between phenylurea herbicides and ClO2 highly depended on the different substituted groups on the aromatic ring. The degradation rates by ClO2 were found as diuron ≫ chlortoluron > isoproturon under circumneutral condition. Degradation of chlortoluron and isoproturon were accelerated considerably under alkaline conditions. The oxidation of phenylureas by ClO2 was subjected to steps including radical intermediates formation, hydroxylation reactions and cleavage of the N–C bond on the ureic side-chain. The degradation pathways of each phenylurea by ClO2 were then proposed. Treatment of phenylureas with ClO2 alone did not produce any chlorinated DBPs. ClO2 preoxidation reduced formation of chloroform (CF), dichloroacetonitrile (DCAN) and dichloropropanone (DCP) during post-chlor(am)ination. However, enhancement of dichloroacetic acid (DCAA) was observed. Meanwhile, the formation of trichloroacetic acid (TCAA) was remarkably induced by ClO2 pretreatment in followed chlor(am)ination processes.

Formation of disinfection by-products after pre-oxidation with chlorine dioxide or ferrate

The effect of pre-oxidation with chlorine dioxide (ClO2) or ferrate (Fe(VI)) on the formation of disinfection by-products (DBPs) during chlorination or chloramination was tested with natural waters from 12 sources (9 surface waters, 1 groundwater, and 2 wastewater effluents). DBPs investigated included trihalomethanes (THM), chloral hydrate (CH), haloketones (HK), haloacetonitriles (HAN) and trichloronitromethane (TCNM), chlorite and chlorate. Chlorite and chlorate were found in the ClO2-treated waters. Application of 1 mg/L ClO2 ahead of chlorination reduced the formation potential for THM by up to 45% and the formation of HK, HAN and TCNM in most of the samples. The CH formation results were mixed. The formation of CH and HK was enhanced with low doses of Fe(VI) (1 mg/L as Fe), but was greatly reduced at higher doses (20 mg/L Fe). Fe(VI) reduced the formation of THM, HAN and TCNM in most of the samples. Reduced potential for the formation of NDMA was observed in most of the samples after both ClO2 and Fe(VI) pre-oxidation.

A comparison of disinfection by-products formation during sequential or simultaneous disinfection of surface waters with chlorine dioxide and chlor(am)ine

The effect of chlorine dioxide (ClO2) oxidation on the formation of disinfection by-products (DBPs) during sequential (ClO2 pre-oxidation for 30 min) and simultaneous disinfection processes with free chlorine (FC) or monochloramine (MCA) was investigated. The formation of DBPs from synthetic humic acid (HA) water and three natural surface waters containing low bromide levels (11–27 μg/L) was comparatively examined in the FC-based (single FC, sequential ClO2-FC, and simultaneous ClO2/FC) and MCA-based (single MCA, ClO2-MCA, and ClO2/MCA) disinfection processes. The results showed that much more DBPs were formed from the synthetic HA water than from the three natural surface waters with comparative levels of dissolved organic carbon. In the FC-based processes, ClO2 oxidation could reduce trihalomethanes (THMs) by 27–35% and haloacetic acids (HAAs) by 14–22% in the three natural surface waters, but increased THMs by 19% and HAAs by 31% in the synthetic HA water after an FC contact time of 48 h. In the MCA-based processes, similar trends were observed although DBPs were produced at a much lower level. There was an insignificant difference in DBPs formation between the sequential and simultaneous processes. The presence of a high level of bromide (320 μg/L) remarkably promoted the DBPs formation in the FC-based processes. Therefore, the simultaneous disinfection process of ClO2/MCA is recommended particularly for waters with a high bromide level.

Formation of disinfection byproducts upon chlorine dioxide preoxidation followed by chlorination or chloramination of natural organic matter

Chlorine dioxide (ClO2) is often used as an oxidant to remove taste, odor and color during water treatment. Due to the concerns of the chlorite formation, chlorination or chloramination is often applied after ClO2 preoxidation. We investigated the formation of regulated and emerging disinfection byproducts (DBPs) in sequential ClO2-chlorination and ClO2-chloramination processes. To clarify the relationship between the formation of DBPs and the characteristics of natural organic matter (NOM), changes in the properties of NOM before and after ClO2 oxidation were characterized by fluorescence, Fourier transform infrared spectroscopy (FTIR), and size and resin fractionation techniques. ClO2 preoxidation destroyed the aromatic and conjugated structures of NOM and transformed large aromatic and long aliphatic chain organics to small and hydrophilic organics. Treatment with ClO2 alone did not produce significant amount of trihalomethanes (THMs) and haloacetic acids (HAAs), but produced chlorite. ClO2 preoxidation reduced THMs, HAAs, haloacetonitriles (HANs) and chloral hydrate (CH) during subsequent chlorination, but no reduction of THMs was observed during chloramination. Increasing ClO2 doses enhanced the reduction of most DBPs except halonitromethanes (HNMs) and haloketones (HKs). The presence of bromide increased the formation of total amount of DBPs and also shifted DBPs to more brominated ones. Bromine incorporation was higher in ClO2 treated samples. The results indicated that ClO2 preoxidation prior to chlorination is applicable for control of THM, HAA and HAN in both pristine and polluted waters, but chlorite formation is a concern and HNMs and HKs are not effectively controlled by ClO2 preoxidation.