Comparison of different disinfection processes for controlling disinfection by-product formation in rainwater

Journal Pre-proof Comparison of different disinfection processes for controlling disinfection by-product formation in rainwater Zhi Liu, Yi-Li Lin, Wen-Hai Chu, Bin Xu, Tian-Yang Zhang, Chen-Yan Hu, Tong-Cheng Cao, Nai-Yun Gao, Cheng-Di Dong

PII:

S0304-3894(19)31572-9

DOI:

https://doi.org/10.1016/j.jhazmat.2019.121618

Reference:

HAZMAT 121618

To appear in:

Journal of Hazardous Materials

Received Date:

25 July 2019

Revised Date:

28 October 2019

Accepted Date:

4 November 2019

Please cite this article as: Liu Z, Lin Y-Li, Chu W-Hai, Xu B, Zhang T-Yang, Hu C-Yan, Cao T-Cheng, Gao N-Yun, Dong C-Di, Comparison of different disinfection processes for controlling disinfection by-product formation in rainwater, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121618

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Comparison

of

different

disinfection

processes

for

controlling disinfection by-product formation in rainwater

Zhi Liua,b, Yi-Li Linc, Wen-Hai Chua,b, Bin Xua,b,*, Tian-Yang Zhanga,b, Chen-Yan Hud, Tong-Cheng Caoe, Nai-Yun Gaoa, Cheng-Di Dongf

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a. State Key Laboratory of Pollution Control and Resource Reuse, Key Laboratory of Yangtze Water Environment, Ministry of Education, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, P.R. China

b. Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092,

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P.R. China

c. Department of Safety, Health and Environmental Engineering, National Kaohsiung

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University of Science and Technology, Kaohsiung 824, Taiwan, R.O.C. d. College of Environmental and Chemical Engineering, Shanghai University of

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Electric Power, Shanghai 200090, P. R. China

e. School of Chemical Science and Engineering, and Key Laboratory of Road and

P.R. China

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Traffic Engineering of Ministry of Education, Tongji University, Shanghai 200092,

f. Department of Marine Environmental Engineering, National Kaohsiung University

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of Science and Technology, Kaohsiung 81157, Taiwan, R.O.C.

*Corresponding author: Bin Xu Email: [email protected] Phone: +86-13918493316

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Graphical abstract

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Highlights 

This study first compared different disinfection processes for controlling DBPs in rainwater.



The application of chloramine could control the yields of C-DBPs, but not N-DBPs in rainwater. K2FeO4 exerted effective control of most identified DBPs in rainwater.



UV-related AOPs could control DBP formation as well as cyto- and genotoxicity.



UV/PS was more effective to reduce DOC and alter DOM in rainwater than

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

UV/H2O2.

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Abstract:

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With increasing shortage of clean water, rainwater has been considered as a precious

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alternative drinking water source. The processes applied to rainwater treatment are responsible for the safety of drinking water. Therefore, we systematically compared

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different disinfection processes to evaluate the control of disinfection by-product (DBP) formation and integrated cyto- and genotoxicity of the treated rainwater for the

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first time. The evaluated disinfection processes included chlorination and chloramination, pre-oxidation by potassium permanganate (KMnO4) and potassium

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ferrate (K2FeO4), ultraviolet/hydrogen peroxide (UV/H2O2), and ultraviolet/persulfate (UV/PS) processes. The results revealed that chloramination was effective for controlling the formation of carbonaceous DBPs (C-DBPs), but not nitrogenous DBPs (N-DBPs). Compared to KMnO4 pre-oxidation, better reduction of almost all DBPs was observed during K2FeO4 pre-oxidation. According to the calculation of 3

cytotoxicity index (CTI) and genotoxicity index (GTI), cyto- and genotoxicity of the samples decreased obviously at the dosage of ≥ 2.0 mg/L KMnO4 and K2FeO4. The control of the cyto- and genotoxicity of the formed DBPs from the two UV-related AOPs was more effective at the dosage of ≥ 1.0 mM PS and ≥ 5.0 mM H2O2. Moreover, UV/PS was much more powerful to alter the structure of DBP precursors

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in rainwater.

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Keywords: Rainwater; Disinfection by-products (DBPs); Chlor(am)ination; Pre-oxidation; Ultraviolet related advanced oxidation process (UV-related AOP)

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1. Introduction With the development of industrial and agricultural activities, natural water has suffered intensive contamination by wastewater effluents containing nutrients, organic matter (e.g., petroleum hydrocarbons, pharmaceuticals, pesticides and herbicides), heavy metals, etc. The growth of population and improvement of living quality result in heavy demand of fresh water. In order to mitigate contradiction between supply and

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demand of water, rainwater has been harvested for potable and non-potable use (such as irrigation, and washing toilet). Household collection systems have been encouraged

by the government in Australia with 23% residents in South Australia using rainwater

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as a drinking water source [1]. In Vietnam, fresh water shortage attributed to heavy

metal pollution can also be alleviated by rainwater recycling [2]. A project

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implemented by Women’s Development Foundation in China called for the

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construction water cellars and water supply facilities to solve the problem of fresh water shortage [3]. Until 2009, approximately 1.6 million people in 23 provinces

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benefited from this project [3]. In brief, the collection and utilization of rainwater has become the hotspot in the world. Besides, a previous study has validated that the

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collected rainwater is often contaminated by either chemical or microbiological

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pollution, which can pose threats to human and domestic animal health [4]. Moreover, Hou et al. [5] also reported that the characteristics of rainwater could be greatly different from that in raw water. Although the rainwater utilization is a universal trend, corresponding studies dealing with rainwater characterizing and upgrading for safe reuse are scarce. Therefore, more attentions should be paid to the treatment processes of rainwater in order to not only provide clean and safe water to the public but also fill 5

the knowledge gap mentioned above. Disinfection is an essential treatment process to avoid diseases caused by aqueous microorganisms. Chlorine is the most prevalent disinfectant, which is commonly used in drinking water treatment plants (DWTPs). Unfortunately, chlorine can also oxidize natural organic matter (NOM) in water and form undesirable disinfection by-products (DBPs), which may process chronic cyto- and genotoxicity

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[6-9]. Compared with chlorine, monochloramine (NH2Cl) is usually used as an alternative disinfectant, and shows superiority in providing longer lasting residual

chlorine in the distribution system [10]. However, NH2Cl can also increase the

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formation of nitrogenous DBPs (N-DBPs), which show much higher toxicity than

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typical carbonaceous DBPs (C-DBPs). In the past decades, researches regarding to the formation of DBPs during the disinfection of drinking water were flourishing [11-16],

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while the relative studies on rainwater are rarely reported. Although the composition

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of rainwater is affected by various parameters [4,5], some typical DBPs are actually produced during the disinfection process. According to a recent study, seven DBPs

(trichloromethane

(TCM),

dichloroacetic

acid

(DCAA),

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chlorinated

trichloroacetic acid (TCAA), chloral hydrate (CH), dichloroacetonitrile (DCAN),

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trichloronitromethane (TCNM), and dichloroacetamide (DCAM)) are the mainly formed DBP species during rainwater chlorination [5] and the order of magnitudes of DBP yields in rainwater is the same as those formed in surface water. Therefore, it is essential to evaluate and control the formation of DBPs in rainwater during the disinfection process. Given that the microbial pathogens contained in rainwater can be 6

efficiently inactivated at low dosage of chlorine (2 mg-Cl2/L) [17], the control of DBP formation in rainwater should be considered as the primary objective in order to ensure the safe supply and utilization of rainwater. However, the information mentioned above was limited, which is the focus of this study. Removing DBP precursors by using alternative treatments (such as, pre-oxidation and advanced oxidation processes (AOPs)) before disinfection has been

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studied and reported to be effective to control the formation of DBPs [18-22]. Being one of the powerful oxidants, potassium permanganate (KMnO4) has been considered an alternative pre-oxidant, which can eliminate the taste and odor problems and

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mitigate the trihalomethane (THM) formation [23]. Potassium ferrate (K2FeO4) is

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another stronger oxidant both in acid and basic conditions. Several investigations have demonstrated that K2FeO4, with redox potential as high as +22V, is a better oxidant

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for water applications [24]. As an environmentally friendly treatment process,

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ultraviolet (UV) irradiation has strong capability to inactivate cryptosporidium and pathogens [25] and oxidize organic compounds [26]. Moreover, UV-related AOPs,

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such as UV/hydrogen peroxide (UV/H2O2) and UV/persulfate (UV/PS), presented excellent capability to degrade contaminants in water by producing highly reactive

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oxidizing species (such as, sulfate radical (SO4•-) and hydroxyl radical (•OH)) [19,27-29]. In addition, the oxidation potential of SO4•- (E0 = 2.5-3.1 V) is stronger than that of •OH (E0 = 1.9-2.7 V) and is more effective to oxidize organic compounds with carbon-carbon double bonds and benzene rings [30]. These reactive oxidizing species have sufficient capacity to change the characteristics of NOM, including size, 7

structure and functional groups, which are critical to the reactivity between organic compounds and disinfectants [31]. As reported by previous studies, UV/H2O2 process can decrease the aromaticity and molecular weight (MW) of NOM, resulting in the formation of biodegradable organic compounds [32-34]. To date, although DBP yields from the precursors in raw water during different pre-treatment processes have been explored [15,19,20], no information reported the DBP formation in rainwater during

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various pre-oxidation processes and AOPs. Therefore, it is necessary to make a further insight of these issues, which will provide essential basis for proper treatment and utilization of rainwater in the future.

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Therefore, the objective of this study was to compare different treatment

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processes for controlling the formation of DBPs and their corresponding formation mechanisms, including TCM, DCAA, TCAA, CH, DCAN, TCNM, and DCAM

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during rainwater chlorination, with emphasis on 1) DBP formation during

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chlor(am)ination, 2) the effects of pre-oxidants (KMnO4 and K2FeO4) on DBP formation during post-chlorination of rainwater, 3) the effects of UV-related AOPs

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(UV/H2O2 and UV/PS) on DBP formation during post-chlorination of rainwater, and 4) evaluation of the integrated cyto- and genotoxicity in rainwater treated by different

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treatment processes. In brief, this work systematically compared different disinfection processes to evaluate the DBP formation and integrated cyto- and genotoxicity of the treated rainwater for the first time. All results of this study not only provide theoretical basis for controlling the DBP formation during rainwater disinfection, but also contribute to basic information for optimizing the rainwater utilization in practice. 8

2. Materials and methods 2.1. Chemicals and reagents All chemicals were at least of analytical grade except as noted. All the DBP standard solutions (TCM, DCAA, TCAA, CH, DCAN, TCNM, and DCAM) were purchased from CanSyn Chemical Corp (Toronto, ON, Canada). Sodium hypochlorite

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(NaOCl), ammonium chloride (NH4Cl), potassium dihydrogen phosphate (KH2PO4), KMnO4, K2FeO4, H2O2 and PS were supplied by Sigma-Aldrich (USA). Sulphuric acid (H2SO4) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai,

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China). Ultrapure water was produced from a Milli-Q water purification system

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produced by Milli-Q water purification system (Millipore, USA).

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2.2.1. Sample collection

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2.2. Rainwater sample collection and characterization

Twenty-five rainwater samples were harvested from summer to winter (from

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August to December) in 2018 using two basins placed on the roof of the Mingjing Building on the campus of Tongji University (Yangpu, Shanghai, China). The

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seasonal variation of the sample characteristics was displayed in Figure S1. Almost all values of the water quality parameters slightly increased from summer to winter, which may be due to the more severe air pollution and less rainfall in winter in China. The DOC ranged from 1.36 to 3.66 mg-C/L. The concentrations of DON, NH3-N, NO3-, and NO2- ranged from 0.24 to 1.34, 0.60 to 1.30, 0.20 to 0.90, and 0.001 to 9

0.007 mg-N/L, respectively. In general, not much differences of the parameters mentioned above were observed. Considering that each parameter values in Sample 1 and 2 (Table S1) were close to the average values of all samples, these two samples were selected to further investigate DBP formation of rainwater treated by different processes. Rainwater were filtered through 0.45-μm glass fiber filters and stored in the dark at 4°C until use.

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2.2.2. Spectroscopic characteristics of rainwater samples

UV absorbance of rainwater samples at the wavelengths of 250, 254, 285, and 365 nm before and after different treatment processes was measured to elucidate the

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changes of dissolved organic matter (DOM) structure. Specific UV absorbance at the

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wavelength 254 nm (SUVA254) and 285 nm (SUVA285) were used as indicators of the aromatic NOM and calculated by dividing the absorbance at wavelength 254 nm and

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285 nm by dissolved organic carbon (DOC), respectively [35]. SUVA285 is reported to

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be a better index for benzene carboxylic acids and phenols [36]. Moreover, the ratio of UV absorbance at 250 nm and 365 nm (E2/E3) is correlated positively with the

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MW of NOM as reported by previous studies (the higher the value, the lower the MW of NOM) [37,38]. Fluorescence excitation-emission matrix (EEM) spectra was

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divided into five regions to differentiate various structures of DBP precursors [39]. Region I: tyrosine-like (λEx < 250 nm, λEm < 330 nm). Region II: tryptophan-like (λEx < 250 nm, 330 nm < λEm < 380 nm). Regions I and II are referred to as regions of aromatic proteins (λEx < 250 nm, λEm < 380 nm). Region III: fulvic acid-like (λEx < 250 nm, λEm > 380 nm). Region IV: soluble microbial product-like (SMP-like) (λEx > 10

250 nm, λEm < 380 nm). Region V: humic acid-like (λEx > 250 nm, λEm > 380 nm).

2.3. Experimental procedures 2.3.1. Chlor(am)ination and pre-oxidation experiments Batch experiments of chlor(am)ination and pre-oxidation were carried out using Sample 1 in 40 mL amber glass volumetric bottles under headspace-free conditions in

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the dark at controlled room temperature (25.0 ± 0.5 °C). The monochloramine (NH2Cl) solution was freshly prepared with the Cl2/N molar ratio of 0.8 at pH 8.5 according to

previous studies [40,41]. Solution pH was adjusted to 7.0 ± 0.2 using KH2PO4 (10 m

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M). For chlor(am)ination, chlor(am)ine (3-15 mg-Cl2/L) was added for disinfection

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for 24-h. For pre-oxidation, the KMnO4 and K2FeO4 (0.5, 1.0, 2.0, 3.0, and 5.0 mg/L) were dosed to samples for 30 min. Then, specific concentration of chlorine (9

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mg-Cl2/L), whose dosage was three times of solution DOC, was added for subsequent

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disinfection for 24-h (see section 3.1). According to a previous study [17], the microbial pathogens can be effectively inactivated at the low chlorine dosage (2

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mg-Cl2/L), therefore, only the DBP formation was analyzed after chlorination process. Ten mL solution of each sample was quenched with overdosed NH4Cl at the molar

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ratio of NH4Cl to disinfectant as 1.2:1 and transferred to a 15 mL bottle spiked with 2 mL methyl tert-butyl ether for liquid-liquid extraction. Then the samples were extracted and analyzed for DBP concentrations immediately to avoid any hydrolytic loss.

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2.3.2. UV/PS and UV/H2O2 AOPs UV/PS and UV/H2O2 experiments were performed in 50 mL quartz tubes at 25.0 ± 0.5 °C. Sample 2 was used to perform AOP treatments. Solution pH was adjusted to 7.0 ± 0.2 using KH2PO4 (10 mM). PS and H2O2 were added right before UV irradiation. UV irradiation was conducted in a photochemical reactor (XPA-7, Xujiang Electromechanical Plant, Nanjing, China) with UV intensity of 0.80 mW/cm2 (36 W,

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4P-SE, Philips) measured by an ultraviolet radiometer (Photoelectric Instrument

Factory, Beijing Normal University) at 254 nm. UV fluence (mJ/cm2) was calculated

as the UV intensity multiplied by the exposure time [42,43]. UV fluence was set as

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240, 480, 960, and 1440 mJ/cm2 for UV/PS and UV/H2O2 AOPs. Enough chlorine

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was used to quench residual H2O2 (with the dosage equal to initial H2O2 concentration). After 24-h post-chlorination, DBP formation was analyzed according

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to above-mentioned procedures.

2.4. Analytical methods

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DOC and total dissolved nitrogen (TDN) were measured separately using a TOC analyzer (Shimadzu TOC-VCSH, Japan) with detection limit of 0.1 mg/L. NH4+ was

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measured using the salicylic acid – cyanuric acid salt colorimetric method [44]. Concentrations of NO2- and NO3- were measured using the corresponding HACH test kits and a UV-vis spectrophotometer (HACH DR6000). Dissolved organic nitrogen (DON) is calculated by the difference between TDN and dissolved inorganic nitrogen (i.e. NH4+, NO2-, and NO3-). UV-vis spectrophotometer (UV-9000S, Metash 12

Instrument, Shanghai) was used to measure the UV absorbance at 250, 254, 285, and 365 nm. EEM spectra was monitored by a three-dimensional spectrofluorometry (F-7100 Fluorescence, HITACHI, Japan). H2O2 solution was calibrating by using photometric method [45]. The concentration of KMnO4 was verified by measuring the absorbance at 526 nm and 546 nm [46]. PS concentration was measured according to our previous study [46]. K2FeO4 was prepared using the method by Li et al. [48].

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TCM, CH, DCAN, and TCNM were measured based on the U.S. EPA method

551.1 [49]. DCAA and TCAA were analyzed according to the U.S. EPA method 552.2 [50]. DCAM was measured by the analytical method reported by a previous study

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[51]. All DBPs were detected by using a gas chromatograph (GC-2010, Shimadzu,

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Japan) that was equipped with an electron capture detector and a HP-5 capillary column (30 m × 0.25 mm i.d., 0.25 μm film thickness, J&W, USA). The injector and

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detector temperatures were set at 200 and 290 °C, respectively. The nitrogen carrier

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gas was at the pressure of 69.8 kPa with the flow rate of 30 mL/min. The temperature program for TCM, CH, DCAN, and TCNM analyses was set at 37 °C for 10 min,

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ramped to 50 °C at the rate of 5 °C/min and hold for 5 min, and then ramp to 260 °C at the rate of 15 °C/min and hold for 10 min. The temperature program for DCAM

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analysis method was set at 50 °C for 8 min, ramped to 120 °C at the rate of 20 °C/min and hold for 2 min, and then ramped to 220 °C at the rate of 40 °C/min and hold for 5 min. The temperature program for haloacetic acid (HAA) analysis method was set at 50 °C for 8 min, ramped to 120 °C at the rate of 20 °C/min and hold for 2 min, and then ramped to 220 °C at the rate of 40 °C/min and hold for 5 min. 13

3. Results and discussion 3.1. DBP formation of rainwater during chlor(am)ination

[Figure 1] DBP formation of rainwater during chlor(am)ination with various disinfection

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dosages is displayed in Fig. 1. As illustrated in Fig. 1 (a), TCM, DCAA, and CH were the dominant C-DBPs formed during chlorination with concentrations of one order of magnitude higher than those of N-DBPs (DCAN, TCNM, and DCAM, Fig. 1 (b)).

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This might be explained by less N-DBPs precursors (such as DON, in Table S1) that

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are available in rainwater. In contrast, the application of chloramine was effective to control the formation of C-DBPs (Fig. 1 (a)), but N-DBP formation from

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chloraminated rainwater was similar to that from chlorinated samples (Fig. 1 (b)). As reported by previous studies, chloramine can also be served as the source of nitrogen

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in N-DBPs, although chloramine has lower oxidant capacity than that of chlorine

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[52,53], suggesting that the application of chloramine can be responsible for some DBP formation, such as DCAM. The formation pathway of DCAM during

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chloramination was reported in a previous study [54], where the nitrogen was originated from the incorporation of NH2Cl by the aldehyde pathway. Of note, almost all the formed DBP species increased with the increasing disinfectant dosages from 3 to 15 mg-Cl2/L, but the increasing trend slowed down as the dosage of disinfectants further increased from 9 to 15 mg-Cl2/L (Fig. 1 (a) and (b)). This phenomenon may be 14

explained by insufficient concentration of DBP precursors with increasing disinfectant dosages (9-15 mg-Cl2/L). Therefore, the dosage of chlorination was chosen to be 3 times of DOC (9 mg-Cl2/L) in the solution. As presented in Fig. 1 (b), the formation of DCAM exhibited the trend that increased at first, and then declined at higher disinfectant dosages, which depended on the relative contributions of its formation and decomposition rates. Previous studies have indicated that the decomposition rate

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of DCAM increased with increasing the chlor(am)ine dosage [55,56]. Therefore, at higher chlor(am)ine dosages, the formed DCAM can be further decomposed, which

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can explain the decreasing trend of DCAM after the peak point.

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3.2. Effects of pre-oxidation on DBP formation in rainwater

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[Figure 2]

DBP formation of rainwater treated by KMnO4 and K2FeO4 pre-oxidation

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followed by post-chlorination is present in Fig. 2 (a) and (b). At low dosage of pre-oxidants (0.5 and 1.0 mg/L), the formation of DBPs in rainwater treated by

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KMnO4 pre-oxidation was slightly lower than that treated by K2FeO4. With the increasing dosage of KMnO4, the formation of all detected DBPs decreased. Similar results were observed at K2FeO4 dosage of 0.5-2 mg/L. Moreover, at 2.0 and 3.0 mg/L of K2FeO4 addition, almost all identified DBP formation was lower than that formed during KMnO4 pre-oxidation under identical condition. However, except CH and 15

DCAM, the yields of other DBPs increased with excessive K2FeO4 addition (5 mg/L). The increase of most DBP yields may due to the poor stability of K2FeO4 and the low oxidizing capacity of Fe(OH)3 formed from K2FeO4 decomposition, which is exhibited as Eq. (1) [57]: 4K2FeO4 + 10H2O → 4Fe(OH)3 + 3O2↑ + 8KOH

(1)

The reduction rates of DBP formation by different pre-oxidants (KMnO4 and

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K2FeO4) were shown in Fig. 2 (c) and (d). With the increase of KMnO4 dosage from 0

to 2 mg/L, the reduction rates of C- and N-DBPs increased continuously. Moreover,

the control of CH, TCAA, DCAN, and DCAM was relatively better than other

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identified DBPs (Fig. 2 (c) and (d)). As for TCM and DCAA, the reduction rates were

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approximately 60% at the dosage of 5.0 mg/L KMnO4 (Fig. 2 (c)). However, the reduction rate of TCNM was even lower than 40% by KMnO4 pre-oxidation,

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indicating the control of N-DBPs was relatively worse than that of C-DBPs. These

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results may be due to the fact that the C-DBP precursors were more degradable than those of N-DBPs, which was consistent with a previous study [58]. The effect of

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K2FeO4 pre-oxidation on DBP formation was quite different from that of KMnO4 pre-oxidation. CH and DCAM were the only two DBPs with reduction rates increased

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with increasing K2FeO4 dosage (Fig. 2 (c) and (d)), indicating that these two precursor groups of CH and DCAM were easier to be removed than the other identified DBPs. The reduction rates of other DBPs followed an increasing and then decreasing trend with the maximum reached at 2.0-3.0 mg/L K2FeO4 dosage, which may be contributed by the poor stability of K2FeO4 (refer to Eq. (1)). Although the reduction 16

rates of all DBPs by KMnO4 pre-oxidation were much higher than those by K2FeO4 pre-oxidation at low pre-oxidants dosages (0.5 and 1.0 mg/L), much better control of almost all DBP formation was observed during K2FeO4 pre-oxidation at dosage of 2.0 and 3.0 mg/L, and the reduction rates of all identified DBPs were higher than 70%. This result is consistent with a previous report that the control of DBP formation by K2FeO4 pre-oxidation was much more significant with low dosages [59]. However,

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with excessive dosage of K2FeO4, K2FeO4 decomposition may intensify the increase of pH and the formation of Fe(OH)3, resulting in worse control of DBP precursors

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[57].

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3.3. Effects of UV/PS and UV/H2O2 AOPs on DBP formation in rainwater

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[Figure 3]

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[Figure 4]

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The effect of UV irradiation alone on DBP formation is displayed in Fig. S2. Almost all identified DBP formation remained relatively stable with increasing UV

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fluence, indicating UV irradiation alone cannot effectively degrade DBP precursors. Besides, according to a previous study, either PS or H2O2 pre-oxidation alone can’t reduce the formation of DBPs [20]. Therefore, UV/PS and UV/H2O2 AOPs were applied to DBP control, and the results were displayed in Figs. 3-6. Figs. 3 and 4 illustrated the effects of UV/PS on the control of C- and N-DBPs, respectively. As an 17

effective oxidant, PS can be activated by UV irradiation to form more powerful free radicals (Reaction (2)): S2O82- + UV → 2 SO4• -

(2)

As displayed in Figs. 3 (a), (b) and (d), TCM, DCAA, and CH formation did not decrease with increasing UV fluence at low PS dosage (≤ 0.5 mM), indicating that increasing the dosage of UV showed negligible effect on TCM, DCAA, and CH

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control at low PS dosage, due to the low capacity of UV for DBP precursors [60]. This finding is in agreement with previous literature [28,61]. However, significant

reduction of TCM, DCAA, and CH yields was observed at PS dosage higher than 5

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mM at all the tested UV fluences, suggesting that SO4• - played a more important role

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compared to UV fluence in TCM, DCAA and CH control. It is well-known that UV/PS AOP can alter the structures of NOM and form lower MW organics, including

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aldehydes, ketones, and carboxylic acids, which can serve as DBP precursors [61-63].

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Therefore, DBP precursors produced from NOM degradation by UV/PS AOP could lead to the increase of DBP yields (TCM, DCAA, TCAA, and CH in Fig. 3) at the

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dosage of ≤ 1 mM PS. At higher PS dosages (≥ 5 mM), further mineralization may occur, leading to lower C-DBP yields.

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The control of N-DBPs is displayed in Fig 4. Similarly, well performance of

UV/PS in DCAN and DCAM control was observed with increasing PS dosage regardless of UV fluence. In the contrast, TCNM yields reduced slowly by UV/PS AOP, which could be attributed to alternative TCNM formation pathways. It has been reported that UV irradiation can transformed NO2- and NO3- to nitrating agents (such 18

as ONOOH, NO2Cl, and N2O4), which could result in nitroalkane formation [64]. Then, nitroalkane can react with chlorine to form TCNM [64]. In sum, at PS dosage of 1 mM and UV fluence of 960 mJ/cm2, the formation of all DBPs was relatively low, and their reduction was promoted significantly with the increases of UV fluence and PS dosage. Under the optimal condition, the reduction rates of identified DBP formation were as follows: 74% for TCM, 6% for DCAA, 100%

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for TCAA, 68% for DCAN, 40% for TCNM, and 35% for DCAM.

The performance of UV/H2O2 on the control of C- and N-DBP was also investigated, and the results are displayed in Figs. 5 and 6. Hydroxyl radical (•OH)

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also has powerful oxidation potential (E0 = 1.9-2.7 V) to mineralize organic matter,

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which is beneficial to DBP control. As displayed in Figs. 5(a)-(c), the concentrations of TCM, DCAA, and TCAA were lower in rainwater samples treated by UV/H2O2

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pre-oxidation, indicating that the control of TCM, DCAA, and TCAA through

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UV/H2O2 was relatively better compared to UV/PS pre-oxidation. In contrast, CH formation was promoted at low H2O2 dosage (0.5-1.0 mM), but its reduction was

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observed at higher H2O2 dosages (5.0-20.0 mM) (Fig. 5 (d)). In the presence of low H2O2 dosage (0.5-1.0 mM), •OH can produce a large amount of aldehydes that are the

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precursors of CH so as to contribute to the formation of CH. As the H2O2 concentration increased from 5.0 to 20.0 mM, mineralization may further increase, leading to the decrease of CH. In short, the results indicated that •OH has an important influence on the control of C-DBPs. However, the application of H2O2 was not effective for the control of N-DBPs (TCNM, DCAN, and DCAM in Fig. 6). 19

According to a previous study [61], organic nitrogen can be transformed to ammonium in the presence of •OH, resulting in the formation of chloramines during post-chlorination and further contributed to the formation of N-DBPs.

[Figure 5]

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[Figure 6]

DOC concentrations, SUVA254, SUVA285, and E2/E3 of the rainwater samples

treated by UV/PS and UV/H2O2 AOPs are displayed in Figs. S3, S4 and S5,

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respectively. As shown in Fig. S3, at low PS and H2O2 dosage of 0.5 mM, DOC

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concentration did not significantly decrease with the increasing of fluence of UV fluence. But at the PS and H2O2 dosages ≥1.0 mM, DOC concentration decreased

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obviously in the rainwater treated by UV/PS than that treated by UV/H2O2. Compared with PS and H2O2, UV fluence played a less important role in DOC reduction. On the

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other hand, SUVA254 and SUVA285 values were related to the concentrations of

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aromatic DOM. SUVA254 and SUVA285 values increased with the increasing dosage of PS and H2O2 (Fig. S4), indicating that non-aromatic compounds were more sensitive

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to those two AOPs. And the values of SUVA254 and SUVA285 of the samples treated by UV/PS were much higher than that treated by UV/H2O2. It revealed that activated PS by UV irradiation could alter the structures of organics more radically. Meanwhile, E2/E3 ratio was a parameter that correlated well with the size of DOM in samples [37,38]. E2/E3 value of rainwater treated by UV/PS increased enormously with 20

increasing PS dosage but only increased moderately with increasing H2O2 dosage (Fig. S5), which also illustrated that UV/PS could transform DOM to lower MW organic compounds [37,38]. In addition, fluorescence EEM spectra are used to investigate the characteristics of DOM in rainwater. As illustrated in Fig. S6, the most intense peak was in the region of aromatic proteins (λEx < 250 nm, λEm < 380 nm), and the second-highest peak was observed in the SMP-like region (λEx > 250 nm, λEm < 380

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nm), followed by the fulvic acid-like (λEx < 250 nm, λEm > 380 nm) and humic

acid-like (λEx > 250 nm, λEm > 380 nm) regions, respectively, which were significantly different from the characteristics of raw water [65]. According to previous studies, the

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peaks (λEx = 230 nm, λEm = 330 nm) in these regions were confirmed to be significant

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DBP precursors [51,53,65], and can be greatly reduced during the pretreatment of UV/PS and UV//H2O2 AOPs. The results further confirmed that UV/PS and UV//H2O2

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processes can effectively change the DOM characteristics in rainwater, leading to

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efficient control of DBP formation. Moreover, UV/PS treatment was more powerful than UV/H2O2 in terms of altering the characteristics of DOM in rainwater at a high

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dosage (5.0 mM), although the peaks in these regions were stronger than those of the

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samples treated by UV/H2O2 at a low dosage (0.5 mM).

3.4. Integrated cyto- and genotoxicity assessment of rainwater before and after different disinfection processes

[Figure 7]

21

Integrated cyto- and genotoxicity assessments were made to evaluate the effect of different disinfection processes on the control of DBP formation in rainwater. Chinese hamster ovary (CHO) cells were commonly used to analyze chronic cytotoxicity and genomic DNA damaging capacity [6,7]. The reduction in the density of the CHO cells during the incubating period can indicate the chronic cytotoxicity of DBPs, and the minimum DBP concentration that can result in half of cell apoptosis

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represented cytotoxicity value (%C1/2). Acute genomic DNA damage induced by DBPs was analyzed by single cell gel electrophoresis, and the midpoint of the

concentration-response curve represented the genotoxicity value (Genotoxic potency)

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of DBPs. %C1/2 and Genotoxic potency investigated by Plewa et al. [12] and other

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researchers of the DBPs are listed in Table S2. Cytotoxicity index (CTI) and genotoxicity index (GTI) of the samples were the sum of CTI and GTI of each DBP

CTI = ∑ [ X=1

1 × Cx ] %C1/2x × Mx

n

GTI = ∑ [

1 × Cx ] Genotoxicity potencyx × Mx

(3)

(4)

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X=1

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n

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compound, which were calculated using as Eqs. (3) and (4), respectively:

where %C1/2X and Genotoxic potencyx are cyto- and genotoxicity of each DBP

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compound; M is the relative molecular mass of each DBP compound; CX is the formation potential (nM) of each DBP compound; X is the specific DBP compound (TCM, DCAA, TCAA, CH, DCAN, TCNM, and DCAM). CTI and GTI of DBP formation resulted from different disinfection processes of rainwater are displayed comprehensively in Fig. 7. For KMnO4 and K2FeO4 22

pre-oxidation (Fig. 7 (a) and (d)), either CTI or GTI decreased with increasing of oxidant dosage and reached a low level at 2.0-3.0 mg/L oxidant dosages. However, the adverse effect on the control of CTI and GTI was observed in the rainwater samples treated by 5 mg/L K2FeO4 pre-oxidation. As for the UV/PS and UV/ H2O2 AOPs, without PS or H2O2 addition, CTI decreased slightly while GTI increased gradually with increasing UV fluence (black lines in Fig.7 (b)-(c) and (e)-(f)). CTI

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and GTI for rainwater samples treated by UV/PS AOP did not decrease at PS dosage

of 0.5 mM (Fig. 7 (b) and (e)). Better control of them was observed at PS dosage of 1.0 mM and UV fluence of 960 mJ/cm2. Of note, CTI and GTI increased with

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increasing fluence of UV at 10.0 mM PS dosage due to higher yields of DBPs. But

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CTI and GTI level reached to the lowest level at 20.0 mM PS dosage. As shown in Fig.7 (e) and (f), CTI and GTI for the samples treated by UV/H2O2 decreased with

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increasing H2O2 dosage, and H2O2 dosage was more significant than UV fluence for

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cyto- and genotoxicity control. High oxidant dosage H2O2 (≥ 10.0 mM) can lead to

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lower CTI and GTI levels in rainwater samples treated by UV/H2O2 AOP.

4. Conclusions

The application of chloramine could effectively reduce the formation of C-DBPs,

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

but not N-DBPs in rainwater, indicating that other alternative disinfectants should be used for N-DBP control. 

The reduction rates of TCM, DCAA, and TCNM during KMnO4 pre-oxidation of

rainwater were low even at high oxidant dosage. Compared to KMnO4, K2FeO4 was a 23

more effective pre-oxidant for the reduction of DBP precursors, but the reduction rates decreased at high K2FeO4 dosage. 

Neither UV irradiation alone nor UV/PS and UV/H2O2 at low oxidant dosage (≤

0.5 mM) could efficiently control DBP formation and cyto- and genotoxicity. However, better performance can be achieved at PS dosage of 1.0 mM and UV fluence of 960 mJ/cm2. Similar control of cyto- and genotoxicity was observed by



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UV/H2O2 treatment at 5.0 mM H2O2 dosage and UV fluence of 960 mJ/cm2.

UV/PS treatment was more powerful than UV/H2O2 to reduce the DOC and alter

the characteristics of DOM in rainwater. The decrease of molecular size and increase

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of the relative content of aromatic compounds were observed.

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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   

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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

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

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Acknowledgements

This study was supported partially by the Natural Science Foundation of China

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(Nos. 51778444, 51978483, 51678354 and 51808222), the National Major Science and Technology Project of China (No. 2017ZX07207004), the Fundamental Research Funds for the Central Universities (No. 22120180123), and the Ministry of Science and Technology in Taiwan (MOST-107-2221-E-992-008-MY3).

24

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Figure Legends Fig. 1. DBP formation during chlor(am)ine disinfection: (a) C-DBPs; (b) N-DBPs. Experimental conditions: pH = 7.0 ± 0.2; temperature = 25.0 ± 0.5 °C; chlor(am)ination time = 24 h. Error bars represent one standard deviation of duplicate measurements. Fig. 2. Reduction rate of DBP formation by adding different pre-oxidants: KMnO4 and K2FeO4. Experimental conditions: post-chlorine dosage = 3 × DOC mg/L; pH =

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7.0 ± 0.2; temperature = 25.0 ± 0.5 °C; pre-oxidation time = 30 min; chlorination time = 24 h. Error bars represent one standard deviation of duplicate measurements.

Fig. 3. The effect of UV/PS on the control of: (a) TCM; (b) DCAA; (c) TCAA; (d)

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CH formation. Experimental conditions: UV fluence = 240, 480, 960, and 1440 mJ/cm2; PS dosage = 0, 0.5, 1, 5, 10, and 20 mM; post-chlorine dosage = 3 × DOC

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mg/L; pH = 7.0 ± 0.2; temperature = 25.0 ± 0.5 °C; chlorination time = 24 h. Error

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bars represent one standard deviation of duplicate measurements. Fig. 4. The effect of UV/PS on the control of: (e) DCAN; (f) TCNM; (g) DCAM

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formation. Experimental conditions: UV fluence = 240, 480, 960, and 1440 mJ/cm2; PS dosage = 0, 0.5, 1, 5, 10, and 20 mM; post-chlorine dosage = 3 × DOC mg/L; pH

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= 7.0 ± 0.2; temperature = 25.0 ± 0.5 °C; chlorination time = 24 h. Error bars represent one standard deviation of duplicate measurements.

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Fig. 5. The effect of UV/ H2O2 on the control of: (a) TCM; (b) DCAA; (c) TCAA; (d) CH formation. Experimental conditions: UV fluence = 240, 480, 960, and 1440 mJ/cm2; H2O2 dosage = 0, 0.5, 1, 5, 10, and 20 mM; post-chlorine dosage = 3 × DOC mg/L; pH = 7.0 ± 0.2; temperature = 25.0 ± 0.5 °C; chlorination time = 24 h. Error bars represent one standard deviation of duplicate measurements.

35

Fig. 6. The effect of UV/H2O2 on the control of: (e) DCAN; (f) TCNM; (g) DCAM formation. Experimental conditions: UV fluence = 240, 480, 960, and 1440 mJ/cm2; H2O2 dosage = 0, 0.5, 1, 5, 10, and 20 mM; post-chlorine dosage = 3 × DOC mg/L; pH = 7.0 ± 0.2; temperature = 25.0 ± 0.5 °C; chlorination time = 24 h. Error bars represent one standard deviation of duplicate measurements. Fig. 7. CTI and GTI of rainwater samples treated by different disinfection processes:

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-p

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(a) & (d): KMnO4 and K2FeO4; (b) & (e): UV/PS AOP; (c) & (f): UV/H2O2 AOP.

Fig. 1.

36

Jo

ur

na

lP

ro of

re

-p

Fig. 2.

Fig. 3.

37

38

ro of

-p

re

lP

na

ur

Jo Fig. 4.

39

ro of

-p

re

lP

na

ur

Jo Fig. 5.

Jo

ur

na

lP

re

Fig. 7.

-p

ro of

Fig. 6.

40