Supporting Information

for

Pilot-scale Evaluation of Oxidant Speciation, 1,4-Dioxane Degradation and Disinfection Byproduct Formation during UV/Hydrogen Peroxide, UV/Free Chlorine and UV/Chloramines Advanced Oxidation Process Treatment for Potable Reuse

Zhong Zhang†1, Yi-Hsueh Chuang†1,2, Aleksandra Szczuka1, Kenneth P. Ishida3, Shannon Roback3,4,

Megan H. Plumlee3, and William A. Mitch1*

1 Department of Civil and Environmental Engineering, Stanford University, 473 Via Ortega, Stanford, California 94305, United States2 Current affiliation: Institute of Environmental Engineering, National Chiao Tung University, Hsinchu City, Taiwan3 Research & Development Department, Orange County Water District, 18700 Ward Street, Fountain Valley, California 92708, United States4 Current affiliation: Department of Health Science, California State University at Dominguez Hills, Carson, California 90747, United States† Those authors contributed equally to this work*Contact Information: email: wamitch@stanford.edu, Phone: 650-725-9298, Fax: 650-723-7058

32 pages3 Texts

13 Tables4 Figures

Table of Contents

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Text S1: Additional materials and methods S3Figure S1: UV reactor scheme S5Table S1: Basic water quality data from field samplings S6 Text S2: Estimation of UV fluence of the pilot-scale UV reactor S7 Table S2: The decay of chloramines and H2O2 under laboratory-scale UV S7irradiation Table S3: Chinese hamster ovary cell comparative cytotoxicity of target S8halogenated DBPsTable S4: Principal reactions in the kinetic model S9Table S5: Applied oxidant dose and concentrations for NH3, free chlorine, S14NH2Cl, total chlorine and H2O2 measured in the influent and effluent of thepilot UV reactor during Event 1.Figure S2: 1,4-Dioxane removal for the UV/H2O2 AOP, the UV/free chlorine S15AOP and the UV/chloramines AOP during Event 2.Table S6: DBP concentrations during AOP treatment for the UV/free chlorine S16AOP during Event 1.Table S7: DBP concentrations after UFC chloramination for the UV/free chlorine S17AOP during Event 1.Table S8: DBP concentrations during AOP treatment for the UV/chloramines S18AOP during Event 1.Table S9: DBP concentrations after UFC chloramination for the UV/chloramines S19AOP during Event 1.Table S10: DBP concentrations during AOP treatment for the UV/H2O2 S20AOP during Event 1.Table S11: DBP concentrations after UFC chloramination for the UV/H2O2 S21AOP during Event 1.Table S12: DBP concentrations during AOP treatment during Event 2 S22Table S13: DBP concentrations after UFC chloramination during Event 2 S23Figure S3: Calculated DBP toxicity during Event 1. S24Figure S4: DBP concentrations and calculated DBP toxicity during Event 2. S25 Text S3: Cost estimate summary. S26References S27

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Text S1: Additional materials and methods

Materials: Ammonium chloride (NH4Cl; ≥99.5%), sodium hydroxide (NaOH; >99%), sodium hypochlorite (5.65–6%) and hydrogen peroxide (30% v/v) were purchased from Fisher Scientific. 1,4-Dioxane (99.8%) was purchased from Acros Organics (Geel, Belgium). L-Ascorbic acid (≥99.0%), 2-bromobutyric acid (97%), bromochloroacetic acid (99.2%), bromodichloroacetic acid (99.6%), bromodichloromethane (99.4%), bromoform (99.4%), chloroacetic acid (99.9%), dibromochloroacetic acid (99.9%), chloroform (99.9%), dibromoacetic acid (99.7%), dibromochloromethane (99.9%), 1,2-dibromopropane (97%), dichloroacetamide (98%), dichloroacetic acid (99.4%), dichloroacetonitrile (98%), 1,1-dichloro-2-propanone (98.2%), EPA 552.2 esters calibration mix, EPA 552 halogenated acetic acids mix (2000 µg/mL each component in methyl tert-butyl ether), EPA 551B halogenated volatiles mix (2000 µg/mL each component in acetone), EPA 501/601 trihalomethanes calibration mix (2000 µg/mL each component in methanol), iodoacetic acid (≥98.0%), methyl tert-butyl ether (MtBE; 99.9%), trichloroacetaldehyde (chloral hydrate; 98.7%), trichloroacetamide (99%), trichloroacetic acid (99.4%), trichloroacetonitrile (98%) and 1,4-dioxane-d8 (99%) were purchased from Sigma-Aldrich (St. Louis, MO). Bromochloroacetonitrile (5015 µg/mL in acetone; 97.4%), dibromoacetonitrile (5015 µg/mL in acetone; 95.9%), trichloronitromethane (chloropicrin, 5018 µg/mL in acetone; 95.0%), and 1,1,1-trichloro-2-propanone (4935 µg/mL in acetone; 98.7%) were purchased from AccuStandard, Inc. (New Haven, CT). Bromodiiodomethane (95.0%), bromochloroiodomethane (99.0%), chlorodiiodomethane (97.0%), dibromoiodomethane (97.0%), dichloroiodomethane (97.8%), dibromoacetamide (100.0%), bromochloroacetamide (100.0%), bromodichloroacetaldehyde (95.6%), dibromochloroacetaldehyde (91.1%), and tribromoacetaldehyde (97.0%) were purchased from CanSyn Chem. Corp. All chemicals were used as received.

Analytical details: A total of 35 halogenated DBPs in 8 classes were analyzed, including the four regulated trihalomethanes (THMs: chloroform (TCM), bromodichloromethane (BDCM), dibromochloromethane (DBCM), bromoform (TBM)), six iodinated trihalomethanes (I-THMs: iodoform (TIM), bromodiiodomethane (BDIM), bromochloroiodomethane (BCIM), chlorodiiodomethane (CDIM), dibromoiodomethane (DBIM), and dichloroiodomethane (DCIM)), ten haloacetic acids (HAAs: bromoacetic acid (BAA), bromochloroacetic acid (BCAA), bromodichloroacetic acid (BDCAA), chloroacetic acid (CAA), dibromochloroacetic acid (DBCAA), dibromoacetic acid (DBAA), dichloroacetic acid (DCAA), tribromoacetic acid (TBAA), trichloroacetic acid (TCAA), and iodoacetic acid (IAA)), four haloacetonitriles (HANs: bromochloroacetonitrile (BCAN), dibromoacetonitrile (DBAN), dichloroacetonitrile (DCAN), and trichloroacetonitrile (TCAN)), four haloacetaldehydes (HALs: trichloroacetaldehyde (chloral hydrate; TCAL), bromodichloroacetaldehyde (BDCAL), dibromochloroacetaldehyde (DBCAL), and tribromoacetaldehyde (TBAL)), four haloacetamides (HAMs: dichloroacetamide (DCAM),

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dibromoacetamide (DBAM), bromochloroacetamide (BCAM), and trichloroacetamide (TCAM)), two haloketones (HKs: 1,1-dichloropropanone (1,1-DCP) and 1,1,1-trichloropropanone (1,1,1-TCP)), and one halonitromethane (HNM: trichloronitromethane (chloropicrin; TCNM)). The DBPs were extracted by modified USEPA Methods 551.1 and 552.3 and analyzed by gas chromatography-mass spectrometry (GC-MS) with ~0.05–0.2 g/L reporting limits.

For analysis of 1,4-dioxane, water samples (40 mL) were spiked with 50 µg/L 1,4-dioxane-d8 as an internal standard and 12 g sodium sulfate for salting out. The samples were then extracted into 3 mL methyl tert-butyl-ether (MtBE) by shaking vigorously for 2 min and further concentrated by nitrogen blowdown to 0.5 mL. The MtBE extract was analyzed by an Agilent 6890N gas chromatography coupled with a HP-5MS column (Agilent, CA) and a 5973N MS in the electron impact mode with a 0.9 g/L method reporting limit.

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Figure S1. UV/AOP reactor scheme, where UVF = UV/AOP pilot feed and UVP = UV/AOP pilot product water.

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Table S1: Basic water quality data for RO permeate samples.

SampleChlorine residual

(mg/L as Cl2)NH2Cl

(mg/L as Cl2)NHCl2

(mg/L as Cl2)pH TOC (mg/L)

UV254

(cm-1)Cl-

(mg/L)NO2

-

(mg/L as N)NO3

-

(mg/L as N) Event 1 3.4 2.3 1.1 5.6 0.23 0.02 5.3 <0.05 0.95Event 2 2.3 1.5 0.8 5.8 0.27 0.02 NM <0.05 1.03

NM = Not Measured

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Text S2: Estimation of the UV fluence for the pilot-scale UV reactor.

To estimate the UV fluence of the pilot-scale UV reactor, RO permeate (containing 3.4 mg/L as Cl2

chloramines) and RO permeate spiked with 3.2 mg/L H2O2 were irradiated with a semi-collimated beam apparatus containing three 15 W Philips low pressure mercury lamps emitting at 254 nm, as described previously (Chuang et al. 2017, McCurry et al. 2016). Briefly, light from the UV lamps shone down through a shutter onto a 750-mL crystallization dish, which was continuously mixed by a magnetic stir bar. Incident irradiance (0.60 mW cm-2) was determined by iodide-iodate actinometry (Bolton et al. 2011, Goldstein and Rabani 2008). The decay of chloramines and H2O2 was compared with that observed in the pilot-scale study. The decay of chloramines and H2O2 at different fluence is provided in Table S2. During the pilot-scale study, chloramines decreased from 3.4 mg/L as Cl2 to 1.3 mg/L as Cl2 while H2O2 concentrations decreased from 3.2 mg/L to 2.7 mg/L. Based on these results, the UV fluence of the pilot-scale UV reactor was estimated at around 1000 mJ/cm2.

Table S2: The decay of chloramines and H2O2 under laboratory-scale UV irradiation.Fluence (mJ/cm2) H2O2 conc. (mg/L)

Chloramines conc. (mg/L as Cl2)

0 3.2 3.4

200 3.1 2.5

600 3.0 1.7

900 2.8 1.3

1100 2.7 1.2

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Table S3: Toxic potency metrics for halogenated DBPs.

DBP LC50 (M)a LECR50 (M)b

THMsTCM 9.17×10−3 NABDCM 1.15×10−2 1.69×10−3

DBCM 5.36×10−3 1.00×10−3

TBM 3.96×10−3 8.59×10−3

HAAsCAA 8.48×10−4 NABAA 9.60×10−6 NADCAA 7.30×10−3 2.75×10−3

TCAA 2.40×10−3 1.53×10−3

BCAA 7.78×10−4 NADBAA 5.21×10−4 NABDCAA 6.85×10−4 NADBCAA 2.00×10−4 NATBAA 8.50×10−5 NAIAA 3.20×10−6 NAI-THMsDCIM 4.13×10−3 NABCIM 2.42×10−3 NADBIM 1.91×10−3 NACDIM 2.41×10−3 NABDIM NA NATIM 6.60×10−5 NAHNMsTCNM 5.36×10−4 NAHKs1,1-DCP NA NA1,1,1-TCP NA NAHALsTCAL 1.16×10−3 NABDCAL 2.04×10−5 NADBCAL 5.15×10−6 NATBAL 3.56×10−6 NAHANsDCAN 5.73×10−5 NABCAN 8.46×10−6 NADBAN 2.85×10−6 NATCAN 1.60×10−4 NAHAMsDCAM 1.92×10−3 NABCAM 1.71×10−5 NADBAM 1.22×10−5 NA

TCAM 2.05×10−3 NA

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a CHO cell cytotoxicity LC50 values (Wagner and Plewa, 2017) b LECR50 values (US EPA, 2019)

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Table S4: Principal reactions in the kinetic model.

No. Reaction Rate constant Reference

Photolysis reactions

1 NH2Cl hv→

•NH2 + •Cl

rradical=EP

0 [1−10−εCd] ∙ Фd

EP0

: photon flux at 254 nm

(mEinstein s-1 cm-2)

d: pathlength (cm)

ε: molar extinction coefficient

Ф: quantum yield (mol/Einstein)

C: concentration of oxidant

(Chuang et al. 2016)

2 NHCl2 hv→

•NHCl + •Cl

3 H2O2 hv→

•OH + •OH

4 HO2- hv

→ •OH + O•-

5 HOCl hv→

•Cl + •OH

6 OCl- hv→

•Cl + •O-

Oxidant scavenging reactions

7 NH2Cl + •OH → •NHCl+ H2O 1.02×109 M-1s-1 (Chuang et al. 2017)

8 NH2Cl + •Cl → •NHCl+ Cl- 1.00×109 M-1s-1 (Chuang et al. 2017)

9 NH2Cl + •Cl2- → •NHCl+ 2Cl- + H+ 1.14×107 M-1s-1 (Patton et al. 2017, Patton et al. 2018)

10 NHCl2 + •OH → NCl2• + H2O 6.21×108 M-1s-1 (Zhang et al. 2019)

11 NHCl2 + •Cl → NCl2• + Cl- + H+ 1.00 ×109 M-1s-1 (Zhang et al. 2019)

12 NHCl2 + •Cl2- → NCl2•+ 2Cl- + H+ 4.4 ×106 M-1s-1 (Patton et al. 2018)

13 •OH + H2O2 → HO2• + H2O 2.70×107 M-1s-1 (Buxton et al. 1988)

14 •OH + HO2- → HO2• + OH- 7.50×109 M-1s-1 (Crittenden et al. 1999)

15 •Cl + H2O2 → HO2• + Cl- + H+ 2.00×109 M-1s-1 (Yu and Barker 2003)

16 Cl2•- + H2O2 → HO2• + 2Cl- + H+ 1.40×105 M-1s-1 (Matthew and Anastasio 2006)

17 HOCl + •OH → ClO• + H2O 5.00×108 M-1s-1 (Chuang et al. 2017)

18 HOCl + •Cl → ClO• + H+ + Cl- 3.00×109 M-1s-1 (Klaning and Wolff 1985)

19 OCl- + •OH → ClO•+ OH- 1.85×109 M-1s-1 (Chuang et al. 2017)

20 •Cl + OCl- → ClO• + Cl- 8.30×109 M-1s-1 (Klaning and Wolff 1985)

ClOx scavenging reactions

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21 ClO• + ClO• → Cl2O2 2.50×109 M-1s-1 (Klaning and Wolff. 1985)

22 Cl2O2 + H2O → ClO2- + HOCl + H+ 1.00×104 s-1 (Mialocq et al. 1973)

23 ClO• + ClO2- → ClO2 + OCl- 9.40×108 M-1s-1 (Alfassi et al. 1988)

24 •Cl + ClO2- → ClO2 + Cl- 7.00×109 M-1s-1 (Chuang et al. 2017)

25 •OH + ClO2- → ClO2 + OH- 7.00×109 M-1s-1 (Eriksen et al. 1981)

26 •OH + ClO2 → ClO3- + H+ 4.00×109 M-1s-1 (Klaning et al. 1985

27 •Cl + ClO2 → products 4.00×109 M-1s-1 (Chuang et al. 2017)

28 •OH + ClO3- → products <1.00×106 M-1s-1 (Buxton and Subhani 1972)

29 •Cl + ClO3- → products <1.00×106 M-1s-1 (Chuang et al. 2017)

Reactions of •NH2 and NHCl•

30 •NH2 + O2 → NH2O2• 1.2 ×108 M-1s-1 (Laszlo et al. 1998, Menkin et al. 1991)

31 •NHCl + O2 → NHClO2• 1.2 ×108 M-1s-1 (Poskrebyshev et al. 2003)

32 NH2O2• → •NO + H2O 1.0 ×108 s-1 (Zhang et al. 2019)

33 NHClO2• → •NO + product 1.0 ×108 s-1 (Zhang et al. 2019)

34 NH2O2• → transient species → N2O 5.98 × 108 s-1 (Zhang et al. 2019)

35 NHClO2• → transient species → N2O 6.7 × 108 s-1 (Zhang et al. 2019)

36 •NO + •OH → NO2- + H+ 1.0 × 1010 M-1s-1 (Seddon et al. 1973)

37 NO2- + •OH →•NO2 + OH- 1.2 × 1010 M-1s-1 (NDRL/NIST Database)

38 •NO + •NO + O2 → 2 •NO2 2.1 × 106 M-2s-1 (Lewis et al. 1995)

39 •NO + •NO2 → N2O3 1.1 × 109 M-1s-1 (Lewis et al. 1995)

40 N2O3 → •NO + •NO2 4.3 × 106 s-1 (Lewis et al. 1995)

41 N2O3 + H2O → 2NO2- + 2H+ 1.6 × 103 s-1 (Lewis et al. 1995)

42 •NO2 + •NO2 → N2O4 4.5 × 108 M-1s-1 (Mack and Bolton 1999)

43 N2O4 + H2O → NO2- + NO3

- + 2H+ 1.0 × 103 s-1 (Mack and Bolton 1999)

Reactions with organic compounds

44 •OH + CH3COO- → •CH2COO- + H2O 7.50×107 M-1s-1 (Fisher and Hamill 1973)

45 •Cl + CH3COO- → •CH2COO- + HCl 3.70×109 M-1s-1 (George V. Buxton 1999)

46 •OH + CH3COOH → •CH2COO- + H2O 1.50×107 M-1s-1 (Stephens 1984, Thomas 1965)

47 •Cl + CH3COOH → •CH2COO- + HCl 2.00×108 M-1s-1 (Gilbert et al. 1988)

48 •O- + CH3COO- → •CH2COO- + OH- 5.00×107 M-1s-1 (Neta and Schuler 1975)

49 1,4-dioxane + •OH → product 3.1×109 M-1s-1 (Patton et al. 2017)

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50 1,4-dioxane + •Cl → product 4.4×106 M-1s-1 (Patton et al. 2017)

51 1,4-dioxane + •Cl2- → product 3.3×106 M-1s-1 (Patton et al. 2017)

52 DOC + •OH → product 2.1 ×104 (mg/L)-1s-1 (Zhang et al. 2019)

Breakpoint chlorination

53 HOCl + NH3 → NH2Cl + H2O 4.17×106 M-1s-1 (Jafvert and Valentine 1992)

54 NH2Cl + H2O → HOCl + NH3 2.11×10-5 s-1 (Jafvert and Valentine 1992)

55 NH2Cl + HOCl → NHCl2 + H2O 2.78×102 M-1s-1 (Jafvert and Valentine 1992)

56 NHCl2 + H2O → NH2Cl + HOCl 6.39×10-7 s-1 (Jafvert and Valentine 1992)

57 NH2Cl + NH2Cl → NHCl2 + NH3 1.1×10-2 M-1 s-1 (Jafvert and Valentine 1992)

58 NHCl2 + NH3 → NH2Cl + NH2Cl 9.51×10-2 M-1 s-1 (Jafvert and Valentine 1992)

59 NHCl2 + H2O → Intermediate 6.94×10-7 s-1 (Jafvert and Valentine 1992)

60 NHCl2 + Intermediate → HOCl + product 2.78×104 M-1 s-1 (Jafvert and Valentine 1992)

61 NH2Cl + Intermediate → product 8.3×103 M-1 s-1 (Jafvert and Valentine 1992)

62 NH2Cl + NHCl2 → product 1.53×10-2 M-1 s-1 (Jafvert and Valentine 1992)

63 HOCl + NHCl2 → NCl3 + H2O 20.8 M-1 s-1 (Jafvert and Valentine 1992)

64 NCl3 + H2O → HOCl + NHCl2 3.2×10-5 s-1 (McCurry et al. 2017)

65 HOCl + NHCl2 → NCl3 + H2O 20.8 M-1 s-1 (Jafvert and Valentine 1992)

66 NCl3 + NHCl2 + 2H2O → 2HOCl + product 3.51×102 M-1 s-1 (Jafvert and Valentine 1992)

67 NCl3 + NH2Cl + H2O → HOCl + product 8.76 M-1 s-1 (Jafvert and Valentine 1992)

68 NHCl2 + 2HOCl + H2O → NO3- + 5H+ + 4Cl- 231 M-1 s-1×[NHCl2]×[OCl-]a (Jafvert and Valentine 1992)

Other reactions

69 H2O → H+ + OH- 1.00×10-3 s-1 (Yang et al. 2014)

70 H+ + OH- → H2O 1.00×1011 M-1s-1 (Yang et al. 2014)

71 H2O2 → H+ + HO2- 1.26×10-1 s-1 (Yang et al. 2014)

72 H+ + HO2- →H2O2 5.00×1010 M-1s-1 (Yang et al. 2014)

73 •OH + •OH → H2O2 5.50×109 M-1s-1 (Buxton et al. 1988)

74 HO2• + H2O2 → O2 + •OH + H2O 3.00 M-1s-1 (Crittenden et al. 1999)

75 O2•- + H2O2 →O2 + •OH + OH- 1.30×10-1 M-1s-1 (Crittenden et al. 1999)

76 HO2• + HO2• → O2 + H2O2 8.30×105 M-1s-1 (Crittenden et al. 1999)

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77 HO2• + O2•- → O2 + HO2- 9.70×107 M-1s-1 (Crittenden et al. 1999)

78 •OH + HO2• → O2 + H2O 6.60×109 M-1s-1 (Crittenden et al. 1999)

79 •OH + O2•- → O2 + OH- 7.00×109 M-1s-1 (Crittenden et al. 1999)

80 •OH + OH- → O•- + H2O 1.20×1010 M-1s-1 (NDRL/NIST Database)

81 •OH + Cl- → ClOH•- 4.30×109 M-1s-1 (Jayson et al. 1973)

82 ClOH•- → Cl- + •OH 6.10×109 s-1 (Jayson et al. 1973)

83 •Cl + H2O → ClOH•- + H+ 2.50×105 s-1 (Mcelroy 1990)

84 •Cl + OH- → ClOH•- 1.80×1010 M-1s-1 (Klaning and Wolff 1985)

85 •Cl + Cl- → Cl2•- 8.00×109 M-1s-1 (Nagarajan and Fessenden 1985)

86 Cl2•- → •Cl + Cl- 6.00×104 s-1 (Yu and Barker 2003)

87 Cl2•- + H2O → Cl- + HClOH 1.30×103 s-1 (Mcelroy 1990)

88 HClOH → ClOH•- + H+ 1.00×108 s-1 (Mcelroy 1990)

89 ClOH•- + Cl- → Cl2•- + OH- 1.00×104 M-1s-1 (Grigorev et al. 1987)

90 ClOH•- + H+ → •Cl + H2O 2.10×1010 M-1s-1 (Jayson et al. 1973)

91 •Cl + •Cl → Cl2 8.80×107 M-1s-1 (Wu et al. 1980)

92 Cl2•- + •OH → HOCl + Cl- 1.00×109 M-1s-1 (Wagner et al. 1986)

93 Cl2•- + Cl2•- → Cl2 + 2Cl- 9.00×108 M-1s-1 (Yu and Barker 2003)

94 Cl2•- + •Cl → Cl2 + Cl- 2.10×109 M-1s-1 (Yu and Barker 2003)

95 Cl2•- + HO2• → 2Cl- + H+ + O2 3.00×109 M-1s-1 (Matthew and Anastasio 2006)

96 Cl2•- + O2•- → 2Cl- + O2 2.00×109 M-1s-1 (Matthew and Anastasio 2006)

97 Cl2•- + OH- → ClOH•- + Cl- 4.50×107 M-1s-1 (Grigorev et al. 1987)

98 Cl3- → Cl2 + Cl- 1.10×105 s-1 (Ershov 2004)

99 Cl3- + HO2• → Cl2•- + HCl + O2 1.00×109 M-1s-1 (Bjergbakke et al. 1981)

100 Cl3- + O2•- → Cl2•- + Cl- + O2 3.80×109 M-1s-1 (Matthew and Anastasio 2006)

101 Cl2 + H2O → HOCl + Cl- + H+ 15 s-1 (Wang and Margerum 1994)

102 Cl2 + Cl- → Cl3- 2.00×104 M-1s-1 (Ershov 2004)

103 Cl2 + H2O2 → 2HCl + O2 1.30×104 M-1s-1 (Matthew and Anastasio 2006)

104 Cl2 + O2•- → Cl2•- + O2 1.00×109 M-1s-1 (Matthew and Anastasio 2006)

105 Cl2 + HO2• → Cl2•- + H+ + O2 1.00×109 M-1s-1 (Bjergbakke et al. 1981)

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106 HOCl → OCl- + H+ 1.41×103 s-1 (Yang et al. 2014)

107 OCl- + H+ → HOCl 5.00×1010 M-1s-1 (Yang et al. 2014)

108 HOCl + H2O2 → HCl + H2O + O2 1.10×104 M-1s-1 (Connick 1947)

109 OCl- + H2O2 → Cl- + H2O + O2 1.70×105 M-1s-1 (Connick 1947)

110 HOCl + O2•- → •Cl + OH- + O2 7.50×106 M-1s-1 (Long and Bielski 1980)

111 HOCl + HO2• → •Cl + OH- + O2 7.50×106 M-1s-1 (Matthew and Anastasio 2006)

112 OCl- + O2•- + H2O → •Cl + 2OH- + O2 2.00×108 M-2s-1 (Matthew and Anastasio 2006)

113 H+ + Cl- → HCl 5.00×1010 M-1s-1 (Yang et al. 2014)

114 HCl →H+ + Cl- 8.60×1016 s-1 (Yang et al. 2014)

115 H+ + NH2Cl + NO2- → NH3 + NO2Cl → NO3

- 1.36 × 107 M-2s-1 (Margerum et al. 1994)

116 •OH + HPO42- → HPO4•- + OH- 1.50×105 M-1s-1 (Crittenden et al. 1999)

117 •OH + H2PO4- → HPO4•- + H2O 2.00×104 M-1s-1 (Crittenden et al. 1999)

118 H2O2 + HPO4•- → H2PO4- + HO2• 2.70×107 M-1s-1 (Crittenden et al. 1999)

119 •OH + HCO3- → CO3•- + H2O 8.50×106 M-1s-1 (Crittenden et al. 1999)

120 •OH + H2CO3 → CO3•- + H2O + H+ 1.00×106 M-1s-1 (Grebel et al. 2010)

121 •OH + CO3•- → product 3.00×109 M-1s-1 (Crittenden et al. 1999)

122 H2O2 + CO3•- → HCO3- + HO2• 4.30×105 M-1s-1 (Crittenden et al. 1999)

123 HO2- + CO3•- →CO3

2- + HO2• 3.00×107 M-1s-1 (Crittenden et al. 1999)

124 O2•- + CO3•- →CO32- + O2 6.00×108 M-1s-1 (Crittenden et al. 1999)

125 CO3•- + CO3•- → product 3.00×107 M-1s-1 (Crittenden et al. 1999)

a The empirical rate expression is provided for this reaction.

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Table S5: Applied oxidant dose and concentrations (average ± range of experiments in the morning and in the afternoon) of NH3, free chlorine, NH2Cl, total chlorine and H2O2 measured in the influent and effluent of the pilot UV reactor during Event 1.

nd = not detectable. a Data was not shown because H2O2 interferes with total chlorine measurement via the DPD method.

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Figure S2. 1,4-Dioxane removal for the UV/H2O2 AOP, the UV/free chlorine AOP and the UV/chloramines AOP during Event 2. The ambient condition featured 2.3 mg/L as Cl2 background chloramines in the RO permeate. Error bars represent the range of analytical duplicates from one pilot run for each AOP.

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S19

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Figure S3. Toxic potency-weighted DBP concentrations in the UV feed (UVF) and product waters (UVP) during AOP treatment and after chloramine UFC treatment for (A) the UV/free chlorine AOP, (B) the UV/chloramines AOP and (C) the UV/H2O2 AOP during Event 1. Experimental conditions for each AOP were repeated in the morning and in the afternoon and error bars represent the range for these duplicate pilot runs. THM4 = four regulated trihalomethanes; HAA9 = nine regulated and unregulated haloacetic acids; TCNM = chloropicrin; HKs = haloketones; HALs = haloacetaldehydes; HANs = haloacetonitriles; HAMs = haloacetamides.

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Figure S4. (A) DBP concentrations and (B) calculated toxic potency-weighted DBP concentrations in the UV feed (UVF) and UV product (UVP) waters during AOP treatment and after chloramine UFC treatment during Event 2. Error bars represent the range of analytical duplicates. THM4 = four regulated trihalomethanes; HAA9 = nine regulated and unregulated haloacetic acids; TCNM = chloropicrin; HKs = haloketones; HALs = haloacetaldehydes; HANs = haloacetonitriles; HAMs = haloacetamides.

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Text S3. Initial cost estimates for AOP alternatives

The cost estimates targeted 0.5-log removal of 1,4-dioxane and also considered that the utility would

attempt to reach a 2.5 mg/L as Cl2 chloramine residual for final distribution.

UV/H2O2 AOP: The low dose (3.1 mg/L H2O2) condition for the UV/H2O2 AOP achieved 0.5-log removal of 1,4-dioxane with ~1000 mJ/cm2 UV fluence during Event 1 (Figure 1). Previous research has indicated that ~0.3 kWh/kgal is needed to achieve 1-log removal of 1,4-dioxane in RO permeate containing 2 mg/L residual chloramines and 3 mg/L hydrogen peroxide (Hokanson, 2018), comparable to the low dose condition considered for this RO permeate. Accordingly, for 0.5-log removal of 1,4-dioxane, we estimated an energy input of ~0.15 kWh/kgal. For a $0.16/kWh price of electricity, the electricity cost would be $6.35/million liters (ML). The chemical cost for 3.1 mg/L H2O2 would be $4.51/million liters (ML), assuming $2.75/gallon for a 50% by weight hydrogen peroxide stock solution (City of Oxnard, 2018). The 2.8 mg/L residual H2O2 after UV exposure (Table 1) must be quenched by free chlorine at a 1:1 molar ratio. The cost of free chlorine would be $8.62/million liters (ML), assuming a price of $0.69/gallon for a 12.5% by weight solution of sodium hypochlorite (Mid American Regional Council, 2018). The total chloramine residual was 1.3 mg/L as Cl2 after UV exposure (Table 1) and thus an additional 1.1 mg/L NH2Cl would be needed for distribution. The additional chloramines could be added using 15.5 M sodium hypochlorite and 18.6 M ammonium sulfate, assuming a ~20% molar excess of ammonia added to form chloramines. The cost of 15.5 M sodium hypochlorite would be $1.68/million liters (ML) and the cost of 18.6 M ammonium sulfate would be $2.19/million liters (ML), assuming a price of $2.70/gallon for a 40% by weight solution of ammonium sulfate (City of San Leandro, 2018). Thus, the total cost for the UV/H2O2 AOP to treat 1 ML RO permeate was estimated to be $23.34/million liters (ML).

UV/free chlorine AOP: To achieve 0.5-log removal of 1,4-dioxane by the UV/free chlorine AOP

with 2.7 mg/L free chlorine (the low dose condition) would require ~770 mJ/cm2 fluence. The

electricity cost would be $4.88/million liters (ML). The cost of adding 2.7 mg/L free chlorine would

be $4.13/million liters (ML). We estimated a timescale between UV treatment and providing the

disinfectant residual at ~30 mins to permit other processes, such as decarbonation. After 30 mins, the

model predicted residuals of 0.82 mg/L as Cl2 free chlorine and 1.15 mg/L as Cl2 NHCl2 in the AOP

effluent. For addition of 7 M ammonium sulfate to convert 0.82 mg/L as Cl2 free chlorine to NH2Cl,

the cost would be $1.64/million liters (ML). An additional 0.5 mg/L as Cl2 NH2Cl would be needed

to reach a 2.5 mg/L as Cl2 residual. This cost would be $1.76/million liters (ML). Thus, the total

cost for the UV/free chlorine AOP to treat 1 ML RO permeate was estimated to be $12.41/million liters (ML).

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UV/chloramines AOP: UV/chloramines AOP treatment using 1000 mJ/cm2 fluence and 3.4 mg/L as

Cl2 background chloramines achieved only ~35% (0.19-log) removal of 1,4-dioxane, leaving

1.4 mg/L as Cl2 residual chloramines (Table 1). This removal is roughly half of the 68% removal

needed (0.5-log removal). The UV/chloramines AOP treatment becomes less efficient for chloramine

concentrations below 3 mg/L as Cl2 (e.g., compare the 1,4-dioxane removal for the ambient

conditions for Events 1 and 2 in Figs. 2 and S2). However, if the chloramine residual within the UV

reactor were boosted back from 1.4 mg/L as Cl2 to 3.4 mg/L as Cl2, 0.5-log removal of 1,4-dioxane

could be achieved by extending the UV fluence to a total of 2000 mJ/cm2. The electricity cost would

be $12.70/million liters (ML). The cost needed to boost the chloramine residual by 2 mg/L as Cl2

within the reactor and by 1.1 mg/L as Cl2 after treatment to achieve a 2.5 mg/L as Cl2 residual for

distribution would be $10.91/million liters (ML). Thus, the total cost for the UV/chloramines AOP

to treat 1 ML RO permeate was estimated to be $23.61/million liters (ML). This cost is comparable

to that of the UV/H2O2 AOP. However, the background chloramine concentration in the RO

permeate fluctuated. For Event 2, there was only 2.3 mg/L as Cl2 chloramines in the RO permeate

such that 1.1 mg/L as Cl2 monochloramine would need to be added prior to AOP treatment to bring

the chloramine residual to 3.4 mg/L as Cl2, an additional cost of $3.87/million liters (ML).

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