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Comparison of micropollutants' removal performance between pre-ozonation and post-ozonation using a pilot study
Accepted Manuscript Comparison of micropollutants' removal performance between pre-ozonation and post-ozonation using a pilot study Kai Yang, Jianwei Yu, Qingyuan Guo, Chunmiao Wang, Min Yang, Zhang Yu, Ping Xia, Dong Zhang, Zhiyong Yu PII:
S0043-1354(16)30987-3
DOI:
10.1016/j.watres.2016.12.043
Reference:
WR 12593
To appear in:
Water Research
Received Date: 1 September 2016 Revised Date:
23 December 2016
Accepted Date: 25 December 2016
Please cite this article as: Yang, K., Yu, J., Guo, Q., Wang, C., Yang, M., Yu, Z., Xia, P., Zhang, D., Yu, Z., Comparison of micropollutants' removal performance between pre-ozonation and post-ozonation using a pilot study, Water Research (2017), doi: 10.1016/j.watres.2016.12.043. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT 1
Comparison
of
micropollutants’
removal
performance
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pre-ozonation and post-ozonation using a pilot study
between
Kai Yang a, c, Jianwei Yu a, c, *, Qingyuan Guo a, c, Chunmiao Wang a,c, Min Yang b,c,
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Zhang Yu a, c, Ping Xia d, Dong Zhang d, Yu Zhiyong b
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a
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Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China.
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b
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Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China.
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c
University of the Chinese Academy of Sciences, Beijing 100019, China.
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d
Shanghai National Engineering Research Center of Urban Water Resources Co., Ltd.,
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Shanghai 200082, China.
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* Corresponding author, E-mail: [email protected]
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Keywords: micropollutants, pre-ozonation, post-ozonation, drinking water
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Abstract: Despite the strong oxidizing ability of ozone, pre-ozonation has seldom
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been employed for the purpose of micropollutant removal in drinking water utilities.
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In this paper, the possibility of using pre-ozonation instead of post-ozonation for the
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removal of micropollutants was explored because of the lower risk of forming
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carcinogenic bromate. A 1.0 m3/h pilot system was utilized to compare the efficacy of
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pre- and post-ozonation in the removal of bulk organic pollutants as well as
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micropollutants, including typical odor-causing compounds, pharmaceuticals, and
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typical pesticides, from one source water (Huangpu River) characterized by the
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occurrence of various micropollutants. Both pre-ozonation and post-ozonation could
Key Laboratory of Drinking Water Science and Technology, Research Center for
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State Key Laboratory of Environmental Aquatic Chemistry, Research Center for
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ACCEPTED MANUSCRIPT achieve similar water purification performance under an ozone dose of 1.5 mg/L, in
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terms of bulk water quality parameters like CODMn (66% in combination with
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biological activated carbon (BAC) treatment, compared to 62% with the
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pre-ozonation-BAC combination) or micropollutants including 27 pharmaceuticals
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(85% in combination with BAC compared to 87% with the pre-ozonation-BAC
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combination) and 25 pesticides (72% in combination with BAC compared to 61%
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with the pre-ozonation-BAC combination). Pre-ozonation exhibited slightly better
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odorant removal performance (100% in combination with BAC compared to 92%
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with the post-ozonation-BAC combination); however, post-ozonation generated
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approximately 6.0 µg/L bromate at an ozone dose of 2.0 mg/L, while pre-ozonation
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did not form bromate even at an ozone dose as high as 3.0 mg/L. So pre-ozonation in
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combination with BAC might be a solution for the removal of micropollutants from
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source water with high bromate formation risk. The results of this study will be
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helpful for the optimization of ozonation processes in the water supply industry.
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1. Introduction
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The occurrence of various micropollutants such as pharmaceuticals and
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pesticides in source water has caused wide concern over drinking water safety
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(Shannon et al. 2008, Luo et al. 2014). Removing micropollutants from source water
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has become increasingly important for water supply. Because of its high oxidation
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potential and ease of handling, ozone treatment has long been considered one of the
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most important technologies for the removal of micropollutants in drinking water
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treatment (Bundschuh and Schulz 2011, Wang et al. 2014). The formation of
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ACCEPTED MANUSCRIPT potentially carcinogenic bromate (BrO3–) in treating bromide-bearing source water,
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however, has caused serious concerns over the safety of ozonation (Myllykangas et al.
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2000). Though the addition of ammonia or hydrogen peroxide has been found to be
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effective in controlling the formation of bromate (von Gunten and Hoigne 1994,
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Wang et al. 2014), such measures may compromise the effectiveness of ozonation and
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makes the treatment process very complicated (von Gunten 2003).
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On the other hand, previous studies have shown that pre-ozonation, which
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introduces ozone before the coagulation unit, has the merits of producing less bromate
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in treating bromide-bearing source water (Pei et al. 2007). Pre-ozonation has been
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known for its ability in enhancing the coagulation process, leading to the increased
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removal of DBP precursors and algae through coagulation (Daldorph 1998) and
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extension of the filtration cycles of sand filters (Zhang et al. 2011). At the same time,
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in comparison with post-ozonation, pre-ozonation may proceed with higher fraction
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of hydroxyl radical (•OH) reactions because of the higher concentration of dissolved
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organic carbon (DOC) in raw water, which may favor the removal of micropollutants
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(von Sonntag and von Gunten 2012). Particularly, higher DOC concentration will
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restrain bromate formation (Allard et al. 2013) since NOM could compete with
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bromide for ozone/•OH in water (Westerhoff et al. 1998). However, few studies have
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explored the potential of pre-ozonation in the removal of micropollutants.
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The purpose of this study was to compare the efficacy of pre- and post-
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ozonation in the removal of micropollutants. The Huangpu River, an important source
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water for Shanghai, was selected for evaluation in this study. It is known for its 3
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septic/swampy odor occurrence and the presence of a variety of micropollutants
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because of pollution (Jia et al. 2010, Jiang et al. 2011, Guo et al. 2016). A pilot system
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consisting
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post-ozonation and BAC in succession was established, and used for comparison of
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the performance of pre- and post-ozonation in treating Huangpu River source water
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by operating the system in different modes: (pre-ozonation mode: pre-ozonation,
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coagulation, sedimentation, sand filtration and BAC in succession; post-ozonation
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mode: coagulation, sedimentation, sand filtration, ozonation and BAC in succession).
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A total of 120 micropollutants including typical odorants, pesticides, pharmaceuticals
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and perfluorinated compounds (PFCs) were selected as the target compounds for the
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evaluation. The results of this study will be helpful for the optimization of the
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ozonation process in drinking water treatment.
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2. Materials and methods
ozonation,
coagulation,
sedimentation,
sand
filtration,
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2.1 Information on the pilot system and experimental design The pilot system (1.0 m3/h; Fig. 1) consisting of pre-ozonation, coagulation,
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sedimentation, sand filtration, post-ozonation and BAC in succession was set up in a
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waterworks in Shanghai, fed with Huangpu River source water. Stainless steel
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columns were used as the pre-ozonation (4300 mm height, 150 mm i.d.) and
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post-ozonation (4300 mm height, 200 mm i.d.) contact reactors, respectively, and
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ozone gas generated from an ozonizer (3S-OW-20, Tonglin Technology Co. China)
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was admitted at the bottom of the columns through a porous titanium plate.
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Coagulation was conducted in a 1240×380×950 (mm), four-chamber, stainless steel
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sheet sedimentation tank: surface load of 3.6 m3/(m2·h); angle of 60°. A transparent
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plexiglass column (3000 mm height, 400 mm i.d.) filled with quartz sand of
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d10=0.798 mm (sand height, 1100 mm) was used as the sand filter. A stainless steel
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column (4500 mm height, 300 mm i.d.) filled with granular activated carbon
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(average particle size 1.25 mm, Shanxi Huaqing activated carbon Group, China) was
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used as the BAC container. The activated carbon was taken from the BAC tanks in
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the waterworks and had been put into use for at least one year, to ensure that the
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adsorption capacity of the activated carbon was almost depleted.
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The pilot system was operated from March 2014 to Feb 2015. Raw water was fed
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into the pre-ozonation reactor at a rate of 1.0 m3/h, with a contact time of 5 min.
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Aluminum sulfate (Al2 (SO4)3) coagulant was fed into the rapid mixing unit at a fixed
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dose of 40 mg/L (as Al2O3 concentration 8.0 mg/L) by a peristaltic pump, and the
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coagulation/flocculation conditions were as follows: rapid mixing at 150 rpm for 1
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min; then slow mixing at 100 rpm for 6 min, at 60 rpm for 6 min, and at 30 rpm for 6
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min. The sedimentation time was 30 min, and sludge discharge duration was 24 h by
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manual operation; the post-ozonation reactor contact time was 15 min; the sand
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filtration velocity was 8 m/h, and backwashing was performed once every day; the
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BAC filtration velocity was 8 m/h, and backwashing was performed once every 7
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days.
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In order to compare the performance of pre-ozonation and post-ozonation, the
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system was operated with two modes, ① pre-ozonation mode: pre-ozonation, 5
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mode: coagulation, sedimentation, sand filtration, ozonation and BAC in succession.
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The performance of each mode was evaluated under six ozone doses: 0, 0.5, 1.0, 1.5,
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2.0 and 3.0 mg/L, and the system was operated for at least one week under each
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condition. For the pre-ozonation mode, sampling points included raw water, sand
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filtration and BAC effluent, while for the post-ozonation mode, sampling points
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included raw water, post-ozonation and BAC effluent. For the determination of
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chemical oxygen demand by KMnO4 titration (CODMn), dissolved organic carbon
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(DOC), UV254 absorbance (UV254), bromide and bromate, samples were taken every
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two days under each ozonation condition. CODMn was determined right after
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sampling, while samples for DOC, UV254, bromide and bromate analyses were stored
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in a refrigerator after filtration with a glass fiber filter and then taken back to the
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laboratory for the analyses. Duplicate samples were taken and preconcentrated using
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solid phase extraction (SPE) for the determination of micropollutants under each
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ozonation condition, and then taken back to the laboratory for analysis.
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2.2. Sample preparation and analysis
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All reagents used in the experiment were of guaranteed grade, and all stock
solutions were prepared with Milli-Q water (Millipore). CODMn, DOC and UV254 were measured on a HACH Model DR2800
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spectrophotometer (HACH, USA), a TOC analyzer (TOC-VCPH, Shimadzu, Japan)
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and a spectrophotometer (T6, PGENERAL, China), respectively. Bromate and
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bromide were determined on an ion chromatography system (Dionex 2100, USA) 6
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using an AC9-SC column with a detection limit of 0.5 and 5.0 µg/L, respectively. Twenty-seven pharmaceuticals (for details see Table S1) were analyzed on an
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UPLC–MS/MS (1290 UPLC system, Agilent, USA) equipped with an SB-C18
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column (100 mm × 2.1 mm, 1.8 µm, Zorbax, Agilent, USA) after SPE
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preconcentration. Water samples (800 mL) filtered through a glass membrane (GF/F,
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Whatman, UK) were spiked with a mixture of four internal standards (400 ng
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SMN-13C6 for SAs, 400 ng OLF-D3 for FQs, 400 ng DMC for TCs, and 400 ng
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CAF-13C3 for other pharmaceuticals), and then extracted using Oasis HLB cartridges
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(6 mL, 500 mg, Waters, USA) at a flow rate of 3 mL/min, which were preconditioned
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sequentially with 5.0 mL of MeOH, 5.0 mL of 0.5 mol/L HCl, and 5.0 mL of Milli-Q
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water. After drying under vacuum, cartridges were eluted with 10 mL methanol. The
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eluate was collected in a 10 mL glass vial, dried under a gentle stream of N2, and then
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dissolved with 400 µL MeOH and 600 µL Milli-Q water. The resulting extract was
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centrifuged at 10,000 r/min for 6 min (Centrifuge 5418, Eppendorf, Germany), and
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the supernatant was filtered through 0.2 µm PES filters (PALL, USA), then analyzed
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according to a previously published method (Yuan et al. 2014).
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Twenty-five typical pesticides (for details see Table S2) were selected and
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determined using GC/MS after SPE preconcentration. Water samples (1000 mL)
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filtered through glass fiber filters (GF/C, Whatman) were preconcentrated using a
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combination of Oasis HLB and C18 cartridges (6 mL, 500 mg, Waters, USA). The
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C18 cartridge was placed on the top of the HLB one and both columns were
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preconditioned sequentially with 6.0 mL of methylene chloride, 6.0 mL of MeOH and 7
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with 10.0 mL of methylene chloride, respectively. The eluate was mixed, dried under
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a gentle stream of N2, and then dissolved with 1.0 mL hexane. Then the resulting
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extract was analyzed by GC/MS according to Yu et al (2011).
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The UPLC–MS/MS method was employed for the analysis of 14 perfluorinated
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compounds（PFCs）(for details see Table S3) as reported by Peng et al. (2010). Five
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hundred mL water samples filtered through glass fiber filters (GF/C, Whatman, UK)
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were spiked with a mixture of 2 internal standards (5 ng for
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13
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Waters, USA) cartridges at a flow rate of 1-2 drops/s. Each WAX cartridge was
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preconditioned with 6.0 mL of methanol containing 0.5% NH4OH, followed by 6.0
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mL of methanol and 6.0 mL of ultrapure water. After being dried with N2, the
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cartridges were eluted with 6.0 mL of methanol containing 0.5% NH4OH. The extract
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was dried under a gentle N2 stream and redissolved with 0.5 mL of methanol for
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UPLC–MS/MS analysis.
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C4-PFOS and
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C4-PFOA, respectively), and then passed through Oasis WAX (6 mL, 500 mg,
The flavor profile analysis (FPA) method was employed to characterize the odors
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according to the description in the Standard Methods for the Examination of Water
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and Wastewater (APHA 2012). Simultaneously, 54 odorants (for details see Table S4)
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were determined by comprehensive two dimensional gas chromatography with
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time-of-flight mass spectrometry (GC x GC-TOFMS) according to our previous study
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(Guo et al. 2015). Odor activity value (OAV), which was calculated by dividing the
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odorant concentration by the corresponding odor threshold concentration (OTC), was
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(Burdack-Freitag and Schieberle 2012).
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3. Results
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3.1 Removal of bulk organic parameters
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Tables S5, 6 and 7 summarize the variations of bulk organic parameters
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including DOC, UV254 and CODMn. Over the study period, the raw water CODMn
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ranged from 4.50 to 6.84 mg/L. The CODMn removal for the two operation modes is
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compared in Fig. 2. Here pre-ozonation performance was evaluated at the point of the
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sand filtration effluent since pre-ozonation has been considered to enhance
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coagulation (Owen et al. 1993). The CODMn removal was over 40% without ozone,
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showing that the combination of coagulation and sand filtration was quite effective for
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the removal of CODMn from Huangpu River source water. The CODMn removal was
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over 55% when pre-ozonation was applied at an ozone dose of 1.0 mg/L, and was
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almost unchanged until the ozone dose was increased to 3.0 mg/L. On the other hand,
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post-ozonation permitted slightly higher CODMn removal (60%) at an ozone dose of
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2.0 mg/L.
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Under both operational modes, further removal of CODMn was achieved by BAC
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treatment, as shown in Fig. 1. The overall CODMn removal was 60% or higher at an
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ozone dose of 1.0 for both operational modes, showing that both the
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pre-ozonation-BAC and post-ozonation-BAC combinations could satisfy the Chinese
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Drinking Water Standard (CODMn, 3 mg/L) (Wei et al. 2002). Under higher ozone
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doses, however, the difference between pre-ozonation and post-ozonation became 9
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in the post-ozonation mode, it was almost unchanged until the ozone dose was
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increased to 3.0 mg/L in the pre-ozonation mode. The DOC removal exhibited similar
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trends. It was assumed that the CODMn removal by BAC was achieved through
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biodegradation since the BAC column was filled with activated carbon used for over
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one year. So it was clear that the increased CODMn removal in the post-ozonation
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process under high ozone dose was mainly attributed to the improved biodegradability
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of the organic compounds in water. Pre-ozonation, on the other hand, was able to
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enhance the CODMn removal by coagulation (Pei et al. 2007), but its ability in
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improving the biodegradability of organic compounds might be limited because part
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of the ozone could be consumed by particles (Chandrakanth et al. 1996).
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3.2 Odors and odorants
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Huangpu River source water has long been known for its taste and odor
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problems (Guo et al. 2015, Guo et al. 2016). Two major odors, septic and
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earthy/musty ones, were detected with an FPA intensity ranging from 4.6- 8.6 and 3.0-
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8.0, respectively during the study period. As shown in Fig. S1, the musty odor was
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easily removed. With an ozone dose of 1.0 or higher, the musty FPA intensity was
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able to be controlled to 2.0 in the pre-ozonation mode. In general, it is thought to be
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acceptable for drinking water when the FPA level is lower than 3 (Suffet et al. 2004).
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When the ozone dose was increased to 2.0 mg/L, pre-ozonation allowed complete
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removal of the musty odor following BAC treatment. It was easier for the
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post-ozonation treatment to remove the musty odor: an ozone dose of 0.5 mg/L
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the treatment of the septic odor, the post-ozonation mode needed an ozone dose of 1.0
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mg/L to control the FPA intensity to 3.0 after ozonation, and an ozone dose of 3.0
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mg/L to completely remove the odor by the combination of ozone and BAC.
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Pre-ozonation at an ozone dose of 1.0 mg/L could control the FPA intensity to 2.0.
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However, further increase of ozone dose could not improve the odor removal
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performance. It is possible that some odorants adsorbed on the particles were released
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into water under high ozone doses.
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In this study, 9 major odorants, including Dimethyl disulfide (DMDS), pyrazine,
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thiazole, bis(2-chloroisopropyl) ether, 2-Methylisoborneol (2-MIB), p-xylene,
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ethylbenzene, 1,4-dichloro-benzene, and tetramethyl-pyrazine, were detected in
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source water, of which two (DMDS, bis(2-chloroisopropyl) ether) were associated
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with the septic odor, and one (2-MIB) with the musty odor. The OAVs of these 3
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odorants in raw water are shown in Fig. S2. Bis(2-chloroisopropyl) ether exhibited the
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highest average OAV of 2.95, followed by 2-MIB and dimethyl disulfide, with
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average OAV of 1.49 and 1.34, respectively. A compound with an OAV over 1.0 is
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considered to be important in constituting the odor profile (Pang et al. 2012). So
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bis(2-chloroisopropyl) ether and dimethyl disulfide should be the key odorants
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responsible for the septic odor, while 2-MIB should be the key odorant related to the
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musty odor. These detected septic odorants were reported in our previous study (Guo
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et al. 2016), while 2-MIB produced by Phormidium spp. was found to be the major
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musty odorant in Huangpu River source water in another study (Sun et al. 2013). Fig.
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odorants’ removal increased with the increase of ozone doses, though the removal was
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a little higher in the post-ozonation mode. Correspondingly, the septic and musty
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odors were reduced from an FPA intensity of 4.6-8.6 and 3.0-8.0 to intensity of lower
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than 3 at the applied ozone dose of 1.0 mg/L and above, respectively. Moreover,
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almost no odorants were detected after BAC (Fig. S3), indicating that they could be
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further removed by adsorption or biodegradation processes (Cook et al. 2001).
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3. 3 Pharmaceuticals
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Among the 27 pharmaceuticals, 14 were detected in the source water, the
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concentrations of which ranged from 0.03 to 200 ng/L. For example, caffeine varied
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between 46.8 and 237.7 ng/L, while the sulfamethoxazole (SMX) concentration
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ranged from 11.3 to 98.0 ng/L. As shown in Table 1, the pre- and post-ozonation
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processes exhibited similar performance in removing 8 major target contaminants.
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The percent removals of 6 in 8 major contaminants in both modes were more than
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88% at an ozone dose of 1.0 mg/L, except for caffeine and metoprolol; when the dose
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rose to 2.0 mg/L, the percent removals were more than 97.6%, except for caffeine.
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Post-ozonation was usually recommended as a suitable mitigation strategy for some
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typical micropollutants in previous studies (Broséus et al. 2009). However, this study
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shows that pre-ozonation might also be effective in the removal of pharmaceuticals.
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3.4 Pesticides
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Among
the
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monocrotophos,
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investigated
hexachlorozene
pesticides,
(HCB), 12
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dimethoate,
(dichlorvos, atrazine,
fenobucarb, chlorothaloni,
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ranging from 14.9 to 419.0 ng/L, and only dimethoate, atrazine, acetochlor and
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machete were detected in all the raw water samples. The pre- and post-ozonation
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modes exhibited similar pesticide removal performance at an ozone dose of 1.5 mg/L
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or lower: almost no pesticide removal was achieved at an ozone dose of 0.5 mg/L, and
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the pesticide removal was over 40% at an ozone dose of 1.5 mg/L (Fig. 4). While
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further increase of the ozone dose improved the post-ozonation performance, the
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effect of ozone dose increase was less effective for the pre-ozonation mode. The slow
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increase of pesticide removal at higher ozone doses in the pre-ozonation mode might
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be associated with the competitive effects of dissolved organic matte (DOM) (Ben et
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al. 2011). Even after BAC treatment, however, approximately 30% of pesticides were
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left in treated water in both operational modes, as shown in Fig. S5.
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3.5 PFCs
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Among the 14 investigated PFCs, the total PFC concentration was 320 ng/L,
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with PFOA as the major component (161 ng/L), followed by PFHxA (41 ng/L),
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PFHxS (33 ng/L) and PFBA (20 ng/L). As shown in Fig. S6, the PFCs could not be
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removed by the combination of ozonation and BAC effectively, which was in
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accordance with previous studies (Shivakoti et al. 2010). The AOPs such as O3,
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O3/H2O2 are ineffective for the removal of PFOA and PFOS due to their relatively
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slow reaction rates with OH radicals (Schröder and Meesters 2005). Limited studies
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have shown that the existing water purification processes are in general not effective
286
for the removal of PFOS and PFOA (Lein et al. 2008), though powdered activated
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(Ochoa-Herrera and Sierra-Alvarez 2008, Yu et al. 2014). So the removal method for
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PFCs should be further explored.
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3.6 Bromate formation
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During the pilot study, the bromide level ranged from 140.4 to 196.5µg/L in raw
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water. No bromate was produced with the pre-ozonation even under the highest ozone
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dose (3.0 mg/L). The bromate production with the post-ozonation mode is shown in
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Fig. 5. Approximately 6.0 and 14.0 µg/L bromate was produced under an ozone dose
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of 2.0 and 3.0 mg/L, respectively. This result indicates that post-ozonation should not
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be operated with an ozone dose above 2.0 mg/L for treating Huangpu River source
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water. Bromate formation may be impacted by different water quality parameters like
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dissolved organic carbon (DOC), alkalinity, and pH (von Gunten and Hoigne 1994).
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The low potential of pre-ozonation in producing bromate might be due to the higher
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level of background compounds including natural organic matters (NOM) which
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could compete with hypobromite to react with ozone (Guo et al. 2007). Previous
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studies have shown that bromate will only be considerably formed at a specific ozone
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dose≥0.4-0.6 mg O3/mg DOC (Soltermann et al. 2016). By adopting pre-ozonation,
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it will be easier to realize a relatively low specific ozone dose to control the formation
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of bromate. Another reason might be related with the short contact time (5 min) in
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comparison with the post-ozonation (15 min).
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4. Discussion
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For ozonation of micropollutants in drinking water, both molecular ozone and 14
ACCEPTED MANUSCRIPT •OH reaction could occur (Lee and von Gunten 2016). DOM can react with ozone and
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influence the ozone life time in drinking water. The decay of ozone in natural waters
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with higher DOC concentration is faster than that with lower concentration, and •OH
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would be continuously produced from a side reaction of ozone with DOC during the
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ozone decomposition in water (von Sonntag and von Gunten 2012). Thus, more •OH
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could be produced during pre-ozonation at the same ozone dose since the DOC
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concentration in raw water was approximately 30% higher than that in the sand filter
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filtrate. Although DOM will compete with micropollutants for •OH, the second order
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rate constants for the reaction of various DOM sources with •OH are reported to have
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an average value of 2.23 × 108 M-1S-1, lower than most of the reported micropollutants
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(1 × 109 to 2 × 1010 M-1S-1) (von Gunten 2003). Thus high ratio of •OH reaction
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toward micropollutants might have occurred in pre-ozonation process, leading to the
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relatively high removals of micropollutants in this study.
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Previous studies have indicated that the elimination efficacy of micropollutants is
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largely determined by the ozone and •OH second-order rate constants (Lee and von
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Gunten 2016). •OH can react rapidly with most pharmaceuticals with a similar rate
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constant (k•OH), for example, 6.9 × 109 M-1s-1 for caffeine, 8.8 × 109 M-1s-1 for
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carbamazepine, and 5.5 × 109 M-1s-1 for sulfamethoxazole (Dodd et al. 2006, Broséus
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et al. 2009, von Sonntag and von Gunten 2012). On the other hand, caffeine exhibited
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a low ozone reactivity (kO3: 650 ± 22 M-1s-1) in comparison with carbamazepine (kO3:
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~3 × 105 M-1s-1) and sulfamethoxazole (kO3: ~2.5 × 106 M-1s-1) (Broséus et al. 2009).
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So the relatively low caffeine elimination efficacy suggested that molecular ozone
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reaction might still be the dominant mechanism for the oxidation of micropollutants
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during pre-ozonation in spite of the relatively high DOC concentrations. Most pesticides with double bonds and activated aromatic systems are generally
334
expected to be destroyed by molecular ozone, while the rate constants cover a range
335
of more than five orders of magnitude (von Gunten 2003). However, for some
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pesticides like atrazine with the structure of an aromatic entities with heteroatoms, the
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reactivity with ozone was very low (kO3: 6 M-1s-1) (Acero et al. 2000, von Gunten
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2003). The atrazine removals were 65% and 77%, respectively, for pre- and
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post-ozonation even at an ozone dose of 3.0 mg/L. So advanced oxidation processes
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(AOPs) like ozone/H2O2 might be a good option for efficient removal of atrazine
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(Acero et al. 2000).
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A function of specific ozone dose (gO3/gDOC) was used to express the ozone
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exposure in previous study (Lee and von Gunten 2016). As shown in Table S8, similar
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removal performance of 34% and 35% for 2-MIB was achieved at a specific ozone
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dose of 0.32 and 0.31 gO3/gDOC in pre- and post-ozonated water, respectively. Due
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to the kinetics of the reaction of ozone with DOM strongly affects the efficiency of
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micropollutant transformation by ozone (Katsoyiannis et al. 2011), the abatement rate
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of a micropollutant could be different in different water matrices (von Sonntag and
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von Gunten 2012).
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As discussed above, both pre-ozonation and post-ozonation could achieve similar
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water purification performance in terms of bulk water quality parameters like CODMn
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or micropollutants like pharmaceuticals and pesticides under an ozone dose of 1.0-1.5 16
ACCEPTED MANUSCRIPT mg/L, though post-ozonation exhibited slightly better odor removal performance.
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However, pre-ozonation does not pose the risk of bromate formation. So a
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combination of pre-ozonation and post-ozonation should be explored for the control
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of micropollutants as well as odors without generating bromate formation risks in the
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future.
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5. Conclusion A comprehensive
comparison
was
conducted
for
pre-ozonation
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and
post-ozonation in terms of their performance in the removal of bulk water quality
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parameters and micropollutants and bromate formation potential using an on-site pilot
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system. Major conclusions are summarized as follows:
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(1) Both pre-ozonation and post-ozonation and their combination with BAC permitted similar CODMn removal performance under an ozone dose of 1.0-1.5 mg/L.
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(2) Both pre-ozonation and post-ozonation and their combination with BAC
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exhibited similar performance in removing pharmaceuticals and pesticides, while
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post-ozonation permitted slightly better performance in the removal of odors/odorants.
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However, the combination of ozonation and BAC was not effective in the removal of
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PFCs.
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(3) No bromate was produced during the pre-ozonation, while approximately 6
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µg/L bromate was formed during post-ozonation under an ozone dose of 2.0 mg/L.
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Acknowledgements
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This study was supported by Funds for the National Natural Science Foundation
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of China (No. 21377144), Major Science and Technology Program for Water 17
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Pollution Control and Treatment (No. 2015ZX07406001), and the “135” Major
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Project of Research Center for Eco-Environment Science (YSW2013A02).
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Corresponding Author
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* Tel: 86-10-62849149;
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Fax: 86-10-62923541;
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E-mail: [email protected]
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AUTHOR INFORMATION
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Westerhoff, P., Song, R., Amy, G. and Minear, R., 1998. NOM's role in bromine and bromate formation during ozonation. J Am Water Works Ass 90(2), 82-94.
Yu, J., He, C., Liu, X., Wu, J., Hu, Y. and Zhang, Y., 2014. Removal of perfluorinated compounds by membrane bioreactor with powdered activated carbon (PAC): Adsorption onto sludge and PAC. Desalination 334(1), 23-28. Yu, Z. 2011. Characteristics of the occurrence and health risk assessment of pesticides in drinking water and fish of China's major cities. Doctor, Graduate University of Chinese Academy of Sciences, Beijing. Yuan, X., Qiang, Z., Ben, W., Zhu, B. and Liu, J., 2014. Rapid detection of multiple class pharmaceuticals in both municipal wastewater and sludge with ultra high performance liquid 20
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chromatography tandem mass spectrometry. J. Environ. Sci. 26(9), 1949-1959. Zhang, J., Yang, M., Guo, Z., Yu, J., Yamaguchi, D., Zhang, X. and Chen, Y., 2011. Effects and mechanism of pre-ozonation on sand filtration performance. Ozone Sci. Eng. 33(1), 66-73.
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ACCEPTED MANUSCRIPT Table 1 Removal of 8 massive pharmaceuticals for pre- and post -ozonation (%) SDZ
CAF
TMP
SMN
MET
SMX
CBZ
ERY-H2O
Pre-O3 water
0.0 0.5 1.0 1.5 2.0 3.0
-2.78 66.83 100.00 100.00 100.00 100.00
3.14 28.37 71.45 69.40 74.37 91.51
7.06 62.51 98.48 98.84 100.00 100.00
2.90 59.74 100.00 100.00 100.00 100.00
-2.95 3.55 56.35 79.81 100.00 100.00
-12.49 62.55 98.37 97.47 97.38 98.59
0.95 52.00 100.00 100.00 100.00 100.00
-15.54 45.69 97.18 98.04 100.00 100.00
Post-O3 water
0.0 0.5 1.0 1.5 2.0 3.0
-12.72 97.95 100.00 100.00 100.00 100.00
-6.35 11.44 28.72 74.10 84.39 88.93
6.71 93.12 96.04 100.00 100.00 100.00
42.14 24.74 88.02 99.18 99.39 99.76
-10.15 23.57 62.59 100.00 100.00 100.00
-10.04 100.00 100.00 100.00 100.00 100.00
-31.36 95.63 100.00 100.00 100.00 100.00
-9.23 89.00 96.38 98.44 97.59 100.00
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dose
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Fig. 1 Flow chart of the pilot system
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1.0 Pre-O3 Post-O3 Pre-O3-BAC Post-O3-BAC 0.6
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COD/COD0
0.8
0.4
0.0 0.0
0.5
1.0
1.5
2.0
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0.2
2.5
3.0
Ozone dose(mg/L)
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Fig.2 COD/COD0 for pre-ozonation, post-ozonation and BAC effluent at different ozone doses. Pre-O3: pre-ozonated water; Post-O3: post-ozonated water; Pre-O3-BAC: BAC filtrate in pre-ozonation mode; Post-O3-BAC: BAC filtrate in post-ozonation mode. Contact times for pre- and post-ozonation were 5 and 15 min, respectively; DOC concentrations for pre- and post-ozonation were 4.38-5.35 mg/L and 3.28-4.62 mg/L, respectively.
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100
Pre-O3
Residual ratio (%)
Residual ratio (%)
80
60
40
20
0
0.5
1
1.5
2
80
60
40
20
0
3
(c)
0.5
1
1.5
2
3
Ozone dose(mg/L)
Ozone dose(mg/L)
Pre-O3 Post-O3
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80
60
40
20
0 0
0.5
1
1.5
2
Ozone dose(mg/L)
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Pre-O3
0
0
100
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Post-O3
Post-O3
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Fig. 3 Residual ratio of typical odorants in pre- and post-ozonated water. (a) DMDS, (b) bis (2-chloroisopropyl) ether, (c) 2-MIB. Pre-O3: pre-ozonated water; Post-O3:post-ozonated water. Contact times for pre- and post-ozonation were 5 and 15 min, respectively; Concentrations of DOC, DMDS, bis (2-chloroisopropyl) and 2-MIB for pre- and post-ozonation were 4.38-5.35 mg/L and 3.28-4.62 mg/L, 41.66-48.11 and 32.25-37.66 ng/L, 24.39-46.04 and 43.22-51.62 ng/L, 8.14-13.26 and 12.34-16.88 ng/L, respectively.
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100
Pre-O3
80
60
40
20
0 0
0.5
1
1.5
2
3
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Ozone dose(mg/L)
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Residual ratio (%)
Post-O3
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Fig. 4 Residual ratio of detected pesticides in pre- and post-ozonated water. Pre-O3: pre-ozonated water; Post-O3: post-ozonated water. Contact times for pre- and post-ozonation were 5 and 15 min, respectively; Concentrations of DOC for pre- and post-ozonation were 4.38-5.35 mg/L and 3.28-4.62 mg/L, respectively.
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25
20
-
Br -Pre Br -Post BrO3 -Pre
80
60
15
-
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BrO3 -Post 40
10
20
5
1.0
1.5
2.0
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0 0.5
Bromate (µg/L)
Risdual ratio of Br (%)
100
2.5
0
3.0
Ozone dose(mg/L)
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Fig. 5 Bromate formation after pre- and post-ozonation at different ozone doses. Br--Pre and Br--Post: Br- residue (%) in pre- and post-ozonated water; BrO3--Pre and BrO3--Post: BrO3- concentration in pre- and post-ozonated water. Contact times for pre- and post-ozonation were 5 and 15 min, respectively; Concentrations of DOC for pre- and post-ozonation were 4.38-5.35 mg/L and 3.28-4.62 mg/L, respectively.
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Highlights ·The efficacy of pre- and post- ozonation in the removal of micropollutants was
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compared ·Both pre- and post-ozonation could achieve similar water purification performance
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from source water with high bromate formation risk
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·Pre-ozonation combining with BAC might be a solution for micropollutant removal
S0043-1354(16)30987-3
DOI:
10.1016/j.watres.2016.12.043
Reference:
WR 12593
To appear in:
Water Research
Received Date: 1 September 2016 Revised Date:
23 December 2016
Accepted Date: 25 December 2016
Please cite this article as: Yang, K., Yu, J., Guo, Q., Wang, C., Yang, M., Yu, Z., Xia, P., Zhang, D., Yu, Z., Comparison of micropollutants' removal performance between pre-ozonation and post-ozonation using a pilot study, Water Research (2017), doi: 10.1016/j.watres.2016.12.043. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT 1
Comparison
of
micropollutants’
removal
performance
2
pre-ozonation and post-ozonation using a pilot study
between
Kai Yang a, c, Jianwei Yu a, c, *, Qingyuan Guo a, c, Chunmiao Wang a,c, Min Yang b,c,
4
Zhang Yu a, c, Ping Xia d, Dong Zhang d, Yu Zhiyong b
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3
5
a
6
Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China.
7
b
8
Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China.
9
c
University of the Chinese Academy of Sciences, Beijing 100019, China.
10
d
Shanghai National Engineering Research Center of Urban Water Resources Co., Ltd.,
11
Shanghai 200082, China.
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* Corresponding author, E-mail: [email protected]
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Keywords: micropollutants, pre-ozonation, post-ozonation, drinking water
14
Abstract: Despite the strong oxidizing ability of ozone, pre-ozonation has seldom
15
been employed for the purpose of micropollutant removal in drinking water utilities.
16
In this paper, the possibility of using pre-ozonation instead of post-ozonation for the
17
removal of micropollutants was explored because of the lower risk of forming
18
carcinogenic bromate. A 1.0 m3/h pilot system was utilized to compare the efficacy of
19
pre- and post-ozonation in the removal of bulk organic pollutants as well as
20
micropollutants, including typical odor-causing compounds, pharmaceuticals, and
21
typical pesticides, from one source water (Huangpu River) characterized by the
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occurrence of various micropollutants. Both pre-ozonation and post-ozonation could
Key Laboratory of Drinking Water Science and Technology, Research Center for
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State Key Laboratory of Environmental Aquatic Chemistry, Research Center for
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ACCEPTED MANUSCRIPT achieve similar water purification performance under an ozone dose of 1.5 mg/L, in
24
terms of bulk water quality parameters like CODMn (66% in combination with
25
biological activated carbon (BAC) treatment, compared to 62% with the
26
pre-ozonation-BAC combination) or micropollutants including 27 pharmaceuticals
27
(85% in combination with BAC compared to 87% with the pre-ozonation-BAC
28
combination) and 25 pesticides (72% in combination with BAC compared to 61%
29
with the pre-ozonation-BAC combination). Pre-ozonation exhibited slightly better
30
odorant removal performance (100% in combination with BAC compared to 92%
31
with the post-ozonation-BAC combination); however, post-ozonation generated
32
approximately 6.0 µg/L bromate at an ozone dose of 2.0 mg/L, while pre-ozonation
33
did not form bromate even at an ozone dose as high as 3.0 mg/L. So pre-ozonation in
34
combination with BAC might be a solution for the removal of micropollutants from
35
source water with high bromate formation risk. The results of this study will be
36
helpful for the optimization of ozonation processes in the water supply industry.
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1. Introduction
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The occurrence of various micropollutants such as pharmaceuticals and
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pesticides in source water has caused wide concern over drinking water safety
40
(Shannon et al. 2008, Luo et al. 2014). Removing micropollutants from source water
41
has become increasingly important for water supply. Because of its high oxidation
42
potential and ease of handling, ozone treatment has long been considered one of the
43
most important technologies for the removal of micropollutants in drinking water
44
treatment (Bundschuh and Schulz 2011, Wang et al. 2014). The formation of
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ACCEPTED MANUSCRIPT potentially carcinogenic bromate (BrO3–) in treating bromide-bearing source water,
46
however, has caused serious concerns over the safety of ozonation (Myllykangas et al.
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2000). Though the addition of ammonia or hydrogen peroxide has been found to be
48
effective in controlling the formation of bromate (von Gunten and Hoigne 1994,
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Wang et al. 2014), such measures may compromise the effectiveness of ozonation and
50
makes the treatment process very complicated (von Gunten 2003).
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On the other hand, previous studies have shown that pre-ozonation, which
52
introduces ozone before the coagulation unit, has the merits of producing less bromate
53
in treating bromide-bearing source water (Pei et al. 2007). Pre-ozonation has been
54
known for its ability in enhancing the coagulation process, leading to the increased
55
removal of DBP precursors and algae through coagulation (Daldorph 1998) and
56
extension of the filtration cycles of sand filters (Zhang et al. 2011). At the same time,
57
in comparison with post-ozonation, pre-ozonation may proceed with higher fraction
58
of hydroxyl radical (•OH) reactions because of the higher concentration of dissolved
59
organic carbon (DOC) in raw water, which may favor the removal of micropollutants
60
(von Sonntag and von Gunten 2012). Particularly, higher DOC concentration will
61
restrain bromate formation (Allard et al. 2013) since NOM could compete with
62
bromide for ozone/•OH in water (Westerhoff et al. 1998). However, few studies have
63
explored the potential of pre-ozonation in the removal of micropollutants.
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The purpose of this study was to compare the efficacy of pre- and post-
65
ozonation in the removal of micropollutants. The Huangpu River, an important source
66
water for Shanghai, was selected for evaluation in this study. It is known for its 3
ACCEPTED MANUSCRIPT 67
septic/swampy odor occurrence and the presence of a variety of micropollutants
68
because of pollution (Jia et al. 2010, Jiang et al. 2011, Guo et al. 2016). A pilot system
69
consisting
70
post-ozonation and BAC in succession was established, and used for comparison of
71
the performance of pre- and post-ozonation in treating Huangpu River source water
72
by operating the system in different modes: (pre-ozonation mode: pre-ozonation,
73
coagulation, sedimentation, sand filtration and BAC in succession; post-ozonation
74
mode: coagulation, sedimentation, sand filtration, ozonation and BAC in succession).
75
A total of 120 micropollutants including typical odorants, pesticides, pharmaceuticals
76
and perfluorinated compounds (PFCs) were selected as the target compounds for the
77
evaluation. The results of this study will be helpful for the optimization of the
78
ozonation process in drinking water treatment.
79
2. Materials and methods
ozonation,
coagulation,
sedimentation,
sand
filtration,
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2.1 Information on the pilot system and experimental design The pilot system (1.0 m3/h; Fig. 1) consisting of pre-ozonation, coagulation,
82
sedimentation, sand filtration, post-ozonation and BAC in succession was set up in a
83
waterworks in Shanghai, fed with Huangpu River source water. Stainless steel
84
columns were used as the pre-ozonation (4300 mm height, 150 mm i.d.) and
85
post-ozonation (4300 mm height, 200 mm i.d.) contact reactors, respectively, and
86
ozone gas generated from an ozonizer (3S-OW-20, Tonglin Technology Co. China)
87
was admitted at the bottom of the columns through a porous titanium plate.
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Coagulation was conducted in a 1240×380×950 (mm), four-chamber, stainless steel
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90
sheet sedimentation tank: surface load of 3.6 m3/(m2·h); angle of 60°. A transparent
91
plexiglass column (3000 mm height, 400 mm i.d.) filled with quartz sand of
92
d10=0.798 mm (sand height, 1100 mm) was used as the sand filter. A stainless steel
93
column (4500 mm height, 300 mm i.d.) filled with granular activated carbon
94
(average particle size 1.25 mm, Shanxi Huaqing activated carbon Group, China) was
95
used as the BAC container. The activated carbon was taken from the BAC tanks in
96
the waterworks and had been put into use for at least one year, to ensure that the
97
adsorption capacity of the activated carbon was almost depleted.
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The pilot system was operated from March 2014 to Feb 2015. Raw water was fed
99
into the pre-ozonation reactor at a rate of 1.0 m3/h, with a contact time of 5 min.
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Aluminum sulfate (Al2 (SO4)3) coagulant was fed into the rapid mixing unit at a fixed
101
dose of 40 mg/L (as Al2O3 concentration 8.0 mg/L) by a peristaltic pump, and the
102
coagulation/flocculation conditions were as follows: rapid mixing at 150 rpm for 1
103
min; then slow mixing at 100 rpm for 6 min, at 60 rpm for 6 min, and at 30 rpm for 6
104
min. The sedimentation time was 30 min, and sludge discharge duration was 24 h by
105
manual operation; the post-ozonation reactor contact time was 15 min; the sand
106
filtration velocity was 8 m/h, and backwashing was performed once every day; the
107
BAC filtration velocity was 8 m/h, and backwashing was performed once every 7
108
days.
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In order to compare the performance of pre-ozonation and post-ozonation, the
110
system was operated with two modes, ① pre-ozonation mode: pre-ozonation, 5
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112
mode: coagulation, sedimentation, sand filtration, ozonation and BAC in succession.
113
The performance of each mode was evaluated under six ozone doses: 0, 0.5, 1.0, 1.5,
114
2.0 and 3.0 mg/L, and the system was operated for at least one week under each
115
condition. For the pre-ozonation mode, sampling points included raw water, sand
116
filtration and BAC effluent, while for the post-ozonation mode, sampling points
117
included raw water, post-ozonation and BAC effluent. For the determination of
118
chemical oxygen demand by KMnO4 titration (CODMn), dissolved organic carbon
119
(DOC), UV254 absorbance (UV254), bromide and bromate, samples were taken every
120
two days under each ozonation condition. CODMn was determined right after
121
sampling, while samples for DOC, UV254, bromide and bromate analyses were stored
122
in a refrigerator after filtration with a glass fiber filter and then taken back to the
123
laboratory for the analyses. Duplicate samples were taken and preconcentrated using
124
solid phase extraction (SPE) for the determination of micropollutants under each
125
ozonation condition, and then taken back to the laboratory for analysis.
126
2.2. Sample preparation and analysis
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All reagents used in the experiment were of guaranteed grade, and all stock
solutions were prepared with Milli-Q water (Millipore). CODMn, DOC and UV254 were measured on a HACH Model DR2800
130
spectrophotometer (HACH, USA), a TOC analyzer (TOC-VCPH, Shimadzu, Japan)
131
and a spectrophotometer (T6, PGENERAL, China), respectively. Bromate and
132
bromide were determined on an ion chromatography system (Dionex 2100, USA) 6
ACCEPTED MANUSCRIPT 133
using an AC9-SC column with a detection limit of 0.5 and 5.0 µg/L, respectively. Twenty-seven pharmaceuticals (for details see Table S1) were analyzed on an
135
UPLC–MS/MS (1290 UPLC system, Agilent, USA) equipped with an SB-C18
136
column (100 mm × 2.1 mm, 1.8 µm, Zorbax, Agilent, USA) after SPE
137
preconcentration. Water samples (800 mL) filtered through a glass membrane (GF/F,
138
Whatman, UK) were spiked with a mixture of four internal standards (400 ng
139
SMN-13C6 for SAs, 400 ng OLF-D3 for FQs, 400 ng DMC for TCs, and 400 ng
140
CAF-13C3 for other pharmaceuticals), and then extracted using Oasis HLB cartridges
141
(6 mL, 500 mg, Waters, USA) at a flow rate of 3 mL/min, which were preconditioned
142
sequentially with 5.0 mL of MeOH, 5.0 mL of 0.5 mol/L HCl, and 5.0 mL of Milli-Q
143
water. After drying under vacuum, cartridges were eluted with 10 mL methanol. The
144
eluate was collected in a 10 mL glass vial, dried under a gentle stream of N2, and then
145
dissolved with 400 µL MeOH and 600 µL Milli-Q water. The resulting extract was
146
centrifuged at 10,000 r/min for 6 min (Centrifuge 5418, Eppendorf, Germany), and
147
the supernatant was filtered through 0.2 µm PES filters (PALL, USA), then analyzed
148
according to a previously published method (Yuan et al. 2014).
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Twenty-five typical pesticides (for details see Table S2) were selected and
150
determined using GC/MS after SPE preconcentration. Water samples (1000 mL)
151
filtered through glass fiber filters (GF/C, Whatman) were preconcentrated using a
152
combination of Oasis HLB and C18 cartridges (6 mL, 500 mg, Waters, USA). The
153
C18 cartridge was placed on the top of the HLB one and both columns were
154
preconditioned sequentially with 6.0 mL of methylene chloride, 6.0 mL of MeOH and 7
ACCEPTED MANUSCRIPT 6.0 mL of ultrapure water. After being dried under vacuum, the cartridges were eluted
156
with 10.0 mL of methylene chloride, respectively. The eluate was mixed, dried under
157
a gentle stream of N2, and then dissolved with 1.0 mL hexane. Then the resulting
158
extract was analyzed by GC/MS according to Yu et al (2011).
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The UPLC–MS/MS method was employed for the analysis of 14 perfluorinated
160
compounds（PFCs）(for details see Table S3) as reported by Peng et al. (2010). Five
161
hundred mL water samples filtered through glass fiber filters (GF/C, Whatman, UK)
162
were spiked with a mixture of 2 internal standards (5 ng for
163
13
164
Waters, USA) cartridges at a flow rate of 1-2 drops/s. Each WAX cartridge was
165
preconditioned with 6.0 mL of methanol containing 0.5% NH4OH, followed by 6.0
166
mL of methanol and 6.0 mL of ultrapure water. After being dried with N2, the
167
cartridges were eluted with 6.0 mL of methanol containing 0.5% NH4OH. The extract
168
was dried under a gentle N2 stream and redissolved with 0.5 mL of methanol for
169
UPLC–MS/MS analysis.
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C4-PFOS and
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The flavor profile analysis (FPA) method was employed to characterize the odors
171
according to the description in the Standard Methods for the Examination of Water
172
and Wastewater (APHA 2012). Simultaneously, 54 odorants (for details see Table S4)
173
were determined by comprehensive two dimensional gas chromatography with
174
time-of-flight mass spectrometry (GC x GC-TOFMS) according to our previous study
175
(Guo et al. 2015). Odor activity value (OAV), which was calculated by dividing the
176
odorant concentration by the corresponding odor threshold concentration (OTC), was
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178
(Burdack-Freitag and Schieberle 2012).
179
3. Results
180
3.1 Removal of bulk organic parameters
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Tables S5, 6 and 7 summarize the variations of bulk organic parameters
182
including DOC, UV254 and CODMn. Over the study period, the raw water CODMn
183
ranged from 4.50 to 6.84 mg/L. The CODMn removal for the two operation modes is
184
compared in Fig. 2. Here pre-ozonation performance was evaluated at the point of the
185
sand filtration effluent since pre-ozonation has been considered to enhance
186
coagulation (Owen et al. 1993). The CODMn removal was over 40% without ozone,
187
showing that the combination of coagulation and sand filtration was quite effective for
188
the removal of CODMn from Huangpu River source water. The CODMn removal was
189
over 55% when pre-ozonation was applied at an ozone dose of 1.0 mg/L, and was
190
almost unchanged until the ozone dose was increased to 3.0 mg/L. On the other hand,
191
post-ozonation permitted slightly higher CODMn removal (60%) at an ozone dose of
192
2.0 mg/L.
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Under both operational modes, further removal of CODMn was achieved by BAC
194
treatment, as shown in Fig. 1. The overall CODMn removal was 60% or higher at an
195
ozone dose of 1.0 for both operational modes, showing that both the
196
pre-ozonation-BAC and post-ozonation-BAC combinations could satisfy the Chinese
197
Drinking Water Standard (CODMn, 3 mg/L) (Wei et al. 2002). Under higher ozone
198
doses, however, the difference between pre-ozonation and post-ozonation became 9
ACCEPTED MANUSCRIPT obvious: while the overall CODMn removal increased with the increase of ozone dose
200
in the post-ozonation mode, it was almost unchanged until the ozone dose was
201
increased to 3.0 mg/L in the pre-ozonation mode. The DOC removal exhibited similar
202
trends. It was assumed that the CODMn removal by BAC was achieved through
203
biodegradation since the BAC column was filled with activated carbon used for over
204
one year. So it was clear that the increased CODMn removal in the post-ozonation
205
process under high ozone dose was mainly attributed to the improved biodegradability
206
of the organic compounds in water. Pre-ozonation, on the other hand, was able to
207
enhance the CODMn removal by coagulation (Pei et al. 2007), but its ability in
208
improving the biodegradability of organic compounds might be limited because part
209
of the ozone could be consumed by particles (Chandrakanth et al. 1996).
210
3.2 Odors and odorants
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Huangpu River source water has long been known for its taste and odor
212
problems (Guo et al. 2015, Guo et al. 2016). Two major odors, septic and
213
earthy/musty ones, were detected with an FPA intensity ranging from 4.6- 8.6 and 3.0-
214
8.0, respectively during the study period. As shown in Fig. S1, the musty odor was
215
easily removed. With an ozone dose of 1.0 or higher, the musty FPA intensity was
216
able to be controlled to 2.0 in the pre-ozonation mode. In general, it is thought to be
217
acceptable for drinking water when the FPA level is lower than 3 (Suffet et al. 2004).
218
When the ozone dose was increased to 2.0 mg/L, pre-ozonation allowed complete
219
removal of the musty odor following BAC treatment. It was easier for the
220
post-ozonation treatment to remove the musty odor: an ozone dose of 0.5 mg/L
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222
the treatment of the septic odor, the post-ozonation mode needed an ozone dose of 1.0
223
mg/L to control the FPA intensity to 3.0 after ozonation, and an ozone dose of 3.0
224
mg/L to completely remove the odor by the combination of ozone and BAC.
225
Pre-ozonation at an ozone dose of 1.0 mg/L could control the FPA intensity to 2.0.
226
However, further increase of ozone dose could not improve the odor removal
227
performance. It is possible that some odorants adsorbed on the particles were released
228
into water under high ozone doses.
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In this study, 9 major odorants, including Dimethyl disulfide (DMDS), pyrazine,
230
thiazole, bis(2-chloroisopropyl) ether, 2-Methylisoborneol (2-MIB), p-xylene,
231
ethylbenzene, 1,4-dichloro-benzene, and tetramethyl-pyrazine, were detected in
232
source water, of which two (DMDS, bis(2-chloroisopropyl) ether) were associated
233
with the septic odor, and one (2-MIB) with the musty odor. The OAVs of these 3
234
odorants in raw water are shown in Fig. S2. Bis(2-chloroisopropyl) ether exhibited the
235
highest average OAV of 2.95, followed by 2-MIB and dimethyl disulfide, with
236
average OAV of 1.49 and 1.34, respectively. A compound with an OAV over 1.0 is
237
considered to be important in constituting the odor profile (Pang et al. 2012). So
238
bis(2-chloroisopropyl) ether and dimethyl disulfide should be the key odorants
239
responsible for the septic odor, while 2-MIB should be the key odorant related to the
240
musty odor. These detected septic odorants were reported in our previous study (Guo
241
et al. 2016), while 2-MIB produced by Phormidium spp. was found to be the major
242
musty odorant in Huangpu River source water in another study (Sun et al. 2013). Fig.
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ACCEPTED MANUSCRIPT 3 shows the removals of the two septic odorants and 2-MIB. In both modes, all the
244
odorants’ removal increased with the increase of ozone doses, though the removal was
245
a little higher in the post-ozonation mode. Correspondingly, the septic and musty
246
odors were reduced from an FPA intensity of 4.6-8.6 and 3.0-8.0 to intensity of lower
247
than 3 at the applied ozone dose of 1.0 mg/L and above, respectively. Moreover,
248
almost no odorants were detected after BAC (Fig. S3), indicating that they could be
249
further removed by adsorption or biodegradation processes (Cook et al. 2001).
250
3. 3 Pharmaceuticals
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Among the 27 pharmaceuticals, 14 were detected in the source water, the
252
concentrations of which ranged from 0.03 to 200 ng/L. For example, caffeine varied
253
between 46.8 and 237.7 ng/L, while the sulfamethoxazole (SMX) concentration
254
ranged from 11.3 to 98.0 ng/L. As shown in Table 1, the pre- and post-ozonation
255
processes exhibited similar performance in removing 8 major target contaminants.
256
The percent removals of 6 in 8 major contaminants in both modes were more than
257
88% at an ozone dose of 1.0 mg/L, except for caffeine and metoprolol; when the dose
258
rose to 2.0 mg/L, the percent removals were more than 97.6%, except for caffeine.
259
Post-ozonation was usually recommended as a suitable mitigation strategy for some
260
typical micropollutants in previous studies (Broséus et al. 2009). However, this study
261
shows that pre-ozonation might also be effective in the removal of pharmaceuticals.
262
3.4 Pesticides
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Among
the
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monocrotophos,
25
investigated
hexachlorozene
pesticides,
(HCB), 12
10
dimethoate,
(dichlorvos, atrazine,
fenobucarb, chlorothaloni,
ACCEPTED MANUSCRIPT acetochlor, chlorpyrifos and machete) were detected in raw water, with concentrations
266
ranging from 14.9 to 419.0 ng/L, and only dimethoate, atrazine, acetochlor and
267
machete were detected in all the raw water samples. The pre- and post-ozonation
268
modes exhibited similar pesticide removal performance at an ozone dose of 1.5 mg/L
269
or lower: almost no pesticide removal was achieved at an ozone dose of 0.5 mg/L, and
270
the pesticide removal was over 40% at an ozone dose of 1.5 mg/L (Fig. 4). While
271
further increase of the ozone dose improved the post-ozonation performance, the
272
effect of ozone dose increase was less effective for the pre-ozonation mode. The slow
273
increase of pesticide removal at higher ozone doses in the pre-ozonation mode might
274
be associated with the competitive effects of dissolved organic matte (DOM) (Ben et
275
al. 2011). Even after BAC treatment, however, approximately 30% of pesticides were
276
left in treated water in both operational modes, as shown in Fig. S5.
277
3.5 PFCs
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Among the 14 investigated PFCs, the total PFC concentration was 320 ng/L,
279
with PFOA as the major component (161 ng/L), followed by PFHxA (41 ng/L),
280
PFHxS (33 ng/L) and PFBA (20 ng/L). As shown in Fig. S6, the PFCs could not be
281
removed by the combination of ozonation and BAC effectively, which was in
282
accordance with previous studies (Shivakoti et al. 2010). The AOPs such as O3,
283
O3/H2O2 are ineffective for the removal of PFOA and PFOS due to their relatively
284
slow reaction rates with OH radicals (Schröder and Meesters 2005). Limited studies
285
have shown that the existing water purification processes are in general not effective
286
for the removal of PFOS and PFOA (Lein et al. 2008), though powdered activated
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288
(Ochoa-Herrera and Sierra-Alvarez 2008, Yu et al. 2014). So the removal method for
289
PFCs should be further explored.
290
3.6 Bromate formation
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During the pilot study, the bromide level ranged from 140.4 to 196.5µg/L in raw
292
water. No bromate was produced with the pre-ozonation even under the highest ozone
293
dose (3.0 mg/L). The bromate production with the post-ozonation mode is shown in
294
Fig. 5. Approximately 6.0 and 14.0 µg/L bromate was produced under an ozone dose
295
of 2.0 and 3.0 mg/L, respectively. This result indicates that post-ozonation should not
296
be operated with an ozone dose above 2.0 mg/L for treating Huangpu River source
297
water. Bromate formation may be impacted by different water quality parameters like
298
dissolved organic carbon (DOC), alkalinity, and pH (von Gunten and Hoigne 1994).
299
The low potential of pre-ozonation in producing bromate might be due to the higher
300
level of background compounds including natural organic matters (NOM) which
301
could compete with hypobromite to react with ozone (Guo et al. 2007). Previous
302
studies have shown that bromate will only be considerably formed at a specific ozone
303
dose≥0.4-0.6 mg O3/mg DOC (Soltermann et al. 2016). By adopting pre-ozonation,
304
it will be easier to realize a relatively low specific ozone dose to control the formation
305
of bromate. Another reason might be related with the short contact time (5 min) in
306
comparison with the post-ozonation (15 min).
307
4. Discussion
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For ozonation of micropollutants in drinking water, both molecular ozone and 14
ACCEPTED MANUSCRIPT •OH reaction could occur (Lee and von Gunten 2016). DOM can react with ozone and
310
influence the ozone life time in drinking water. The decay of ozone in natural waters
311
with higher DOC concentration is faster than that with lower concentration, and •OH
312
would be continuously produced from a side reaction of ozone with DOC during the
313
ozone decomposition in water (von Sonntag and von Gunten 2012). Thus, more •OH
314
could be produced during pre-ozonation at the same ozone dose since the DOC
315
concentration in raw water was approximately 30% higher than that in the sand filter
316
filtrate. Although DOM will compete with micropollutants for •OH, the second order
317
rate constants for the reaction of various DOM sources with •OH are reported to have
318
an average value of 2.23 × 108 M-1S-1, lower than most of the reported micropollutants
319
(1 × 109 to 2 × 1010 M-1S-1) (von Gunten 2003). Thus high ratio of •OH reaction
320
toward micropollutants might have occurred in pre-ozonation process, leading to the
321
relatively high removals of micropollutants in this study.
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Previous studies have indicated that the elimination efficacy of micropollutants is
323
largely determined by the ozone and •OH second-order rate constants (Lee and von
324
Gunten 2016). •OH can react rapidly with most pharmaceuticals with a similar rate
325
constant (k•OH), for example, 6.9 × 109 M-1s-1 for caffeine, 8.8 × 109 M-1s-1 for
326
carbamazepine, and 5.5 × 109 M-1s-1 for sulfamethoxazole (Dodd et al. 2006, Broséus
327
et al. 2009, von Sonntag and von Gunten 2012). On the other hand, caffeine exhibited
328
a low ozone reactivity (kO3: 650 ± 22 M-1s-1) in comparison with carbamazepine (kO3:
329
~3 × 105 M-1s-1) and sulfamethoxazole (kO3: ~2.5 × 106 M-1s-1) (Broséus et al. 2009).
330
So the relatively low caffeine elimination efficacy suggested that molecular ozone
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reaction might still be the dominant mechanism for the oxidation of micropollutants
332
during pre-ozonation in spite of the relatively high DOC concentrations. Most pesticides with double bonds and activated aromatic systems are generally
334
expected to be destroyed by molecular ozone, while the rate constants cover a range
335
of more than five orders of magnitude (von Gunten 2003). However, for some
336
pesticides like atrazine with the structure of an aromatic entities with heteroatoms, the
337
reactivity with ozone was very low (kO3: 6 M-1s-1) (Acero et al. 2000, von Gunten
338
2003). The atrazine removals were 65% and 77%, respectively, for pre- and
339
post-ozonation even at an ozone dose of 3.0 mg/L. So advanced oxidation processes
340
(AOPs) like ozone/H2O2 might be a good option for efficient removal of atrazine
341
(Acero et al. 2000).
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A function of specific ozone dose (gO3/gDOC) was used to express the ozone
343
exposure in previous study (Lee and von Gunten 2016). As shown in Table S8, similar
344
removal performance of 34% and 35% for 2-MIB was achieved at a specific ozone
345
dose of 0.32 and 0.31 gO3/gDOC in pre- and post-ozonated water, respectively. Due
346
to the kinetics of the reaction of ozone with DOM strongly affects the efficiency of
347
micropollutant transformation by ozone (Katsoyiannis et al. 2011), the abatement rate
348
of a micropollutant could be different in different water matrices (von Sonntag and
349
von Gunten 2012).
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As discussed above, both pre-ozonation and post-ozonation could achieve similar
351
water purification performance in terms of bulk water quality parameters like CODMn
352
or micropollutants like pharmaceuticals and pesticides under an ozone dose of 1.0-1.5 16
ACCEPTED MANUSCRIPT mg/L, though post-ozonation exhibited slightly better odor removal performance.
354
However, pre-ozonation does not pose the risk of bromate formation. So a
355
combination of pre-ozonation and post-ozonation should be explored for the control
356
of micropollutants as well as odors without generating bromate formation risks in the
357
future.
358
5. Conclusion A comprehensive
comparison
was
conducted
for
pre-ozonation
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and
post-ozonation in terms of their performance in the removal of bulk water quality
361
parameters and micropollutants and bromate formation potential using an on-site pilot
362
system. Major conclusions are summarized as follows:
364
(1) Both pre-ozonation and post-ozonation and their combination with BAC permitted similar CODMn removal performance under an ozone dose of 1.0-1.5 mg/L.
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(2) Both pre-ozonation and post-ozonation and their combination with BAC
366
exhibited similar performance in removing pharmaceuticals and pesticides, while
367
post-ozonation permitted slightly better performance in the removal of odors/odorants.
368
However, the combination of ozonation and BAC was not effective in the removal of
369
PFCs.
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(3) No bromate was produced during the pre-ozonation, while approximately 6
371
µg/L bromate was formed during post-ozonation under an ozone dose of 2.0 mg/L.
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Acknowledgements
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This study was supported by Funds for the National Natural Science Foundation
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of China (No. 21377144), Major Science and Technology Program for Water 17
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Pollution Control and Treatment (No. 2015ZX07406001), and the “135” Major
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Project of Research Center for Eco-Environment Science (YSW2013A02).
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Corresponding Author
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* Tel: 86-10-62849149;
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Fax: 86-10-62923541;
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E-mail: [email protected]
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AUTHOR INFORMATION
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ACCEPTED MANUSCRIPT Table 1 Removal of 8 massive pharmaceuticals for pre- and post -ozonation (%) SDZ
CAF
TMP
SMN
MET
SMX
CBZ
ERY-H2O
Pre-O3 water
0.0 0.5 1.0 1.5 2.0 3.0
-2.78 66.83 100.00 100.00 100.00 100.00
3.14 28.37 71.45 69.40 74.37 91.51
7.06 62.51 98.48 98.84 100.00 100.00
2.90 59.74 100.00 100.00 100.00 100.00
-2.95 3.55 56.35 79.81 100.00 100.00
-12.49 62.55 98.37 97.47 97.38 98.59
0.95 52.00 100.00 100.00 100.00 100.00
-15.54 45.69 97.18 98.04 100.00 100.00
Post-O3 water
0.0 0.5 1.0 1.5 2.0 3.0
-12.72 97.95 100.00 100.00 100.00 100.00
-6.35 11.44 28.72 74.10 84.39 88.93
6.71 93.12 96.04 100.00 100.00 100.00
42.14 24.74 88.02 99.18 99.39 99.76
-10.15 23.57 62.59 100.00 100.00 100.00
-10.04 100.00 100.00 100.00 100.00 100.00
-31.36 95.63 100.00 100.00 100.00 100.00
-9.23 89.00 96.38 98.44 97.59 100.00
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Fig. 1 Flow chart of the pilot system
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1.0 Pre-O3 Post-O3 Pre-O3-BAC Post-O3-BAC 0.6
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COD/COD0
0.8
0.4
0.0 0.0
0.5
1.0
1.5
2.0
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0.2
2.5
3.0
Ozone dose(mg/L)
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Fig.2 COD/COD0 for pre-ozonation, post-ozonation and BAC effluent at different ozone doses. Pre-O3: pre-ozonated water; Post-O3: post-ozonated water; Pre-O3-BAC: BAC filtrate in pre-ozonation mode; Post-O3-BAC: BAC filtrate in post-ozonation mode. Contact times for pre- and post-ozonation were 5 and 15 min, respectively; DOC concentrations for pre- and post-ozonation were 4.38-5.35 mg/L and 3.28-4.62 mg/L, respectively.
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100
Pre-O3
Residual ratio (%)
Residual ratio (%)
80
60
40
20
0
0.5
1
1.5
2
80
60
40
20
0
3
(c)
0.5
1
1.5
2
3
Ozone dose(mg/L)
Ozone dose(mg/L)
Pre-O3 Post-O3
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40
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0 0
0.5
1
1.5
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Ozone dose(mg/L)
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Pre-O3
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100
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Post-O3
Post-O3
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Fig. 3 Residual ratio of typical odorants in pre- and post-ozonated water. (a) DMDS, (b) bis (2-chloroisopropyl) ether, (c) 2-MIB. Pre-O3: pre-ozonated water; Post-O3:post-ozonated water. Contact times for pre- and post-ozonation were 5 and 15 min, respectively; Concentrations of DOC, DMDS, bis (2-chloroisopropyl) and 2-MIB for pre- and post-ozonation were 4.38-5.35 mg/L and 3.28-4.62 mg/L, 41.66-48.11 and 32.25-37.66 ng/L, 24.39-46.04 and 43.22-51.62 ng/L, 8.14-13.26 and 12.34-16.88 ng/L, respectively.
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Pre-O3
80
60
40
20
0 0
0.5
1
1.5
2
3
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Ozone dose(mg/L)
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Residual ratio (%)
Post-O3
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Fig. 4 Residual ratio of detected pesticides in pre- and post-ozonated water. Pre-O3: pre-ozonated water; Post-O3: post-ozonated water. Contact times for pre- and post-ozonation were 5 and 15 min, respectively; Concentrations of DOC for pre- and post-ozonation were 4.38-5.35 mg/L and 3.28-4.62 mg/L, respectively.
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20
-
Br -Pre Br -Post BrO3 -Pre
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15
-
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BrO3 -Post 40
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1.0
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Bromate (µg/L)
Risdual ratio of Br (%)
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Ozone dose(mg/L)
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Fig. 5 Bromate formation after pre- and post-ozonation at different ozone doses. Br--Pre and Br--Post: Br- residue (%) in pre- and post-ozonated water; BrO3--Pre and BrO3--Post: BrO3- concentration in pre- and post-ozonated water. Contact times for pre- and post-ozonation were 5 and 15 min, respectively; Concentrations of DOC for pre- and post-ozonation were 4.38-5.35 mg/L and 3.28-4.62 mg/L, respectively.
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Highlights ·The efficacy of pre- and post- ozonation in the removal of micropollutants was
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compared ·Both pre- and post-ozonation could achieve similar water purification performance
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from source water with high bromate formation risk
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·Pre-ozonation combining with BAC might be a solution for micropollutant removal