UV/chlorine control of drinking water taste and odour ...

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operations could be relatively simple and cost-effective compared ... (2015) reported in two case studies that less than 14 μg LА1 trihalomethane (THM) and haloacetic acid ... Aldrich) and d3-caffeine (99 atom % D, CDN Isotopes) were used.
Chemosphere 136 (2015) 239–244

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UV/chlorine control of drinking water taste and odour at pilot and full-scale Ding Wang a,⇑, James R. Bolton b, Susan A. Andrews a, Ron Hofmann a a b

Department of Civil Engineering, University of Toronto, 35. St. George St., Toronto, Ontario M5S 1A4, Canada Bolton Photosciences Inc., 628 Cheriton Cres., NW, Edmonton, AB T6R 2M5, Canada

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 UV/chlorine is more efficient than

UV/H2O2 at pH 6.5 for geosmin and MIB removal.  UV/chlorine efficiency is comparable to UV/H2O2 at pH 7.5 and 8.5.  Caffeine is a suitable surrogate for geosmin and MIB.

a r t i c l e

i n f o

Article history: Received 24 March 2015 Received in revised form 13 May 2015 Accepted 18 May 2015

Keywords: UV Chlorine Advanced oxidation Taste and odour Efficiency

a b s t r a c t Advanced oxidation processes (AOPs) can be used to destroy taste and odour-causing compounds in drinking water. This work investigated both pilot- and full-scale performance of the novel ultraviolet (UV)/chlorine AOP for the destruction of geosmin, 2-methylisoborneol (MIB) and caffeine (as a surrogate) in two different surface waters. The efficiency of the UV/chlorine process at pH 7.5 and 8.5 was comparable to that of the UV/hydrogen peroxide (UV/H2O2) process under parallel conditions, and was superior at pH 6.5. Caffeine was found to be a suitable surrogate for geosmin and MIB, and could be used as a more economical alternative to geosmin or MIB spiking for site-specific full-scale testing. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Geosmin and 2-methylisoborneol (MIB) can be destroyed during drinking water treatment by advanced oxidation processes (AOPs), which generate hydroxyl radicals (OH) (reaction rate constants of 7.8  109 and 5.1  109 M1 s1, respectively) (Peter and

⇑ Corresponding author. E-mail address: [email protected] (D. Wang). http://dx.doi.org/10.1016/j.chemosphere.2015.05.049 0045-6535/Ó 2015 Elsevier Ltd. All rights reserved.

von Gunten, 2007). Ultraviolet light (UV)-based AOPs—usually UV with hydrogen peroxide (H2O2)—are becoming more popular. The UV/H2O2 process is used at the City of Cornwall Water Purification Plant (Ontario, Canada) for the control of seasonal taste and odour events that occur in late summer. The UV/H2O2 process, while effective, can create operational problems. Most of the applied H2O2 survives UV exposure, and the residual therefore needs to be quenched to avoid a strong chlorine demand during secondary disinfection. At Cornwall, the residual H2O2 is quenched

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with a stoichiometric excess of chlorine, but operationally this is challenging because of an inability to continuously measure or accurately predict the H2O2 residual, issues with different residual H2O2 in the effluent from multiple UV reactors, H2O2 handling challenges, and other factors. The UV/chlorine AOP may be an alternative to the UV/H2O2 AOP for water and wastewater treatment. It has been investigated in laboratory studies to treat contaminants such as para-chlorobenzoic acid, benzoic acid, nitrobenzene, phenol, maleic acid, and trichloroethylene (Watts et al., 2007; Jin et al., 2011; Fang et al., 2014; Zhao et al., 2011; Wang et al., 2012), emerging contaminants in drinking water (Sichel et al., 2011), and naphthenic acids and fluorophore organic compounds in oil sands wastewater (Chan et al., 2012; Shu et al., 2014). For a drinking water treatment plant, operations could be relatively simple and cost-effective compared to the UV/H2O2 process, especially if the chlorine dose is selected to provide adequate photolysis for the control of taste and odour compounds, with remaining chlorine serving as a secondary disinfectant (Watts et al., 2012). Research on the UV/chlorine process, however, is still largely at the theoretical level or laboratory-scale. One important issue with the UV/chlorine AOP is the potential formation of disinfection (chlorination) by-products (DBPs), especially considering that a relatively high chlorine dose of 5–10 mg L1 may be required, compared to the more traditional 0.2–2 mg L1 used for chlorine disinfection. Tending to mitigate this effect, however, is the very short (i.e. seconds) chlorine contact time during the AOP process, during which much or all of the higher chlorine dose undergoes photolysis or may be quenched immediately downstream of the UV reactors. Wang et al. (2015) reported in two case studies that less than 14 lg L1 trihalomethane (THM) and haloacetic acid (HAA) formation was observed when applying UV/chlorine at doses required for approximately 90% geosmin and MIB reduction, but that 2–17% of the photolyzed chlorine was oxidized to chlorate. While such results are preliminary, they suggest that with careful management of chlorate formation (such as by avoiding overdosing free chlorine), DBP formation may not be a limiting factor for the UV/chlorine AOP, justifying further research into the use of UV/chlorine for treatment. In this study, the effectiveness of a medium pressure (MP) UV/chlorine AOP for taste and odour control was investigated at the Cornwall Water Purification Plant through full-scale trials, with a comparison of the UV/chlorine AOP to the UV/H2O2 AOP under parallel conditions. Caffeine as a potential surrogate for geosmin and MIB was also evaluated at the Cornwall plant, as well as in pilot-scale testing in a RayoxÒ reactor using water from a second natural source (Lake Simcoe). Given the lack of experience with UV/chlorine and UV-AOPs in general, utilities may wish to conduct pilot-scale or full-scale tests to confirm site-specific performance. Spiking low concentrations of caffeine into the flow for such tests, if appropriate, would be far less expensive than spiking geosmin and MIB, and likely to receive approval from local regulators.

as internal standards for quantification of geosmin/MIB and caffeine, respectively. Sodium hypochlorite solution (NaOCl) (10– 15 wt.%, reagent grade, Sigma–Aldrich) and H2O2 solution (50 wt.%, Sigma–Aldrich) were used in the pilot-scale tests. Industrial grade NaOCl solution (12.5 wt.%, NSF 60 certified, Olin Chlor Alkali) and H2O2 solution (35 wt.%, NSF 60 certified, Arkema Inc.) were used in the Cornwall full-scale tests. Sulphuric acid (H2SO4, 95–98%, A.C.S. grade, Sigma–Aldrich) and sodium hydroxide (NaOH, P97.0%, A.C.S. grade, Sigma–Aldrich) were freshly diluted to appropriate concentrations for pH adjustment in both full- and pilot-scale tests. Other compounds used in experiments and sample analyses were all analytical reagent grade or higher. Milli-QÒ water was used in all experiments and analytical determinations. 2.2. Experimental facilities and procedures 2.2.1. Cornwall full-scale tests Full-scale experiments were carried out at the Cornwall Water Purification Plant in early summer (May) and late summer (September, when taste and odour events typically occur), and are referred to as the 1st and 2nd full-scale tests in the following text. One of the MP UV reactors (Model: UVSwift 8L24, TrojanUV) was isolated for the experiments. The UV dose was estimated to be 2000 ± 150 mJ cm2 (200–400 nm) for an exposure of 7.2 s at a water flowrate of 100 L s1, based on the free chlorine photolysis rate determined by Wang et al. (2012). Although this estimation is approximate, the actual UV dose is not required when comparing the efficiencies of UV/chlorine and UV/H2O2 processes under equivalent conditions. The water was drawn from the St. Lawrence River and had been treated by conventional treatment (prechlorination, alum coagulation, flocculation, settling, and conventional sand/anthracite filtration). Water quality parameters are summarized in Table 1. In the 1st full-scale test, approximately 400 ng L1 geosmin and MIB, and 20 lg L1 caffeine were spiked into the water flow upstream of the UV reactor, along with a free chlorine dose of 2, 6, or 10 mg L1, or an H2O2 dose of 1.0, 2.9, or 4.8 mg L1 (equimolar concentrations as chlorine) and pH adjustment to 6.5, 7.5, or 8.5. Additional trials in the absence of chlorine and H2O2 were carried out to evaluate the stability of geosmin, MIB, and caffeine to UV exposure alone and to confirm the inertness of the system to other reactions that could result in losses of the target analytes. Preliminary tests (not shown) indicated that caffeine, geosmin and MIB were all stable in the presence of 40 mg L1 free chlorine or 10 mg L1 H2O2 for at least 20 min, with the decay less than 0.5%. The 2nd full-scale test of the UV/chlorine process at Cornwall was more limited than the first, with the performance of UV/chlorine process (only) monitored for controlling the existing

Table 1 Post-filtration water quality parameters for full- and pilot-scale tests.

2. Material and methods

Cornwall 1st full-scale

Cornwall 2nd full-scale

RayoxÒ pilot-scale

St. Lawrence River 8.1 0.03 88

Lake Simcoe 7.5 0.2 123

1.8 Not measured 18 ng L1 geosmin

3.5 0.67 20 lg L1 caffeine

2.1. Reagents and materials

Source

St. Lawrence River

Geosmin and MIB (>95% purity) from Dalton Chemical Laboratories were dissolved together in Milli-QÒ water to make a stock solution at a concentration of approximately 80 mg L1 for full-scale tests. Caffeine stock solutions at concentrations of 10 g L1 and 1 g L1 for the full- and pilot-scale tests, respectively, were prepared from purchased caffeine (ReagentPlusÒ grade, Sigma–Aldrich). Deuterated d3-geosmin (99 atom % D, Sigma– Aldrich) and d3-caffeine (99 atom % D, CDN Isotopes) were used

pH Turbidity (NTU) Alkalinity (mg CaCO3 L1) TOC (mg C L1) 1 Nitrate (NO ) 3 ) (mg L Target chemicals

7.9 0.02 92 1.5 1.2 400 ng L1 geosmin 400 ng L1 MIB 20 lg L1 caffeine

D. Wang et al. / Chemosphere 136 (2015) 239–244

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18 ng L1 of geosmin in the incoming water (MIB was below detection limits). Chlorine doses of 2, 6, or 10 mg L1 at pH 6.5, 7.5, and 8.5 were investigated. The main purpose of this second test was to monitor potential disinfection by-product formation impacts (reported in Wang et al., 2015), but it was also used as another opportunity to validate the UV/chlorine process performance under more limited conditions. 2.2.2. RayoxÒ pilot-scale test A 40 L RayoxÒ completely-mixed batch reactor (Model: PS1-1-120, Calgon Carbon Corporation), equipped with a 1 kW MP UV lamp positioned in the centre of the water (Heraeus Noblelight GmbH, Germany) was used in the pilot-scale experiments to evaluate the UV/chlorine process efficiency, using spiked caffeine as a performance indicator. Water was collected post-filtration from the Keswick Water Treatment Plant (Ontario, Canada), which draws water from Lake Simcoe. Treatment at the Keswick plant is similar to that at Cornwall, except that no prechlorination is used. The water contained approximately twice the total organic carbon (TOC) as the St. Lawrence River water (Table 1). The experimental conditions in the RayoxÒ reactor were similar to those applied in the full-scale tests. Caffeine at 20 lg L1 was spiked into 40 L water and treated by UV alone, UV/chlorine (2, 6, or 10 mg L1) or UV/H2O2 (1.0, 2.9, or 4.8 mg L1) at pH 6.5, 7.5, or 8.5. The UV exposure time in the RayoxÒ reactor was 40 s, which was predicted to deliver a UV dose (200–400 nm) of 1820 ± 110 mJ cm2, using UVCalcÒ software version 2B (from Bolton Photosciences Inc.). The UV dose varied slightly with different chlorine or H2O2 doses and pH values. 2.3. Sample analysis Free chlorine was determined using a HACHÒ spectrophotometer (Model: DR/2500, HACH), according to Standard Method 4500-Cl G (APHA et al., 2012). A Cecil UV/vis spectrophotometer (Model: CE3055, Cecil Instruments) was used to determine H2O2 concentrations based on the triiodide method described by Klassen et al. (1994). An Agilent 8453 UV/vis photodiode array spectrophotometer (Model: G1103A, Agilent Technologies) was used to determine solution absorbances in the UV range. Geosmin and MIB samples spiked with 100 ng L1 d3-geosmin were extracted and concentrated using the headspace solid phase micro-extraction (HS-SPME) method and quantified according to Standard Method 6040D (APHA et al., 2012) using a Varian 3800 gas chromatograph coupled with a Varian 4000 ion-trap mass spectrometer (GC–MS) operated in electron ionization (EI) mode. Caffeine samples with 1 lg L1 d3-caffeine added were extracted and concentrated using solid-phase extraction (SPE) cartridges and analyzed using the same GC–MS, but in the positive chemical ionization (CI) mode, based on the method described by Verenitch et al. (2006). Method detection limits (MDLs) for geosmin, MIB, and caffeine were 2, 9, and 31 ng L1, respectively.

Fig. 1. Percentage of free chlorine photolysis by UV exposure. Error bars represent the values of experimental duplicates.

undergo greater photolysis at a higher pH, with about 1.5–2 times the total decay at pH 8.5 compared to pH 6.5. This can be explained by the higher OCl concentration relative to HOCl at the higher pH (pKa of HOCl = 7.54 at 25 °C). OCl absorbs MP UV light about 4.5 times more than HOCl, and photolyzes at approximately the same rate as HOCl (quantum yield of 0.9, compared to 1.0 for HOCl photolysis) (Wang et al., 2012). It was also observed that chlorine photodecomposition was faster at a lower initial concentration, especially at pH 8.5. This may arise from a higher average UV dose at a lower chlorine concentration.

3.2. Geosmin and MIB decay The destruction of geosmin and MIB arising from UV photolysis alone, the UV/chlorine process, or the UV/H2O2 process for the 1st full-scale test is shown in Fig. 2. Approximately 20% and 10% of spiked geosmin and MIB, respectively, were destroyed by UV exposure alone (2000 mJ cm2). When an AOP was applied, geosmin and MIB destruction was increased because of the additional OH oxidation. The UV/chlorine and UV/H2O2 processes led to similar amounts (difference within 10% in most cases) of geosmin and MIB destruction at pH 7.5 and 8.5. At pH 6.5, however, the UV/chlorine process was superior to the UV/H2O2 process, resulting in 10–25% more destruction for all applied doses. The superior UV/chlorine process performance at lower pH has been reported by Watts and Linden (2007), Watts et al. (2007), and Wang et al.

3. Results and discussion 3.1. Free chlorine decay One of the potential advantages of the UV/chlorine process over the UV/H2O2 process is the predicted greater photolysis of chlorine across the UV reactor than H2O2, minimizing the need to quench excess oxidant. For both the full-scale and pilot-scale tests, the chlorine concentrations decreased by approximately 40–80% across the UV reactors at UV doses (200 nm to 400 nm) of 1800– 2000 mJ cm2 (Fig. 1). The H2O2 decay in parallel tests was at most approximately 5% (data not shown). Chlorine was observed to

Fig. 2. Geosmin (a) and MIB (b) decay in the 1st full-scale test. Error bars represent the values of experimental duplicates.

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(2012), and is probably because of the conversion of OCl to HOCl at the lower pH. HOCl absorbs MP UV light about 2.3 times more efficiently than H2O2 (Wang et al., 2012) and produces OH at a similar efficiency (quantum yield of OH formation for HOCl: 0.79 vs. 1.11 for H2O2) (Wang et al., 2012; Goldstein et al., 2007), but also reacts with OH more slowly than H2O2 (rate constant with OH for HOCl: 8.46  104 M1 s1 vs. 2.7  107 M1 s1 for H2O2) (Watts and Linden, 2007; Goldstein et al., 2007). This all leads to a higher efficiency of the UV/HOCl process compared to that of the UV/H2O2 process. In contrast, the much higher reaction (scavenging) rate of OCl with OH than either HOCl or H2O2 (rate constant: 9.0  109 M1 s1, Buxton and Subhani, 1972) more than offsets the benefit of OCl0 s stronger UV absorption. In these previous studies, it has been proposed that the UV/chlorine process would be superior to the UV/H2O2 process at pH < 6 in pure water (i.e. laboratory grade water, containing no OH scavengers). It was also proposed that the pH at which the UV/chlorine process remained competitive relative to the UV/H2O2 process would increase with increasing OH scavenger concentration (Wang et al., 2012). The Cornwall water had an OH scavenging potential 2–17 times higher than the three previous studies as estimated using the total organic carbon (TOC) and alkalinity concentrations reported in Table 1. As such, this result tends to corroborate the theory that the UV/chlorine process remains competitive with the UV/H2O2 process at higher pH—up to pH 8.5 in this case—with more OH scavengers present. Similar to the UV/chlorine process, a pH effect on the UV/H2O2 process efficiency was observed, with reduced effectiveness at higher pH (Fig. 2). However, unlike the UV/chlorine process, whose efficiency is largely dependent on the differential OH scavenging of HOCl and OCl, the UV/H2O2 process efficiency is likely changed by the bicarbonate/carbonate equilibrium. With the increase of pH from 6.5 to 8.5, carbonic acid (H2CO3) and bicarbonate (HCO 3 ) present in the water partially convert to carbonate (CO2 3 ) (pKa1 of 6.37, pKa2 of 10.36) (Fanghänel et al., 1996; Kolthoff and Bosch, 1928). CO2 is a much stronger OH scavenger than H2CO3 and 3 2 HCO is 3.9  108 M1 s1, 3 . The rate constant of OH with CO3 6 1 1  compared to 8.5  10 M s for HCO3 and negligible for H2CO3 (Buxton et al., 1988; Liao et al., 2001). In the presence of an alkalinity of 92 mg CaCO3 L1 in the Cornwall water, the 2 H2CO3/HCO 3 /CO3 species was calculated to increase the OH scavenging potential of the H2O2 solution by approximately 12% at pH 8.5 relative to 6.5. However, this is still less than the 40% decrease of the rates of geosmin and MIB decay in the UV/H2O2 process when the pH is increased from 6.5 to 8.5, as suggested by the data in Fig. 2. This implies that there might be other OH scavengers present in the water that could also increase the OH scavenging potential of the solution at a higher pH. In addition, the effect of carbonate species was also present in water treated by the UV/chlorine process. However, since OCl is a much stronger OH scavenger than carbonate species, the contribution of carbonate species to the increase of the chlorine solution scavenging potential was estimated to be minimal, compared to OCl (theoretically less than 1%). In the 2nd full-scale test, only the UV/chlorine process performance was evaluated, and only in terms of the destruction of the 18 ng L1 geosmin already present in the water. The trend in geosmin destruction (Fig. 3) was similar to the first testing campaign. The UV/chlorine process was observed to be more efficient at lower pH, but substantial geosmin destruction was still accomplished at pH 7.5 and 8.5 (70% and 45%, respectively, on average). Since the natural pH at Cornwall is approximately 8, these data suggest that chlorine doses in the order of 6 mg L1 could reduce the incoming geosmin of 18 ng L1 to below the common detection threshold of 7–10 ng/L (Rosenfeldt et al., 2005).

Fig. 3. Geosmin decay in the 2nd full-scale test. Error bars represent the values of experimental duplicates.

3.3. Caffeine decay Caffeine does not degrade appreciably by photolysis (data not shown), but reacts with OH. As an OH probe, it can be used to determine the OH concentration generated by an AOP. It therefore could serve as a convenient surrogate for geosmin and MIB when conducting future AOP tests at pilot or full-scale given that its cost is several orders of magnitude lower than geosmin and MIB when spiked into a large flow, and that it is environmentally safe. Caffeine was spiked into the 1st full-scale test to compare its degradation to that of geosmin and MIB, and was also used at pilot-scale in a second water matrix (Lake Simcoe) that contained approximately twice the TOC as at Cornwall, and therefore a presumed higher scavenging potential. The full-scale results (Fig. 4(a)) show that a UV dose (alone) of 2000 mJ cm2 led to 10–15% caffeine decay, which was approximately the same as the destruction of geosmin (20%) and MIB (10%). Past studies have reported the rapid reaction between caffeine and OH, with an average rate constant of 5.0  109 M1 s1 (Shi et al., 1991; León-Carmona and Galano, 2011). The rate constant is in the same order of magnitude as those for geosmin (7.8  109 M1 s1), and is almost identical to MIB (5.1  109 M1 s1). This means that the rate of caffeine decay by OH should be approximately the same as that of MIB, but 36% less

Fig. 4. Caffeine decay in the 1st full-scale (a) and pilot-scale (b) tests. Error bars represent the values of experimental duplicates.

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D. Wang et al. / Chemosphere 136 (2015) 239–244 Table 2 Full- and pilot-scale EEO values (kWh m3 order1) for geosmin, MIB, and caffeine removal. Treatment

1st full-scale test UV/chlorine 2 mg L1 UV/chlorine 6 mg L1 UV/chlorine 10 mg L1 UV/H2O2 1.0 mg L1 UV/H2O2 2.9 mg L1 UV/H2O2 4.8 mg L1

pH 6.5

pH 7.5

Geosmin

MIB

Caffeine

Geosmin

MIB

Caffeine

Geosmin

MIB

Caffeine

0.26 0.22 0.16 0.47 0.30 0.23

0.35 0.29 0.22 0.66 0.46 0.31

0.43 0.26 0.21 0.94 0.49 0.36

0.49 0.39 0.28 0.60 0.37 0.29

0.68 0.51 0.34 0.90 0.53 0.34

0.65 0.47 0.42 1.2 0.69 0.41

0.65 0.59 0.40 1.0 0.51 0.36

1.0 0.81 0.58 2.2 0.73 0.52

0.87 0.67 0.54 1.9 0.87 0.52

Pilot-scale test UV/chlorine 2 mg L1 UV/chlorine 6 mg L1 UV/chlorine 10 mg L1 UV/H2O2 1.0 mg L1 UV/H2O2 2.9 mg L1 UV/H2O2 4.8 mg L1

0.98 0.53 0.39 2.1 1.2 0.80

1.5 1.0 0.88 2.7 1.3 0.91

than that of geosmin. The results shown in Fig. 4(a) generally reflect the theory, which illustrated that the experimental rate of caffeine decay by OH was approximately 90% and 70% (on average) compared to those observed for MIB and geosmin (shown in Fig. 2), respectively, when comparing the natural logarithms of the final over the initial concentrations of these compounds. After including UV photolysis, the caffeine destruction by the UV/chlorine and UV/H2O2 processes was on average equivalent to 95% and 67% of MIB and geosmin decay, respectively. Caffeine, therefore, is an almost perfect surrogate to estimate MIB destruction by the UV/chlorine or UV/H2O2 process, with the geosmin decay expected to be approximately 50% greater. Based on the caffeine destruction in the Lake Simcoe water in the pilot-scale test (Fig. 4(b)), which was similar to that in the Cornwall water, the UV/chlorine process was predicted to have similar performance for geosmin and MIB destruction in both waters, further validating the effectiveness of UV/chlorine. 3.4. Electrical energy per order (EEO) Electrical energy per order (EEO) (kW h m3 order1) is the electrical energy in kilowatt hours (kW h) required to degrade a target contaminant by one order of magnitude (90%) in 1 m3 of water (Bolton et al., 2001), as given by Eqs. (1) and (2):

EEO

P ¼ for the full-scale reactor Q lgðC i =C f Þ

EEO ¼

pH 8.5

1000Pt for the pilot-scale RayoxÒbatch reactor V lgðC i =C f Þ

ð1Þ

ð2Þ

where P is the electrical power input (kW) into the UV reactor, Q is the flowrate (m3 h1), Ci and Cf are the initial and final concentrations (M) of the target contaminant before and after the AOP, t is the UV exposure time (h) in the batch reactor, and V is the volume of treated water (L) in the batch reactor. In this study, P was 83.5 kW for the full-scale test and 1.8 kW for the pilot-scale test; F was 360 m3 h1 converted from 100 L s1; t was 0.011 h converted from 40 s; and V was 40 L. The calculated EEO values for geosmin, MIB, and caffeine are given in Table 2. EEO reflects the relative efficiency of an AOP. Comparing the EEO values for each compound in the full-scale test summarized in Table 2, it was found that EEO values for the UV/chlorine process were lower than those for the UV/H2O2 process in most cases, which demonstrates the superior efficiency of the UV/chlorine process. The only case where the UV/chlorine process was less efficient (higher EEO) than the UV/H2O2 process occurred at the highest pH (8.5) with the highest oxidant dose (chlorine:

2.1 1.4 1.1 3.1 1.7 1.1

10 mg L1 and H2O2: 4.8 mg L1). Similarly, in the pilot-scale test using a different water matrix, the EEO values for the UV/chlorine process were all lower than or the same as those for the UV/H2O2 process. This tends to verify the theory proposed by Wang et al. (2012) that the UV/chlorine process becomes more competitive relative to the UV/H2O2 process with more OH scavengers, because the levels of two major scavengers, TOC and alkalinity, in the Lake Simcoe water were approximately 2 and 1.3 times higher than those of the Cornwall water. Acknowledgements This work was funded by the Natural Sciences and Engineering Research Council of Canada through the Industrial Research Chair program. The authors express their appreciation to the staff at the Cornwall Water Purification Plant, including Owen O’Keefe, Daniel Drouin, and Morris McCormick, for their help with the full-scale experiments. The help of Zhen (Jim) Wang, Jiafan (Kevin) Yang, Hong Zhang, and A.H.M. Anwar Sadmani is also gratefully acknowledged. References APHA, AWWA, WEF, 2012. Standard Methods for the Examination of Water & Wastewater, 22nd ed., American Public Health Association, American Water Works Association, and Water Environment Federation, Washington, D.C., USA. Bolton, J.R., Bircher, K.G., Tumas, W., Tolman, C.A., 2001. Figures-of-merit for the technical development and application of advanced oxidation technologies for both electric- and solar-driven systems. Pure Appl. Chem. 73 (4), 627–637. Buxton, G.V., Subhani, M.S., 1972. Radiation chemistry and photochemistry of oxychlorine ions. Part 1. – Radiolysis of aqueous solutions of hypochlorite and chlorite ions. J. Chem. Soc., Faraday Trans. 1 (68), 947–957. Buxton, G.V., Greenstock, C.L., Helman, W.P., Ross, A.B., 1988. Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (OH/O) in aqueous solution. J. Phys. Chem. Ref. Data 17 (2), 513–886. Chan, P.Y., Gamal El-Din, M., Bolton, J.R., 2012. A solar-driven UV/chlorine advanced oxidation process. Water Res. 46 (17), 5672–5682. Fang, J., Fu, Y., Shang, C., 2014. The roles of reactive species in micropollutant degradation in the UV/free chlorine system. Environ. Sci. Technol. 48 (3), 1859– 1868. Fanghänel, T., Neck, V., Kim, J.I., 1996. The ion product of H2O, dissociation constants 2  of H2CO3 and Pitzer parameters in the system Na+/H+/OH/HCO 3 /CO3 /ClO4 / H2O at 25°C. J. Solut. Chem. 25 (4), 327–343. Goldstein, S., Aschengrau, D., Diamant, Y., Rabani, J., 2007. Photolysis of aqueous H2O2: quantum yield and applications for polychromatic UV actinometry in photoreactors. Environ. Sci. Technol. 41 (21), 7486–7490. Jin, J., Gamal El-Din, M., Bolton, J.R., 2011. Assessment of the UV/chlorine process as an advanced oxidation process. Water Res. 45 (4), 1890–1896. Klassen, N.V., Marchington, D., McGowan, H.C.E., 1994. H2O2 determination by the I 3 method and by KMnO4 titration. Anal. Chem. 66 (18), 2921–2925. Kolthoff, I.M., Bosch, W., 1928. The influence of neutral salts on acid-salt equilibria: III. The second dissociation constant of carbonic acid and the influence of salts on the activity of the hydrogen ions in a bicarbonate-carbonate mixture. Recl. Trav. Chim. Pays-Bas 47 (10), 819–825.

244

D. Wang et al. / Chemosphere 136 (2015) 239–244

León-Carmona, J.R., Galano, A., 2011. Is caffeine a good scavenger of oxygenated free radicals? J. Phys. Chem. B 115 (15), 4538–4546. Liao, C.-H., Kang, S.-F., Wu, F.-A., 2001. Hydroxyl radical scavenging role of chloride and bicarbonate ions in the H2O2/UV process. Chemosphere 44 (5), 1193–1200. Peter, A., von Gunten, U., 2007. Oxidation kinetics of selected taste and odor compounds during ozonation of drinking water. Environ. Sci. Technol. 41 (2), 626–631. Rosenfeldt, E.J., Melcher, B., Linden, K.G., 2005. UV and UV/H2O2 treatment of methylisoborneol (MIB) and geosmin in water. J. Water Supply Res. Technol.Aqua 54 (7), 423–434. Shi, X., Dalal, N.S., Jain, A.C., 1991. Antioxidant behaviour of caffeine: efficient scavenging of hydroxyl radicals. Food Chem. Toxicol. 29 (1), 1–6. Shu, Z., Li, C., Belosevic, M., Bolton, J.R., Gamal El-Din, M., 2014. Application of a solar UV/chlorine advanced oxidation process to oil sands process-affected water remediation. Environ. Sci. Technol. 48 (16), 9692–9701. Sichel, C., Garcia, C., Andre, K., 2011. Feasibility studies: UV/chlorine advanced oxidation treatment for the removal of emerging contaminants. Water Res. 45 (19), 6371–6380. Verenitch, S.S., Lowe, C.J., Mazumder, A., 2006. Determination of acidic drugs and caffeine in municipal wastewaters and receiving waters by gas

chromatography-ion trap tandem mass spectrometry. J. Chromatogr. A 1116 (1–2), 193–203. Wang, D., Bolton, J.R., Hofmann, R., 2012. Medium pressure UV combined with chlorine advanced oxidation for trichloroethylene destruction in a model water. Water Res. 46 (15), 4677–4686. Wang, D., Bolton, J.R., Andrews, S.A., Hofmann, R., 2015. Formation of disinfection by-products in the ultraviolet/chlorine advanced oxidation process. Sci. Total Environ. 518–519, 49–57. Watts, M.J., Hofmann, R., Rosenfeldt, E., 2012. Low-pressure UV/Cl2 for advanced oxidation of taste and odor. J. Am. Water Works Assoc. 104 (1), E58–E65. Watts, M.J., Linden, K.G., 2007. Chlorine photolysis and subsequent OH radical production during UV treatment of chlorinated water. Water Res. 41 (13), 2871–2878. Watts, M.J., Rosenfeldt, E.J., Linden, K.G., 2007. Comparative OH radical oxidation using UV-Cl2 and UV-H2O2 processes. J. Water Supply Res. Technol.-Aqua. 56 (8), 469–477. Zhao, Q., Shang, C., Zhang, X., Ding, G., Yang, X., 2011. Formation of halogenated organic byproducts during medium-pressure UV and chlorine coexposure of model compounds. Water Res. 45 (19), 6545–6554.