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Photo-assisted electrochemical treatment of municipal wastewater reverse osmosis concentrate
Chemical Engineering Journal 249 (2014) 180–188
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Photo-assisted electrochemical treatment of municipal wastewater reverse osmosis concentrate Gil Hurwitz a,⇑, Eric M.V. Hoek a,b, Kai Liu c, Linhua Fan c, Felicity A. Roddick c a
Department of Civil & Environmental Engineering, California NanoSystems Institute and Institute of the Environment & Sustainability, University of California, Los Angeles, CA, USA Department of Applied Chemistry, University of Johannesburg, Johannesburg, South Africa c RMIT University, School of Civil, Environmental & Chemical Engineering, Melbourne, Victoria, Australia b
h i g h l i g h t s Organics in RO concentrate were treated by a hybrid advanced oxidation process. Hybrid photolytic and electrochemical process showed synergistic DOC degradation. Electrochemical breaks down fluorescence and photolysis destroys core structure. Hybrid treatment consumes less energy than photolysis alone.
a r t i c l e
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Article history: Received 4 December 2013 Received in revised form 19 March 2014 Accepted 21 March 2014 Available online 3 April 2014 Keywords: Photolysis Electrochemical oxidation Synergy Wastewater Reverse osmosis concentrate
a b s t r a c t The combination of a photochemical (UV) and electrochemical (EL) process led to enhanced degradation of dissolved organic matter in reverse osmosis (RO) concentrate produced from municipal wastewater. Treatment by UV and EL alone resulted in 25% and 35% removal of dissolved organic carbon (DOC) after 5 h, respectively. However, the hybrid process (UVEL) degraded more than 80% of DOC after the same treatment time. Fluorescence excitation–emission matrix spectroscopy and size exclusion chromatography suggest simultaneous and cooperative degradation of backbone aliphatic bonds by UV and aromatic ring cleavage by EL within the UVEL process. Overall, UVEL treatment led to efficient and non-selective degradation of dissolved organics over a wide range of molecular weights. Further, energy consumption and halogenated by-product formation (typically a limiting factor for the application of oxidation technology) were reduced in the UVEL process relative to the UV and EL processes, respectively. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Municipal wastewater effluent treated by RO produces highly purified product water with many valuable uses (e.g., landscape irrigation, industrial process water, and aquifer recharge). However, the RO process also produces a highly concentrated version of the wastewater effluent, which may contain elevated concentrations of contaminants such as dissolved and suspended solids comprising BOD, COD, pathogens, and trace organics (e.g., pharmaceuticals and personal care products, endocrine disrupting compounds, and disinfection byproducts) not fully removed by the preceding biological treatment process [1–3]. The potential human and environmental health hazards associated with discharging ⇑ Corresponding author. Current address: Water Planet Engineering, 721 Glasgow Ave, Unit D, Los Angeles, CA 90301, USA. Tel.: +1 (310) 215 8960; fax: +1 (310) 215 8961. E-mail address: [email protected] (G. Hurwitz). http://dx.doi.org/10.1016/j.cej.2014.03.084 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.
wastewater RO concentrate containing elevated levels of pathogens, BOD/COD, and trace organics are not fully understood [4–10]. Therefore, it seems prudent to explore treatment technologies that can ensure effluent discharge standards are met, particularly when dilution (i.e., mixing the RO concentrate with large volumes of unconcentrated wastewater treatment plant effluent) is not possible. In particular, further disinfection and organic removal may be necessary. Both photochemical and electrochemical processes can be used for disinfection and organic removal. Direct UV irradiation of wastewater gives rise to photo-activated chemical reactions that translate absorbed photonic energy into direct chemical bond breakage or the production of free radicals [11,12]. Utilization of photolysis for advanced oxidation is typically achieved by the addition of catalysts or UV reactive compounds (i.e., H2O2 and TiO2) to provide a source of the requisite hydroxyl radicals [12]. Electrochemical oxidation targets mineralization of organic compounds by electron abstraction at the anode surface or by the
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production of highly reactive oxidants, such as OH, produced in situ electrochemically [13]. In situ production of chemical reactants eliminates the need to purchase, transport, and store chemicals produced off-site, which can save money and energy as well as reduce carbon emissions and potential security vulnerabilities [14]. Electrochemical oxidation may be an attractive option for treating wastewater RO concentrate because it becomes more efficient and effective with increased conductivity and chloride concentration. High conductivity reduces the voltage needed to achieve a target current density, which results in considerable energy savings. However, electrochemical processes traditionally suffer from high investment costs, especially for advanced anode materials, such as boron-doped diamond. For this reason, improving electrochemical oxidation efficiency is paramount in order to reduce the required active electrode area and, thus, the initial capital cost for a given electrochemical treatment system [15–17]. Advanced anode materials have garnered considerable attention recently due to their ability to improve oxidation efficiency by enhancing the rate of direct oxidation at or near the electrode surface. Generally, advanced anodes are characterized as ‘‘nonactive’’ materials with high oxygen evolution overpotentials, such as antimonydoped tin oxide, lead dioxide, and boron-doped diamond, and favor the complete oxidation of organics to CO2 due to their weak surface interactions with hydroxyl radicals [18]. Boron-doped diamond also shares the ability of some active materials, such as Ti/RuO2, to promote indirect bulk oxidation by the production of intermediate oxidants, of which chlorine is commonly targeted [19,20]. Specifically, chloride ions become oxidized at the anode; thus, introducing an entire family of potential secondary oxidants and disinfectants. Indirect organic oxidation may occur via reaction with chlorine radicals (Cl) adsorbed on the anode surface or by reactive chlorine species such as Cl2, HOCl, or OCl in the bulk solution [13]. However, the formation of halogenated by-products is an obvious concern when treating waters containing both halide ions and dissolved organic matter. Significant research has been undertaken to determine the effectiveness of hybrid photo-assisted electrochemical oxidation processes with particular emphasis on the UV excitation of photoanodes (e.g., thin-film TiO2, DSAÒ) [21–25]. However, due to its minimal anodic photoactivity, we have not been able to find previous research studying the use of boron-doped diamond (a highly promising material with superior electrochemical properties) electrodes in hybrid photo-assisted electrochemical oxidation processes [26]. Herein, we tested a hybrid photo-assisted electrochemical process (UVEL) with the electrochemical and photochemical reactors placed in series within a batch recirculation system. This unique reactor configuration can, therefore, take advantage of the electrochemical benefits of boron-doped diamond electrodes (wide potential window, low background current, high chemical stability, high resistance to deactivation [27–30]) without the need to invest energy and resources to enhance its photoactivity. The aim of this research is to study the practical limitations (extent of mineralization, energy consumption, by-product formation) of a photoassisted electrochemical reactor, which integrates highly efficient boron-doped diamond electrodes to treat a target water of growing public and environmental concern. 2. Methodology Biologically treated wastewater effluent was provided by Western Treatment Plant (Victoria, Australia), which used a sequential activated sludge-lagoon treatment (AS-lagoon) process. RO concentrate was prepared from the effluent using a laboratory scale plate-and-frame unit (Sepa CF, GE-Osmonics, Minnetonka, MN) and a commercial interfacial composite RO membrane (AG; GE-Osmonics, Minnetonka, MN) operated at a constant pressure
and temperature of 17 bar and 20 °C and an average permeate flux of 46.8 LMH. The effluent was concentrated to a recovery of 65%. Major water quality parameters are given in Table 1. Electrochemical oxidation (EL) experiments were performed in a flow-through, undivided, spacer-filled electrochemical cell. A DiaChemÒ (Electrocell, Amherst, NY) boron-doped diamond anode was used with a geometric active area of 10 cm2. Electrochemical oxidation was performed at a constant current density of 20 mA/ cm2 with a flow rate of 0.033 m3/h and Reynolds number of 360. Photolysis (UV) experiments were conducted using an annular reactor with a centrally mounted lamp. A low-pressure mercuryvapor lamp emitting at 254 nm with an intensity of 8500 lW/ cm2 was used as the UV-C irradiation source (G36T15NU; Australian Ultra Violet Services, Victoria, Australia). The average irradiated area was 464 cm2 with a path length of 1.94 cm; other UV reactor conditions were reported elsewhere [11]. A Titan 1500 chiller (Aqua-Medic, Bissendorf, Germany) was used to maintain a constant solution temperature of 20 °C during all the experiments. The UVEL reactor subjected the wastewater RO concentrate (WWROC) to both electrochemical oxidation and UV photolysis in series, recirculation mode (Fig. 1). Specific power consumption was estimated from the power consumed over the operating time and normalized by the total volume processed. Dissolved organic carbon (DOC) was determined by measuring the total organic carbon (TOC) content (Sievers 5310 C, GE, Boulder, Colorado, USA) of 0.45 lm filtered samples. The TOC analyzer was equipped with an auto-sampler and inorganic carbon was internally purged automatically (Sievers 900 ICR; GE, Boulder, Colorado, USA) prior to DOC analysis. The absorbance was measured using a Unicam UV–vis spectrophotometer with a quartz cell of 1 cm pathlength. The excitation–emission matrix (EEM) spectra of the samples were obtained with a fluorescence spectrometer (LS55, Perkin Elmer, Waltham, Massachusetts, USA). The size exclusion chromatography (SEC) analyses were carried out at the Water Research Centre of the University of New South Wales, Australia. The SEC system (LC-OCD Model 8, DOC-Labor Dr. Hüber, Germany) consisted of a SEC column (Toyopearl TSK HW-50S, diameter 2 cm, length 25 cm) to separate organic molecules by size (i.e., 100–200,000 Da). Trihalomethane concentrations were determined by the Australian Water Quality Centre in Adelaide, Australia using previously described methods [31].
3. Results 3.1. DOC removal and energy consumption After 5 h of treatment of the WWROC, the electrochemical and photochemical processes resulted in approximately 25% and 35% Table 1 Water quality of secondary wastewater RO concentrate. Parameter
Unit
Value
Conductivity pH DOC A254 Na+ Ca2+ Mg2+ Cl Br SO24 TN, as N TP, as P Alkalinity, as CaCO3 Color (Pt–Co) Turbidity
mS/cm – mg/L /cm mg/L mg/L mg/L mg/L mg/L mg/L mg N/L mg P/L mg CaCO3/L mg Pt–Co/L NTU
3.8 8.8 22 0.42 600 47 53 954 3.9 207 32 8 242 55 0.5
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Fig. 1. Schematic diagram of the hybrid photo-assisted electrochemical reactor. (1) Jacketed beaker; (2) magnetic stirrer; (3) thermometer; (4) temperature control device; (5) pump; (6) commercial undivided electrochemical cell; (7) mesh turbulence promoter; (8) cathode; (9) anode; (10) DC power supply; (11) annular photoreactor; (12) UV-C lamp; and (13) quartz sleeve.
DOC removal, respectively; however, the UVEL process achieved approximately 80% removal (Fig. 2a). If the photochemical and electrochemical degradation rates were mathematically combined, the hypothetical DOC removal by addition would be only 60%; hence there was a synergistic effect of combining the two processes and a previously unrecorded and potentially powerful oxidation mechanism at play. Moreover, both the electrochemical and UVEL treatments were less energy intensive (per unit of DOC removal) than the photochemical process (Fig. 2b). The slopes of
Fig. 2. Electrochemical oxidation (EL), UV-C photolysis (UV), hypothetical summation of electrochemical oxidation and photolysis (UV + EL), and hybrid photoassisted electrochemical oxidation (UVEL) performance: (a) DOC removal versus time and (b) specific power versus conversion.
photochemical and electrochemical power consumption plotted against DOC conversion were 1160 and 24 kW h/m3, respectively. The UVEL process consumed an intermediate amount of energy up to 60% DOC removal (176 kW h/m3); however, at conversions above 60% the hybrid process energy consumption increased to 1170 kW h/m3, which was almost identical to that of UV-C irradiation alone.
3.2. Mechanisms of organic degradation EEM spectroscopy was used to better understand the unique degradation pathway observed during the UVEL process by tracking the relative rate of removal for different fluorescent functional groups. Two major fluorescence peaks were distinguished for the WWROC (at kEm/kEx 430/340 and 440/250) (Fig. 3). These peaks derive predominantly from humic and fulvic compounds, respectively [32–36]. After 1 h of electrochemical treatment the intensity of fluorescence was greatly reduced and after 5 h no fluorescence remained. On the other hand, UV treatment led to less reduction in the fluorescence of the humic and fulvic compounds after 1 h with a residual remaining after 5 h. There was a similar effect on the minor fluorescent peaks, such as those for soluble microbial products (kEm/kEx 380/250–280) and aromatic proteins (kEm/kEx 340–355/235–240 and 320–335/280–285) [37]. The combined UVEL treatment virtually eliminated all fluorescent peaks within 1 h (Fig. 3). Looking more closely at the first hour, photo-assisted electrochemical treatment degraded humics and fulvics at comparable rates (Fig. 4), producing nearly complete reduction of fluorescence after 40 min. SEC was used to directly observe how effective each treatment option was on structural degradation and the conversion of larger organic molecules into smaller derivatives (Table 2). The high molecular weight fraction (>20,000 Da) represents biopolymers, such as polysaccharides, proteins, and aminosugars. The medium-sized molecular weight fractions represent humic substances (1000 Da) and ‘‘building block’’ molecules (300–500 Da), which signify breakdown products formed from humic degradation. Finally, the low molecular weight (LMW) fractions for molecules less than 350 Da represent LMW neutrals (i.e., mono-oligosaccharides, alcohols, aldehydes, and ketones) and LMW acids (i.e., monoprotic organic acids). The UVEL process resulted in the conversion of large macromolecules into smaller derivatives at a rate considerably higher than photochemical or electrochemical treatments alone. Specifically, 1 h of UVEL treatment resulted in
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Fig. 3. Excitation–emission matrix spectroscopy for the UV-C photolysis (UV), electrochemical oxidation (EL), and photo-assisted electrochemical oxidation (UVEL) of secondary wastewater effluent RO concentrate.
significant degradation of biopolymers and humic substances and after 5 h there were only low concentrations of building block and low molecular weight organics remaining. In contrast, within the first hour of treatment EL only targeted the rapid removal of humic aromaticity while UV targeted the conversion of biopolymers and humics to smaller secondary molecules (i.e., building blocks, LMW neutrals, and LMW acids). However, both electrochemical and photochemical treatments were less effective beyond 1 h in that significant concentrations of organics across a wide range of molecular weights remained, even after 5 h of treatment. 3.3. Tracking changes in ultraviolet absorbance UV–vis absorption spectra were analyzed to gather more insight into the UVEL degradation mechanism and the role of oxidation intermediates (Figs. 5 and 6). During UV-C photolysis alone, the absorbance profile between 190 and 350 nm displayed a low, yet steady rate of decline (Fig. 5a). On the other hand, electrochemical treatment produced a new absorption band at 291 nm (Fig. 5b). UVEL oxidation accelerated chromophore destruction with a
significantly reduced absorbance peak at 291 nm (Figs. 5c and 6). Solution pH was relatively constant throughout all the experiments, remaining within 0.5 units of the original pH of 8.8. This suggests that the synergy achieved during UVEL treatment may have been related to the destruction of electrochemically-derived intermediates with minimal interference due to molecular protonation or deprotonation. Additional electrochemical and photochemical degradation experiments were performed to elucidate the mechanism responsible for the apparent synergy in the hybrid UVEL process (Figs. 7 and 8). Two scenarios were explored. First, photochemical treatment was performed for 5 h followed by electrochemical treatment for another 5 h (Figs. 7a and 8a). Second, electrochemical treatment was performed for 5 h followed by photochemical treatment for another 5 h (Figs. 7b and 8b). Regardless of the order of operation, electrochemical treatment quickly produced intermediates with a characteristic absorbance at 291 nm. Photochemical treatment could not prevent the formation of electrochemicallyproduced intermediates (Fig. 7a), but proved very effective at removing them (Fig. 7b). Indeed, UV treatment alone required only
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Fig. 4. Excitation–emission matrix spectroscopy for the hybrid photo-assisted electrochemical treatment of secondary wastewater effluent RO concentrate.
Table 2 Size exclusion chromatography data from UV, EL, and UVEL processes.
WWROC feed 1 h UV 2 h UV 5 h UV 1 h EL 2 h EL 5 h EL 10 min UVEL 1 h UVEL 2 h UVEL 5 h UVEL
Biopolymers
Humics
Building blocks
LMW neutrals
LMW acids
DOC (mg/L)
DOC (mg/L)
SUVA (L/mg-m)
DOC (mg/L)
DOC (mg/L)
DOC (mg/L)
1.20 0.940 0.910 0.870 1.075 0.940 0.760 1.13 0.730 0.250 0.016
7.55 5.94 5.12 4.58 7.33 6.61 5.74 7.41 5.32 4.03 n.q.
2.49 1.68 1.33 0.65 0.78 0.75 0.79 1.58 0.40 0.15 n.q.
2.74 3.59 3.32 3.54 2.79 2.59 2.89 3.92 3.50 2.79 1.40
11.42 13.92 9.38 9.99 8.06 9.07 9.86 8.54 4.28 4.10 1.65
0.09 0.95 0.80 1.50 0.44 0.67 1.60 2.09 0.98 1.69 1.75
n.q. = not quantifiable (<1 ppb).
approximately 15 min to photolyze more than 90% of the intermediates formed after 5 h of electrochemical oxidation. However, no synergistic effect on DOC removal was observed when electrochemical and photochemical treatments were performed sequentially (Fig. 8). This suggests that the simultaneous electrochemical production and photolytic destruction of oxidation intermediates was a major mechanism leading to the synergistic behavior of the hybrid photo-assisted electrochemical process.
3.4. Formation of halogenated by-products Unlike electrochemical and UVEL treatments, photolysis alone resulted in negligible trihalomethane (THM) formation (Table 3). Electrochemical oxidation resulted in dramatically increased concentrations of all THMs, particularly heavily chlorinated molecules ([CHCl3], [CHBrCl2] > [CHBr2Cl] > [CHBr3]). This was expected due to the significantly greater reactivity of chlorine radicals over
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Fig. 5. UV–vis absorbance profile for (a) UV-C photolysis (UV), (b) electrochemical oxidation (EL), and (c) photo-assisted electrochemical oxidation (UVEL) of secondary wastewater effluent RO concentrate.
bromine radicals [38]. However, the UVEL process resulted in a diminished concentration of THMs after 5 h of treatment and more heavily favored the production of brominated species ([CHBr3] > [CHBr2Cl] > [CHBrCl2] > [CHCl3]). 4. Discussion
Fig. 6. Normalized UV–vis absorbance at 254 and 291 nm comparing electrochemical oxidation (EL), UV-C photolysis (UV), and photo-assisted electrochemical oxidation (UVEL) of wastewater reverse osmosis concentrate.
UVEL treatment was proven to be significantly superior to conventional electrochemical and photochemical processes due to the emergence of a unique synergistic degradation mechanism attained by their simultaneous operation (Fig. 2). The UVEL degradation mechanism was studied by the use of a series of in-depth analytical techniques, such as EEM fluorescence, SEC, and UV–vis absorbance. EEM spectroscopy results (Figs. 3 and 4) indicated that electrochemical oxidation was much more effective than UV photolysis at targeting molecular bonds responsible for the fluorescence of large parent organics found in WWROC. SEC showed that this occurred via cleavage of conjugated bonds and aromatic rings rather than complete molecular degradation as demonstrated by the greater reduction in specific ultraviolet absorbance (SUVA)
Fig. 7. UV–vis absorbance at 254 and 291 nm for the sequential treatment of wastewater reverse osmosis concentrate: (a) UV-C photolysis (UV) followed by electrochemical oxidation (EL) and (b) EL followed by UV.
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Fig. 8. Comparison of DOC removal for the independent and sequential treatment of wastewater reverse osmosis concentrate: (a) electrochemical oxidation (EL) only and UV-C photolysis (UV) followed by EL and (b) UV only and EL followed by UV.
Table 3 Trihalomethane (THM) formation for UV, EL, and UVEL processes. Concentration in lg/L
WWROC feed UV after 2 h UV after 5 h EL after 2 h EL after 5 h UVEL after 2 h UVEL after 5 h
Bromoform
Chloroform
Dibromochloroform
Dichlorobromoform
Total THM
<10 <6 <6 48 24 228 66
<10 <6 <6 138 72 12 12
<10 <6 <6 108 48 138 36
<10 <6 <6 144 60 48 6
<40 <24 <24 438 204 426 120
of the humics for EL than for UV treatment (Table 2). However, electrochemical oxidation was less effective at breaking down parent macromolecules beyond fluorescence removal. Photolysis was much more effective at destroying core structural bonds, which led to the conversion of large macromolecules to smaller secondary molecules. Photolysis removed more humics and biopolymers within the first hour of treatment than did EL, which was marked by the subsequent increase in building blocks, LMW neutrals, and LMW acids. Overall, the results suggest that EL and UV target different molecular bonds and that the synergy observed during the hybrid UVEL treatment could be due to cooperative (or complementary) attack of these bonds. The UVEL pathway improved removal of organic chromophores and enhanced total mineralization. Fluorescence results not only showed that UVEL treatment removed large parent organic molecules (i.e., humics, fulvics, soluble microbial products, and aromatic proteins), but that it did so much more efficiently than either electrochemical or photochemical treatments alone (Figs. 3 and 4). SEC further confirmed that UVEL treatment effectively degraded a variety of organic molecules over a wider range of molecular weights (Table 2). Quantitatively, this synergistic mechanism can be observed as the difference between the hypothetical and observed UVEL rates of DOC removal (Fig. 2a). Absorbance spectroscopy showed that reactive oxidation intermediates also played a role in the overall UVEL degradation mechanism. UVEL degradation of organic chromophores could be related
to the destruction of electrochemical intermediates observed at 291 nm (Figs. 5 and 6). Since this characteristic absorbance band was not present initially in the WWROC, it was most likely associated with the formation of electrochemical oxidation intermediates. Production of these intermediates was not affected by photochemical pretreatment (Fig. 7a), but they were rapidly broken down by photochemical post-treatment (Fig. 7b). These oxidation intermediates may be associated with the formation of carbonylated organochlorines, such as chloranil [39,40], as well as hypochlorite ions [41–45]. The role of chloride is of specific importance due to the potential impact of active chlorine species, such as Cl2, HOCl, and OCl . Chloride can be oxidized into active chlorine through redox reactions at both the anode and cathode [46–48]. This secondary oxidation pathway was observed for both electrochemical and UVEL processes (Fig. 6). Electrochemical treatment exhibited higher removal rates of organic chromophores within the first 15 min compared with UV treatment. However, after 15 min only electrochemical treatment produced intermediates with absorbance profiles centered around 291 nm. This indicates that the formation of active chlorine was, indeed, unique to the electrochemical system and that supplementary oxidation by active chlorine was best employed in the presence of UV-C radiation. The transition point observed in the energy demand for the hybrid UVEL system (Fig. 2b) may be characteristic of this effect. It may represent a point of depletion of active chlorine, which would limit the
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effectiveness of electrochemically-driven secondary oxidation at which the slope of the specific energy consumption mirrored the UV process. Other synergistic oxidation mechanisms include the formation of highly reactive hydroxyl and chlorine radicals from the photolysis of electrogenerated active chlorine [24,49]. The UVEL process was seen to improve the energy efficiency of UV photolysis and mitigate the electrochemical formation of disinfection by-products (DBPs) at residence times exceeding 2 h. UVEL resulted in a noticeable production of DBPs, specifically THMs, but at a rate lower than EL alone (Table 3). Although total THM concentrations were comparable after the first hour of treatment for both electrochemical and UVEL treatments, the UVEL process showed more efficient removal rates with time (120 and 204 ppb for UVEL and electrochemical oxidation, respectively, after 5 h of treatment). Photo-irradiation reduced the production of heavily chlorinated THMs during UVEL treatment, but favored the formation of brominated THMs. UV-C photolysis dramatically reduced the residence times of electrochemically-generated HOCl in the UVEL system, which allowed greater interaction between reactive bromine species and organic matter to selectively form brominated THMs. The increased steric obstruction of bromine compared with chlorine may have led to C–Br bonds that were less accessible to further radical or photolytic reactions [50]. Therefore, the overall effect of UV irradiation within the UVEL system may have been an increased formation rate and minimal destruction of brominated organics. The mitigation of the formation of DBPs by electrochemical oxidation is a promising result and should be further exploited in future UVEL studies, as should optimization of the high rate of mineralization and low selectivity for organic removal. Although a significant majority of the UVEL energy demand was due to the power consumption of the UV lamp, which consumed over 95% of the total power for the process (Fig. 2), the electrochemical cell will also be of focus for future optimization studies. The applied current density and resultant current efficiency play dominant roles in the overall kinetics and energy efficiency of the UVEL process. In this research, the target current density of 20 mA/cm2 was chosen after a thorough literature review of comparable EL applications [4,13,28,51,52]. However, future work will be devoted to studying if the unique UVEL oxidation mechanism can benefit with operation at current densities beyond what is recommended by conventional electrochemical practice (i.e., upwards of 100 mA/cm2). The role of current density will also be analyzed with respect to its effect on the required electrode area in order to drive down the UVEL capital investment costs. Further studies should also focus on reducing the power consumption of the photochemical component of the UVEL process by exploring lower energy UV sources, such as UV-A and UV-B.
5. Conclusions The combination of UV photolysis with anodic oxidation by boron-doped diamond led to non-selective and synergistic degradation of effluent organic matter in wastewater RO concentrate. The UVEL process effectively treated a wide array of organics found in wastewater effluent at a rate that exceeded treatment by UV and EL individually or additively. The observed synergy was due in part to the cooperative destruction of select chemical bonds favored by each process individually. Electrochemical oxidation was shown to be less effective for the structural degradation of large macromolecules, but more effective for ring cleavage and the removal of aromatic functionality, while photolysis preferentially targeted the destruction of backbone aliphatic bonds and led to less aromatic degradation. As a result, UVEL was able to effectively mineralize not only large aromatic macromolecules, such as biopolymers and humics, but also smaller derivative compounds, such as low
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molecular weight organic acids, without significant intermediate bottlenecking. Overall oxidation was further aided by the production of highly reactive hydroxyl and chlorine radicals. The chlorine radicals were produced by the photolysis of electrochemically generated active chlorine and introduced a new class of reactants with both high oxidation potentials and low selectivities. The overall result of this research was the development of an advanced oxidation process that utilized conventional plug-in processes with a boron-doped diamond electrode material to effectively degrade a wide range of dissolved organic matter found in wastewater RO concentrate. In doing so, energy efficiency was enhanced and DBP formation mitigated compared with UV and EL alone, respectively. The authors find these results highly promising and suggestive of future work focusing on the targeted removal of recalcitrant emerging organic contaminants, such as pharmaceuticals and endocrine disrupting compounds, in wastewater RO concentrate prior to discharge into receiving bodies.
Acknowledgements This research was supported in part by the UCLA Water Technology Research Center through the California Department of Water Resources Proposition 50 Grant Program (Agreement No. 4600004120). We are also grateful to Dr. Sam Aroni from the UCLA International Institute’s Special Academic Cooperative Project as well as the UCLA Department of Civil & Environmental Engineering Horn fund and RMIT University for providing additional financial support.
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Photo-assisted electrochemical treatment of municipal wastewater reverse osmosis concentrate Gil Hurwitz a,⇑, Eric M.V. Hoek a,b, Kai Liu c, Linhua Fan c, Felicity A. Roddick c a
Department of Civil & Environmental Engineering, California NanoSystems Institute and Institute of the Environment & Sustainability, University of California, Los Angeles, CA, USA Department of Applied Chemistry, University of Johannesburg, Johannesburg, South Africa c RMIT University, School of Civil, Environmental & Chemical Engineering, Melbourne, Victoria, Australia b
h i g h l i g h t s Organics in RO concentrate were treated by a hybrid advanced oxidation process. Hybrid photolytic and electrochemical process showed synergistic DOC degradation. Electrochemical breaks down fluorescence and photolysis destroys core structure. Hybrid treatment consumes less energy than photolysis alone.
a r t i c l e
i n f o
Article history: Received 4 December 2013 Received in revised form 19 March 2014 Accepted 21 March 2014 Available online 3 April 2014 Keywords: Photolysis Electrochemical oxidation Synergy Wastewater Reverse osmosis concentrate
a b s t r a c t The combination of a photochemical (UV) and electrochemical (EL) process led to enhanced degradation of dissolved organic matter in reverse osmosis (RO) concentrate produced from municipal wastewater. Treatment by UV and EL alone resulted in 25% and 35% removal of dissolved organic carbon (DOC) after 5 h, respectively. However, the hybrid process (UVEL) degraded more than 80% of DOC after the same treatment time. Fluorescence excitation–emission matrix spectroscopy and size exclusion chromatography suggest simultaneous and cooperative degradation of backbone aliphatic bonds by UV and aromatic ring cleavage by EL within the UVEL process. Overall, UVEL treatment led to efficient and non-selective degradation of dissolved organics over a wide range of molecular weights. Further, energy consumption and halogenated by-product formation (typically a limiting factor for the application of oxidation technology) were reduced in the UVEL process relative to the UV and EL processes, respectively. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Municipal wastewater effluent treated by RO produces highly purified product water with many valuable uses (e.g., landscape irrigation, industrial process water, and aquifer recharge). However, the RO process also produces a highly concentrated version of the wastewater effluent, which may contain elevated concentrations of contaminants such as dissolved and suspended solids comprising BOD, COD, pathogens, and trace organics (e.g., pharmaceuticals and personal care products, endocrine disrupting compounds, and disinfection byproducts) not fully removed by the preceding biological treatment process [1–3]. The potential human and environmental health hazards associated with discharging ⇑ Corresponding author. Current address: Water Planet Engineering, 721 Glasgow Ave, Unit D, Los Angeles, CA 90301, USA. Tel.: +1 (310) 215 8960; fax: +1 (310) 215 8961. E-mail address: [email protected] (G. Hurwitz). http://dx.doi.org/10.1016/j.cej.2014.03.084 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.
wastewater RO concentrate containing elevated levels of pathogens, BOD/COD, and trace organics are not fully understood [4–10]. Therefore, it seems prudent to explore treatment technologies that can ensure effluent discharge standards are met, particularly when dilution (i.e., mixing the RO concentrate with large volumes of unconcentrated wastewater treatment plant effluent) is not possible. In particular, further disinfection and organic removal may be necessary. Both photochemical and electrochemical processes can be used for disinfection and organic removal. Direct UV irradiation of wastewater gives rise to photo-activated chemical reactions that translate absorbed photonic energy into direct chemical bond breakage or the production of free radicals [11,12]. Utilization of photolysis for advanced oxidation is typically achieved by the addition of catalysts or UV reactive compounds (i.e., H2O2 and TiO2) to provide a source of the requisite hydroxyl radicals [12]. Electrochemical oxidation targets mineralization of organic compounds by electron abstraction at the anode surface or by the
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production of highly reactive oxidants, such as OH, produced in situ electrochemically [13]. In situ production of chemical reactants eliminates the need to purchase, transport, and store chemicals produced off-site, which can save money and energy as well as reduce carbon emissions and potential security vulnerabilities [14]. Electrochemical oxidation may be an attractive option for treating wastewater RO concentrate because it becomes more efficient and effective with increased conductivity and chloride concentration. High conductivity reduces the voltage needed to achieve a target current density, which results in considerable energy savings. However, electrochemical processes traditionally suffer from high investment costs, especially for advanced anode materials, such as boron-doped diamond. For this reason, improving electrochemical oxidation efficiency is paramount in order to reduce the required active electrode area and, thus, the initial capital cost for a given electrochemical treatment system [15–17]. Advanced anode materials have garnered considerable attention recently due to their ability to improve oxidation efficiency by enhancing the rate of direct oxidation at or near the electrode surface. Generally, advanced anodes are characterized as ‘‘nonactive’’ materials with high oxygen evolution overpotentials, such as antimonydoped tin oxide, lead dioxide, and boron-doped diamond, and favor the complete oxidation of organics to CO2 due to their weak surface interactions with hydroxyl radicals [18]. Boron-doped diamond also shares the ability of some active materials, such as Ti/RuO2, to promote indirect bulk oxidation by the production of intermediate oxidants, of which chlorine is commonly targeted [19,20]. Specifically, chloride ions become oxidized at the anode; thus, introducing an entire family of potential secondary oxidants and disinfectants. Indirect organic oxidation may occur via reaction with chlorine radicals (Cl) adsorbed on the anode surface or by reactive chlorine species such as Cl2, HOCl, or OCl in the bulk solution [13]. However, the formation of halogenated by-products is an obvious concern when treating waters containing both halide ions and dissolved organic matter. Significant research has been undertaken to determine the effectiveness of hybrid photo-assisted electrochemical oxidation processes with particular emphasis on the UV excitation of photoanodes (e.g., thin-film TiO2, DSAÒ) [21–25]. However, due to its minimal anodic photoactivity, we have not been able to find previous research studying the use of boron-doped diamond (a highly promising material with superior electrochemical properties) electrodes in hybrid photo-assisted electrochemical oxidation processes [26]. Herein, we tested a hybrid photo-assisted electrochemical process (UVEL) with the electrochemical and photochemical reactors placed in series within a batch recirculation system. This unique reactor configuration can, therefore, take advantage of the electrochemical benefits of boron-doped diamond electrodes (wide potential window, low background current, high chemical stability, high resistance to deactivation [27–30]) without the need to invest energy and resources to enhance its photoactivity. The aim of this research is to study the practical limitations (extent of mineralization, energy consumption, by-product formation) of a photoassisted electrochemical reactor, which integrates highly efficient boron-doped diamond electrodes to treat a target water of growing public and environmental concern. 2. Methodology Biologically treated wastewater effluent was provided by Western Treatment Plant (Victoria, Australia), which used a sequential activated sludge-lagoon treatment (AS-lagoon) process. RO concentrate was prepared from the effluent using a laboratory scale plate-and-frame unit (Sepa CF, GE-Osmonics, Minnetonka, MN) and a commercial interfacial composite RO membrane (AG; GE-Osmonics, Minnetonka, MN) operated at a constant pressure
and temperature of 17 bar and 20 °C and an average permeate flux of 46.8 LMH. The effluent was concentrated to a recovery of 65%. Major water quality parameters are given in Table 1. Electrochemical oxidation (EL) experiments were performed in a flow-through, undivided, spacer-filled electrochemical cell. A DiaChemÒ (Electrocell, Amherst, NY) boron-doped diamond anode was used with a geometric active area of 10 cm2. Electrochemical oxidation was performed at a constant current density of 20 mA/ cm2 with a flow rate of 0.033 m3/h and Reynolds number of 360. Photolysis (UV) experiments were conducted using an annular reactor with a centrally mounted lamp. A low-pressure mercuryvapor lamp emitting at 254 nm with an intensity of 8500 lW/ cm2 was used as the UV-C irradiation source (G36T15NU; Australian Ultra Violet Services, Victoria, Australia). The average irradiated area was 464 cm2 with a path length of 1.94 cm; other UV reactor conditions were reported elsewhere [11]. A Titan 1500 chiller (Aqua-Medic, Bissendorf, Germany) was used to maintain a constant solution temperature of 20 °C during all the experiments. The UVEL reactor subjected the wastewater RO concentrate (WWROC) to both electrochemical oxidation and UV photolysis in series, recirculation mode (Fig. 1). Specific power consumption was estimated from the power consumed over the operating time and normalized by the total volume processed. Dissolved organic carbon (DOC) was determined by measuring the total organic carbon (TOC) content (Sievers 5310 C, GE, Boulder, Colorado, USA) of 0.45 lm filtered samples. The TOC analyzer was equipped with an auto-sampler and inorganic carbon was internally purged automatically (Sievers 900 ICR; GE, Boulder, Colorado, USA) prior to DOC analysis. The absorbance was measured using a Unicam UV–vis spectrophotometer with a quartz cell of 1 cm pathlength. The excitation–emission matrix (EEM) spectra of the samples were obtained with a fluorescence spectrometer (LS55, Perkin Elmer, Waltham, Massachusetts, USA). The size exclusion chromatography (SEC) analyses were carried out at the Water Research Centre of the University of New South Wales, Australia. The SEC system (LC-OCD Model 8, DOC-Labor Dr. Hüber, Germany) consisted of a SEC column (Toyopearl TSK HW-50S, diameter 2 cm, length 25 cm) to separate organic molecules by size (i.e., 100–200,000 Da). Trihalomethane concentrations were determined by the Australian Water Quality Centre in Adelaide, Australia using previously described methods [31].
3. Results 3.1. DOC removal and energy consumption After 5 h of treatment of the WWROC, the electrochemical and photochemical processes resulted in approximately 25% and 35% Table 1 Water quality of secondary wastewater RO concentrate. Parameter
Unit
Value
Conductivity pH DOC A254 Na+ Ca2+ Mg2+ Cl Br SO24 TN, as N TP, as P Alkalinity, as CaCO3 Color (Pt–Co) Turbidity
mS/cm – mg/L /cm mg/L mg/L mg/L mg/L mg/L mg/L mg N/L mg P/L mg CaCO3/L mg Pt–Co/L NTU
3.8 8.8 22 0.42 600 47 53 954 3.9 207 32 8 242 55 0.5
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Fig. 1. Schematic diagram of the hybrid photo-assisted electrochemical reactor. (1) Jacketed beaker; (2) magnetic stirrer; (3) thermometer; (4) temperature control device; (5) pump; (6) commercial undivided electrochemical cell; (7) mesh turbulence promoter; (8) cathode; (9) anode; (10) DC power supply; (11) annular photoreactor; (12) UV-C lamp; and (13) quartz sleeve.
DOC removal, respectively; however, the UVEL process achieved approximately 80% removal (Fig. 2a). If the photochemical and electrochemical degradation rates were mathematically combined, the hypothetical DOC removal by addition would be only 60%; hence there was a synergistic effect of combining the two processes and a previously unrecorded and potentially powerful oxidation mechanism at play. Moreover, both the electrochemical and UVEL treatments were less energy intensive (per unit of DOC removal) than the photochemical process (Fig. 2b). The slopes of
Fig. 2. Electrochemical oxidation (EL), UV-C photolysis (UV), hypothetical summation of electrochemical oxidation and photolysis (UV + EL), and hybrid photoassisted electrochemical oxidation (UVEL) performance: (a) DOC removal versus time and (b) specific power versus conversion.
photochemical and electrochemical power consumption plotted against DOC conversion were 1160 and 24 kW h/m3, respectively. The UVEL process consumed an intermediate amount of energy up to 60% DOC removal (176 kW h/m3); however, at conversions above 60% the hybrid process energy consumption increased to 1170 kW h/m3, which was almost identical to that of UV-C irradiation alone.
3.2. Mechanisms of organic degradation EEM spectroscopy was used to better understand the unique degradation pathway observed during the UVEL process by tracking the relative rate of removal for different fluorescent functional groups. Two major fluorescence peaks were distinguished for the WWROC (at kEm/kEx 430/340 and 440/250) (Fig. 3). These peaks derive predominantly from humic and fulvic compounds, respectively [32–36]. After 1 h of electrochemical treatment the intensity of fluorescence was greatly reduced and after 5 h no fluorescence remained. On the other hand, UV treatment led to less reduction in the fluorescence of the humic and fulvic compounds after 1 h with a residual remaining after 5 h. There was a similar effect on the minor fluorescent peaks, such as those for soluble microbial products (kEm/kEx 380/250–280) and aromatic proteins (kEm/kEx 340–355/235–240 and 320–335/280–285) [37]. The combined UVEL treatment virtually eliminated all fluorescent peaks within 1 h (Fig. 3). Looking more closely at the first hour, photo-assisted electrochemical treatment degraded humics and fulvics at comparable rates (Fig. 4), producing nearly complete reduction of fluorescence after 40 min. SEC was used to directly observe how effective each treatment option was on structural degradation and the conversion of larger organic molecules into smaller derivatives (Table 2). The high molecular weight fraction (>20,000 Da) represents biopolymers, such as polysaccharides, proteins, and aminosugars. The medium-sized molecular weight fractions represent humic substances (1000 Da) and ‘‘building block’’ molecules (300–500 Da), which signify breakdown products formed from humic degradation. Finally, the low molecular weight (LMW) fractions for molecules less than 350 Da represent LMW neutrals (i.e., mono-oligosaccharides, alcohols, aldehydes, and ketones) and LMW acids (i.e., monoprotic organic acids). The UVEL process resulted in the conversion of large macromolecules into smaller derivatives at a rate considerably higher than photochemical or electrochemical treatments alone. Specifically, 1 h of UVEL treatment resulted in
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Fig. 3. Excitation–emission matrix spectroscopy for the UV-C photolysis (UV), electrochemical oxidation (EL), and photo-assisted electrochemical oxidation (UVEL) of secondary wastewater effluent RO concentrate.
significant degradation of biopolymers and humic substances and after 5 h there were only low concentrations of building block and low molecular weight organics remaining. In contrast, within the first hour of treatment EL only targeted the rapid removal of humic aromaticity while UV targeted the conversion of biopolymers and humics to smaller secondary molecules (i.e., building blocks, LMW neutrals, and LMW acids). However, both electrochemical and photochemical treatments were less effective beyond 1 h in that significant concentrations of organics across a wide range of molecular weights remained, even after 5 h of treatment. 3.3. Tracking changes in ultraviolet absorbance UV–vis absorption spectra were analyzed to gather more insight into the UVEL degradation mechanism and the role of oxidation intermediates (Figs. 5 and 6). During UV-C photolysis alone, the absorbance profile between 190 and 350 nm displayed a low, yet steady rate of decline (Fig. 5a). On the other hand, electrochemical treatment produced a new absorption band at 291 nm (Fig. 5b). UVEL oxidation accelerated chromophore destruction with a
significantly reduced absorbance peak at 291 nm (Figs. 5c and 6). Solution pH was relatively constant throughout all the experiments, remaining within 0.5 units of the original pH of 8.8. This suggests that the synergy achieved during UVEL treatment may have been related to the destruction of electrochemically-derived intermediates with minimal interference due to molecular protonation or deprotonation. Additional electrochemical and photochemical degradation experiments were performed to elucidate the mechanism responsible for the apparent synergy in the hybrid UVEL process (Figs. 7 and 8). Two scenarios were explored. First, photochemical treatment was performed for 5 h followed by electrochemical treatment for another 5 h (Figs. 7a and 8a). Second, electrochemical treatment was performed for 5 h followed by photochemical treatment for another 5 h (Figs. 7b and 8b). Regardless of the order of operation, electrochemical treatment quickly produced intermediates with a characteristic absorbance at 291 nm. Photochemical treatment could not prevent the formation of electrochemicallyproduced intermediates (Fig. 7a), but proved very effective at removing them (Fig. 7b). Indeed, UV treatment alone required only
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Fig. 4. Excitation–emission matrix spectroscopy for the hybrid photo-assisted electrochemical treatment of secondary wastewater effluent RO concentrate.
Table 2 Size exclusion chromatography data from UV, EL, and UVEL processes.
WWROC feed 1 h UV 2 h UV 5 h UV 1 h EL 2 h EL 5 h EL 10 min UVEL 1 h UVEL 2 h UVEL 5 h UVEL
Biopolymers
Humics
Building blocks
LMW neutrals
LMW acids
DOC (mg/L)
DOC (mg/L)
SUVA (L/mg-m)
DOC (mg/L)
DOC (mg/L)
DOC (mg/L)
1.20 0.940 0.910 0.870 1.075 0.940 0.760 1.13 0.730 0.250 0.016
7.55 5.94 5.12 4.58 7.33 6.61 5.74 7.41 5.32 4.03 n.q.
2.49 1.68 1.33 0.65 0.78 0.75 0.79 1.58 0.40 0.15 n.q.
2.74 3.59 3.32 3.54 2.79 2.59 2.89 3.92 3.50 2.79 1.40
11.42 13.92 9.38 9.99 8.06 9.07 9.86 8.54 4.28 4.10 1.65
0.09 0.95 0.80 1.50 0.44 0.67 1.60 2.09 0.98 1.69 1.75
n.q. = not quantifiable (<1 ppb).
approximately 15 min to photolyze more than 90% of the intermediates formed after 5 h of electrochemical oxidation. However, no synergistic effect on DOC removal was observed when electrochemical and photochemical treatments were performed sequentially (Fig. 8). This suggests that the simultaneous electrochemical production and photolytic destruction of oxidation intermediates was a major mechanism leading to the synergistic behavior of the hybrid photo-assisted electrochemical process.
3.4. Formation of halogenated by-products Unlike electrochemical and UVEL treatments, photolysis alone resulted in negligible trihalomethane (THM) formation (Table 3). Electrochemical oxidation resulted in dramatically increased concentrations of all THMs, particularly heavily chlorinated molecules ([CHCl3], [CHBrCl2] > [CHBr2Cl] > [CHBr3]). This was expected due to the significantly greater reactivity of chlorine radicals over
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Fig. 5. UV–vis absorbance profile for (a) UV-C photolysis (UV), (b) electrochemical oxidation (EL), and (c) photo-assisted electrochemical oxidation (UVEL) of secondary wastewater effluent RO concentrate.
bromine radicals [38]. However, the UVEL process resulted in a diminished concentration of THMs after 5 h of treatment and more heavily favored the production of brominated species ([CHBr3] > [CHBr2Cl] > [CHBrCl2] > [CHCl3]). 4. Discussion
Fig. 6. Normalized UV–vis absorbance at 254 and 291 nm comparing electrochemical oxidation (EL), UV-C photolysis (UV), and photo-assisted electrochemical oxidation (UVEL) of wastewater reverse osmosis concentrate.
UVEL treatment was proven to be significantly superior to conventional electrochemical and photochemical processes due to the emergence of a unique synergistic degradation mechanism attained by their simultaneous operation (Fig. 2). The UVEL degradation mechanism was studied by the use of a series of in-depth analytical techniques, such as EEM fluorescence, SEC, and UV–vis absorbance. EEM spectroscopy results (Figs. 3 and 4) indicated that electrochemical oxidation was much more effective than UV photolysis at targeting molecular bonds responsible for the fluorescence of large parent organics found in WWROC. SEC showed that this occurred via cleavage of conjugated bonds and aromatic rings rather than complete molecular degradation as demonstrated by the greater reduction in specific ultraviolet absorbance (SUVA)
Fig. 7. UV–vis absorbance at 254 and 291 nm for the sequential treatment of wastewater reverse osmosis concentrate: (a) UV-C photolysis (UV) followed by electrochemical oxidation (EL) and (b) EL followed by UV.
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Fig. 8. Comparison of DOC removal for the independent and sequential treatment of wastewater reverse osmosis concentrate: (a) electrochemical oxidation (EL) only and UV-C photolysis (UV) followed by EL and (b) UV only and EL followed by UV.
Table 3 Trihalomethane (THM) formation for UV, EL, and UVEL processes. Concentration in lg/L
WWROC feed UV after 2 h UV after 5 h EL after 2 h EL after 5 h UVEL after 2 h UVEL after 5 h
Bromoform
Chloroform
Dibromochloroform
Dichlorobromoform
Total THM
<10 <6 <6 48 24 228 66
<10 <6 <6 138 72 12 12
<10 <6 <6 108 48 138 36
<10 <6 <6 144 60 48 6
<40 <24 <24 438 204 426 120
of the humics for EL than for UV treatment (Table 2). However, electrochemical oxidation was less effective at breaking down parent macromolecules beyond fluorescence removal. Photolysis was much more effective at destroying core structural bonds, which led to the conversion of large macromolecules to smaller secondary molecules. Photolysis removed more humics and biopolymers within the first hour of treatment than did EL, which was marked by the subsequent increase in building blocks, LMW neutrals, and LMW acids. Overall, the results suggest that EL and UV target different molecular bonds and that the synergy observed during the hybrid UVEL treatment could be due to cooperative (or complementary) attack of these bonds. The UVEL pathway improved removal of organic chromophores and enhanced total mineralization. Fluorescence results not only showed that UVEL treatment removed large parent organic molecules (i.e., humics, fulvics, soluble microbial products, and aromatic proteins), but that it did so much more efficiently than either electrochemical or photochemical treatments alone (Figs. 3 and 4). SEC further confirmed that UVEL treatment effectively degraded a variety of organic molecules over a wider range of molecular weights (Table 2). Quantitatively, this synergistic mechanism can be observed as the difference between the hypothetical and observed UVEL rates of DOC removal (Fig. 2a). Absorbance spectroscopy showed that reactive oxidation intermediates also played a role in the overall UVEL degradation mechanism. UVEL degradation of organic chromophores could be related
to the destruction of electrochemical intermediates observed at 291 nm (Figs. 5 and 6). Since this characteristic absorbance band was not present initially in the WWROC, it was most likely associated with the formation of electrochemical oxidation intermediates. Production of these intermediates was not affected by photochemical pretreatment (Fig. 7a), but they were rapidly broken down by photochemical post-treatment (Fig. 7b). These oxidation intermediates may be associated with the formation of carbonylated organochlorines, such as chloranil [39,40], as well as hypochlorite ions [41–45]. The role of chloride is of specific importance due to the potential impact of active chlorine species, such as Cl2, HOCl, and OCl . Chloride can be oxidized into active chlorine through redox reactions at both the anode and cathode [46–48]. This secondary oxidation pathway was observed for both electrochemical and UVEL processes (Fig. 6). Electrochemical treatment exhibited higher removal rates of organic chromophores within the first 15 min compared with UV treatment. However, after 15 min only electrochemical treatment produced intermediates with absorbance profiles centered around 291 nm. This indicates that the formation of active chlorine was, indeed, unique to the electrochemical system and that supplementary oxidation by active chlorine was best employed in the presence of UV-C radiation. The transition point observed in the energy demand for the hybrid UVEL system (Fig. 2b) may be characteristic of this effect. It may represent a point of depletion of active chlorine, which would limit the
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effectiveness of electrochemically-driven secondary oxidation at which the slope of the specific energy consumption mirrored the UV process. Other synergistic oxidation mechanisms include the formation of highly reactive hydroxyl and chlorine radicals from the photolysis of electrogenerated active chlorine [24,49]. The UVEL process was seen to improve the energy efficiency of UV photolysis and mitigate the electrochemical formation of disinfection by-products (DBPs) at residence times exceeding 2 h. UVEL resulted in a noticeable production of DBPs, specifically THMs, but at a rate lower than EL alone (Table 3). Although total THM concentrations were comparable after the first hour of treatment for both electrochemical and UVEL treatments, the UVEL process showed more efficient removal rates with time (120 and 204 ppb for UVEL and electrochemical oxidation, respectively, after 5 h of treatment). Photo-irradiation reduced the production of heavily chlorinated THMs during UVEL treatment, but favored the formation of brominated THMs. UV-C photolysis dramatically reduced the residence times of electrochemically-generated HOCl in the UVEL system, which allowed greater interaction between reactive bromine species and organic matter to selectively form brominated THMs. The increased steric obstruction of bromine compared with chlorine may have led to C–Br bonds that were less accessible to further radical or photolytic reactions [50]. Therefore, the overall effect of UV irradiation within the UVEL system may have been an increased formation rate and minimal destruction of brominated organics. The mitigation of the formation of DBPs by electrochemical oxidation is a promising result and should be further exploited in future UVEL studies, as should optimization of the high rate of mineralization and low selectivity for organic removal. Although a significant majority of the UVEL energy demand was due to the power consumption of the UV lamp, which consumed over 95% of the total power for the process (Fig. 2), the electrochemical cell will also be of focus for future optimization studies. The applied current density and resultant current efficiency play dominant roles in the overall kinetics and energy efficiency of the UVEL process. In this research, the target current density of 20 mA/cm2 was chosen after a thorough literature review of comparable EL applications [4,13,28,51,52]. However, future work will be devoted to studying if the unique UVEL oxidation mechanism can benefit with operation at current densities beyond what is recommended by conventional electrochemical practice (i.e., upwards of 100 mA/cm2). The role of current density will also be analyzed with respect to its effect on the required electrode area in order to drive down the UVEL capital investment costs. Further studies should also focus on reducing the power consumption of the photochemical component of the UVEL process by exploring lower energy UV sources, such as UV-A and UV-B.
5. Conclusions The combination of UV photolysis with anodic oxidation by boron-doped diamond led to non-selective and synergistic degradation of effluent organic matter in wastewater RO concentrate. The UVEL process effectively treated a wide array of organics found in wastewater effluent at a rate that exceeded treatment by UV and EL individually or additively. The observed synergy was due in part to the cooperative destruction of select chemical bonds favored by each process individually. Electrochemical oxidation was shown to be less effective for the structural degradation of large macromolecules, but more effective for ring cleavage and the removal of aromatic functionality, while photolysis preferentially targeted the destruction of backbone aliphatic bonds and led to less aromatic degradation. As a result, UVEL was able to effectively mineralize not only large aromatic macromolecules, such as biopolymers and humics, but also smaller derivative compounds, such as low
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molecular weight organic acids, without significant intermediate bottlenecking. Overall oxidation was further aided by the production of highly reactive hydroxyl and chlorine radicals. The chlorine radicals were produced by the photolysis of electrochemically generated active chlorine and introduced a new class of reactants with both high oxidation potentials and low selectivities. The overall result of this research was the development of an advanced oxidation process that utilized conventional plug-in processes with a boron-doped diamond electrode material to effectively degrade a wide range of dissolved organic matter found in wastewater RO concentrate. In doing so, energy efficiency was enhanced and DBP formation mitigated compared with UV and EL alone, respectively. The authors find these results highly promising and suggestive of future work focusing on the targeted removal of recalcitrant emerging organic contaminants, such as pharmaceuticals and endocrine disrupting compounds, in wastewater RO concentrate prior to discharge into receiving bodies.
Acknowledgements This research was supported in part by the UCLA Water Technology Research Center through the California Department of Water Resources Proposition 50 Grant Program (Agreement No. 4600004120). We are also grateful to Dr. Sam Aroni from the UCLA International Institute’s Special Academic Cooperative Project as well as the UCLA Department of Civil & Environmental Engineering Horn fund and RMIT University for providing additional financial support.
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