Environ Sci Pollut Res DOI 10.1007/s11356-014-3907-3

RESEARCH ARTICLE

In situ reactive oxygen species production for tertiary wastewater treatment Léa Guitaya & Patrick Drogui & Jean François Blais

Received: 1 September 2014 / Accepted: 24 November 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract The goal of this research was to develop a new approach for tertiary water treatment, particularly disinfection and removal of refractory organic compounds, without adding any chemical. Hydrogen peroxide can indeed be produced from dissolved oxygen owing to electrochemical processes. Using various current intensities (1.0 to 4.0 A), it was possible to in situ produce relatively high concentration of H2O2 with a specific production rate of 0.05×10−5 M/min/A. Likewise, by using ultraviolet-visible absorption spectroscopy method, it was shown that other reactive oxygen species (ROS) including HO* radical and O3 could be simultaneously formed during electrolysis. The ROS concentration passed from 0.45×10−5 M after 20 min of electrolysis to a concentration of 2.87×10−5 M after 100 min of electrolysis. The disinfection and the organic matter removal were relatively high during the tertiary treatment of municipal and domestic wastewaters. More than 90 % of organic compounds (chemical oxygen demand) can be removed, whereas 99 % of faecal coliform abatement can be reached. Likewise, the process was also effective in removing turbidity (more than 90 % of turbidity was removed) so that the effluent became more and more transparent.

Responsible editor: Bingcai Pan Electronic supplementary material The online version of this article (doi:10.1007/s11356-014-3907-3) contains supplementary material, which is available to authorized users. L. Guitaya : P. Drogui (*) : J. F. Blais Institut National de la Recherche Scientifique (INRS-Eau Terre et Environnement), Université du Quebec, 490 rue de la Couronne, Quebec, QC, Canada G1K 9A9 e-mail: [email protected] L. Guitaya e-mail: [email protected] J. F. Blais e-mail: [email protected]

Keywords Reactive oxygen species . Electrooxidation . Wastewater . Carbon felt . Tertiary treatment . Clean technology Abbreviations CF Carbon felt GF Graphite felt COD Chemical oxygen demand RNO p-Nitrosodimethylaniline DWW Domestic wastewater MWW Municipal wastewater SS Suspended solids FC Faecal coliform C Concentration of RNO at time t C0 Initial concentration of RNO kR RNO decomposition rate apparent constant N0 Initial concentration of faecal coliform Nt Concentration of faecal coliforms at time t

Introduction Domestic and municipal wastewaters (DWW and MWW) contain varied and high amounts of organic, inorganic and microbial pollutants (Table 1) that can escape to conventional wastewater treatment processes (Vlyssides et al. 2002). Their exposure and accumulation of pollutants in the aquatic environment lead to adverse effect towards human life and nature and cause eutrophication of surface waters and transmission of waterborne diseases (Konnerup et al. 2009; Denny 1997). Therefore, the major concern is to treat the wastewater before it is discharged into the aquatic environment. The stricter restrictions imposed by new legislations have caused extensive effort to focus on the development of powerful treatment process as an alternative methods applied for achieving the complete removal of organic pollutants from domestic and

Environ Sci Pollut Res Table 1 Characteristics of the municipal wastewaters (MWW) and domestic wastewaters (DWW) Parameters pH Conductivity (μS cm−1) Turbidity (NTU) SS (mg L−1) COD (mg L−1) VS (mg L−1) FC (CFU mL−1)

MWW 7.6±0.1 756±11 12.7±0.2 50±4 45.3±8.8 128±6 (6.88±0.29)×103

DWW 8.3±0.2 1240±20 52±3 75±21 132±29 212±15 (1.19±0.07)×104

wastewaters. Several methods can be used for tertiary wastewater treatment. For instance, advanced oxidation processes (AOPs) are used for the treatment of toxic residual wastewaters that are difficult to biodegrade (Gogate and Pandit 2004; Ikehata and El-Din 2006). The aim of AOPs (including, O3/ H2O2, UV/O3, UV/H2O2, H2O2/Fe2+, etc.) is to produce the hydroxyl radical in water, a very powerful oxidant capable of oxidising a wide range of organic compounds with one or many double bonds. In spite of good oxidation of refractory organic compounds, the complexity of these methods (AOPs), high chemical consumption and relatively higher treatment cost constitutes major barriers in the field application (Martinez-Huitle and Ferro 2006; Panizza and Cerisola 2004). Conventional methods such as ultrafiltration and adsorption using activated carbon can be also applied for tertiary water treatment. The main disadvantage of such methods is that they do not destroy the pollutants but rather transfer the pollutant from one phase to another (Ozcan et al. 2004; Tahir and Rauf 2006). Nowadays, electrochemical oxidation processes have been proposed and identified as an attractive option for wastewaters treatment (Guinea et al. 2008; Isarain-Chàvez et al. 2011; Peralta-Hernàndez et al. 2006; Sirés et al. 2007). The electrochemical oxidation processes have been found to be a promising environmental remediation technology to remove organic and microbial pollutants (Zhang et al. 2008; Wang et al. 2005; Martinez-Huitle and Ferro, 2006). The electrochemical method takes advantage of coupling chemistry (in situ generation of oxidant) with electronic science (electron transfer). This technique has widely proved to be a clean, flexible and powerful tool for the development of new methods for waste and water treatment (Grimm et al. 1998). Likewise, electrochemical treatment is generally characterised by simple equipment, easy operation, brief retention time and negligible equipment for adding chemicals. According to Rajeshwar and Ibanez (1997), benefits from using electrochemical techniques include: environmental compatibility, versatility, safety, amenability to automation and cost effectiveness. The interest of using electrochemical oxidation processes is based on its capability of reacting on pollutants by using both direct

and indirect effect of electrical current (Martinez-Huitle and Ferro, 2006; Brillas et al. 2004; Zaviska et al. 2011). Direct oxidation may be achieved through mineralisation with hydroxyl radical (OH°) produced at the electrode surface by dimensionally stable anodes having high oxygen overvoltage, such as IrO2, PbO2 and BDD, among others (Panizza et al. 2000; Comninellis and Pulgarin, 1991; Panakoulias et al. 2010; Comninellis, 1994). In fact, HO* radicals are exclusively generated on the anode electrodes from the oxidation of water and organic compounds can be completely transformed or degraded by reacting with adsorbed HO* radicals (Dirany et al. 2010; Hamza et al. 2009; Oturan et al. 2013). The hydroxide radicals are species capable of oxidising numerous complex organics, non-chemically oxidisable or difficulty oxidisable (Pulgarin et al. 1994). They efficiently react with the double bonds -C=C- and attack the aromatic nucleus, which are the major component of refractory organic compounds. Furthermore, indirect oxidation can be achieved through electrochemical generation of a mediator in solution such as reactive oxygen species (ROS), including H2O2, O3, H2S2O8, and among others (Drogui et al. 2001; Rajeshwar and Ibanez 1995). The ROS are some powerful oxidants capable of oxidising a wide range of organic compounds (Jeong et al. 2009; Guinea et al. 2010). In particular, H2O2 electrochemically generated at the cathode is an environmentally metastable molecule. At high concentrations, H2O2 presents high disinfecting and oxidising properties, and it is able to convert toxic organics to less harmful products (Drogui et al. 2001; Khataee et al., 2011). In this context, to increase the quality of effluent from wastewater treatment plants and from decentralised systems, an electrochemical process capable of simultaneously using anodic and cathodic reactions to generate several ROS was developed for tertiary water treatment of domestic and municipal wastewaters. The main objective of this study is firstly to evaluate the capacity of an electrolytic cell to produce ROS and secondly, to evaluate its performance in treating DWW and MWW as tertiary treatment. The process that we propose uses the environmentally friendly oxidants (H2O2, HO*, etc.). This process should be capable of simultaneously removing refractory organic pollutants, bacteria and turbidity from effluents without any chemical addition.

Experimental Synthetic effluent The synthetic solution was prepared from distilled water along with 500 mg L−1 Na2SO4 to improve electrical conductivity. This type of solution was used to evaluate the capacity of the electrolytic cell’s to produce ROS.

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Municipal and domestic wastewaters The MWW were effluents obtained from a physicochemical processing plant that was installed in the Ange Gardien City in the province of Québec. It is a conventional wastewater treatment plant comprised of a pretreatment by screening and grit removal followed by coagulation/flocculating and clarifier system. The samples were collected after the clarification step. The DWW were effluents obtained from a biofiltration processing plant that was installed in the in the St. Joseph of Kamouraska. This biofiltration process was used to treat wastewaters from isolated residences. The process takes advantage of using diversified microflora of wastewater as well as the sorption capacity aerated trickling biofilter filled with organic support. The organic support was essentially made of peat. Samples were collected in polypropylene bottles, shipped cold and kept at 4 °C until further use Table 1. Electrolytic cell The reactor unit used had 2 L of capacity and was made of polyvinyl chloride (PVC) material with a dimension of 15 (height)×14 cm (diameter). The electrolytic cell was comprised of two anode and two cathode electrodes in the form of expanded metal, each one having a solid surface area of 65 cm2 and a void surface area of 45 cm2. The anode electrodes were circular disks (12 cm diameter×0.1 cm thick). The circular anode electrodes (12 cm diameter×0.1 cm thick) were platinum-coated with titanium (Ti/Pt) or coated with lead oxide (Ti/PbO2). The cathode electrode was comprised of carbon felt (CF) or graphite (GF). The carbon felt had a high surface area, associated with high porosity, so had low flow resistance and a thickness of 6 mm with 10–20 μm diameter fibers. The bulk density is about 500 g m−2 and porosity of 0.94 %. This carbon felt consist of 99 to 99.7 % of carbon and 0.02 to 0.25 % of ashes. The electrical resistance is equal to 0.5Ω. The inter-electrode gap was 10 mm in the electrolytic cell. The electrodes were horizontally installed inside the electrolytic cell, and each anode was immediately followed by a cathode. The electrodes were supplied by Electrolytica Inc (Amherst, NY, USA). Experimental setup The tests were carried out in a closed loop with the cylindrical cell depicted schematically in Fig. 1. A 10-L PVC reservoir (1), a recycling pump (2), the electrolytic cell (8), and 1 L of PVC constituted the loop. The recycle flow rate (QR) studied varied between 1.0 and 4.0 L min−1. The excess gas generated during electrolysis was rejected out of the system by means of a venting pipe (12) fixed to the 1-L PVC reservoir. The system also provided an injection of air or oxygen into the

recirculation loop in order to saturate in oxygen the effluent subjected to treatment. The air flow meter (5), connected to an oxygen cylinder (6), measured the flow rate of oxygen injected in the experimental unit. An oximeter (7) installed in the hydraulic pipe allows to monitor the concentration of dissolved oxygen during the electrolysis. A manometer (9) followed by needle valve (10) measured and controlled the hydrostatic pressure inside the electrolytic cell. The concentrations of dissolved oxygen ranged from 8.0 to 14 mg L−1. The working volume was 7.0 L. The electrochemical cells were operated under galvanostatic conditions, with current intensities imposed during a period of treatment ranging from 10 to 90 min. Current intensities were imposed by means of a DC power source, Xantrex XFR40-70 (Aca Tmetrix, Mississauga, Ontario, Canada) with a maximum current rating of 70 A at an open circuit potential of 40 V. Analytical techniques The pH was determined using a pH-meter (Fisher Acumet model 915) equipped with a double junction Cole-Palmer electrode with Ag/AgCl reference cell. A conductivity meter (Oakton Model 510) was used to determine the ionic conductivity of the effluent. Turbidity (in nephelometric units (NTU)) was measured using a turbidimeter Hach 2100 AN. Chemical oxygen demand (COD) determination was measured based on APHA (1999) and a reading spectrophotometer UV 0811 M136 (Varian, Australia). TSS was measured using standard methods (MA. 115-S.S.1.1) from Environmental Center of Analysis of Quebec (ECAQ). Whatman 934AH filter was used for determining of SS. The H2O2 concentration was measured by volumetric dosage in accordance with the method proposed by Sigler and Masters (1957). A cerium ion solution (Ce(SO 4 ) 2 , 2(NH4)2SO4 and 2H2O) was used under acidic conditions (H2SO4) in the presence of Fe(ophen)32+ indicator. To quantitatively determine the concentration of H2O2, the calibration curve was obtained by plotting the cerium concentration as a function of H2O2 concentration. The gradual change of the solution colour from red to the blue indicates a total oxidation of hydrogen peroxide using the cerium solution. The ozone concentrations were determined using the indigo method (Bader and Hoigné 1981). A stock solution of the indigo reagent was prepared by dissolving 770 mg/L of potassium indigo trisulphonate in 1 mL of phosphoric acid. The ozone calibration curve was obtained by plotting the absorbance as a function of O3 concentration. The standard solutions of ozone were prepared using an ozone generator (OzoStar, model WL7, Ozocan Corporation, Ontario, Canada). Ultraviolet(UV)-visible absorption spectroscopy method was applied to investigate the hydroxyl radicals (Jiang et al.

Environ Sci Pollut Res Fig. 1 Schematic view of the electrochemical unit with a recirculation loop

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12 1 Feed Tank 2 Centrifugal pump 11 3 Water flowmeter 4 Filter pump 10

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2006; Xiang et al. 2011; Simonsen et al. 2010; Kraljié and Trumbore 1965; Zang et al. 1997). p-Nitrosodimethylaniline (RNO) is an organic dye molecule having a strong yellow colour in aqueous solution and is easy to detect using UVvisible absorption spectroscopy (Muff et al. 2011). RNO is bleached selectively by oxidation with hydroxyl radicals and does not react with singlet oxygen (1O2), superoxide anions (O2−) or other peroxy compounds (Simonsen et al. 2010; Muff et al. 2011). The bleaching rate was monitored by absorbance measurements of RNO at 440 nm using an UV-visible absorption spectrophotometer (UV 0811 M136, Varian, Australia). The RNO calibration curve was obtained by plotting the RNO absorbance at 440 nm as a function of RNO concentrations.

Results and discussion

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the best cathode material (CF versus GF) for H2O2. Figure 2 shows the changes in H2O2 concentration for CF and GF respectively used as cathode materials. The H2O2 concentration increased over the first 20 min (from 0.0 to 5.59× 10−5 M), and then remain quite stable until the end of experiment using GF cathode electrode. By comparison, in the case of CF, H2O2 concentration increased from 0.0 to 22.6×10−5 M and then tended towards a plateau. CF was more appropriate for H2O2 formation than GF. For instance, after 60 min of electrolysis time, a concentration of 19.6×10−5 M was recorded with CF, whereas 5.59×10−5 M was measured for GF (a concentration three times lower). The amorphous structure of CF (Kuramitza et al. 2004) facilitates the trapping of dissolved oxygen so that it can be reduced into H2O2, compared with the hexagonal structure of GF less appropriate for oxygen trapping (Yue et al. 1999). Consequently, CF was retained for the next step of the study.

Electrochemically hydrogen peroxide production The current intensity effect Cathode material effects The primary objective of the tests was to evaluate the efficacy of the electrolytic cell to produce H2O2. Such an oxidant can be generated at the cathode (Pozzo et al. 2005; Zhao et al. 2012; Zhou et al. 2008). According to previous studies (Guinea et al. 2010; Zhou et al. 2008; Drogui et al. 2001), several cathode materials including graphite (Gr), vitreous carbon and CF can be used. The first tests consist to identify

Figure 3 shows the change in H2O2 concentration for different current intensities (1.0, 2.0, 3.0 and 4.0A). The H2O2 concentration linearly increased (between 10 and 40 min of electrolysis time) and the slope KP increased with current intensity. The H2O2 concentration can be written as follows: C H202 ¼ K P :t

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Fig. 2 Hydrogen peroxide production against time: carbon felt (CF) electrode versus graphite felt (GF) electrode; I=3.0 A; QR =2 L min−1

Graphite felt (GF) Carbon Felt (CF)

H2O2 x 10-5 M

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The recycling flow rate effect The performance and the behaviours of the electrolytic cell for H2O2 generation at various flow rates (2.0, 4.0 and 6.0 L min−1) were studied at a current intensity of 3.0 A for a period of 90 min. The influence of electrolyte flow rate on H2O2

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production is shown in the Electronic supplementary material Figure SM1. For the three flow rates tested, the H2O2 concentration increased with electrolysis time. The H2O2 concentration rapidly increases over first 20 min and tended towards a plateau from 40 min of electrolysis time while imposing relatively higher flow rates (4.0 and 6.0 L.min−1). The H2O2 concentration remained constant around 14.8×10−5 M for a recycling flow rate of 4.0 L.min−1, whereas a steady state around 11.5×10−5 M was recorded for a recycling flow rate of 6 L.min−1. By comparison, when a relatively low flow rate (2.0 L min−1) was imposed, H2O2 continue increasing after 40 min of electrolysis time and then tended towards a plateau around 20.0 × 10 −5 M H 2 O 2 after 60 min. The H 2 O 2

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Fig. 3 Hydrogen peroxide production against time for different current intensities using CF cathode electrode; QR =2 L min−1

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concentration decreased as the recycling flow rate increased from 2.0 to 6.0 L min−1. It is worth noting that an increase in the recirculation rate is accompanied by higher velocity in the electrolytic cell. For instance, a linear velocity of 0.58 cm s−1 was imposed for 6.0 L min−1 compared with 0.19 cm s−1 measured for 2.0 L min−1. During electrolysis, two possible extreme limitation processes can be considered: mass transfer control and electrochemical reaction control. The electro-active species (here O2 molecules) must be transported toward the cathode electrode surface first, and then be reduced there into H2O2 (electrochemical reaction side), followed by its transportation in solution. Regarding the influence of recycling flow rate, it is believed that the process is not under mass transfer control, but it is probably limited by the electrochemical reaction. Likewise, while increasing the recycling flow rate, the liquid arrived rapidly on cathode electrodes so that the oxygen molecules cannot be efficiently reduced to produce H2O2. The dissolved oxygen injection effect It is worth underlining that the results discussed above were obtained without any oxygen injection in the close loop. Then, some experiments were carried out by injecting oxygen (6 L O2 min−1) in the close loop at the condition of 3.0 A current intensity, 2 L/min flow rate and carbon felt cathode material. The interest of continuously injecting oxygen in the system was to saturate the solution in oxygen and to avoid the limitation of the process by electro-active species concentration (i.e. dissolved oxygen). Figure 4 compares the changes in H2O2 concentrations with and without O2 injection. As expected, the residual H2O2 concentrations were more important while injecting O2 in the close loop. For instance, over 60 min of electrolysis, H2O2 concentration recorded (44×10−5 M) with oxygen injection was almost two times higher than the results obtained (20×10−5 M) without oxygen injection. Other reactive oxygen species formation Other ROS (e.g., O3, HO*, etc.) can be formed during electrolysis (Jeong et al. 2009). In particular, hydroxyl radical (HO*) can be generated by anodic decomposition of water using some catalytic electrodes having high oxygen overvoltage (such as Pt, Ti/IrO2, Ti/RuO2, Ti/SnO2, PbO2, Gr, etc.) (Cantrell et al. 1995; Gandini et al., 1998; Rajkumar et al. 2003). RNO is selectively bleached by oxidation with hydroxyl radicals and does not react with singlet oxygen (1O2), superoxide anions (O 2 − ) or other peroxy compounds (Simonsen et al. 2010). Hydrogen peroxide cannot react with RNO. However, ozone, hypochlorous acid and hypochlorite ions have been shown to bleach RNO (Simonsen et al. 2010). In our experimental conditions, hypochlorous acid and hypochlorite ion could not be formed since sodium sulphate (Na2SO4) was used as supporting electrolyte in the synthetic

solution. Ozone was one of the ROS that could be formed during electrolysis. Thus, ozone could simultaneously react with RNO (in addition to the reaction between RNO and HO*). Several authors showed that ozone could be formed by anodic oxidation of water (Stucki, 1991; Kraft et al. 2006; Onda et al., 2005; Jeong et al. 2009). A current intensity of 1.0 A and treatment time of 100 min were imposed during electrolysis of RNO solution. The experiment unit without oxygen injection was used. The ROS production rate (VROS) is equal to the RNO disappearance rate according to Eq. 1. V ðROSÞ ¼

d ½RN O ¼ −k R :½RN O dt

ð1Þ

Where [RNO] is the concentration of RNO, kR is the firstorder reaction rate constant (t−1) and VROS is the production rate of oxidising species (including mainly HO* and O3), but other ROS could be formed during electrolysis. Integration of Eq. 1 gives:   C0 Ln ð2Þ ¼ k R :t C Where C0 is the initial concentration of RNO, C is the concentration of RNO at time t, and t is the reaction time. The reaction rate constant kR could be calculated from the slope of a plot of (t) versus Ln (C0/C) from Eq. 2. Figure SM2 shows that the RNO disappearance rate follows a first-order kinetic model. The first-order kinetic reaction rate constant (kR) for ROS formation was 0.011 min−1. The ROS (HO* and O3) concentration increased with time. The ROS concentration passed from 0.45×10−5 M after 20 min of electrolysis to a concentration of 2.84×10−5 M after 100 min of electrolysis (Fig. 5). To specifically quantify the ozone (O3) produced, additional experiments were carried out using a synthetic solution containing 0.5 g L−1 of Na2SO4. This sulphate synthetic solution was electrolyzed during a period of 100 min, and a current intensity of 1.0 A was imposed. The electrolytic cell was used without carbon felt electrode in order to avoid H2O2 formation. Titanum was used as cathode electrode. During electrolysis, some sub-samples (90 mL) were withdrawn at the outlet of the electrolytic cell at regular interval time. The electrolyzed samples were immediately transferred into some volumetric flasks containing 10 mL of a solution of indigo. Ozone decolourised rapidly the solution of indigo, and the residual peak absorbance (λmax =600 nm) was measured. The fraction of the absorbance ([Abs]0 −[Abs]t) that disappeared corresponded to the amount of indigo that reacted with ozone. The ozone concentration increased with time. The ozone concentration passed from 0.016×10−5 M after 20 min of electrolysis to a concentration of 0.28×10−5 M after 100 min of electrolysis (Fig. 5). The process has been developed for tertiary treatment of municipal and domestic wastewaters

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Fig. 4 Influence of injection of oxygen in a close loop on hydrogen peroxide production; I=3.0 A; QR =2.0 L min−1

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current intensities (0.5, 1.0 and 3.0 A) with a recycling flow rate of 2.0 L min−1. CF was used as cathode electrode. Figure 6 shows the change in Nt/N0 (abatement of faecal coliforms) during the treatment of MWW. N0 and Nt respectively represent the initial and residual concentrations of bacteria. Two control tests (CONT-1 and CONT-2) were used. The control CONT-1 test consisted in recirculating the effluent in the electrolytic cell during a period of 90 min (in the absence of CF electrode inside the reactor) and without imposing any current intensity. The CONT-2 test consisted in recirculating the effluent in the electrolytic cell during a period of 90 min (in the presence of CF electrode inside the reactor) and without imposing any current intensity. As it can be seen from Fig. 6, faecal coliform

whose pH was generally around the neutral value. However, it could be interesting to test the pH effect on ROS production. Application to disinfection and wastewater treatment Treatment of municipal wastewater effluent (MWW) The objective of this part of the study was to examine the feasibility of the electrochemical process for tertiary water treatment of MWW containing different types of pollutants (microbial, organic and inorganic pollutants, etc.). The experiment unit without oxygen injection and without adding any chemical was used. The system was operated by using three 3.5

Fig. 5 Other reactive oxygen species (ROS) production against time during electrolysis; I=1.0 A; QR =2.0 L min−1

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Environ Sci Pollut Res Fig. 6 Removal of faecal coliforms using different intensities (municipal wastewater effluent); N0 =6.88±0.29× 103 CFU mL−1; QR =2.0 L min−1; CF cathode electrode

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abatement increased rapidly over the first 20 min of the treatment, and almost complete disinfection could be reached after 40 min of electrolysis (3 log inactivation), using either a current intensity of 1.0 or 3.0 A. When a relatively low current intensity (0.5 A) was imposed, the ratio Nt/N0 decreased slowly and 1 log abatement (98 % of faecal removal) was recorded after 90 min of treatment. High current intensity induced larger peroxide concentration. This is in conformity with the established relationship between hydrogen peroxide concentration and disinfection (Jeong et al. 2009). As the current intensity increased, the disinfection efficiency increased. By comparison, in the absence of current intensity (CONT-1 and CONT-2), faecal

coliform abatements were also recorded (37 % and 72 %, respectively) after 90 min of electrolysis. The decrease in faecal coliform concentration during the control tests was probably attributed to the deposition of a fraction of bacteria on the electrolytic tank, on the pipes of the experimental unit or inside the CF electrode. Considering the possible deposition of bacteria on the walls of the reactor and inside the CF electrode (by considering CONT-2), the real contribution of the electrolysis (direct and indirect effects combined) can be obtained by subtracting the yields of faecal coliform removal (while imposing a current intensity) from the yields recorded without current intensity. For instance, in our experimental conditions, the real

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Fig. 7 Oxidation of organic compounds using different current intensities (municipal wastewater); initial COD, 45.3±8.8 mg L−1; QR =2.0 L min−1 Residual COD/ inial COD

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Environ Sci Pollut Res Fig. 8 Removal of faecal coliforms using different current intensities (domestic wastewater effluent); N0 =1.19±0.07× 104 CFU mL−1; QR =2.0 L min−1; CF cathode electrode

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yield of faecal removal by electrolysis was more than 27 % (compared with CONT-2) when a current intensity of 3.0 Awas imposed. The CF electrode acted as a filter and contributed to remove a non-negligible fraction (35 %) of microorganisms. The turbidity was also removed by electrolysis. The initial brown colour disappeared, and water became transparent while a current intensity was imposed. At the end of the treatment, a residual turbidity varying between 2.2 and 3.3 NTU was recorded compared with the initial value of 12.7 measured in the raw water (Figure SM3). More than 80 % of turbidity was removed during electrolysis compared with 45 % and 76 % recorded during the control tests (CONT-1 and CONT-2,

respectively). The electrolytic cell could simultaneously oxidise refractory organic compounds (refractory COD) (Fig. 7). For a relatively low current intensity imposed (0.5 A), the COD abatement was low and was in the range of the values (27 % to 30 % of COD removal) recorded during the control tests. In this case, the fraction of COD removed was mainly in form of colloid or particles. However, when a relatively high current intensity was imposed (3.0 A), the soluble faction of COD could also be removed and the efficiency of COD reached 70 %. Indeed, in such oxidation process, organic pollutants were subjected to two effects: direct anodic oxidation where the organic compounds are destroyed at the anode surface and indirect

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Fig. 9 Oxidation of organic compounds at different current intensities (domestic wastewater); initial COD, 132±29 mg L−1; QR =2.0 L min−1

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oxidation where a mediator was electrochemically generated (i.e. H2O2 or O3) to carry out the oxidation of organic pollutants (Zaviska et al. 2012).

test. The electrolyzed water can then be safely reused for soil irrigation in agricultural areas.

Treatment of domestic wastewaters (DWW) Conclusion The performances of the electrochemical process have been also evaluated for tertiary water treatment of DWW relatively loaded with organic matter, turbidity and bacteria (compared to MWW). The same current intensities (0.5, 1.0 and 3.0 A) were applied. The faecal coliform abatement depended on the current intensity (Fig. 8). These results were quite similar to those measured while treating MWW. At low current intensity, the coliform abatement was relatively low. When relatively high current intensities of 1.0 and 3.0 A were imposed, 1 log and 3 log abatements of faecal coliform were recorded, respectively. By comparison, for control tests (CONT-1 and CONT-2), the log-inactivation was lower than 1 unit. Over a period of 90 min of electrolysis, 67 % to 93 % of faecal coliform removals were recorded dependently on the current intensity imposed, whereas 38 % and 53 % were respectively removed during the control tests. The explanations mentioned above to justify faecal coliform abatement in control tests during the treatment of MWW are also valid for DWW. The real contribution of electrolysis for bacteria inactivation was estimated between 14 % and 40 % dependently on the current intensity imposed. The changes in COD were also measured during the electrolysis of DWW. The results were summarised in Fig. 9. A percentage of COD removal up to 78 % could be reached during the control tests (CONT-2 test). By comparison, 80 % to 94 % of COD removals were measured over 90 min of electrolysis. The COD abatement increased with the current intensity imposed. Indeed, a non-negligible fraction of COD was in form of insoluble compounds (particles or colloids), which were easily adsorbed inside the CF electrode and on the walls of the experimental unit (in the absence and in presence of current intensity). It was one of the main reasons for which there was a very few discrepancy between the control tests and electrolysis tests (in terms of COD removal). The Figure SM4 shows also the changes in turbidity in function of time. There was not a significant difference between the CONT-2 test and the results obtained when a current intensity was applied. In fact, the CF electrodes acted as sorbent, so that it was quite impossible to really appreciate the effect of current intensity on the decoulorisation of DWW. The turbidity decreased rapidly over the first 5 min of the treatment and then remained quite stable until the end of experiment, while either applying a current intensity or in the absence of current intensity (CONT-2 test). The initial black colour disappeared, and water became transparent. For instance, 92 % to 94 % of turbidity removals were measured over 90 min of electrolysis, compared with 92 % of the value recorded during the CONT-2

Hydrogen peroxide was electrochemically generated with relatively high concentration (up to 22×10−5 M) without adding any chemical. Other reactive oxygen species such as ozone (0.016×10−5 M to 0.28×10−5 M) were also formed during electrolysis. The disinfection and COD removal were relatively high during tertiary water treatment of MWW and DWW. More than 90 % of refractory organic compounds (COD) can be removed, whereas 99 % of faecal coliform abatement can be reached. Likewise, the process was also effective in removing turbidity (more than 90 % of turbidity was removed), so that the effluent became more and more transparent. Thus, this process could be the basis of a process for water disinfection, turbidity and refractory organic carbon removal for tertiary water treatment of municipal, domestic and industrial wastewaters. Acknowledgements Sincere thanks are extended to the National Sciences, Engineering Research Council of Canada and Premier Tech Ltée for their financial to this study.

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