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Electrochemical treatment of biologically pre-treated dairy wastewater using dimensionally stable anodes
Journal of Environmental Management 202 (2017) 217e224
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Research article
Electrochemical treatment of biologically pre-treated dairy wastewater using dimensionally stable anodes Vlasia Markou a, Maria-Christina Kontogianni a, Zacharias Frontistis a, *, Athanasia G. Tekerlekopoulou b, Alexandros Katsaounis a, Dimitris Vayenas a, c a b c
Department of Chemical Engineering, University of Patras, Caratheodory 1, GR-26504 Patras, Greece Department of Environmental & Natural Resources Management, University of Patras, Agrinio, Greece Institute of Chemical Engineering Sciences, Foundation for Research and Technology, PO Box 1414, GR-26504 Patras, Greece
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2017 Received in revised form 21 June 2017 Accepted 17 July 2017
In this work, electrochemical oxidation of aerobically pre-treated dairy wastewaters using IrO2-Pt coated dimensionally stable anodes was investigated. It was found that IrO2/Ti electrode outperforming Pt/Ti and IrO2-Pt/Ti at lower current densities, while Pt/Ti achieved better efficiency at higher current density. Among the different parameters which were studied, the current density was the most crucial for the efficiency of the process. A current density of 100 mA/cm2 led to almost complete removal of 3700 mg/L COD after 360 min of treatment using IrO2/Ti electrode and 0.2 M of sodium chloride while complete decolorization was achieved in less than 60 min. Electrolytes also found to significantly affect the process. More specific, the use of sodium chloride instead of sodium sulfate enhanced both COD and color removal due to the formation of active chlorine species. The effect of temperature was relative low; the process was favourable at elevated temperatures while increasing COD loading resulted in a decrease of COD and color removal. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Dairy Wastewater Electrochemical Dimensionally stable anodes
1. Introduction Since ancient times the consumption of dairy products constitutes an important part of human nutrition. In recent years, due to the intensification of the dairy industry the management of dairy effluents has become a major problem since dairy wastewaters can be a significant threat, mostly for the aquatic environment (Carvalho et al., 2013; Prazeres et al., 2012). Consequently, there is a significant need to improve the management and treatment of dairy effluents in order to reduce environmental problems and to ensure the economic viability of this essential agricultural industry. (Sarkar et al., 2006). Several research groups have studied the biological treatment of dairy wastewater, either under aerobic condition (Bumbac et al., 2015; Tatoulis et al., 2015) or anaerobic conditions (Venetsaneas et al., 2009; Demirel et al., 2005) with the simultaneous production of hydrogen and methane. However, even though biological processes may lead to a significant reduction of the organic
* Corresponding author. E-mail address: [email protected] (Z. Frontistis). http://dx.doi.org/10.1016/j.jenvman.2017.07.046 0301-4797/© 2017 Elsevier Ltd. All rights reserved.
material, usually they fail to meet the limits settled by environmental legislations for the disposal of effluents into the environment. Therefore, the combination of biological treatment with other treatment processes such as constructed wetlands (Sultana et al., 2016) and intermittent sand filters (Healy et al., 2007) have been proposed in literature. At the same time, a considerable research on agro-industrial wastewaters treatment using physical processes like membrane rez and technology, mainly reverse osmosis and nanofiltration (Sua szlo et al., 2007) or adsorption Riera, 2015; Vourch et al., 2008; La (Kushwaha et al., 2010) has been carried out. Nevertheless, the main drawback of the physical processes is the fact that they transfer the problem from one phase to another resulting in the necessity either of the further concentrated solution treatment (Perez et al., 2010), or the regeneration of the adsorption material (Ehrenmann et al., 2011). Advanced Oxidation Processes (AOPs) is a family of technologies based on the in situ production of highly reactive radicals. Among various AOPs most studies related to dairy wastewaters have been conducted with Fenton, photo Fenton and electro Fenton like reactions (Davarnejad and Nikseresht, 2016; Loures et al., 2014; Prazeres et al., 2013; Vlyssides et al., 2013), photocatalysis (Lamas
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Samanamud et al., 2012), ozonation (Varga and Szigeti, 2016), electro-coagulation and electro flocculation (Melchiors et al., 2016; € S¸engil and Ozacar, 2006). More specific, Loures et al. (2014) studied the photo Fenton process for the treatment of dairy effluents with an initial COD (chemical oxygen demand) concentration of 9500 mg/L. They found that more than 90.7% COD and 78.8% BOD can be removed under optimal conditions (35 g H2O2, 3.6 g Fe2þ, pH lu and Yonar (2017) applied ozon3.5 and UV light 28 W). Sivriog ation for the degradation of dairy wastewater with initial COD concentration of 6300 mg/L. Ozonation led to COD removal up to 70% after 240 min of treatment with ozone dose equal to 2 gr/h. € S¸engil and Ozacar (2006) treated dairy wastewaters by electrocoagulation using steel electrodes. COD efficiency reached the value of 98% while the optimum current density, pH and treatment time for COD equal to 18,300 mg/L were 0.6 mA/cm2, 7 and 1 min, respectively. Melchiors et al. (2016) examined the efficiency of electroflocculation method for the treatment of dairy wastewater with initial COD of 8303 mg/L. A significant removal of organic matter was achieved (97.4%) using iron electrodes at final pH of 7.4. However, despite the high COD removal the main problem of electroflocculation and electrocoagulation processes remains firstly the ‘’sacrificial’’ electrodes and their dilution into the wastewater streams (because of their oxidation) and secondly the generation of significant quantities of sludge during the process (Bensadok et al., 2011). On the other hand, electrochemical oxidation (EO) doesn't suffer from these drawbacks and seems to be a promising alternative approach, mainly due to its high efficiency, and ease of use and €rkk€ control (Sa a et al., 2015; Valero et al., 2014; Anglada et al., 2009). EO is divided into direct and indirect oxidation (depending on the anode and its participation of the oxidation process), while the oxidizing agents include among others hydroxyl radicals, hypochlorite and ozone. In recent years, the use of dimensionally stable anodes (DSA) like RuO2/Ti, PbO2/Ti and TiO2eRuO2eIrO2/Ti has increased significantly against other types of electrodes, due to their high stability and activity (Subba Rao and Venkatarangaiah, 2014; Turro et al., 2012; Chatzisymeon et al., 2009; FaridaYunus et al., 2009). n et al. (2014) studied the oxidation of dairy wastewater Borbo using electrochemistry over an IrO2-Ta2O5/Ti electrode. They used different electrochemical techniques like cyclic voltammetry, pulsed differential voltammetry and chronoamperometry and concluded that two different processes/pathways exist using the above DSA-type electrode: one indirect (via hydroxyl radicals) and one direct oxidation (with the active participation of the electrode) n et al., 2014). However, they suggested that further research (Borbo is needed to study the variables that affect the efficiency of the system. Considering that the kinetics of electrochemical oxidations of industrial effluents with high organic loading follows near zeroorder rate with respect to the organic loading (Chatzisymeon et al., 2009) the approach to couple an electrochemical process and a biological treatment seems to be rather advantageous. Indeed, several researchers (Zhang et al., 2011; Feki et al., 2009) have demonstrated the potential of the combined biological and physicochemical processes to treat industrial effluents like landfill leachates. Lei and Maekawa (2007) studied the electrooxidation of anaerobic digestion effluents using Pt-IrO2/Ti electrode. They found that ammonia could be completely removed at 5 h under 1A and 1% NaCl, while the TOC and turbidity removal reached 51.4% and 95.5%, respectively. Under this perspective, in this study, the electrochemical oxidation of an aerobically pretreated dairy wastewater is investigated giving attention to issues that have not been dealt with before; these include working electrode material (different DSA
electrodes), current density, electrolytes, pH, treatment time, temperature and initial COD loading. According to our knowledge, this is the first report on the combination of a biological and an electrochemical process for the treatment of dairy effluents. 2. Materials and methods 2.1. Dairy effluent Second cheese whey from cottage cheese production was obtained from a Greek cheese factory (Papathanasiou A.B.E.E.). The dairy effluent was firstly treated aerobically using a pilot-scale packed-bed bioreactor. The bioreactor as well as the bioprocess have been described in previous study (Tatoulis et al., 2015). The COD and pH of the pilot plant effluent were in the range of 2500e15.000 mg/L and 6e7.5, respectively, depending on the operating conditions of the bioreactor. 2.2. Electrodes The preparation method of the DSA-type electrodes has been explained extensively in previous studies (Kapałka et al., 2010; Papastefanakis et al., 2010; Chatzisymeon et al., 2009). Briefly, the IrO2 or Pt working electrodes was prepared by thermal decomposition of 250 mM H2IrCl6 (Acros Organics, 40%) or H2PtCl6 metal precursors dissolved in isopropanol (Fluka, 99.5%) on a squaredshaped titanium support, taking into account that the deposition yield of Pt and IrO2 is 60% and 100% respectively (Comninellis and Vercesi, 1991). The titanium substrate was sandblasted to ensure good adhesion of the deposit on its surface and chemically treated using oxalic acid solution (1M) to clean its surface from residual sands. The substrate was then dried in an oven at 70 C and weighed. The precursor solution was spread on the titanium substrate forming a thin film layer on the electron surface. Afterwards, the sample was treated in an oven for 10 min to allow solvent evaporation. This step was followed by treatment in the furnace for the thermal decomposition of the precursor solution at 500 C in air for 10 min. The same procedure was repeated several times and, after the last IrO2 or Pt coating, the electrode remained at 500 C for 1 h. The final metal loading in all cases was 1.2e1.3 mg/cm2. For the Pt-IrO2 binary electrode, (50/50 molar ratio), the above method was applied using solutions of the precursors in the appropriate ratio. 2.3. Scanning electron microscopy (SEM) The morphology of the surface was explored with Scanning electron microscopy (SEM) using a FEI, FEG QUANTA 520 scanning electron microscope. 2.4. Electrochemical oxidation experiments Experiments were conducted in a double wall glass reactor. The appropriate electrode (Pt/Ti, IrO2/Ti or Pt-IrO2/Ti) had working surface of 12.5 cm2 and two zirconium bars served as cathodes. The distance between the anode and the cathodes was 1.5 cm while the electrodes were connected with a DC power supply (model QJ3005C). The volume of the solution inside the reactor was 150 mL and the biologically pretreated dairy wastewater stirred with a magnetic bar. The temperature remained constant using a water bath. All experiments were conducted with NaCl (>99%, Aldrich) as the electrolyte, while, in some cases, Na2SO4 (>99%, Aldrich) was also added. The majority of the experiments were performed in triplicate and the relative error was less than 5%.
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2.5. Analytical methods
3. Results and discussion
2.5.1. Measurement of COD and color COD concentration was determined by the dichromate method according to the Standard Methods (APHA, 2012) and the concentration was measured colorimetrically using a DR5000 spectrophotometer (Hach Company, USA). Sample absorbance was also scanned in the 200e800 nm wavelength band on a DR 5000 spectrophotometer. Changes in sample absorbance at 600 nm were monitored to assess the extent of decolorization during electrochemical treatment.
3.1. Surface morphology
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Scanning Electron Microscopy was used to explore the surface structure of the samples and have an estimation on the size of the metal particles. Fig. 1 show the surface before and after the deposition of the anodic films. SEM images shown in Fig. 1a and b reveal a porous Ti surface (substrate) mainly due to the chemical treatment. Deposition of IrO2 (Fig. 1c and d), Pt (Fig. 1e and f) and Pt-IrO2 (Fig. 1g and h) via thermal treatment at 500 C leads to the development of metal or metal oxide islands on the porous surface.
Fig. 1. SEM images of (a, b) Ti support, (c, d) IrO2/Ti, (e, f) Pt/Ti and (g, h) Pt- IrO2/Ti.
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These islands are not uniform while the diameter of metal or metal oxide particles varied between 10 and 100 nm. According to the SEM, Pt and IrO2 particles appear together rather than two distinguished phases. In general, the images are similar with images taken from DSA type electrodes reported previously in literature both by our group (Turro et al., 2012) and other researchers (Zhou et al., 2011; Song et al., 2004).
3.4. Effect of initial COD concentration The effect of the initial organic loading on the process using IrO2 electrode, 0.2M NaCl and 100 mA/cm2 was investigated and the results are illustrated in Fig. 4. The COD percentage removal decreased with increasing organic loading. Indeed, COD reduction was 93% at initial COD concentration of 3700 mg/L (after 360 min), while it dropped down significantly at the value of 47% for an initial COD equal to 15,000 mg/L.
3.2. Effect of electrodes material Preliminary experiments were carried out to assess the effect of the electrode material on the treatment efficiency. Fig. 2 presents the experiments conducted at initial COD ~4000 mg/L, with 0.2 M NaCl at 50, 100 and 200 mA/cm2. Iridium-based electrode shows a significantly higher activity at the lower current density of 50 mA/ cm2 (COD removal of 96.7%). In contrary, platinum electrodes have an advantage at the high current density of 200 mA/cm2, achieving almost complete COD removal, mostly due to the larger working potential window of Pt electrodes. According to the Jeong et al. (2009), who studied the effect of the electrode materials on the electro generation of active chlorine, IrO2 is a better electrode than Pt. The following order has been found concerning the ability of the electrode to produce active chlorine: IrO2 > RuO2 > PteIrO2 > BDD > Pt. They also concluded that most of the chlorine produced using IrO2 as anodic electrode coming from direct oxidation of Cl ions rather than reaction with hydroxyl radicals as for example at the case of Boron Doped Diamond (BDD) electrodes. Recently, Yoon et al. (2014) evaluated the electro-generation of oxidants and potential dangerous by-products using Pt, RuO2, and IrO2 electrodes. The higher concentration of hydrogen peroxide was observed using IrO2 electrode while at the same time both ClO3 and ClO4, which are hazardous by-products of the electrochemical oxidation of the chloride mediums, were pronounced electro generated on Pt. Considering the above conclusions together with the fact that iridium is almost 30% cheaper than platinum, IrO2/Ti electrode was selected for the rest of this study.
3.3. Effect of current density The effect of current density (in the range of 50e200 mA/cm2) on the treatment of aerobically pre-treated dairy wastewater was also investigated by examining both COD and color removal. Experiments with an initial COD concentration equal to 3750 mg/L and 0.2 M NaCl were performed and the results are presented in Fig. 3. It was found that the current density is a crucial parameter for the process. More specific, COD removal increases with increasing current density (82.4%, 89% and 96.7% for 50, 100 and 200 mA/cm2, respectively). As the current increasing, higher concentrations of reactive species (like hydroxyl radicals and hypochlorite) are produced (Li et al., 2011). Resulted in a faster oxidation of the organic pollutants (Fig. 3a). On the contrary, utilization of high current densities result in low current efficiencies, especially in the case of DSA-type anodes (Comninellis and Chen, 2010) due to side reactions like oxygen evolution and thus to a non-economically favored process. Color removal was significantly faster than COD removal. As shown in Fig. 3b almost complete decolorization was achieved in less than 60 min for all current densities. These results are in agreement with Rajkumar and Kim (2006) who reported the vital effect of increasing current on the electrochemical oxidation of textile wastewater on DSA electrodes.
Fig. 2. Effect of electrode material on the electrochemical oxidation of aerobically pretreated dairy effluent. CODo ¼ 4000 mg/L, operating volume ¼ 150 mL, [NaCl] ¼ 0.2 M, for current density (a) 50 mA/cm2, (b) 100 mA/cm2 and (c) 200 mA/cm2.
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Fig. 5. Effect of supporting electrolyte on the electrochemical oxidation of biological pre treated dairy effluent. CODo ¼ 3750 mg/L, operating volume ¼ 150 mL, [NaCl] ¼ 0.2 M.
concentration of the reactive radicals becomes the limiting factor at high concentrations of organic matter. Moreover, at heterogeneous systems, the mass transfer limitations during the migration of the organic effluent from the bulk to the electrode or catalyst surface plays a significant role. Another explanation is the possible formation of polymeric compounds during the electrochemical oxidation process and their deposition on the anodic surface leads to electrode fouling and thus to lower electrocatalytic performance.
3.5. Effect of the type of the electrolyte Fig. 3. Effect of applied current density on the electrochemical oxidation of the aerobically pre-treated dairy effluent in the terms of (a) COD removal and (b) color removal. CODo ¼ 3750 mg/L, operating volume ¼ 150 mL, [NaCl] ¼ 0.2 M.
Fig. 4. Effect of COD loading on the electrochemical oxidation of biological pre-treated dairy effluent. CODo ¼ 3750 mg/L, operating volume ¼ 150 mL, [NaCl] ¼ 0.2 M.
This behavior has been extensively observed and discussed using various advanced oxidation process like photocatalysis (Dimitrakopoulou et al., 2012), Fenton reaction (Youssef et al., 2016) and electrochemical remediation (Gotsi et al., 2005). The common factor between different advanced oxidation processes is that the
Experiments with two different types of electrolytes (0.2 M NaCl and 0.2 M of Na2SO4 who are the most common electrolytes for environmental applications) were carried out to examine their effect on the electrochemical oxidation of pre-treated dairy wastewater. The pH change before and after the addition of the above electrolytes was very small. In all experiments IrO2 electrode was used as anode while current density and initial COD concentration were 100 mA/cm2 and 3750 mg/L, respectively. Sodium sulfate presented lower COD removal efficiency (57.7%) than sodium chloride (89%) (Fig. 5). This is possibly due to the different mechanism of the reaction. In the case of sodium chloride, the significant oxidation pathway includes the production of active chlorine. On the other hand, in the case of Na2SO4 the dominant oxidation species are the sulfate radicals which are considered as selective oxidants with relative low oxidation power. However, high concentration of sodium chloride can lead to effluents with high toxicity due to the presence of organochlorides which are considered as potential dangerous byproducts (Frontistis et al., 2016). It is worth mentioning that the evolution of the compounds that absorb at wavelength of 280 nm were much higher (after the first 20 min of treatment) in the case of sodium chloride than in the case of sodium sulfate where their concentration was negligible after 2 h of treatment (Fig. 6a). Although these compounds can be associated with organochlorinated molecules their exact identification is very difficult due to the complexity of the industrial effluent used in this study. In agreement with COD removal, the color reduction is depended significantly on the electrolyte. In the presence of sodium chloride, the decolorization was almost complete at 60 min (Fig. 6b), in contrast with the case of sodium sulfate which was only 70%.
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Fig. 7. Effect of temperature on the electrochemical oxidation of biological pre-treated dairy effluent. CODo ¼ 3750 mg/L, operating volume 150 mL, [NaCl] ¼ 0.2 M.
Fig. 6. Effect of supporting electrolyte on the electrochemical oxidation of biological pre-treated dairy effluent in the terms of (a) byproducts at 290 nm and (b) color removal at 600 nm. CODo ¼ 3750 mg/L, operating volume ¼ 150 mL, [NaCl] ¼ 0.2 M.
3.6. Effect of temperature All the above experiments were conducted at room temperature using a water bath. However, another set of experiments was also conducted in order to be examined the effect of temperature on the process. Three different temperatures were tested (25 C, 40 C and 60 C) using IrO2 as anode, initial COD concentration of 3750 mg/L, 0.2 M NaCl and current density of 100 mA/cm2. It was found that the reaction was almost unaffected at the range between 25 and 40 C (achieving COD removal about 90% in 360 min), while there is an improvement at higher temperature, 60 C (Fig. 7). This behavior can be explained by the Arrhenius equation between reactive oxygen species/radicals and pollutants. In addition, according to Dridi Gargouri et al. (2013) increase of temperature reduces the viscosity, thus enhances the diffusion rate and the mass transport of the pollutant to the electrode surface. Color removal followed the same behavior, however complete decolorization of the effluent achieved in less than 60 min in every studied temperature. 3.7. Perspectives The economy of many European countries is based on agricultural and agro-industries. Of all agro-industrial activities, the food sector has one of the highest consumptions of water, producing large quantities of wastewaters. Especially, dairy industry is one of
the most polluting of agro-industries, with high volume of effluent generated. (Shete Bharati and Shinkar, 2013a, 2013b). Dairy effluents present a relatively high organic load (COD in the range of 0.1e100 kg/m3) with an index of biodegradability (BOD5/COD) typically in the range 0.4e0.8 (Prazeres et al., 2012). Consequently, these wastewaters cannot be discharged directly to water bodies or even into municipal sewerage systems without some preliminary treatment. Biological degradation is considered one of the most promising methods of organic matter removal from dairy effluents (Tatoulis et al., 2015). The majority of the studies have been conducted under anaerobic conditions (with significant COD removal), however drawbacks like low values of alkalinity, formation of granular sludge, high cost and requirement of skilled personnel make them difficult to applied in small industrial plants. For these reasons, studies have been carried out under aerobic or anaerobic/aerobic conditions. However, the final effluent after these treatment methods was still not suitable for discharge, as COD concentration wasn't under the permissible limit of 125 mg L1 (for municipal and industrial effluents, Gazette of the Government (GR) 2011/354B) and decolorization could not be achieved even under the most favourable conditions (Tatoulis et al., 2015). In recent years AOPs have been investigated as possible treatment option for agroindustrial wastewaters. Oxidation processes are not recommended when dealing with raw dairy effluents, however, is suggested to perform better as a post-treatment process, after biodegradation for the complete purification of dairy effluents (Carvalho et al., 2013). Unfortunately, due to economic reasons, almost all of the existing technologies are not recommended for small to- mediumsized dairy industries that are dispersed over different regions. In this study a hybrid system comprising an aerobic biological process and electrochemical oxidation for the treatment of dairy wastewaters has been tested. Packed bed reactors under sequencing batch operation with recirculation proved very promising devices providing a support material for consistent biofilm structure development while minimizing the operating cost since physical aeration is adequate for microorganism needs. Also, the use of mixed indigenous microorganisms originating from the dairy wastewater provided an advantage ensuring durability under various environmental and operating conditions. Experiments in pilot-scale bioreactor led to high degradation rates up to 6.41 g dCOD/(L day) (Tatoulis et al., 2015). Then, electrochemical oxidation can completely mineralize and decolorize the biologically pre-
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treated effluent. Iridium electrodes, which are durable and cheap, under favourable conditions led to significant COD removals up to 96.7%. It is worth mentioning that the efficacy of the proposed system to follow the legislations limits depends strongly on the operating conditions. The COD limit of 125 mg/L achieved only when the initial COD before the electrochemical oxidation was below 4000 mg/L. This implies that further research and optimization is needed in order to choose the optimum conditions for the transition between biological and electrochemical treatment. Concerning the produced sludge from the above-mentioned treatment process; a suitable composting process could be designed. In addition, co-composting with sludge generated from the treatment of other agro-industrial wastewater (winery, raisin, table olive wastewater etc.) can also be a beneficial solid waste treatment option. 4. Conclusions The combination of biological and physicochemical hybrid treatment is conceptually very attractive in order to overcome the limitations of both technologies for the wastewater treatment and to provide a viable alternative for the treatment of agro-industrial effluents. This study investigated the effect of several process parameters on electrochemical treatment of aerobically pre-treated dairy wastewaters in terms of COD and color removal. The main conclusions are as follows: Among different DSA electrode materials, iridium electrodes provide a certain advantage at relative low current density (50 mA/cm2), achieving high COD removal (up to 96.7%) The role of current density is a crucial operating parameter for the process of the electrochemical treatment, since COD removal increases with increasing current density. (Current density of 50, 100 and 200 mA/cm2 led to COD removal up to 82.4%, 89% and 96.7%, respectively), Temperature has an effect on the electrochemical oxidation of aerobically pre-treated dairy wastewaters only for temperatures above 40 C, Sodium chloride seems to enhance the COD removal rate mainly due to the electro generation of active chlorine however, further research is needed due to possibly formation of organochlorinated compounds. References Anglada, Angela, Urtiaga, A., Ortiz, I., 2009. Contributions of electrochemical oxidation to waste-water treatment: fundamentals and review of applications. J. Chem. Technol. Biotechnol. http://dx.doi.org/10.1002/jctb.2214. APHA/AWWA/WEF, 2012. Standard Methods for the Examination of Water and Wastewater, Standard Methods doi:ISBN 9780875532356. Bensadok, K., El Hanafi, N., Lapicque, F., 2011. Electrochemical treatment of dairy effluent using combined Al and Ti/Pt electrodes system. Desalination 280, 244e251. http://dx.doi.org/10.1016/j.desal.2011.07.006. n, B., Oropeza-Guzman, M.T., Brillas, E., Sire s, I., 2014. Sequential electroBorbo chemical treatment of dairy wastewater using aluminum and DSA-type anodes. Environ. Sci. Pollut. Res. Int. 21, 8573e8584. http://dx.doi.org/10.1007/s11356014-2787-x. Bumbac, C., Ionescu, I.A., Tiron, O., Badescu, V.R., 2015. Continuous flow aerobic granular sludge reactor for dairy wastewater treatment. Water Sci. Technol. 71, 440e445. http://dx.doi.org/10.2166/wst.2015.007. Carvalho, F., Prazeres, A.R., Rivas, J., 2013. Cheese whey wastewater: characterization and treatment. Sci. Total Environ. http://dx.doi.org/10.1016/ j.scitotenv.2012.12.038. Chatzisymeon, E., Dimou, A., Mantzavinos, D., Katsaounis, A., 2009. Electrochemical oxidation of model compounds and olive mill wastewater over DSA electrodes: 1. The case of Ti/IrO2 anode. J. Hazard. Mater. 167, 268e274. http://dx.doi.org/ 10.1016/j.jhazmat.2008.12.117. Comninellis, Ch., Chen, G., 2010. Electrochemistry for the Environment. Electrochemistry for the Environment. http://dx.doi.org/10.1007/978-0-387-68318-8.
223
Comninellis, Ch., Vercesi, G.P., 1991. Problems in DSA coating deposition by thermal decomposition. J. Appl. Electrochem. http://dx.doi.org/10.1007/BF01464294. Davarnejad, R., Nikseresht, M., 2016. Dairy wastewater treatment using an electrochemical method: experimental and statistical study. J. Electroanal. Chem. 775, 364e373. http://dx.doi.org/10.1016/j.jelechem.2016.06.016. Demirel, B., Yenigun, O., Onay, T.T., 2005. Anaerobic treatment of dairy wastewaters: a review. Process Biochem. 40, 2583e2595. http://dx.doi.org/10.1016/ j.procbio.2004.12.015. Dimitrakopoulou, D., Rethemiotaki, I., Frontistis, Z., Xekoukoulotakis, N.P., Venieri, D., Mantzavinos, D., 2012. Degradation, mineralization and antibiotic inactivation of amoxicillin by UV-A/TiO 2 photocatalysis. J. Environ. Manag. 98, 168e174. http://dx.doi.org/10.1016/j.jenvman.2012.01.010. Dridi Gargouri, O., Samet, Y., Abdelhedi, R., 2013. Electrocatalytic performance of PbO2 films in the degradation of dimethoate insecticide. Water SA 39. Ehrenmann, J., Henninger, S.K., Janiak, C., 2011. Water adsorption characteristics of MIL-101 for heat-transformation applications of MOFs. Eur. J. Inorg. Chem. 2011, 471e474. http://dx.doi.org/10.1002/ejic.201001156. FaridaYunus, R., Zheng, Y.-M., Nanayakkara, K.G.N., Chen, J.P., 2009. Electrochemical removal of rhodamine 6G by using RuO2 coated Ti DSA. Ind. Eng. Chem. Res. 48, 7466e7473. http://dx.doi.org/10.1021/ie801719b. Feki, F., Aloui, F., Feki, M., Sayadi, S., 2009. Electrochemical oxidation post-treatment of landfill leachates treated with membrane bioreactor. Chemosphere. http:// dx.doi.org/10.1016/j.chemosphere.2008.12.013. Frontistis, Z., Antonopoulou, M., Yazirdagi, M., Kilinc, Z., Konstantinou, I., Katsaounis, A., Mantzavinos, D., 2016. Boron-doped diamond electrooxidation of ethyl paraben: the effect of electrolyte on by-products distribution and mechanisms. J. Environ. Manag. http://dx.doi.org/10.1016/ j.jenvman.2016.06.044. Gotsi, M., Kalogerakis, N., Psillakis, E., Samaras, P., Mantzavinos, D., 2005. Electrochemical oxidation of olive oil mill wastewaters. Water Res. 39, 4177e4187. http://dx.doi.org/10.1016/j.watres.2005.07.037. Healy, M.G., Rodgers, M., Mulqueen, J., 2007. Treatment of dairy wastewater using constructed wetlands and intermittent sand filters. Bioresour. Technol. http:// dx.doi.org/10.1016/j.biortech.2006.07.036. Jeong, J., Kim, C., Yoon, J., 2009. The effect of electrode material on the generation of oxidants and microbial inactivation in the electrochemical disinfection processes. Water Res. 43, 895e901. http://dx.doi.org/10.1016/j.watres.2008.11.033. Kapałka, A., Katsaounis, A., Michels, N.-L., Leonidova, A., Souentie, S., Comninellis, C., Udert, K.M., 2010. Ammonia oxidation to nitrogen mediated by electrogenerated active chlorine on Ti/PtOx-IrO2. Electrochem. Commun. http:// dx.doi.org/10.1016/j.elecom.2010.06.019. Kushwaha, J.P., Srivastava, V.C., Mall, I.D., 2010. Treatment of dairy wastewater by commercial activated carbon and bagasse fly ash: parametric, kinetic and equilibrium modelling, disposal studies. Bioresour. Technol. 101, 3474e3483. http://dx.doi.org/10.1016/j.biortech.2010.01.002. Lamas Samanamud, G.R., Loures, C.C.A., Souza, A.L., Salazar, R.F.S., Oliveira, I.S., rioFilho, H.J., 2012. Heterogeneous photocatalytic degradation of Silva, M.B., Iza dairy wastewater using immobilized ZnO. ISRN Chem. Eng. 2012, 1e8. http:// dx.doi.org/10.5402/2012/275371. szlo , Z., Kerte sz, S., Mlinkovics, E., Hodúr, C., 2007. Dairy waste water treatment by La combining ozonation and nanofiltration. Sep. Sci. Technol. 42, 1627e1637. http://dx.doi.org/10.1080/01496390701290508. Lei, X., Maekawa, T., 2007. Electrochemical treatment of anaerobic digestion effluent using a Ti/PteIrO2 electrode. Bioresour. Technol. 98, 3521e3525. http:// dx.doi.org/10.1016/j.biortech.2006.11.018. Li, G., Zhu, M., Chen, J., Li, Y., Zhang, X., 2011. Production and contribution of hydroxyl radicals between the DSA anode and water interface. J. Environ. Sci. 23, 744e748. http://dx.doi.org/10.1016/S1001-0742(10)60470-6. rioFilho, H.J., Oliveira, I. dos S., Lamas Samanamud, G.R., Souza, A.L. Loures, C.C.A., Iza de, Silva, M.B., 2014. The application of response surface methodology in the study of photodegraded industrial dairy effluents by the photo-Fenton process: optimization and economic viability. ISRN Chem. Eng. 2014. Melchiors, M.S., Piovesan, M., Becegato, V.R., Becegato, V.A., Tambourgi, E.B., Paulino, A.T., 2016. Treatment of wastewater from the dairy industry using electroflocculation and solid whey recovery. J. Environ. Manag. 182, 574e580. http://dx.doi.org/10.1016/j.jenvman.2016.08.022. Papastefanakis, N., Mantzavinos, D., Katsaounis, A., 2010. DSA electrochemical treatment of olive mill wastewater on Ti/RuO2 anode. J. Appl. Electrochem 40, 729e737. http://dx.doi.org/10.1007/s10800-009-0050-9. Perez, G., Fern?ndez-Alba, A.R., Urtiaga, A.M., Ortiz, I., 2010. Electro-oxidation of reverse osmosis concentrates generated in tertiary water treatment. Water Res. 44, 2763e2772. http://dx.doi.org/10.1016/j.watres.2010.02.017. Prazeres, A.R., Carvalho, F., Rivas, J., 2012. Cheese whey management: a review. J. Environ. Manag. http://dx.doi.org/10.1016/j.jenvman.2012.05.018. Prazeres, A.R., Carvalho, F., Rivas, J., 2013. Fenton-like application to pretreated cheese whey wastewater. J. Environ. Manag. 129, 199e205. http://dx.doi.org/ 10.1016/j.jenvman.2013.07.016. Rajkumar, D., Kim, J.G., 2006. Oxidation of various reactive dyes with in situ electrogenerated active chlorine for textile dyeing industry wastewater treatment. J. Hazard. Mater. 136, 203e212. http://dx.doi.org/10.1016/j.jhazmat.2005.11.096. Sarkar, B., Chakrabarti, P.P., Vijaykumar, A., Kale, V., 2006. Wastewater treatment in dairy industries - possibility of reuse. Desalination 195, 141e152. http:// dx.doi.org/10.1016/j.desal.2005.11.015. €, M., 2015. Recent developments of electroS€ arkk€ a, H., Bhatnagar, A., Sillanp€ aa oxidation in water treatment d a review. J. Electroanal. Chem. 754, 46e56.
224
V. Markou et al. / Journal of Environmental Management 202 (2017) 217e224
http://dx.doi.org/10.1016/j.jelechem.2015.06.016. € S¸engil, I.A., Ozacar, M., 2006. Treatment of dairy wastewaters by electrocoagulation using mild steel electrodes. J. Hazard. Mater. 137, 1197e1205. http://dx.doi.org/ 10.1016/j.jhazmat.2006.04.009. Shete Bharati, S., Shinkar, N.P., 2013a. Anaerobic reactor to treat dairy industry wastewater. Int. J. Curr. Eng. Technol. 3, 1257e1263. Shete Bharati, Shinkar, N.P., 2013b. Dairy industry wastewater sources, characteristics & its effects on environment. Int. J. Curr. Eng. Technol. 3, 1611e1615. ISSN 2277-4106. € Yonar, T., 2017. Determination of the acute toxicities of physicolu, O., Sivriog chemical pretreatment and advanced oxidation processes applied to dairy effluents on activated sludge. J. Dairy Sci. 98, 2337e2344. http://dx.doi.org/ 10.3168/jds.2014-8278. Song, Y.-S., Lee, D.Y., Kim, B.-Y., 2004. Effect of glass frit addition on corrosion resistance of Ti/TiO2/IrO2eRuO2 films. Mater. Lett. http://dx.doi.org/10.1016/ j.matlet.2003.07.019. rez, A., Riera, F.A., 2015. Production of high-quality water by reverse osmosis of Sua milk dairy condensates. J. Ind. Eng. Chem. 21, 1340e1349. http://dx.doi.org/ 10.1016/j.jiec.2014.06.004. Subba Rao, A.N., Venkatarangaiah, V.T., 2014. Metal oxide-coated anodes in wastewater treatment. Environ. Sci. Pollut. Res. 21, 3197e3217. http:// dx.doi.org/10.1007/s11356-013-2313-6. Sultana, M.-Y., Mourti, C., Tatoulis, T., Akratos, C.S., Tekerlekopoulou, A.G., Vayenas, D.V., 2016. Effect of hydraulic retention time, temperature, and organic load on a horizontal subsurface flow constructed wetland treating cheese whey wastewater. J. Chem. Technol. Biotechnol. 91, 726e732. http://dx.doi.org/ 10.1002/jctb.4637. Tatoulis, T.I., Tekerlekopoulou, A.G., Akratos, C.S., Pavlou, S., Vayenas, D.V., 2015. Aerobic biological treatment of second cheese whey in suspended and attached growth reactors. J. Chem. Technol. Biotechnol. 90, 2040e2049. http:// dx.doi.org/10.1002/jctb.4515. Turro, E., Giannis, A., Cossu, R., Mantzavinos, D., Katsaounis, A., 2012. Reprint of:
electrochemical oxidation of stabilized landfill leachate on DSA electrodes. J. Hazard. Mater. 207, 73e78. http://dx.doi.org/10.1016/j.jhazmat.2012.01.083. sito, E., Aldaz, A., Montiel, V., 2014. ElectroValero, D., García-García, V., Expo chemical treatment of wastewater from almond industry using DSA-type anodes: direct connection to a PV generator. Sep. Purif. Technol. 123, 15e22. http://dx.doi.org/10.1016/j.seppur.2013.12.023. Varga, L., Szigeti, J., 2016. Use of ozone in the dairy industry: a review. Int. J. Dairy Technol. 69, 157e168. http://dx.doi.org/10.1111/1471-0307.12302. Venetsaneas, N., Antonopoulou, G., Stamatelatou, K., Kornaros, M., 2009. Using cheese whey for hydrogen and methane generation in a two-stage continuous process with alternative pH controlling approaches. Bioresour. Technol. 100, 3713e3717. http://dx.doi.org/10.1016/j.biortech.2009.01.025. Vlyssides, A., Tsimas, E., Barampouti, E.M., Mai, S., Stamatoglou, A., 2013. Implementation of Fenton process on wastewater from a cheese-making factory. Desalin. Water Treat. 51, 3069e3075. http://dx.doi.org/10.1080/ 19443994.2012.748459. Vourch, M., Balannec, B., Chaufer, B., Dorange, G., 2008. Treatment of dairy industry wastewater by reverse osmosis for water reuse. Desalination 219, 190e202. http://dx.doi.org/10.1016/j.desal.2007.05.013. Yoon, Y., Cho, E., Jung, Y., Kwon, M., Yoon, J., Kang, J.W., 2014. Evaluation of the formation of oxidants and by-products using Pt/Ti, RuO2/Ti, and IrO2/Ti electrodes in the electrochemical process. Environ. Technol. 36, 317e326. http:// dx.doi.org/10.1080/09593330.2014.946098. Youssef, N.A., Shaban, S.A., Ibrahim, F.A., Mahmoud, A.S., 2016. Degradation of methyl orange using Fenton catalytic reaction. Egypt. J. Pet. 25, 317e321. http:// dx.doi.org/10.1016/j.ejpe.2015.07.017. Zhang, H., Ran, X., Wu, X., Zhang, D., 2011. Evaluation of electro-oxidation of biologically treated landfill leachate using response surface methodology. J. Hazard. Mater. 188, 261e268. http://dx.doi.org/10.1016/j.jhazmat.2011.01.097. Zhou, M., Liu, L., Jiao, Y., Wang, Q., Tan, Q., 2011. Treatment of high-salinity reverse osmosis concentrate by electrochemical oxidation on BDD and DSA electrodes. Desalination 277, 201e206. http://dx.doi.org/10.1016/j.desal.2011.04.030.
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Research article
Electrochemical treatment of biologically pre-treated dairy wastewater using dimensionally stable anodes Vlasia Markou a, Maria-Christina Kontogianni a, Zacharias Frontistis a, *, Athanasia G. Tekerlekopoulou b, Alexandros Katsaounis a, Dimitris Vayenas a, c a b c
Department of Chemical Engineering, University of Patras, Caratheodory 1, GR-26504 Patras, Greece Department of Environmental & Natural Resources Management, University of Patras, Agrinio, Greece Institute of Chemical Engineering Sciences, Foundation for Research and Technology, PO Box 1414, GR-26504 Patras, Greece
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2017 Received in revised form 21 June 2017 Accepted 17 July 2017
In this work, electrochemical oxidation of aerobically pre-treated dairy wastewaters using IrO2-Pt coated dimensionally stable anodes was investigated. It was found that IrO2/Ti electrode outperforming Pt/Ti and IrO2-Pt/Ti at lower current densities, while Pt/Ti achieved better efficiency at higher current density. Among the different parameters which were studied, the current density was the most crucial for the efficiency of the process. A current density of 100 mA/cm2 led to almost complete removal of 3700 mg/L COD after 360 min of treatment using IrO2/Ti electrode and 0.2 M of sodium chloride while complete decolorization was achieved in less than 60 min. Electrolytes also found to significantly affect the process. More specific, the use of sodium chloride instead of sodium sulfate enhanced both COD and color removal due to the formation of active chlorine species. The effect of temperature was relative low; the process was favourable at elevated temperatures while increasing COD loading resulted in a decrease of COD and color removal. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Dairy Wastewater Electrochemical Dimensionally stable anodes
1. Introduction Since ancient times the consumption of dairy products constitutes an important part of human nutrition. In recent years, due to the intensification of the dairy industry the management of dairy effluents has become a major problem since dairy wastewaters can be a significant threat, mostly for the aquatic environment (Carvalho et al., 2013; Prazeres et al., 2012). Consequently, there is a significant need to improve the management and treatment of dairy effluents in order to reduce environmental problems and to ensure the economic viability of this essential agricultural industry. (Sarkar et al., 2006). Several research groups have studied the biological treatment of dairy wastewater, either under aerobic condition (Bumbac et al., 2015; Tatoulis et al., 2015) or anaerobic conditions (Venetsaneas et al., 2009; Demirel et al., 2005) with the simultaneous production of hydrogen and methane. However, even though biological processes may lead to a significant reduction of the organic
* Corresponding author. E-mail address: [email protected] (Z. Frontistis). http://dx.doi.org/10.1016/j.jenvman.2017.07.046 0301-4797/© 2017 Elsevier Ltd. All rights reserved.
material, usually they fail to meet the limits settled by environmental legislations for the disposal of effluents into the environment. Therefore, the combination of biological treatment with other treatment processes such as constructed wetlands (Sultana et al., 2016) and intermittent sand filters (Healy et al., 2007) have been proposed in literature. At the same time, a considerable research on agro-industrial wastewaters treatment using physical processes like membrane rez and technology, mainly reverse osmosis and nanofiltration (Sua szlo et al., 2007) or adsorption Riera, 2015; Vourch et al., 2008; La (Kushwaha et al., 2010) has been carried out. Nevertheless, the main drawback of the physical processes is the fact that they transfer the problem from one phase to another resulting in the necessity either of the further concentrated solution treatment (Perez et al., 2010), or the regeneration of the adsorption material (Ehrenmann et al., 2011). Advanced Oxidation Processes (AOPs) is a family of technologies based on the in situ production of highly reactive radicals. Among various AOPs most studies related to dairy wastewaters have been conducted with Fenton, photo Fenton and electro Fenton like reactions (Davarnejad and Nikseresht, 2016; Loures et al., 2014; Prazeres et al., 2013; Vlyssides et al., 2013), photocatalysis (Lamas
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Samanamud et al., 2012), ozonation (Varga and Szigeti, 2016), electro-coagulation and electro flocculation (Melchiors et al., 2016; € S¸engil and Ozacar, 2006). More specific, Loures et al. (2014) studied the photo Fenton process for the treatment of dairy effluents with an initial COD (chemical oxygen demand) concentration of 9500 mg/L. They found that more than 90.7% COD and 78.8% BOD can be removed under optimal conditions (35 g H2O2, 3.6 g Fe2þ, pH lu and Yonar (2017) applied ozon3.5 and UV light 28 W). Sivriog ation for the degradation of dairy wastewater with initial COD concentration of 6300 mg/L. Ozonation led to COD removal up to 70% after 240 min of treatment with ozone dose equal to 2 gr/h. € S¸engil and Ozacar (2006) treated dairy wastewaters by electrocoagulation using steel electrodes. COD efficiency reached the value of 98% while the optimum current density, pH and treatment time for COD equal to 18,300 mg/L were 0.6 mA/cm2, 7 and 1 min, respectively. Melchiors et al. (2016) examined the efficiency of electroflocculation method for the treatment of dairy wastewater with initial COD of 8303 mg/L. A significant removal of organic matter was achieved (97.4%) using iron electrodes at final pH of 7.4. However, despite the high COD removal the main problem of electroflocculation and electrocoagulation processes remains firstly the ‘’sacrificial’’ electrodes and their dilution into the wastewater streams (because of their oxidation) and secondly the generation of significant quantities of sludge during the process (Bensadok et al., 2011). On the other hand, electrochemical oxidation (EO) doesn't suffer from these drawbacks and seems to be a promising alternative approach, mainly due to its high efficiency, and ease of use and €rkk€ control (Sa a et al., 2015; Valero et al., 2014; Anglada et al., 2009). EO is divided into direct and indirect oxidation (depending on the anode and its participation of the oxidation process), while the oxidizing agents include among others hydroxyl radicals, hypochlorite and ozone. In recent years, the use of dimensionally stable anodes (DSA) like RuO2/Ti, PbO2/Ti and TiO2eRuO2eIrO2/Ti has increased significantly against other types of electrodes, due to their high stability and activity (Subba Rao and Venkatarangaiah, 2014; Turro et al., 2012; Chatzisymeon et al., 2009; FaridaYunus et al., 2009). n et al. (2014) studied the oxidation of dairy wastewater Borbo using electrochemistry over an IrO2-Ta2O5/Ti electrode. They used different electrochemical techniques like cyclic voltammetry, pulsed differential voltammetry and chronoamperometry and concluded that two different processes/pathways exist using the above DSA-type electrode: one indirect (via hydroxyl radicals) and one direct oxidation (with the active participation of the electrode) n et al., 2014). However, they suggested that further research (Borbo is needed to study the variables that affect the efficiency of the system. Considering that the kinetics of electrochemical oxidations of industrial effluents with high organic loading follows near zeroorder rate with respect to the organic loading (Chatzisymeon et al., 2009) the approach to couple an electrochemical process and a biological treatment seems to be rather advantageous. Indeed, several researchers (Zhang et al., 2011; Feki et al., 2009) have demonstrated the potential of the combined biological and physicochemical processes to treat industrial effluents like landfill leachates. Lei and Maekawa (2007) studied the electrooxidation of anaerobic digestion effluents using Pt-IrO2/Ti electrode. They found that ammonia could be completely removed at 5 h under 1A and 1% NaCl, while the TOC and turbidity removal reached 51.4% and 95.5%, respectively. Under this perspective, in this study, the electrochemical oxidation of an aerobically pretreated dairy wastewater is investigated giving attention to issues that have not been dealt with before; these include working electrode material (different DSA
electrodes), current density, electrolytes, pH, treatment time, temperature and initial COD loading. According to our knowledge, this is the first report on the combination of a biological and an electrochemical process for the treatment of dairy effluents. 2. Materials and methods 2.1. Dairy effluent Second cheese whey from cottage cheese production was obtained from a Greek cheese factory (Papathanasiou A.B.E.E.). The dairy effluent was firstly treated aerobically using a pilot-scale packed-bed bioreactor. The bioreactor as well as the bioprocess have been described in previous study (Tatoulis et al., 2015). The COD and pH of the pilot plant effluent were in the range of 2500e15.000 mg/L and 6e7.5, respectively, depending on the operating conditions of the bioreactor. 2.2. Electrodes The preparation method of the DSA-type electrodes has been explained extensively in previous studies (Kapałka et al., 2010; Papastefanakis et al., 2010; Chatzisymeon et al., 2009). Briefly, the IrO2 or Pt working electrodes was prepared by thermal decomposition of 250 mM H2IrCl6 (Acros Organics, 40%) or H2PtCl6 metal precursors dissolved in isopropanol (Fluka, 99.5%) on a squaredshaped titanium support, taking into account that the deposition yield of Pt and IrO2 is 60% and 100% respectively (Comninellis and Vercesi, 1991). The titanium substrate was sandblasted to ensure good adhesion of the deposit on its surface and chemically treated using oxalic acid solution (1M) to clean its surface from residual sands. The substrate was then dried in an oven at 70 C and weighed. The precursor solution was spread on the titanium substrate forming a thin film layer on the electron surface. Afterwards, the sample was treated in an oven for 10 min to allow solvent evaporation. This step was followed by treatment in the furnace for the thermal decomposition of the precursor solution at 500 C in air for 10 min. The same procedure was repeated several times and, after the last IrO2 or Pt coating, the electrode remained at 500 C for 1 h. The final metal loading in all cases was 1.2e1.3 mg/cm2. For the Pt-IrO2 binary electrode, (50/50 molar ratio), the above method was applied using solutions of the precursors in the appropriate ratio. 2.3. Scanning electron microscopy (SEM) The morphology of the surface was explored with Scanning electron microscopy (SEM) using a FEI, FEG QUANTA 520 scanning electron microscope. 2.4. Electrochemical oxidation experiments Experiments were conducted in a double wall glass reactor. The appropriate electrode (Pt/Ti, IrO2/Ti or Pt-IrO2/Ti) had working surface of 12.5 cm2 and two zirconium bars served as cathodes. The distance between the anode and the cathodes was 1.5 cm while the electrodes were connected with a DC power supply (model QJ3005C). The volume of the solution inside the reactor was 150 mL and the biologically pretreated dairy wastewater stirred with a magnetic bar. The temperature remained constant using a water bath. All experiments were conducted with NaCl (>99%, Aldrich) as the electrolyte, while, in some cases, Na2SO4 (>99%, Aldrich) was also added. The majority of the experiments were performed in triplicate and the relative error was less than 5%.
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2.5. Analytical methods
3. Results and discussion
2.5.1. Measurement of COD and color COD concentration was determined by the dichromate method according to the Standard Methods (APHA, 2012) and the concentration was measured colorimetrically using a DR5000 spectrophotometer (Hach Company, USA). Sample absorbance was also scanned in the 200e800 nm wavelength band on a DR 5000 spectrophotometer. Changes in sample absorbance at 600 nm were monitored to assess the extent of decolorization during electrochemical treatment.
3.1. Surface morphology
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Scanning Electron Microscopy was used to explore the surface structure of the samples and have an estimation on the size of the metal particles. Fig. 1 show the surface before and after the deposition of the anodic films. SEM images shown in Fig. 1a and b reveal a porous Ti surface (substrate) mainly due to the chemical treatment. Deposition of IrO2 (Fig. 1c and d), Pt (Fig. 1e and f) and Pt-IrO2 (Fig. 1g and h) via thermal treatment at 500 C leads to the development of metal or metal oxide islands on the porous surface.
Fig. 1. SEM images of (a, b) Ti support, (c, d) IrO2/Ti, (e, f) Pt/Ti and (g, h) Pt- IrO2/Ti.
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These islands are not uniform while the diameter of metal or metal oxide particles varied between 10 and 100 nm. According to the SEM, Pt and IrO2 particles appear together rather than two distinguished phases. In general, the images are similar with images taken from DSA type electrodes reported previously in literature both by our group (Turro et al., 2012) and other researchers (Zhou et al., 2011; Song et al., 2004).
3.4. Effect of initial COD concentration The effect of the initial organic loading on the process using IrO2 electrode, 0.2M NaCl and 100 mA/cm2 was investigated and the results are illustrated in Fig. 4. The COD percentage removal decreased with increasing organic loading. Indeed, COD reduction was 93% at initial COD concentration of 3700 mg/L (after 360 min), while it dropped down significantly at the value of 47% for an initial COD equal to 15,000 mg/L.
3.2. Effect of electrodes material Preliminary experiments were carried out to assess the effect of the electrode material on the treatment efficiency. Fig. 2 presents the experiments conducted at initial COD ~4000 mg/L, with 0.2 M NaCl at 50, 100 and 200 mA/cm2. Iridium-based electrode shows a significantly higher activity at the lower current density of 50 mA/ cm2 (COD removal of 96.7%). In contrary, platinum electrodes have an advantage at the high current density of 200 mA/cm2, achieving almost complete COD removal, mostly due to the larger working potential window of Pt electrodes. According to the Jeong et al. (2009), who studied the effect of the electrode materials on the electro generation of active chlorine, IrO2 is a better electrode than Pt. The following order has been found concerning the ability of the electrode to produce active chlorine: IrO2 > RuO2 > PteIrO2 > BDD > Pt. They also concluded that most of the chlorine produced using IrO2 as anodic electrode coming from direct oxidation of Cl ions rather than reaction with hydroxyl radicals as for example at the case of Boron Doped Diamond (BDD) electrodes. Recently, Yoon et al. (2014) evaluated the electro-generation of oxidants and potential dangerous by-products using Pt, RuO2, and IrO2 electrodes. The higher concentration of hydrogen peroxide was observed using IrO2 electrode while at the same time both ClO3 and ClO4, which are hazardous by-products of the electrochemical oxidation of the chloride mediums, were pronounced electro generated on Pt. Considering the above conclusions together with the fact that iridium is almost 30% cheaper than platinum, IrO2/Ti electrode was selected for the rest of this study.
3.3. Effect of current density The effect of current density (in the range of 50e200 mA/cm2) on the treatment of aerobically pre-treated dairy wastewater was also investigated by examining both COD and color removal. Experiments with an initial COD concentration equal to 3750 mg/L and 0.2 M NaCl were performed and the results are presented in Fig. 3. It was found that the current density is a crucial parameter for the process. More specific, COD removal increases with increasing current density (82.4%, 89% and 96.7% for 50, 100 and 200 mA/cm2, respectively). As the current increasing, higher concentrations of reactive species (like hydroxyl radicals and hypochlorite) are produced (Li et al., 2011). Resulted in a faster oxidation of the organic pollutants (Fig. 3a). On the contrary, utilization of high current densities result in low current efficiencies, especially in the case of DSA-type anodes (Comninellis and Chen, 2010) due to side reactions like oxygen evolution and thus to a non-economically favored process. Color removal was significantly faster than COD removal. As shown in Fig. 3b almost complete decolorization was achieved in less than 60 min for all current densities. These results are in agreement with Rajkumar and Kim (2006) who reported the vital effect of increasing current on the electrochemical oxidation of textile wastewater on DSA electrodes.
Fig. 2. Effect of electrode material on the electrochemical oxidation of aerobically pretreated dairy effluent. CODo ¼ 4000 mg/L, operating volume ¼ 150 mL, [NaCl] ¼ 0.2 M, for current density (a) 50 mA/cm2, (b) 100 mA/cm2 and (c) 200 mA/cm2.
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Fig. 5. Effect of supporting electrolyte on the electrochemical oxidation of biological pre treated dairy effluent. CODo ¼ 3750 mg/L, operating volume ¼ 150 mL, [NaCl] ¼ 0.2 M.
concentration of the reactive radicals becomes the limiting factor at high concentrations of organic matter. Moreover, at heterogeneous systems, the mass transfer limitations during the migration of the organic effluent from the bulk to the electrode or catalyst surface plays a significant role. Another explanation is the possible formation of polymeric compounds during the electrochemical oxidation process and their deposition on the anodic surface leads to electrode fouling and thus to lower electrocatalytic performance.
3.5. Effect of the type of the electrolyte Fig. 3. Effect of applied current density on the electrochemical oxidation of the aerobically pre-treated dairy effluent in the terms of (a) COD removal and (b) color removal. CODo ¼ 3750 mg/L, operating volume ¼ 150 mL, [NaCl] ¼ 0.2 M.
Fig. 4. Effect of COD loading on the electrochemical oxidation of biological pre-treated dairy effluent. CODo ¼ 3750 mg/L, operating volume ¼ 150 mL, [NaCl] ¼ 0.2 M.
This behavior has been extensively observed and discussed using various advanced oxidation process like photocatalysis (Dimitrakopoulou et al., 2012), Fenton reaction (Youssef et al., 2016) and electrochemical remediation (Gotsi et al., 2005). The common factor between different advanced oxidation processes is that the
Experiments with two different types of electrolytes (0.2 M NaCl and 0.2 M of Na2SO4 who are the most common electrolytes for environmental applications) were carried out to examine their effect on the electrochemical oxidation of pre-treated dairy wastewater. The pH change before and after the addition of the above electrolytes was very small. In all experiments IrO2 electrode was used as anode while current density and initial COD concentration were 100 mA/cm2 and 3750 mg/L, respectively. Sodium sulfate presented lower COD removal efficiency (57.7%) than sodium chloride (89%) (Fig. 5). This is possibly due to the different mechanism of the reaction. In the case of sodium chloride, the significant oxidation pathway includes the production of active chlorine. On the other hand, in the case of Na2SO4 the dominant oxidation species are the sulfate radicals which are considered as selective oxidants with relative low oxidation power. However, high concentration of sodium chloride can lead to effluents with high toxicity due to the presence of organochlorides which are considered as potential dangerous byproducts (Frontistis et al., 2016). It is worth mentioning that the evolution of the compounds that absorb at wavelength of 280 nm were much higher (after the first 20 min of treatment) in the case of sodium chloride than in the case of sodium sulfate where their concentration was negligible after 2 h of treatment (Fig. 6a). Although these compounds can be associated with organochlorinated molecules their exact identification is very difficult due to the complexity of the industrial effluent used in this study. In agreement with COD removal, the color reduction is depended significantly on the electrolyte. In the presence of sodium chloride, the decolorization was almost complete at 60 min (Fig. 6b), in contrast with the case of sodium sulfate which was only 70%.
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Fig. 7. Effect of temperature on the electrochemical oxidation of biological pre-treated dairy effluent. CODo ¼ 3750 mg/L, operating volume 150 mL, [NaCl] ¼ 0.2 M.
Fig. 6. Effect of supporting electrolyte on the electrochemical oxidation of biological pre-treated dairy effluent in the terms of (a) byproducts at 290 nm and (b) color removal at 600 nm. CODo ¼ 3750 mg/L, operating volume ¼ 150 mL, [NaCl] ¼ 0.2 M.
3.6. Effect of temperature All the above experiments were conducted at room temperature using a water bath. However, another set of experiments was also conducted in order to be examined the effect of temperature on the process. Three different temperatures were tested (25 C, 40 C and 60 C) using IrO2 as anode, initial COD concentration of 3750 mg/L, 0.2 M NaCl and current density of 100 mA/cm2. It was found that the reaction was almost unaffected at the range between 25 and 40 C (achieving COD removal about 90% in 360 min), while there is an improvement at higher temperature, 60 C (Fig. 7). This behavior can be explained by the Arrhenius equation between reactive oxygen species/radicals and pollutants. In addition, according to Dridi Gargouri et al. (2013) increase of temperature reduces the viscosity, thus enhances the diffusion rate and the mass transport of the pollutant to the electrode surface. Color removal followed the same behavior, however complete decolorization of the effluent achieved in less than 60 min in every studied temperature. 3.7. Perspectives The economy of many European countries is based on agricultural and agro-industries. Of all agro-industrial activities, the food sector has one of the highest consumptions of water, producing large quantities of wastewaters. Especially, dairy industry is one of
the most polluting of agro-industries, with high volume of effluent generated. (Shete Bharati and Shinkar, 2013a, 2013b). Dairy effluents present a relatively high organic load (COD in the range of 0.1e100 kg/m3) with an index of biodegradability (BOD5/COD) typically in the range 0.4e0.8 (Prazeres et al., 2012). Consequently, these wastewaters cannot be discharged directly to water bodies or even into municipal sewerage systems without some preliminary treatment. Biological degradation is considered one of the most promising methods of organic matter removal from dairy effluents (Tatoulis et al., 2015). The majority of the studies have been conducted under anaerobic conditions (with significant COD removal), however drawbacks like low values of alkalinity, formation of granular sludge, high cost and requirement of skilled personnel make them difficult to applied in small industrial plants. For these reasons, studies have been carried out under aerobic or anaerobic/aerobic conditions. However, the final effluent after these treatment methods was still not suitable for discharge, as COD concentration wasn't under the permissible limit of 125 mg L1 (for municipal and industrial effluents, Gazette of the Government (GR) 2011/354B) and decolorization could not be achieved even under the most favourable conditions (Tatoulis et al., 2015). In recent years AOPs have been investigated as possible treatment option for agroindustrial wastewaters. Oxidation processes are not recommended when dealing with raw dairy effluents, however, is suggested to perform better as a post-treatment process, after biodegradation for the complete purification of dairy effluents (Carvalho et al., 2013). Unfortunately, due to economic reasons, almost all of the existing technologies are not recommended for small to- mediumsized dairy industries that are dispersed over different regions. In this study a hybrid system comprising an aerobic biological process and electrochemical oxidation for the treatment of dairy wastewaters has been tested. Packed bed reactors under sequencing batch operation with recirculation proved very promising devices providing a support material for consistent biofilm structure development while minimizing the operating cost since physical aeration is adequate for microorganism needs. Also, the use of mixed indigenous microorganisms originating from the dairy wastewater provided an advantage ensuring durability under various environmental and operating conditions. Experiments in pilot-scale bioreactor led to high degradation rates up to 6.41 g dCOD/(L day) (Tatoulis et al., 2015). Then, electrochemical oxidation can completely mineralize and decolorize the biologically pre-
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treated effluent. Iridium electrodes, which are durable and cheap, under favourable conditions led to significant COD removals up to 96.7%. It is worth mentioning that the efficacy of the proposed system to follow the legislations limits depends strongly on the operating conditions. The COD limit of 125 mg/L achieved only when the initial COD before the electrochemical oxidation was below 4000 mg/L. This implies that further research and optimization is needed in order to choose the optimum conditions for the transition between biological and electrochemical treatment. Concerning the produced sludge from the above-mentioned treatment process; a suitable composting process could be designed. In addition, co-composting with sludge generated from the treatment of other agro-industrial wastewater (winery, raisin, table olive wastewater etc.) can also be a beneficial solid waste treatment option. 4. Conclusions The combination of biological and physicochemical hybrid treatment is conceptually very attractive in order to overcome the limitations of both technologies for the wastewater treatment and to provide a viable alternative for the treatment of agro-industrial effluents. This study investigated the effect of several process parameters on electrochemical treatment of aerobically pre-treated dairy wastewaters in terms of COD and color removal. The main conclusions are as follows: Among different DSA electrode materials, iridium electrodes provide a certain advantage at relative low current density (50 mA/cm2), achieving high COD removal (up to 96.7%) The role of current density is a crucial operating parameter for the process of the electrochemical treatment, since COD removal increases with increasing current density. (Current density of 50, 100 and 200 mA/cm2 led to COD removal up to 82.4%, 89% and 96.7%, respectively), Temperature has an effect on the electrochemical oxidation of aerobically pre-treated dairy wastewaters only for temperatures above 40 C, Sodium chloride seems to enhance the COD removal rate mainly due to the electro generation of active chlorine however, further research is needed due to possibly formation of organochlorinated compounds. References Anglada, Angela, Urtiaga, A., Ortiz, I., 2009. Contributions of electrochemical oxidation to waste-water treatment: fundamentals and review of applications. J. Chem. Technol. Biotechnol. http://dx.doi.org/10.1002/jctb.2214. APHA/AWWA/WEF, 2012. Standard Methods for the Examination of Water and Wastewater, Standard Methods doi:ISBN 9780875532356. Bensadok, K., El Hanafi, N., Lapicque, F., 2011. Electrochemical treatment of dairy effluent using combined Al and Ti/Pt electrodes system. Desalination 280, 244e251. http://dx.doi.org/10.1016/j.desal.2011.07.006. n, B., Oropeza-Guzman, M.T., Brillas, E., Sire s, I., 2014. Sequential electroBorbo chemical treatment of dairy wastewater using aluminum and DSA-type anodes. Environ. Sci. Pollut. Res. Int. 21, 8573e8584. http://dx.doi.org/10.1007/s11356014-2787-x. Bumbac, C., Ionescu, I.A., Tiron, O., Badescu, V.R., 2015. Continuous flow aerobic granular sludge reactor for dairy wastewater treatment. Water Sci. Technol. 71, 440e445. http://dx.doi.org/10.2166/wst.2015.007. Carvalho, F., Prazeres, A.R., Rivas, J., 2013. Cheese whey wastewater: characterization and treatment. Sci. Total Environ. http://dx.doi.org/10.1016/ j.scitotenv.2012.12.038. Chatzisymeon, E., Dimou, A., Mantzavinos, D., Katsaounis, A., 2009. Electrochemical oxidation of model compounds and olive mill wastewater over DSA electrodes: 1. The case of Ti/IrO2 anode. J. Hazard. Mater. 167, 268e274. http://dx.doi.org/ 10.1016/j.jhazmat.2008.12.117. Comninellis, Ch., Chen, G., 2010. Electrochemistry for the Environment. Electrochemistry for the Environment. http://dx.doi.org/10.1007/978-0-387-68318-8.
223
Comninellis, Ch., Vercesi, G.P., 1991. Problems in DSA coating deposition by thermal decomposition. J. Appl. Electrochem. http://dx.doi.org/10.1007/BF01464294. Davarnejad, R., Nikseresht, M., 2016. Dairy wastewater treatment using an electrochemical method: experimental and statistical study. J. Electroanal. Chem. 775, 364e373. http://dx.doi.org/10.1016/j.jelechem.2016.06.016. Demirel, B., Yenigun, O., Onay, T.T., 2005. Anaerobic treatment of dairy wastewaters: a review. Process Biochem. 40, 2583e2595. http://dx.doi.org/10.1016/ j.procbio.2004.12.015. Dimitrakopoulou, D., Rethemiotaki, I., Frontistis, Z., Xekoukoulotakis, N.P., Venieri, D., Mantzavinos, D., 2012. Degradation, mineralization and antibiotic inactivation of amoxicillin by UV-A/TiO 2 photocatalysis. J. Environ. Manag. 98, 168e174. http://dx.doi.org/10.1016/j.jenvman.2012.01.010. Dridi Gargouri, O., Samet, Y., Abdelhedi, R., 2013. Electrocatalytic performance of PbO2 films in the degradation of dimethoate insecticide. Water SA 39. Ehrenmann, J., Henninger, S.K., Janiak, C., 2011. Water adsorption characteristics of MIL-101 for heat-transformation applications of MOFs. Eur. J. Inorg. Chem. 2011, 471e474. http://dx.doi.org/10.1002/ejic.201001156. FaridaYunus, R., Zheng, Y.-M., Nanayakkara, K.G.N., Chen, J.P., 2009. Electrochemical removal of rhodamine 6G by using RuO2 coated Ti DSA. Ind. Eng. Chem. Res. 48, 7466e7473. http://dx.doi.org/10.1021/ie801719b. Feki, F., Aloui, F., Feki, M., Sayadi, S., 2009. Electrochemical oxidation post-treatment of landfill leachates treated with membrane bioreactor. Chemosphere. http:// dx.doi.org/10.1016/j.chemosphere.2008.12.013. Frontistis, Z., Antonopoulou, M., Yazirdagi, M., Kilinc, Z., Konstantinou, I., Katsaounis, A., Mantzavinos, D., 2016. Boron-doped diamond electrooxidation of ethyl paraben: the effect of electrolyte on by-products distribution and mechanisms. J. Environ. Manag. http://dx.doi.org/10.1016/ j.jenvman.2016.06.044. Gotsi, M., Kalogerakis, N., Psillakis, E., Samaras, P., Mantzavinos, D., 2005. Electrochemical oxidation of olive oil mill wastewaters. Water Res. 39, 4177e4187. http://dx.doi.org/10.1016/j.watres.2005.07.037. Healy, M.G., Rodgers, M., Mulqueen, J., 2007. Treatment of dairy wastewater using constructed wetlands and intermittent sand filters. Bioresour. Technol. http:// dx.doi.org/10.1016/j.biortech.2006.07.036. Jeong, J., Kim, C., Yoon, J., 2009. The effect of electrode material on the generation of oxidants and microbial inactivation in the electrochemical disinfection processes. Water Res. 43, 895e901. http://dx.doi.org/10.1016/j.watres.2008.11.033. Kapałka, A., Katsaounis, A., Michels, N.-L., Leonidova, A., Souentie, S., Comninellis, C., Udert, K.M., 2010. Ammonia oxidation to nitrogen mediated by electrogenerated active chlorine on Ti/PtOx-IrO2. Electrochem. Commun. http:// dx.doi.org/10.1016/j.elecom.2010.06.019. Kushwaha, J.P., Srivastava, V.C., Mall, I.D., 2010. Treatment of dairy wastewater by commercial activated carbon and bagasse fly ash: parametric, kinetic and equilibrium modelling, disposal studies. Bioresour. Technol. 101, 3474e3483. http://dx.doi.org/10.1016/j.biortech.2010.01.002. Lamas Samanamud, G.R., Loures, C.C.A., Souza, A.L., Salazar, R.F.S., Oliveira, I.S., rioFilho, H.J., 2012. Heterogeneous photocatalytic degradation of Silva, M.B., Iza dairy wastewater using immobilized ZnO. ISRN Chem. Eng. 2012, 1e8. http:// dx.doi.org/10.5402/2012/275371. szlo , Z., Kerte sz, S., Mlinkovics, E., Hodúr, C., 2007. Dairy waste water treatment by La combining ozonation and nanofiltration. Sep. Sci. Technol. 42, 1627e1637. http://dx.doi.org/10.1080/01496390701290508. Lei, X., Maekawa, T., 2007. Electrochemical treatment of anaerobic digestion effluent using a Ti/PteIrO2 electrode. Bioresour. Technol. 98, 3521e3525. http:// dx.doi.org/10.1016/j.biortech.2006.11.018. Li, G., Zhu, M., Chen, J., Li, Y., Zhang, X., 2011. Production and contribution of hydroxyl radicals between the DSA anode and water interface. J. Environ. Sci. 23, 744e748. http://dx.doi.org/10.1016/S1001-0742(10)60470-6. rioFilho, H.J., Oliveira, I. dos S., Lamas Samanamud, G.R., Souza, A.L. Loures, C.C.A., Iza de, Silva, M.B., 2014. The application of response surface methodology in the study of photodegraded industrial dairy effluents by the photo-Fenton process: optimization and economic viability. ISRN Chem. Eng. 2014. Melchiors, M.S., Piovesan, M., Becegato, V.R., Becegato, V.A., Tambourgi, E.B., Paulino, A.T., 2016. Treatment of wastewater from the dairy industry using electroflocculation and solid whey recovery. J. Environ. Manag. 182, 574e580. http://dx.doi.org/10.1016/j.jenvman.2016.08.022. Papastefanakis, N., Mantzavinos, D., Katsaounis, A., 2010. DSA electrochemical treatment of olive mill wastewater on Ti/RuO2 anode. J. Appl. Electrochem 40, 729e737. http://dx.doi.org/10.1007/s10800-009-0050-9. Perez, G., Fern?ndez-Alba, A.R., Urtiaga, A.M., Ortiz, I., 2010. Electro-oxidation of reverse osmosis concentrates generated in tertiary water treatment. Water Res. 44, 2763e2772. http://dx.doi.org/10.1016/j.watres.2010.02.017. Prazeres, A.R., Carvalho, F., Rivas, J., 2012. Cheese whey management: a review. J. Environ. Manag. http://dx.doi.org/10.1016/j.jenvman.2012.05.018. Prazeres, A.R., Carvalho, F., Rivas, J., 2013. Fenton-like application to pretreated cheese whey wastewater. J. Environ. Manag. 129, 199e205. http://dx.doi.org/ 10.1016/j.jenvman.2013.07.016. Rajkumar, D., Kim, J.G., 2006. Oxidation of various reactive dyes with in situ electrogenerated active chlorine for textile dyeing industry wastewater treatment. J. Hazard. Mater. 136, 203e212. http://dx.doi.org/10.1016/j.jhazmat.2005.11.096. Sarkar, B., Chakrabarti, P.P., Vijaykumar, A., Kale, V., 2006. Wastewater treatment in dairy industries - possibility of reuse. Desalination 195, 141e152. http:// dx.doi.org/10.1016/j.desal.2005.11.015. €, M., 2015. Recent developments of electroS€ arkk€ a, H., Bhatnagar, A., Sillanp€ aa oxidation in water treatment d a review. J. Electroanal. Chem. 754, 46e56.
224
V. Markou et al. / Journal of Environmental Management 202 (2017) 217e224
http://dx.doi.org/10.1016/j.jelechem.2015.06.016. € S¸engil, I.A., Ozacar, M., 2006. Treatment of dairy wastewaters by electrocoagulation using mild steel electrodes. J. Hazard. Mater. 137, 1197e1205. http://dx.doi.org/ 10.1016/j.jhazmat.2006.04.009. Shete Bharati, S., Shinkar, N.P., 2013a. Anaerobic reactor to treat dairy industry wastewater. Int. J. Curr. Eng. Technol. 3, 1257e1263. Shete Bharati, Shinkar, N.P., 2013b. Dairy industry wastewater sources, characteristics & its effects on environment. Int. J. Curr. Eng. Technol. 3, 1611e1615. ISSN 2277-4106. € Yonar, T., 2017. Determination of the acute toxicities of physicolu, O., Sivriog chemical pretreatment and advanced oxidation processes applied to dairy effluents on activated sludge. J. Dairy Sci. 98, 2337e2344. http://dx.doi.org/ 10.3168/jds.2014-8278. Song, Y.-S., Lee, D.Y., Kim, B.-Y., 2004. Effect of glass frit addition on corrosion resistance of Ti/TiO2/IrO2eRuO2 films. Mater. Lett. http://dx.doi.org/10.1016/ j.matlet.2003.07.019. rez, A., Riera, F.A., 2015. Production of high-quality water by reverse osmosis of Sua milk dairy condensates. J. Ind. Eng. Chem. 21, 1340e1349. http://dx.doi.org/ 10.1016/j.jiec.2014.06.004. Subba Rao, A.N., Venkatarangaiah, V.T., 2014. Metal oxide-coated anodes in wastewater treatment. Environ. Sci. Pollut. Res. 21, 3197e3217. http:// dx.doi.org/10.1007/s11356-013-2313-6. Sultana, M.-Y., Mourti, C., Tatoulis, T., Akratos, C.S., Tekerlekopoulou, A.G., Vayenas, D.V., 2016. Effect of hydraulic retention time, temperature, and organic load on a horizontal subsurface flow constructed wetland treating cheese whey wastewater. J. Chem. Technol. Biotechnol. 91, 726e732. http://dx.doi.org/ 10.1002/jctb.4637. Tatoulis, T.I., Tekerlekopoulou, A.G., Akratos, C.S., Pavlou, S., Vayenas, D.V., 2015. Aerobic biological treatment of second cheese whey in suspended and attached growth reactors. J. Chem. Technol. Biotechnol. 90, 2040e2049. http:// dx.doi.org/10.1002/jctb.4515. Turro, E., Giannis, A., Cossu, R., Mantzavinos, D., Katsaounis, A., 2012. Reprint of:
electrochemical oxidation of stabilized landfill leachate on DSA electrodes. J. Hazard. Mater. 207, 73e78. http://dx.doi.org/10.1016/j.jhazmat.2012.01.083. sito, E., Aldaz, A., Montiel, V., 2014. ElectroValero, D., García-García, V., Expo chemical treatment of wastewater from almond industry using DSA-type anodes: direct connection to a PV generator. Sep. Purif. Technol. 123, 15e22. http://dx.doi.org/10.1016/j.seppur.2013.12.023. Varga, L., Szigeti, J., 2016. Use of ozone in the dairy industry: a review. Int. J. Dairy Technol. 69, 157e168. http://dx.doi.org/10.1111/1471-0307.12302. Venetsaneas, N., Antonopoulou, G., Stamatelatou, K., Kornaros, M., 2009. Using cheese whey for hydrogen and methane generation in a two-stage continuous process with alternative pH controlling approaches. Bioresour. Technol. 100, 3713e3717. http://dx.doi.org/10.1016/j.biortech.2009.01.025. Vlyssides, A., Tsimas, E., Barampouti, E.M., Mai, S., Stamatoglou, A., 2013. Implementation of Fenton process on wastewater from a cheese-making factory. Desalin. Water Treat. 51, 3069e3075. http://dx.doi.org/10.1080/ 19443994.2012.748459. Vourch, M., Balannec, B., Chaufer, B., Dorange, G., 2008. Treatment of dairy industry wastewater by reverse osmosis for water reuse. Desalination 219, 190e202. http://dx.doi.org/10.1016/j.desal.2007.05.013. Yoon, Y., Cho, E., Jung, Y., Kwon, M., Yoon, J., Kang, J.W., 2014. Evaluation of the formation of oxidants and by-products using Pt/Ti, RuO2/Ti, and IrO2/Ti electrodes in the electrochemical process. Environ. Technol. 36, 317e326. http:// dx.doi.org/10.1080/09593330.2014.946098. Youssef, N.A., Shaban, S.A., Ibrahim, F.A., Mahmoud, A.S., 2016. Degradation of methyl orange using Fenton catalytic reaction. Egypt. J. Pet. 25, 317e321. http:// dx.doi.org/10.1016/j.ejpe.2015.07.017. Zhang, H., Ran, X., Wu, X., Zhang, D., 2011. Evaluation of electro-oxidation of biologically treated landfill leachate using response surface methodology. J. Hazard. Mater. 188, 261e268. http://dx.doi.org/10.1016/j.jhazmat.2011.01.097. Zhou, M., Liu, L., Jiao, Y., Wang, Q., Tan, Q., 2011. Treatment of high-salinity reverse osmosis concentrate by electrochemical oxidation on BDD and DSA electrodes. Desalination 277, 201e206. http://dx.doi.org/10.1016/j.desal.2011.04.030.