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Enhancement of electrocoagulation-flotation process for urban wastewater treatment using Al and Fe electrodes: techno-economic study
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 13 (2019) 549–555
www.materialstoday.com/proceedings
ICMES 2018
Enhancement of electrocoagulation-flotation process for urban wastewater treatment using Al and Fe electrodes: techno-economic study M. Elazzouzi1, 2*, K. Haboubi2, M.S. Elyoubi1 1
Laboratory of Electrochemistry and Environmental Materials, Faculty of Sciences, Kenitra, Morocco 2 Group of material sciences, Energy and environnement, ENSAH, Alhoceima, Morocco
Abstract The performance of batch electrocoagulation-flotation (ECF) process using aluminum (Al) and iron (Fe) electrodes with monopolar configuration was investigated.The effect of several operational parameters (initial pH, current density, initial NaCl concentration and operating time) on the removal efficiency of chemical oxygen demand (COD) from real urban wastewater was studied. At optimum conditions (initial pH of 5 and 7, current density of 20mA.cm-2, NaCl concentration of 2 g.L-1 and electrolysis time of 6 minutes), COD removal from urban wastewater containing 1000 mg.L-1was found to be 84% and 80% using Al and Fe electrodes, respectively. The operating cost at optimum conditions was calculated as 0.80 $.Kg-1 and 0.82 $.Kg-1 using Al and Fe electrodes, respectively. It can be concluded that COD removal using Al electrode is as effective and low cost as using Fe electrodes. © 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the International Conference on Materials and Environmental Science, ICMES 2018. Keywords: Urban wastewater; Electrocoagulation-flotation (ECF); operating cost; Energy consumption
* Corresponding author. Tel.: +212 676788409. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the International Conference on Materials and Environmental Science, ICMES 2018.
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1. Introduction The recovery and treatment of urban wastewater is inevitable to ensure the preservation of water quality and public health [1]. The most commonly used technologies for treating different types of wastewater are coagulation [2], aerobic and anaerobic treatment [3], advanced oxidation processes [4], adsorption [5], activated sludge [6] and membrane bioreactor [7]. All the previous technologies need important investment cost, which can be expressed by the cost of raw material and energy consumption. Recently, a special attention has been attributed to electrocoagulation (EC) technique due to its attractive advantages as: simplicity, reliability and cost efficiency. EC process was successfully employed for the treatment of various types of wastewater such as textile wastewater [8], semiconductor manufacturing wastewater [9], Dairy effluent [10], Groundwater [11], Landfill leachate [12], hydraulic fracturing wastewater [13], almond wastewater [14]. However, the treatment of urban wastewater by electrocoagulation, in the literature, is scarce [15]. The process is based on the in situ formation of the metal ions as the sacrificial anode oxidizesdue to applied current, the producedcoagulant react with organic matter and nutrients to form a complex that can easily be separated from wastewater by decantation. While the, formed oxygen and hydrogen bubbles at the cathode lead to increase the separation process efficiency through flotation [16]. The main objective of the present work is to investigate the effect of current density, electrolysis time and supporting electrolyte on ECF performance for the removal of COD from urban wastewater using Al and Fe electrodes. In order to evaluate the economy of the present technology, energy consumption and operating cost for the removal of COD were calculated. 2. Materials and methods 2.1 Wastewater sample and characteristic Urban wastewater sample was obtained from the wastewater treatment plant of Al-Hoceima city, located in the northeast part of Morocco. The main characteristics of studied urban wastewater are shown in Table 1. Conductivity, pH and COD were analyzed according to standard methods for examining water and wastewater [17]. The COD was measured by closedreflux method using potassium dichromate. The Conductivity and pH of solutions were measured by pH/ion/con 750 WTW Inolab. Table 1.Characteristics of the urban wastewater used in this study Parameter COD BOD TSS NaCl Conductivity pH
Unit -1
mg.L mg. L -1 mg. L -1 mg. L -1 µs.cm-1 pH
values 1000 540 400 2 3100 7.2
3 Results and discussion 3.1 Effect of initial pH It has often been reported that initial pH is one of the most sensitive operating parameter that can directly affect the pollutants removal efficiencies using EC treatment [18]. The effect of initial pH of urban wastewater on the removal of COD was first explored within the pH values of 5, 7and 9 at a current density of 20mA.m-2 and an operating time varied between (2–10 min). Fig. 1 represents the variation of COD removal efficiency with initial pH, and it is clear from this figure that initial pH has significant effect on COD removal. Although the highest COD removal efficiencies of 80% and 84% was observed at 6 min of operating time, pHi of 7and pHi of 5 using Fe and Al electrodes, respectively. According to the activity-pH diagrams for Al(III) and Fe(III), the predominate species Al(OH)3 were formed at the pH range of 5.0 – 8.5 . Therefore, the solubility of Al(OH)3 increases when the solution
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becomes either more acidic pH (<5) or alkaline pH (>8.5) [19]. This behavior can be explained by the amphoteric character of aluminum hydroxide which does not precipitate at very low pH. Moreover, high pH leads to the formation of Al OH , which is soluble and useless for adsorption of pollutants. Therefore, further increase of the influent pH would decrease the pollutants removal efficiencies. Inthe case of iron electrodes, the soluble Fe OH and Fe OH were produced at acidic pH (pH<5), which are not capable to remove pollutantsfrom urban wastewater. These species are transformed into Fe(OH)3 in the pH aqueous solution of 6–9.5. Fe(OH)3 is an insoluble metal hydroxide and removes the pollutants through precipitation and adsorption [20]. Fig. 2 shows the evolution of pHiwith time. It was observed that when electrolysis time varied from 2 to 6 min, the pH increased from 5 to 6 and from 7 to 8. In other word, when the pH influent is alkaline, the pH effluent dropped from 9 to 8.2 for both Al and Fe electrodes, respectively. It was also noticed that the pH values of the treated urban wastewater increased in the acidic and neutral conditions while it decreased in alkaline medium. Thus, the pH value of the effluent will be brought closer to neutral which is due to the buffering effect of ECF. It is extremely important to notice that the evolution of hydrogen and the OH- liberation at the cathode are responsible of the increase of pH values in the acidic and neutral conditions. Moreover, the decrease of pH values of the effluent is due to the formation of hydroxide precipitates that release H+ cations at the anode vicinity and to the secondary reactions such as water oxidation and chlorine production [21].
Fig 1. Effect of pHi and time on COD removal using Al and Fe electrodes: Applied by (CD 20 mA.cm-2, NaCl 2g.L-1 and T 25°C).
Fig 2.Evolution pH with time during ECF process using Al and Fe electrode
3.2 Effect of current density Current density is one of the operational parameters which influence greatly the removal efficiencies of the pollutants from urban wastewater using EC process. It is considered as an important parameter for the control of
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reaction rate, production rate of coagulants and growth of flocs [22]. Current density values of 6, 15 and 20 mA.cm-2 were applied to the electrochemical reactor in orderto examine its effect on COD removal efficiency.Fig.3 shows that,whenthe current densityincreased from 6 to 20mA.cm-2 whilekeeping the time at a constant value of 6 min, the percentage of COD removal increased from 55 to 80% for Fe electrode and 58to 84% for Al electrode.This behavior was ascribed to the fact that at high current density, the extent of anodic dissolution of Al and Fe increasedwhich implies more release of aluminum orferric ions, resulting in a greater amount of Fe(OH)3(s) or Al(OH)3(s) particles. Moreover, the bubble generation rate increased and the bubble size decreased with increasing the current densities. These effects were both beneficial for high pollutant removal by H2(g) flotation[23,24].
Fig 3. Effect of current density and operating time on COD removal efficiency using Al and Fe electrodes: Applied by (pHi 5 for Al and pHi 7.2 for Fe, NaCl 2 g.L-1 and T 25°C)
3.3 Effect of initial NaCl concentration Numerous researches have discussed the effect of conductivity on the pollutants removal from wastewater using EC process. To assess the influence of initial NaCl on the ECF process, the initial NaCl concentration of wastewater was varied from 2 to 4mg.L-1. Fig. 4 shows the effect of NaCl concentration on the COD removal efficiency for various operating time of electrolysis (2 – 10 min). At pH 7 and 20 mA.cm-2 of current density, the increase of initial NaCl concentration from 2 to 4g.L-1 led to the decrease of COD removal efficiency from 84 to 70% for Al electrode. Under the same operating conditions, it was also noted that the COD removal efficiency decreased from 80 to 70% for Fe electrode. These results indicate that the COD removal efficiency decreased when more NaCl was added to the solution. This may be due to the fact that the Cl- ions in the solution containing Al(OH)3 will form some transitory compounds, namely Al(OH)2Cl, Al(OH)Cl2 and AlCl3. The transitory compounds that could finally be dissolved into the solution with excess chloride ions, as a form of Al [25]. Thus, the amount of Al(OH)3 coagulants decreased, resulting in the decrease of the removal efficiency. 4. Energy consumption and cost analysis The estimation of operating cost is also an important parameter to compare the economic performance of the EC using Al and Fe. The operating cost includes the cost of energy, chemical additives, electrode and sludge. To calculate the operating cost of both process (Al and Fe electrodes) for the removal of COD from urban wastewater, Eq. (1) was applied :
(1) -3
ENC is energy consumption (kWh.m ); ELC is electrode consumption (kg.m-3) and CC is chemical consumption (kg.m-3).
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Fig.4. Effects of NaCl Concentration on COD removal efficiencies using Al and Fe electrodes: Applied by (pHi 5 for Al and pHi 7.2 for Fe, CD 20mA.cm-2 and T 25°C)
Unit prices a, b and c given from the morocco market, are as follows: (a) electrical energy price 0.125 $.kWh-1, (b) electrodes material Al or Fe price are 2.23 and 1.55 $.kg-1, (c) chemical consumption price 1.01 $.kg-1 for NaOH and 0.40 $.kg-1 for H2SO4. ENC and ELC were calculated from Eqs. (2) and (3). kwh. Kg kg. Kg
(2) (3)
Mw is molecular mass of electrode (Mw,Al= 0.02698 kg.mol-1, Mw,Fe = 0.05585 kg.mol-1), tEC is operating time (s), z is number of electron transferred (zAl = 3, zFe = 2), F is Faraday’s constant (96.487 C.mol-1), Ci and Cf are pollutants concentration of wastewater in the inlet and outlet stream, respectively. In addition, the faradic yield (φAl or Fe = ∆mexp/∆mth) of electrode(Al or Fe) dissolution was calculated as the ratio of the weight loss of the electrodes during the experiments (∆mexp) and the amount of electrode consumed theoretically (∆mth) at the anode. As illustrated in Fig. 5, when operating time increased from 2 to 10 min, energy consumption for COD using Al electrode increased from 3.33 to 12.65 KWh.Kg-1, respectively.
Fig.5. Effect of operating time on energy consumption and operating cost: Applied by (Current density: 20mA.m-2, initial pH 5 for Al and 7for Fe; NaCl concentration: 2g.L-1and temperature: 25°C)
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These values can be expressed by operating cost as follow: 0.33 to 1.18 $.kg-1 COD. Regarding the Fe electrode, the energy consumption increased from 3.63 to 13.33 KWh.kg-1and operating cost increased from 0.40 to 1.23 $.kg-1 COD. Under optimum condition of 20 mA.cm-2 current density, NaCl concentration of 2 g.L-1, and 6 min of operating time, the energy consumption was noted to be 7.05 kWh.kg-1 CODusing Al electrode and 8.21 kWh.kg1 COD using Fe electrode which can be expressed by operating cost as 0.80 $.kg-1and0.82 $.kg-1, respectively. 5. Conclusion The effect of current density, initial pH, operating time and NaCl concentration on the removal of COD from urban wastewater using electrocoagulation withAland Fe electrodes have been examined. The results showed that under optimal operating conditions such: initial pH of 7 using Fe electrodes and pH of 5 using Al electrodes, current density of 20 mA.cm-2, time of 6 min and NaCl concentration of 2g.L-1, the removal efficiency of COD using Fe and AL electrodeswas 80% and 84%, respectively. It was observed that the COD removal increases with increasing current density and operating time. However, the increases in NaCl concentration lead to the decreases in COD removal efficiency. Also, under optimal conditions, the operating cost was calculated as 0.80 and 0.82 US $.Kg-1COD using Fe and Al electrodes, respectively. From the experimental result it was found that ECF technique using Al electrodescould be very effective, low-cost, eco-friendly and viable alternative process for the COD removalfrom urban wastewater. Acknowledgments The authors would kindly thanks Dr. Achraf El Kasmi fromFaculty of Sciences and Techniques (Tangier) for his insightful contributions and discussions and also Dr. AbdellahAouaaram from ONEEP (Al Hoceima) for his technical support
References [1] Carpenter S, Caraco N, Correll D, Howarth R, Sharpley A, Smith V. Nonpoint pollution of surface waters with phosphorus and nitrogen. Issues Ecol. 8 (3) (1998) 559-568 [2] Yu T, Li G, Lin W, Hu HY, Lu Y. Coagulation increased the growth potential of various species bacteria of the effluent of a MBR for the treatment of domestic wastewater, Environ. Pollut. 24 (2017) 5126-5133. [3] Petropoulos E, Dolfing J,Davenport RJ, Bowen EJ, Curtis TP.Developing cold-adapted biomass for the anaerobic treatment of domestic wastewater at low temperatures (4, 8 and 15° C) withinocula from cold environments. Water Res. 112 (2017) 100-109. [4] S. Giannakis, F. A. G. Vives, D. Grandjean, A. Magnet, L. F. De Alencastro, C. Pulgarin, Effect of advanced oxidation processes on the micropollutants and the effluent organic matter contained in municipal wastewater previously treated by three different secondary methods, Water Res. 84 (2015) 295-306. [5] Hussain S, Aziz HA, Isa MH, Ahmad A, Van Leeuwen J, Zou L, Umar M, Orthophosphate removal from domestic wastewater using limestone and granular activated carbon. Desalination. 271 (2011) 265-272. [6] Liu JJ, Wang XC, Fan B. Characteristics of PAHs adsorption on inorganic particles and activated sludge in domestic wastewater treatment. Bioresource technology.102(2011) 5305-5311. [7] Jeong Y, Hermanowicz SW, Park C. Treatment of food waste recycling wastewater using anaerobic ceramic membrane bioreactor for biogas production in mainstream treatment process of domestic wastewater. Water Res.123 (2017) 86-95. [8] Ghanbari F, Moradi MA.comparative study of electrocoagulation, electrochemical Fenton, electro-Fentonandperoxi-coagulation for decolorization of real textilewastewater: electrical energy consumption andbiodegradability improvement. J. Environ. Chem. Eng. 3(2015)499– 506. [9] Aoudj S, Khelifa A, Drouiche N, Belkad R, Miroud D. Simultaneous removal of chromium (VI) and fluoride by electrocoagulation– electroflotation: application of a hybrid Fe-Al anode. Chem. Eng. J.; 267(2015) 153-162. [10] Bassala HD, Dedzo GK, Bememba CBN, Seumo PMT, Dazie JD, Nanseu-Njiki CP, Ngameni E. Investigation of the efficiency of a designed electrocoagulation reactor: Application for dairy effluent treatment. PROCESS SAF ENVIRON. 111 (2017) 122-127 [11] Lacasa E, Cañizares P, Sáez C, Fernández FJ, Rodrigo MA. Removal of nitrates from groundwater by electrocoagulation. Chem. Eng. J. 171(2011) 1012-1017.
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[12] Li X, Song J, Guo J, Wang Z, Feng Q. Landfill leachate treatment using electrocoagulation. Procedia Environ Sci. 10 (2011) 1159-1164. [13] Sari MA, Chellam S. Mechanisms of boron removal from hydraulic fracturing wastewater by aluminum electrocoagulation. J. Colloid Interface Sci.458 (2015) 103-111. [14] Valero D, Ortiz JM, García V, Expósito E, Montiel V, Aldaz A. Electrocoagulation of wastewater from almond industry. Chemosphere. 84(2011)1290–1295. [15] Elazzouzi M, Haboubi K, Elyoubi MS. Electrocoagulation flocculation as a low-cost process for pollutants removal from urban wastewater. Chem. Eng. Res. Des. 117 (2017)614-626. [16] Cansares P, Martinez F, Jimenez C, Saez C, Rodrigo M. Coagulation and electrocoagulation of oil-in-water emulsion, J. Hazard.Mater. 151 (2008) 44–51 [17] APHA. Standard methods for examination of water and wastewater.American Water Work Association.New York. 2005. [18] Carmona M, Khemis M, Leclerc JP, Lapicque F.A simple model to predict the removal of oil suspensions from water using the electrocoagulation technique, Chem. Eng. Sci. 61(2006)1237-1246. [19] Yilmaz AE, Boncukcuoğlu R, Kocakerim MM, A quantitative comparison between electrocoagulation and chemical coagulation for boron removal from boron-containing solution, J. Hazard. Mater. 149 (2007)475-481. [20] Mook WT, Aroua MK, Szlachta M, Lee CS..Optimisation of Reactive Black 5 dye removal by electrocoagulation process using response surface methodology. Water Sci. Technol. 75(4) (2017)952-962 [21] Chen G, Electrochemical technologies in wastewater treatment. Sep.purify. Technol. 38 (2004)11-41. [22] Aleboyeh A, Daneshvar N, Kasiri MB. Optimization of CI Acid Red 14 azo dye removal by electrocoagulation batch process with response surface methodology. Chem. Eng. Process: Process Intensification. 47 (2008): 827-832. [23] Un UT, Koparal AS, Ogutveren UB.Electrocoagulation of vegetable oil refinery wastewater using aluminum electrodes. J. Environ. Manage. 90 (2009): 428-433. [24] An C, Huang G, Yao Y, Zhao S, Emerging usage of electrocoagulation technology for oil removal from wastewater: a review. Sci. Total Environ. 579 (2017): 537-556.
ScienceDirect Materials Today: Proceedings 13 (2019) 549–555
www.materialstoday.com/proceedings
ICMES 2018
Enhancement of electrocoagulation-flotation process for urban wastewater treatment using Al and Fe electrodes: techno-economic study M. Elazzouzi1, 2*, K. Haboubi2, M.S. Elyoubi1 1
Laboratory of Electrochemistry and Environmental Materials, Faculty of Sciences, Kenitra, Morocco 2 Group of material sciences, Energy and environnement, ENSAH, Alhoceima, Morocco
Abstract The performance of batch electrocoagulation-flotation (ECF) process using aluminum (Al) and iron (Fe) electrodes with monopolar configuration was investigated.The effect of several operational parameters (initial pH, current density, initial NaCl concentration and operating time) on the removal efficiency of chemical oxygen demand (COD) from real urban wastewater was studied. At optimum conditions (initial pH of 5 and 7, current density of 20mA.cm-2, NaCl concentration of 2 g.L-1 and electrolysis time of 6 minutes), COD removal from urban wastewater containing 1000 mg.L-1was found to be 84% and 80% using Al and Fe electrodes, respectively. The operating cost at optimum conditions was calculated as 0.80 $.Kg-1 and 0.82 $.Kg-1 using Al and Fe electrodes, respectively. It can be concluded that COD removal using Al electrode is as effective and low cost as using Fe electrodes. © 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the International Conference on Materials and Environmental Science, ICMES 2018. Keywords: Urban wastewater; Electrocoagulation-flotation (ECF); operating cost; Energy consumption
* Corresponding author. Tel.: +212 676788409. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the International Conference on Materials and Environmental Science, ICMES 2018.
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M. Elazzouzi et al / Materials Today: Proceedings 13 (2019) 549–555
1. Introduction The recovery and treatment of urban wastewater is inevitable to ensure the preservation of water quality and public health [1]. The most commonly used technologies for treating different types of wastewater are coagulation [2], aerobic and anaerobic treatment [3], advanced oxidation processes [4], adsorption [5], activated sludge [6] and membrane bioreactor [7]. All the previous technologies need important investment cost, which can be expressed by the cost of raw material and energy consumption. Recently, a special attention has been attributed to electrocoagulation (EC) technique due to its attractive advantages as: simplicity, reliability and cost efficiency. EC process was successfully employed for the treatment of various types of wastewater such as textile wastewater [8], semiconductor manufacturing wastewater [9], Dairy effluent [10], Groundwater [11], Landfill leachate [12], hydraulic fracturing wastewater [13], almond wastewater [14]. However, the treatment of urban wastewater by electrocoagulation, in the literature, is scarce [15]. The process is based on the in situ formation of the metal ions as the sacrificial anode oxidizesdue to applied current, the producedcoagulant react with organic matter and nutrients to form a complex that can easily be separated from wastewater by decantation. While the, formed oxygen and hydrogen bubbles at the cathode lead to increase the separation process efficiency through flotation [16]. The main objective of the present work is to investigate the effect of current density, electrolysis time and supporting electrolyte on ECF performance for the removal of COD from urban wastewater using Al and Fe electrodes. In order to evaluate the economy of the present technology, energy consumption and operating cost for the removal of COD were calculated. 2. Materials and methods 2.1 Wastewater sample and characteristic Urban wastewater sample was obtained from the wastewater treatment plant of Al-Hoceima city, located in the northeast part of Morocco. The main characteristics of studied urban wastewater are shown in Table 1. Conductivity, pH and COD were analyzed according to standard methods for examining water and wastewater [17]. The COD was measured by closedreflux method using potassium dichromate. The Conductivity and pH of solutions were measured by pH/ion/con 750 WTW Inolab. Table 1.Characteristics of the urban wastewater used in this study Parameter COD BOD TSS NaCl Conductivity pH
Unit -1
mg.L mg. L -1 mg. L -1 mg. L -1 µs.cm-1 pH
values 1000 540 400 2 3100 7.2
3 Results and discussion 3.1 Effect of initial pH It has often been reported that initial pH is one of the most sensitive operating parameter that can directly affect the pollutants removal efficiencies using EC treatment [18]. The effect of initial pH of urban wastewater on the removal of COD was first explored within the pH values of 5, 7and 9 at a current density of 20mA.m-2 and an operating time varied between (2–10 min). Fig. 1 represents the variation of COD removal efficiency with initial pH, and it is clear from this figure that initial pH has significant effect on COD removal. Although the highest COD removal efficiencies of 80% and 84% was observed at 6 min of operating time, pHi of 7and pHi of 5 using Fe and Al electrodes, respectively. According to the activity-pH diagrams for Al(III) and Fe(III), the predominate species Al(OH)3 were formed at the pH range of 5.0 – 8.5 . Therefore, the solubility of Al(OH)3 increases when the solution
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becomes either more acidic pH (<5) or alkaline pH (>8.5) [19]. This behavior can be explained by the amphoteric character of aluminum hydroxide which does not precipitate at very low pH. Moreover, high pH leads to the formation of Al OH , which is soluble and useless for adsorption of pollutants. Therefore, further increase of the influent pH would decrease the pollutants removal efficiencies. Inthe case of iron electrodes, the soluble Fe OH and Fe OH were produced at acidic pH (pH<5), which are not capable to remove pollutantsfrom urban wastewater. These species are transformed into Fe(OH)3 in the pH aqueous solution of 6–9.5. Fe(OH)3 is an insoluble metal hydroxide and removes the pollutants through precipitation and adsorption [20]. Fig. 2 shows the evolution of pHiwith time. It was observed that when electrolysis time varied from 2 to 6 min, the pH increased from 5 to 6 and from 7 to 8. In other word, when the pH influent is alkaline, the pH effluent dropped from 9 to 8.2 for both Al and Fe electrodes, respectively. It was also noticed that the pH values of the treated urban wastewater increased in the acidic and neutral conditions while it decreased in alkaline medium. Thus, the pH value of the effluent will be brought closer to neutral which is due to the buffering effect of ECF. It is extremely important to notice that the evolution of hydrogen and the OH- liberation at the cathode are responsible of the increase of pH values in the acidic and neutral conditions. Moreover, the decrease of pH values of the effluent is due to the formation of hydroxide precipitates that release H+ cations at the anode vicinity and to the secondary reactions such as water oxidation and chlorine production [21].
Fig 1. Effect of pHi and time on COD removal using Al and Fe electrodes: Applied by (CD 20 mA.cm-2, NaCl 2g.L-1 and T 25°C).
Fig 2.Evolution pH with time during ECF process using Al and Fe electrode
3.2 Effect of current density Current density is one of the operational parameters which influence greatly the removal efficiencies of the pollutants from urban wastewater using EC process. It is considered as an important parameter for the control of
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reaction rate, production rate of coagulants and growth of flocs [22]. Current density values of 6, 15 and 20 mA.cm-2 were applied to the electrochemical reactor in orderto examine its effect on COD removal efficiency.Fig.3 shows that,whenthe current densityincreased from 6 to 20mA.cm-2 whilekeeping the time at a constant value of 6 min, the percentage of COD removal increased from 55 to 80% for Fe electrode and 58to 84% for Al electrode.This behavior was ascribed to the fact that at high current density, the extent of anodic dissolution of Al and Fe increasedwhich implies more release of aluminum orferric ions, resulting in a greater amount of Fe(OH)3(s) or Al(OH)3(s) particles. Moreover, the bubble generation rate increased and the bubble size decreased with increasing the current densities. These effects were both beneficial for high pollutant removal by H2(g) flotation[23,24].
Fig 3. Effect of current density and operating time on COD removal efficiency using Al and Fe electrodes: Applied by (pHi 5 for Al and pHi 7.2 for Fe, NaCl 2 g.L-1 and T 25°C)
3.3 Effect of initial NaCl concentration Numerous researches have discussed the effect of conductivity on the pollutants removal from wastewater using EC process. To assess the influence of initial NaCl on the ECF process, the initial NaCl concentration of wastewater was varied from 2 to 4mg.L-1. Fig. 4 shows the effect of NaCl concentration on the COD removal efficiency for various operating time of electrolysis (2 – 10 min). At pH 7 and 20 mA.cm-2 of current density, the increase of initial NaCl concentration from 2 to 4g.L-1 led to the decrease of COD removal efficiency from 84 to 70% for Al electrode. Under the same operating conditions, it was also noted that the COD removal efficiency decreased from 80 to 70% for Fe electrode. These results indicate that the COD removal efficiency decreased when more NaCl was added to the solution. This may be due to the fact that the Cl- ions in the solution containing Al(OH)3 will form some transitory compounds, namely Al(OH)2Cl, Al(OH)Cl2 and AlCl3. The transitory compounds that could finally be dissolved into the solution with excess chloride ions, as a form of Al [25]. Thus, the amount of Al(OH)3 coagulants decreased, resulting in the decrease of the removal efficiency. 4. Energy consumption and cost analysis The estimation of operating cost is also an important parameter to compare the economic performance of the EC using Al and Fe. The operating cost includes the cost of energy, chemical additives, electrode and sludge. To calculate the operating cost of both process (Al and Fe electrodes) for the removal of COD from urban wastewater, Eq. (1) was applied :
(1) -3
ENC is energy consumption (kWh.m ); ELC is electrode consumption (kg.m-3) and CC is chemical consumption (kg.m-3).
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Fig.4. Effects of NaCl Concentration on COD removal efficiencies using Al and Fe electrodes: Applied by (pHi 5 for Al and pHi 7.2 for Fe, CD 20mA.cm-2 and T 25°C)
Unit prices a, b and c given from the morocco market, are as follows: (a) electrical energy price 0.125 $.kWh-1, (b) electrodes material Al or Fe price are 2.23 and 1.55 $.kg-1, (c) chemical consumption price 1.01 $.kg-1 for NaOH and 0.40 $.kg-1 for H2SO4. ENC and ELC were calculated from Eqs. (2) and (3). kwh. Kg kg. Kg
(2) (3)
Mw is molecular mass of electrode (Mw,Al= 0.02698 kg.mol-1, Mw,Fe = 0.05585 kg.mol-1), tEC is operating time (s), z is number of electron transferred (zAl = 3, zFe = 2), F is Faraday’s constant (96.487 C.mol-1), Ci and Cf are pollutants concentration of wastewater in the inlet and outlet stream, respectively. In addition, the faradic yield (φAl or Fe = ∆mexp/∆mth) of electrode(Al or Fe) dissolution was calculated as the ratio of the weight loss of the electrodes during the experiments (∆mexp) and the amount of electrode consumed theoretically (∆mth) at the anode. As illustrated in Fig. 5, when operating time increased from 2 to 10 min, energy consumption for COD using Al electrode increased from 3.33 to 12.65 KWh.Kg-1, respectively.
Fig.5. Effect of operating time on energy consumption and operating cost: Applied by (Current density: 20mA.m-2, initial pH 5 for Al and 7for Fe; NaCl concentration: 2g.L-1and temperature: 25°C)
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These values can be expressed by operating cost as follow: 0.33 to 1.18 $.kg-1 COD. Regarding the Fe electrode, the energy consumption increased from 3.63 to 13.33 KWh.kg-1and operating cost increased from 0.40 to 1.23 $.kg-1 COD. Under optimum condition of 20 mA.cm-2 current density, NaCl concentration of 2 g.L-1, and 6 min of operating time, the energy consumption was noted to be 7.05 kWh.kg-1 CODusing Al electrode and 8.21 kWh.kg1 COD using Fe electrode which can be expressed by operating cost as 0.80 $.kg-1and0.82 $.kg-1, respectively. 5. Conclusion The effect of current density, initial pH, operating time and NaCl concentration on the removal of COD from urban wastewater using electrocoagulation withAland Fe electrodes have been examined. The results showed that under optimal operating conditions such: initial pH of 7 using Fe electrodes and pH of 5 using Al electrodes, current density of 20 mA.cm-2, time of 6 min and NaCl concentration of 2g.L-1, the removal efficiency of COD using Fe and AL electrodeswas 80% and 84%, respectively. It was observed that the COD removal increases with increasing current density and operating time. However, the increases in NaCl concentration lead to the decreases in COD removal efficiency. Also, under optimal conditions, the operating cost was calculated as 0.80 and 0.82 US $.Kg-1COD using Fe and Al electrodes, respectively. From the experimental result it was found that ECF technique using Al electrodescould be very effective, low-cost, eco-friendly and viable alternative process for the COD removalfrom urban wastewater. Acknowledgments The authors would kindly thanks Dr. Achraf El Kasmi fromFaculty of Sciences and Techniques (Tangier) for his insightful contributions and discussions and also Dr. AbdellahAouaaram from ONEEP (Al Hoceima) for his technical support
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