Treatment of the textile wastewater by electrocoagulation Economical evaluation
Treatment of the textile wastewater by electrocoagulation Economical evaluation
Mahmut Bayramoglu a, Murat Eyvaz b, Mehmet Kobya b,∗
Electrocoagulation (EC) is an attractive method for the treatment of various kinds of wastewater, by virtue of various benefits including environmental compatibility, versatility, energy efficiency, safety, selectivity, amenability to automation, and cost effectiveness. This process is characterized by simple equipment, easy operation, a shortened reactive retention period, a reduction or absence of equipment for adding chemicals and decreased amount of precipitate or sludge which sediments rapidly.
Textile wastewaters are one of the significant pollutants for the environment due to their characteristics such as high COD concentration, strong color, high pH and temperature, and low biodegradability. Several conventional methods have been carried out for this purpose such as adsorption, coagulation, flocculation, biological and chemical oxidation. Although these methods have been widely applied, they have some disadvantages. For example, adsorbents are usually difficult to regenerate, chemical coagulation causes additional pollution due to the undesired reactions in treated water and produces large amounts of sludge and biological methods are not suitable for most textile wastewaters due to the harmful effects of some commercial dyes on the organisms used in the process. Moreover, these conventional methods are also usually expensive and treatment efficiency is inadequate because of the large variability of the composition of textile wastewaters.
Despite the impressive amount of scientific research on the treatment of various industrial wastewaters by EC, a few research have been done on the economic analysis. Thus, detailed research has been performed in order to assess both the technical and economic aspects of EC for the treatment of textile wastewater.
2. Experimental
2.1. Materials
The wastewater was obtained from a tank containing a mixture of exhaust dyeing solutions at a textile factory in Gebze (Turkey) producing approximately 1000 m3 of wastewater per day. The electrocoagulator was made of plexiglas with the dimensions of 120 mm × 110 mm × 110 mm. Al and Fe electrode materials were used as sacrificial electrode in parallel and serial connection modes. Both aluminum or iron cathodes and anodes were made from plates with dimensions of 45 mm × 53 mm × 3 mm. The total effective electrode area was 143 cm2 and the spacing between electrodes was 20 mm. The electrodes were connected to a digital dc power supply (Topward 6306D; 30 V, 6 A) operated at galvanostatic mode. The EC reactor is operated in batch mode. Characteristics of wastewater used have the following properties: chemical oxygen demand (COD): 2031 mg l−1, total suspended solids (TSS): 102 mg l−1, Conductivity: 2310 mS cm−1, turbidity: 671 NTU, pH: 8.88 and procedures are reported elsewhere.
2.2. Economic analysis
Total operation cost has been calculated for a plant with a capacity of 1000 m3 wastewater per day which includes direct cost items such as electricity, material (electrodes and chemical reagents), sludge transportation and disposal costs, as well as indirect cost items such as labor, maintenance and depreciation of the major equipment including rectifier and electrocoagulator. Values of the total operating cost are calculated based on economic data obtained from Turkish market in 2005.
3. Results and discussion
This study is focused on the selection of the sacrificial electrode type and electrode connection mode with respect to the treatment efficiency and operating cost. Before the economic analysis, important costs items namely, sacrificial electrode and electrical energy consumptions and amount of the sludge formed are represented as function of important design variables such as, pH of wastewater, current density, operating time and connection mode, respectively.
3.1. Effect of initial pH
pH is an important operating factor influencing the performance of EC process. In this study, experiments have been conducted to determine the pH effect on cost items at constant current density of 30 A m−2 and operating time of 15 min. For a given connection mode, the electrode consumption is found to be weakly dependent on the initial pH (Fig. 1). The highest electrode consumption is obtained with bipolar series mode (BP-S); approximately 0.27 kg m−3 for Fe electrode and between 0.18–0.23 kg m−3 for Al electrode. Monopolar parallel (MP-P) mode shows the lowest electrode consumption for both electrode materials; 0.12 kg m−3 for Al electrode and 0.16 kg m−3 for Fe electrode. When energy consumptions are compared, as seen in Fig. 2, weak dependence on pH is observed for all the systems; it may be concluded that, for Al electrodes, there were clear dependencies on pH for MP-S and BP-S modes. MP-S and BP-S modes exhibit high consumptions since consequence of the serial connection requires made higher potential. The lowest consumption values are approximately 0.63 kWh m−3 for Fe electrode and 0.7 kWh m−3 for Al electrode when MP-P mode is used. The effect of the initial pH on the sludge amounts is depicted in Fig. 3. Sludge amounts vary from 0.65 to 1.0 kg m−3 for Fe electrode and from 0.9 to 1.3 kg m−3 for Al electrode. In general, more sludge is produced with BP-S mode and less sludge with MP-P mode. As seen in Figs. 4 and 5, MP-P mode for both electrode materials is economically more feasible owing to having low electrical energy consumptions and amount of sludge produced.
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3.2. Effect of current density
Experiments have been performed to determine the effect of the current density at pH 7 for Al electrode and at pH 5 for Fe electrode at 15 min of operating time. Electrode consumption values increase with increasing current density, as shown Fig. 6. Electrode consumption value for BP-S mode reaches its highest value of 0.58 kg m−3 for Fe electrode and 0.32 kg m−3 for Al electrode at 60 A m−2 current density, while the lowest electrode consumption with MP-P mode is 0.27 kg m−3 for Fe electrode and 0.11 kg m−3 for Al electrode, respectively. It is clear that electrode consumptions are higher with Fe electrode material, based on a weight basis. By considering atomic weights of Al (27 g mol−1) and Fe (56.5 g mol−1), the consumption on molar basis is not very different; with MP-P mode, for example, the calculated values are 4.78 mol g m−3 for Fe electrode and 4.07 mol g m−3 for Al electrode, respectively. As shown in Fig. 7, all of the systems exhibit similar trends with respect to energy consumption as a function of current density; it increases with increasing current density. Similar to pH effects on energy consumption, BP-S mode requires higher energy values, while MP-P mode is the most economical at 30 A m−2 of current density. Its consumption values are 0.68 kWh m−3 for Fe electrode and 0.72 kWh m−3 for Al electrode. Sludge formation dependence on the current density is depicted in Fig. 8. Amount of sludge increases with increasing current density for all connection modes and electrode materials. BP-S and MP-S modes have similar values. MP-P system produces the lowest amount of sludge as 0.81 kg m−3 and 0.9 kg m−3 for Fe and Al electrodes at 30 A m−2. When these latter values are evaluated on a molar basis with respect molar electrode consumptions, it is seen that aluminum hydroxide flocs bound more water, chemically or physically, than iron hydroxide flocs do.
The effect of the current density on the operating cost of EC process is presented in Figs. 9 and 10. For both electrode materials, operating cost increases more rapidly for MP-S and BP-S modes when compared with MP-P mode. BP-S mode exhibits the highest cost values as 1.02$ m−3 and 1.48$ m−3 for Fe and Al electrodes, respectively. MP-P mode with Fe electrode material is the most economic with having 0.2$ m−3 operating cost at 30 A m−2
3.3. Effect of operating time
Operating time experiments were carried out at pH 7 for Fe electrode, at pH 5 for Al electrode at 30 A m−2. As seen in Fig. 11, Electrode consumption values are higher for Fe electrode. The electrode consumption is the highest in BP-S mode as compared to the other two modes. Energy consumption versus operating time is presented in Fig. 12. Linear dependencies are observed. MP-S and BP-S modes have similar slopes, almost 4–6.5 times greater than that of the MP-P mode. Thus, longer operating times may be used with MP-P mode, to obtain higher wastewater treatment efficiencies using less electric energy. The effect of the operating time on sludge formation is presented in Fig. 13. It is not very profound as in the case of energy consumption. Generally, a greater amount of sludge is formed when Al electrode is used. BP-S mode gives rise to a higher amount of sludge and MP-P mode to lower ones. Operating cost values with regard to operating time are shown in Figs. 14 and 15. As expected, operating costs increase with increasing of operating time. All connection modes with Fe electrode exhibit lower-cost values than Al electrode. For an operating time of 25 min, the operating cost for Al electrode with BP-S mode is 5.5 times higher than the operating cost for Fe electrode with MP-P mode.
Optimum conditions from the experimental results are summarized in Table 2; from technical point of view, MP-P connection mode is the most appropriate one for both materials exhibiting similar performance in reducing removal efficiency of COD and turbidity. Fe electrode requires slightly acidic medium (pH 5), while neutral medium (pH 7) is more suitable for Al electrode. 30 A m−2 of current density and 15 min of operating time are sufficient for both electrodes. From the operating cost point of view, Fe electrode is clearly a more economic material type than Al electrode. For a concluding economic analysis, it is worth comparing the technical–economical performances of EC and CC. Comparative results are given in Table 3. For CC, FeCl3 is the preferable salt for its techno-economic performance.
(1) The COD removal performance of CC is 10% higher than EC, the turbidity removal is nearly the same, but having 60% longer retention time for CC.
(2) With the same initial pH, the final pH is 7.9 in EC, but 2.9 in CC. The final acidic and chloride bearing medium is an important drawback of CC, causing severe corrosion problems which may necessitate high-cost building materials. From this point, Fe2(SO4)3·7H2O may be used despite its higher operating cost.
(3) High coagulant consumption in CC means high chloride concentration in the effluent.
4. Conclusion
In this study, Al and Fe electrodes were used as sacrificial electrodes with parallel and series connection modes. Various cost items were considered in the calculation of the total cost for the treatment of wastewater from the textile plant. Results showed that MP-P mode was the most cost-effective for both electrodes. Similar results were obtained from Al and Fe electrodes for reducing COD and turbidity, but Fe electrode was found to be a low-cost material. Optimum operating conditions were obtained for both electrodes as pH 7 for Fe, pH 5 for Al electrode, 30 A m−2 of current density and 15 min of operating time. A comparative study between EC and CC was carried out to determine for consumed electrode material and production of sludge with the same connection mode used. The EC process was faster and more economic than CC. The treatment costs of EC and CC at the optimum operating conditions were 0.25 and 0.80$ m−3, respectively. The operating cost for CC was 3.2 times more expensive than that of EC.
References
[1] G. Chen, Electrochemical technologies in wastewater treatment, Sep. Purif. Technol. 38 (2004) 11–41.
[2] M.Y.A. Mollah, R. Schennach, J.R. Parga, D.L. Cocke, Electrocoagulation (EC)—science and applications, J. Hazard. Mater. B84 (2001) 29–41.
[3] M.Y.A. Mollah, P. Morkovsky, J.A.G. Gomes, M. Kesmez, J. Parga, D.L. Cocke, Fundamentals, present and future perspectives of electrocoagulation, J. Hazard. Mater. B114 (2004) 199–210.
[4] S.H. Lin, M.L. Chen, Treatment of textile wastewater by electrochemical methods for reuse, Water Res. 31 (1997) 868–876.
[5] J. Jia, J. Yang, J. Liao, W. Wang, Z. Wang, Treatment of dyeing wastewater with ACF electrodes, Water Res. 33 (1999) 881–884.
[6] N. Daneshvar, H.A. Sorkhabi, M.B. Kasiri, Decolorization of dye solution containing Acid Red 14 by electrocoagulation with a comparative investigation of different electrode connections, J. Hazard. Mater. B112 (2004) 55–62.
[7] A.G. Vlyssides, M. Loizidou, P.K. Karlis, A.A. Zorpas, D. Papaioannou, Electrochemical oxidation of a textile dye wastewater using a Pt/Ti electrode, J. Hazard. Mater. B70 (1999) 41–52.
[8] M. Bayramoglu, M. Kobya, O.T. Can, M. Sozbir, Operating cost analysis of electrocoagulation of textile dye wastewater, Sep. Purif. Technol. 37 (2004) 117–125.
[9] J.C. Donini, J. Szynkarczuk, J. Kan, T.A.T. Hassan, Electrochemical coagulation of clay suspensions, Clay Clay. Miner. 42 (1994) 667–673.
[10] M. Eyvaz, M. Bayramoglu, M. Kobya, Treatment of the textile wastewater by electrocoagulation: technical and economic evaluation, Xth National Symposium: Industrial Pollution Control, Istanbul Technical University, 7–9 June 2006. Istanbul, pp. 175–182.
[11] J.S. Do, M.L. Chen, Decolorization of dye-containing solutions by electrocoagulation, J. Appl. Electrochem. 24 (1994) 785–790.
[12] C.T. Tsai, S.T. Lin, Y.C. Shue, P.L. Su, Electrolysis of soluble organic matter in leachate from landfills, Water Res. 31 (1997) 3073–3081.
Mahmut Bayramoglu a, Murat Eyvaz b, Mehmet Kobya b,∗
Electrocoagulation (EC) is an attractive method for the treatment of various kinds of wastewater, by virtue of various benefits including environmental compatibility, versatility, energy efficiency, safety, selectivity, amenability to automation, and cost effectiveness. This process is characterized by simple equipment, easy operation, a shortened reactive retention period, a reduction or absence of equipment for adding chemicals and decreased amount of precipitate or sludge which sediments rapidly.
Textile wastewaters are one of the significant pollutants for the environment due to their characteristics such as high COD concentration, strong color, high pH and temperature, and low biodegradability. Several conventional methods have been carried out for this purpose such as adsorption, coagulation, flocculation, biological and chemical oxidation. Although these methods have been widely applied, they have some disadvantages. For example, adsorbents are usually difficult to regenerate, chemical coagulation causes additional pollution due to the undesired reactions in treated water and produces large amounts of sludge and biological methods are not suitable for most textile wastewaters due to the harmful effects of some commercial dyes on the organisms used in the process. Moreover, these conventional methods are also usually expensive and treatment efficiency is inadequate because of the large variability of the composition of textile wastewaters.
Despite the impressive amount of scientific research on the treatment of various industrial wastewaters by EC, a few research have been done on the economic analysis. Thus, detailed research has been performed in order to assess both the technical and economic aspects of EC for the treatment of textile wastewater.
2. Experimental
2.1. Materials
The wastewater was obtained from a tank containing a mixture of exhaust dyeing solutions at a textile factory in Gebze (Turkey) producing approximately 1000 m3 of wastewater per day. The electrocoagulator was made of plexiglas with the dimensions of 120 mm × 110 mm × 110 mm. Al and Fe electrode materials were used as sacrificial electrode in parallel and serial connection modes. Both aluminum or iron cathodes and anodes were made from plates with dimensions of 45 mm × 53 mm × 3 mm. The total effective electrode area was 143 cm2 and the spacing between electrodes was 20 mm. The electrodes were connected to a digital dc power supply (Topward 6306D; 30 V, 6 A) operated at galvanostatic mode. The EC reactor is operated in batch mode. Characteristics of wastewater used have the following properties: chemical oxygen demand (COD): 2031 mg l−1, total suspended solids (TSS): 102 mg l−1, Conductivity: 2310 mS cm−1, turbidity: 671 NTU, pH: 8.88 and procedures are reported elsewhere.
2.2. Economic analysis
Total operation cost has been calculated for a plant with a capacity of 1000 m3 wastewater per day which includes direct cost items such as electricity, material (electrodes and chemical reagents), sludge transportation and disposal costs, as well as indirect cost items such as labor, maintenance and depreciation of the major equipment including rectifier and electrocoagulator. Values of the total operating cost are calculated based on economic data obtained from Turkish market in 2005.
3. Results and discussion
This study is focused on the selection of the sacrificial electrode type and electrode connection mode with respect to the treatment efficiency and operating cost. Before the economic analysis, important costs items namely, sacrificial electrode and electrical energy consumptions and amount of the sludge formed are represented as function of important design variables such as, pH of wastewater, current density, operating time and connection mode, respectively.
3.1. Effect of initial pH
pH is an important operating factor influencing the performance of EC process. In this study, experiments have been conducted to determine the pH effect on cost items at constant current density of 30 A m−2 and operating time of 15 min. For a given connection mode, the electrode consumption is found to be weakly dependent on the initial pH (Fig. 1). The highest electrode consumption is obtained with bipolar series mode (BP-S); approximately 0.27 kg m−3 for Fe electrode and between 0.18–0.23 kg m−3 for Al electrode. Monopolar parallel (MP-P) mode shows the lowest electrode consumption for both electrode materials; 0.12 kg m−3 for Al electrode and 0.16 kg m−3 for Fe electrode. When energy consumptions are compared, as seen in Fig. 2, weak dependence on pH is observed for all the systems; it may be concluded that, for Al electrodes, there were clear dependencies on pH for MP-S and BP-S modes. MP-S and BP-S modes exhibit high consumptions since consequence of the serial connection requires made higher potential. The lowest consumption values are approximately 0.63 kWh m−3 for Fe electrode and 0.7 kWh m−3 for Al electrode when MP-P mode is used. The effect of the initial pH on the sludge amounts is depicted in Fig. 3. Sludge amounts vary from 0.65 to 1.0 kg m−3 for Fe electrode and from 0.9 to 1.3 kg m−3 for Al electrode. In general, more sludge is produced with BP-S mode and less sludge with MP-P mode. As seen in Figs. 4 and 5, MP-P mode for both electrode materials is economically more feasible owing to having low electrical energy consumptions and amount of sludge produced.
3.2. Effect of current density
Experiments have been performed to determine the effect of the current density at pH 7 for Al electrode and at pH 5 for Fe electrode at 15 min of operating time. Electrode consumption values increase with increasing current density, as shown Fig. 6. Electrode consumption value for BP-S mode reaches its highest value of 0.58 kg m−3 for Fe electrode and 0.32 kg m−3 for Al electrode at 60 A m−2 current density, while the lowest electrode consumption with MP-P mode is 0.27 kg m−3 for Fe electrode and 0.11 kg m−3 for Al electrode, respectively. It is clear that electrode consumptions are higher with Fe electrode material, based on a weight basis. By considering atomic weights of Al (27 g mol−1) and Fe (56.5 g mol−1), the consumption on molar basis is not very different; with MP-P mode, for example, the calculated values are 4.78 mol g m−3 for Fe electrode and 4.07 mol g m−3 for Al electrode, respectively. As shown in Fig. 7, all of the systems exhibit similar trends with respect to energy consumption as a function of current density; it increases with increasing current density. Similar to pH effects on energy consumption, BP-S mode requires higher energy values, while MP-P mode is the most economical at 30 A m−2 of current density. Its consumption values are 0.68 kWh m−3 for Fe electrode and 0.72 kWh m−3 for Al electrode. Sludge formation dependence on the current density is depicted in Fig. 8. Amount of sludge increases with increasing current density for all connection modes and electrode materials. BP-S and MP-S modes have similar values. MP-P system produces the lowest amount of sludge as 0.81 kg m−3 and 0.9 kg m−3 for Fe and Al electrodes at 30 A m−2. When these latter values are evaluated on a molar basis with respect molar electrode consumptions, it is seen that aluminum hydroxide flocs bound more water, chemically or physically, than iron hydroxide flocs do.
The effect of the current density on the operating cost of EC process is presented in Figs. 9 and 10. For both electrode materials, operating cost increases more rapidly for MP-S and BP-S modes when compared with MP-P mode. BP-S mode exhibits the highest cost values as 1.02$ m−3 and 1.48$ m−3 for Fe and Al electrodes, respectively. MP-P mode with Fe electrode material is the most economic with having 0.2$ m−3 operating cost at 30 A m−2
3.3. Effect of operating time
Operating time experiments were carried out at pH 7 for Fe electrode, at pH 5 for Al electrode at 30 A m−2. As seen in Fig. 11, Electrode consumption values are higher for Fe electrode. The electrode consumption is the highest in BP-S mode as compared to the other two modes. Energy consumption versus operating time is presented in Fig. 12. Linear dependencies are observed. MP-S and BP-S modes have similar slopes, almost 4–6.5 times greater than that of the MP-P mode. Thus, longer operating times may be used with MP-P mode, to obtain higher wastewater treatment efficiencies using less electric energy. The effect of the operating time on sludge formation is presented in Fig. 13. It is not very profound as in the case of energy consumption. Generally, a greater amount of sludge is formed when Al electrode is used. BP-S mode gives rise to a higher amount of sludge and MP-P mode to lower ones. Operating cost values with regard to operating time are shown in Figs. 14 and 15. As expected, operating costs increase with increasing of operating time. All connection modes with Fe electrode exhibit lower-cost values than Al electrode. For an operating time of 25 min, the operating cost for Al electrode with BP-S mode is 5.5 times higher than the operating cost for Fe electrode with MP-P mode.
Optimum conditions from the experimental results are summarized in Table 2; from technical point of view, MP-P connection mode is the most appropriate one for both materials exhibiting similar performance in reducing removal efficiency of COD and turbidity. Fe electrode requires slightly acidic medium (pH 5), while neutral medium (pH 7) is more suitable for Al electrode. 30 A m−2 of current density and 15 min of operating time are sufficient for both electrodes. From the operating cost point of view, Fe electrode is clearly a more economic material type than Al electrode. For a concluding economic analysis, it is worth comparing the technical–economical performances of EC and CC. Comparative results are given in Table 3. For CC, FeCl3 is the preferable salt for its techno-economic performance.
(1) The COD removal performance of CC is 10% higher than EC, the turbidity removal is nearly the same, but having 60% longer retention time for CC.
(2) With the same initial pH, the final pH is 7.9 in EC, but 2.9 in CC. The final acidic and chloride bearing medium is an important drawback of CC, causing severe corrosion problems which may necessitate high-cost building materials. From this point, Fe2(SO4)3·7H2O may be used despite its higher operating cost.
(3) High coagulant consumption in CC means high chloride concentration in the effluent.
4. Conclusion
In this study, Al and Fe electrodes were used as sacrificial electrodes with parallel and series connection modes. Various cost items were considered in the calculation of the total cost for the treatment of wastewater from the textile plant. Results showed that MP-P mode was the most cost-effective for both electrodes. Similar results were obtained from Al and Fe electrodes for reducing COD and turbidity, but Fe electrode was found to be a low-cost material. Optimum operating conditions were obtained for both electrodes as pH 7 for Fe, pH 5 for Al electrode, 30 A m−2 of current density and 15 min of operating time. A comparative study between EC and CC was carried out to determine for consumed electrode material and production of sludge with the same connection mode used. The EC process was faster and more economic than CC. The treatment costs of EC and CC at the optimum operating conditions were 0.25 and 0.80$ m−3, respectively. The operating cost for CC was 3.2 times more expensive than that of EC.
References
[1] G. Chen, Electrochemical technologies in wastewater treatment, Sep. Purif. Technol. 38 (2004) 11–41.
[2] M.Y.A. Mollah, R. Schennach, J.R. Parga, D.L. Cocke, Electrocoagulation (EC)—science and applications, J. Hazard. Mater. B84 (2001) 29–41.
[3] M.Y.A. Mollah, P. Morkovsky, J.A.G. Gomes, M. Kesmez, J. Parga, D.L. Cocke, Fundamentals, present and future perspectives of electrocoagulation, J. Hazard. Mater. B114 (2004) 199–210.
[4] S.H. Lin, M.L. Chen, Treatment of textile wastewater by electrochemical methods for reuse, Water Res. 31 (1997) 868–876.
[5] J. Jia, J. Yang, J. Liao, W. Wang, Z. Wang, Treatment of dyeing wastewater with ACF electrodes, Water Res. 33 (1999) 881–884.
[6] N. Daneshvar, H.A. Sorkhabi, M.B. Kasiri, Decolorization of dye solution containing Acid Red 14 by electrocoagulation with a comparative investigation of different electrode connections, J. Hazard. Mater. B112 (2004) 55–62.
[7] A.G. Vlyssides, M. Loizidou, P.K. Karlis, A.A. Zorpas, D. Papaioannou, Electrochemical oxidation of a textile dye wastewater using a Pt/Ti electrode, J. Hazard. Mater. B70 (1999) 41–52.
[8] M. Bayramoglu, M. Kobya, O.T. Can, M. Sozbir, Operating cost analysis of electrocoagulation of textile dye wastewater, Sep. Purif. Technol. 37 (2004) 117–125.
[9] J.C. Donini, J. Szynkarczuk, J. Kan, T.A.T. Hassan, Electrochemical coagulation of clay suspensions, Clay Clay. Miner. 42 (1994) 667–673.
[10] M. Eyvaz, M. Bayramoglu, M. Kobya, Treatment of the textile wastewater by electrocoagulation: technical and economic evaluation, Xth National Symposium: Industrial Pollution Control, Istanbul Technical University, 7–9 June 2006. Istanbul, pp. 175–182.
[11] J.S. Do, M.L. Chen, Decolorization of dye-containing solutions by electrocoagulation, J. Appl. Electrochem. 24 (1994) 785–790.
[12] C.T. Tsai, S.T. Lin, Y.C. Shue, P.L. Su, Electrolysis of soluble organic matter in leachate from landfills, Water Res. 31 (1997) 3073–3081.
09 Jul 2024
