Document Type : Research Paper
Abstract
Introduction
The aim of this study was to evaluate the efficiency of electrochemical systems for removing of Acid red 14 contaminants, in which electrical coagulation and flotation methods by steel electrodes, are used simultaneously. It is expected that, simultaneously using of electrical flotation and electrical coagulation process eliminate gravity sedimentation unit for the separation of the clot, and result both in the separation of emissions and reducing the cost of the treatment. Researches in the field of electrochemically dye removing methods, are based on electrical coagulation, and flotation property of bubbles are rarely used. In this research, innovations such as gridded horizontal were used to improve the performance of flotation. The impact of key parameters on electrochemical system performance including current density and initial dye concentration was examined and optimized based on the amount of energy consumption and anodes consumption and ensuring the proper functioning in terms of coagulants and bubbles. In optimum conditions, in order to evaluate the performance of coagulation and flotation process in treating of real wastewater plants, electrochemically treatment of the actual colored wastewater was evaluated.
Materials and methods
Cubic Plexiglas electrochemical cells with small dimensions of 15×7×7, with a pureed volume of 735 ml was used for electrical coagulation and electrical floatation process. Two mesh stainless steel 316 electrodes with a purity of 99%, with horizontal monopole arrangement was used as the anode and cathode in the reactor. Because hydrogen gas at the cathode, play a key role in floating suspended particulates, current flow was introduced in a way that, the cathode and anode was placed on top and down, respectively.
Acid red 14 dyestuff, which was used as the main contaminants to create artificial wastewater, is anionic and has an Azo group with chemical formula of C20H12N2Na2O7S2 and molecular weight of 502.4 gr/mol. In addition, in order to evaluate the effectiveness of the system in optimal conditions, discoloration of real wastewater were studied.
In order to determine the optimal electrical current density and optimal initial concentration of dye by using of single-element method, synthetic wastewater was prepared with desired specifications.
Results and discussion
In order to determine the optimal amount of electrical current density, experiments are performed in different electric current densities when other affecting parameters considered to be constant. According to the observations, by increasing the amount of electric current, the speed of dye removal is increased. This is because, production speed of coagulants and hydrogen and oxygen gases has been increased by increasing current density, which leads to coagulation, flocculation and faster separation of contaminants. The specific energy consumption and reduction of the anode metal mass are evaluated as a criteria for better comparison of the economic and environmental terms according to which, by removal efficiency of 99% in 10, 20, 30 ,40 and 50 mA/cm2 electrical current densities and for kg dye removal, energy consumption is respectively 3.57, 3.22, 3.19, 2.52 and 2.26 kWh and anode consumption rate is 0.4, 0.34, 0.63,0.54 and 0.67 kg Fe/kg Dye Removed, respectively. Finally, the optimal electrical current density was selected equal to 20 mA / cm2 (0.4A) according to the results, in which 99% of dye was removed in less than 20 minutes, and the amount of specific energy consumption was 3.22kWh / kg Dye Removed, anode’s consumption was 0.34kg Fe/kg Dye Removed and the TSS of sludge was 3820mg/L.
Various experiments with different concentrations of dye take place in order to determine the optimal value of this parameter with optimized electrical current density where other parameters were kept constant. According to these tests, dye removal rate decreased with increasing initial concentration and more time is required in order to get fixed removal efficiency. This is because at a constant electric current density, the produced coagulant metal hydroxide is constant with time and this amount of metal hydroxide is not sufficient to coagulation and flocculation greater amount of pollutants. It should be noted that at very high concentrations (500mg/L) dye removal process go slower but the performance is much better in compare with other studies. By comparing of energy and anode’s consumption with efficiency of dye removing in terms of time, the optimum concentration was selected equal to 250 mg/L, in which 99% of dye was removed in less than 30 minutes, and the amount of specific energy consumption, anode’s consumption and TSS of sludge were 1.9kWh/kg Dye Removed, was 0.95kg Fe/kg Dye Removed and 3700mg/L, respectively .
Using the optimal conditions obtained from previous experiments, to evaluate the performance of coagulation and electrical flotation process in real waste water treatment, real sewage electrochemical treatment for actual dyeing is considered, in which, after 30 minutes treatment by removal efficiency of 63%, the primary COD equal to 803 mg/L reached to 296 mg/L. In synthesized sample, after 30 minutes treatment with removal efficiency of 79%, the primary COD magnitude decreased from 278 to 58mg/L. This amount is lower than the standard limit of discharge to surface water and catchy wells. After 30 minutes treatment with removal efficiency of 77%, the color of actual wastewater decreased from 1.5 Gardner in the beginning to 2.1Gardner. The energy and anode’s consumption in actual wastewater treatment for removing of 63% of COD in 30 minutes, were 3.11 kWh/kg COD Removed and 0.565 kg Fe/kg COD Removed respectively. These values for synthetic wastewater with removal efficiency of 79% in the same time were 2.10kWh/kg COD Removed and 1.38kg Fe/kg COD Removed, respectively.
Conclusion
Electrical coagulation and flotation has the advantages of low sludge production, in comparison with similar methods and fully automatic and continuous operation is possible. The parameters involved in the process can be controlled easily and the safety of equipment is high. Tiny bubbles with the same size are produced and there is a little or no need to add chemicals. Also, very high tolerance against organic, hydraulic and toxins shocks and reduced number of processing plants and thus reducing the required surface for treatment plant and reducing in operating costs are another advantages of this method.
By increasing of electrical current density, production speed of coagulants and gases were increased, which lead in faster coagulation, flocculation and flotation. However, when electrical current density is increased in very high rate, the amount of iron in the separated sludge is increased with high speed. Increasing of iron clots in the water containing sludge, in spite of their high volume, and because of lightness, reduce the TSS of separated sludge and therefore the ability of system in flotation and separation of clots is reduces.
Due to the limited amount of coagulants produced at a time of electrolysis and consistency of electrical current density, with increasing of initial concentration of dye (from 50 to 500mg/L), the removal efficiency was decreased. However, according to the amount of energy used and the high dissolution of anode at lower concentrations, concentration of 250 mg / L was chosen as the optimal concentration.
Anode consumption and the need for its renewing and cathode corrosion are the disadvantages of this method. However, according to results, 99% of dye was removed in less than 30minutes, where specific energy consumption was 1.9kWh/kg Dye Removed, the anode’s consumption and the TSS of sludge were 0.95kg Fe/kg Dye Removed and 3700mg/L, respectively. These results proved the ability of electrical flotation and coagulation methods in treatment of wastewater containing dye, with low consumption of materials and energy. The proper functioning of this procedure in real waste water treatment, which are containing different combinations of colored dyes, suggest that, this method can be used for wastewater treatment in dyeing industry, textile and other industries. In addition, low sludge produced in this treatment reduces sludge’s treatment, disposal, and its relative problems costs. As a result, using of this method as an alternative option to conventional methods is considerable.
Keywords
Main Subjects
)
