عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Municipal wastewater is the mixture of domestic wastewater, small amounts of industrial and storm water, drain water, surface infiltration, and ground water. It usually consists of a number of contaminants, such as suspended solids, biodegradable organics, pathogens, nutrients, refractory organics, heavy metals and dissolved inorganics. Direct discharge of untreated wastewater into the natural water bodies is not desirable, as the decomposition of the organic waste would seriously deteriorate the water quality. In addition, communicable diseases can be transmitted by the pathogenic microorganisms. Nutrients such as nitrogen and phosphorous, along with organic material can lead to excessive growth of undesirable aquatic life, when discharged into the aquatic environment, and can also lead to the pollution of groundwater, when discharged in excessive amounts on the lands. For these reasons, treatment of wastewater has become necessary for the protection of the environment, keeping in view public health, economic, social and political concerns. In recent years, various methods for eliminating pollutants have been investigated, in which the electrocoagulation process with proper removal efficiency and low sludge production is considered as one of the newest and most desirable methods. The purpose of this study was to evaluate the efficiency of removal of COD, BOD5 and Coliform bacteria from wastewater of Urmia refinery using electrocoagulation method and optimization of the process. To do so, the effect of different parameters such as time of electrolysis, current density, pH, flow rate and conductivity of the solution was investigated. This experimental study was carried out in an electrochemical recirculating tubular reactor with an iron pipe as the cathode and some iron rods inside it as the anode. For sampling, the inlet of the Urmia wastewater refinery was considered. In order to increase the electrical conductivity of the aqueous solution, sodium chloride was applied as the electrolyte. After performing the electrochemical reactions, the treated wastewater was evaluated for clotting and COD and BOD5 analysis. To determine the characteristics of the sludge the SEM and BET analysis was done.
Results showed that, increasing the time of electrolysis increases the amount of metal and hydroxide ions produced in the anode and cathode of an electrochemical cell. Therefore, with the large amount of hydroxide flocs, the removal efficiency increases. With an increase in current density, the rate of removal increased, and then it did not change significantly, so it can be probably said that there is a limit to the rate of adsorption of pollutants to the flocs. It has been established that the effluent pH is one of the important factors influencing the performance of electrocoagulation process. The maximum removal was observed at a pH around of 7; considering the nature of the reaction between ferrous and hydroxide ions, this was not unexpected. When pH is lower than 4, Fe(OH)3 is in its soluble form (Fe+3), and when it is higher than 6, Fe(OH)3 is in its soluble form of Fe(OH)4-. Since Fe(OH)3 has a major role in removing dye, when pH is 7, the rate of dye removal is the highest. With an increase in electrolyte concentration, electrical conductivity of the solution increases, and the voltage needed for accessing a certain value of electric current density decreases; less electric energy is also consumed. With an increase in the rate of the flow passing through the reactor, the removal efficiency decreases because the high flow rate of circulation of the solution in the reactor causes decrease in retention time of the solution in the reactor; therefore, the solution is not treated completely. The released cations from the electrocoagulation process lead to infertility of the negative electrical charge produced by the cell wall of the bacterium.
The final optimum conditions during the electrolysis time 40 min, the current density 7.036 A/m2, pH 7.8, conductivity of solution 1477 ms/cm and flow rate 1.97 L/min was obtained. In these conditions the removal efficiency 93% of COD, 94% of BOD5 and 100% of Coliform bacteria was determined, which is completely agreeable with the environmental standards. As it is evident, the size of the constituent particles of the sludge as a cluster is about 0.1 mm. Based on the results of BET analyses, the specific surface area, total pore volume, and mean pore diameter of the sludge are 56.4 m2/g, 0.27 cm3/g, and 19 nm, respectively.