Document Type : Original Article
Authors
Department of Environmental Engineering, School of Environment, College of Engineering, University of Tehran, Tehran, Iran
Abstract
Introduction:
Oil refinery is one of the industrial centers and its wastewater has a lot of environmental pollutants which are a serious threat to the environment and water resources. In this study, an electrocoagulation reactor was used to remove and reduce the amount of Chemical Oxygen Demand (COD) in the wastewater from the API unit of the Shazand Oil Refinery in Arak, which is a gravity separator of water and oil.
Material and methods:
In order to simulate the electrocoagulation process in experimental conditions, a 15 × 14 × 13 cm Plexiglas pilot was designed, in which six 10 × 12 cm aluminum electrodes with a thickness of 2 mm were put as sacrificial electrodes during the process. The distance between the electrodes was 2 cm. The monopolar arrangement was selected for electrodes. At each stage of the experiment, after measuring the pH and COD of the initial wastewater, the volume of 2 L was poured out as an internal sample into the coagulation reactor; after the electrochemical process, the secondary pH and COD were measured to determine the effect of the electrocoagulation process.
Results and discussion:
In this research, the effect of time, pH, and voltage in the electro-flocculation process has been evaluated. The amount of COD, pH, and lead were measured 450 mg/L, 5.5 and 5.27 mg/L in initial wastewater, respectively. To avoid the overlapping effect of time, pH, and voltage during the process, the value of one of the parameters was changed and two other parameters were kept constant and the optimum value was determined. In order to minimize the errors during the testing process, each step has been repeated three times. After performing different stages of the test and collecting and analyzing the results, the optimum time of the test, optimum pH, and optimum voltage were considered as 90 min, 6 and 30 V, respectively. Consequently, the COD decreased from 450 to 193 mg/L and 99.05% of lead was removed. The results of the experiments showed that by increasing the duration of electrolysis, the removal rate also increases. The results of other research also showed that the flow density is directly related to the reaction time, and by increasing the time, more removal rate can be obtained. As the results demonstrate, the removal rate is directly related to the applied voltage, which is due to the more production of aluminum hydroxides in the wastewater, which plays the role of coagulant. In previous studies, the results also indicated an increase in the removal rate of pollutants by increasing the applied voltage. The pH also had an important effect on the results. At low pH values, cationic monomeric species such as Al3+ and Al(OH)2+ were obtained from the dissolution of the aluminum anode. On the other hand, with the excessive increase in the pH of the solution, such compounds as Al(OH)4- were formed, which caused disruption and sluggishness in the removal process.
Conclusion:
The results of this study showed that aluminum hydroxides as coagulants, which are produced by the electrochemical dissolution of the sacrificial electrodes, play a major role in the pollutant removal process. The higher their production rates, the more removal will occur. To do this, the optimal parameters for producing them should be optimally set. By increasing the electrolysis time, the amount of aluminum hydroxides also increases. Also, adjusting the pH of the solution at an appropriate range provides optimum conditions for the production and formation of gelatin aluminum hydroxide polymer.
Keywords
)