Electrocoagulation Treatment of Oily Wastewater in the Oil Industry

Electrocoagulation has turned out to be a rapidly growing area of wastewater treatment due toits ability to remove contaminants that are generally more difficult to remove by filtration orchemical treatment systems. These pollutants may include emulsified oil, petroleumhydrocarbons, suspended solids, and heavy metals. In this novel technology, metal cations arereleased into the wastewater through dissolving metal electrodes. Simultaneously, beneficialside reactions can help in removing flocculated material from the water.Oily wastewater represents a dangerous threat when discharged to the environment; thereforetreating it becomes vital in oil industry. This research has investigated electrocoagulation as asimple, effective and economic technique for treatment of such wastewater. Bench scale reactorwas used to evaluate the factors that may affect the treatment of oily wastewater. Aluminumwas used as a sacrificial anode. Electrodes were arranged at different configurations to selectthe optimal one. Other tested operation parameters include time of treatment, current density,distance between the electrodes, and the electrolyte concentration of the emulsion. Theexperimental results indicated that electrocoagulation was very efficient and able to achieve94% turbidity removal in less 60 min, pH:7, current density of 45 (A/m2).Also the effect of initial concentration of oil (100-1000 ppm) in wastewater, applied voltage(10-20 v), the dose of NaCl electrolyte (0-1 g) and temperature (35-60 oC) were optimized.What distinguishes this work is the the type of the electrodes configuration. Circular horizontalAl/Al electrodes (Ø12 cm) with 17 holes (Ø7 mm) have been found to give the bestelectrocoagulation environment and thus optimum results. The associated electrical energyconsumption is 1.6 kW h/m3 for flotation times of 60 min.


No.20 Journal of Petroleum Research & Studies
(JPR&S) 277 intricate process depending highly on the efficiency to meet the environmental standards for disposal and the characteristics necessary for reuse [1].
In the oil industry, oily water occurs in many phases of production, transportation and refining, as well as during the usage of oil derivatives. However, the production phase is the largest source of this contamination. During the production process, oil is commonly mined along with water and gas. The accompanying water may grasp half of the volume produced, or even higher, approaching 100% at the end of the wells productive life. The release or reinjection of this co-produced water is only allowed after the removal of most of the oil and suspended solids. The composition of this produced water is very complex. Depending on its origin it may comprise a variety of substances such as salts, hydrocarbons, oils, and metals This extreme production of water has become a foremost apprehension in the oil industry. Before disposal into the environment or use, it is necessary to treat this water because the large amounts of pollutants cannot be discharged into the receiving environment.

Electrocoagulation
Electrolysis is a chemical decomposition in which oxidation and reduction reactions take place when electric current is applied to an electrolytic solution [14] . The electrocoagulation (EC) is a novel technology that has been successfully used to treat industrial wastewater containing different contaminants such as arsenic, phosphate, boron, dyes and viruses [14][15][16][17][18][19].
Furthermore, it has been also effective in breaking oil emulsions in water and treating paper mill effluent, olive mill effluent, landfill leachate and dairy effluent. However, few studies have investigated the removal of hydrocarbons using EC process. EC is based on dissolution of the electrode material used as an anode. The so-called "sacrificial anode" yields metal ions which preform as coagulant agents in the aqueous solution. For many reasons, EC is an alternative method to the conventional chemical coagulation. EC is capable of reducing the need for chemicals due to the fact that the electrodes facilitate the coagulant. However, many individuals still use chemical coagulants to attempt to enhance treatment. Conventionally, chemical coagulation involves the use of alum, ferric chloride, or ferrous sulfate. These chemicals can be very expensive depending on the volume of water treated. When applying the coagulant, it

No.20 Journal of Petroleum Research & Studies (JPR&S)
278 performs a similar function as the electrodes, neutralizing the charge of the particulates, thereby allowing them to agglomerate and settle down at the bottom of the tank. In addition, EC is capable of reducing waste production from wastewater treatment and also reduces the time necessary for treatment.
In an EC process, the coagulating ions are generated involving three stages:

Experimental
Schematic diagram for the experimental set up is given in Figure (1) [19]. The EC tests were done in a batch system using a 2000 ml Pyrex beaker by horizontal circular Al/Al electrodes. The electrodes were connected to a digital DC power supply having an input of 220 V and variable output of 0-20 V, with variable current 0-2 A. Effective area of electrode used was 220 cm 2 . The optimum gap between the electrodes was 6 mm. Before each experiment, electrodes were washed with acetone to remove surface grease-oily and pollutants were washed, then matters on electrode surfaces were cleaned by dipping for 1 min into a diluted HCl solution then washed with pure water for the removal of the residuals on their surfaces and dried by oven. Oil for this study was obtained using common lubricant oil from a local market.
A synthetic emulsion mixture of oil/water was prepared by mixing an appropriate amount of oil (with density 0.89 g/L and 28 o API) in 10 liter of water to obtain 100 NTU turbidity. This mixture was then subjected to vigorous mechanical stirring for 15 minutes to form a stable oil/water emulsion. The desired amount of oil mixed with fresh tap water was used under ultrasound agitation for 30 min, for obtaining very stable or soluble synthetic oil wastewater. This quantity will be stored to carry out a number of experiments. All tests were performed at

Results and Discussion
In this study, electrocoagulation using aluminum sacrificial anode was used for the treatment of oily emulsion. In order to evaluate the decreasing of turbidity and increasing oil removal from the emulsion, various important electrochemical factors were investigated: applied voltage, the behavior of the current during the operation, the initial concentration of oil in the wastewater, initial pH, gaps between the electrodes, the added NaCl electrolyte, volume of wastewater and temperature.

Effect of time on oil separation efficiency:
In this research, turbidity was used as a measure of the oil concentration in the wastewater. The change in turbidity with time during the electrocoagulation process is depicted by Figure 2. As seen in the figure the oil separation efficiency has a direct relationship with the time. Increasing the treating time caused considerable decrease in water turbidity. During this period of time of 60 min, the turbidity has been reduced from 100 NTU to 6 NTU with an efficiency of 94%. These hydrogen bubbles will push the oil drops upwards by bouncy force thus increasing the efficiency of oil separation and decreasing turbidity of the emulsion.

Effect of distance between electrodes on oil separation efficiency:
The relation between oil separation efficiency and the distance between the electrodes is shown in Figure 3 carried out at 35 o C and initial oil concentration of 100 NTU and applied voltage of 15v. The inter-electrode distance is an important variable in order to optimize the operating costs of electrolysis systems. Researchers report that when the conductivity of the effluent is high, a larger spacing between the electrodes is possible. On the other hand, when conductivity is low as the case in this work, the spacing should be smaller [20].

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In this study, the inter-electrode distance was varied (4 mm, 6 mm and 8 mm) while the other factors remained unmodified. Our results indicate that the inter-electrode distance did affect the performance. Increasing the distance between electrodes has led to reduce removal efficiency, in this case, the ohmic resistance has decreased because of the shortening of the path of the electric current between the anode and the cathode [22]. In addition, the energy consumption also increases due to the higher resistivity of the solution [21].
The results in Figure (3) show an improvement in the efficiency of removal for lower inter-electrode distance between 4 and 8 mm. A distance of 6 mm has been chosen as an optimum distance since the use of larger distances would involve greater energy consumption.
Previous results have shown that increasing the inter-electrode distance to about 15 mm has reduced considerably the removal efficiency [18]. Similar results have been reported from other researchers [22].

Effect of applied Voltage on oil separation efficiency:
The using of different applied voltages (10, 15, 20 v) with fixed electrodes distance of (6 mm) has been investigated here. Figure (4)

Conclusion
In this study, the efficiency of the electrocoagulation process applied to the treatment of oily wastewater emulsion was investigated. It was observed that the electrocoagulation treatment achieves a fast and effective removal of oil turbidity. The treatment efficiency was found to be a function of the initial pH, inter-electrode distance, applied voltage, NaCl electrolyte dose, temperature and operating time under the optimal values of the process parameters.
The results showed removal efficiencies of 94% turbidity with an energy consumption of 1.6 kWh/m 3 with electrode consumption of 0.05 g could be achieved at a current density of 45 A/m 2 with an operation time of 60 min and initial pH of 7.0.
This technique thus seems to be a promising alternative for the treatment of oil-in-water emulsions of the petroleum industry.