Treating oil wastewater with pulse electro-coagulation flotation technology

Transcription

1 Journal of Chongqing University (English Edition) [ISSN ] Vol. 9 No. 1 March 2010 Article ID: (2010) To cite this article: XIANG Ya-fang, XIE Zhao-ming, Zou Yong. Treating oil wastewater with pulse electro-coagulation flotation technology [J]. J Chongqing Univ: Eng Ed [ISSN ], 2010, 9(1): Treating oil wastewater with pulse electro-coagulation flotation technology XIANG Ya-fang, XIE Zhao-ming, ZOU Yong College of Chemistry & Chemical Engineering, Chongqing University, Chongqing , P. R. China Received 26 November 2009; received in revised form 22 January 2010 Abstract: was used to treat oil wastewater of high oil content. Different operational conditions were examined, including current density, reactive time, electrode distance, ph and pole switching time. Orthogonal tests were carried out to identify the optimal operational conditions for this technique. Considering the treatment cost and efficiency together, the optimal operational conditions were an electrode distance of 3.3 cm, ph of 4, current density of ma/cm 2, reaction time of 15 min and pole switching time of 10 s. The removal efficiency of oil wastewater under normal conditions reached up to 96.21%. The influences of different factors on removal efficiency were in the following decreasing sequence: ph> current density > pole switching time > reactive time > board distance. Keywords: oil wastewater; electro-coagulation flotation; oil removal efficiency CLC number: X703.1 Document code: A 1 Introduction Oil-bearing wastewater causes environmental pollution if discharged without proper treatment. Direct discharge into municipal drainages creates a huge extra burden to municipal wastewater collection and treatment systems. Sticky in nature, oil tends to clog drainpipes and sewer lines, causing odor nuisance and corrosion of sewer lines under anaerobic conditions [1]. Oil in wastewater aggregates, fouls sewer systems and generates an unpleasant odor. Oil, as stable organic matter, can not be easily decomposed biologically or simply treated by other conventional means due to its consistency. Electro-coagulation flotation, as an emerging tech- XIANG Ya-fang ( 向亚芳 ): xiangyafang2006@163.com. Corresponding author, XIE Zhao-ming ( 谢昭明 ): xiezm@cqu.edu.cn. Funded by the Natural Science Foundation of Chongqing (No.2006BB6183). nology, is an efficient method for treating many types of wastewater contaminated with oils, solids, dyes or other organics [2-5]. It is more competitive than other flotation techniques such as dissolved air flotation and dispersed air flotation. In fact, electro-coagulation flotation supports the electrolysis of a small quantity of water by the passage of an electric current between an anode and a cathode producing fine oxygen and hydrogen bubbles from the following reactions. + Anodic oxidation: 2H2O O2 + 4H + 4e (1) Cathodic reduction: 4H2O + 4e 2H2 + 4OH (2) This process is complex because of its dependence on several factors. Indeed, current density influences directly the number and size of bubbles. Chen [6] has demonstrated that current density and the mass of bubbles produced are proportional. The ph is also a parameter which influences the mechanism of electrocoagulation flotation owing to the fact that the hydrogen bubbles are the smallest at neutral ph and for 41

2 the oxygen bubbles their sizes increase with ph. Besides ph and current density, there are several other parameters affecting electro-coagulation flotation, such as the state and arrangement of electrodes, the nature of water to be treated and processing duration. It has been noted that electro-coagulation flotation has been used primarily to treat wastewater from pulp and paper, mining, and metal-processing industries [7-8]. In addition, it has been applied to treating water containing foodstuff wastes, dyes, suspended particles, chemical and mechanical polishing waste, organic matter from landfill leachates and mine wastes [9-11]. Electro-coagulation flotation process is characterized by its fast rate of pollutant removal, compact size of equipment, simplicity in operation, and low capital and operating costs. Moreover, it is particularly effective in treating wastewaters containing small and light suspended particles (e.g. oily plant wastewater) because of the accompanying electro-flotation effect. Hence, it is expected that electro-coagulation flotation will be an ideal choice for treating oil wastewaters. In this research, we used pulse electro-coagulation flotation to treat oil wastewater, and conducted orthogonal tests to examine different operational conditions including the current density, the reaction time, ph, the electrode distance and the pole switching time for identifying the optimal operational conditions under which the best removal efficiency was achieved at a relatively low operation cost.. 2 Materials and methods The motor oil was collected from car-repair shops. Anhydrous sodium sulfate was dried for 12 h in the oven. Analytical reagent (AR) sodium chloride was used as the supporting electrolyte to increase the conductance of samples, and AR petroleum ether as the oil-extracting agent. Concentrated sulfuric acid and hydrochloric acid were from Chuandong Chemical Co., Ltd. The KL-DNJ-1 Electro-coagulation Flotation Experimental Device was made by Wuhan Colin Highteaching Equipment Co., Ltd. The JJ-1 Timed Motor Stirrer and the FA2004 Electronic Balance were used in the experiments. Ultraviolet-visible spectrophotometer was taken as the analysis equipment to detect the ultraviolet-visible absorption of the samples was recorded on a GBC UV Vis Spectrophotometer (TU- 1901, Beijing Purkinje General Instrument Co., Ltd.). The schematic diagram of the experimental setup is shown in Fig. 1. Wastewater flew upward in the electrochemical reactor and downward in the separator. The reactor volume was 240 L. Several electrodes were connected in a dipolar mode at a specific distance in the electrochemical reactor, all with dimensions of 200 mm 50 mm 2 mm. The electro-coagulating reactor was composed of an electrolytic cell, an anode and a cathode. When connected to an external power source, the anode material would be electrochemically corroded due to oxidation; whereas the cathode would be subject to passivation. Fig. 1 Schematic diagram of the electrochemical reactor Oil wastewater samples of an oil concentration varied from 120 mg L 1 to 250 mg L 1 were made with the motor oil in the laboratory; their colors were slightly yellow.. A stock solution of each sample was prepared by digesting 12 g oil wastewater in 50 ml concentrated H 2 SO 4 for 30 min and diluted to a final volume of 200 ml. Sample working solutions were prepared by diluting 10 ml stock solution to a volume of 50 ml and extracted with petroleum ether at ph below 2, and then examined by the ultraviolet-visible spectrophotometer at nm. 3 Results 3.1 Effect of current density Current density has been reported to influence the treatment efficiency of the electrochemical process [4]. 42 J. Chongqing Univ. Eng. Ed. [ISSN ], 2010, 9(1): 41-46

3 This conclusion was based on the experimental condition of changing charge load. In the present study, we tested different intensity of electro-coagulation flotation power supply at the same cell voltage to change the current density of Al board. As evidenced in Fig. 2, when the current density increased, the quantity of Al 3+ was larger and oil removal rate was also higher; a maximum removal rate of 86% was achieved with a current density of ma/cm 2 after 30 min of reaction, The Al anode began to be passivated and oil removal rate started to decrease when the current density was further increased. The optimal current density turned out to be (30 to 40) ma/cm Effect of board distance The change of removal efficiency with different Al board (electrode) distance is shown in Fig. 4. At the same voltage and same board area, the smaller the electrode distance is, the larger the current density and the quantities of A1 3+ and H 2. When the electrode working surface was fully utilized, however, it easily led to passivation, and increased uneven distribution of the electric field. The oil removal efficiency and power consumption went down when the electrode distance increased. As a result, the required power decreased. The optimal electrode distance was (3 to 4) cm. Fig. 2 Effect of current density on oil removal under the conditions: electrode distance of 3.3 cm, reaction time of 30 min, ph of 7, and pole switching time of 0 s. Fig. 3 Effect of reaction time on oil removal under the conditions: electrode distance of 3.3 cm, current density of ma/cm 2, ph of 7, and pole switching time of 0 s. 3.2 Effect of reaction time Reaction time also influenced the treatment efficiency of the electrochemical process (Fig. 3). The oil removal efficiency grew up quickly at the beginning of the electro-coagulation flotation process. More than 90% of oil was degraded within 40 min, and the main removal occurred in the first 15 min to 30 min. With the reaction time further increased, the increase of the oil removal efficiency became slower, bubbles in water were saturated with a lower utilization. Experiment load and energy consumption increased. Thus, work efficiency decreased and the treatment cost increased. Therefore, the optimal reaction time was (15 to 30) min for this process considering the treatment cost and efficiency. Fig. 4 Effect of electrode distance on oil removal under the conditions: current density of ma/cm 2, reaction time of 30 min, ph of 7, and pole switching time of 0 s. J. Chongqing Univ. Eng. Ed. [ISSN ], 2010, 9(1):

4 3.4 Effect of ph The variation of removal efficiency with different influent ph is illustrated in Fig. 5. It is believed that ph influences the electrochemical process in a complicated way. To examine its effect, the wastewater was adjusted to a desired ph for each experiment by using sodium hydroxide or sulfuric acid. It is observed in Fig. 5. that low ph value was in favor of the dissolution of oxide film and palliated electrode passivation. However, high PH was conducive to the formation of aluminum complexes to achieve good flocculation. Although the maximum removal of oil was achieved at ph around 7, the ph effect was not so significant in the range from 3 to 9. Fig. 5 Effect of ph on oil removal under the conditions: current density of ma/cm 2, reaction time of 10 min, electrode distance of 3.3 cm, and pole switching time of 0 s. 3.5 Effect of pole switching time conditions of electro-coagulation flotation. An L 16 (4 5 ) orthogonal table was used, with the removal efficiency as an index (Table 1). The removal efficiency (Y) is given by Y W W 1 2 /% = 100, W1 where W 1 is the amount of oil wastewater before treatment; and W 2 is that after treatment. Based on the orthogonal test results, we calculated the extremum difference R j by R k k, j = ijmax ijmin where i = 1, 2, 3, 4 represents different value of a tested factor adopted in the orthogonal tests; j = 1, 2, 3, 4, 5 stands for the five factors: current density, reaction time, electrode distance, ph, and pole switching time, respectively; k ij is the sum of all Y corresponding to the ith value of the jth factor; and k ij is the average of k ij.. The calculation results are listed in Table 2. R j is an important index; its measurement determines the importance of a factor. The influences of the tested factors on the oil removal efficiency turned out to be in the following sequence: ph> current density > pole switching time > reactive time > electrode distance. The maximum k ij of different factors represented the optimal conditions, which were an electrode distance of 3.3 cm, ph of 4, current density of ma/cm 2, reaction time of 15 min and pole switching time of 10 s were the optimal conditions. The variation of removal efficiency with different pole switching time is presented in Fig. 6. Pole switching was applied to prevent the Al board passivation. In the electrolysis process, the imposed pulse signal produced pulsed electrolysis, promoted pole dissolution, and facilitated flocculation between metal ions and colloidal particles. The oil removal efficiency was lower when the poles were switched less frequently. Pole switching time is a complicated and important factor, and its effect on oil removal efficiency is dependent on other relevant factors. 3.6 Orthogonal test Based on different factor tests, orthogonal tests were designed for identifying the optimum operational Fig. 6 Effect of pole switching time on oil removal under the conditions: current density of ma/cm 2, reaction time of 10 min, electrode distance of 3.3 cm, and ph of J. Chongqing Univ. Eng. Ed. [ISSN ], 2010, 9(1): 41-46

5 Table 1 Orthogonal test parameters with oil removal efficiency as an index Test No. Current density/(ma/cm 2 ) Reaction time/min Electrode distance/cm ph Pole switching time/s Removal efficiency/% Table 2 Extremum difference analyzing chart j Factor k 1 j k 2 j k 3 j k 4 j k 1 j k 2 j k 3 j k 4 j R j 1 Current density Reaction time Electrode distance ph Pole switching time Notes: k ij is the sum of all removal efficiencies corresponding to the ith value of the jth factor ( i = 1, 2,3, 4 ); k ij is the average value of k ij ; and R j is the extremum difference of the jth factor 4 Conclusions 1) Electro-coagulation flotation is a promising process for treating oil wastewater characterized by high oil content, fluctuating chemical oxygen demand and high suspended solid concentration. For usual oil wastewater, an oil removal efficiency up to 96.21% can be achieved with this technique using aluminum electrodes. 2) Considering treatment cost and oil removal efficiency together, the optimal operation conditions of this pulse electro-coagulation flotation process are: a electrode distance of 3.3 cm, ph of 4, a current density of ma/cm 2, reaction time of 15 min and pole switching time of 10 s. 3) The influences of different factors on oil removal J. Chongqing Univ. Eng. Ed. [ISSN ], 2010, 9(1):

6 efficiency are: ph> current density > pole switching time > reactive time > electrode distance. References [1] Chen XM, Chen GH, Yue PL. Separation of pollutants from restaurant wastewater by electrocoagulation [J]. Separation and Purification Technology, 2000, 19(1-2): [2] Tsai CT, Lin ST, Shue YC, et al. Electrolysis of soluble organic matter in leachate from landfills [M].Water Research, 1997, 31(12): [3] Chen GH, Chen XM, Yue PL. Electrocoagulation and electro-flotation of restaurant wastewater [J]. Journal of Environmental Engineering, 2000, 126(9): [4] Kobya M, Can OT, Bayramoglu M. Treatment of textile wastewaters by electrocoagulation using iron and aluminum electrodes [J]. Journal of Hazardous Materials B, 2003, 100(1-3): [5] Bektaş N, Akbulut H, Inan H, et al. Removal of phosphate from aqueous solutions by electrocoagulation [J]. Journal of Hazardous Materials, B, 2004, 106(2-3): [6] Chen G. Electrochemical technologies in wastewater treatment [J]. Separation and Purification Technology, 2004, 38(1): [7] Jiang JQ, Graham N, André C, et al. Laboratory study of electro-coagulation flotation for water treatment [J]. Water Research, 2002, 36(16): [8] Barrera-Diza C, Ureña-Nuñez F, Campos E, et al. A combined electrochemical-irradiation treatment of highly colored and polluted industrial wastewater [J]. Radiation Physics and Chemistry, 2003, 67(5): [9] Pouet MF, Grasmick A. Urban wastewater treatment by electrocoagulation and flotation [J]. Water Science and Technology, 1995, 31(3): [10] Kovatcheva VK, Parlapanski MD. Sonoelectrocoagulation of iron hydroxides [J]. Colloids and Surfaces A: Physicochemical and Engineering Aspects [ ], 1999, 149(1-3): [11] Mills D. A new process for electrocoagulation [J]. American Water Works Association, 2000, 92(1): Edited by LUO Min 46 J. Chongqing Univ. Eng. Ed. [ISSN ], 2010, 9(1): 41-46