Treatment of oil refinery wastewater containing spent caustic soda using electrocoagulation method

A vital stage in oil refining is elimination of hydrogen sulfide, which is done by means of sodium hydroxide solution in petrochemical industries, leaving a spent caustic soda (NaOH solution) as the product. In the process, hazardous gases react with sodium hydroxide and hydrogen sulfide solutions and Thiol compounds to form a rich brown to nearly black effluent demonstrating the fragrant toxic components such as methanethiol, enzene, toluene and phenol. Despite all these odorous noxious organosulfur compounds, spent caustic soda leads to environmental problems due to its alkalinity (pH>12), salinity (5-12 wt.%) and high sulfide content (1-4 wt.%). Spent NaOH was registered as industrial dangerous waste in resource conservation and recovery act law. Inefficient and inappropriate management in spent NaOH treatment and disposal causes stability challenges, reduction in energy resources and water security attenuation. Techniques for spent caustics treatment have been neutralization with acid, wet air oxidation, combination of neutralization and Fenton (i.e. electro-Fenton), biologic treatment and ignition each of which would face some limitations. In recent decades, electrocoagulation (EC) has engrossed much attention as an Environmental-friendly and effective process.  In addition, the EC process is a potential suitable way for treatment of wastewater with a view to costs and environment. Furthermore, EC offers further advantages as simple operation facilities, small occupying area, dispensability of chemical additives and short treatment time. EC often consists of anodes and DC cathodes a part of which are immersed in wastewater container. Shape, number and configuration of electrodes may be different but rectangular types are preferred. The widespread anodes are iron and aluminum based for their availability, reasonable cost and harmless media. In electrocoagulation, electrolysis takes place to dissolve metal anode (sacrificial electrode) in wastewater. Metal ion flow from sacrificial electrode as coagulant surrounds wastewater particles. After release of Al3+ and Fe2+, the ions react with hydroxide groups and metal hydroxides turn to insoluble agglomerates able to trap contaminants and increase particle size by complexation of electrostatic attraction. In addition, hydrogen gas produced in cathode, allows agglomerates to float on surface. The object of this study is electrochemical evaluation of COD removal from refinery wastewater, specifically refinery spent caustic, using iron and aluminium (anode) and graphite (cathode) electrodes. Therefore, the effect of key variables including electrode arrangement (bipolar-serie, monopolar-serie and monopolar-parallel), anode electrode material (iron and aluminium), using pierced anode electrode and cathode graphitem initial pH (7-11), electrolysis time (0 to 120 minute), current density (15.6 to 28.125 mA/cm2) was evaluated. The three parameters of current density, electrolysis time and initial pH has been modeled with design expert software with response surface method (RSM) and central composite design (CCD). Impact of other variables has been investigated with single parameter method. According to the results, the optimum conditions including, parallel mono-polar electrode arrangement, aluminium electrode has been achieved. In designing experiments in accordance with the model provided by the software, quadratic analysis design with R2=0.96 had a high accuracy in designing the experiment. According to the model analysis and laboratory work, optimum electrolysis time was 116 min, current density was 25 mA/cm2 and initial pH=8 reached COD removal percentage of 85.1% in vitro and 88.9% for model.