Phenol and COD removal from petroleum refinery wastewater using electrocoagulation and electrochemical oxidation

Abstract

The petroleum industry is among the prime consumers of freshwater, through which a large quantity of its discharge enters into the aquatic environment. Immediate disposal of such wastewaters is concerning as it results in odour and the spreading of diseases in local rivers and freshwater sources. Petroleum refinery wastewater (PRW) produced, contains a significant amount of COD, phenol, FOG and BTEX that can lead to environmental deterioration if not properly treated before discharge. Due to the high concentration of organic matter and suspended solids in the wastewater, it is necessary to pre-treat the PRW prior to sequential electrooxidation treatment. Conventional treatment processes are not capable of treating contaminants and pollutants in PRW to sufficient concentrations, and hence advanced treatment processes are necessary. For this study, a lab-scale integrated treatment process was used to investigate the successful reduction of pollutants, COD, phenol, colour, FOG and BTEX. The integrated treatment process used in this research consisted of two consecutive steps; electrocoagulation (EC) and electrochemical oxidation (EO). In the electrocoagulation process with aluminium anodes, the experimental runs were conducted in a batch reactor. The effect of the operating parameters, such as the applied current and initial pH of the solution was examined. The efficiency of the pollutant removal was measured through COD, phenol, and colour with 67.5%, 98.7% and 88.5%, respectively. This was achieved at experimental conditions of an initial pH of 5, applied current of 2.5A and electrolysis time of 3 hours. The energy consumption of the EC process was found to be 0.8 kWh/m3. The electrocoagulation mechanism was modelled using adsorption isotherms. The adsorption of the pollutants in the wastewater on the surface of flocs was modelled using the Freundlich, Langmuir, Temkin, and Dubinin – Radushkevich isotherms. The Freundlich isotherm model matched satisfactorily with the experimental observations for the electrocoagulation process. The electrochemical oxidation process with Ti/IrO2 -Ta2O5 electrodes was applied to treat the wastewater effluent from the electrocoagulation process. The experimental runs were also carried out in a batch reactor, and a working volume of 1L. The highest pollutant removal conditions were the current density of 7.5 mA/cm2, supporting electrolyte (sodium chloride) of 4 g/L, and temperature of 40 °C. The energy consumption of 5.8 kWh/m3 was estimated at these optimal conditions. It was observed that the integrated EC-EO treatment system was able to reduce COD, phenol and colour levels by 96%, 100% and 100% respectively. This concludes that the treated PRW effluent complies with local industrial effluent discharge standards, which could be disposed safely without further treatment