The treatment of carwash wastewater using a combined process of chemical coagulation and electrochemical oxidation

Abstract

Carwashes are found in every city across the world, whether informal or formal stations, a large amount of water is required to effectively clean a car to the owner’s satisfaction. Typically, 150 to 600 L of water is required per car, depending on the size of the vehicle. This in turn generates complex wastewater which contains high levels of pollutants which is discharged into water sources. This creates serious environmental concerns that have a devasting effect on aquatic life. Therefore an effective, low-cost solution is required for the remediation of carwash wastewater (CWW) for re-use application. This would reduce operational cost and conserve fresh water. Due to the high concentration of organic matter and suspended solids in the wastewater, it is necessary to pre-treat the CWW prior to sequential electrochemical oxidation treatment. Conventional treatment processes are not capable of treating contaminants and pollutants in CWW to sufficient concentrations, and hence advanced treatment processes are necessary. In this study, a lab-scale integrated treatment process was used to treat carwash wastewater to reduce high levels of pollutants such as COD, FOG, anionic surfactants, and turbidity. The integrated treatment process used, consisted of a chemical coagulation (CC) pre-treatment and an electrochemical oxidation (EO) process. Polyaluminium chloride (PAC) was selected as the coagulant in the chemical coagulation process, where experimental runs were conducted in a batch reactor. The effect of PAC dosage was examined. The efficiency of the pollutant removal was measured through COD, anionic surfactants, FOG’s, and turbidity, which were found to be 68.44, 19.88, 97.93 and 95.70%, respectively. This was achieved at experimental condition where the PAC concentration was 100 mg/L. The PAC sludge samples generated after the CC process were characterized with Fourier transform infrared (FTIR) spectroscopy. The analyses showed the presence of alcohols, phenols and alkanes which is strongly associated with pollutants and heavy metal ions. 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, at a constant temperature of 60oC with a working volume of 1 L. The highest COD removal percentage of 97.13 was achieved at a pH of 2, current density 10 mA/cm2, and supporting electrolyte (NaCl) concentration of 0.055 M. The highest anionic surfactant removal percentage of 99.22 was achieved at experimental conditions at pH 7, current density 10 mA/cm2, and supporting electrolyte (NaCl) concentration of 0.1 M. The electrochemical oxidation experiments were characterized by a Box-Behnken design (BBD). Polynomial quadratic models were successfully developed for the removal of COD and anionic surfactants. They were identified as the major pollutants in this study. Their removal was found to be significant. It was observed that the integrated CC-EO treatment system was able to reduce COD, FOG, anionic surfactants, and turbidity levels by 97.13%, 100%, 98.49%, and 99.41%, respectively. This concludes that the treated CCW effluent complies with the industrial effluent discharge standards for disposal or recycling.