Comparison of two methods of neutralization and wet air oxidation for treating wastewater spent caustic produced by oil refineries

Today, problems of treating wastewater spent caustic produced by refinery units with high toxic compounds and chemical oxygen demand (COD) become concerns of many managers of these industries and environmental experts. Hence, emission without application of treatment methods will have adverse environmental impacts. In this study, two direct acid neutralization (DAN) and wet air oxidation (WAO) processes were selected to treatment the wastewater of the Bandar Abbas oil refinery in southern Iran. The aim was to reduce COD and harmful substances, compare the two methods, optimize processes, and evaluate their performance. Experimental experiments were performed in a reactor system with different input variables related to two different methods. Parameter optimization was performed based on Box-Behnken (BBD) method and Design Expert software. The analysis of the results based on statistical methods and response procedure diagrams was used to evaluate the status of COD changes to the parameters. The best operating conditions of temperature, pressure, residence time, and stoichiometric coefficient of air were 148.269 °C, 15.716 bar, 3.563 h, and 8.415 l/h respectively in WAO with 68% reduction in COD, and for DAN process, temperature, pH, and agitation speed were 30.082 °C, 2.008, and 203.672 rpm, respectively, with 43% reduction in COD. Results of rapid impact assessment matrix (RIAM) showed that WAO process with a higher score is a more environmentally friendly method and DAN process has been considered by experts due to its popularity, ease of testing, less equipment requirements, and lower cost. Graphical abstract