Complete gravity pre-treatment of vegetable oil refinery wastewater: kinetic and RSM studies

The vegetable oil refinery wastewater (VORW) has high amounts of oil and grease (O&G), turbidity, chemical oxygen demand (COD), and low biodegradability. This study presents gravity separation as an inexpensive pre-treatment method for VORW, with emphasis on pH and retention time effects. The maximum removal efficiencies were experimentally obtained at pH 1.0 and 60 min, achieving 99% O&G removal, 99% turbidity removal, and 87% COD removal. However, the RSM analysis identified pH 2.0 and 18 min as the optimal operational conditions, representing a practical compromise between treatment efficiency, retention time, and process feasibility. Kinetic analysis using Michaelis-Menten model gave excellent fitting (R2 > 0.97), representing fast pollutant removal in acidic conditions. A central composite design (CCD) in framework of response surface methodology (RSM) was also utilized to optimize experimental parameters. The statistical analysis showed that pH and retention time significantly influenced treatment performance, with high correlation coefficients (R2 = 0.998 for O&G and 0.993 for COD). The results showed that gravity separation was a rapid, effective, and eco-friendly primary treatment plan for VORW, decreasing contaminant load before the physicochemical processes. These gravity separations showed potential as a primary treatment step for vegetable oil refinery wastewater. Furthermore, the pilot-scale and techno-economic studies are required for larger-scale water treatment.