Optimal synthesis and operation of low-cost polyvinyl chloride/bentonite ultrafiltration membranes for the purification of oilfield produced water

The low-cost PVC/bentonite ultrafiltration membranes are prepared by phase inversion method using different coagulation baths involving de-mineralized water and aqueous saturated solution of NH₄Cl, NaCl, KCl, MgCl₂ and CaCl₂. Effect of coagulation bath on membrane performance is investigated by evaluating ternary-phase diagram, membrane morphology, hydrophilicity, porosity, mean pore size, pore density, pure water flux and antifouling behavior using oilfield produced water. Better performing membrane with 5% bentonite loading is obtained from KCl-coagulation bath with pure water permeance-467.5 L m⁻² h⁻¹ atm⁻¹. Response surface method based on central composite design is employed to enhance the ultrafiltration membrane performance using the optimum design of experiments through maximizing permeate flux as well as oil rejection separately considering following decision variables: PVC and bentonite loading in casting solution, membrane thickness, pH and trans-membrane pressure (TMP). The maximum permeation flux of 421.3 L m⁻² h⁻¹ is achieved using PVC-loading-15.01 g, bentonite-loading-1.37 g, membrane thickness-100 µm, pH-9 and TMP-250 kPa, while maximum oil rejection is 98.6% at PVC-loading-18.0 g, bentonite-loading-0.0 g, membrane thickness-250 µm, pH-5.1 and TMP-54.4 kPa. Due to conflicting nature of permeation and oil rejection, a multi-objective optimization problem is formulated and solved using an elitist non-dominated sorting genetic algorithm (NSGA-II), resulting the Pareto-optimal solution.