Cross-flow ultrafiltration of stable oil-in-water emulsion using polysulfone membranes

This paper focuses on the treatment of oily wastewater coming out from the post-treatment unit of petroleum industries using cross-flow mode of ultrafiltration. Four types of polysulfone (PSf) membranes which had been synthesized using different combinations of additives and solvents were used and their performances were evaluated by treating with laboratory made oil-in-water (o/w) emulsion and later the selected membranes were tested for their applicability to the industrial oily water also known as ‘produced water’. Experiments were carried out mainly to study the influence of cross-flow rate on membrane performance. The study showed that with increase in cross-flow rate, the flux increases significantly; but the oil rejection (%) shows a decreasing trend. Also change in morphological properties of membranes due to addition of different molecular weight PVP and PEG are found to have a significant impact on the permeate flow rate and hence subsequent oil removal. The oil retentions of all the four membranes were over 90% and oil concentration in the permeate was below 10 mg L−1 with the synthetic oily water, which met the requirement for discharge. However, the rejection was found to be below 80% for all the membranes with the ‘produced water’ although the flux was reasonably high. This suggests that the membranes need further modifications to improve their properties such as average pore size and pore size distribution in order to tackle the difficulty of lower oil separation due to penetration of the smaller oil droplets along with the permeate.