Treatment of oily wastewater using photocatalytic membrane reactors: A critical review

Oily wastewater is generated from various sources such as oil/gas exploration (produced water), oil refining, pharmaceutical, food industries, and household waste. Toxins in oily wastewater often percolate into drinking water, seawater, and groundwater. This becomes a source of environmental and public health concern. Due to its hazardous nature, the discharge of oily wastewater into the environment is strictly regulated. This work critically reviews progress in photocatalytic membrane reactor (PMR) for oily wastewater treatment, regulations on allowable oil discharge, various factors that affect PMR performance, and its self-cleaning and anti-fouling properties in oily wastewater treatment. Their removal performance for stabilized oil emulsion and trace oil contaminants is highlighted. This work also evaluates trends of integrated techniques, utilization of functional materials, PMR scale-up and the outlook of PMR. It was evident from 226 published articles (1976–2022) that oily waste water contamination has been a source of concern and PMR which integrates both membrane filtration and photodegradation processes, has emerged as a promising technology for oily wastewater treament, simultaneously degrading oil emulsion and undertaking separation. The PMRs attained over 96% oil rejection. Both the UV and visible light aided the degradation of oil using the PMR. High membrane surface area, provides additional sites for the photocatalyst to occupy, contributing to an efficient degradation. Concludingly, PMRs can exhibit a high flux of recovery ratio after several filtration cycles under UV/Vis irradiation, and with proper design and fabrication methods, the membranes can do self-cleaning and be re-used for several cycles of filtration with high efficiency.