Photocatalytic membrane filtration and its advantages over conventional approaches in the treatment of oily wastewater: A review

Clean water supply has become one of the biggest challenges of the 21st century; therefore, water source protection is of increasing importance. Beyond environmental protection reasons, economic concerns-derived from increasing costs of processing water and wastewater discharge-also prompt industries to use advanced wastewater treatment methods, which ensure higher purification efficiency or even the recycling of water. Therefore, highly effective treatment of oily wastewaters has become an urgent necessity because they are produced in high quantities and have harmful effects on both the environment and human population. However, high purification efficiency can be difficult to achieve, because some compounds are hard to eliminate. Conventional methods are effective for the removal of floating and dispersed oil, but for finely dispersed, emulsified and dissolved oil advanced methods must be used, such as membrane filtration which exhibits several advantages. The application of this technology is restricted by fouling-the major limiting factor-which jeopardizes the membrane performance. In order to reduce fouling, in-depth research is being conducted to make the treatment of oil-contaminated water technically and economically feasible. The present work aims to review the conventional oil separation methods with their limitations and to focus on membrane filtration, which ensures significantly higher purification efficiencies, including the main problem: the flux reduction caused by fouling. This paper also discusses promising solutions, such as membrane modification methods, mostly with hydrophilic and/or photocatalytic nanoparticles and nanocomposites, overviewing the efforts that are being made to develop feasible technologies to treat oil-contaminated waters.