Process intensification with reactive membrane distillation: A review of hybrid and integrated processes

Membrane distillation (MD) is a promising thermally-driven desalination process for various applications in food, pharmaceutical, textile, and chemical industries. However, membrane fouling and wetting are still major chal-lenges, which limit the stability of MD membranes over long-term use. From 2005, researchers have developed many strategies to enhance membrane fouling resistance by simultaneously engaging reactions, but these works have not been systematically reviewed and categorized. Therefore, this review aims to identify research gaps and provide an overview of the state-of-the-art reactive MD processes, which are categorized broadly based on two configurations: 1 - "hybrid" involving coupling MD with an oxidation or biological process as a pre-treatment step and 2 - "integrated" involving immobilizing reactive materials on the membrane.The reactive MD processes were sub-classified into five categories based on reaction technology then analysed in terms of working mechanisms, advantages, limitations, and their application potentials. It shows that mini-mizing fouling in both hybrid and integrated processes can be attributed to the interaction of the organics/ degraded by-products with the membrane surface/immobilized reactive layer, and the capability of the pollut-ants to be oxidized. In addition, depending on the types of the coupled process, oxidation can be accompanied by other mechanisms, such as electrostatic repulsion and electro flotation which is the most promising mechanism. Overall, despite the current developments in hybrid and integrated MD processes, many challenges exist including flux decline in the case of the complex feed solutions, a trade-off between oxidation and other accompanied mechanisms (e., electrostatic repulsion), and the possibility of membrane alteration. Future op-portunities can be explored in terms of dedicated fouling studies, energy efficiency, removing/converting waste streams to valuable by-products, durability of the process equipment against oxidation, and targeting unique applications such as the treatment of concentrated organic waste streams.