The evolution of feed spacer role in membrane applications for desalination and water treatment: A critical review and future perspective

The membrane research community has witnessed a significant research growth on feed spacers since 2010, both in quantity and scope. Increasingly novel spacer geometries and spacer chemistries are being reported in literature to tackle key challenges facing membrane processes, including concentration polarization, energy consumption, low flux, and fouling. This review aims to analyze the recent developments of spacer research, starting with a review of the various roles spacers can and do play in membrane systems and their increased prominence in tackling membrane fouling and scaling. We then review the continued need for novel spacer designs, discuss the contribution spacers can make to energy saving, and the necessity for new generations of spacers to facilitate the deployment of next generation, ultra-permeable membranes. Qualitative and quantitative spacer performance assessment tools are described. The key trends in spacer modifications over the period of 2010 to 2021 are categorized into: i) design modifications of both conventional and novel geometries, and ii) spacer surface modifications, including chemical coatings and new spacer materials. Finally, the future trends of spacer research are highlighted, with an emphasis on the development of functionalized feed spacers and new spacers made via 4D printing and smart materials.