Engineered spacers for fouling mitigation in pressure driven membrane processes: Progress and projection

Membrane fouling remains a major obstacle in liquid-based membrane processes, which restricts widespread applications of membrane-based processes. Approaches for membrane fouling control have been reported, including development of spacers. Typically, a spacer is required to provide a flow channel for the feed and permeate, to induce local mixing, and more recently to facilitate membrane fouling control. Those critical roles of a spacer could be enhanced by improving spacer design and geometry as discussed in the present paper. This study provides a comprehensive review on recent development of spacers incorporated in the plate-and-frame as well as in the spiral wound modules (SWMs), particularly on the spacer roles in membrane fouling control. The hydraulic performances of filtration systems incorporating new spacer along with the spacer's impacts against biofouling are extensively discussed. Lastly, perspective on the future development of spacer in aiding membrane fouling control has also been proposed. Research results demonstrated the possibility to modify spacer geometry - more recently - made from 3D printing to enhance the module throughput. The spacer geometry has been modified to reduce pressure loss, improve local mixing and promote turbulence, and to reduce dead zones. Application of new spacer geometries with reduced stagnation zone and coating of spacer surface with biocide agent have shown effective in reducing the propensity of biofouling. In the plate-and-frame module, spacers have successfully been used to enhance air bubble distribution in liquid/gas streams, promote turbulence as well as projecting the air bubbles toward the membrane surface for effective membrane cleaning.