Going from Inner‐Skinned to Outer‐Skinned Polyelectrolyte Multilayer Based Hollow Fiber Nanofiltration Membranes

Abstract Recently, hollow fiber polyelectrolyte multilayer membranes (PEMMs) have shown great potential for removing organic micropollutants from wastewater streams. Currently, these PEMMs are made with the selective layer on the inside of the hollow fiber. However, to reduce the physical footprint of PEMM modules, it would be beneficial to have the selective layer on the outside. This will lead to an increased surface area per fiber and enable the use of smaller mechanically stronger fibers, which further increases the surface area per membrane module. To prove that the concept of an outer‐skinned PEMM is feasible, the dry‐jet wet‐spinning process is used to fabricate charged outer‐skinned hollow fiber supports, and they are coated through the layer‐by‐layer self‐assembly process to form a PEMM. Scanning electron microscopy images of the hollow fibers confirm the existence of an asymmetric structure with an outer skin, while fluorescence imaging confirms that the polyelectrolyte multilayer is located on the outside of the hollow fiber. Filtration experiments showed that the PEMMs exhibited nanofiltration properties similar to the conventional inner‐skinned PEMMs. Overall, these membranes show a 3.8x increase in active membrane surface area to volume ratio compared to commercial PEMMs, clearly highlighting the benefits in terms of footprint.