Impacts of feed spacer design on UF membrane cleaning efficiency

Abstract

In this study, the impacts of three feed spacer design parameters; thickness, porosity and architecture, on the cleaning efficiency of ultrafiltration (UF) membranes were studied, both in backwash and relaxation cleaning modes, using sodium alginate solution dosed with CaCl2 as the process feed. Both commercial, net-type spacers and 3D-printed spacers based on triply periodic minimal surfaces (TPMS) architectures were utilized in the study. Increased spacer thickness and porosity were found to increase the cleaning resistance of the membrane, while TPMS spacers were found to yield a lower cleaning resistance than net-type spacers, even with the latter being thicker. The root causes of these observations were analyzed based on resistance analysis of the fouling layer on the membranes. To do that, overall, residual, reversible and irreversible fouling resistances were quantified at the end of a 20-cleaning cycle test. Statistical correlations were then established between these resistances and spacer properties. The results showed that the consequential impacts of spacer design on the shear stresses, created by the feed flow on the fouling layer, underscore the observed fouling mechanism and the consequent cleaning efficiency. The spacer design can impact these shear stresses through its hydraulic diameter and spacer-membrane contact area. When the spacer design leads to lower shear stresses, a cake filtration mechanism prevails, a more irreversible fouling occurs and more challenging cleaning becomes. A recently published work on combined intermediate pore blockage and cake filtration model for fouling was used to construe these observations.