3D printed spacers based on TPMS architectures for scaling control in membrane distillation

In this study, the performance of novel 3D printed spacers was investigated for scaling control in direct contact membrane distillation (DCMD). The spacers, designed as triply periodic minimal surfaces (TPMS), were tested under calcium sulfate scaling conditions, with brine recycling leading to continuous feed concentration increase. The DCMD experiments were done using 1900 mg/L calcium sulfate as the starting feed solution at feed and permeate inlet temperatures of 65 and 35 °C, respectively, and feed and permeate flow velocity of 0.1 m/s. The best performing TPMS spacer, the tCLP design, resulted in a 50% flux increase (47 L m⁻².h⁻¹) in comparison to a commercial spacer, but at the expense of increased pressure drop (0.52 bar vs. 0.04 bar). The membrane in contact with the commercial spacer had higher scalant deposition than those in contact with the TPMS spacers. On the other hand, the surface micro-roughness of the TPMS spacers contributed to increased scalant deposition on the spacer itself. The calcium sulfate scalant deposition patterns on the fouled membranes were visualized by utilizing alizarin red S (ARS) staining, which was applied herein for the first time in characterizing membrane fouling. The ARS stains proved that the spacer contact region functioned as the scaling initiation sites. A hybrid spacer design combining two TPMS architectures, tCLP and Gyroid, was then investigated, which resulted in high flux performance on par with tCLP, but at a lower pressure drop penalty. The results highlight the prospective applications of 3D printed TPMS designs to control scaling in MD.