Scaling mitigation in direct contact membrane distillation using air microbubbles

Abstract

Membrane distillation (MD) is a promising process for high-salinity waters. However, water recovery in MD treatments of these feed waters is often limited by their high scaling potential, which results in the need for extensive pre-treatment or the use of antiscalants for scaling control. In this study, we show the potential of using air microbubbles (MB) for scaling mitigation in MD. Addition of MB in the feed was found to reduce membrane scaling, which was confirmed by electron microscopy and quantification of salt deposits on the membrane. In the presence of MB, scaling-induced flux decline was observed at a critical concentration factor of 2.70, compared to 1.97 in the absence of MBs. Scaling mitigation by MB was found to be dependent on the MD operating conditions, with better performance at lower feed temperatures (50 °C) and flow rate (0.4 L/min). Continuous MB injection was more effective in preventing scaling than a periodic on/off MB regime, which emphasized that MB mitigate scaling by preventing salt crystal formation on the membrane instead of dislodging the scaling layer formed on the surface. These results highlight the potential of air MB as a chemical-free scaling control strategy for MD treatment of high scaling potential waters.