In-situ monitoring and understanding of wetting in membrane distillation by means of optical coherence tomography

Despite the rapid development of membrane distillation (MD) as an advanced water treatment technology, membrane wetting is still a major problem, especially for applications with low-surface-tension contaminants. High rejections can no longer be maintained as the feed solution penetrates the membrane pores. Successful wetting prevention requires a comprehensive understanding, which is unattainable with conventional methods. In this work, optical coherence tomography (OCT) was applied to non-invasively study the development of wetting, visualizing the heterogeneity of wetting distribution for the first time. Cetyltrimethylammonium bromide (CTAB) was used to induce wetting in MD experiments. The results confirm that the CTAB concentration strongly influences the wetting rate and distribution. Increasing the concentration from 10 to 50 mg/L led to increasing wetting within hours to minutes. At concentrations ≤ 20 mg/L, wetting was heterogeneously distributed, while at a concentration of 50 mg/L, wetting was relatively homogeneous. Wetting was more pronounced near the inlet of the MD cell than in the middle and near the outlet, thus exhibiting a good correlation with the salt rejection. Additionally, wetting reversibility tests were attempted using in-situ approaches, emphasizing that incomplete restoration of membrane pore hydrophobicity could thwart separation in the subsequent operation.