Investigation of concentration polarization and initial membrane fouling during crossflow ultrafiltration using the Micro-LIF and Micro-PIV techniques

Abstract

Membrane filtration has been increasingly used in water purification and wastewater treatment and reclamation. However, concentration polarization (CP) and membrane fouling remain to be a major concern for the application of membrane filtration in water industry. The formation and development of CP and membrane fouling is closely related to the hydraulic conditions in crossflow filtration. In order to develop effective strategies to mitigate CP and membrane fouling, a sound understanding is needed for the interplay between the mass transport and the hydraulic conditions near the membrane surface. However, a lack of real-time detection tools makes it difficult to characterize the CP and fouling dynamics near the membrane surface. In this study, non-invasive laser-based techniques, i.e., microscopic laser-induced fluorescence (micro-LIF) and microscopic particle image velocimetry (micro-PIV), were applied for the first time to investigate the CP details and initial membrane fouling during crossflow ultrafiltration. Using micro-LIF, the reversible, highly dynamic nature of CP and its sensitive response to the filtration conditions were investigated and validated by in situ visualization of the CP layer near the membrane surface. The CP profile near the membrane surface was well determined by image processing and analysis. The results showed that CP layer varied with the filtration condition and it reached a new steady state in a short period after the filtration condition changed. A higher cross-flow velocity and/or a lower transmembrane pressure (TMP) decreased the CP concentration and thickness. Further quantitative analysis of the filtration test results helped to obtain the particle concentration at the membrane surface and the thickness of the CP layer (30-50 μm). Accordingly, the nature of CP dynamics was characterized and the deficiency of the traditional CP model was explored. The effect of the fluid shear rate on shear-induced diffusion of particles of different sizes and related particle deposition and membrane fouling was investigated with the collaborate application of micro-LIF and micro-PIV. Filtration tests were conducted with 100, 600 and 800 nm particles under a high and a low shear conditions. The particle concentration profile near the membrane surface was depicted by the micro-LIF and the local shear rate at the interface was obtained from the vector map of the micro-PIV. Results showed that membrane fouling by particle deposition could be mitigated by shear-induced diffusion that is affected by the shear rate and particle size. A high shear rate and large particle size resulted in a greater shear-induced diffusion, leading to less fouling. Moreover, membrane modification by surface patterning was experimented for the fouling control purpose. The results showed that the growth of the particle deposition layer on the micro-patterned membrane was much slower than that on the non-patterned membrane. Fluid vortex was observed in the groove between the patterns on the membrane surface and there was a high shear rate region in the upper region of the micro-patterned membrane surface, which explained the mechanism of the observed fouling mitigation by the membrane surface patterning.