Unveiling the role of stratified extracellular polymeric substances in membrane-based microalgae harvesting: thermodynamic and computational insights

Abstract

Membrane separation technology has emerged as a highly energy-efficient method for microalgae enrichment and harvesting in wastewater treatment. However, membrane fouling caused by algal cells and stratified extracellular polymeric substances (EPS) remains a critical barrier to its industrial-scale application. This study meticulously investigates the micro process of algae-derived pollutants stacking to the membrane surface affected by stratified EPS. The fouling process resulting from algal cell particle deposition and cake layer formation are clearly simulated using a semi-coupled computational method of Computational Fluid Dynamics (CFD)-Discrete Element Method (DEM) for the first time. The results reveal that the hydrophilic component and spatial network structure of soluble EPS (S-EPS) effectively impede the algae-membrane adhesion, and enable the algal cake layer exhibit "dynamic membrane" characteristic to enhance the organic matter retention. In contrast, bound EPS (B-EPS) with higher protein content exhibits a stronger fouling potential and adhesion tendency of algal cells. The influence of stratified EPS on the variation of thermodynamic interaction with contact scale in the sphere-plane/sphere-sphere model is inventively conducted. Based on different algal cell filtration modes, a sequential increase in the eigenvalue n was observed by delaminating EPS layer by layer, indicative of a more severe membrane pore blockage. The semi-coupled CFD-DEM method provides a quantitative analysis of the deposition process, offering spatial resolution and force analysis for algal-derived pollutants. Additionally, we propose a novel calculation method to reverse the deposition process based on the particle stress, providing a valuable reference for simulating membrane-based microalgae harvesting under the influence of stratified EPS.