In sand filtration systems used in land-based aquaculture, backwashing is a critical performance factor for maintaining effective operation, but flow conditions and design guidelines to improve backwashing efficiency remain unclear. This study evaluates the motion of sand and sludge driven by seawater flow and examines the influence of the strainer geometry through experiments and three-dimensional computational fluid dynamics (CFD) analysis. In the CFD model, seawater and sand were treated as a Eulerian multiphase flow to compute their interactions, while sludge was treated as a discrete phase model whose motion was calculated based on the seawater–sand flow. The results show that reducing residual sludge requires two key mechanisms: (1) increasing the average flow velocity around the tank bottom to enhance the mobility of settled sludge, and (2) expanding the distribution area of upward flow to promote vertical transport of suspended sludge toward the outlet. The residual sludge rate predicted by the numerical model was within a maximum error of 9%. Based on further detailed analysis, design guidelines for the strainer were proposed: specifically, the strainer diameter should be set to maximize the area-averaged flow velocity at the tank bottom, and the slit height should be designed to maximize the distribution of upward flow within the tank.
ABE et al. (Wed,) studied this question.