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Performance of a sand filter in removal of micro-algae from seawater in aquaculture production systems

In this study, a sand filter was used to remove micro-algae from seawater feeding aquaculture ponds. A lab-scale sand filter was used to filter 30,000 cells/mL of Heterocapsa triquetra suspension, a non-toxic micro-alga that has morphological and dimensional (15–20 μ m) similarities with Alexandrium sp., one of the smallest toxic micro-algae in seawater. Removal efficiency and capture mechanisms for a fixed superficial velocity (3.5 m/h) were evaluated in relation to size distribution and mean diameter of the sand. Various sands (average diameter ranging between 200 μ m and 600 μm) were characterized and used as porous media. The structural parameters of the fixed beds were evaluated for each medium using experimental measurements of pressure drop as a function of superficial velocity over a range of Reynolds numbers covering Darcy's regime and the inertial regime. For a filtration cycle of six hours, the best efficiency (E=90%) was obtained with the following sand characteristics: sieved sand with a range of grain diameter of 100 and 300 μm and a mean grain diameter equal to 256 μ m. Results obtained show the influence of the size distribution of sand on the quality of retention of the micro-algae studied.