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 mu m) similarities with Alexandrium sp., one of the smallest toxic micro-algae in seawater. Removal efficiency and capture mechanisms for a fixed superficial velocity (3.5m/h) were evaluated in relation to size distribution and mean diameter of the sand. Various sands (average diameter ranging between 200 mu m and 600 mu m) 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 mu m and a mean grain diameter equal to 256 mu m. Results obtained show the influence of the size distribution of sand on the quality of retention of the micro-algae studied.