Procédés de traitement de l'eau de mer en conchyliculture pour la sauvegarde et le maintien de la qualité des mollusques bivalves

The aim of this work is the study of seawater filtration process to preserve commercial bivalves and main tain their quality during harmful algal blooms. The originality of this work is based on the study of rustic processes, capable of removing toxic microalgae from seawater. Two filtration processes have been studied, the immersed hollow fibres membranes and a sand filter. The origin and nature of fouling mechanisms have been identified in connection with the selectivity and energy consumption . Suspensions of toxic and nontoxic microalgae were reconstituted and then filtered through 10 kDa, 300 kDa and 0.2 µm membranes and a sand filter (mean grain diameter dg equal to 250-520 µm). Microfiltration membrane 0.2 µm has emerged as the most suitable for the filtration of seawater : more than 99 % of micro-algae were eliminated for a specific energy consumption below 0.5 kWh/m3. Fouling mechanisms were investigated by membranes autopsies and the use of membrane filtration models. Sand filtration does not allow to retain ail microalgae. Nevertheless, a decrease in the average size of sand grains significantly increases the toxic micro -algae retention (Rejection rate equal to 90 % after 6 h of filtration with dg = 256 µm). Sand filtration does not allow a total retention of toxic microalgae, then it can only be proposed as a seawater pretreatment system before membrane filtration. Despite a higher cost, immersed hollow fibres membranes can be considered for the protection of shellfish basins