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The culture of microalgae is important for the production and maintenance of bivalves. One of the major challenges is to maintain the reliability of microalgae forages over the long term. The aim of this work is to use Ultrafiltered (UF) seawater to cultivate them. Thus, cultures in a volume of 300 L of 2 species of microalgae Tetraselmis and T-isochrysis, were monitored in UF water (membrane pore size: 20 nm) and in sea water usually used on the Ifremer mollusk experimental platform of Bouin (France) (Prefiltration, 3 filtrations and 2 UV). The major result is the securing of microlagae cultures with the absence of parasites in all cultures supplied with ultrafiltered water, unlike analyses of the various control cultures. In the case of T-isochrysis, 3 cultures out of 4 resulted in higher microalgae concentrations, up to 30%, in ultrafiltered water thus bringing a benefit on the algal density. These conclusions and the ease of recovering water (linked to the reduction in treatment stages) allowed a transfer of technology. In fact the 300 L cultures hitherto carried out on the experimental platform are now produced in ultrafiltered water since early 2019.

Shellfish profession is jeopardized by water quality problem that concerns inlet, with the need to protect the animals from pathogens contaminations, and effluents potentially harmful for the environment with the presence of pathogens, nutrients or organic matter. In this study, ultrafiltration was tested to answer these issues. The objective of the work was twofold: (i) treat a real effluent from an oyster breeding, the pilot had to continuously face a water containing organic matter and pathogens and (ii) use ultrafiltered water to feed an oyster spat. The process was proved to be efficient in terms of total suspended solids (TSS) and bacterial retention, and especially for Vibrio bacteria, some of whom are potentially harmful for shells. The sustainability of the process facing this pollution was demonstrated and thus for different filtration conditions. Indeed, backwashes and air-backwashes performed were efficient enough to control the fouling generated, so a chemical cleaning was necessary about every 12 h. Water quality parameters, physico-chemical and bacterial, of ultrafiltered effluents were similar to the one obtained with a classical seawater used to feed oyster spats. Ultrafiltration was efficient to treat an effluent from oyster farm and produce water allowing the grown of juveniles. This process could be a solution to reuse effluents in shellfish farms. Highlights ► Ultrafiltration process was efficient to treat shellfish effluents. ► The sustainability of the process facing this pollution was demonstrated. ► An oyster spat was successfully bred with ultrafiltered effluents. ► A protection of the juveniles from pathogens was obtained.

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