World Aquaculture Magazine - March 2021

66 MARCH 2021 • WORLD AQUACULTURE • WWW.WA S .ORG concentrations reflecting conditions observed in commercial oyster spat production. For chronic release, filtration of spermatozoa (spz) and oocyte (oo) solutions was monitored for 30 h with initial concentrations around 10,000 spz/mL or 69 oo/mL. For accidental release, effluents were treated for five hours with membrane inlet concentrations around 200,000 spz/mL and 300 oo/mL. To obtain these concentrations, a highly concentrated solution of spermatozoa or oocytes was prepared and then fed into a buffer tank for supplying the membrane filter. This concentrated solution was obtained from sexually mature oyster broodstock (Ifremer, plateforme de Bouin, France). Oocytes were around 30-40 µm and spermatozoa were around 1 µm. Aquasource hollow-fiber membranes were used. The pore size of this polyethersulfone UF membrane is 0.02 µm and the initial permeability is 1000 L/h m 2 bar. With this pore size, ultrafiltration is expected to retain oyster gametes. Each hollow fiber consists of seven channels of 0.9-mm internal diameter, for a total surface area of the UF module of 8 m 2 . The membrane module has been integrated into a semi-industrial pilot and is able to treat 20 m 3 /d. The installation is completely automated and is able to adapt filtration parameters to water quality. A feed tank was continuously supplied with pretreated seawater (Fig. 1). In the case of chronic release, a pump injected a concentrated solution into the feed tank with a controlled flow rate that was a function of the production flux to keep the gamete concentration at the inlet of the module constant. In the case of accidental release, gametes were suddenly injected into the feed tank. To eliminate fouling, one of three membrane cleaning methods was automatically performed by the pilot: classic backwashes (CB), air backwashes (AB), which consists of a first step of membrane draining by air injection of the membrane before backwashing, and chemical cleaning (CEB). Filtration conditions, permeate flux (60 L/h m 2 ), and filtration time (60 min) were pre-selected according to the literature (Guilbaud et al. 2018). The process goal was to have a sustainable flux rate, which occurs at a modest degree of fouling. This rate represents a compromise between capital expenditure, which can be reduced by using a high flux rate, and operating expenses, which can be reduced FIGURE 2. Evolution of permeability versus time (J = 60 L/h m 2 ; t filtration = 60 min). a-c: treatment of chronic release of gametes, b-d: comparison with seawater filtration. A. B. C. D. The objective of this study was to evaluate the efficiency of ultrafiltration for the retention of spermatozoa and oocytes of Crassostrea gigas oysters in the case of continuous release at low concentrations, or intermittent, accidental release at high concentrations, as both may occur in effluents from oyster hatcheries. The viability of these species was studied after treatment by flow cytometry and microscopic analysis. by restricting the fouling rate.