World Aquaculture Magazine - March 2021

68 MARCH 2021 • WORLD AQUACULTURE • WWW.WA S .ORG the treatment. As expected, the concentrations obtained in conventional backwashes were greater than the membrane inlet concentrations. Video analysis of spermatozoa indicated that, while an average of 18 percent of the spermatozoa viewed before filtration were motile, only 7 percent were motile in backwash water. With a volume of water almost identical to CB, it was expected that a much greater concentration of gametes would be found in AB water. Nonetheless, the concentrations were much lower. Flow cytometry clearly showed a strong degradation of the integrity of spermatozoa and this was confirmed by video imaging, where no moving spermatozoa were observed. For accidental release, the increase of accumulation of spermatozoa on the membrane surface resulted in a permeability drop on the first cycle of around 300 and 350 L/h m 2 bar, in comparison to 150 L/h m 2 bar for chronic release treatment tests. If fouling was expected for treatment of accidental release, Figures 4 (a-b) showed the effectiveness of the backwash with a previous air injection for control of fouling. Whatever the operating conditions, the concentration in the permeate was always lower than the detection limit of the flow cytometer. Abatements were, as expected, greater and varied between 3 to more than 5 log. These results confirmed the retention of oyster spermatozoa for different ultrafiltration concentration conditions. Concentrations in backwash waters were greater than in the inlet membrane. As shown for chronic release, spermatozoa were strongly degraded by air injection during the backwashing phases. Near total retention of spermatozoa occurred under the different conditions tested. With control of fouling by special backwashes, ultrafiltration of high concentrations of oyster spermatozoa in hatchery effluent was demonstrated. Ultrafiltration of Oyster Oocytes As with spermatozoa, the evolution of permeability during filtration of chronic release of oocytes versus time demonstrated that fouling was moderate during treatment of those effluents. Indeed, the permeability drop over the test duration was 200 L/h m 2 bar for a filtration time of 60 min (Figs. 2 c-d). An example of evolution of concentration versus time during a filtration cycle is shown in Figure 5. Oocyte concentrations at the inlet of the membrane were close to target concentrations between 45 and 150 oo/mL, with an average of 82 oo/mL. These analyses highlight the absence of oocytes in permeate, irrespective of test conditions. For these tests, oocytes were completely retained by ultrafiltration. The calculated abatement over a filtration cycle varied between 2.5 and 4.4 log. Concentrations recovered in backwash waters were greater than those measured at the membrane inlet. Previous air injection during backwash to drain the membrane had no effect on the number of oocytes recovered during backflush. However, flow cytometry revealed an impact of air injection on the integrity of oocytes recovered in the backwash water. Image analysis (Fig. 6) clearly showed the degradation of oocyte structure in the case of backwash preceded by air injection. For accidental release, two tests were carried out for the same operating conditions of filtration. Permeability monitoring over the 5 h of tests was presented in Figures 4c-d. The permeability drop observed on the first cycle, 350 and 250 L/h m 2 bar, compared to the test simulating a chronic release (<100 L/h m 2 bar) reflected the impact of oocyte concentration on membrane fouling. The importance of backwash with or without a previous air injection was, as with spermatozoa, illustrated by the emergence of fouling. The effect of air backwashes was more marked at high concentrations. The absence of oocytes detected by flow cytometry in the permeate demonstrated the total retention of oyster oocytes by ultrafiltration, even in the case of a highly concentrated effluent, a concentration four times greater than the one in the case of chronic release under the same filtration conditions. The calculated abatements were between 4.6 and 5.1 log. As with chronic release, flow cytometry revealed the impact of air backwash on the physical integrity of oocytes. FIGURE 5. Evolution of oocytes concentration versus time (chronic release; J = 60 L/h m 2 ; t filtration = 60 min). FIGURE 6. Oocytes before filtration and in backwash water.