ULTRAFILTRATION TO TREAT GAMETE EFFLUENTS FROM HATCHERIES

Hatcheries might produce non endemic species, such as polyploid oysters, that induces the risk of gametes or larvae propagation in the environment. To protect marine biodiversity, effluents from these structures must be treated but conventional processes present disadvantages: an efficiency dependent on water quality (UV treatment) or a byproducts generation (chemical oxidation). The objective of this work was to evaluate the efficiency of ultrafiltration for gametes removal from shellfish hatchery effluents. The retention by ultrafiltration membranes was determined and the oyster gametes viability was evaluated after treatment for chronic and accidental pollutions.

Tests were carried out with a pilot able to treat 20 m³.d^-1, continuously fed with seawater. This automated filtration unit, equipped with hollow fibre membrane with a pore size of 0.02 µm, included filtration, backwash and cleaning steps. First, sustainable conditions for which a moderate degree of fouling occurs, have been determined with seawater filtration under different conditions of flux and cleaning frequency. Then, two types of effluents were treated by addition of spermatozoa (spz) or oocytes from oyster Crassostrea gigas, to simulate (a) a chronic pollution: low concentrated effluents during a long time and (b) an accidental pollution: highly concentrated effluents during a short time to reproduce an accidental release of biological material during oyster maturation process. Flow cytometry analyses were performed to determine gametes concentrations and assess their integrity before and after ultrafiltration.

Whatever the gamete treated or their concentration, retention rate was 100 % with a removal superior to 3 log. For instance, spermatozoa concentration measured in permeate was lower than detection limit (Figure 1). Hydraulic performances, continuously monitored on the period of the tests (>6 months), remained stable confirming that the process is adapted for this application. Furthermore, flow cytometry analyses and microscopic observations highlighted an impact on integrity of oocytes and spermatozoa in the case of backwash and more especially when membrane was first drained with air. In the case of spermatozoa, a drastic reduction of the number of these species was observed, from 50545 to 10340 spz.mL^-1, reflecting a loss of integrity (Figure 1).

To conclude, ultrafiltration is an efficient process to protect environment biodiversity towards biological contamination because (i) a total retention is obtained whatever the concentrations (i.e. type of pollution) and type of gametes (ii) an impact on integrity of these contaminants was highlighted for particular cleaning procedure of membranes and (iii) the sustainability of the process was demonstrated.