This study demonstrates that, whatever the quality of the seawater, the ultrafiltration process delivers disinfected water with a quality adapted to shellfish culture
Ultrafiltration process was tested to treat inlet water of shellfish hatcheries and nurseries farms. The aim of the study was to protect animals from pathogens linked to mass mortalities in shellfish culture. A semi industrial process was confronted to two qualities of water: natural pre-treated seawater and an effluent from an oyster breeding. The control of total load bacteria and elimination of vibrio was confirmed in both cases. Indeed, whatever the quality of inlet water, a removal of bacteria was obtained. Moreover, the ultrafiltration sustainability confronted to these two water qualities was validated. Chemical cleanings carried out every 12 hours minimum on hardest conditions of water qualities, led to a performance recover on the period of the tests. Finally, ultrafiltered seawater and effluent were used for fecundation and oyster breedings application. The results obtained, in comparison to prefiltered (1 μm) - UV treated seawater, showed that the ultrafiltration could be applied to shellfish culture.
Shellfish culture is imperilled by the degradation of coastal water quality. Indeed, the profession must face pollution from different origins, variable with season and weather, with different impacts depending on the shellfish species. Since 2008, mass mortality episodes strike oyster cultures at different steps of their lives. Two pathogens were shown as partially responsible of these deleterious phenomena for the profession: Vibrio aestuarianus bacteria and an herpes virus, OsHv-1 (Cochennec-Laureau and Baud, 2011; Renault, 2011). In-land shellfish structures, hatcheries and nurseries may be exposed to these contaminants and then need to control and treat inlet waters to protect animals. Nowadays, physical disinfection using UV treatment is commonly used and more generally in aquaculture, chemical oxidation with chlorine or ozone is also employed (Summerfelt, 2003). These processes were proved efficient against the targeted pathogens, but they present disadvantages such as a treatment efficiency linked to the water quality (UV treatment) or, in the case of chemical oxidation, the generation of disinfection by products which could potentially be harmful for the animals (Gullian et al., 2012; Powell and Scolding, 2016). The consequence is the need of a pre-treatment when the disinfection is carried out with UV, to ensure the satisfying eliminating of contaminants or a post treatment to eliminate byproducts created with chemical oxidation (Lekang, 2013; Ozawa et al., 1991). Therefore, in shellfish culture, there is a need of a disinfection process able to deliver pathogen free water with a quality adapted for the breedings.
The aim of the study is to evaluate the performances of the ultrafiltration process to treat inlet waters in shellfish hatcheries or nurseries. This new process must (i) remove troublesome pathogens, (ii) deliver water with a quality adapted to the different stages of development of the Crassostrea gigas oyster and (iii) demonstrate its ability to face different water qualities and remain stable over time.
Material and Methods
The first part of the study was realised with a semi industrial unit able to treat 20 m.3.d-1 as presented in Figure 1. Membranes were PES hollow fibres (Aquasource) with a Molecular Weight Cut Off (MWCO) of 0.02 μm and a filtration surface of 8 m². Frontal filtration was carried out. Backwashes, air backwashes (backwash with a pre-injection of air in membranes) were performed at a chosen frequency and chemical cleaning was operated when the permeability decreased lower than 300 L.h-1.m-2.bar-1. This pilot was confronted to two qualities of feed water during this study: natural seawater pre-treated with sand filtration (25 – 30 μm) and an effluent of an adult oyster breeding. In the case of natural seawater filtration, the aim was to validate the retention of Vibrio bacteria and total bacteria load. Moreover, the ultrafiltered water was used to realise fecundation of oysters. Fecundation rates were calculated and compared to the ones obtained with a control water treated with a filtration at 1 μm and UV. In the case of the treatment of the effluent of oyster breeding, the retention of Vibrio and bacteria was controlled with harder quality of water to treat. The ultrafiltered effluent was used for oyster spat breedings: the growth of shells in this water quality was compared to a control spat supplied with seawater filtered 1 μm and UV treated. Water quality parameters (physico chemicals and bacteriologic) and the evolution of spats were followed for a period superior to one month. In both studies, bacterial measurements were realised by petri dish culture in marine agar for total bacteria load and TCBS for Vibrio. These Vibrio measurements concerned the whole family of Vibrio and not only the targeted one, Vibrio aestuarianus. Hydraulic performances (transmembrane pressure and permeability) of the filtration unit were continuously monitored, taking account of the temperature, to validate the stability of the process.
The second part of the study was focused on the retention of specific pathogens Vibrio aesturianus and herpes virus OsHv-1 with a lab scale pilot. The same membranes were used and the working surface of 0.14 m² was determined in order to work with the same volumetric concentration factor as the semi industrial unit but with a higher pathogen and virus concentrations. The retention of pathogens was controlled by (i) dipping oyster in permeate water and (ii) injecting permeate in oyster. The mortality of the animals was followed and compared with positive and negative controls.
Results and Discussion
Filtration of Natural Seawater
The conditions of filtration were based on previous studies and literature (Cordier et al., 2018, 2019; Guilbaud et al., 2013). A flux of 60 L.h-1.m-2 and a time of 60 min between two backwashes were then imposed. The evolution of permeability in these conditions and the turbidity on the same period are presented in Figure 2. The turbidity of seawater feeding was not constant because dependant on the environment (weather, tides). Moreover, twice a day, spikes of turbidity generated by the cleaning of the sand filtered that could reach 20 NTU were monitored. Under these conditions of filtration and quality of water, a chemical cleaning of the membranes was carried out about every 40 h and the hydraulic performances remained stable.
Figure 2: Evolution of permeability vs. time [filtration of seawater; J = 60 L.h-1.m-2, tfiltration = 60 min]
The bacterial water quality of the feed and the initial and final permeate was followed under these filtration conditions. The results obtained are presented in Figure 3 a. and b. Ultrafiltration led to a removal of the total load of bacteria and in the case of Vibrio, the graph puts in light a total retention of these microorganisms potentially harmful for oysters and thus, whatever the concentration in feed water.