Optimisation de deux systèmes de production piscicole : biotransformation des nutriments et gestion des rejets

Sustainability of fish aquaculture is currently subjected to environmental protection enhancement and to increasing restrictions of water and energy availability. The aim of this work was to study the biotransformations and waste management in a salmonid french farm (Charles Murgat S.A.S, Isère). Quantification of the wastes is necessary for the farmers and administrations to define control tools and treatment systems in view of water legislation respect (French water low, EC Water Framework Directive). The nutritional method gives a direct relation between feed and effluent fluxes. This method is easier to use than the hydrobiological method, which requires precise flow rate and concentration measurements (difficult and expensive). Solid waste treatment and valorisation (land deposit) are possible. The effluent treatment system (mechanical and biological filtrations and decantations) removed more than half of the solid waste from the rearing water. After one year, the biological and physico-chemical qualities of the recipient river were improved. In the constructed wetlands (horizontal and vertical), the removal rates of suspended solids and biological oxygen demand of the supernatant were 41-44 g.m-2.d-1 and 13 g.m-2.d-1 respectively. With hypoxic conditions in both types of constructed wetlands, thenitrification was only partial. Further experiments on the treatment of supernatants and filtrated waste water by constructed wetlands are necessary in order to meet the effluent standards and/or toreuse the treated waste water. The possibility of trout rearing in a recirculating system was demonstrated. Fish performance and welfare were similar to those of the flow through system of the farm. With the recirculating system, the water consumption was reduced by 6 to 20 times, the rearing tank surface was also reduced, releasing some space for the treatment systems and the water treatment became easier (lower waste flow rate and higher effluent concentration). Life Cycle Assessment (LCA) demonstrated that the recirculating system has a lower environmental impact than the flow through system, except for energy use. Nevertheless, improvements of water and dissolved gas treatment systems could reduce the energy consumption of recirculating systems (16 kWh per kg of fish produced), which is similar to the quantity of energy consumed per kg fish trawled. It could reach in a near future the same (or lower) level of energy consumption as the flow through system (10-12 kWh per kg of fish produced).