Synthetic fibre as biological support in freshwater recirculating aquaculture systems (RAS)

Abstract The aim of this study was to evaluate the use of synthetic fibre as a biological support for the adhesion of nitrifying bacteria in an aquaculture recirculation system (RAS). It was developed from three assays over 120 days. In the first assay, the synthetic fibres used as biological support were introduced in tanks of biological filtration of the system for posterior respirometry analysis and scanning electron microscopy (SEM). Respirometry and SEM were performed 10 days after inoculation with nitrifying microorganisms. Water quality parameters were monitored daily, and the respirometry showed that the bacteria in this assay were consuming the following amounts of ammonium: concentrations [C1]35.369 mg NH 3 /L, R 2 = 0.9912; [C2]51.628 mg NH 3 /L, R 2 = 0.9883; [C3]79.494 mg NH 3 /L, R 2 = 0.986; and [C4]215.225 mg NH 3 L, R 2 = 0.9934. In the second assay, a 1920-L tank was stocked with 120 Nile tilapia, Oreochromis niloticus , with an initial weight 32.11 ± 7.6 g and a biomass of 3.8 kg. After 60 days, the tank and its contents were assessed to evaluate zootechnical parameters and physical–chemical parameters of water quality. From these results, a third assay was developed in which the biomass of fish was increased to challenge the recirculation system. The tank was stocked with 480 jundia Rhamdia quelen (initial weight 11.34 ± 2.4 g and biomass 5.4 kg) for 60 days. In both the tilapia and jundia assays, the fish were fed four times per day with a commercial diet of 35% crude protein and 42% crude protein, respectively, at 5% of each individual fish’s body weight. At the end of the zootechnical assays, the synthetic fibres used showed efficient biological support for bacterial growth, as confirmed by scanning electron microscopy. The fibres also demonstrated maintenance of the water quality, which allowed good fish growth in the recirculating aquaculture system, and the maintenance of up to 11.19 kg/m³ of biomass of fish.