The performance of an aerobic three phase fluidized bed reactor for the treatment of surface water in the culture tanks effluent, and an anoxic-anaerobic reactor for the bottom effluent of the rearing tanks treatment is show in this study. The main objective of the study was to evaluate the organic matter and nitrogen removal efficiency in a recirculating aquaculture system (RAS) for intensive rainbow trout laboratory culture. The treatment system comprised of an upflow anoxic-anaerobic reactor operated at hydraulic detention time (HDT) of 18 and 36 min, a pre-filtration unit with HDT of 4.9 and 9.8 min, before the three phase airlift fluidized bed reactor operated at HDT between 5 and 11 min, and a granular unit for the anoxic-anaerobic and the fluidized bed reactors effluent filtration with 5.8 and 11.6 min as HDT, along with an ultraviolet (UV) unit for the final effluent disinfection. Plastic material was used as support media in the anoxic-anaerobic reactor, and granular zeolite with effective size of 1.30 mm in an 80 g/L constant concentration was used as carrier for the fluidized bed reactor. Mean removal efficiencies of biochemical oxygen demand (BOD), chemical oxygen demand (COD), total ammonia nitrogen, nitrite, nitrate and total nitrogen were 94.4, 91.7, 52.5, 13.4, 1.3, and 6.0% respectively. At the culture units, with water volume of 125 L and water exchange rates of 125 and 250 L/h, there were no registered mortalities; the calculated daily weight gains were 1.55 and 1.51 g/d and the final stocking densities were respectively 20.87 and 20.58 kg/m3. The results suggested the system capability to develop nitrification process for maintaining water quality characteristics within the recommended values for rainbow trout farming, but total nitrogen was not effectively removed due to the weak denitrification process due to the low values of nitrite and overall nitrogen removal.

Keywords: Fish culture, biochemical oxygen demand (BOD) removal, nitrogen removal, recirculation system, biological treatment.