Recirculating aquaculture system with three phase fluidized bed reactor: Carbon and nitrogen removal

The core objective of the study was to evaluate the organic matter and nitrogen removal efficiency in a recirculating aquaculture system for the intensive laboratory-bred rainbow trout. The treatment system consisted of an upflow reactor which operated at a hydraulic retention time (HRT) of 18 and 36 min, a pre-filtration unit with HRT of 4.9 and 9.8 min, a three-phase airlift fluidized bed reactor operated at HRT between 5 and 11 min, a granular unit for the upflow and the fluidized bed reactors effluents filtration with 5.8 and 11.6 min as HRT, and an ultraviolet (UV) unit for the final effluent disinfection. A plastic material was used as support media in the upflow reactor, and granular zeolite with an effective size of 1.30 mm in an 80 g/L constant concentration was used as a carrier for the fluidized bed reactor. Average removal efficiencies of biochemical oxygen demand (BOD) chemical oxygen demand (COD), ammonium, nitrite, nitrate, and total nitrogen were 94.4, 91.7, 52.5, 13.4, 1.3 and 6.0% respectively. In the rainbow trout rearing tanks, there was a 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/day and the final stocking densities were respectively 20.87 and 20.58 kg/m3. The results suggested that the system had the capability to develop a 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, since there were modest values of nitrite and overall nitrogen removal.