Recirculating aquaculture system applications of oxygen absorption equipment require consideration of the combined effects of the system's physical, chemical and biological components. Interactions of this type were modeled within a recirculating system incorporating a mixed-flow type rearing vessel, a multi-tube clarifier, a rotating biological contactor (RBC) and a U-tube oxygenator. Finite difference mass transfer calculations, based on reactor theory, were used to predict steady-state dissolved gas levels in component effluents given system operating conditions. The model was calibrated and its predictions verified with data obtained from a pilot scale system of 14 m3 capacity: errors in calibrated model predictions (N = 45) average −1·2 mg l−1 (range −4·0 to 0·1 mg l−1). Model use indicated oxygen transfer costs are reduced 48% through recycle of U-tube off-gas. Further savings are provided by increasing the water recirculation rate from 250 to 350 l min−1 with low to moderate fish feed rates and by regulating oxygen injection based on diel variations in fish respiration. Increasing the gas transfer coefficient (KLa) of the RBC reduced oxygen transfer costs despite resultant elevations in dissolved nitrogen and argon concentrations. Carbon dioxide stripping across the RBC was substantial, varied with KLa, and increased with water recirculation rates.
