MODELING GAS TRANSFER AND BIOLOGICAL RESPIRATION IN A RECIRCULATING AQUACULTURE SYSTEM

Recirculating aquaculture system applications of oxygen absorption equ ipment require consideration of the combined effects of the system's p hysical, 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 con tactor (RBC) and a U-tube oxygenator Finite difference mass transfer c alculations, based on reactor theory, were used to predict steady-stat e dissolved gas levels in component effluents given system operating c onditions. The model was calibrated and its predictions verified with data obtained from a pilot scale system of 14 m(3) capacity: errors in calibrated model predictions (N = 45) averaged -1.2 mg l(-1) (range - 4.0 to 0.1 mg l(-1)). Model use indicated oxygen transfer costs are re duced 48% through recycle of U-tube off-gas. Further savings are provi ded by increasing the water recirculation rate from 250 to 350 l min(- 1) with low to moderate fish feed rates and by regulating oxygen injec tion based on diel variations in fish respiration. Increasing the gas transfer coefficient (K(L)a) of the RBC reduced oxygen transfer costs despite resultant elevations in dissolved nitrogen and argon concentra tions. Carbon dioxide stripping across the and RBC was substantial, va ried with K(L)a, and increased with water recirculation rates.