Application of down-flow hanging sponge - Upflow sludge blanket system for nitrogen removal in Epinephelus bruneus closed recirculating aquaculture system

A high-rate nitrogen removal process is required to maintain the water quality in aquaculture tanks used to produce high-density fish populations in recirculating aquaculture systems. In this study, a combined pilot-scale system of down-flow hanging sponge (DHS) reactors and upflow sludge blanket (USB) reactors was applied as a nitrogen removal system for a longtooth grouper Epinephelus bruneus recirculating aquaculture system. The DHS reactors demonstrated high total ammonium nitrogen (TAN) removal efficiencies and the TAN concentration in the aquaculture tank was maintained at 0.32 0.12 mg-N.L-1. The USB reactor fed with sodium acetate acted as a denitrification unit and adjusted the pH through denitrification reaction. All E. bruneus survived the entire experimental period and the daily weight gain rate of 0.6%"day(-1) was comparable to that in marine aquaculture. This result shows that the DHS-USB system to be a novel applicable system for nitrogen removal processes in recirculating aquaculture systems and further study at a large scale should be recommended.