Nitrogenous waste removal in closed freshwater recirculating aquaculture systems using an innovative DHS-USB bioreactor

This study aimed to evaluate the efficacy of a combined Down-flow Hanging Sponge and Up-flow Sludge Blanket (DHS-USB) system in managing nitrogenous waste in aquaculture and reducing the need for frequent water changes by mimicking natural nitrification and denitrification processes. The setup included a fish tank, water pumps, and an underground tank for storing filtered water. Fish growth parameters—body weight gain (BWG), standard growth rate (SGR), and feed conversion ratio (FCR)—were measured biweekly, while water quality metrics such as dissolved oxygen (DO), temperature, pH, turbidity, and electrical conductivity (EC) were monitored daily. Total Ammonium Nitrogen (TAN), nitrites, and nitrates were analysed using a UV–VIS spectrophotometer. Results showed that fish in the DHS-USB system had optimal growth, with an SGR of 1.98 and an FCR of 2.22, outperforming those in a pumice aquaponics setup. The DHS-USB system achieved reductions of 34.68 %, 17.54 %, and 43.71 % for TAN, Total Inorganic Carbon, and Total Suspended Solids, respectively. Bacterial density was highest within the DHS system (3.54 ± 0.27 ⨯ 10⁵ CFUs mL⁻ 1 ), supporting efficient waste processing. Overall, the DHS-USB system demonstrated significant potential for sustainable aquaculture waste management, enhancing water quality without compromising fish growth. • DHS-USB bioreactor study contributes valuable insights to the fields of aquaculture engineering and bioreactor design. • The DHS-USB bioreactor system resulted in reduced concentration of nitrogenous waste, inorganic carbon and suspended solids. • The system used mechanical filtration, nitrification, and denitrification to reduce aquaculture waste. • DHS-USB bioreactor boosts productivity and sustainability in fish farming operations.