Performance characterization of a diffused aeration basin for carbon dioxide removal in RAS

Fish produce dissolved carbon dioxide (CO 2 ) as a normal outcome of aerobic metabolism and transfer this gas through their gills into the surrounding water. Dissolved carbon dioxide control unit processes are necessary to maintain safe levels of CO 2 in the culture tanks of recirculating aquaculture systems (RAS) with high stocking densities. The project's goal was to evaluate the removal of CO 2 using low-head technology and to identify design criteria for optimizing such technologies. The study examined an aeration basin that used diffused air as the stripping gas in a shallow water column. Diffused aeration basins are often used in current RAS designs after a moving bed biofilter. This research evaluated hydraulic loading rates (204, 306, 407, 611, 815, and 1182 L min⁻¹ m⁻² (5, 7.5, 10, 15, 20, and 29 gal min⁻¹ ft⁻²)), three influent CO 2 levels (10, 15, and 25 mg L⁻¹), two diffused airflow rates (G:L ratio of 2 and 5), and two basin depths (0.97 and 0.48 m (38 and 19 in)) to assess CO 2 removal efficiencies in a research-scale aeration basin. Results showed that aeration basins designed with hydraulic loadings of 407–815 L min⁻¹ m⁻² (10–20 gal min⁻¹ ft⁻²) and a G:L ratio of 5 achieved 50–60 % CO 2 removal but decreased to 30–35 % as hydraulic loading reached 1182 L min⁻¹ m⁻² (29 gal min⁻¹ ft⁻²). Basins with hydraulic loadings of 407–815 L min⁻¹ m⁻² (10–20 gal min⁻¹ ft⁻²) and a G:L ratio of 2 achieved 40–50 % CO 2 removal, but removals fell to 20–30 % at a hydraulic loading of 1182 L min⁻¹ m⁻² (29 gal min⁻¹ ft⁻²). These removal efficiencies suggest that aeration basins can be designed to control CO 2 over the typical range of concentrations found in RAS. • This project identified design criteria for dissolved carbon dioxide (CO 2 ) removal using diffused aeration basins. • Aeration basins 0.97 m (38 in) deep with a hydraulic loading rate of 407–815 L min⁻¹ m⁻² (10–20 gal min⁻¹ ft⁻²) achieve 50–60 % CO 2 removal. • Shallower basins exhibited lower performance but were operational within practical design limits. • Hydraulic loading rates over 1222 L/min⁻¹ m⁻² (30 gal min⁻¹ ft⁻²) and retention times < 60 s significantly reduce CO 2 removal efficiency.