Fish produce dissolved carbon dioxide (CO2 ) as a normal outcome of aerobic metabolism and excrete this gas through the gills into the surrounding water. In recirculating aquaculture systems (RAS) with high stocking densities, CO2 accumulates to unsafe levels unless some form of carbon dioxide control is used. CO2 control unit processes are needed to maintain safe levels of CO2 in the culture tank at high stocking densities. Typical CO2 removal unit processes include a force-ventilated column with or without packing media, a shallow diffused aeration basin, and a passive ventilation trickling filter. Force-ventilated packed columns can remove 40% to 60% dissolved CO2 from RAS water when properly designed (Summerfelt et al., 2000). However, pumping is required to transport water to the top of this unit process. Pumps require capital investment, 24/7 operating costs, and routine maintenance . In this configuration it is critical to maximize removal efficiency of CO2 to minimize the flow that must be pumped . Alternatively, unit processes that remove CO2 using less energy can be utilized if enough performance criteria are known to properly design them to control CO2 in the system.
The goal of the project was to evaluate the removal of CO2 using a low head technology and develop design criteria to optimize such technologies. The project examined a diffused aeration basin that uses diffused air as the stripping gas in a relatively shallow basin. Diffused aeration basins are often used in current RAS designs after the moving bed biofilter. Water gravity flows from the moving bed into the diffused aeration basin and no pumping is required. This study evaluated three different hydraulic retention times (67, 90, and 135 sec), influent CO2 levels (10, 15, and 25 mg/L), and diffused airflow (G:L of 2 and 5) to characterize CO2 removal efficiencies in a research-scale aeration basin that was 1.2 m by 0.9 m by 0.9 m deep (1 m3). The project setup consisted of an influent tank using a low head oxygenator to inject carbon dioxide to achieve the desired dissolved CO2 level and a diffused aeration test tank with a pump sump for returning treated water to the influent tank. OxyGuard International A/S portable CO2 sensors monitored the dissolved carbon dioxide in the influent tank and immediately after the diffused aeration test tank. Diffused air for aeration was provided by a regenerative blower and air manifold at the bottom of the test tank. The results of the tests will be reported in this presentation along with recommendations for optimizing the design of diffused aeration basins.