Effective control of dissolved carbon dioxide is essential for maintaining optimal water quality in recirculating aquaculture systems (RAS). In low-intensity aquaculture systems that rely on air-based aeration, carbon dioxide removal occurs alongside the addition of dissolved oxygen. However, high-intensity systems that use pure oxygenation and operate with low water exchange rates face the challenge of carbon dioxide accumulation, which can reach toxic levels. The safe threshold for dissolved carbon dioxide varies depending on species, developmental stage, and water quality, but generally, concentrations should remain below 15–20 mg/L to prevent adverse effects.
Unlike oxygen and nitrogen, carbon dioxide concentration in water is governed not only by gas-liquid equilibrium but also by acid-base reactions. Gas-liquid equilibrium influences the transfer of carbon dioxide between air and water, and the acid-base reactions determine the chemical form in which dissolved inorganic carbon is present in water. As a result, dissolved carbon dioxide levels are a function of both total dissolved inorganic carbon and the water’s pH.
This presentation will address the fundamentals of carbon dioxide control in intensive recirculating aquaculture systems, focusing on state-of-the-art strategies and real-world applications. Topics will include physical gas transfer mechanisms and the implications of carbonate chemistry on carbon dioxide management. A steady-state mass balance approach will be presented as a framework for future system designs, with case studies of unit processes and control strategies to illustrate key design considerations.