Using Oxygen Gas Transfer Coefficients to Predict Carbon Dioxide Removal

The purpose of this research was to determine if oxygen gas transfer coefficients, as reflected by overall mass transfer coefficient (KLa) values, could be used to predict carbon dioxide (CO2) removal by degassing in aquaculture production systems. The motivation for this approach was that while there is ample literature related to oxygen gas transfer, there is limited information on CO2 removal. A series of tests was conducted to determine the ratio (φE) of KLa for CO2 to that of oxygen for two commonly used surface aerators and then compare φE to the theoretical 22 International Journal of Recirculating Aquaculture, Volume 8, June 2007 ratio, φT, which is 0.90 based upon gas molecular diameters. Experiments were conducted in a 10,000 L circular tank aerated by means of two different surface agitators. The two aerators were selected to represent aeration patterns with high and moderate water to gas interface exposures or breakup patterns (photos supplied, Figures 2 and 3). The results showed that φE/φT ratios were 96% (for high air exposure) and 74% (for moderate air exposure) for water with an alkalinity of ~130 mg/L as CaCO3. The φE/φT ratio decreased to 0.84 and 0.51 for the high and moderate air exposures, respectively, when higher alkalinity waters (~1,000 mg/L as CaCO3) were used.