Mass transfer efficiency of a vacuum airlift—Application to water recycling in aquaculture systems

In aquaculture, oxygen transfer and carbon dioxide stripping are the first limiting factors to fish rearing intensification. In this study we measured the O2 and CO2 mass transfer coefficient (KLa) for a vacuum airlift in fresh (<1‰ salinity) and sea water (35‰ salinity) recirculating aquaculture systems (RAS). The airlift was composed of two concentric tubes: an inner riser tube and an external downcomer tube and can be adjusted at three different heights: 2, 4 or 6 m. Several types of air injectors were tested, delivering different sizes of bubble swarms depending on their porosity and functioning conditions (low or high injection pressure), with air flows varying from 0 to 80 L min−1. Experiments were also carried out at different water circulation velocities and with cold (7 °C) and warm water (22 °C). The best transfer coefficient (KLa) value was obtained at a high air flow rate, a high temperature and with reduced bubble size. Results showed that KLa was not affected by water salinity, but it was slightly affected by water flow (Qw), airlift inner pipe length and vacuum. The presence of vacuum reduces gas solubility in water and facilitates CO2 stripping. The comparison between O2 and CO2 transfers showed that higher KLa values were obtained for O2 than for CO2 in fresh and sea water, probably due to chemical reactions between the CO2 and water. For RAS, the vacuum airlift provides a Standard Aeration Efficiency (SAE) of 1.13 kgO2 kW h−1 and a Standard Stripping Efficiency (SSE) of 1.8 kgO2 kW h−1 or 0.023 kgCO2 kW h−1. In rearing water, CO2 and O2 transfers were negatively affected when feed was added. An empirical model for CO2 mass transfer coefficient prediction was developed and calibrated. Simulation shows a good correlation between predicted and measured values (R2 = 0.87).

► The O2 and CO2KLa, SOTR, SCTR and SAE/SSE of a vacuum airlift were calculated in absorption and desorption and they were compared to other gas transfer systems. ► For other vacuum airlift configurations (pipe lengths and bubble diameters) and working conditions (air flow rates, depression levels, water characteristics), CO2 mass transfer coefficient (KLa) was calculated. ► In rearing conditions, differences in mass transfer efficiency were observed before and after feeding. ► An empirical model for CO2 mass transfer coefficient prediction of the vacuum airlift was developed. A high correlation was observed between predicted and experimental data.