Continuous bubble reactor using carbon dioxide and its mixtures for ballast water treatment

The treatment of ballast water is indispensable for preventing ecological and economic damage from the spread of invasive species. In this study, a continuous gas bubble reactor (CBR) system was developed for the efficient disinfection of microorganisms in ballast water. Ballast water treatment (BWT) in the CBR was experimentally performed to disinfect Artemia salina in seawater by using 1) pure CO₂ and 2) mixtures with CO₂, N₂, and/or SO₂ as a simulated flue gas (CO₂/N₂: 20%/80% and CO₂/N₂/SO₂: 19.2%/77.0%/3.8%). The BWT efficiency was improved with an increase in gas flowrate, residence time, gas pressure, and CO₂ concentration in the gas. The toxicity of SO₂ in the CO₂ mixture significantly improved the mortality of microorganisms. Since good dispersion of bubbles and effective contact between bubbles and liquid were important factors in the BWT, a 100% mortality rate of microorganisms could be achieved by controlling the operating conditions in the vertical-type CBR with a counter-current flow between the gas bubbles and seawater. The CO₂ gas distribution, CO₂ solubility, and gas bubble size distribution in the CBR were determined using computational fluid dynamics (CFD) and experimentally confirmed using a high-speed camera. Since excess gas can be recovered from a gas-liquid separator before a ballast tank, the CBR system can be operated without using any toxic or explosive gases in an eco-friendly and energy saving manner.