CFD-Based Investigation of the Effects of Aeration Mode and Superficial Gas Velocity on Dissolved Oxygen Distribution and Oxygen Transfer in an Airlift Reactor

Abstract Dissolved oxygen (DO) regulation is essential for one-stage partial nitrification–anammox (PNA), which requires oxygen supply for nitrite production while protecting anammox bacteria from excessive oxygen exposure. In this study, a computational fluid dynamics (CFD) model coupled with a population balance model, species transport model, and gas–liquid mass transfer model was developed to investigate DO regulation in an airlift reactor (ALR). The effects of aeration mode and superficial gas velocity on DO distribution, oxygen transfer, flow-field characteristics, and bubble behavior were analyzed. The results showed that annulus-rising aeration mode promoted broad oxygen dispersion, producing a relatively homogeneous DO field with a low-DO zone fraction of only 5%. In contrast, center-rising aeration mode confined oxygen transfer mainly to the riser and maintained lower DO in the downcomer, increasing the low-DO zone fraction to 52%. Under center-rising aeration, increasing superficial gas velocity from 0.5 to 3.0 cm/s enhanced gas holdup, liquid circulation, interfacial area, and kLa. Passive-tracer analysis further showed that increasing superficial gas velocity shortened the residence times in both the riser and downcomer, while regional tracer transport was faster in the CR-ALR than in the AR-ALR at 1.5 cm/s. A superficial gas velocity of 1.5 cm/s achieved the best balance between oxygen transfer enhancement and low-DO zone preservation. This study provides guidance for constructing spatially differentiated oxygen environments in airlift reactors for one-stage PNA.