Hydrodynamic–interfacial-area coupling governs oxygen mass transfer in fine-bubble jet aeration systems

Micro- and nano-bubbles (MNBs) have attracted increasing attention for enhancing gas–liquid mass transfer. However, a unified understanding of how hydrodynamic conditions govern interfacial-area generation and the associated oxygen-transfer and energy-efficiency responses is still lacking. This, in turn, limits scenario-oriented optimization and engineering scale-up of MNB aeration systems. In this study, a fine-bubble jet aeration system was investigated at pressures (P) of 0.3–0.6 MPa, recirculation frequencies (f_rec) of 4–87 h⁻¹, and liquid-to-gas ratio (L/G) of 8.6–50. The responses of k_La, standard oxygen-transfer rate (SOTR), standard oxygen-transfer efficiency (SOTE), and standard aeration efficiency (SAE) were evaluated using regression, principal component, and SOTE–SAE Pareto-frontier analyses. The Pareto frontier revealed a clear tradeoff between oxygen utilization and energy efficiency: SOTE was dominantly enhanced by increasing L/G, whereas SAE was primarily constrained by L/G and f_rec, with standardized effect coefficients of approximately 0.70, −0.65 and −0.50, respectively. Bubble-size analysis showed that oxygen generated a 1.12-fold larger effective interfacial area and a 1.10-fold higher liquid-side mass-transfer coefficient than air, resulting in a 1.23-fold increase in k_La. These findings demonstrate that oxygen transfer in fine-bubble jet aeration is governed by the coupled effects of operating parameters and interfacial-area generation, and they provide a multi-metric framework for process optimization, operation control, and scale-up design of aeration systems.