Bubble mass transfer is a common phenomenon in industrial applications. In this paper, bubble dynamics in both still and turbulent flow were introduced first, followed by the mass transfer properties of a single bubble and bubble swarms. Then, bubble mass transfer models for different scenarios were summarized, including three classical models, extended models, eddy diffusion and whirlpool theoretical models, and semi- or empirical correlations. Finally, existing methods for mass transfer intensification in industries were reviewed. Despite extensive researches, the mechanism for bubble mass transfer has not been fully understood. Models are commonly limited to some specific conditions and the accuracy is limited, especially for bubble swarms and bubble mass transfer in turbulent and non-Newtonian fluids. Also, the mass transfer intensification methods have their own limitations. Additional exploration of knowledge on bubble mass transfer models and further improvement in mass transfer intensification technologies are still required in the future.

Energy dissipation is one of the main considerations in the design of drop manholes and is closely related to the flow regimes observed. Four flow regimes in a drop manhole are classified according to different flow conditions at the inlet and outlet: free overfall flow, orifice flow, pressurized outflow, and fully submerged manhole condition. Theoretical predictions of energy dissipation in drop manholes under different flow regimes compare well with previous experimental data. The drop height limitation for drop manholes is suggested to be 3 m in this study, as larger drop heights can cause the water flow to disintegrate into drops and result in excessive air entrainment. Designs of drop manholes based on different flow regimes are presented. A single drop manhole is also compared with a prior reported stacked drop manhole in terms of energy dissipation. It is found that a well-designed single drop manhole has a similar performance in energy dissipation compared with a stacked drop manhole.