Gas transfer from air diffusers

The bubble and surface volumetric mass transfer coefficients for oxygen, k_La_b and k_La_s, are separately determined for 179 aeration tests, with diffuser depths ranging from 2.25 to 32 m, using the DeMoyer et al. [2003. Impact of bubble and free surface oxygen transfer on diffused aeration systems. Water Res 37, 1890-1904] mass transfer model. Two empirical characterization equations are developed for k _La_b and k_La_s, correlating the coefficients to air flow, Q_a, diffuser depth, h_d, cross-sectional area, A_cs, and volume, V. The characterization equations indicate that the bubble transfer coefficient, k_La _b, increases with increasing gas flow rate and depth, and decreases with increasing water volume. For fine bubble diffusers, k_La _b is approximately six times greater than k_La_b for coarse bubble diffusers. The surface transfer coefficient, k _LA_s, increases with increasing gas flow rate and diffuser depth. The characterization equations make it possible to predict the gas transfer that will occur across bubble interfaces and across the free surface with a bubble plume at depths up to 32 m and with variable air discharge in deep tanks and reservoirs.