Local surface aeration at hydraulic structures

In water pollution abatement, the natural gas exchange at the free surface is sometimes not sufficient to balance the oxygen consumption due to pollutants. In such cases, local aeration is needed in order to increase the oxygen transfer into the water body. In this context, hydraulic structures leading to local surface aeration provide the increased oxygen transfer by way of the entrained air bubbles. Thus the air-entrainment and oxygen-transfer characteristics have become an important design criterion for many hydraulic structures. The following material is a summary of the available information about local air entrainment and oxygen transfer, and it is intended to serve as an orientation for preliminary design considerations. Limited results can be presented for a small number of flow geometries. These results can only be used for similar geometries; the flow configurations must really be comparable.