Aeration device

An apparatus for aerating a liquid, especially wastewater located in a basin, comprising a basin containing the liquid defining a free surface above which a gas space filled with oxygen mixture is provided, a vertically disposed hollow shaft, means operatively connected for rotating said shaft about a substantially vertical axis, a rotatable surface aeration device mounted on said shaft and immersible into said liquid for circulating and aerating said liquid near the surface, a flow guide tube vertically disposed and extending upwardly from a point just above the basin floor to a level just below said surface aerator, said guide tube surrounding said shaft and defining an inlet region at its top end thereof, at least one down pumping impeller disposed in said guide tube and mounted on said shaft for creating a downward flow in said guide tube, said hollow shaft providing a passageway for communicating said gas space above the liquid surface with the inlet region of said guide tube, restriction means for guiding flow of liquid into the inlet region, said restriction means located between the surface aerator and the top end of said guide tube, wherein said downward flow creates a low pressure in said inlet region of said guide tube so that the oxygen mixture is entrained therein by self-aspiration through said passageway, and the mixture of entrained gas and liquid thusly formed and traveling downwardly in said guide tube is discharged at its lower end into said basin.