In mass transfer processes such as waste treatment and bio-reactions, it is common to carry out these processes in an aeration vessel in which gas, such as oxygen and/or air, is introduced into a biodegradable liquid for treatment. These aforementioned processes are oftentimes utilized by municipalities and industry to treat waste water wherein the object of the process is to introduce air to the liquid and then micro-organisms in the liquid proceed to use this oxygen to digest the waste. The gas is commonly introduced by way of impellers wherein the impellers aerate the liquid.
During the aeration of a liquid, for example, waste water treatment, it is common to employ impellers which are especially adapted for use in surface aeration of liquids in an open tank where the impellers are disposed at the surface of the liquid in vessel. Typical surface aerators commonly used in the art are generally either radial flow impellers or pitched blade turbines. The blades are usually flat rectangular plates which are pitched usually at an angle of 45° to the axis of rotation of the impeller. This is frequently termed the static level of the liquid. The aforementioned impellers are commonly located close to the static level liquid surface and a small portion of the width of the blade may project up through the surface. Typically, when the impeller is pitched forwardly, the upper edge of the blade is termed the leading edge while lower edge is termed the trailing edge. Alternatively, typically when the impeller is pitched backwardly, the upper edge is the trailing edge while the lower edge is the leading edge. The liquid is usually either pushed out in front of the angled blade and/or scooped by the blade and discharged radially across the surface of the tank with some of the liquid being sprayed into the atmospheric air from the outer upper surfaces of the blade. As a result of the spraying of the liquid into the atmospheric air, the liquid becomes aerated.