Pump-stirred aerator

Abstract

In entrainment-type aerators, liquid pumped by a centrifugal pump is mixed with air, usually entrained through a venturi; the bubbly mixture is then discharged into a tank. As an alternative, the possibility of admitting air directly into the centrifugal pump is considered in this article. In such an aerator - referred to here as a pump-stirred aerator - mass transfer occurs in the tank as well as in the pump. Mass transfer rates are therefore higher than for the case employing air introduction downstream of the pump. For a given air flow rate, the electrical power consumed by the pump is also lower and therefore 20-100% higher values of oxygen transfer efficiency (kg O₂/kWh) have been obtained. The introduction of air into the pump, however, reduces its liquid pumping capacity and hence the mixing intensity in the tank. Comparison of the power dissipation levels in the tank, calculated for the cases - one with air introduction into the pump and the other with air introduction after the pump at the same flow rate - shows that the mixing intensity is reduced by 20% when the enhancement in oxygen transfer efficiency is about 80%. Introduction of air into the pump also lowers noise due to flow-induced vibrations and cavitation noise; this is likely to extend the working "life" of the pump.