Top impeller is provided for maximum oxygen transfer.
The extra bottom impeller is provided for thorough mixing. It works as a mixer, so mixing energy is reduced & due to thorough mixing the oxygen transfer efficiency of the impeller is increased & hence with lesser HP more oxygen transfer is achieved, hence energy saving. Secondly, due to the bottom impeller the whole mask is mixed thoroughly & it can cater the higher depths upto 5.5 mtrs. SWD in aeration tank.