BACKGROUND: The hydrodynamic conditions of a surface aeration system can be characterized by interpreting the power consumption of the impeller. Power consumed by aeration tanks depends on the geometries of the impeller and tank, which is generally denoted by the power number. RESULTS: The various geometric and dynamic parameters affecting the power number have been studied experimentally. Three flat-bottomed cylindrical tanks of 1, 0.518 and 0.168 m(2) cross sectional area, fitted with six flat bladed impellers were used as baffled and unbaffled circular tanks. CONCLUSIONS: Scale-up criteria have been developed for baffled surface aeration tanks. General correlations have been developed for both baffled and unbaffled aeration systems which are useful in the design of surface aeration tanks. (C) 2010 Society of Chemical Industry