The performance of fine bubble diffused aeration systems are conventionally evaluated by such characteristic criteria as specific standard oxygen transfer efficiency (SSOTE), transfer number (N-T) and oxygen transfer coefficient (K-L alpha(20)), but these criteria cannot directly show the variation of air demand with wastewater volume. The ratio of air demand to the wastewater volume (named as air-water ratio) can directly relate the air demand to the wastewater volume, and it is more convenient for designing the aeration systems. The standard oxygenation performances of fine bubble diffused aeration systems in clean water and wastewater, measured in 47 cylindrical tanks, are analyzed in this study. Two corresponding relationships between air-water ratio and dissolved oxygen (DO) concentration are established to estimate the effect of air-water ratio on DO concentration. Within the ranges of parameters studied, the DO concentration is an increasing function of air-water ratio which increases with such two factors: (1) the internal pressure difference between the initial air bubbles entering aeration tank and the terminal air bubbles breaking up from the water surface; (2) the diameters of the initial air bubbles. To verify the validity of air-water ratio, this paper compares the performance of three fine bubble diffused aeration systems evaluated by the ratio with those by SSOTE, N-T and K-L alpha(20), and the process of the calculation of their design and operation based on air-water ratio with those based on the three criteria. The results show that their evaluations are consistent with each other and the process of the calculation based on air-water ratio is more convenient and direct than that based on the other three criteria. (c) 2007 Elsevier B.V. All rights reserved.