Scale-up of surface aerators for waste water treatment
Laboratory-scale experiments using flat-blade turbines of different sizes (scale 1:3) working on the surface of the liquid have shown that the sorption number Y and the power number Ne are only dependent on the Froude number Fr. Using this information, the sorption correlation Y(Fr) and the power correlation Ne(Fr) for different types of surface aerators were measured on the laboratory scale. Some additional geometric parameters, namely the form and number of blades, the degree of blade enclosure and the ratio of liquid height/aerator diameter were also varied. The validity of these scale-up correlations has been proved by measurements on one large surface aerator of 4.3 m diameter, this being exactly 10 times larger than its geometrically similar model used in laboratory tests.
The conclusion that the sorption number Y and the power number Ne are functions of the Froude number Fr implies that the dimensionless number for the efficiency E [kg O2/kWh], which is derived from these three numbers, is also a function of Fr. From the definition of this number it is apparent that the efficiency of surface aerators is inversely proportional to the square root of the aerator diameter, i.e. the efficiency decreases as the aerator size increases. This implies that only relatively small surface aerators with diameters of approx. 1 m can be recommended.
The paper ends with a critical comparison of the findings of the recently published pertinent literature of the years 1977/78 with the conclusions drawn from this work.
KeywordsWaste Water Treatment Froude Number Surface Aerator Power Number Aerator Type
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