Abstract
The grains of binary or ternary-media water filters are cleansed by backwashing with water at a rate sufficient to fluidize the entire bed without mixing two or three kinds of filter media. Theoretical study which regards a backwashing of filters as a binary or ternary solid liquid fluidized bed shows that superficial velocity of backwashing water must be (1) greater than the minimum fluidization velocity of grain, (2) less than the terminal velocity, (3) less than the velocity at which bulk density of the heavier and smaller grain's layer is equal to that of the lighter and larger grain's layer and (4) less than the critical velocity at which mixing of different kinds of grains begins. In order to calculate the critical velocity, the model is developed on the basis of the force balance equations of each grain and validity of the model is confirmed by experimental results. The range of superficial velocity and diameters satisfying the segregation conditions of sand, anthracite and garnet are illustrated.
