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The grains of a rapid water filter are best cleansed by washing upward with water consistently at a rate sufficient to fluidize the entire bed and for a period sufficient to wash the dislodged floc particles out of the bed and into the wash water gutters. Effective washing cannot be accomplished in operation unless the means are provided in the design and construction of the filter plant. Mathematical relationships are developed for beds fluidized by water, beds fluidized by an air-water mixture, and fluidized multi-media beds. Experimental procedures and data are presented, which were used to develop and substantiate the mathematical relationships, or both, and provide useful design information. Suggestions are given for the design of the bed and filter bottom to achieve effective washing. The value of surface wash and air wash is analyzed. It is noted that when air is applied to a bed which is fluidized with water, the bed expansion is reduced.