Optimum Backwashing of Sand Filters

A fundamental theoretical and experimental study of backwashing is presented showing that backwashing with water alone is an inherently weak process because of the absence of particle collision in the fluidized state. Thus, the principal mechanism of cleaning is hydrodynamic shear. Fluidization and backwashing literature are reviewed to substantiate the concepts of: (1) Insignificant particle collisions; and (2) the existence of a maximum for most turbulence parameters at a porosity, ϵ=0.65-0.70. A mathematical theory is developed indicating a maximum hydrodynamic shear and consequently optimum cleaning at an expanded porosity, ϵ around 0.70. Experimental evidence confirming the validity of the theoretical result is presented. The importance of air scour or surface wash in terms of the preceding concepts are examined.