Developments in Backwashing of Granular Filters

Reasons for the renewed interest in granular filter backwashing are presented, especially the more difficult backwashing problem encountered in wastewater filtration. Water fluidization backwash alone is inherently weak because of the absence of particle collisions in a fluidized bed, and the cleaning results from hydraulic shear alone. Wastewater studies of the benefits of backwash auxiliaries such as air-scour prior to water backwash or surface wash (before and during water backwash) show: (1) That such auxiliaries are essential in wastewater filtration; (2) that the two auxiliaries are about equal in their effectiveness; (3) that all evidence of dirty filters is not eliminated by their use; and (4) that the use of air-scour and water fluidization simultaneously is an effective backwash but poses some design and operating problems. Filter design and operating considerations for each backwashing auxiliary are examined.