How are backwash rates determined?
The determination of appropriate backwash rates is dependent upon the type of bed involved and the type of backwash employed. In general, while dual and mixed media configurations require that the minimum fluidization velocities of each constituent media material be exceeded, this may not be necessary for mono media beds. When air scour is included in the backwash in a mono media design, it is not necessary to achieve fluidization. However, if a lower backwash rate is used, the length of wash must be increased in order to adequately remove the accumulated solids. As a general rule of thumb, it takes between 100 and 150 gallons of water to backwash one square foot of filter depending upon many factors. In addition, the backwash rate must be sufficient to raise and wash out the solids removed by the filtration process. It also must be of long enough duration to do so.
