The Water Cycle
Water in a Pool
Backwashing passes pool water back through the filters, hence cleaning them. This should be done whenever the loss of the pressure across the filter reaches the recommended level set by the manufacturer, but at least once a week. This should not be done during bathing, as active disinfection stops during the backwash.
The flow must be fast enough to fluidize sand the sand bed, and must be allowed to settle once the process has been done so that sand does not pass through the rest of the system.
Backwash water is a trade effluent, so consent must be granted from the relevant authority about nature, volume and frequency of the discharge from backwashing and dilution. The local pool in my home town, has permission to dump the effluent straight out to sea, provided it is a certain distance from the shoreline.