Backwash (BW) is the upward flow of water through a resin or filter bed in the water treatment process. It is used to cleanse, expand and regenerate the resin or filter bed.
When fluid is passed through a filter, particulate is left behind and as it accumulates, a couple things could happen. They can either cause an increase of the pressure drop across the filter (and upsetting attached components) or water could be forced through the filter with little or no filtering. This means that the turbidity could increase.
It becomes necessary to flush the filter by backwashing. This can be done automatically by a differential pressure transmitter or on a timed basis.