Backwash (physical phenomenon)

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Backwash is the backwards flow of air or water. Backwash can be created naturally by waves rebounding off a beach or other obstacle. It is created by propulsion devices such as an oar or engine. Unlike a slipstream, a backwash is harder to travel through than normal, and in sports such as rowing avoiding a competitor's backwash is important.

In terms of water treatment, backwash refers to the reversal pumping of water, and sometimes intermittent with air, through the process. Filters get dirty and must be maintained. This can be Rapid Filters, Pressure Filter, GAC filters etc. To keep them functional, they have to be washed periodically. The process, called "backwashing," involves several steps. First, the filter is taken off line and the water is drained down to the filter bed. Then, the air wash cycle is started which pushes air up through the filter bed causing the filter bed to appear to boil. This breaks up the compacted filter bed and forces the accumulated particles into suspension. The air wash cycle lasts for about three minutes. After the air wash cycle stops, the backwash cycle starts with water flowing up through the filter bed. Clean water or 'sweetened' water is passed through the filter bed in order to wash the media and remove most of the accumulated particles deposited on the media. This cycle continues until the backwash water looks clean. Backwashing is a form of maintenance and increases the lifespan, which is the useful life, of the media.