Backwash (physical phenomenon)

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 water treatment

In terms of water treatment, backwash refers to the reversal pumping of water, and sometimes intermittent with compressed air, through the process. Filters get dirty and must be maintained. Filters can be rapid sand filters, pressure filters, granulated activated carbon (GAC) filters (Brita is GAC), 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 material causing the filter bed to appear to boil. This breaks up the compacted filter bed and forces the accumulated particles into suspension. After the air wash cycle stops, the backwash cycle starts with water flowing up through the filter bed. Clean water is passed through the filter bed in order to wash the material and remove most of the accumulated particles. This cycle continues a fixed time or until the turbidity of backwash water is below a set value. In some cases, the additional step of air/water wash (simultaneously) is done after air wash cycle and followed by rinse water wash. These use less water compare to traditional step and have higher removal efficiency which result the cleaner filter.

Backwashing is a form of preventive maintenance to prevent further clogging of the filter medium. Backwashing in water treatment plants is an automated process, usually run by programmable logic controllers (PLCs). The backwash cycle is usually triggered when the differential pressure over the filter exceeds a set value.