Backwash

In a filtration system, backwash, abbreviated as BW, refers to the process of reversing the flow of water or another fluid through the filter to clean and remove accumulated impurities or contaminants.  The primary purpose of backwashing is to dislodge and flush out the trapped particles from the filter media, thereby restoring the filtration efficiency of the system.

Here's how the backwash process typically works

• Normal Filtration Operation  -  During regular filtration, water flows through the filter medium, which could be sand, gravel, activated carbon, or another material designed to capture impurities.  The filter media traps particles, debris, and contaminants from the water, allowing clean water to pass through.
• Accumulation of Contaminants  -  Over time, the filter medium becomes saturated with trapped impurities, reducing its effectiveness.
• Pressure Drop  -  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.
• Initiation of Backwash  -  To clean the filter, the direction of water flow is reversed temporarily.  Instead of flowing through the filter in the usual direction, water is forced backward through the filter media.
• Dislodging and Removal of Particles  -  The reverse flow of water dislodges and carries away the accumulated particles, debris, and contaminants from the filter medium.  The dislodged material is then directed out of the system as part of the backwash water.
• Disposal of Backwash Water  -  The water containing the dislodged contaminants, known as backwash water, is usually directed to a drain or a separate waste disposal system.  Proper disposal of backwash water is needed to prevent the reintroduction of contaminants into the filtered water or the environment.
• Resumption of Normal Filtration  -  After the backwash process is complete, the normal flow of water through the filter is restored.  The filter is now clean and ready to resume its regular operation of removing impurities from the incoming water.

Backwashing is a maintenance step in various filtration systems, including those used in water treatment plants, swimming pools, and industrial processes.  It helps extend the life of the filter media and ensures the continued effectiveness of the filtration system.