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In water treatment, including water purification and sewage treatment, backwashing refers to pumping water back through the filter media, sometimes including the intermittent use of compressed air in the process.Backwashing is a form of preventive maintenance so the filter media can be reused.In water treatment plants, backwashing can be an automated process run by a local programmable logic controller (PLC).After a set time interval, a backwash cycle is triggered when the filter effluent turbidity is greater than the treatment guidelines or the pressure differential (head loss) of the filter exceeds the set value.Backwashable water treatment filters include quick sand filters,water softening systems,pressure filters and granular activated carbon (GAC) filters.Diatomaceous earth filters are backwashed according to a proprietary arrangement of pumps,valves and filters associated with the filtration system.Slow sand filters and self-cleaning mesh filters use mechanisms other than backwashing to remove trapped particles.In order to maintain the functionality of water treatment filters, they must be cleaned regularly to remove particulates.Ineffective backwashing is one of the main causes of water treatment filter failure.
Procedure 
Backwashing of particulate media filters involves several steps.First, the filter is taken offline and the water is discharged to a level above the surface of the filter bed. Next,compressed air is pushed up through the filter material,causing the filter bed to expand, breaking down the compacted filter bed and forcing accumulated particles into suspension.After the air flush cycle, clean backwash water is forced upward through the filter bed, continuing the bed expansion and carrying suspended particles into the backwash tank suspended above the filter surface.In some applications, air and water streams are simultaneously pushed up through the granular media, which is then rinsed with rinse water.Backwashing continues for a fixed period of time, or until the turbidity of the backwash water falls below a predetermined value.At the end of the backwash cycle, upward water flow ceases and the filter bed returns to its original state by gravity.The water to be filtered is then applied to the filter surface until the filter becomes clogged and repeated backwash cycles are required.Some water treatment filters use surface cleaning systems to break down heavily clogged surface layers of granular media.Surface cleaning systems are either buried on top of the filter media or suspended above the surface of the filter media.John R. Baylis developed a fixed mesh system consisting of pipes with nozzles that injected water jets into the filter material during expansion.The swivel arm uses water jets to break up clogged filter surfaces and rotate the movable arm through the filter material.The surface cleaning step in the backwash cycle usually occurs at the beginning of the filter bed cleaning process.
Treatment
The used backwash water is either discharged untreated to the sewage sewer system or treated and recycled at the plant.: ‰22.37–22.40‰ Historically,backwash water was discharged directly to surface water sources; however, direct discharge is now strictly regulated through NPDES discharge permits and is generally discouraged.Used backwash water contains high concentrations of particulate matter.Typical treatment processes include coagulation,flocculation and sedimentation.High molecular weight synthetic organic polymers are sometimes added to promote the formation of settleable flocs.Failure of the backwash treatment process and reintroduction of the resulting poor quality water into the main water treatment plant stream can mess up the entire process and result in the production of poor quality treated drinking water.
Recycling
As a water conservation measure, many water purification plants recycle filter backwash water and other product streams from the sludge treatment process back to the beginning of the plant the US EPA issued a final rule on acceptable practices for recycling backwash water.The purpose of the regulation is to improve the control of microbial contaminants such as Cryptosporidium by reducing the potential for recycled product streams to disrupt the removal efficiency of primary treatment processes.The regulations require that used filter backwash water be recycled to the front of the treatment plant so that all available particle removal treatment processes can be employed to remove microbial and particulate matter from the backwash water.
