Backwashing (water treatment)

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In the case of water treatment, including water purification and waste treatment, backwashing refers to pumping water backwards through the filter media, sometimes including the use of disconnected from compressed air during the process. Backwashing is a form of preventive maintenance so that the filter media can be reused. In a water treatment plant, backwashing can be an automated process run by locally programmable logic controllers (PLCs). The backwash cycle is triggered after the specified time interval, when the filter effluent effluence is greater than the maintenance guideline or when the head loss in the filter exceeds the specified value.

The washable water treatment filters include quick sand filters, pressure filters and granular activated carbon filters (GAC). Diatomaceous earth filters are backwashed according to the pump settings, valves and filters associated with the filtration system. Slow sand filters and self-cleaning screen filters use mechanisms other than backwashing to remove trapped particles. To keep the water treatment filters functioning, they should be cleaned periodically to remove particulates. Ineffective backwashing is one of the main reasons why water treatment filters fail.

Video Backwashing (water treatment)

Procedures

The washing of granular media filters involves several steps. First, the filter is taken off line and water flowed to a level that is above the surface of the filter bed. Furthermore, the compressed air is pushed upward through the filter material which causes the filter bottom to split up to break down the compacted base layer and force the accumulated particle into suspension. After the air braking cycle, the clean backwash water is forced upward through the filter bed to continue expanding the filter bed and carrying the particles in the suspension into the backwash trough suspended above the filter surface. In some applications, air and water flow are simultaneously pushed upward through granular media followed by washing rinse water. Laundering continues for a fixed time, or until the turbidity of the backwash water is below a predetermined value. At the end of the backwash cycle, the upward flow of water is terminated and the filter bed settles by gravity to the initial configuration. The water to be filtered is then applied to the filter surface until the filter clogs and the backwash cycle needs to be repeated.

Some water treatment filters use a surface wash system that breaks the highly blocked surface coating of the media. Surface leaching systems are buried at the top of the filter media or suspended above the surface of the filter media. John R. Baylis developed a fixed network system consisting of pipes with nozzles that inject the water jets into the filter material during expansion. The rotating arm uses a jet of water to break down the clogged filter surface and rotate the arm that moves through the filter material. The surface washing steps in the backwash cycle usually occur at the beginning of the filter bed cleaning process.

Maps Backwashing (water treatment)

Treatment

Water used for backwashes is disposed of without maintenance to sanitary sewer systems or treated and recycled in the factory. Historically, backwash water was discharged directly into surface water supply; however, direct debit is now highly regulated through NPDES release permissions and is often not recommended. Used backwash water contains high concentrations of particulate materials. Common treatment processes include coagulation, flocculation, and sedimentation. High molecular weight synthetic organic polymers are sometimes added to facilitate the formation of a settleable floc. Failure of backwash treatment processes and poor quality water reintroduction generated into mainstream water purification streams can cause overall process disruption and result in poor quality drinking water production.

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Recycle

As a water conservation measure, many water purification plants recycle the backwash filter water and other product streams from the mud treatment process back to the beginning of the plant. On June 8, 2001, the USEPA issued a final regulation governing acceptable practices for recycling water backwash. The purpose of this regulation is to improve the control of microbial contaminants such as Cryptosporidium by reducing the potential flow of recycled products to disrupt the efficiency of the main treatment process allowance. The regulation requires that recycled spray water should be recycled to the front of the processing plant so that all available particle processing processes can be used to remove microbes and particulate materials from backwash water.

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References

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External links

• National Environmental Services Center - Filter Backwashing Accessed 2012-06-20.
• Backwash Filter - Seymour Capilano Water Plant Video. Retrieved 2012-06-20.
• High Impact Savings Filter Backwash Water Treatment Information Tool Accessible 2012-06-20.
• Filtration: Water Treatment Basics Session III - Technical Higher Learning Videos. Retrieved 2012-06-20.

Source of the article : Wikipedia