Backwashing (water treatment)

Overview

Water treatment filters that can be backwashed include rapid sand filters, water softening systems, pressure filters and granular activated carbon (GAC) filters. Diatomaceous earth filters are backwashed according to the proprietary arrangement of pumps, valves and filters associated with the filtration system. Slow sand filters and self-cleaning screen filters employ mechanisms other than backwashing to remove trapped particles. To keep water treatment filters functional, they have to be cleaned periodically to remove particulates. Ineffective backwashing is one of the main reasons that water treatment filters fail.

Procedure

Backwashing of granular media filters involves several steps. First, the filter is taken offline and the water is drained to a level that is above the surface of the filter bed. Next, compressed air is pushed up through the filter material causing the filter bed to expand breaking up the compacted filter bed and forcing the accumulated particles into suspension. After the air scour cycle, clean backwash water is forced upwards through the filter bed continuing the filter bed expansion, and carrying the particles in suspension into backwash troughs suspended above the filter surface. In some applications, air and water streams are simultaneously pushed upwards through the granular media followed by a rinse water wash.

Some water treatment filters use surface wash systems that break up the heavily clogged, granular media surface layer. Surface wash systems are buried in the top of the filter media or are suspended above the filter media surface. John R. Baylis developed a fixed grid system which consisted of pipes with nozzles that injected jets of water into the filter material during expansion. Rotating arms use jets of water to break up the clogged filter surface and to rotate a movable arm through the filter material. A surface wash step in a backwash cycle usually takes place at the beginning of the filter bed cleaning process.

Treatment

Spent backwash water is either discharged without treatment to a sanitary sewer system or is treated and recycled within the plant. High molecular weight synthetic organic polymers are sometimes added to facilitate the formation of settleable floc. Failure of a backwash treatment process and reintroduction of the resulting poor quality water into the main water purification plant flow stream can cause overall process upsets, 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 sludge treatment processes back to the beginning of the plant. The purpose of the regulation was to improve the control of microbial contaminants such as Cryptosporidium, by reducing the potential for recycled product streams to upset the removal efficiency of the main treatment processes. The regulation requires that spent 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 the microbial and particulate material from the backwash water.

References

References

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2. [http://www.poolcenter.com/de-pool-filter-information.html Diatomaceous Earth Pool Filters] Accessed 2012-06-20.
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6. Baruth, Edward E., ed. (1990). ''Water Treatment Plant Design.'' 4th Edition. New York:McGraw-Hill. {{ISBN. 0-07-141872-5
7. Williams, Robert B. and Culp, Gordon L., eds. (1986). ''Handbook of Public Water Systems.'' New York:Van Nostrand Reinhold. {{ISBN. 0-442-21597-5
8. Baylis, John R. (1959). "Review of Filter Bed Design and Methods of Washing." ''Journal AWWA.'' 51:11 1433-54.
9. "Basic Chemistry of Water & Analysis". Watertreatmentservices.co.uk.
10. "Filter Backwash Recycling Rule". Water.epa.gov.