Backwashing (water treatment)

In terms of water treatment, including water purification and sewage treatment, backwashing refers to pumping water backwards through the filters media, sometimes including intermittent use of compressed air during the process. Backwashing is a form of preventive maintenance so that the filter media can be reused. In water treatment plants, backwashing can be an automated process that is run by local programmable logic controllers (PLCs). The backwash cycle is triggered after a set time interval, when the filter effluent turbidity is greater than a treatment guideline or when the differential pressure (head loss) across the filter exceeds a set value.

Water treatment filters that can be backwashed include rapid sand filters, 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.

Backwashing of granular media filters involves several steps. First, the filter is taken off line 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. Backwashing continues for a fixed time, or until the turbidity of the backwash water is below an established value. At the end of the backwash cycle, the upward flow of water is terminated and the filter bed settles by gravity into its initial configuration. Water to be filtered is then applied to the filter surface until the filter clogs and the backwash cycle needs to be repeated.

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