Backwashing (water Treatment) - Procedure

Procedure

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 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.

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. Fixed grid systems consist of pipes with nozzles that inject 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.