Gravity Filtration

Gravity filters, known also as rapid sand filters, use relatively coarse sand and other granular media to remove particles and impurities that have been trapped in a floc through the use of flocculation chemicals (typicallyalum). The unfiltered water flows through the filter medium under gravity or under pumped pressure and the floc material is trapped in the sand matrix.

Mixing, flocculation and sedimentation processes are typical treatment stages that precede filtration. Chemical additives, such as coagulants, are often used in conjunction with the filtration system

Rapid sand filters must be cleaned frequently, often several times a day, by backwashing, which involves reversing the direction of the water and adding compressed air. During backwashing, the bed is fluidized and care must be taken not to wash away the media.

Advantages of gravity filters include:

  • Much higher flow rate than a slow sand filter.
  • Requires relatively small land area.
  • Less sensitive to changes in raw water quality
  • Requires less quantity of sand.


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