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What is bed expansion?

Bed expansion in media filters is defined as the level of increase in the volume of filter media. There is a resting or a working bed depth and an expanded bed depth. This is the level to which the media needs to rise to during upflow or backwash.

Types of media that require backwashing and expansion to remove trapped foreign particles include:
  1. Filter Sand
  2. Zeolites
  3. Glass
  4. Ion Exchange Resins
  5. Anthracite
  6. Granular Activated Carbon
  7. Greensand and others
Expansion is caused by a flow of water (and air where used) at a certain velocity (depending on the bulk density of the media) to lift and separate the media bed. Usually, bed expansion is expressed as the percent of increase in bed depth.