Rapid sand filter

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The rapid sand filter is commonly used in municipal water treatment facilities.

Water flows down through the filter medium, which is sand with suitable size and uniformity characteristics. The water may flow under gravity or pressure.

Rapid sand filters must be cleaned frequently, usually daily, by backwashing, which involves reversing the direction of the water. During backwashing, the bed is fluidized and care must be taken not to wash away the media.

[edit] Advantages

It has a much higher flow rate than a slow sand filter;

[edit] Disadvantages

The rapid sand filter is not an adequate treatment on its own.
It requires greater maintenance than a slow sand filter and generally requires mechanical pumping of water, at least for backwashing. For this reason, it is not usually classed as an appropriate technology.