Gravity Filtration: Water flows from the top of the filter bed, through layers of sand and gravel, and out the bottom, with the filter media trapping particles as it passes.

 In water pretreatment, a sand gravity filter, also known as a rapid gravity filter, uses gravity to draw water through a bed of sand and gravel, removing suspended solids, turbidity, and some microorganisms, and is commonly used in municipal drinking water facilities. 

Here's a more detailed explanation:

How it works:

Gravity Filtration:

Water flows from the top of the filter bed, through layers of sand and gravel, and out the bottom, with the filter media trapping particles as it passes. 

Filter Media:

The sand bed is typically made of coarse sand and other granular media. 

Underdrain System:

A system of perforated pipes at the bottom collects the filtered water and distributes backwash water during cleaning. 

Backwashing:

As the filter clogs with trapped solids, it requires backwashing, where water flows upwards through the filter, dislodging the solids and allowing them to be flushed away.