How are layers of gravel and sand arranged in a gravity sand-bed filter?

In a gravity sand-bed filter, the arrangement of materials is designed to support the filtration process and facilitate cleaning. The filter media—typically layers of sand or other granular materials—are supported at the bottom by a layer of coarse gravel.1

Modern rapid gravity filters often utilize a multi-layered approach to enhance in-depth filtration:

• Dual-media filters: These consist of a layer of anthracite coal positioned above a layer of fine sand. The upper coal layer captures larger floc, while the lower sand layer traps smaller impurities.1
• Mixed-media filters: Some systems incorporate a third, bottom layer consisting of a fine-grained, dense mineral called garnet.1

Beneath these media layers, the coarse gravel serves as a structural support. The entire bed is situated above an underdrain system, which uses perforated pipes or porous tile blocks to distribute backwash water uniformly when the filter requires cleaning.1