The most common higher-rate filter media consists of the following (bottom to top)

The most common higher-rate filter media consists of the following (bottom to top)

• A. Sand, Silica Sand, and Anthracite
• B. Garnet Sand, Silica Sand, and Anthracite
• C. Silica Sand, Garnet Sand, Gravel, and Anthracite
• D. Cobble, Gravel, Silica Sand, and Anthracite

Answer: (B)

In rapid filtration, the media are layered by density and size to create a stable, efficient bed that can handle high flow rates. The densest material goes at the bottom, with progressively lighter and finer materials above it, so the bed stays stable during backwashing and provides effective filtration at higher velocities. Garnet on the bottom provides a high-density base that resists compaction and helps maintain bed structure under backwash. Silica sand sits above it to perform the main filtration with its finer grains, while the top layer of anthracite, being coarser, offers good initial particle interception and reduces head loss while protecting the finer layers beneath. Other options introduce gravel or cobble, which are typically considered part of the support underlayer rather than the main filter media stack, so they don’t reflect the common triple-layer arrangement used for high-rate filtration.

In rapid filtration, the media are layered by density and size to create a stable, efficient bed that can handle high flow rates. The densest material goes at the bottom, with progressively lighter and finer materials above it, so the bed stays stable during backwashing and provides effective filtration at higher velocities.

Garnet on the bottom provides a high-density base that resists compaction and helps maintain bed structure under backwash. Silica sand sits above it to perform the main filtration with its finer grains, while the top layer of anthracite, being coarser, offers good initial particle interception and reduces head loss while protecting the finer layers beneath.

Other options introduce gravel or cobble, which are typically considered part of the support underlayer rather than the main filter media stack, so they don’t reflect the common triple-layer arrangement used for high-rate filtration.