Which material sits on the top in the standard bottom-to-top arrangement of higher-rate filter media?

Which material sits on the top in the standard bottom-to-top arrangement of higher-rate filter media?

• A. Garnet sand
• B. Silica sand
• C. Anthracite
• D. Gravel

Answer: (C)

In higher-rate multimedia filtration, media are layered from bottom to top by increasing porosity and decreasing density, so the top layer is the lightest and most porous. Anthracite fits this role because it has a lower density and a coarse, highly porous surface. This lets it act as the first line of interception for larger particles while allowing high flow rates with lower headloss. Underneath, denser media like silica sand and garnet provide progressively finer filtration and structural stability, and gravel sits at the very bottom as a support layer for the underdrain system. Gravel is too coarse to be on top because it would not effectively remove fine particles and would clog or carry through at higher rates, while the denser media belong lower to do the finer filtration.

In higher-rate multimedia filtration, media are layered from bottom to top by increasing porosity and decreasing density, so the top layer is the lightest and most porous. Anthracite fits this role because it has a lower density and a coarse, highly porous surface. This lets it act as the first line of interception for larger particles while allowing high flow rates with lower headloss. Underneath, denser media like silica sand and garnet provide progressively finer filtration and structural stability, and gravel sits at the very bottom as a support layer for the underdrain system. Gravel is too coarse to be on top because it would not effectively remove fine particles and would clog or carry through at higher rates, while the denser media belong lower to do the finer filtration.