Garnet sand is used at the bottom medium in multimedia filters because it is finer and greater density compared to the other sand in the filter. Which option best describes this characteristic?

• A. finer and greater density
• B. coarser and greater density
• C. finer and lower density
• D. coarser and lower density

Answer: (A)

In multimedia filtration, layers are chosen with different grain sizes and densities to optimize removal across particle sizes and to keep the bed stable during backwashing. Garnet sits at the bottom because it is both finer than the other sand and denser. The finer grain increases the surface area and reduces pore size, helping trap smaller particles that pass through the upper layers. The greater density keeps this bottom layer in place and resists being lifted or mixed during backwash, which helps maintain a stable, effective filtration bed and improves overall backwash performance.

So describing this characteristic as finer and greater density captures why garnet is used at the bottom. If it were coarser or less dense, it wouldn’t provide the same depth filtration effect or bed stability.