Greensand filters are often capped to enhance filter performance and backwash efficiency. It is common to cap them with...

• A. 18 in anthracite
• B. 6 in anthracite
• C. 12 in GAC
• D. 1-2 in GAC

Answer: (A)

Greensand filters use a top cap to create a protective, pre-filtration layer that helps distribute water evenly and keeps the finer bed material from moving during backwash. Anthracite coal is ideal for this cap because its larger, more porous particles form a coarse, stable layer that resists compaction and channeling, while still allowing water to flow through easily.

A typical cap uses a thick layer of anthracite—about 18 inches. That depth provides enough porosity and depth to trap loose fines, protect the greensand underneath, and improve backwash efficiency by allowing the top layer to expand and lift during backwashing without breaking up the main bed.

Using other materials or thinner caps (like a small layer of GAC or only a few inches of any cap) won’t offer the same benefits. GAC adds adsorption and can complicate bed operation, and a much thinner cap wouldn’t prevent fines from migrating or improve distribution and backwash as effectively.