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

• A. 1-2 in GAC
• B. 12 in GAC
• C. 6-8 in anthracite
• D. 18 in anthracite

Answer: (D)

When refining greensand filtration, a top cap layer is used to create a stable, high-porosity surface that protects the bed and aids backwash. The cap should be a lighter, coarse media so it doesn’t compact under flow and so it can expand well during backwash, effectively washing fines from the bed and maintaining filtration efficiency. Anthracite is commonly chosen for this cap because its larger, smooth particles form a durable cushion on top of the greensand, reducing bed compaction and promoting even flow paths. An 18-inch depth provides enough material to establish a robust surface, trap fines, and support effective bed expansion during backwash without wasting headroom. A thinner cap, such as 6–8 inches, may not prevent fines from migrating and can lead to channeling; using another media like GAC isn’t standard for this cap because it doesn’t offer the same hydraulic characteristics or surface stability as anthracite.