In manganese greensand filters, the greensand is usually capped with at least _____

• A. 6 in anthracite
• B. 6 in of coarse sand and 6 in anthracite
• C. 12 in garnet
• D. 6 in coarse sand and 12 in anthracite

Answer: (A)

In manganese greensand filtration, the active filtration happens in the greensand, which oxidizes and removes iron and manganese. To keep that active bed stable and provide a clean, even surface for filtration, a coarse cap of media is placed on top. A six-inch layer of anthracite is used because its coarse,High-porosity nature creates a protective surface that resists compaction, helps prevent fines from migrating into the underdrain during backwash, and promotes more uniform flow through the bed. It also reduces wear on the greensand and improves backwash effectiveness by allowing the bed to expand and settle more predictably.

Other options introduce different depths or media that don’t offer the same protective benefits, which can lead to higher headloss, more channeling, or faster degradation of the active greensand layer. Hence, at least six inches of anthracite is the standard cap.