In manganese greensand filtration, what is the typical layer above greensand?

• A. 6 inches of anthracite
• B. 12 inches of sand
• C. No layer
• D. 6 inches of gravel

Answer: (A)

In manganese greensand filtration, a protective top layer of coarse media sits above the greensand to prefilter the water and protect the bed from fines that could clog or wear the greensand. Anthracite coal is commonly used for this top layer because it is lightweight, highly porous, and effective at trapping larger particles without greatly impeding flow. A 6-inch thickness strikes a balance between providing filtration and keeping head loss manageable during operation and backwashing.

Choosing a thicker sand layer would raise head loss and complicate backwashing, while a gravel top wouldn’t filter meaningful particulate and wouldn’t protect the greensand. Having no top layer leaves the greensand exposed to fines and wear, reducing longevity and performance. Hence, six inches of anthracite on top is the typical arrangement.