What is the primary advantage of placing a properly sized anthracite coal layer over manganese greensand?

• A. greater iron-holding capacity by the greensand
• B. longer filter run times
• C. higher degree of coal and greensand mixing, aiding adsorption
• D. less floc penetration into the greensand; thus, more iron sticks to the greensand

Answer: (B)

The main idea is that a properly sized anthracite layer on top of manganese greensand creates a two-media bed that works like a built-in prefilter. Anthracite has high porosity and larger, lighter particles, so it provides a more open, forgiving top layer that traps larger solids and slows the passage of fines into the greensand. This protects the reactive greensand bed from clogging and keeps the overall bed permeability higher for longer, which means headloss builds up more slowly. With the greensand staying cleaner longer and flow being distributed more evenly, you can run the filter longer between backwashes. That extended filter run time is the primary practical advantage.

The other ideas don’t capture the main effect. The greensand’s iron-holding capacity isn’t increased by the coal layer itself; the layer’s benefit isn’t to mix the media or enhance adsorption on greensand, and while the top layer may reduce some floc penetration, the key outcome you’re aiming for is longer run times between maintenance.