Continuous regeneration (CR) is usually used in greensand filters when ________ is mostly being removed and intermittent regeneration (IR) when __________ is mainly being removed. The _________ regeneration is used while the filter is offline.

• A. Iron, manganese, IR
• B. Manganese, iron, CR
• C. Iron, manganese, CR
• D. Manganese, iron, IR

Answer: (A)

Greensand filtration relies on a manganese oxide coating to oxidize and remove iron and manganese from water. How the coating is regenerated depends on which contaminant dominates.

Continuous regeneration keeps the coating effectively cycling on-line, which works best when iron is the primary contaminant being removed. Iron is readily oxidized and precipitated on the MnO2 surface, so a steady, on-line regeneration helps maintain consistent iron removal without interrupting service.

When manganese is the main target, the coating needs periodic restoration, which is accomplished by regenerating the bed offline. This intermittent regeneration allows a more thorough recharge of the coating with MnO2 and is used to keep manganese removal effective over time.

Therefore, the correct fit is: continuous regeneration is used when iron is mostly being removed; intermittent regeneration is used when manganese is mainly being removed; and intermittent (offline) regeneration is used while the filter is offline.