During greensand regeneration, continuous regeneration is typically used while the filter is offline.

• A. While online
• B. While offline
• C. During backwash
• D. During startup

Answer: (B)

Greensand filtration uses a manganese dioxide coating on the media to oxidize iron, manganese, and hydrogen sulfide. When this coating loses activity, the bed is regenerated with potassium permanganate (KMnO4) to restore its oxidizing capability. Because KMnO4 is a strong oxidant and you need to flush out the reaction byproducts, regeneration is done while the filter is offline. This allows the chemical to contact the media fully, lets you waste the spent solution, and prevents any KMnO4 from entering the treated water. After regeneration, the bed is rinsed and the system can be brought back online.

If regeneration were done online, KMnO4 could contaminate the treated water and affect water quality; backwash is a separate cleaning step, not the regeneration step; startup is not the usual time for reactivating the media.