The ideal alkalinity for manganese greensand filters for the removal of iron and manganese is _______ as CaCO3.

• A. >120 mg/L
• B. >75 mg/L
• C. >110 mg/L
• D. >100 mg/L

Answer: (A)

Alkalinity acts as a buffer that keeps the water’s pH from dropping during the oxidation steps used by manganese greensand filters. When iron and manganese are oxidized (and when the filter media is regenerated), alkalinity is consumed. Keeping a high alkalinity—greater than about 120 mg/L as CaCO3—provides enough buffering capacity so the pH stays in the range where the manganese oxide coating on the greensand remains active and the removal reactions proceed efficiently. If alkalinity is too low, pH can fall, the media may not oxidize and retain contaminants effectively, and the filter performance can drop. If needed, alkalinity can be raised with lime or other additives to reach that level.