What is the required minimum alkalinity (as CaCO3) for manganese greensand filters?

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

Answer: (B)

Alkalinity serves as a buffering shield for pH during manganese oxidation and the regeneration cycle in a manganese greensand filter. When manganese is oxidized, acidity is produced, and the regeneration step with potassium permanganate consumes alkalinity further. If the water doesn’t have enough buffering capacity, the pH can drop, which harms the media’s ability to remove manganese and shortens its life. To keep the process reliable and the coating stable, a relatively high minimum alkalinity is specified—greater than 120 mg/L as CaCO3. If alkalinity were lower, pH could fall too much during treatment, leading to poorer manganese removal and more frequent maintenance.