In a two-stage recarbonation process, CO2 is added after lime to lower the pH to approximately which range?

• A. 9.0-9.5
• B. 10.0-10.6
• C. 9.5-9.9
• D. 10.4-10.7

Answer: (B)

In a two-stage recarbonation, lime is added first to drive the water to a strongly alkaline level, building up alkalinity and promoting precipitation of unwanted hardness. The second stage uses CO2 to dial the pH back down to a controlled high-alkalinity range. The target pH is about 10.0 to 10.6 because at this level the carbonate system remains well buffered, maintaining adequate alkalinity and stability for downstream processes while avoiding excessive corrosion or excessive reduction of hardness. This specific range provides a balance where carbonation has reduced the pH from its peak after lime, but water still retains enough buffering capacity and carbonate species to prevent rapid pH swings or scale issues.