In the two-stage recarbonation process, the raw water pH is elevated to ____ or greater with lime; after that stage, carbon dioxide is added to lower the pH to ____

In the two-stage recarbonation process, the raw water pH is elevated to ____ or greater with lime; after that stage, carbon dioxide is added to lower the pH to ____

• A. 10, 9.0-9.5
• B. 11, 10.0-10.6
• C. 10, 9.5-9.9
• D. 12, 10.4-10.7

Answer: (B)

In this two-stage recarbonation, the goal is to shape the carbonate system of the water: first create conditions that promote precipitation and removal of hardness and other impurities, then adjust the carbonate balance so the water is stable and less corrosive in the distribution system. Lime is added in the first stage to push the pH up to about 11 or higher. At these alkaline conditions, calcium carbonate and other compounds precipitate, removing hardness and other impurities and increasing the water’s buffering capacity. This elevated pH also drives the formation of carbonate species that will be useful in the next step. Next, carbon dioxide is bubbled in to lower the pH to a controlled, still-alkaline range. Reducing the pH with CO2 converts some carbonate to bicarbonate and establishes a stable carbonate system that helps buffer the water and reduces corrosion in the distribution system. The intended finished water pH after this stage is typically around 10.0–10.6. So the correct pair reflects raising to about 11 with lime, then lowering to roughly 10.0–10.6 with CO2.

In this two-stage recarbonation, the goal is to shape the carbonate system of the water: first create conditions that promote precipitation and removal of hardness and other impurities, then adjust the carbonate balance so the water is stable and less corrosive in the distribution system.

Lime is added in the first stage to push the pH up to about 11 or higher. At these alkaline conditions, calcium carbonate and other compounds precipitate, removing hardness and other impurities and increasing the water’s buffering capacity. This elevated pH also drives the formation of carbonate species that will be useful in the next step.

Next, carbon dioxide is bubbled in to lower the pH to a controlled, still-alkaline range. Reducing the pH with CO2 converts some carbonate to bicarbonate and establishes a stable carbonate system that helps buffer the water and reduces corrosion in the distribution system. The intended finished water pH after this stage is typically around 10.0–10.6.

So the correct pair reflects raising to about 11 with lime, then lowering to roughly 10.0–10.6 with CO2.