What substance can cause the consumption of lime when present in high concentrations in water?

• A. Free carbon dioxide
• B. Hydrogen sulfide
• C. Chlorine
• D. Amonium

Answer: (A)

The consumption of lime in water treatment processes is significantly influenced by the presence of free carbon dioxide. When free carbon dioxide is dissolved in water, it forms carbonic acid, which leads to a decrease in pH. In order to neutralize this acidity and increase the pH back to desirable levels, lime (calcium hydroxide) is added. The lime reacts with the carbonic acid to form calcium carbonate, thereby consuming the lime in the process.

High concentrations of free carbon dioxide can lead to an increased requirement for lime in water treatment plants because it directly contributes to the acidity of the water. Therefore, the presence of free carbon dioxide necessitates the addition of more lime to balance the pH and ensure optimal water chemistry for treatment.

In contrast, while other substances like hydrogen sulfide, chlorine, and ammonium can interact with water treatment processes, they do not primarily cause the consumption of lime in the same manner as free carbon dioxide. Hydrogen sulfide may create issues in terms of odor and toxicity but does not have a direct effect on lime consumption for pH adjustment. Chlorine is typically used for disinfection and does not require lime for its function. Ammonium can convert to ammonia, especially in alkaline conditions, but again,