Florida Drinking Water Operator "B" Practice Test

If the hardness of the finished water from the lime softening process is similar to the raw water, what might have occurred?

1. Low water temperature

2. Excessive carbon dioxide dose

3. Insufficient lime added

4. High levels of alkalinity

The correct answer is: Excessive carbon dioxide dose

When evaluating the lime softening process, the goal is to reduce water hardness, which primarily consists of calcium and magnesium ions. If the hardness of the finished water is similar to that of the raw water, it indicates that the process was not effective in achieving its intended outcome. An excessive carbon dioxide dose can lead to a condition where the calcium and magnesium ions remain in solution rather than precipitating out as carbonate salts. Carbon dioxide reacts with water to form carbonic acid, which can lower the pH of the water. If the pH is not raised sufficiently by the addition of lime, the precipitation reaction necessary for softening may not occur effectively, thereby allowing hardness ions to remain in the water. This scenario highlights the importance of balancing lime and carbon dioxide in the softening process. Without the proper control of these chemicals, the expected reduction in hardness may not take place, leading to finished water that remains hard, akin to the raw source.