Florida Drinking Water Operator "B" Practice Test

What does the application of sodium hexametaphosphate to the feedwater of an RO membrane help to do?

1. Increase membrane pressure

2. Decrease the amount of calcium sulfate precipitation

3. Enhance flavor

4. Eliminate chlorine residual

The correct answer is: Decrease the amount of calcium sulfate precipitation

The application of sodium hexametaphosphate to the feedwater of a reverse osmosis (RO) membrane primarily helps to decrease the amount of calcium sulfate precipitation. Sodium hexametaphosphate acts as a threshold inhibitor, effectively preventing scale formation by binding to calcium and sulfate ions in the water. This action reduces the likelihood of these ions combining to form insoluble calcium sulfate, which can lead to scaling on the membrane surface. Scaling can significantly decrease the efficiency and operational lifespan of RO membranes, so using sodium hexametaphosphate is a common practice to maintain optimal performance and reduce maintenance costs associated with cleaning and membrane replacement. While some of the other options may seem plausible, they do not directly relate to the primary function of sodium hexametaphosphate. It does not increase membrane pressure since its function is more about preventing precipitation rather than affecting pressure dynamics. Enhancing flavor is not a role of sodium hexametaphosphate, as it is primarily used for its technical benefits rather than for modifying taste. Additionally, sodium hexametaphosphate does not eliminate chlorine residual; chlorine can be harmful to RO membranes, but sodium hexametaphosphate is not used in this context for chlorine removal.