Tennessee Wastewater Operator Practice Exam

How much chlorine must be added to wastewater for effective disinfection?

1. Only the desired residual amount

2. Enough to satisfy the demand plus desired residual

3. Only the amount that is typically used

4. A fixed amount for all situations

The correct answer is: Enough to satisfy the demand plus desired residual

For effective disinfection of wastewater, it is essential to add enough chlorine to not only meet the desired residual levels but also to satisfy the chlorine demand of the wastewater. Chlorine demand refers to the amount of chlorine needed to react with substances in the water, such as organic matter, ammonia, and other constituents that can consume chlorine before it can achieve disinfection. Therefore, simply adding the desired residual amount would not ensure that sufficient disinfectant is present to kill pathogens, as some of the chlorine would be used up in reactions rather than contributing to disinfection. By calculating the total amount of chlorine needed, which includes both satisfying the demand of the wastewater and providing the desired residual, operators ensure that the chlorine reaches a concentration high enough to effectively disinfect the wastewater. This approach promotes better treatment outcomes and protects public health. The other options suggest either insufficient (such as focusing only on the desired residual) or arbitrary methods that do not account for the variability in wastewater composition, which can significantly impact chlorine demand.