Chlorine applied minus _________ equals chlorine residual

The correct answer is chlorine demand. Chlorine residual refers to the amount of chlorine that remains in the water after it has been added and some of it has reacted with contaminants or substances in the water. Chlorine demand represents the quantity of chlorine that is consumed by reactions with organic material, ammonia, and other impurities present in the water.

When you subtract the chlorine demand from the total chlorine applied, what you are left with is the chlorine residual, which is the amount of chlorine still available to act as a disinfectant or to help with water treatment. Understanding this relationship is crucial for effectively managing water treatment processes and ensuring safe drinking water.

The other options do not accurately reflect this relationship. For instance, chlorine dose refers to the total amount of chlorine added to the water, which does not take into account the reactions it undergoes. Combined chlorine refers specifically to the portion of chlorine that has reacted with ammonia or other nitrogenous compounds, while free chlorine refers to the chlorine that remains unreacted and is available for disinfection, neither of which directly defines the remaining residual after demand is met.