In the context of chemical dosing, what does the term 'residual' refer to?

In the context of chemical dosing, 'residual' specifically refers to the amount of chemical that remains in the water after it has been mixed with the source water and reacted with any contaminants or demands present. This is a critical concept in water treatment processes because the residual concentration indicates how effective the chemical was in treating the water and how much of it is still available to perform further reactions or provide ongoing disinfection.

Understanding the residual helps operators determine if they have added the right amount of chemicals to achieve the desired water quality. It also assists in ensuring that chemicals are dosed adequately without excessive amounts that could lead to adverse effects or waste. The other options describe different aspects of chemical dosing but do not capture the essence of what a residual is, which is inherently about what is left over after the chemical has served its purpose in the treatment process.