What does the term "demand + residual" equal in water treatment terminology?

In water treatment terminology, the term "demand + residual" refers specifically to the total dose of a chemical applied to the water. The "demand" is the amount of chemical needed to react with contaminants in the water, while the "residual" is the amount of that chemical that remains in the water after the treatment process occurs. The sum of these two components gives the complete dosage required to ensure that sufficient levels of the chemical are present for effective treatment, allowing for both the demand met by the chemical and the desired residual left in the water. This is a crucial concept in ensuring that water treatment processes are effective and that water quality meets regulatory standards.

Understanding this relationship helps water treatment professionals determine how much of a chemical must be added to achieve the desired effect while still leaving an acceptable residual for further treatment stages or for the end-use of the water.