What is meant by "residual" in water treatment terminology?

In water treatment terminology, "residual" refers to what is left over after a specific process has taken place. This term can apply to various contexts within water treatment, such as the remaining disinfectant, like chlorine, after water has been treated, or the leftover materials or byproducts following a filtration or chemical process. Understanding the concept of residual is crucial in evaluating the effectiveness of water treatment processes and ensuring that the water meets safety standards for consumption.

For instance, measuring the chlorine residual in treated water helps operators determine if there is enough disinfection to kill remaining pathogens while ensuring the water is safe. Additionally, being aware of the residual contaminants helps in assessing water quality and the need for further treatment or remediation.

The other options do describe certain aspects of water treatment but do not capture the full scope of what "residual" signifies in this context. The residual refers specifically to leftover materials or substances after treatment processes.