What is the definition of chlorine residual testing?

Chlorine residual testing refers to measuring the concentration of chlorine species that remain in water after the chlorine has reacted with contaminants and has met the demand of the water. This measurement is crucial because it indicates how much chlorine is still available to provide disinfection and protect against pathogens. The residual is essential for maintaining the safety and quality of drinking water, as it ensures that there is enough chlorine present to continually disinfect the water throughout the distribution system.

In contrast, the amount of chlorine added pertains to the dosage applied initially, which does not reflect what is left in the system after all reactions. The volume of water after treatment does not provide any information about residual chlorine, nor does it measure chlorine's effectiveness in disinfection. Additionally, the time taken for chlorine to decompose is not relevant to residual testing itself but rather considers the stability and longevity of chlorine in various conditions. Thus, the focus of chlorine residual testing is specifically on the concentration of chlorine that remains effective after it has interacted with the water.