What is 'residual chlorine' and why is it monitored?

Residual chlorine refers to the amount of chlorine that remains in water after the disinfection process has been completed. This measurement is critical because it indicates the effectiveness of the disinfection treatment used to eliminate pathogenic microorganisms from the water. Monitoring residual chlorine is essential for ensuring that water treatment processes effectively kill harmful bacteria and viruses, while also providing ongoing protection as the water moves through distribution systems.

Maintaining an adequate level of residual chlorine helps prevent recontamination of the water supply after treatment, ensuring that it remains safe for consumption until it reaches the end-user. The presence of residual chlorine also serves as a proxy for overall water quality, reflecting the conditions within the distribution system.

In contrast, the total chlorine used in the process is not indicative of the ongoing effectiveness of disinfection, and the chlorine content in untreated water does not reflect how well chlorination has succeeded in killing pathogens. Monitoring chlorine levels that pose health risks is significant, but this is separate from measuring residual chlorine, which focuses specifically on ensuring safe drinking water post-disinfection. Therefore, understanding residual chlorine highlights the importance of its role in maintaining the safety and quality of drinking water.