Which of the following is a common indicator of effective disinfection?

Free chlorine residual is a critical indicator of effective disinfection in water treatment processes. It refers to the amount of unreacted, available chlorine in the water after disinfection has occurred. This residual is necessary to ensure that chlorine is still present in the water to prevent microbial regrowth as it moves through the distribution system. Maintaining a measurable level of free chlorine is essential for safeguarding public health, as it indicates that the water has been adequately treated to kill harmful pathogens.

Chlorine residual should be monitored regularly because it confirms that the disinfection process was effective and that the water remains safe for consumption. Levels that are too low indicate insufficient disinfection, while excessively high levels can pose other health risks. Therefore, maintaining an appropriate free chlorine residual is a fundamental practice in water treatment facilities.

Other indicators, such as color change, presence of chlorine odor, and settling of particles, do not provide direct evidence of effective disinfection. These factors might be associated with water quality, but they do not definitively indicate whether harmful microorganisms have been adequately eliminated.