What is the minimum contact time required for adequate disinfection using chlorine when the residual is 0.2 mg/l?

The minimum contact time required for adequate disinfection using chlorine, particularly with a residual of 0.2 mg/l, is tied to its effectiveness in eliminating pathogens in water. At this level of chlorine concentration, studies and guidelines indicate that a contact time of 20 minutes is necessary to achieve satisfactory disinfection.

This minimum contact time is based on the ability of chlorine to penetrate and inactivate various microorganisms effectively. In this context, the time factor is crucial because the chlorine needs sufficient duration to interact with and kill harmful bacteria, viruses, and other pathogens present in the water.

Shorter contact times, such as 5 or 10 minutes, may not provide sufficient exposure for chlorine to achieve effective disinfection at this low residual concentration. Therefore, the selected answer of 20 minutes aligns with established guidelines for maintaining water safety and ensuring that harmful microorganisms are effectively disinfected at a chlorine residual of 0.2 mg/l.