Which is the proper detention time for disinfecting a water storage tank filled with already chlorinated water such that the free chlorine residual is 10 mg/L after the proper detention time is completed?

The proper detention time for disinfecting a water storage tank filled with chlorinated water, ensuring that a free chlorine residual of 10 mg/L is achieved, is typically around 6 hours. This timeframe is based on common disinfection protocols which account for factors such as the concentration of chlorine, the quality of the water, temperature, and contact time necessary for effective disinfection.

For effective disinfection, maintaining a free chlorine residual at a specified level over an adequate period is crucial. A higher concentration of residual chlorine will allow for a shorter contact time, but a level of 10 mg/L at 6 hours is generally accepted as a standard practice to ensure that pathogens are eliminated effectively.

Longer detention times are not necessary under typical operational conditions, and while shorter times may achieve adequate disinfection, they may not guarantee the desired residual. Hence, selecting a duration of 6 hours aligns with established practices for maintaining effective water quality during storage disinfection.