How long can disinfection residual fall below 0.2 mg/L?

The appropriate duration for disinfection residual to fall below 0.2 mg/L is typically no more than 4 hours. Maintaining a residual concentration of disinfectant is critical for effective pathogen removal and ensuring safe drinking water. When levels drop below this threshold, it could compromise the disinfection process, allowing potential pathogens to survive. The 4-hour limit is generally recognized in water treatment guidelines to maintain the effectiveness of disinfection and ensure public health safety. It balances the need for adequate disinfection while recognizing that occasional, brief drops can happen without immediate risk, provided the duration is limited. Therefore, the choice that specifies this duration aligns with established best practices in water treatment.