How long should free chlorine be used for disinfection of water wells, and at what concentration?

Free chlorine is a commonly used disinfectant in water treatment, particularly for the disinfection of water wells to eliminate pathogens and ensure water safety. The recommended practice is to maintain free chlorine in the well for a sufficient duration to achieve effective disinfection, which balances both time and concentration.

Using free chlorine at a concentration of 50 mg/L for 12-24 hours provides an adequate level to inactivate a range of microorganisms, including bacteria and viruses. This concentration is effective in penetrating biofilms and overcoming any disinfectant demand present in the water system.

The duration of 12-24 hours is crucial because it allows sufficient contact time for the chlorine to act on pathogens. If the contact time is too short, the chlorine may not effectively kill all harmful microorganisms, posing a health risk. Conversely, using excessively high concentrations or extended durations, as in some other options, may lead to lingering tastes or odors in the water and exceed safe thresholds for human exposure when the well is put back into service.

Therefore, the combination of 12-24 hours at a concentration of 50 mg/L is the optimal protocol for disinfection of water wells, ensuring both effectiveness and safety.