For follow-up testing after chlorination of a contaminated domestic water system, when should the first test be conducted?

The timing for follow-up testing after chlorination of a contaminated domestic water system is critical for assessing the effectiveness of the disinfection process. Conducting the first test 3-7 days after chlorination is essential because this timeframe allows sufficient time for any residual chlorine to dissipate and for the system to stabilize. It also provides an opportunity to evaluate whether the chlorination effectively reduced or eliminated the presence of Legionella and other pathogens.

Testing too soon, immediately after chlorination, may not yield accurate results because residual chlorine levels might remain high, potentially masking any existing contamination. Waiting for a longer period, such as 14 days or more, may increase the risk of the potential reintroduction of contaminants or conditions that could foster bacterial regrowth. Therefore, testing within the 3-7 day range strikes a balance that allows for an accurate assessment while minimizing the risk of false negatives. This approach is aligned with best practices for ensuring the long-term safety and reliability of the water system following chlorination.