Legionella is a bacterium that can cause Legionnaires' disease. Which setting is least effective in reducing Legionella risk?

• A. Maintaining hot water above 60°C (140°F), cold water below 20°C (68°F), maintain chlorine residual, flushing dead legs, and system cleaning.
• B. Flushing dead legs regularly and maintaining hot and cold temperatures as recommended.
• C. Allowing water to stagnate.
• D. Using only UV treatment without maintaining temperature or residual chlorine.

Answer: (C)

Preventing Legionella requires keeping water moving and maintaining conditions that inhibit its growth throughout the system. Allowing water to stagnate creates pockets where disinfectant residuals decay, nutrients and biofilms accumulate, and temperatures can drift into the range that supports Legionella growth. In these stagnant areas, bacteria can proliferate and disinfection reaching them becomes unreliable, so the overall risk remains high even if other parts of the system are well controlled.

Keeping water moving through regular flushing, maintaining hot water above about 60°C (140°F) and cold water below about 20°C (68°F), and ensuring a chlorine residual helps continuously suppress Legionella and prevent biofilm formation. Relying on UV treatment alone is insufficient because UV inactivates organisms at the moment of exposure but does not provide ongoing residual disinfection, so bacteria can regrow downstream if temperatures or residual chlorine are not maintained.