Water Age and Its Effects on Legionella

Water age is a critical risk factor that needs to be monitored and controlled to reduce chances of Legionella growth. It should be key component in any Water Management Plan. As water sits stagnant, a number of things begin to occur.

Dissipation of chemicals in the water:

Most municipal water facilities inject disinfectants into the water supply to protect it from excessive bacterial growth. As water sits in pipes, these chemicals, often chlorine (Cl2) or monochloramines (NHCl2), will dissipate leaving the water unprotected. If water is continuously moving into and out of a building, it allows a continual stream of protective chemicals into the building. Without this reintroduction of disinfectants, the chemicals will dissipate leaving the water at a higher risk of Legionella and pathogen growth.

Water age risk is exacerbated in warmer or hot water due to the acceleration of disinfectant dissipation for two reasons.

  1. Legionella grow faster in warmer water using up the disinfectants.
  2. Chlorine based chemicals naturally break down faster in hot water.


In piping, stagnation allows bacteria to colonize easier and grow at a much faster rate. Water naturally has Legionella and other bacteria in it, including water treated by the municipality. It is estimated that 20% – 70% of commercial buildings have some level of Legionella present in the water. The real danger lies in water supplies that don’t move in and out of the building. When water is continuously moving, Legionella doesn’t have the opportunity to rapidly colonize and begin to reproduce.

See the diagram below of biofilm growth due to stagnant water in a section of pipe.

How do you manage water age for Legionella control? Ideally, water should not be in a building for longer than 72 hours. To manage water age, it is necessary to monitor how much water enters the building relative to the size of the water system. Hot and cold water lines may need to be put on a flushing protocol depending on the building plumbing, application, occupancy, temperature and other factors.

In addition to water age, be sure to consider scale/biofilm and sediment in your water system.

Legionella / Pathogen Control and Water Management Consulting

Want to keep your customers, patients and employees safe? Contact Legionella Control Systems at info@legionellacontrolsystems.com or 888-416-8626 for Legionella controls systems, including patent-pending water softeners, disinfectant systems, engineering design, consulting, and sales of Legionella control water systems in Indiana, the Midwest and throughout the U.S.

See all of the Legionella risk factors: Water Age (stagnant water), Temperature, Disinfectant Levels, and Scale/Sediment.

Contact Legionella Control Systems for CDC Legionella risk assessment and Legionella testing.