Is City Water Treatment Effective at Killing Legionella?

Water treatment is a crucial process that municipalities use to ensure the water we consume is clean, safe, and free from potentially harmful microorganisms. One such microorganism of concern is Legionella pneumophila which can cause Legionnaires’ disease. The disease is a severe form of pneumonia, which can be fatal if not treated promptly. This raises the question: Is city water treatment effective at killing Legionella?

Understanding Legionella

Before diving into the effectiveness of city water treatment, it’s essential to understand the nature of Legionella. These bacteria thrive in warm, stagnant water and can colonize various water systems, including cooling towers, hot tubs, showers, and even large plumbing systems. It’s primarily transmitted to humans when they inhale aerosolized water containing the bacteria, like mist from a shower or a cooling tower.

The City Water Treatment Process and Potential Legionella Treatment Issues

City water treatment typically involves several steps: coagulation, sedimentation, filtration, and disinfection. The disinfection step is most important to our question about Legionella and municipal water treatment, as this is where many microorganisms, including bacteria, viruses, and other waterborne pathogens, are targeted.

A recent study examined the occurrence of Legionella in source water, partially treated water after conventional water treatment using granular activated carbon (GAC), and distribution water that had been treated by the city. Here is what it found:

  • Between 80% and 100% of source water samples were positive for Legionella depending on the testing method.
  • Between 20% and 40% of granular activated carbon (GAC) water samples were positive for Legionella depending on the testing method.
  • The analysis of city water samples yielded mixed results but 20% of biofilms from the distribution system tested positive for Legionella.

The conclusion of the study: It confirmed the potential for human infection by Legionella from city water.

Why is this? Chlorine is one of the most widely used disinfectants in municipal water treatment. It can be effective at inactivating many pathogens including Legionella, so how can Legionella make it through the chlorination process?

To answer this, it’s important to understand that the efficacy of chlorine is influenced by factors like water temperature, pH, and the presence of other organic materials. For example:

  • Legionella can reside in biofilms, which are protective layers formed by groups of microorganisms. These biofilms can shield the bacteria from chlorine, making them harder to eliminate.
  • Legionella proliferates in warm water, between 77°F and 108°F. If there are points in the water distribution system where water remains stagnant and within this temperature range, it provides an environment for the bacteria to multiply, potentially negating the effectiveness of the initial Legionella disinfection.
  • In older cities with aging water infrastructures, there are higher chances of pipe breaks, leaks, and other issues that might introduce contaminants or allow for the growth of Legionella.

So, while city water treatment processes are designed to eliminate a wide range of pathogens, including Legionella, they aren’t foolproof. Factors like water temperature, biofilms, and infrastructure quality can affect their effectiveness. That is why many facilities such as hospitals and assisted living facilities are required to have a Legionella water management plan, conduct regular testing, and have a ready response for Legionella remediation. In reality, every facility should have these to improve safety and reduce potential liability. People who are older or have immunocompromised immune systems are at particularly high risk for Legionnaire’s disease, and virtually every facility in the United States will regularly have those types of visitors, whether they realize it or not.

To help prevent and control Legionella in your facility, contact the independent experts at Legionella Control Systems today at or 888-416-8626. We can help you with Legionella testing, mitigation, remediation, water management plans, and everything you need to keep your facility safe from waterborne pathogens such as Legionella.