The bacterium Legionella pneumophila is the primary cause of Legionnaires’ disease, a severe form of pneumonia that occurs when contaminated water droplets are inhaled. This bacterium is naturally present in freshwater environments, but it proliferates when it enters and colonizes man-made water systems. Chlorine is the most common disinfectant used in water treatment and it possesses the chemical ability to kill Legionella. However, the effectiveness of chlorine in real-world plumbing and water systems is not a simple yes or no answer, as various environmental factors often reduce its lethal impact.
Where Legionella Bacteria Live
Legionella bacteria thrive in the complex plumbing and water systems found in large buildings, hospitals, hotels, and industrial settings. Their preferred habitat includes cooling towers, hot tubs, showerheads, decorative fountains, and large, intricate plumbing networks where water can stagnate. The bacteria grow rapidly within a specific temperature range, typically between 77°F and 113°F (25°C and 45°C). Temperatures above 140°F (60°C) are generally lethal to the microbes.
The bacteria can live and multiply inside protozoa, providing Legionella with nutrients and significant protection from environmental stressors, including chemical disinfectants. The presence of sediment, scale, and other microorganisms creates an ideal, nutrient-rich environment for the bacteria to colonize.
How Chlorine Kills Legionella
Chlorine kills Legionella through oxidation. When chlorine gas or a chlorine compound is added to water, it primarily forms hypochlorous acid (HOCl) and hypochlorite ions (\(OCl^-\)). Hypochlorous acid is the more potent disinfectant because its neutral charge allows it to readily penetrate the bacterial cell wall. Once inside the cell, HOCl disrupts the bacteria’s metabolism and oxidizes essential proteins and DNA, causing irreparable damage.
Successful disinfection is defined by the “CT value,” which represents the product of the disinfectant concentration (C) and the time (T) the water is exposed to it. Maintaining a specific concentration of free chlorine for a sufficient contact time can achieve a high kill rate. However, Legionella is significantly more resistant to chlorine than many other common waterborne bacteria, such as E. coli. For instance, achieving a 99% kill of L. pneumophila at 70°F (21°C) with a very low chlorine residual of 0.1 mg/L can require approximately 40 minutes of contact time.
Why Chlorine Treatment Can Fail
Despite its chemical potency, chlorine often struggles to eliminate Legionella in complex water systems due to physical and chemical barriers. The most significant barrier is biofilm, a protective matrix of microorganisms and organic material that adheres to the inner surfaces of pipes. Legionella often hides within this biofilm, which chlorine has difficulty penetrating. This protective layer can even contain protozoa where the Legionella multiply, making them resistant to chlorine concentrations as high as 50 parts per million (ppm).
Water chemistry, specifically high pH levels, complicates treatment. As the pH increases, the balance shifts away from the highly effective hypochlorous acid (HOCl) toward the less potent hypochlorite ion (\(OCl^-\)). A higher total chlorine dose or a longer contact time is then needed to achieve the same level of disinfection. Physical issues within the plumbing system, such as dead legs, areas of low flow, and accumulated sediment, also prevent chlorine from reaching all parts of the system, allowing bacteria to survive in sheltered locations.
Practical Methods for Water Disinfection
Water management strategies employ chlorine in two primary ways to combat Legionella: continuous residual chlorination and shock chlorination. Continuous residual chlorination involves maintaining a low, steady concentration of free chlorine, typically between 0.5 and 2.0 ppm, throughout the entire water distribution system. This method prevents bacterial growth by ensuring a constant disinfectant presence at the point of water use. However, maintaining this level can be challenging in large systems and may lead to corrosion over time.
Shock chlorination is a high-dose, short-term treatment used to eradicate an existing outbreak or significant contamination. This involves introducing very high chlorine concentrations, often ranging from 10 to 50 ppm, into the system for a defined period. While effective at killing free-floating Legionella, this aggressive approach may not fully penetrate established biofilm. Some facilities now prefer alternative disinfectants like monochloramine or chlorine dioxide for long-term control, as these can be more effective at penetrating biofilm and maintaining a residual throughout the system.