Drinking water is not sterile. Legionella and other environmental-source pathogenic (disease-causing) microorganisms are present in most public water supplies, typically in very low or even undetectable concentrations. In the absence of control, conditions in building water systems may support significant growth of these pathogens, which can then exit the system by splashing or in microscopic droplets, for example from showers, sinks, and ornamental fountains.

The microbial ecology of building water systems is defined primarily by biofilm, the slime layer found on all wet surfaces, including the inside walls of pipes and plumbing components. Plumbing biofilms are complex, dynamic microbial communities where most plumbing-associated pathogens flourish. The biofilms impair the effectiveness of most physical and chemical control methods. Legionella, Pseudomonas, Non-tuberculous mycobacteria (NTM) and other clinically important pathogens are known to invade and replicate within protozoa that are associated with biofilms. While inside these protozoa, the bacteria are protected from chemical disinfectants and temperature extremes.

In the absence of control, certain physical and chemical conditions of building water systems can support the growth of Legionella and other waterborne pathogens. These conditions, represented by the acronym STAR*, include accumulation of sediment (S), tepid water temperatures (T), excessive water age (A) and lack of a persistent disinfectant residual (R).


The accumulation of sediment, such as scale, dirt, mineral deposits, etc., in a building water system can significantly increase the survival and growth of Legionella and other waterborne pathogens. Sediment provides a high-surface area structure on which biofilm can grow. It also acts as a thermal insulator protecting Legionella and other waterborne pathogens from high temperatures and as a barrier to chemical disinfectants. Plumbing components that are prone to accumulating sediment include, but are not limited to expansion tanks, water heaters and dead legs.


Water temperature is a significant factor that influences the survival and growth of Legionella and other waterborne pathogens. In laboratory-controlled conditions, Legionella can grow at water temperatures between about 68°F (°C) and 122°F (°C), temperatures often found in many parts of buildings’ potable water systems. The optimal water temperatures for Legionella growth are between about 86°F (30°C) and 108°F (42°C). Legionella begins to slowly die at about 122°F (42°C) and die immediately at temperatures at or above about 158°F (70°C). Maintaining water temperatures that are sufficiently hot or cold throughout a building’s potable water system can be practically challenging, especially when the system includes un-insulated pipes, dead legs, long pipe runs and locations where some taps may go unused for extended periods. Biofilm, debris and accumulated sediment may provide thermal insulation sufficient for Legionella to continue to grow, even when water is colder than 68°F (20°C) or hotter than 122°F (50°C). In building water systems, the temperature and the speed at which Legionella dies are affected by numerous environmental variables, such as pH, salts and minerals, Legionella species, Legionella growth phase and association with biofilms. Similar factors affect the temperature tolerance of other waterborne pathogens. Therefore, the temperature guidance based on laboratory studies may not necessarily hold true for any particular building.

Water Age

Water age, the residence time of the water in the building’s potable water system, is an important factor in the growth of Legionella and other waterborne pathogens. The probability of colonization by and proliferation of Legionella and other waterborne pathogens increases with water age. Excessive water age, frequently associated with low flow conditions, can lead to water temperatures favorable to growth, accumulation of nutrients and loss of disinfectant residual. System design and component selection can affect water age. Water age also is affected by water use patterns, such as variations in occupancy of individual rooms, floors or sections of buildings and fluctuations in water usage associated with seasonality and business cycles. Excessive water age also is common in new construction where building water systems are completed, filled with water and tested, but then remain largely unused for extended periods before the building is occupied and normal water usage is established. Repurposing of facilities can also adversely affect water age. For example, an area used for laundry will use significantly less water if it is repurposed to provide file storage. The extent of colonization by and proliferation of Legionella and other waterborne pathogens due to excessive water age can be significant.

Disinfectant Residual

Drinking water utilities typically add a chemical disinfectant, such as chlorine or monochloramine to the water leaving the treatment plant. The disinfectant is intended to persist and control microbial growth in water throughout the distribution system. The disinfectant residual level (net concentration of disinfectant) begins to decline as soon as water leaves the treatment facility. Buildings farthest from the municipal treatment facility may receive water with little or no remaining disinfectant residual. The lower the concentration of disinfectant residual in the supply water entering the building’s potable water system, the more likely it will contain microorganisms, including Legionella. Even when water entering the building has a high disinfectant residual, this residual may not persist throughout the building water systems all the way to the points of use. Disinfectant residual declines as water age and water temperature increase. Typically, there is no residual disinfectant remaining after the water is heated for use in the building’s potable water system. Lack of a persistent disinfectant residual throughout the building water systems increases the likelihood of growth of Legionella and other waterborne pathogens.

Water Management to Prevent Waterborne Disease

According to CDC, the implementation of water management programs consistent with the methodology in ANSI/ASHRAE 188 can prevent 9 out of 10 waterborne disease outbreaks associated with buildings, including Legionnaires’ disease.

In general, the most effective water management programs employ multiple barriers that address the four key factors (STAR) that support microbial growth: accumulation of sediment (S), tepid water temperatures (T), excessive water age (A) and lack of a disinfectant residual (R) throughout the building’s hot and cold potable water. By controlling these factors in a comprehensive, integrated program, growth-supportive conditions can be effectively managed and plumbing-associated waterborne disease can be prevented.

Contact Us To Schedule a Consultation

Micromanagement offers comprehensive turnkey services for the reliable, regulation-compliant treatment of potable water systems in buildings. We help you protect your patients, residents, guests, staff and visitors from Legionnaires’ disease and other plumbing-associated infections. Call or write to us today to learn more about our services.

*Use of the acronym STAR© in connection with building water systems is subject to copyright owned by Gordon & Rosenblatt, LLC. It is used here with permission.