Legionella is a risk when designing water systems.
by James Peterson
May 14, 2019

Over the last few years, there has been a steady increase in drinking-water associated disease and outbreaks, particularly those caused by bacterial microorganisms. According to the Centers for Disease Control (CDC), a total of 42 outbreaks were reported across 19 states between 2013 and 2014.

More than half of those (57 percent) were due to Legionella, a bacteria that has been determined by the World Health Organization (WHO) to be the most detrimental waterborne pathogen to human health in the developed world.

While aging infrastructure and poor maintenance practices play a role in these statistics, pipes are not the only reason waterborne pathogens are on the rise.

water systemImage 1. Example of a water system design (Image courtesy of Crystal IS)

A new report in Opflow by the American Water Works Association (AWWA) found that approximately 50 percent of all building water systems can harbor Legionella. These systems could include equipment such as water heaters and coolers, hot water tanks, booster pumps, beverage equipment, washing machines and even central heating and air conditioning systems. Therefore, the question around controlling for Legionella is no longer a hypothetical “if,” but rather a known risk that needs to be addressed within the initial design of building water systems.

This new challenge makes disinfection a critical part of water systems design. Both the pathogenic nature of Legionella as well as new guidelines from the CDC and the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) for specific applications of water use in buildings open up new methods of controlling risk at the point-of-use (POU), instead of using more costly point-of-entry (POE) systems.

How Legionella Grows & Spreads

Any vessel or appliance that can store or transfer water can be a potential breeding ground for bacteria. Most water utilities add chlorine to water to control and mitigate bacteria growth as water passes through the pipes and underlying infrastructure.

While this method controls significant growth, opportunistic bacterial growth begins to be supported by dead ends, leaks and aging infrastructure as water moves from the treatment plant to the building.

Without a POE disinfection method, these microorganisms move freely into the premise plumbing. However, once water reaches an appliance, it typically passes through a carbon filtration device, which removes the chlorine, again creating a potentially unsafe environment susceptible to more rapid bacterial growth.

In appliances or taps, where water sits static when not running and warms to the ambient temperature, biofilm can continue to form and harbor the growth of Legionella. Biofilm sticks to and grows on any surface that remains continually damp or wet, providing an ideal environment for a range of bacteria, algae and other microorganisms to grow and spread. The water that subsequently passes through is at risk for contamination. As equipment is turned on and the water begins flowing again, pathogens are released into the water stream. However, flowing water itself is not the only path for Legionella to spread. It can also become airborne through steam, mist or splashing, spreading rapidly to the air throughout the entire building.

Identifying Key Areas at Risk for Legionella Growth

To understand the best solutions to fit specific building needs, identify the areas most at risk. In alignment with CDC and the ASHRAE 188 guidelines, building owners must determine which appliances and pieces of equipment within the building pose potential for harboring bacteria.

Depending on the building, there are different areas to consider. Across most buildings, areas to flag include water storage tanks, water heaters, water filters, faucets, pipes, valves and external hoses and decorative fountains. Hospitals and health care facilities should be conscious of medical devices that rely on water, such as continuous positive airway pressure (CPAP) machines, hydrotherapy equipment, bronchoscopes and even eye-washing stations in medical labs.

In residential buildings, consider showerheads, water filters, humidifiers and air washers, ice machines and hot tubs. Then, there are use-cases such as cooling towers for a data center or centrally installed misters at a theme park.

Once these areas are identified, the next step is to prioritize them based on certain risk factors and criteria.

For instance, health care equipment used in sensitive occupant situations such as elderly or immunocompromised patients should be a high priority due to the increased likelihood of patient infection.

Appliances where stagnation occurs are critical to address, as they can lead to significant concentrations of microorganisms due to uninterrupted growth time.

Locations such as pipes or storage containers where water temperature may approach ambient temperatures create a more supportive environment for bacterial growth and therefore need to be protected.

Apart from the equipment, there are external factors that can lead to bacteria growth. Consider a construction site or, in the instance of a water main break, vibrations and changes in water pressure that can break down biofilm and release Legionella into the water.

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