Legionella is a risk when designing water systems.

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.

Designing Water Systems with Disinfection in Mind

Once present in the water system, containing and eliminating bacteria is time consuming and expensive. It is critical to have the right tools and technology in place to mitigate the risk of growth in the first place. There are few regulations that address the threat of Legionella since it has only recently become a growing problem. However, both the CDC and ASHRAE 188 are beginning to define risk management strategies that help water system designers identify and prioritize high-risk areas and situations.

As a result, designers can now use this approach as a quantitative method to decide what scale of solution is most appropriate for a specific system.

A localized disinfection facility for the entire building is certainly an option. One approach is to use chemical dosing that increases the concentration of disinfectants in the water. This can be an effective choice for facilities like hospitals that need to assure safe water at a majority of locations, but the costs and operating requirements can be a burden beyond what many building managers can justify. System designs that find a more limited number of high-risk locations can quickly compare the costs and maintenance of a full building solution to the costs and maintenance of POU systems deployed at each high-risk location.

There are several key technologies available for POU disinfection. UltraViolet (UV) mercury lamp systems are highly effective at reducing microorganisms; however, they require at least annual replacements of lamps at each location and can be prone to scaling, which necessitates additional maintenance and potentially reduces disinfection performance. UV light-emitting diode (LED) systems use similar UV disinfection technology as mercury lamps but with smaller LED chips. This newer technology offers benefits comparable to the LED lightbulbs in houses: less frequent maintenance, consistent performance from on/off cycling and a smaller footprint for installation. Most of these systems are only available for POU flow rates—0.5 to 2 gallons per minute (gpm)—and from a limited number of suppliers.

Finally, high-performance microbial filtration cartridges are made for bacterial reduction, using fine membranes to filter single organisms. These advanced membranes and their small pore sizes generally require expensive annual replacements and have the potential to clog if particulate levels in the water exceed filter ratings.

Taking Ownership of Water Quality in Buildings

Bacteria growth is inevitable in many water appliances, equipment and infrastructure, and water safety in buildings is critical to ensure what ultimately reaches the end-user is safe. Working with building operators to understand these detailed needs and risk potential allows both system designers and equipment providers to better tailor solutions to the needs of water safety and occupant health, while potentially expanding the scope of their contract. Water system designers and suppliers now have a responsibility to collaborate with building owners and operators to ensure protection across the facility.

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