Legionella bacteria can be very persistent in building HVAC and water systems, as recent tragedies have shown. In order to avoid such problems, designers need to consider not just a system’s functionality, but also how it can be maintained.
From the December 2015 print edition, p. 25
Recent outbreaks of Legionnaire’s Disease in Quebec and in New York City have heightened awareness that commercial buildings can be a breeding ground for the potentially lethal bacteria Legionella. Legionella have been associated with buildings’ water systems, including main water lines, cooling towers, evaporative condensors, potable water systems, water heaters and showers.
The transmission of Legionnaires’ disease is not completely understood. Legionella bacteria are present in fresh water and soil, but the problems of an outbreak are generally associated with someone inhaling the bacteria in aerosols, i.e. small particles of contaminated water. Aspiration is another way that the Legionella microbes enter the lungs. Aspiration means choking during drinking, ingesting or swallowing. Aspiration allows fluids and particles, including Legionella, to inadvertently enter the lungs instead of going into the stomach.
Assessing the health risks is difficult because there are no known exposure limits, either in water or air, for the Legionella bacteria. In addition, evidence of person-to-person transmission has not been found. Therefore, attention has focused on the spread of the bacteria in building water systems. Legionella can thrive in warm stagnant water, with optimal growth conditions between 25°C to 42°C. When the circulated air flow of the ventilation system picks up droplets of contaminated water, the bacteria can be transported throughout a building. If the droplets are small enough, they can be inhaled, thus providing a way for the bacteria to enter the lungs.
Many steps can be taken to remove or limit the health risks of Legionella. However, the importance of the design of an HVAC system cannot be overstated since occupants’ health depends on how well the system can be operated and maintained. Designers should therefore take into account not only the functionality of the system, but also the impact of the design on its operation and maintenance.
Guidelines and standards for controlling the problem
To aid in the control of Legionella growth, government agencies and professional organizations have published guidelines not only for the design, but also the commissioning requirements for cooling towers and water systems. These include the Cooling Technology Institute’s “Legionellosis. Guideline: Best Practices for Control of Legionella,” published in 2008.
Very recently ASHRAE produced its ANSI/ASHRAE Standard 188-2015, “Legionellosis: Risk Management for Building Water Systems.”
In Canada, guidance is available from Public Works and Government Services Canada: “PWGSC MD 15161–2013, Control of Legionella in Mechanical Systems.”
There is also a requirement in ASHRAE Standard 188-2015 for commissioning building water systems. It includes procedures for flushing and disinfection, as per the requirements of the American Water Works Association (AWWA) Standards C651 or C652, or compliance with applicable national, regional, and local regulations.
In terms of the ongoing operation and maintenance of building water systems, various guidelines recommend developing a management plan, commonly referred to as a Legionella Bacteria Control Management Plan (LBCMP). There are also standardized preventive measures and proactive water sampling (PWGSC 2013 and AIHA 2015). An LBCMP addresses all aspects of the Legionella issue, including: risk and hazard assessment; periodic water testing; the use of biocides; and the documentation of all actions taken to prevent Legionella growth in the building’s water systems.
The building water systems primarily include:
• heating, ventilation and air conditioning (HVAC) system and components;
• domestic water systems; and
• open water systems and non-potable water storage.
An outbreak in New York cooling towers
As part of large air-conditioning systems, cooling towers produce warm water and aerosols. Legionella bacteria can grow in the warm water, especially if biofilm and scale are present. As well, water droplets, known as drifts, are produced that might carry Legionella and disperse it as far as 10 kilometres from the source.
Measures to control the growth of Legionella within cooling towers and to minimize or eliminate the entrainment of water droplets are well established. They include the proper selection and location of equipment, regular maintenance, and water disinfection with a combination of biocide, rust and scale inhibitors.
However, in an outbreak in the Bronx, New York that occurred this summer, disinfection alone was shown to be insufficient in controlling Legionella re-growth in cooling towers. After an initial outbreak of the disease killed 12 people in July and August in the South Bronx, the city required that every building with cooling towers in the city had to be cleaned within two weeks. A second outbreak in the Bronx in September killed one individual and sickened a dozen. The city health department found that there were at least 15 cooling towers near the second outbreak where the Legionella bacteria had re-grown within a month following disinfection.
Disinfection specialists said the bacteria’s quick return was not surprising because Legionella bacteria will thrive in warmer weather. They also said that the re-growth supported the notion that the cleanings were only a short-term fix. It was also noted that if a treatment program and risk management program are not in place after the disinfection, nine out of 10 times the bacteria will re-grow.
Other potential sources of water within HVAC systems are humidifiers and condensate pans. Steam humidifiers are typically not a problem, although standing water within the temperature range for growth combined with biofilm and rust/scale can be a source for Legionella growth. Humidifiers should be completely drained when they are not in use. Also, condensate pans should be properly sloped and trapped for drainage. Access to condensate pans for visual inspection and maintenance is important but not always possible.
Domestic water system
The PWGSC document states that hot water shall be maintained or stored above 60°C, distributed to each outlet at a minimum of 50°C, and reduced to below 43°C at the point of use. More often than not, however, visual inspections find that there is no thermostat on the hot water tank, or that the hot water tank temperature is kept as low as 45°C to save energy and to prevent scalding.
Cold water systems are generally not a problem for Legionella growth since the water is usually stored below 20°C. However, the water in “point of use” systems such as irregularly used spigots, drinking fountains, emergency eye wash stations, etc. can be found at higher temperatures. These systems often have minimum maintenance performed on them to limit bacterial growth.
The water distribution system should be designed to minimize “dead legs” (sections of pipe that are no longer in use but continue to contain stagnant water) and to reduce the water residence time with the use of recirculating pumps.
Indoor water features – not always pretty
Indoor or outdoor water features such as fountains, waterfalls, and vertical green walls can facilitate Legionella bacterial growth. In decorative water features, the water sprays or cascades over rocks or other materials, which can result in aerosols contaminated with bacteria. The location of these features in foyers or common areas increases the concern for a Legionella outbreak. The water is also often recirculated, potentially increasing the concentration of bacteria or organic matter that feeds the bacteria.
Non-potable water storage is becoming a more common feature in new buildings. Rain water is collected and treated for toilet flushing or outdoor watering.
In these systems, keeping the water below 20°C is important since the Legionella bacteria are dormant below that temperature. It is not always possible to control the temperature of the water feeding green walls, however, since most of the plants have exposed roots that are sensitive to cool water temperatures.
Easy access to the water storage tanks for regular cleaning and removing sediments are essential in controlling Legionella. Some of these water features’ storage systems are considered as confined space and are not easily accessible to maintenance staff. A water treatment program should be implemented with appropriate chemical treatment. If this is not possible because the water is to be used for watering greenery, other control measures such as filtration or reverse osmosis should be considered.cce
Lan Chi Nguyen Weekes, P.Eng., is a partner and senior mechanical engineer at InAIR Environmental, an indoor environmental consulting company in Ottawa. Donald M. Weekes, CIH, CSP, is president and a Certified Industrial Hygienist and Certified Safety Professional at InAIR Environmental.
CTI Guidelines WTB-I48 (08). Legionellosis. Guideline: Best Practices for Control of Legionella. 2008, Cooling Technology Institute, Houston, TX.
Recognition, Evaluation, and Control of Legionella in Building Water Systems. 2015, American Industrial Hygiene Association, Fairfax, VA.
ANSI/ASHRAE Standard 188-2015. Legionellosis: Risk Management for Building Water Systems. 2015, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Atlanta, GA.
PWGSC MD 15161 – 2013. Control of Legionella in Mechanical Systems. 2013, Public Works and Government Services Canada.
AWWA/ANSI C651. Disinfecting Water Mains. 2014, American Water Works Association, Denver, CO.
AWWA/ANSI C652. Disinfection of Water Storage Facilities. 2011, American Water Works Association, Denver, CO.