In recent years, headlines blaring news of Legionella outbreaks and deaths in hospitals and other public buildings have been raising the alarm of owners and occupants alike about the risks of bacteria in both potable water systems and HVAC cooling towers.
Outbreaks of Legionnaires’ disease across the U.S. have increased 450 percent between 2000 and 2015. According to the Centers for Disease Control and Prevention, about 6,000 cases were reported in 2015 alone. However, because Legionnaires’ disease is likely underdiagnosed, this number is probably underestimated and the actual cases may be higher.
A recent memorandum from the U.S. Department of Health & Human Services’ Centers for Medicare & Medicaid Services Division directed state agency survey directors to reduce the risk of Legionella in healthcare facility water systems.
“CMS expects Medicare-certified healthcare facilities to have water management policies and procedures to reduce the risk of growth and spread of Legionella and other opportunistic pathogens in building water systems,” the memo states. “The Centers for Medicare & Medicaid Service is aware of multiple recent [Legionnaires' disease] outbreaks in hospitals and long-term care facilities as reported by the CDC, state and local health departments, or investigated by State Survey Agencies. Outbreaks generally are linked to environmental reservoirs in large or complex water systems, including those found in healthcare facilities such as hospitals and long-term care facilities.”
Biofilm and Legionella
The problem originates with biofilm, a slimy glue-like substance that can harbor bacteria such as Legionella. It forms on piping materials when biomass adhere to surfaces in wet conditions and provides the bacteria protection from thermal and oxidative disinfection methods including chlorine, monochloramine, chlorine dioxide, and ultraviolet and copper and/or silver ion treatments.
Open circuit cooling towers, closed circuit cooling towers and evaporative condensers have been linked to outbreaks of Legionnaires’ disease. Among the reasons for the outbreaks are the following:
• Periods of sporadic operation.
• Systems that are idle without draining.
• Lack of water treatment.
• Supply water that is contaminated.
The building’s potable water can also affect cooling water because it is typically used to initially fill the system and as make-up water to replace water loss due to evaporation and bleed.
Temperature is another key factor. Legionella’s ideal growth range is 68-120 degrees F. The disinfection range for Legionella starts at approximately 158 degrees F. Temperatures of the water in open-circuit cooling towers can range from around 70 degrees F and reach above 120 degrees F. Water temperatures in closed-circuit cooling towers and evaporative condensers are typically lower.
The good news is that Legionella outbreaks are preventable. In fact, investigations by the CDC found that almost all Legionella outbreaks were caused by problems preventable with more effective water management.
Among the directives in the HHS memo is for surveyors to verify that facilities “conduct a facility risk assessment … ; implement a water management program that considers the ANSI/ASHRAE industry standard and the CDC Toolkit, and includes control measures such as physical controls, temperature management, disinfectant level control, visual inspections, and environmental testing for pathogens; and specify testing protocols and acceptable ranges for control measures, and document the results of testing and corrective actions taken when control limits are not maintained.”
The “ANSI/ASHRAE Standard 188-2015, Legionellosis: Risk Management for Building Water Systems” establishes minimum requirements for design, commissioning, operation, maintenance, repair, replacement, and expansion of new and existing buildings concerning their associated potable and nonpotable water systems and components. It is predominantly intended for owners, managers and designers of building water systems.
Among the places where the standard applies are “human-occupied buildings” containing cooling towers or evaporative condensers.
Project designers meeting ANSI/ASHRAE 188-2015 must identify hazardous conditions, including diagrams for monitoring and controlling the water system and its components.
Designers must provide final installation documents, including monitoring points for confirming proper design temperatures and safety data sheets for materials used. Finally, they must define detailed instructions for commissioning all systems, including procedures for flushing and disinfection.
For facility managers, the following control methods are recommended:
• Maintain hot water storage above 140 degrees F.
• Reduce temperature of water in evaporative condensers and cooling towers to 77-113 degrees F; below the ideal Legionella growth temperature range. (Note: This does not eliminate the need for a cooling water treatment program.)
• Maintain disinfectant residual throughout water system with supplemental disinfection treatment of chlorine, chloramines, chlorine dioxide or copper silver ions.
Where decontamination is necessary, the following counteractive treatments are suggested:
• Chemical shock using elevated concentrations of chlorine or chlorine dioxide for a period of time.
• Thermal shock maintaining elevated temperatures at the outlets while flushing for a period of time. (Note: The ASHRAE Guideline is in process of being revised and was not finalized at the time of this writing. Thermal shock is highly cautioned and the final wording for proper recommendation has not been determined.)
Pipe material considerations
Another major consideration is the material of the piping itself, whose characteristics, such as surface roughness and impact from sanitization, can impact the likelihood of a Legionella issue.
Biofilm provides a safe harbor for Legionella growth, and some materials create more biofilm-friendly environments than others, due, in part, to surface roughness – pipe materials with a smoother surface have less potential for biofilm growth. CPVC is one of the smoothest, with consistent surface roughness of 12.1 micrometers (µm).
Treatments recommended for disinfection also can have an impact on all plumbing system components. Disinfection for Legionella requires chemical treatments including elevated levels of chlorination and/or temperatures requiring a deeper understanding of piping material limitations and strengths.
CPVC is exceptionally useful for handling corrosive fluids up to 180 degrees F. CPVC is a recommended piping for chlorine dioxide and sodium hypochlorite conveyance in water treatment facilities as well as used by the chlor-alkali industry for chlorine production piping, ducting and vessel lining.
However, other plastics such as PEX and polypropylene require testing per “ASTM F2023, Oxidative Resistance to Hot Chlorinated Water,” and are evaluated at 4 ppm chlorine, pH 6.8, 80 psi and temperatures 73.4 degrees F and 140 degrees F. Long-term requirements may be reduced due to frequent or continuous exposure to conditions of aggressive water quality, higher pressures or temperatures, which may cause premature oxidation and eventual brittleness. Polypropylene manufacturers don’t recommend the use of chlorine dioxide without their prior approval.
While it is vital to conserve water and energy, the number one requirement will always be to protect the health of the public. Designing new, and protecting existing, plumbing systems is no exception to our welfare.
Considerations of biofilm growth potential, disinfection treatments, temperatures, and system materials all need to be taken into account. Resources such as ANSI-ASHRAE 188-2015 and the CDC Toolkit, along with industry associations and manufacturer representatives, provide support in designing systems and implementing long-term maintenance strategies to prevent the growth of Legionella in water systems.
Constant research and education will help bridge the science and engineering communities, resulting in maintaining safe water supply in all forms of human-occupied buildings.
Jeff Ramey is a piping systems consultant for Lubrizol.