The safety of the U.S. water drinking system is regulated by the standards set by the U.S. Environmental Protection Agency (EPA) under the Safe Drinking Water Act. Those standards are designed to protect public health by limiting the levels of contaminants in drinking water, including microorganisms, organic and inorganic chemicals, and disinfectants and their byproducts.

According to the Center for Disease Control, over 90% of Americans get their tap water from municipal water systems subject to these standards, with chlorine being the most widely used disinfectant. However, when water sits stagnant in pipes, disinfectant residuals that protect water as it moves through the distribution system dissipate and bacteria can begin to proliferate. As water conservation efforts increase, stagnant water becomes more likely, which can provide more time for nutrients like assimilable organic carbon compounds and minerals to accumulate, serving as food sources for bacterial growth. When these microorganisms aggregate and attach to a surface, they form biofilm.

Piping Materials and Biofilm Formation

In the right conditions, biofilm can form inside the pipes of potable water systems and host bacteria and viruses, including legionella. Biofilm develops slowly over time and can form a complete film coating the pipe interior or exist as small patches on the pipe’s inner surface

The rougher the surface area of the pipe’s inner wall, the easier it is for the compounds secreted by microbes to attach themselves to the pipe surface and form biofilm. As a result, biofilm formation potential—the likelihood of a surface to develop biofilm growth—varies by material type and can also be impacted by the age of the pipe. Copper, for example, has a very low biofilm formation potential early in its life when the inner wall of the pipe is smooth but becomes much more hospitable to biofilm as it ages and corrosion makes the inner surface rougher.

The biofilm formation potential of plastic piping systems is more a function of the material and its microscopic surface characteristics than age. A controlled study conducted by Kiwa, a respected international testing and inspection institute in The Netherlands, found that several times more legionella developed in PEX and PP-R piping compared to CPVC over the same time period.

Piping Materials and Biofilm Mitigation When legionella is detected in a potable water supply system, ASHRAE 188-2018 requires corrective actions which may include tactics like elevated temperatures above 140° F and/or shock chlorination to raise the oxidative reduction potential (ORP) of the water for accelerated disinfection. Unfortunately, these tactics may have harmful effects on some plastic and metal piping systems.

The Plastics Pipe Institute (PPI), in their Technical Note 53, defines multiple conditions in which chlorinated water can increase the likelihood of premature failure in PEX. Those conditions include temperatures above 140° F, pressures above 80 psi and an ORP above 825 mv. These factors should be kept in mind when specifying piping materials and, for facilities required to have a legionella mitigation plan, when developing that plan.

The ORP of the water flowing through pipes is outside the control of plumbing engineers and contractors but is relevant because a higher ORP can indicate a higher volume of disinfectant residuals, which helps protect against bacterial growth. That’s good for water safety but creates conditions that can be aggressive to other piping, such as PEX, PP-R and copper.

The temperature limitations defined by PPI TN-53 are important because the ideal growth range for legionella is 95° to 115° F and legionella can be killed most effectively at water temperatures above 140° F (Figure 3). If legionella mitigation plans include flushing pipes with hot chlorinated water, those plans may exceed the PPI TN-53 limits and “may cause premature oxidation and eventual brittleness of the PEX material, reducing its ability to meet long-term requirements”.

Water Temperature              Effect on legionella

Above 158° F                                           Disinfection range

151° F                                                        Legionella die within two minutes

140° F                                                       Legionella die within 32 minutes

Above 122° F                                           Legionella can survive but do not multiply

68° F to 122°                                           Legionella growth range

Below 68° F                                            Legionella can survive but are dormant

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Also, be aware that chlorine dioxide can pose additional concerns for several piping materials. Chlorine Dioxide is often recommended for legionella mitigation and is being used as a secondary disinfectant by some municipal water systems due to growing concern over the disinfectant byproducts created by chlorine.

The PPI recently released Technical Note 67 that warns that chlorine dioxide has the potential to reduce the service life of most plumbing materials, including copper, steel, PEX, PE-RT, and PP-R. Leading Some PEX manufacturers now explicitly advise against the use of their products as part of a potable water distribution system in buildings where chlorine dioxide is used for secondary disinfection.

According to PPI TN-67, “chlorine dioxide is not known to be aggressive to CPVC at elevated temperatures of 200° F and below”. Due to its chemical composition, CPVC is inherently immune to corrosion or degradation from water treated with chlorine, chloramines and chlorine dioxide. That immunity means CPVC pipes do not have to be de-rated to protect them from chlorine degradation. Both Copper-tube size SDR 11 and IPS schedule 80 CPVC pipes and fittings are available with temperature ratings of 180° F at 100 psi with no limitations for hot, chlorinated water.

Designing for Legionella Mitigation

Facilities with a high percentage of at-risk residents, such as senior living centers, or those where water is more likely to sit stagnant for extended periods, such as hotels, should be designed to minimize the risk of biofilm formation and enable effective risk mitigation.

Those buildings typically use both IPS schedule 80 and copper-tube size SDR 11 piping and there can be a tendency to mix materials based on plumbing contractor preference. For example, some contractors may want to use copper for main lines and copper-tube size CPVC for distribution to fixtures while others use schedule 80 CPVC for mains and PEX to the fixtures.

Technically, the transition from one material to another is straightforward, but mixing materials may compromise the overall performance of the system. When materials with a higher biofilm formation potential are used for the mains, the bacterial colonies that form in those pipes are carried downstream, increasing the potential for biofilm formation in smaller diameter pipes. Then, when it becomes necessary to treat for legionella, the most effective treatments are likely to damage the materials that are vulnerable to chlorine degradation.

IPS Schedule 80 and copper-tube size SDR CPVC work together to provide consistently low biofilm formation potential across the entire system as well as immunity to water treated with chlorine, chloramine and chlorine dioxide, enabling facility staff to mitigate legionella growth proactively and aggressively. These systems also benefit from having a single material throughout the system with similar performance and design principles across all pipe sizes.

For information on copper-tube size SDR 11 CPVC, visit FlowGuardGold.com. For information on IPS schedule 80 CPVC, visit Corzan.com. To view the full findings from PPI, visit their recently released Technical Note 67.