The majority of community water systems (CWSs) in North America use one or more chemical disinfectants to treat surface or groundwater for potable use. In the United States and Canada, approximately 99% of disinfected potable water systems use free chlorine or chloramine as the disinfectant, according to the American Water Works Association (AWWA).

Chlorine is typically supplied in a liquid form as sodium hypochlorite, the same chemical found in bleach. It is also available in gas form. Chloramine is formed when ammonia is added to water first treated with chlorine. These compounds have proven to be effective methods for keeping our drinking water safe.

The plastic piping materials CPVC (chlorinated polyvinyl chloride), PEX (cross-linked polyethylene), PE-RT (polyethylene of raised temperature resistance), PP-R (polypropylene random copolymer) and PP-RCT (polypropylene random copolymer with modified crystallinity and temperature resistance) have proven to be resistant to these disinfectants when used in hot and cold water plumbing distribution, water services, building supply lines and other related applications. 

However, what about other water disinfectants? 

Chlorine Dioxide in Municipal Systems

A less common secondary water disinfectant is chlorine dioxide (ClO₂). While often used as a primary water disinfectant in water treatment plants, chlorine dioxide is used as a secondary (i.e., residual) disinfectant in fewer than 1% of the public potable water systems in the United States. 

The 2021 AWWA Journal article, “Understanding Community Water System Disinfection Practices in the United States,” contains recent information about the use of water disinfectants (https://bit.ly/3XLWTQG).

Findings of this report include: “Free chlorine … is by far the most utility-reported residual disinfectant for very large, large, medium and small CWSs” and “Chloramines are the second-most reported secondary disinfectant in large and very large CWSs … We estimate that around 18% of CWSs, excluding very small systems, use chloramines for secondary disinfection.” 

Chlorine dioxide is a dissolved gas used in different concentrations than free chlorine or chloramines. It is highly volatile and efficient as an oxidizing agent for disinfection. ClO₂ has a different attack mechanism on the various materials to which it is exposed. It has been shown to be aggressive to most piping materials, including copper and polyolefin-based plastics.

“Chlorine dioxide is a yellow to reddish-yellow gas that can decompose rapidly in air,” the Centers for Disease Control and Prevention (CDC) explains. “Because it is a hazardous gas, chlorine dioxide is always made at the location where it is used. Chlorine dioxide is used as a bleach at pulp mills, which make paper and paper products, and in public water-treatment facilities, to make water safe for drinking. It has also been used to decontaminate public buildings.”

The U.S. EPA has set the maximum level of chlorine dioxide for potable water at 0.8 parts per million (ppm), but it is rarely used at such high levels in residual treatment. It is normal for disinfectant levels to diminish depending on water age, water temperature, distance from the treatment plant, piping material, organic matter in the water and other factors. 

In community water systems treated with chlorine dioxide, the actual residual level reaching most buildings is typically less than 0.4 ppm. Still, this level of ClO₂ could be aggressive to certain piping system components, so buildings connected to water supplies with ClO₂ should have piping components carefully selected. 

However, plumbing systems have another risk of exposure to ClO₂ — from within the building itself (cue lines from 1970s horror films here).

Where and Why is Chlorine Dioxide Used in Buildings

Plumbing distribution system design is a dynamic challenge with several interactive variables such as fixture demands and water flow rates, pipe diameters and lengths, water temperatures, recirculation velocities, water and energy conservation demands, and other factors. 

The bacterium Legionella pneumophila has been a topic of much research in recent years, with experts in the public health and plumbing fields focusing on piping designs that eliminate dead legs and use properly sized pipes to help prevent its growth by reducing the volume of stagnant water. 

According to the CDC and other sources, Legionella can grow in water between 68 F and 120 F (20 C to 48 C) with an ideal growth range of 85 F to 110 F (29 C to 43 C).

In large facilities such as hospitals, nursing homes, hotels, apartment buildings and large office buildings, chlorine dioxide is sometimes added to the hot and cold water plumbing distribution systems to treat or control outbreaks of harmful bacteria. Specialized chlorine dioxide generation devices can be added to inject ClO₂ in measured doses directly into the piping system before water is delivered throughout the building. These injection devices are more commonly found in hot water systems. 

Chlorine dioxide is one of several plumbing system treatment options to combat pathogen growth. Other options include chlorination, UV treatment of water, thermal shock to kill existing colonies of Legionella, and combinations thereof. Several industry resources are available to give guidance on these issues, including: 

• ASHRAE Guideline 12, Managing the Risk of Legionellosis Associated with Building Water Systems;

• ASHRAE 188, Legionellosis: Risk Management for Building Water Systems;

• ASHRAE 514, Risk Management for Building Water Systems: Physical, Chemical and Microbial Hazards;

• IAPMO Uniform Plumbing Code, Appendix N, Impact of Water Temperature on the Potential for Scalding and Legionella Growth;

• ASPE Engineering Methodologies to Reduce the Risk of Legionella in Premise Plumbing Systems.

The injection of chlorine dioxide into the plumbing distribution system within buildings may be occasional, such as to treat an outbreak. The process may also be maintained for recurring or permanent system treatment, especially in large facilities with complex potable water distribution layouts where persistent biofilms are established within metallic piping components, and outbreaks are recurring. 

Therefore, the exposure (e.g., concentration, time and temperature) of piping materials to chlorine dioxide can vary significantly from one building to the next. What are the potential effects on the piping of frequent or constant injection of ClO₂ into a plumbing system? A new report helps to address this topic. 

Effects of ClO₂ on Plumbing Distribution Piping Materials

A team of plastic pipe industry experts began a research project in 2020 to investigate the potential effects of chlorine dioxide on pressure piping materials used in plumbing distribution systems. Their investigation found that multiple research teams worldwide have performed laboratory testing to evaluate the effects of ClO₂ on these materials. 

Research showed that ClO₂ water treatment can be aggressive to most plumbing distribution materials, including metallic piping such as copper, where its oxidative effects can impact long-term performance. Chlorine dioxide may also affect elastomeric materials such as seals and gaskets found in pumps and appurtenances such as valves. 

The 2019 report “Chlorine Dioxide Degradation Issues on Metal and Plastic Water Pipes Tested in Parallel in a Semi-Closed System,” available from the International Journal of Environmental Research and Public Health ((https://bit.ly/3LhIMLJ), states that “four different kinds of water [pipe], two based on plastics, namely random polypropylene (PPR) and polyethylene of raised temperature (PERT/aluminum multilayer), and two made of metals, i.e., copper and galvanized steel, were put in a semi-closed system where ClO₂ was dosed continuously. …

“Results show that ClO₂ has a deep effect on all the materials tested (plastics and metals) [and severe] damage occurs due to its strong oxidizing power in terms of surface chemical modification of metals and progressive cracking of plastics.”

 While such testing was experimental in nature and did not precisely simulate the exposure of hot, treated flowing water through pressurized piping systems, the published results indicate that chlorine dioxide has the potential to reduce the service life of most plumbing distribution materials to below-normal expected lifetimes. 

For many hot and cold water distribution piping materials, the actual effects of chlorine dioxide are dependent upon the combination of factors such as water temperature, water velocity, system pressure, the use of other water treatment chemicals, the concentration of ClO₂ itself and other potential variables unique to each system. 

Data indicates that chlorine dioxide has the potential to be substantially more aggressive than free chlorine or chloramines, even at comparatively lower concentrations, to piping materials copper, PEX, PE-RT, PP-R and PP-RCT. Evaluation by Plastics Pipe Institute (PPI) member firms indicates that chlorine dioxide is not known to be aggressive to CPVC at elevated temperatures of 200 F (93 C) and below. 

The summary of the PPI research, including links to referenced studies, has been published in PPI Technical Note TN-67, “Chlorine Dioxide and Plastic Hot- And Cold- Water Plumbing Distribution Pipes” (https://bit.ly/3LhIMLJ).

Based on these findings, specifiers and designers should use caution when considering the use of chlorine dioxide as a chemical disinfectant to treat water for the control of Legionella or other pathogens. 

PPI recommends contacting each piping system manufacturer for guidance on how to use their pipe and fitting material(s) when chlorine dioxide has been selected as the disinfection chemical. 

There may be cases where a particular piping material is not suitable for a specific application due to the factors listed above, or insufficient data is available to indicate if a given pipe or fitting material is suitable. 

Lance MacNevin, PEng, is the director of engineering for the Plastics Pipe Institute’s Building & Construction Division and has been in the plastic pipe industry since 1993 in various technical roles. He serves on technical committees within ASHRAE, ASPE, ASTM International, AWWA, CSA Group, IAPMO, ICC, IGSHPA, NSF International and RPA, helping to develop codes and piping standards.