Because of reduced water flows, waterborne illnesses associated with building water systems, such as Legionnaires’ disease, have increased significantly in the years since the Energy Policy Act of 1992 began being implemented in the mid- to late 1990s. In recent years, building owners have started learning that if they are near the end of a main on a large water utility distribution system, they will not likely be getting water from the utility that contains sufficient levels of water treatment chemical residuals to control bacteria growth within their building. Particular attention has been paid to Legionella bacteria, the microorganism that causes Legionellosis, which includes both Legionnaires’ disease and Pontiac fever (a milder, influenza-like illness in the Legionellosis family).
Legionella bacteria requires certain conditions for growth, such as temperature, stagnation, and a food source, such as those found in biofilm and scale on pipe walls. Temperature is a significant factor in Legionella bacteria growth. Warm water, in the ideal growth range, leaves a water system especially vulnerable to Legionella bacteria colonization and growth. Table A shows the effects of temperature on Legionella bacteria.
Control of the growth of Legionella bacteria in hot water systems requires maintenance of elevated temperatures to limit colonization and growth. Several studies demonstrate the overarching benefit of elevated temperature for Legionella bacteria control in hot water systems. However, control of bacteria in hot water systems by temperature is a factor of both the raised hot water temperature and the exposure time of the bacteria to those temperatures. For storage-type hot water systems, storage temperatures of greater than 140 degrees Fahrenheit (60 degrees Celsius) are a key factor for reducing positive detection of Legionella bacteria. At 140 degrees Fahrenheit (60 degrees Celsius), it takes approximately 32 minutes to kill 100% of Legionella bacteria in a laboratory setting. Exposure time of the bacteria to hot water at elevated temperatures is generally achievable in storage-type hot water systems because they are designed to store hot water for long periods until the peak demand period. For instantaneous hot water systems, outlet temperatures of greater than 160 degrees Fahrenheit (71 degrees Celsius) — the temperature at which Legionella bacteria is killed instantaneously — should be achieved. This is because, in instantaneous hot water systems, the exposure time of the bacteria to elevated temperatures is dependent upon the rate of flow through a heat exchanger, which is generally less than one minute. The temperature of hot water in instantaneous hot water systems must be brought up to a temperature that will kill or pasteurize the bacteria in the time it takes for the water to pass through the heat exchanger.
A hot water system design that uses temperature to control the growth of Legionella bacteria should incorporate a thermostatic mixing valve conforming to industry standards listed in the model plumbing codes to ensure a stable hot water distribution temperature that is greater than the growth temperature. To prevent scalding, the hot water should be distributed to all points in the circulated system, including distal points and the hot water return to the water heater, with the installation of point-of-use mixing valves, conforming to industry standards listed in the model plumbing codes.
It is not practical to control the growth of Legionella bacteria in cold water systems year-round by maintaining cool temperatures because these temperatures can get well into the growth ranges for Legionella bacteria. Therefore, measures to control the colonization and growth of Legionella bacteria in cold water systems is generally achieved by chemical methods. If the water supplied by the utility does not have an adequate water treatment chemical residual to control the growth of Legionella bacteria throughout a building water distribution system, the building owner should consider installing a secondary or supplementary water treatment disinfection system to help control microbial or bacterial growth within the building water distribution system. Secondary chemical disinfection methods include: chlorine; monochloramine; chlorine dioxide, sodium hypochlorite, copper silver ionization, ultraviolet light, and ozone. However, because water treatment chemicals oxidize at a higher rate in hot water, a secondary or supplementary water treatment disinfection system does not ensure control of the growth of Legionella bacteria in hot water systems.
EMERGENCY DISINFECTION OF BUILDING WATER SYSTEMS
As with methods to control bacteria growth under normal operation, the methods to kill or eradicate Legionella bacteria as an emergency disinfection or emergency remediation procedure also takes the form of temperature controls and chemical controls. However, an emergency remediation procedure to kill or eradicate Legionella bacteria requires temperatures and chemical dosages at significantly greater levels than those employed to prevent the growth of Legionella bacteria as part of an ongoing daily operation.
Methods to kill Legionella bacteria, as part of an emergency remediation, generally involve elevating the temperature temporarily as a “high-temperature thermal disinfection” approach and/or dosing the system with high levels of chemicals, in a process known as “hyperchlorination.” “High-temperature thermal disinfection” may be performed one time or many times, for various durations, and over a range of temperatures (It should be noted that eradication of Legionella species can only be achieved at very high temperatures).
Typically, this requires flushing water from each fixture, first, to remove stagnant and contaminated water from the piping system. Next, water in excess of 160 degrees Fahrenheit is flowed from each fixture for a duration of 20-30 minutes. Thermal disinfection requires evaluation of the existing heating equipment to see if it has the capacity to raise the temperature to the disinfection temperature range while maintaining a flow of water through the fixtures for a period long enough to kill the bacteria in the piping. The number of fixtures that is possible to be flowed, simultaneously, depends upon the energy input to the water heaters. Some buildings have water heating systems that cannot achieve disinfecting hot water temperatures, and only heat water to a usage temperature (for example, an instantaneous water heater that only heats water to a usage temperature, which is in the ideal growth temperature range for Legionella bacteria). For this reason, some buildings cannot utilize high temperature thermal disinfection, so they must use hyperchlorination to disinfect the hot water systems. This requires turning off the water heater and flushing disinfection chemicals through the hot water system.
Hyperchlorination is a process in which the free chlorine level is raised and held for a stated duration; the higher the free chorine level, the shorter the holding time necessary to kill Legionella bacteria in the water. Hyperchlorination involves flushing chemicals through the system, starting at a connection at the building water service entrance or through the secondary or supplementary water treatment disinfection system, if installed, and flowing water through each fixture until the free chlorine level is met. After the free chorine level is met at each fixture, the water must be held in the pipes for the stated duration.
EXAMPLES OF EMERGENCY REMEDIATION PITFALLS
- An insufficient volume of water is flushed through the building’s water distribution mains (hot and cold) in order to draw in “fresh” water (water with adequate water treatment chemicals) from the water utility main before remediation efforts begin. Bear in mind that water in the building’s water service pipe may be stagnant (and contain Legionella bacteria), especially after periods of non-use of a building water system or part thereof, so to ensure that “fresh” water from the water utility main enters and displaces the contaminated water in the building’s water distribution mains (hot and cold), the existing water in both the water service pipe and the building’s water distribution mains (hot and cold) needs to be purged. The aim should be to flush at least two to three times the calculated volume of the service pipe at a minimum of 3 feet per second (fps). Of course, you should confirm a disinfectant residual at the building’s water service entrance before continuing to perform remediation efforts. For further guidance on how to flush for sufficient volume and velocity, please refer to https://www.phcppros.com/articles/11467-flushing-bacteriafrom-stagnant-building-water-piping.
- Not flowing water from every fixture or not flowing water long enough from every fixture to properly flush the piping system.
- Not having a hot water system capable of providing a thermal disinfection, where the water heater is not capable of heating the water up to a thermal disinfecting temperature or maintaining an adequate supply of hot water at a thermal disinfecting temperature during the course of the remediation.
- Where chemical disinfection is performed, chemical levels are so high that it destroys the piping system. Chlorine, monochloramine, chlorine dioxide, sodium hypochlorite and other water treatment chemicals are oxidizers and can be very corrosive to plumbing systems when used for disinfection. Plumbing materials are an important factor to consider in Legionella control, as high heat and high levels of chemicals can accelerate the corrosion or failure of some piping systems, depending on the piping materials, chemicals, and temperatures.
There is no plumbing code section or standard addressing emergency flushing and/or disinfection of existing or in-use building water/premise plumbing systems. Rather (and since at least 1997), both model codes have contained sections addressing flushing and chlorine disinfection of potable water distribution systems prior to utilization. Section 610.1 of the 2021 International Plumbing Code (IPC) addresses new systems; Section 609.10 of the 2021 Uniform Plumbing Code (UPC®) addresses new or repaired systems. Both of these code sections suggest minimum chemical levels — no maximum levels are mentioned — for new plumbing systems. See the excerpt below from Section 609.10.2 of the UPC®:
The system or parts thereof shall be filled with a water chlorine solution containing not less than 50 parts per million of chlorine, and the system or part thereof shall be valved off and allowed to stand for 24 hours; or, the system or part thereof shall be filled with a water chlorine solution containing not less than 200 parts per million of chlorine and allowed to stand for 3 hours.
Additionally, both codes allow for disinfection methods that may be prescribed by a local authority such as the health authority, water purveyor, or authority having jurisdiction, and Section 610.1 of the 2021 IPC explicitly allows for the “procedure described in either AWWA C651 or AWWA C652.” However, AWWA C651, Disinfection of Water Mains, is intended to be applied to water mains within public water systems and AWWA C652, Disinfection of Storage Facilities, is intended to be applied to public water system tanks; neither is intended for premise plumbing systems within buildings, a point that was explicitly clarified in the 2019 editions of these standards.
Although it’s not specifically mentioned in the “disinfection” sections of the codes, be sure to follow the piping, fitting, valve, and equipment manufacturer’s chemical resistance requirements, and note that both codes require that the manufacturer’s instructions be followed (see Section 303.2 of the 2021 IPC; Section 309.4 of the 2021 UPC). Many manufacturers have maximum chemical exposure or resistance levels far below the levels indicated in the the codes.
