Plumbing engineers can minimize this prevalent hazard through education and preventive measures.

Draining water from infrequently used hospital dialysis hookup lines is one example of how to combat the formation of Legionella bacteria. Photos courtesy Matt Freije

Most of us have made the mistake of ignoring a problem, hoping it will just go away, instead of doing what we can to improve the circumstances. And then, once we devise a plan and get to work, we find the problem is more manageable than we realized.

Legionella is a problem that will not vanish without intervention but can be greatly minimized with proper design, construction, operation and maintenance of plumbing systems. Because plumbing engineers play a key role in prevention, they must stay up to date to protect health and life and to reduce their legal risk.

Here is the latest on Legionella and a forecast of what is likely to come.


Because more than 90 percent of Legionnaires’ disease cases are not even detected, let alone reported to health authorities or by the media, many people perceive the disease as rare, but it is actually quite prevalent and possibly on the rise. The number of reported cases rose 70 percent from 2002 to 2003, with the largest increase in the Eastern states, according to a report in 2008 by Neil Berkelman. Of the reported cases occurring from 2000 to 2005, 63 percent were in people less than 65 years old.

Studies have sown electronic faucets to be more conducive to Pseudomonas and Legionella than manual faucets.


Many people who get Legionnaires’ disease (or lose loved ones to it) and find out that it could have been prevented sue the people responsible for the whirlpool spa, plumbing system or cooling tower that made them sick.

In addition to building owners, defendants have included general contractors, plumbing contractors, water-treatment companies, engineers, architects, manufacturers and water utilities.

As the number of diagnosed Legionnaires’ cases increases because of physician awareness and better and cheaper clinical tests, the number of lawsuits is likely to increase as well. Plumbing engineers should check their professional liability (errors and omissions) insurance policies to ensure coverage for claims related to bacteria are not excluded.

An electronic mixing valve to better control temperature in a hot-water loop may be a good idea, but must be thoroughly evaluated.

Regulations and Guidelines

A number of countries have issued or updated Legionella regulations recently. In the United States, Legionella preventive measures are not mandated by law but are outlined in several government and industry publications. 

This year and next, the document likely to affect plumbing engineers the most is the new ASHRAE Legionella standard, expected to be released for public review late this year. The current ASHRAE Legionella document, issued in 2000, is a guideline whereas the new one will be a standard, which has more muscle.

The ASHRAE standard is likely to advocate a proactive approach to the prevention of Legionnaires’ disease, emphasizing the need for a water-management plan similar to what is recommended by the World Health Organization. Although ASHRAE will not likely give a specific recommendation for water testing, the standard may require an owner to validate preventive measures, which can best be done by testing water for Legionella.

The ASHRAE standard will likely increase demand for Legionella assessments and management plans. Type here

Water Testing Technology

The culture method is still considered the standard but as newer technologies are refined, validated and accepted by the scientific community, Legionella water test results will be available in one or two days (instead of seven) after a sample is collected, more accurately quantified, and more specific in the species and serogroups reported. It won’t happen this year but quite likely will happen before the decade is half over. If the new testing technologies are also lower in cost, Legionella sampling will be more widely used.

Copper-silver ionization is still a method primarily used in domestic water remediation.

Green Building Initiatives

Just as some of the energy-efficient measures adopted in the 1970s were a catalyst for indoor air quality issues, water-saving measures that promote stagnation may exacerbate problems with Legionella and other waterborne pathogens. Recently I was on the phone with a hospital project manager who wanted to install solenoid valves to drain water daily from 30-some infrequently used lines serving dialysis hookups but faced an uphill battle getting approval from a local board for draining water into the sanitary sewer. To flush five times the water volume from 35 10-foot, 3/4-inch lines on a daily basis would require about 40 gallons a day. Waste should definitely be minimized, but compared to the 80,000 gallons used by a hospital of that size each day (not counting the cooling towers), that’s a small amount of water to protect people from waterborne illness.

Hospitals are not the only buildings facing choices between waste and stagnation. Hotels, for example, can minimize Legionella bacteria by running the water in vacant rooms.

Complexity in Plumbing Systems

Extra piping, fixtures and valves add surface area and crevices where biofilm can develop and stagnation can occur. Thus, heat recovery systems, mixing valves, extra faucets to encourage hand washing and other complexities may ultimately do more harm than good.

For systems with a storage tank that holds a day or more of hot water, a master thermostatic mixing valve may help to minimize Legionella by storing water at 140°F or higher and then mixing it down to a safer temperature for delivery to fixtures.

But if smaller water heaters are used (such as semi-instantaneous heat exchangers that hold only about 20 gallons), heating the water to 140°F and then mixing to a lower temperature will do little or nothing to control Legionella because the water will not be at 140°F for enough time to kill the bacteria (Legionella die in about 32 minutes at 140°F). The mixing valves in such a scenario could actually increase the risk of Legionella by adding surface area to the system.

Electronic faucets are another example. Studies have shown them to be significantly more conducive to Pseudomonas and Legionella than manual faucets. Legionella was found, not only in the water flowing out of the electronic faucets, but in the biofilm on the inner components.

Technologies for maintaining temperatures and flow in various parts of plumbing systems, including right at shower fixtures, have recently been introduced in Europe but testing must be conducted to determine whether the designs actually minimize Legionella and other biofilm-associated bacteria.

Complexity, however, is not always a bad thing. An electronic mixing valve to better control temperature in a hot-water loop and solenoid valves to bleed water at stagnant points in a system may be a good idea, but each design needs to be thoroughly evaluated. Everything else being the same, engineers should prefer the simple to the complex.

A chlorine dioxide generator

Water Testing for New Construction

Plumbing specifications for pre-occupancy chlorination of a new plumbing system should be specific and reasonable. Section 610 of the 2009 International Plumbing Code refers to the American Water Works Association document C651 for a chlorination procedure, but C651 is written for public water utilities, not for building water systems. The two key issues are the specifications for the chlorination procedure and the proper test to validate the chlorination was effective.

The AWWA C651 recommendation to hold a chlorine concentration of 50 parts per million in the entire system for 24 hours, or 200 ppm chlorine for three hours, is appropriate for most buildings and agrees with the ASPE’s handbook. But some of the options described for post-chlorination validation testing are either vague or totally inappropriate for building plumbing systems.

For example, the document states samples will be collected from every 1,200 feet of the new water main, at the end of the line and from each branch. If this guideline was to be applied to a building plumbing system, as was recently proposed by the designer of a new hospital, a ridiculous number of costly samples would be required. Also, an HPC (heterotrophic plate count) test, which the AWWA states is optional, is not an appropriate post-chlorination validation test.

HPC is a poor indicator of health risk. In fact, studies of cooling towers and home plumbing systems have shown HPC bacteria may actually protect against pathogenic (disease causing) organisms.

In April 2002, beginning one day after the International Symposium on HPC Bacteria, the World Health Organization held a meeting in Geneva, Switzerland, on the scientific basis for interpretations of HPC measurements. The experts arrived at a consensus that HPC bacteria in drinking water was not associated with a significant health risk. The U.S. EPA subsequently removed the requirement for HPC levels below 500 cfu/ml in public water supplies.

Additionally, the AWWA states the system will show the absence of coliform organisms. Although coliforms are certainly not perfect indicator organisms, testing for them is appropriate since a coliform test is standard for U.S. drinking water.

Some engineers have written specifications requiring “no live bacteria” following the chlorination procedure, but that is not feasible or reasonable in a plumbing system. First, “no live bacteria” is vague because it does not state which type of test should be used or the number of points in the system to sample. Second, to require a plumbing system to have zero live bacteria would put it on the level of a high-purity pharmaceutical or electronics manufacturing system.

A proper specification for plumbing disinfection is important. If the requirements are lax, the people who occupy the building may get sick. If the requirements are excessive, it will unnecessarily add to the project cost and possibly delay the opening of the building. here

A point-of-use submicron filter may be needed on some fixtures that remain Legionella-positive.

Domestic Water Remediation

Continuous disinfection technology for plumbing systems has not changed much within the last 12 months. Copper-silver ionization and chlorine dioxide are still the methods primarily used. Deposit control - removing the environment (biofilm) required for Legionella growth rather than just trying to kill the organisms - is being tested and may become more common. No single method is the best choice for every situation. With any method, a flushing program is usually required to ensure the disinfectant flows throughout the system, and point-of-use submicron filters may be needed on some fixtures that remain Legionella-positive.

With the aging population and increasing threat of waterborne organisms resistant to antibiotics, plumbing engineers will continue to play a key role in minimizing disease.