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Hunt for Legionella harbors

Commercial Water Booster Pumps

Hunt for legionellae “harbors” in the mechanical   room.


By Dennis Whitelaw, President, Towle Whitney LLC

After months of the COVID-19 pandemic, numerous articles have HIGHLIGHTED the need to properly re-open vacant buildings. The plumbing industry has been presented a great       opportunity to better understand the environments inside of the plumbing systems.  The focus here is simply on domestic cold water in the mechanical room:

1) What does plumbing system look like after the last two months?

2) What is the temperature of the different components?

3) What cultures are growing inside of our equipment?

4) Most important: if we can determine a baseline for a building, can we then quantify the effects of different decontamination/sterilization strategies?



Like a spy novel, where the spy is under our nose and tries to remain inconspicuous, the biofilms, pathogens, amoebae, and pathogens act the same.  The most attractive hibernation  locations that create these “harbors” will be found in larger diameter waterways with slower         velocities.  The big question: how much of these biofilms and legionellae are located in our “cooler water” mechanical rooms.



In February at the TREEO backflow conference in Florida, I learned from Dr. Frank Sidari that SLOW moving water is the best environment for growing legionellae.  Dr. Sidari highlighted research that showed that slow moving water replenishes the oxygen and   micro-nutrients necessary to grow Legionellae.  Recent articles I’ve read on LinkedIn threads strongly support Frank’s presentation.  



No-flow water may not be the best environment for legionellae, but it’s still a breeding ground for anaerobic bacteria which are also health hazards. But as long as the dead-legs are isolated and left alone, the bad water will stay in place. Almost like asbestos; as long as it’s left alone, it’s okay.



If you’ve ever looked at a boat’s bottom, it’ll be covered in algae, seaweed, and  barnacles.  Even after a week or two at sea, the water washing over the bottom has little effect. But a pressure-washer does a good job of cleaning the bottom. So, the BIG questions:

1) What velocity of water will keep the biofilms from originally growing?

2) What’s the threshold of velocity that will clean the inside of a backflow preventer, strainer?


Where to look? Taking water samples is great, but I believe it’s not enough. We need to be  aggressive and hunt for the contamination “harbors” that are hiding in the system.  After shutting down the water supply, components can be taken apart in hopes of finding “harbors”. 



As we’ve reviewed in other articles, these are typically oversized, and are known  “harbors”.  Many large diameter (2.5”+) backflow preventers are designed with a “Y” pattern check valve, allowing the check valve to move into the “Y” cylinder, which gets the check valve out of the waterway.  But there’s a huge UNINTENDED CONSEQUENCE: the top portion of the “Y” is shielded (red circles) from the moving water, even at higher velocities

legionella mitigation
legionella mitigation

Two backflow preventers plumbed in parallel will be the best opportunity to find a nasty “harbor”. 



Most mechanical rooms include meter strainers and backflow strainers. Like the backflow  preventers, many of these use the same “Y” pattern, which will yield  similar  findings.  Samples would be best taken from the basket after  removing the cover.


Depending on the size, construction, and piping, an expansion tank may yield a treasure-trove of samples. 


Humans are awful at maintenance, which is why our refrigerators have lights to notify us when the filter needs replacement. Filters in the mechanical room require the same care; they just don’t come with a  warning light. 


Towle Whitney Triplex Water Pressure Booster Pump System

Our systems are programmed to alternate the lead pump after 24 hours of run time. In a building with minimal    usage, the pumps may not have alternated over a two month span.  If the “stand-by” pump(s) have not run in two months, the samples may yield some interesting  cultures. 



It may be too late, but aggressively sampling the  plumbing systems around the country can provide insights into the consequences of vacant buildings. From these baseline studies, we can better formulate sterilization/decontamination plans for future events.