Is your inbox packed with warnings about flushing COVID-closed buildings before reopening? The warnings have been issued because buildings shut down or occupied less than normal become more prone to Legionella and other pathogens due to stagnation.
But the warnings omit a crucial caveat: flushing will not solve a Legionella problem.
Flushing certainly has an important role in building water management programs (WMPs). In an occupied building that is implementing a comprehensive WMP, flushing infrequently used fixtures often enough (e.g., weekly) and long enough (e.g., for 2 minutes) with chlorinated water will help control Legionella by minimizing stagnation and replacing “old water” with “new water” that has an adequate disinfectant residual (ASHRAE 2020. HSE 2014).
If performed properly, flushing can also be important after plumbing system repairs, in the opening of dead lines, and in responding to incidents (e.g., water pressure shock, excavation work, water main breaks) that have been associated with Legionella contamination and outbreaks of Legionnaires’ disease.
However, based on experience and scientific studies, expecting flushing to solve a Legionella problem in a shutdown building is unrealistic.
The problem is biofilm. If Legionella is living within biofilms attached to pipe walls and in the small crevices of valves, fixtures, and fittings, flushing will not remove it. Even if flushing can be performed at high enough flow rates to move stagnation-loosened biofilm through all the pipes and out the faucets — which is difficult in a large building (Proctor 2020) — Legionella within the intact biofilm will remain.
Even flushing with high levels of chlorine (i.e., hyperchlorination procedure) has been shown to control Legionella for less than two months (Muzzi 2020). Consultants and water treatment professionals with experience disinfecting Legionella-contaminated plumbing systems have seen it happen time after time. Samples collected approximately 3 days after a properly performed hyperchlorination procedure test Legionella-negative, but test results for samples collected two months later are not much better, if at all than pre-hyperchlorination findings.
It is easy to understand why this happens. Biofilm provides a layer of protection for Legionella. Legionella that is free-floating, within loosened biofilm, or even in the biofilm’s outer layer may be killed by chemicals or flushed out during a hyperchlorination procedure, but the bacteria living deeper within the biofilm, though perhaps weakened for a bit, survive and eventually multiply. Thus, expecting hyperchlorination—let alone flushing without chemicals—to remove Legionella living deep within biofilms is like trying to remove gophers from your property by mowing the grass.
Testing for Legionella provides the best validation for a flushing procedure. Test twice: Once shortly (3-5 days) after the flushing procedure (WADOH 2020) and again about two months later.
Buildings that followed CDC’s 2016 recommendation to implement a comprehensive WMP per ASHRAE Standard 188 will be fine when reopening post-COVID, assuming the WMP has effective control measures for minimizing stagnation during shutdowns and partial occupancy. And, many of the COVID-closed buildings that failed to implement stagnation preventive measures while shutdown will be fine after a basic pre-opening flush, simply because they’re one of the few buildings–due to the water supply or other factors–that are not prone to Legionella growth. Other buildings, unfortunately, may have to take more expensive measures to protect people in their buildings from exposure to Legionella.
What have you experienced in flushing Legionella-contaminated buildings? Please comment below.
Nice timely guidance Matt – just one comment, I’m not sure what is meant in the second to last sentence of the final paragraph: “And, many of the Covid-closed buildings that failed to implement stagnation preventative measures will be fine, too.” ?????
Thanks for asking, Bill. I should have been clearer. I meant, basically, that some buildings will just be lucky. 🙂 Perhaps because of the water supply or other factors, they might do a simple flush upon reopening and not have an ongoing Legionella problem, even if the system was stagnant for a couple of months.
I agree with Bill. This sentence is a bit confusing. Maybe simplify it to…”Although some buildings that failed to implement stagnation preventive measures will be fine, others may have to take more expensive measures to protect people in their buildings from exposure to Legionella.”
Overall great article we are having these discussions daily and there does seem to be prevalence of people want to run the water for a few minutes and move on. Understandable, but hopefully they will listen to the experts like this group and really provide a better environment for returning to normal operations.
Thanks for the feedback and comments, George! I will look at revising that sentence later today.
Thanks again for the question. I just changed the second to last sentence:
Original version: And, many of the COVID-closed buildings that failed to implement stagnation preventive measures will be fine, too.
New version: And, many of the COVID-closed buildings that failed to implement stagnation preventive measures while shutdown will be fine after a basic pre-opening flush, simply because they’re one of the few buildings–due to the water supply or other factors–that are not prone to Legionella growth.
I can only concur with you Matt: biofouling and biofilm management is challenging in most building water systems.
Definitely a health concern on the drinking side, and contracting legionellosis is one of the many risks that exists.
There is also an increased risk of microbiological influenced corrosion (MIC) of these pipework and I am observing and investigating these failures on copper and steel systems every week. But plastic systems are not fully immune either unfortunately.
Thanks for sharing from your experience, Martin. Much appreciated.
Hello Matt,
My experience is this, to remove the biofilm completely you must have the proper contact time. This is contact with the disinfectant and the biofilm. This cannot be achieved with the standard hyper chlorination methods. I suggest a much longer but not permanent disinfection method.
I know you and G-C have a lot of experience with this so thanks for sharing, Mike. I agree that lower disinfectant levels (e.g., 2-3 ppm chlorine) over a longer period, while the building is occupied (if allowed by regulations), is often a much better solution than hyperchlorination.
Aren’t you worried about the integrity of the piping system when exposing it to 2-3 ppm of free chlorine over several months?
I wouldn’t recommend 2-3 ppm chlorine for several months but in many cases it can make sense for two months (in lieu of hyperchlorination) while determining the best long term strategy. Thanks for commenting, Mitch.
See Bill McCoy’s book commissioned by the IWA titled-“Sustainable Water Management Programs for the Built Environment.” in the potable water section you will find a summary of the PROCESS of Continuous-Supplemental-Chlorination (#CSCPROCESS) this process is a combined synergistic application of liquid sodium hypochlorite as disinfectant combined with a soluble silicate-based corrosion inhibitor. For an example of its performance, see my comment above regarding Henry Ford Hospital in Detroit Michigan and it’s 33 years and ongoing experience with the PROCESS.
An example of this is the ongoing successful program that I established at Henry Ford Hospital in Detroit Michigan in 1987 that utilizes the “PROCESS” of Continuous-Supplemental-Chlorination (#CSCPROCESS). This program is still operating effectively to date…..
That is now 33 years and running.
#LEGIONELLA #LDB #NTM #OPPP #CRDHWS #CSCPROCESS
#DEADLEGSCENARIO #FAILEDLEGIONELLAPOLICY
I’ve come across a few positive Legionella samples in the domestic water systems. We were able to follow the HC
-Info WMP guidelines for flushing the systems and resampled within 7 days after the flush. The retest came back negative. If the retest had come back positive then we would have taken a different approach maybe with chlorine dioxide.
Thanks for sharing, Heidi. You do good work.
Matt, This a great article. I have a client who insists that if they open all the taps at once the high volume of flow will eliminate the entrenched biofilm. This, while, better than opening single taps sequentially, is not the answer to long term buildling health. This will help with my argument.
Your client should listen to you, Pat! You could probably write a book on biofilm! Thanks for commenting
Hello Matt. I have been following all of these articles, procedures, guidelines, etc. I have heard all the theories and it has become evident that they all seem to follow similar practice of flushing. The largest problem is ALL miss the Shower Head. There has not been ONE (that I have seen), that calls to REPLACE. It was determined only 40 years ago in The Annals Of Internal Medicine that Shower Heads(1981) Could NOT be Flushed and declared Legionella Free. Actually, it repopulated after Ethylene Oxide flushing. For that matter, they cannot be flushed of any biofilm as determined by The University of Colorado Study on NTM and Shower Heads (2018). I’m the Only Company that went into business to fix the problem.
Thanks for mentioning that, Steve. I thought the Proctor and WADOH documents mentioned removing aerators and fixtures and either disinfecting or replacing them, but I’d have to go back to look to confirm.
I would be curios since there is no Science that can prove you can clean the inside of a Shower Head you cannot see inside regardless if the aerator is cleaned or not. We all know after cutting open Shower Heads that they accumulate calcium which is an additional platform for Biofilm to Grow. And, even if you could clean it, what day does it become toxic again? Again, Science = Proof.
Matt, biofilm is difficult (if not impossible to irradicate safely in a building water system). The chemicals which will remove the biofilm will make the water undrinkable. We have to manage the situation. Stagnation is the most important issue here. During prolonged periods of buildings being temporarily taken out of use (Mothballing) the water system should be managed so that microbial growth, including legionella in the water, is appropriately controlled. In general, systems are normally left filled with water for mothballing and not
drained down as moisture will remain within the system enabling biofilm to develop
where there are pockets of water or high humidity. The water in the system also
helps to avoid other problems associated with systems drying out, including failure
of tank joints and corrosion in metal pipework. The systems should be
recommissioned as though they were new (ie thoroughly flushed, cleaned and
disinfected) before returned to use. A competent risk assessment should already be in place which should identify measures that need to be implemented and monitored on an ongoing basis. There is no magic bullet to control Legionella it requires an active management plan.
You made some good points, Steve. Here in the US, though, we don’t have the regulations you have in the UK for ongoing water system management, so it’s likely that many building water systems have not been properly managed during the shutdown.
Flushing is just one piece of the puzzle. Since Legionella and other pathogens grow best in hot water maintainability of residual disinfectant is very difficult.
Proactive treatment to remove the dirt/sediment and microbial contamination from source water followed with continuous microbial filtration on hot water loop is the key to controlling the problem. Thus along with continually flushing low use locations automatically will also contribute to improvement of buildings health level. Of course validation of your plan must be ongoing process throughout the building quarterly at minimum.
Good point that flushing is just one part of a comprehensive water management program. Thanks for commenting, Andrew!
Thanks for this, Matt. I worry that those buildings without an ASHRAE 188/CDC Toolkit WMP will think a ‘once and done’ approach to flushing will protect them and their occupants long-term, which we know isn’t true. Let’s hope folks will pay more attention to their building water quality, biofilm, disinfection and Legionnaires’ disease risk going forward. We’ll keep educating…Thanks again. Patsy
Yes, hopefully the attention to reopen buildings safely will be followed with emphasis for ongoing water system management. Thanks for all you do to educate, Patsy!
Nelson Kelly here, I still believe that a engineered potable water system that has a “LOOP SYSTEM” where a end of main water line is piped to a frequently used fixture would be helpful in preventing bacteria build up. Especially true in multi story buildings. Areas that have a tourist season where water flows drop off post season – such as where I live – would be especially helpful. Also how about a potable water circulator-would not have to be full size- from main by most distant fixture to service entering building would keep water in motion and have a place to test for bacteria and disinfect if necessary. I am not a engineer or scientist but I am a retired Plumbing Inspector. I attended Matt’s LD training when he first started and relayed to our State Inspectors Association. For your consideration
A few manufacturers and engineering firms have looked at novel designs and products to reduce stagnation and some are now in use. Studies will eventually determine which ones have a net benefit considering the increased surface area required for the stagnation-reducing piping and components versus the reduced stagnation. I’m thankful for the people who are looking for new approaches and testing them. Thanks for commenting, Nelson. Enjoy your retirement!
This is a viable point for a one-way “cold” water system and is part of my proposed #FCPDP protocol.
A properly designed “hot” water system in larger buildings will already have existing recirculation. Many people do not comprehend that there are in fact two separate and distinctly different potable water systems that operate within a building.
Chlorine is unstable in “hot” water. Any chlorine residual present in incoming “cold” water
that is the “make-up” for the hot water system will be depleted as it passes through a heater. LITTLE, IF ANY CHLORINE RESIDUAL EXISTS IN A HOT WATER SYSTEM TO WHICH IT HAS NOT BEEN SUPPLEMENTALLY ADDED!
Good point, Michael, and absolutely true about little to no chlorine residuals in domestic hot water systems. Thanks for commenting
Our experiences are exactly as described. Flushing alone, whether in idled or occupied premise water systems, cannot manage the biofilm and its “inhabitants”. In most instances, the weeks succeeding the temporary, reactive offline or the online chlorination have indicated the return of water borne pathogens (LD, HPCs, etc.) with a vengeance. It seemed the weaker “competitive strains” were reduced during the one time procedure thus allowing the stronger/protected survivors to recolonize and flourish unchecked. Regardless of disinfection levels it is imperative the disinfectant reach as much of the premise water systems/surfaces as possible to displace the stale water and minimize stagnant conditions as frequently and consistently as possible.
Thanks for sharing that, Anthony. From your experience, you all at g-c.com certainly have a lot of data. Keep up the good work!
Matt – Good article. It is our experience that lack of biofilm control can cause many water quality issues, including microbial induced corrosion of lead and copper, formation of disinfection byproducts/VOCs and creation of a hiding place for Legionella, coliforms and other pathogens. We see that most Legionella interventions actually promote biofilm formation and creation of a hiding place, particularly in hot water recirculation systems with extremely high water age. True Legionella control needs to include biofilm control and biofilm monitoring. Back to topic – closed buildings need periodic flushing during the closure, hot and cold, to keep water quality in check, followed by aggressive flushing prior to opening. A complete plumbing drain-down followed by rapid filling does a good job of aggressive pipeline scouring and biofilm removal. A more advanced method involves air-assisted flushing, which should be performed by a professional.
Thanks for commenting from your experience, Andrew. Could you please provide examples of “Legionella interventions [that] actually promote biofilm formation”? I think people reading your comments will be curious.
Matt,
Biofilm, at its most basic definition, is the result of bacterial response to stress. We see biofilm formation occur 1) in response to a nutrient imbalance, say to a limitation in required carbon source relative to available nitrogen, 2) as defense against competing organisms, say to lock-up nutrients in a game of “keep away” from other organisms, 3) in water with a high nutrient load, which causes an outright battle for supremacy, and 4) to defend against stressors such as metals, disinfection, temperature, oxygen, antibiotics, etc. We have seen the addition of a disinfectant cause biofilm formation, particularly by acid producing bacteria, which provides a target for the oxidant and creates a protective layer for the bacteria. We have seen the same issue with elevated heat and UV. When any intervention/system has a malfunction or is modified/changed, biofilm sloughing occurs along with a release of its inhabitants to the bulk water. The point of the matter is that unless we monitor for biofilm in a system and assess whether a Legionella-targeted intervention is controlling biofilm present, we cannot confidently say we have controlled the environment that could harbor Legionella.
I love the mowing grass analogy. Nice article!
Haha! Thanks Marge!
Matt: Great article and very timely information! I do believe that many out there do have an unrealistic expectation of “clean” piping with simply flushing the lines. I think your comment of ” …flushing lines to remove Legionella living deep within biofilms is like trying to remove gophers from your property by mowing the grass” hits the nail on head. The key is testing post and “post-post” testing, and additional remediation if necessary.
Good point about Legionella testing at those two times providing the best validation, Tom. Thanks for commenting, and for your good work. We at HC Info hear really positive feedback from companies that have utilized your onsite water management plan survey training.
The article seems to beg the question: If Legionella is indeed ubiquitous, and if water stagnation within plumbing systems can lead to Legionella build-up, why do we not see more incidents of legionellosis in second homes that are often uninhabited for long periods of time?
Good question, Robert. When I was a child, our family had a cabin on a lake in Northern Indiana. When we would arrive there each visit, my dad would insist we run faucets for several minutes before using the water. This was (dating myself) before Legionella was even discovered, but my dad had the sense to do the flushing. In a small cabin, especially one supplied by a well, as it was, flushing is likely to be more effective in removing Legionella than it will be in larger and more complex systems. But perhaps most people do not flush water when arriving to a vacation home. By the way, the term “ubiquitous” for Legionella has been misunderstood. It simply means that Legionella can be found in pretty much all TYPES of water systems within certain temperature ranges as well as in lakes and rivers and ground water, and in any geographic location. It does not mean that Legionella is a problem in every building. Over the years I’ve tested a few buildings that based on risk factors should have been proliferated with Legionella, but none was found. Thanks for commenting.
Robert,
your observation can most likely be attributed to die-off due to the depletion of nutrients over time.
Great article, Matt. You’re absolutely right that Legionella validation testing is essential to measure the effectiveness of procedures being done to mitigate Legionella risk in COVID-closed buildings. Thanks for sharing.
-Shaimaa
Thanks Shaimaa!
I completely agree with Shaimaa. The public does not need to be subjected to more health problems having endured (and more time ahead to come) this horrible COVID-19 pandemic. Let us hope the facilities management personnel are reading your article and are implementing comprehensive WMPs.
Matt,
As a facility manager, I would be interested in your assessment of which types of buildings ( and areas within them) are at highly risk for legionella issues as we bring buildings back on line.
Much of the health issues associated with legionella relate to it’s presence as an aerosol.
Consequently, the risks for a hotel or hospital with a number of showers would be much higher than for a typical office building.
What are the aerosol risks from a common bathroom faucet or a drinking fountain?
Also, it would seem that restaurant operations, specifically dishwashing areas, etc. would be high risk as well.
Your thoughts as to building/area types and concerns?
That’s a good question, Robert. Thanks for asking. Domestic (potable) plumbing systems, cooling towers, whirlpool spas, and to some extent decorative fountains are inherently prone to growth of Legionella, and must be managed to reduce risk. The key difference between building types is not so much the types of systems, but the susceptibility of the occupants and the degree and type of water use. This is why ASHRAE Standard 188 bases risk primarily on the type of water systems and requires that certain types of water systems, including the above, be managed to reduce risk. What will vary, though, among building types, to some extent, are the details of the control measures. In LAMPS, water management plan control measures are based on the building type (the user selects among seven categories, e.g., hotel versus school/university) as well as the types of water systems. In any of those building types, supply feeds, water heaters, faucets, showers, drinking fountains, etc. should be properly managed to minimize risk, but the details vary for some control measures. Dishwashing areas should be considered but those are typically at lower risk assuming the water is supplied at > 60C (140F).
Great point Matt to write an article to expose lacks of the Warnings about flushing COVID-closed buildings before reopening. I, myself, wrote a letter to the government of Quebec not to be negligent in the prevention of Legionnaires’ Disease during this period of COVID-19. No reply. We may experience a surprise…
Is flushing of building water systems enough to prevent Legionnaires’ Disease? No one can answer it without doubt. It may be enough in some cases. It may not in others. Flushing procedure has to be validated. As you mentioned, “Testing for Legionella provides the best validation for a flushing procedure.” In other words, a statement that water systems have been flushed is not enough. Building Water Systems must be proved Safe for Public Health before their reopening.
Then, results interpretation is of first importance. Robert question goes in this sense, I guess. Is there any difference of criterion or operational target according to types of buildings or water usage being toilets and faucets only or showers like in hotels and gyms? On the subject, the National Academies of Sciences, Engineering, and Medicine 2019. Management of Legionella in Water Systems. Washington, DC: The National Academies Press. https://doi.org/10.17226/25474 stated at page 161 : “The Committee’s analysis of studies on Legionella occurrence that collected concentration data suggests that a Legionella concentration of 5 x 10 000 CFU/L should be considered an “action level,” that is, a concentration high enough to warrant serious concern and trigger remediation.” At page 275, the report stated:”At this point, urgent action to discontinue exposure and implement a remediation plan is appropriate. For individuals who are highly susceptible (e.g., immunocompromised patients in hospital and healthcare settings), or where a greater safety factor is desired, it would be appropriate to reduce this action level multifold.”
The report also presents the quantitative microbial risk assessment (QMRA) to develop routine operational targets for buildings with just toilets and sinks or buildings with showers. Those who have comprehensive WMP may know what it is all about. However, this is over a simple comment or answer to this question. To conclude, as other people may have stated before: Great for those who already have a WMP and Good Luck to others!
Thanks Mario!
Robert,
We’ve found Legionella lurking in any water-based system (hot or cold) with high water age, decaying biofilm (protein source) and exposed metal. In one location, it infested the galvanized piping and cast iron control valve of a softener system so bad that the softeners had to be replaced (multiple high-dose chemical treatments did not work). In another system, we found the cast iron main at the influent to a building had Legionella lurking in corrosion tubercles, which was resolved by adding a flushing port to the influent line and aggressive flushing. At another location, we found it lurking in corrosion scale on a steel well casing. So, to answer your question, I’d focus on parts of the system with galvanized/iron piping and high water age as these could be sources for aerosols at a shower head.
Great article, Matt! There is definitely an important role for flushing in Legionella prevention, but consideration needs to be made on when and how it is being utilized for each facility. Although guidance can be provided in general, many facilities need to be assessed individually to determine what type of risk is present in each water system and then consider best actions moving forward for that particular situation. I have seen facilities of varying sizes and water system complexities have different patterns of Legionella test results and even vary over years of routine testing. Each facility requires a personalized review of their routine testing to consider testing history and previous remediation actions taken in order to determine what level of intervention is necessary. Flushing can play a critical role in preventing stagnation in facilities with very low and sporadic Legionella positivity and concentrations, but it is usually clear within a couple of quarters of testing with pre and post samples when flushing won’t be enough to fix a problem. Routine testing paired with the control measures outlined in the LAMPS program help to create a synthesized approach to preventative and responsive action to Legionella prevention.
Thanks for commenting, Erica. And thanks for the good work you at watermanagementadvisors.com are doing for your clients!
An example of this is the ongoing successful program that I established at Henry Ford Hospital in Detroit Michigan in 1987 that utilizes the “PROCESS” of Continuous-Supplemental-Chlorination (#CSCPROCESS). This program is still operating effectively to date…..
That is now 33 years and running.
#LEGIONELLA #LDB #NTM #OPPP #CRDHWS #CSCPROCESS
#DEADLEGSCENARIO #FAILEDLEGIONELLAPOLICY
Hi Matt – as always, well-written and insightful content – I do think (from an accreditation standpoint), the management of building water systems in healthcare has the potential for becoming a significant vulnerability for organizations that don’t have a well-organized approach. If you don’t mind, I’m going to provide links to your site for this weeks’ blog (how anyone could have missed you by now is beyond me, but when it comes to information, I would much rather err on reiteration) – keep up the good work and thanks for keeping us informed.
Thanks very much, Steve. Yes, please provide the links. I appreciate it.
I’ve had great success using gas stripped chlorine dioxide in fouled potable water systems; the ClO2 molecule has an affinity for the disulfide linkages found in the amino acids of cell walls, so it actually “seeks out” the organism prior to penetrating the biofilm and killing the anaerobic organisms that are protected by the film. The organisms die, the biofilm loses it’s attachment and the now film free surface is exposed. For the whole story (I wrote the outline for this on my way to Hawaii to visit a customer with positive Legionella and a VERY upset Dept. of Health manager), needed to have a document that addressed the how’s and why’s for personnel of varying responsibilities and knowledge levels. Have a look in my dropbox, any questions, feel free to get back to me!
Thanks Steve!
Thanks for all of the discussion surrounding this issue. I’ve learnt much by reading them. There is a lot of information out there that is poorly presented. For example, a manager of a number of buildings called to ask how long he should flush the lines in his buildings. He made this enquiry after reading on Facebook that building owners should flush the lines before reopening. In his case, these buildings all utilized water tanks. These tanks were left filled with water during the shutdown period(6weeks) and that’s the water that he would have flushed the buildings with.
Thanks for commenting, Maria!
Testing (culturing) for Legionella residing within the undisturbed piping biofilms of recently unoccupied low or no flow buildings, especially via first draw sample collections as recommended by some authorities, is fraught with limitations, namely: culturing will only capture planktonic free floating viable Legionella, and will often miss sessile Legionella replicating within biofilms. The absence of viable Legionella in a water sample doesn’t necessarily mean that Legionella is not present within the system, nor does it mean the water is indeed ‘safe.’ Therefore, flushing accomplishes at least a few useful things such as: 1. Helps to re-establish disinfectant residuals at distal points of use, 2. Overcomes hydraulic blockages caused by no flow/low flow conditions, 3. Provides an opportunity to capture free floating Legionella via sampling (provided viable and culturable organisms are present during sampling.) It is important to measure and record incoming disinfectant levels, and compare/contrast them to distal fixtures in order to help determine organic disinfectant loading throughout the building. In addition to the WA Dept of Health guidelines referenced in the blog, the following documents provide additional useful guidance: CDC Guidance for Building Water Systems; ESPRI; ESGLI; AWWA; and Purdue University Considerations for Large Building Water Quality after Extended Stagnation, among others. All of these should be consulted and taken into consideration before re-opening/recommissioning a previously stagnant building.
Excellent points about the limitations of Legionella testing and the need to monitor POE and POU disinfectant levels, Dominick. Thanks for commenting, and for the good work that you and ChemAqua do.
Maybe I missed it…but what about Ice Pigging (IP) to clean a water line and then, of course, flush and disinfect? My understanding is that IP removes biofilm but does not damage the pipeline.
I don’t have experience with ice pigging and haven’t seen reports of it used in large domestic (potable) plumbing systems. Although it might be beneficial for removing deposits in main lines, it wouldn’t reach the small piping and crevices with valves and fittings where biofilm can be especially problematic. Also, for control of Legionella and other biofilm-associated pathogens, continuous biofilm control is needed. Biofilms would likely be re-established within days after a procedure.
The idea behind your suggestion is excellent, though. A few years ago, a consulting client (hospital) with a Legionella problem began treating its domestic water system with a chemical deposit remover and it worked quite well. (They’re probably still using it.) However, I’ve seen Legionella counts return very soon after such treatment is stopped. It needs to be continuous.
Thanks very much for commenting, Mike.