Flushing Closed Buildings after COVID 19Is 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.


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