Regular Flushing Helps Combat Legionella in Buildings

Low- and no-flow conditions occur more often than you might think. Consider a school or residence hall during summer break, an office building closed over a holiday, or a hospital room not used for patients for a few weeks. During that time, water moving slowly or not at all from the system inlet to where it's consumed can cause stagnation and allow bacteria and microorganisms to proliferate and concentrate to unhealthy levels.

Essential to a flushing program are the right protocols, including testing to confirm the water quality at both the water inlet and outlet. Here's more information on the effectiveness of flushing and the procedures to follow.

Pipe Flushing's Role in Combatting Pathogens

All life depends on water, including harmful pathogens, such as Legionella, Pseudomonas spp., and nontuberculous mycobacteria. Especially when it's stagnant, tepid, or both, water makes an excellent disease carrier. Legionnaires' is probably the best known building-related disease, but other microorganisms may also subject occupants to lung, skin, and gastrointestinal infections. Risk is particularly high for immunocompromised or elderly individuals.

It's important to note that conditions for accelerated growth of waterborne pathogens don't occur only during low occupancy. Water circulation can be poor or inconsistent even during business as usual. For instance, greater use of hand sanitizer may mean less water used for handwashing in a hospital. Conservation measures that save water and energy often require lower water flow and temperature, increasing disease risk.

The WRF report noted three key factors that contribute to microorganisms survival and growth:

• Biofilms in building pipes that provide the carbon and nutrients bacteria need to grow
• Water temperature between 70ºF to 120ºF (20ºC to 50ºC), which provides a hospitable environment for Legionella
• Loss or absence of residual disinfectant over time.

So how does flushing help? Flushing removes biofilms, keeps water either colder or hotter than the temperatures at which Legionella bacteria are active, and replenishes residual disinfectant to an effective concentration. Most significantly, flushing helps reset water age, which affects other factors such as biomass accumulation, temperature, and disinfectants.

Considerations for Effective Water Flushing

Factors crucial to using flushing effectively as a corrective action or ongoing control include frequency and timing. The researchers recommended a maximum water age of seven days if residual disinfectant is present and three days if not. To minimize hazards, flushing should be conducted before water users could be exposed to pathogens and outside normal or peak hours of use. The report provided a number of additional protocol recommendations for flushing efficacy.

The researchers also explored the duration of flushing—whether it should be fixed or adapted based on the water quality. Either way, it's essential to confirm that the outlet water quality is the same as the inlet quality, which requires field measurements. The team set the fixed duration flush at 30 minutes and compared that with the adaptive approach. For the adaptive duration, each outlet was flushed until baseline water quality was observed, plus an additional five minutes before stopping. The researchers found that the adaptive duration flush could save about 80% of water and energy compared to the fixed duration.

Understanding the baseline quality of inlet water to the building is crucial to a decision on flushing for correction or control. Local water utilities can typically provide detailed information for a specific location. Before flushing, it's vital to ensure that the quality of the water in the distribution network justifies the effort and resources required.