Flushing outlets within healthcare facilities is a commonly undertaken measure that involves discharging a volume of water – hot and cold – at selected locations as a compensating behaviour and to reduce hazards such as stagnancy, poor water quality indicators (microorganisms, turbidity, odour, taste), engineering works, commissioning and handover phases of new equipment, a new build or refurbishment project. However, flushing activities are often undertaken without a rationale and water is not sequentially refreshed through the building. Additionally, flushing activities are often unverified as efficacious lacking necessary achievement of temperatures, biocide levels or microbial counts, and rarely evidenced as completed as opposed to a ticked compliance sheet or scanned barcode.
Our experience with continuous remote sensor monitoring across multiple healthcare sites has clearly illustrated significant underuse at tap, shower and plumbed equipment outlets. Even where rooms are occupied by presumed mobile and active patients, the supporting water services are often not in daily use. Extended periods of non-use are common. Clinical staff are less likely to recognise low or no use outlets when rooms are occupied, housekeeping teams assumed to undertake a “wet” cleaning process are frequently evidenced otherwise, and flushing teams may not flush all expected outlets or do not flush for long enough durations. Are outlets facing persistent waterborne pathogen challenges linked with excessive stagnancy due to low use? Certainly, where there is excessive stagnancy there are conditions conducive to microbial amplification.
So, how can Water Safety Groups improve water movement and reduce risk to patients in their facilities? Removing underused outlets is the sustainable approach which eliminates waterborne pathogen risks to patients. However, there will always be circumstances where regular flushing is required. How can these outlets best be managed including flushing? Two recently published papers may help shed some light on improving flushing practice.
Last month a team from Belgium (Waegenaar et al., 2025) published on post-flush microbial and biofilm outcomes. They set up a pilot scale drinking water system in a laboratory setting allowing well controlled conditions. Three identical x 100 meter PVC plumbed recirculating loops were built and the impact of cold water flushing on microorganism counts, types and biofilm densities assessed in the loops following:
- Loop 1 – initially flushed with tap water containing sodium hypochlorite (0.5 mg/L) followed by two flushing cycles of tap water only
- Loop 2 – no flushing activity, and acted as the control
- Loop 3 – flushed three times with tap water only (no biocide)
The team also monitored the survival of three unwanted indicator microorganisms (Aeromonas media, Pseudomonas putida, and Serratia fonticola), and the susceptibility of biofilms to these bacterial invaders. Additionally, a coupon system was used to sample and analyse biofilm cell densities and microbial composition. After the flushing procedures, water quality was followed in the loops for 2 weeks and the experiments were performed twice.
Flushing with chlorination (loop 1) led to increased cell counts and a lower diversity of microorganisms vs loops 2 and 3. This was noted after a period of 3 days. After 7 days the results indicated that the highest microbial regrowth was in the sodium hypochlorite loop, with the lowest (unsurprisingly) from loop 3. Many studies have reported that systemic disinfection can lead to uncontrolled regrowth with the presence of dead organic material likely promoting growth of the protected or biocide tolerant microorganism community.
Flushing with chlorination also resulted in lower microbial diversity over the longer term, indicating that overall biostability was affected. However, the indicator organisms did not amplify proportionally in the chlorinated loop. Higher temperatures (20 degrees vs 16 degrees) instead were found to enhance the persistence of the unwanted indicator microorganisms. Overall, these authors found that flushing with chlorinated waters promoted microbial regrowth, highlighting a trade-off between part control and regrowth.
It is a complex picture and biostability of a water system can be achieved without systemic disinfection, as is the practice in the Netherlands and Germany for example. Understanding such dynamics is essential for optimising flushing SOPs and reducing impacts of unintended consequences such as regrowth. Often microbial levels are not agreed prior to the initiation of a flushing regimen – this can be crucial, particularly when managing an aging infrastructure.
The second paper is from a Portuguese research team (Silva et al., 2025) who reflect on controlling Legionella and Legionnaires Disease through effective water movement and biofilm mitigation. In their laboratory model which utilised biofilms grown in well-plates, Legionella was found to migrate deep within the biofilm where there were stagnant conditions, whereas higher flow conditions favoured the transition of Legionella pneumophila to Viable But Non-Culturable status. Under either condition it could be difficult to assess true risk of pathogen transmission from the water system using traditional culture plate methods of analysis. This research highlights the increased risk posed by biofilms and stagnation, emphasising the importance of more effective monitoring and prevention strategies.
Is it time to rethink your flushing regimens? If you would like to discuss either of these papers in more detail, consider improvements and evidencing with current flushing activities or review the removal of water services from patient care areas to reduce the risk of healthcare acquired infections, please contact us at info@harperwater.com
References
Waegenaar F, Pluym T, Vermeulen E, De Gusseme B, Boon N. Impact of flushing procedures on drinking water biostability and invasion susceptibility in distribution systems. Appl Environ Microbiol. 2025 Jun 18;91(6):e0068625. doi: 10.1128/aem.00686-25. Epub 2025 May 13. PMID: 40358239; PMCID: PMC12175504.
Silva AR, Keevil CW, Pereira A. Legionella pneumophila response to shifts in biofilm structure mediated by hydrodynamics. Biofilm. 2025 Jan 24;9:100258. doi: 10.1016/j.bioflm.2025.100258. PMID: 39957834; PMCID: PMC11830327.
https://www.sciencedirect.com/science/article/pii/S2590207525000061?via%3Dihub