WATER QUALITY
irrelevant. What matters is whether microorganisms can move from either side into the space around them.
H2 0 Inlet Water seal
Aerosol formation in standard trap
Water seal
Aerosol formation in Hygiene-Siphon
Aerosols and the drain problem This is where the issue becomes especially important for healthcare design and operation. When water from a tap strikes the basin and drain, it creates turbulence, splash, and fine droplets. Depending on the geometry of the sink, the drain, the force of the water, and the level of contamination present, microorganisms associated with drain biofilms may be dispersed into the surrounding space.
Figure 5: Aerosol formation in standard traps versus hygiene siphons.
Recent research by Kotay et al. (2025) is especially
relevant here. The study demonstrated that hospital sink drain biofilms could contribute directly to the microbiome of room air and surfaces, with microorganisms transmitted from drain sources into the surrounding environment. That matters because it moves the drain issue from a theoretical reservoir problem into a plausible exposure pathway. This is reflected in current NHS guidance. NETB
No.2024/3 emphasises basin designs that minimise splashing, avoid direct discharge into the drain, and prevent backflow and standing water. It also highlights the risk of contamination from drain contact and splashing, even when filtered water is used. This underlines a broader point: safe water is not only about what comes out of the tap, but also about what happens at and below the drain. Stagnation-related risk must therefore be understood
as a system issue. While upstream stagnation promotes biofilm formation in pipework, drains can act as downstream reservoirs. Splashing and aerosols may then transfer microorganisms back into the patient environment.
Joining up the risk picture Once supply-side stagnation and drainage-side aerosolisation are viewed together, a more complete picture emerges.
Stagnation in pipework promotes biofilm formation and microbial persistence upstream, while low-use outlets increase water age and create favourable conditions
for opportunistic pathogens. At the same time, drainage systems can act as downstream reservoirs, with splashback or aerosols redistributing microorganisms into the surrounding environment. These are not separate issues, but connected parts of
the same water system. In high-risk clinical settings, this matters. Patients are more vulnerable, and ‘close enough’ is not sufficient. It also challenges the traditional divide between engineering and infection prevention – stagnation sits at the intersection of both, with consequences that directly affect patient safety.
From single measures to layered protection An effective response to stagnation cannot rely on a single intervention. It requires a layered approach. At system level, good design remains essential – avoiding oversizing and dead legs, ensuring proper circulation, and minimising splash. Operationally, maintaining regular water turnover through structured flushing remains a core control measure, supported where necessary by monitoring or automation. At the point of use, additional barriers may be required. NETB No.2024/3 highlights that high-risk patients should receive sterile water or water via 0.2 µm point- of-use filtration. More broadly, end-of-line filters provide protection where immediate microbiological assurance is needed. The drainage side must also be considered. Where splash or aerosolisation from contaminated drains is a credible risk, hygienic siphon design can help reduce aerosol generation and limit contamination of the surrounding area. Together, flushing, filtration, and hygienic drainage form a complementary, layered strategy – often the most practical way to manage risk in complex healthcare environments.
The human factor in water safety Technical documents are essential, but they do not by themselves keep water safe. People do. This is sometimes forgotten in conversations that become dominated by temperatures, flow rates, and sampling plans. In practice, water safety depends on how well estates, engineering, infection prevention, clinical teams, and contractors understand one another’s concerns and translate guidance into routine action. A little-used outlet is not just an engineering observation. It may be the result of a changed clinical workflow. A sink that is difficult to use without splash may be telling a design story. A repeated need for manual flushing may indicate a deeper mismatch between system layout and actual service delivery. The organisations that manage stagnation well are often
If stagnation were solely the result of poor design, the answer would be comparatively straightforward. But stagnation in healthcare is not only a design issue. It is operational, organisational, and structural.
44 Health Estate Journal August 2026
those that treat water safety as a live operational issue rather than a paper exercise. They ask not only whether there is a policy, but whether the policy fits the building as it is really being used. They look for recurring weak points. They review how spaces are changing. They recognise that water risk is not static simply because the pipework is hidden. This is particularly relevant when handovers occur
between project teams and operational teams. A system that looks compliant at commissioning may behave very differently six months later once the space is occupied in a less predictable way than originally planned. Likewise, a ward redesign may solve one clinical problem while unintentionally creating another through awkward outlet positioning, changed use patterns, or poor sink-drain
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