When water stands still: the invisible risks

In healthcare estates, risk does not always arrive with a siren. More often, it develops quietly, within systems that appear to be functioning normally while gradually drifting away from the conditions they were designed to maintain. Few issues illustrate this better than stagnation in potable water systems.

Water, in a healthcare setting, is almost instinctively associated with cleanliness and safety. It is the medium of hand hygiene, patient washing, clinical cleaning, food preparation, and countless routine activities that support care. Yet that sense of safety is tied to a basic assumption: that water is moving, turning over, and remaining within the parameters intended by design and operation.

In reality, it often is not.

Across modern healthcare estates, water regularly sits idle. Taps are used less often than expected. Clinical areas are refurbished, decanted, or repurposed. Rooms may stand empty for days, sometimes weeks. Some outlets are installed for flexibility, only to become rarely used in practice. In those moments, the system does not simply pause. It changes. And those changes are rarely benign.

Stagnation is not an unusual event. It is a routine feature of complex buildings. That is precisely what makes it dangerous. It does not arise only from failure. It emerges from ordinary operational realities: changing occupancy, intermittent use, legacy infrastructure, over-capacity, and the unavoidable mismatch between how systems were planned and how buildings are actually used. Despite this, stagnation is still too often treated as a secondary concern, something to be managed through occasional flushing, rather than recognised as something that can materially increase risk in healthcare water systems.

What happens when water stops moving

One reason stagnation remains underestimated is that it is often imagined as a problem of prolonged neglect. The phrase itself can suggest water left untouched for weeks. In practice, the process begins much earlier.

Once flow is reduced or absent, the conditions within the system begin to shift. Water age increases, temperatures begin to drift, and the hydraulic forces that normally act on internal surfaces are reduced.

These changes do not initiate biofilm formation — biofilms are a natural and ubiquitous feature of water systems — but they significantly influence how they develop and persist.

Under conditions of low or no flow, reduced shear allows microorganisms to remain attached more easily, while increased residence time provides greater opportunity for growth and interaction with surfaces. What may begin as initial adhesion can more readily progress into stable colonisation. This is the point at which biofilms become more consequential.

Biofilm is not simply a coating of bacteria on a pipe wall. It is a structured microbial community embedded in a protective matrix. Once established, it changes the behaviour of the system. It provides protection from environmental fluctuations and physical disturbance, retains nutrients, and enables microorganisms to persist and multiply in ways that free-floating cells cannot.

Stagnation + biofilm = invisible risk for drinking water hygiene

In practical terms, this means that a water system affected by stagnation is no longer behaving only as engineered pipework. It has become, in part, a biological habitat.

That distinction matters in healthcare. Opportunistic pathogens such as Legionella pneumophila and Pseudomonas aeruginosa do not require dramatic system failures. They depend on favourable conditions: time, suitable temperatures, and the presence of stable microbial communities. Stagnation can help provide those conditions.

Just as important, biofilms do not remain static. They can intermittently release microorganisms back into the water — often unpredictably. This makes them difficult to detect and even harder to control. A sample taken at a single point in time may appear reassuring, while the underlying reservoir persists within the system.

In this sense, stagnation does not create biofilms, but it creates the conditions in which they become more established, more resilient, and more difficult to manage.

Why healthcare estates are particularly vulnerable

If stagnation were solely the result of poor design, the answer would be comparatively straightforward. Remove the dead legs, correct the oversizing, improve flow, and the problem would largely be solved. But stagnation in healthcare is not only a design issue. It is operational, organisational, and structural.

Hospitals are dynamic places. Wards open and close. Service lines shift. Departments expand, contract, merge, and move. A basin that is heavily used one year may be largely redundant the next. A room designed for one type of patient pathway may end up serving another. Even where the original design is sound, the actual use of the building changes over time, often significantly.

There are also practical realities that estates and infection prevention teams know all too well: temporary decants, delayed openings, phased refurbishments, seasonal pressure, and the need to preserve flexibility in the estate. Every one of these can leave parts of the water system under-used.

This is not a niche issue. It is normal healthcare operation.

Stronger focus on prevention

UK guidance has long recognised that safe water depends on both design and operation. HTM 04-01 and HSG274 Part 2 set out the core principles for maintaining hot and cold water hygiene in healthcare and other complex premises. Their emphasis on temperature control, circulation, maintenance, and regular use reflects a long-standing understanding that microbiological risk is shaped by conditions within the system, not merely by end-point testing.

What has become clearer in recent years, however, is that the bar is rising, especially where high-risk patients are concerned.

The NHS Estates Technical Bulletin (NETB) No.2024/3 sharpened that focus. Issued in the wake of serious concern about waterborne infection risk in specialised clinical environments, it places notable emphasis on design and operation that avoid stagnation, minimise splashing, reduce exposure to drains, and ensure that water systems serving vulnerable patients do not create avoidable opportunities for contamination. It is not only a technical supplement. It is, in effect, a reminder that safe water infrastructure must be thought through as part of patient safety, not just building compliance.

That shift in tone matters. It moves the conversation beyond ‘Have we sampled?’ or ‘Have we documented flushing?’ and towards a more searching question: are we actively preventing the conditions in which harmful microorganisms can establish, persist, and reach patients?

The operational burden of flushing

Flushing remains one of the most familiar and widely applied control measures, and for good reason. The principle is simple: replace stagnant water with fresh water, restore the intended thermal and hydraulic conditions, and reduce the opportunity for microbial growth.

There is nothing wrong with this principle. In many cases, it is essential.

The difficulty lies in implementation. Manual flushing programmes rely on people, and people work in real organisations under real pressures. Staff need clear responsibility, sufficient time, access to outlets, reliable records, and a process that can actually be sustained week after week. In a large estate, that is no small ask.

It is one thing to say that every low-use outlet should be flushed at the required interval. It is another to achieve that consistently across a changing hospital footprint with competing priorities, staffing constraints, refurbishments, and intermittent service disruption.

This is not an argument against flushing. It is an argument against pretending that flushing is easy.

In practice, the more complex the estate, the more important it becomes to distinguish between policy and dependable delivery. A flushing regime is only as good as its execution. Where manual regimes are difficult to sustain, additional support may be needed if organisations want the control measure to be more than aspirational.

That is where automated support can enter the conversation in a reasonable, non-dramatic way. In areas with persistently low use, temporary closures, phased openings, or awkwardly located outlets, automated flushing can help maintain turnover more consistently than a regime that relies entirely on busy teams remembering and recording every action. Used well, it is not a substitute for good system design — it is a practical way of supporting intended operation where the estate itself makes consistency difficult.

Water age, design, and the reality of mismatch

The concept of water age is useful here. The longer water remains in a system, the greater the potential for changes in its characteristics — driven by temperature, materials, nutrient availability, and hydraulic behaviour. In complex buildings, water age can vary significantly between outlets.

This is where design and operation intersect. Systems may be sized for demand that never materialises, include capacity for future expansion, or be affected by changing clinical use. Components such as expansion vessels or dead ends can create areas of poor turnover, even where the wider system appears to function adequately.

NETB No.2024/3 is explicit on several of these issues in higher-risk settings. It stresses the need for design with no potential for stagnation, hot water delivery that reaches safe temperatures quickly, wastewater arrangements that do not allow standing water or backflow, and collection points that avoid contamination from splashing and drain contact. That emphasis is important because it recognises a reality often missed in day-to-day discussion: stagnation is not always a maintenance problem that can be corrected later. Sometimes it is built into the way the system has been conceived, sized, or adapted.

It is also one reason why overly simple advice can be misleading. ‘Just flush it’ may be useful as an immediate instruction, but it is not a strategic answer where the underlying issue is a system that no longer matches building use. In those cases, stagnation is a symptom of a deeper misalignment between design assumptions and operational reality.

Looking beyond the tap

For many years, the dominant conversation around water hygiene centred on the supply side: incoming water, stored water, calorifiers, distribution temperatures, little-used outlets, showerheads, and terminal fittings. All of that remains important. But it is no longer enough.

Increasingly, attention is also being directed to the drainage side of the system, and with good reason.

Sink drains, traps, and siphons are microbiologically active environments. They contain moisture, nutrients, and surfaces well suited to biofilm formation. In hospitals, they can become persistent reservoirs for Gram-negative organisms and other water-associated pathogens. This is not just a laboratory curiosity. It has direct relevance to clinical risk, especially where sinks are close to patient activity and hand hygiene.

The key point is that the drain is not separate from the care environment simply because it sits below the basin. Under the right conditions, what grows there does not necessarily stay there.

That matters because healthcare has historically been more comfortable discussing supply-water hygiene than wastewater-associated risk. Yet the patient does not experience those systems as separate worlds. They meet at the washbasin, the shower, the sink, and the surrounding care environment. From the patient’s perspective, the distinction between ‘fresh water side’ and ‘drain side’ is irrelevant. What matters is whether microorganisms can move from either side into the space around them.

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.

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 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 geometry. Without that feedback loop between design, operation, and infection prevention, stagnation remains something people react to rather than something they anticipate.

From compliance to resilience

For years, much of the sector’s effort was understandably framed around compliance: risk assessments completed, records maintained, temperatures checked, sampling reviewed, actions closed. None of that is unimportant. But compliance alone is a limited ambition.

Resilience is a better one.

A resilient water system is not one that merely passes its next inspection. It is one that can continue to maintain safe conditions despite evolving building use, staffing pressures, refurbishments, heat stress, delayed occupancy, and the inevitable surprises of healthcare operation. It is a system that is designed with realistic use in mind, supported by workable controls, and reviewed when assumptions change.

That is where the discussion around stagnation now needs to sit. Not at the edge of water management, but at its centre.

In practical terms, resilience means accepting that stagnation will always try to reappear wherever complexity, change, or inconsistency create the opportunity. It means designing systems that are less vulnerable to low use, choosing control measures that can realistically be sustained, and recognising that both outlets and drains deserve attention if patient exposure is to be reduced.

A final thought

For all the technical detail surrounding stagnation, the governing principle is surprisingly simple. Water that moves is generally safer than water that stands still. Yet turning that principle into reliable practice across a living, changing healthcare estate is anything but simple.

That is why stagnation still deserves more attention than it often receives. It is quiet, commonplace, and easy to underestimate. But it shapes the conditions in which pathogens can persist, multiply, and in some cases reach the patient environment. It affects not only the supply side of the water system, but also the drainage side, where splash and aerosols may carry consequences that were too long overlooked.

In healthcare, water does not need to appear contaminated to become a risk, as standing water can create conditions that favour microbial growth.

Bibliography

• Bacterial recolonization of hospital sink biofilms, Journal: Journal of Hospital Infection. DOI: 10.1016/j.jhin.2025.05.013, PubMed: PMID 40466811
• NHS England. NHS Estates Technical Bulletin (NETB) No.2024/3: Supplement to HTM 04-01 Safe water in healthcare premises. The bulletin emphasises avoidance of stagnation, minimising splashing, controlling exposure to drains and aerosols, and the use of 0.2 µm point-of-use filtration for high-risk patients. (https://www.england.nhs.uk/publication/nhs-estates-technical-bulletin-netb-no-2024-3/)
• NHS England. HTM 04-01: Safe water in healthcare premises. The core UK framework for design, operation, temperature control, circulation, and safe management of hot and cold water systems in healthcare settings. (https://www.england.nhs.uk/publication/safe-water-in-healthcare-premises-htm-04-01/)
• Health and Safety Executive. HSG274 Part 2: The control of legionella bacteria in hot and cold water systems. Practical UK guidance on managing legionella risk, including the role of flushing, temperature control, and maintenance. (https://www.hse.gov.uk/pubns/priced/hsg274part2.pdf)