What Is Biofilm and Why It Matters for Water System Safety

What is biofilm? Biofilm is the slimy layer that builds up on the inside surfaces of pipes, tanks, fittings and other wetted parts of a water system. It is a community of bacteria and other microorganisms held together in a protective layer they produce themselves, and it sticks firmly to almost any surface that stays wet. Once it establishes, it becomes the main reservoir where waterborne bacteria, including Legionella, can shelter and multiply.

For anyone responsible for a building’s water systems, this matters for a simple reason: you can hold perfect temperature records and still have a colonised system if biofilm has taken hold somewhere out of sight. Most of the systems we assess have biofilm present to some degree. The question is rarely whether it exists, but whether it is being controlled well enough to keep risk down.

Understanding what biofilm is, where it forms and why it shelters Legionella is the difference between managing a water system properly and just filling in a logbook. It also explains why some buildings keep returning positive sample results despite apparently good control.

What Biofilm Actually Is

At its simplest, biofilm is bacteria living together on a surface inside a protective matrix. The bacteria attach to the surface, multiply, and secrete a sticky substance often described as extracellular polymeric substance. That matrix is what gives biofilm its slimy texture and, more importantly, what makes it so difficult to shift.

It is not a single organism. A mature biofilm can contain dozens of different bacterial species, along with amoebae, fungi and other microorganisms, all living in the same layer. Some feed off each other’s waste products. Some protect others from disinfection. This mixed community is part of why biofilm is so resilient.

The practical point for water hygiene is that biofilm behaves very differently from free-floating bacteria in the water itself. Bacteria sitting in the flow are relatively easy to kill with the right temperature or biocide. Bacteria embedded in biofilm are not. The matrix shields them, slows the penetration of disinfectants, and lets them survive conditions that would normally control them. That is the core problem the rest of this article comes back to.

How Biofilm Forms Inside Water Systems

Biofilm formation follows a fairly predictable sequence, and understanding it helps explain why certain parts of a system are far more prone to colonisation than others.

It starts when a few bacteria attach loosely to a wetted surface. If the conditions suit them, they stay, multiply and begin producing the protective matrix. Over time the layer thickens, traps nutrients and sediment from the water, and develops its own internal structure. Mature biofilm can periodically shed cells back into the water, which is how a colonised section of pipework keeps reseeding the rest of the system.

Several conditions speed this up:

• water sitting still for long periods rather than flowing
• temperatures in the range that bacteria prefer, broadly between 20 and 45 degrees celsius
• nutrients such as sediment, scale, rust and organic debris
• surfaces that are rough, corroded or made from materials that support growth
• poor design with dead legs, oversized tanks or low-use outlets

Stagnation is usually the biggest single factor. Moving water disturbs early colonisation and helps maintain disinfectant or temperature control throughout the system. Water that sits, whether in a little-used shower, a capped-off branch or an oversized storage tank, gives biofilm the time and stability it needs to establish. This is why water system stagnation features so heavily in any serious discussion of bacterial growth in pipework.

Why Biofilm Creates a Legionella Risk

Legionella bacteria are naturally present in the environment and find their way into building water systems through the mains supply at low levels. On their own, in clean flowing water at controlled temperatures, they struggle to multiply to dangerous numbers. Biofilm changes that.

The relationship between Legionella and biofilm works in a few ways. The matrix provides shelter and a steady supply of nutrients. Other organisms in the biofilm, particularly amoebae, can actually host Legionella and allow it to multiply inside them, which protects it further from disinfection. And because biofilm resists the temperatures and biocides that would normally control free bacteria, a Legionella biofilm can persist in a section of pipework even when the rest of the system appears to be under control.

This is the part that catches people out. A facilities manager may be running hot water at the right temperature and recording it diligently, but if biofilm has established in a dead leg or a poorly used outlet, that area can remain a source. When water moves through it, or when the outlet is finally used, contaminated water and shed bacteria enter the wider system. If that water is then aerosolised through a shower, tap or cooling system, it can be inhaled. That is the route by which Legionnaires’ disease develops.

It also explains a frustration we see regularly: a building that keeps returning low-level positive samples despite good temperature control. In those cases the issue is almost always biofilm sitting somewhere the monitoring regime does not reach, quietly reseeding the system.

Where Biofilm Builds Up in Buildings

Biofilm does not form evenly across a system. It concentrates in the parts that combine stagnation, suitable temperature and available nutrients. Knowing where to look is half the job.

Common locations include:

• dead legs and blind ends left behind after alterations or removed equipment
• infrequently used outlets such as showers in spare rooms, cleaners’ sinks and seldom-used taps
• cold water storage tanks with poor turnover or that are oversized for the building
• calorifiers and hot water cylinders where sediment and scale collect at the base
• shower heads and flexible hoses, which trap water and warm up between uses
• thermostatic mixing valves, where blended water sits in the ideal growth range
• flexible hoses and rubber components, where the materials themselves can support growth

In real buildings, the worst offenders are often the areas nobody thinks about. A shower in a disused changing room. A branch that fed a piece of plant that was decommissioned years ago and never removed. A tank in a loft space that turns over far too slowly because demand has dropped since the building was first occupied. These are the spots where biofilm builds undisturbed, and they are often the same spots that get missed during routine checks because access is awkward or because they were never recorded properly in the first place.

This is also why a thorough Legionella risk assessment matters. A desk review of the logbook will never find the capped dead leg behind a riser panel. Someone has to go and look at the actual system.

Controlling and Removing Biofilm in Practice

The honest position is that you cannot keep a water system permanently free of biofilm. The realistic aim is to keep it suppressed to a level where bacterial growth, including Legionella, stays under control. That is what good water hygiene management is actually about.

The most effective control is also the most basic: keep water moving and keep it at the right temperature. Hot water should be hot enough and cold water cold enough to discourage bacterial growth, and water should not be allowed to sit. The control regimes set out in the HSE’s approved code of practice, L8, and the supporting HSG274 guidance are built around exactly this principle. Regular flushing of low-use outlets, sensible tank turnover and the removal of redundant pipework all reduce the conditions biofilm needs.

Where biofilm has already established and is causing problems, more direct intervention is usually needed:

• cleaning and disinfection of tanks, calorifiers and affected sections
• chemical or thermal disinfection of the system to knock back the bacterial load
• physical removal of scale, sediment and debris that feed the biofilm
• descaling and cleaning of shower heads and hoses, or replacing them where they are heavily fouled
• removing dead legs and correcting design faults that drive stagnation

It is worth being realistic about disinfection. A one-off chlorination will reduce bacterial numbers, but if the underlying cause, usually stagnation or a design fault, is not addressed, the biofilm will re-establish and the problem will come back. Effective biofilm removal is rarely a single event. It is the combination of removing the source conditions and then keeping the system properly managed afterwards. Disinfection without addressing the cause buys time, not a solution.

Common Mistakes That Let Biofilm Take Hold

Most biofilm problems we come across are not caused by anything exotic. They come from gaps in day-to-day management and from systems that have drifted out of line with how the building is actually used.

The recurring ones are worth setting out plainly:

• treating temperature monitoring as the whole job, when a system can be colonised regardless of recorded temperatures
• logging failed readings without anyone actually investigating or acting on them
• leaving little-used outlets unflushed because nobody has been told they need it
• keeping dead legs and redundant pipework in place after equipment is removed
• running storage tanks that are far too large for current demand, so water turns over slowly
• treating a contractor visit as compliance, without anyone reviewing what the contractor actually found
• filing monitoring records and survey reports without checking that the recommendations were ever closed out

That last point comes up constantly. A risk assessment will identify a dead leg or an oversized tank, the report goes on the shelf, and the action tracker stays blank. Two years later the same finding appears in the next assessment, and the biofilm has had two more years to settle in. The records exist, but nobody has been driving the actions to completion.

The other common failure is assuming responsibility has been delegated cleanly. A landlord may believe the managing agent has the water hygiene side covered, the agent may believe the contractor is making the judgement calls, and the contractor is simply turning up to take readings. Nobody is reviewing the system as a whole. Biofilm thrives in exactly that kind of gap.

When to Bring in Professional Support

Plenty of routine water hygiene work can be handled in-house with the right training, but there are points where specialist input is sensible rather than optional.

It is usually worth bringing in support when you are seeing repeated positive Legionella sample results despite apparently good control, when temperature readings keep failing in particular areas, or when you genuinely do not know what your system looks like because records are incomplete or out of date. The same applies after building alterations, changes in occupancy or any period where parts of the system have been left unused, such as a prolonged closure.

A competent assessor will physically inspect the system rather than relying on paperwork, identify the dead legs and stagnation points that drive biofilm, and set out a control scheme that fits how the building is actually used. Where colonisation has taken hold, they can also specify and oversee the cleaning, disinfection and remedial works needed to deal with it properly, not just suppress it for a few weeks.

The commercial case is straightforward. A managed water system with controlled biofilm is far cheaper to run than one that lurches from positive result to emergency disinfection, and it keeps you on the right side of your duties under L8 and HSG274. Reacting to a problem after a confirmed case of illness is the most expensive route of all, both financially and in terms of reputation and potential enforcement.

Frequently Asked Questions About Biofilm

Can you see biofilm in a water system?

Sometimes, but not always. Heavy biofilm can show as a visible slimy film inside a tank or on a removed shower head, and you may feel it as a slippery surface. Much of it, though, sits inside pipework where you cannot see it at all. Its presence is more reliably inferred from sample results, the condition of accessible components and the design of the system than from a visual check alone.

Does disinfecting the system get rid of biofilm permanently?

No. Disinfection reduces the bacterial load and knocks the biofilm back, but it does not remove the conditions that allowed it to form. If stagnation, dead legs or poor temperature control remain, the biofilm will re-establish. Lasting control comes from fixing those underlying causes and then maintaining the system, with disinfection used as part of that, not as a standalone fix.

Is biofilm a problem in small or domestic-style systems?

It can be. Biofilm forms wherever water sits in suitable conditions, so a small system with infrequently used outlets or an oversized tank can still develop it. Risk depends far more on how the water is used, stored and aerosolised than on the size of the building. Smaller premises with vulnerable occupants, such as care settings, often warrant closer attention rather than less.

How quickly does biofilm form after a period of stagnation?

It can begin within days where conditions are favourable, particularly in warm, still water. This is why outlets that have been unused for a while, or systems left dormant during a closure, are treated as a higher risk and why flushing regimes and recommissioning checks matter so much before a building is brought back into normal use.

Will more frequent sampling solve a biofilm problem on its own?

No. Sampling tells you what is happening in the water at a point in time, but it does not control anything by itself. It is a monitoring tool, useful for confirming whether your controls are working. If sampling keeps flagging issues, the answer is to investigate and correct the cause, not simply to sample more often.

If your water system keeps returning positive results, your records no longer match how the building is used, or you are not confident biofilm is being kept under control, Acorn Safety Services (Legionella) can carry out a thorough risk assessment, pinpoint the stagnation points and design faults driving the problem, and oversee the remedial work needed to bring it back under control. Contact us to talk through the specifics of your system, or get a quote directly.