The inside of a pipe is rarely the clean bore the drawings imply. Years of hard water, oxygen and dissimilar metals leave a crusted, pitted, roughened surface, and that surface, not the water passing over it, is where biofilm takes hold and where Legionella shelters.
This matters because most control effort points at the water: temperatures, flushing, the odd disinfection. The wall the water touches gets far less attention, yet it is the wall that decides how readily a biofilm forms, how much food it has, and how hard it is for a disinfectant to reach the bacteria living in it.
Why a rough, scaled wall is different from clean pipe
Biofilm needs a surface to attach to. A smooth, intact internal bore gives micro-organisms little purchase; a rough or fouled one gives them a vast, sheltered landscape. Three surface conditions push that the wrong way.
Mineral scale. In hard-water areas, much of the south and east of England, calcium and magnesium come out of solution as limescale, fastest where water is heated. A calorifier base, an immersion, the first metre of a hot run: these scale up over time. Scale is not inert decoration. It is porous and enormously rough at the microscopic level, multiplying the surface area available for attachment and creating shielded pockets that the bulk flow never scours.
Internal roughness. Even without heavy scale, an aged or poorly jointed bore carries ridges, burrs and weld lines. Rougher walls are colonised more readily and hold biofilm against shear better than smooth ones. The general direction of the evidence is that smoother surfaces resist fouling and rougher ones accumulate it.
Corrosion. Where pipe metal degrades, pitting in copper, tuberculation and rust nodules in steel, galvanic attack where copper meets galvanised steel, the products are bulky, irregular and chemically active. Corrosion deposits add roughness and food, and the oxidising demand of bare, corroding metal can consume disinfectant that was meant for the organisms.
Put together, scale, roughness and corrosion turn a pipe wall from a barrier into a habitat: more surface, more shelter from flow, more nutrients, and a chemistry that works against the very treatments used to clean it [1][2].
What a cross-section of a scaled, corroding pipe wall looks like
Picture cutting a fouled hot-water pipe in half lengthways and looking at the wall from the inside out. Four layers stack up, and the order explains the risk.
- The base metal (or plastic) wall. The original pipe. In corroded steel this layer is no longer flat, it carries pits and nodules; in copper it may show pinprick pitting.
- The deposit layer: scale and corrosion products. Sitting on the wall, a crust of limescale and/or rust that is porous, irregular and far rougher than the metal beneath. Its valleys and pores are dead-water micro-spaces the flushing flow never reaches.
- The biofilm: a hydrated matrix anchored in that crust. Micro-organisms embed in a slime of their own making, threading into the pores of the scale. Within this matrix sit protozoa, amoebae, and inside those amoebae, protected further still, Legionella can multiply.
- The diffusion boundary at the surface, facing the flowing water. Between bulk water and biofilm lies a thin, near-stationary layer. A disinfectant in the bulk water has to cross that boundary, then diffuse down through the biofilm matrix while the scale and corroding metal consume some of it on the way. By the time it reaches the deepest cells, far less is left than the dose at the tap [1].
Sketch those four bands and the lesson is plain: the bacteria sit at the bottom of a stack that is built to shield them, and surface condition decides how thick and how protective that stack becomes. This is the surface-condition side of the story; for how Legionella exploits the resulting biofilm habitat itself, see Biofilms: how Legionella hides in plumbing systems.
Why this beats biofilm with disinfectant alone
A free-floating Legionella cell is comparatively vulnerable. The same cell inside a mature biofilm, in a scaled and corroding pipe, is not, the matrix and deposit slow chemical penetration and the corroding metal soaks up oxidising capacity, so a dose lethal in clean water under-performs against the colony underneath [1]. That is the mechanical reason eradication from heavily fouled systems is so stubborn, explored from the biofilm side in Legionella in biofilms: why eradication is difficult.
It also reframes maintenance. Where heavy scale or corrosion is present, removing the deposit, descaling a calorifier, replacing a tuberculated section, cutting out a corroded dead leg, addresses the habitat itself, not just the passing water. Cleaning the surface before or alongside disinfection gives the chemical a fighting chance of reaching the organisms rather than being spent on the crust.
What surface condition can, and cannot, tell you
Here is the line that matters. A heavily scaled calorifier, rust-stained water, or visibly corroded pipework raises the risk and warrants closer inspection. It is a prompt, not a finding. Pipe condition does not tell you whether Legionella is present, how much, or whether the system is safe, only sampling under controlled conditions, alongside the statutory controls on temperature, stagnation and cleanliness, speaks to that.
So treat condition as a prioritisation aid. Deposits, scale, sludge and corrosion are recognised factors when assessing a system’s condition and when deciding where to look hardest, and they belong in the risk assessment as such [2]. They feed where you direct sampling, inspection and remedial work, they never replace it.
Where to look first in an older or hard-water building
Concentrate on the places scale and corrosion concentrate. The calorifier or cylinder base and the immersion, where heat drives scale out of solution. The first stretch of hot pipework off the heat source. Galvanised tanks and steel pipe nearing end of life, and any copper-to-galvanised junction. Infrequently used branches and dead legs, where there is no flow to limit deposit build-up at all. An inspection that opens an inspection hatch and actually looks at the internal surface, or samples deposit from a base, tells you more about habitat than a temperature log alone.
The mechanism behind all of this is the organism’s own growth biology; the surface conditions here are what make the building’s particular pipework hospitable to it, and the two are read together in Legionella life cycle and growth conditions.
A closing caveat, freshly put: everything above is general explanation of mechanism. Which deposits matter in your building, whether descaling, repair or replacement is warranted, and how that sits within your monitoring regime are judgements for a competent person assessing your specific system, in line with BS 8580-1 and the ACoP. This is not legal, medical or engineering design advice.
FAQ
Does limescale itself harbour Legionella, or just provide a surface?
Both effects matter, but the surface effect is the point. Scale is porous and very rough, so it multiplies the area and the sheltered micro-spaces available for biofilm to attach and resist flushing. The biofilm anchored in and on that scale is what hosts the amoebae and bacteria. So scale is best understood as habitat-builder rather than a reservoir in its own right, which is why removing it changes the conditions for growth.
Will descaling a calorifier on its own remove the Legionella risk?
No. Descaling reduces the habitat and improves how well subsequent disinfection and heat penetrate, which is worth doing, but it is not a control on its own, and it does not confirm the system is clear. It sits alongside, never instead of, temperature control, flushing and any disinfection your risk assessment specifies, with verification by sampling where appropriate.
Are plastic pipes immune to biofilm?
No. Plastics such as PEX, polybutylene and uPVC do not corrode or scale the way metals do, which removes those particular roughness and nutrient sources. But biofilm forms on every wetted material, and some plastics and elastomeric fittings can themselves leach compounds that feed growth. Smoother and corrosion-free helps; immune it is not.
Can I tell from pipe condition whether a system is contaminated with Legionella?
No, and this is the key limit. Scale, rust and corrosion are risk indicators that should trigger inspection and inform where you sample. They are never a measurement of Legionella and never proof a system is contaminated or safe. Only controlled sampling, read against your temperature and cleanliness control, addresses presence.
Sources
- World Health Organization, “Legionella and the prevention of legionellosis”. https://www.who.int/publications/i/item/9241562978
- HSE, HSG274 Part 2 (2024), “Operation and inspection of hot and cold water systems”, p.70. https://www.hse.gov.uk/pubns/books/hsg274.htm