Most UK buildings that control Legionella well never dose a drop of biocide. They keep hot water hot, cold water cold, water moving, and fittings clean, and that engineering does the job. Water treatment — chemical or physical disinfection added on top — is what you reach for when those fundamentals genuinely cannot carry the load, not a shortcut for when nobody wants to fix the pipework.

That distinction is the whole of best practice here. Treatment is a supplement to good design and temperature control, never a substitute for it. Get the order wrong and you spend money disinfecting a system whose real problem is a forest of dead legs and a calorifier running cool.

When temperature control stops being enough

UK guidance treats temperature as the first-line control: hot water generated and stored hot, distributed so it stays hot to the outlet, and cold water kept genuinely cold [1][3]. As a general expectation that usually means storing hot water at or above 60°C, reaching at least 50°C at the tap within a minute or so, and holding cold below 20°C — but the figures that bind you are the ones your risk assessment sets for your system [1][3].

Supplementary treatment earns its place when you cannot reliably hit or hold those conditions. Sprawling distribution where the far outlets always run tepid. A large, low-occupancy building where flow is too intermittent to keep temperatures up. Systems serving people who are especially vulnerable, where the risk assessment justifies a second barrier. Or a system that keeps returning positive samples despite decent temperatures, where something structural is feeding the bacteria — a case for confirming what you are dealing with through proper detection methods before you act. HSG274 sets out the recognised control options for exactly these situations [2].

The trap is reaching for treatment first. If the underlying cause is stagnation or a cool return, dosing masks the symptom while the reservoir keeps growing in the biofilm.

The main options, and what each is actually good at

There is no single best treatment. Each suits a different system and brings its own running burden [2].

Chlorine dioxide is an oxidising biocide dosed into the supply, valued for penetrating biofilm and holding a residual across a distribution system. It needs careful generation, dosing control and residual monitoring to stay both effective and within limits.

Copper-silver ionisation releases metal ions that persist in the water, which can suit complex hot-water systems where maintaining a chemical residual all the way to distant outlets is hard. It demands attention to ion balance and water chemistry; performance drifts quietly if that slips.

Monochloramine is a more stable chloramine residual used in some large systems for its persistence and lower by-product profile, again dependent on tight dosing control.

Ultraviolet disinfection kills organisms passing the lamp but leaves no residual downstream, so it protects a point in the system rather than the whole network — useful at a tank inlet or feeding one high-risk asset, not as whole-building cover.

Point-of-use filtration physically removes bacteria at the tap or shower and is the pragmatic last barrier for the most vulnerable settings. Cartridges have a finite life and a recurring cost, so it is a control with an ongoing bill attached.

The choice is an engineering decision about your system, the outlets you must protect, your water chemistry and what your team can realistically run and monitor — not a preference for one chemistry over another [2].

Mapping treatment onto the system

The clearest way to test whether a treatment regime is sound is to sketch it. Draw the system as a line from the incoming main: storage, then the dosing or treatment point, then distribution, then the outlets. Mark where treatment is introduced — usually downstream of storage so the whole served network sees it — and shade the part of the system it actually protects. UV protecting only the run past the lamp reveals itself immediately; a dosing point that leaves a whole wing untreated does too.

Now add the feedback loop. At representative and worst-case (sentinel) outlets, mark where you measure the residual or the relevant parameter, how often, who reads it, the acceptable range, and what happens when a reading falls outside it. A treatment system without that loop drawn in is dosing blind. Any outlet the protected zone does not reach, and any monitoring point with no owner and no action threshold, is a gap you have just made visible. Making the gaps visible is the point of drawing it.

Where treatment regimes go wrong

The water treatment practices that fail are rarely undone by chemistry. They are undone by management. The recurring patterns:

  • Dosing a dirty system, so the biocide is consumed fighting sludge and scale instead of protecting outlets — clean first, treat second.
  • Letting treatment justify deferring the descale, the dead-leg removal or the calorifier repair that would have fixed the root cause for good.
  • Losing residual at the outlets that matter most, because they are the furthest and least used — the exact spots a programme is meant to cover.
  • Commissioning a system, then quietly under-monitoring it until the dosing has drifted for months unnoticed.
  • Deskilling: assuming the contractor’s kit runs itself, so nobody on site can say whether it is working today.

Monitoring is the control, not the paperwork around it. Treatment that is applied but never verified at the outlet is an assumption, and HSE is clear that what you test, and how often, follows the system and the risk assessment rather than habit [4]. Frame the whole thing as a water management programme — assess, control, monitor, review — with treatment as one controlled barrier inside it [5].

A necessary caveat

Selecting and running a treatment regime is a competent-person task, not a catalogue purchase. Any chemical disinfectant must be a product approved for the use and dosed within the relevant limits; treatments interact with pipe materials and with each other, and the wrong choice can corrode a system or create unwanted by-products. Specify and validate any regime through a competent, site-specific risk assessment, and use providers who can show recognised competence such as Legionella Control Association registration [6]. The methods, limits and monitoring outlined above are general guidance, not a design for your building.

So before you cost any of it, pull the last twelve months of temperature logs and sample results and ask one plain question: are the fundamentals actually failing, or just unmonitored? More often than not, the cheaper fix sits upstream of any dosing pump.

FAQ

If we add water treatment, can we ease off on temperature control?

No. Treatment is an extra barrier, not a replacement for the fundamentals. Temperature control, removing stagnation and keeping the system clean all stay in place; the treatment sits on top of them. Easing the basics usually just hands the bacteria back the conditions you were trying to remove [1][2].

Is chlorine dioxide better than copper-silver ionisation?

Neither is universally better. Chlorine dioxide holds an oxidising residual through distribution; ionisation can suit complex hot-water systems where a chemical residual is hard to maintain to distant outlets. The right answer depends on your system, water chemistry, the outlets you must protect and what your team can run and monitor day to day [2].

Do we need a specialist to operate a treatment regime?

In practice, yes. Generation, dosing and residual monitoring need competence to keep a regime effective and within limits. The duty holder still owns oversight and records; a competent contractor performs the work, and recognised registration is a reasonable thing to ask for before you sign [6].

Sources

[1] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [2] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [3] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [4] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm [5] CDC, “Controlling Legionella”. https://www.cdc.gov/control-legionella/index.html [6] Legionella Control Association, “Code of Conduct for Service Providers”. https://www.legionellacontrol.org.uk/