Legionella treatment

Updated 12 September 2026

Legionella treatment is the phrase that catches everything you do to remove or control legionella in a water system. It includes the routine work of monitoring temperatures and dosing biocide, the one-off work of disinfecting a system after a bad sample, and the corrective work after an outbreak investigation. Which method you choose depends on the system, the reason you are treating, and the consequences of getting it wrong.

This guide covers the methods in common use in the UK, when each is appropriate, and how they relate to the requirements in the Approved Code of Practice L8 and in HSG274. It is written for duty holders who are working with a water hygiene contractor, not as a do-it-yourself manual.

The two meanings of "treatment"

Most confusion about legionella treatment comes from conflating two different activities.

Routine treatment is the continuous work of keeping a system below the threshold for legionella growth. For a domestic hot and cold water system this is almost entirely temperature control: keeping stored hot water above 60°C, cold water below 20°C, and flushing outlets that sit idle. For a cooling tower it is continuous biocide dosing plus weekly or monthly water quality checks.

Remedial treatment is the one-off action taken after something has gone wrong: a sample above the action threshold, a case of Legionnaires'' disease linked to the premises, or the restart of a system after extended shutdown. Remedial treatment is usually thermal disinfection or chlorination, applied once, with sampling to confirm it has worked.

The same method (for example, raising the calorifier to 70°C) can be a routine control or a remedial action depending on why you are doing it.

Thermal disinfection

Thermal disinfection is the standard remediation method for domestic hot and cold water systems. The procedure in HSG274 Part 2 is to raise the calorifier to at least 70°C, then run each outlet in turn at its maximum flow for five minutes at a temperature of at least 60°C. This kills legionella in the pipework and in the tap or shower head itself.

Done properly, thermal disinfection is highly effective and leaves no chemical residue. The limitations are practical:

• You need a calorifier that can actually reach 70°C, and pipework rated to handle that temperature. Older lead-soldered systems and some plastic plumbing are not suitable.
• Running hot water at 60°C through an outlet creates a serious scalding hazard. The disinfection must be done out of hours or with outlets isolated, and someone has to physically run every tap.
• Thermostatic mixing valves on bath and shower outlets must be bypassed or removed for the disinfection to reach the outlet itself.
• Cold water pipework is not disinfected by this method. If the cold side is the source of the problem, chlorination is needed instead.

For healthcare premises under HTM 04-01, the thermal disinfection procedure includes specific requirements for recording the temperature achieved at every outlet, not just at the calorifier, and for sampling the system after disinfection to confirm the treatment has worked.

Chlorination

Chlorination uses sodium hypochlorite (the active ingredient in most household bleach, at much higher concentration) to disinfect the water system. The standard procedure is to drain the system, refill it with water dosed to 50 ppm free residual chlorine, hold that concentration for at least one hour, flush with fresh water to remove the chlorine, and take confirmation samples.

Chlorination has the advantage over thermal disinfection that it treats hot and cold pipework equally and does not create a scalding hazard. It is the default method for any system where thermal disinfection is impractical.

The limitations:

• Chlorine can damage metal fittings, gaskets, and some plastic components at the concentrations needed for disinfection. Pre-1985 galvanised pipework is particularly vulnerable.
• The flush-through to remove chlorine produces a significant volume of treated water that has to be discharged to foul drain under the terms of the local water authority''s trade effluent consent.
• Chlorine is less effective against biofilm than thermal disinfection, so a chlorinated system may need retreating sooner if the underlying biofilm has not been physically removed.
• Chlorine gas is hazardous to the operator and to anyone else in the building. Ventilation and PPE are essential.

Chlorination is usually a contractor-only activity because of the hazardous substances handling and the trade effluent compliance.

Continuous chemical treatment

Where temperature control is not practical and periodic disinfection is not enough, continuous chemical treatment is used. The three methods in common UK use are:

Chlorine dioxide dosing. A low level of chlorine dioxide (typically 0.3 to 0.5 ppm) is dosed continuously into the incoming mains supply. Chlorine dioxide penetrates biofilm better than free chlorine and works at lower concentrations. It is the standard continuous chemical treatment for healthcare premises where thermal control cannot be maintained on all outlets.

Silver-copper ionisation. Trace silver and copper ions are released into the water by electrolytic cells installed on the hot water system. Silver-copper is effective against legionella in biofilm, works at low concentrations, and does not affect the taste of the water significantly. It is expensive to install and requires regular monitoring to maintain the correct ion concentration.

Ultraviolet disinfection. UV lamps installed in the pipework kill legionella passing through the irradiated zone. UV only treats the water at the point of irradiation, so it does not address biofilm or downstream colonisation. It is used most often for point-of-use protection in dialysis or other sensitive clinical applications, not as a whole-system control.

Treatment of cooling towers

Cooling tower treatment is a separate regime under HSG274 Part 1. The standard continuous treatment is a combination of an oxidising biocide (usually sodium hypochlorite or bromine) and a non-oxidising biocide (such as DBNPA or isothiazolinone), rotated or alternated to prevent resistance developing. Scale inhibitors and corrosion inhibitors are dosed alongside.

Clean and disinfection of a cooling tower is done at least every six months and follows a detailed procedure: drain, mechanical clean of the pack and sump, high-level disinfection at 50 ppm free chlorine for an hour, rinse, and refill. The procedure, the chemicals used, and the operatives carrying out the work all have to be documented, and the work has to be signed off by a competent person.

When to treat: action thresholds

For domestic hot and cold water systems, HSG274 Part 2 Table 2.3 sets action thresholds for legionella sample results:

• Below 100 cfu/litre: no treatment. Continue routine monitoring.
• 100 to 1,000 cfu/litre: review the control scheme, take repeat samples, consider localised treatment of the outlet if repeats confirm.
• Above 1,000 cfu/litre: treat the affected sub-system. Resample after treatment.

For cooling towers, any detection of legionella at 1,000 cfu/litre or above requires immediate shutdown and disinfection. Lower detections trigger review of the biocide regime.

For healthcare premises, HTM 04-01 sets stricter thresholds and requires a positive clinical case to be treated as a possible nosocomial event requiring investigation.

After treatment: confirmation sampling

Any remedial treatment should be followed by confirmation sampling. Take samples at the same outlets that were positive before, plus a representative sample of others in the treated zone. Send to a UKAS-accredited laboratory using the standard ISO 11731 method. A single post-treatment sample is not usually enough: HSG274 Part 2 recommends repeat sampling at two and six weeks to confirm the treatment has held.

If repeat samples come back positive despite treatment, the problem is usually biofilm or a structural feature (a dead leg, a poorly designed calorifier, a failing TMV) that treatment alone cannot fix. At that point the focus should shift from repeated disinfection to investigation and physical remediation.