How does chlorination control legionella?
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Key points
- Chlorination disinfects a water system by dosing it with a high concentration of free chlorine, a strong oxidiser, and holding that concentration long enough to kill legionella throughout the pipework.
- The standard shock regime is to dose to around 50 ppm free residual chlorine, hold for at least one hour, flush clear, then confirm the result by sampling.
- It beats thermal disinfection where you need to treat hot and cold water equally and where running outlets at 60°C would create a scald hazard.
- It is weaker against established biofilm, can attack some fittings and seals, and produces flush water that cannot go straight to drain.
- Because it means handling hazardous chemicals and neutralising the flush water, chlorination is a competent-contractor activity, not a DIY step.
Chlorination controls legionella by flooding the water system with a high concentration of free chlorine, a powerful oxidising disinfectant, and holding it there long enough to kill the bacteria throughout the pipework. In the standard procedure the system is drained, refilled with water dosed to around 50 ppm of free residual chlorine, held for at least an hour, then flushed clear and the result confirmed by sampling. Because the chlorine is carried by the water rather than by heat, it reaches every part of the system the water reaches, hot side and cold side alike.
This page covers disinfection chlorination, the one-off "shock" dose used to clean out a system, for example after a positive sample, after building work, or before a system is brought back into use. It sits alongside thermal disinfection as one of the two main ways of disinfecting a system, and it is a different thing from continuous background chlorination, where a low level of chlorine is dosed permanently to keep a system under control. For the wider picture of control options, see legionella treatment.
What chlorination actually does
Free chlorine in water forms hypochlorous acid, which destroys the cell walls and internal proteins of bacteria on contact. Two things decide whether it works: the concentration of free chlorine and how long the bacteria are exposed to it. This is why a disinfection is always specified as a dose held for a contact time, never as a dose alone. A high concentration for a short period and a lower concentration for a longer period can achieve a similar kill, which is why you will see alternative regimes such as 50 ppm for one hour or 20 ppm for two hours.
The strength of the method is that the disinfectant travels with the water. Heat only reaches the parts of a system that can be brought up to temperature, but a chlorine dose reaches every outlet you can draw it through, including cold-water pipework that thermal disinfection cannot touch.
The standard disinfection procedure
A competent contractor follows a set sequence, broadly in line with the disinfection guidance in the British Standards for water installations. The steps are:
- Clean and drain. Remove debris, descale where needed, and drain the section to be treated. Chlorine is consumed by dirt and fouling, so a system full of sediment wastes the dose.
- Refill and dose. Refill with water dosed to around 50 ppm free residual chlorine, usually from sodium hypochlorite. The fill point needs an air gap or other backflow prevention, so this heavily chlorinated water cannot be pulled back into the mains supply.
- Draw to every outlet. Open each tap, shower, and branch in turn until dosed water is confirmed at the outlet. Every dead leg and fitting has to see the chlorine, or it is not disinfected.
- Hold the contact time. Leave the dosed water in place for at least one hour. The free chlorine residual is checked at the end of the hour, and if fouling has consumed it so that the reading has dropped away, the system is re-dosed and the contact time restarts.
- Flush out. Flush thoroughly until the chlorine level at the outlets returns to that of the incoming supply.
- Confirm by sampling. Once the system is back in normal use, take microbiological samples to check the disinfection worked. See legionella testing for how those samples are taken and read.
The sampling step is the one people skip, and it is the one that matters. A disinfection certificate records that the work was carried out to the specified dose and contact time. It does not, on its own, evidence that the water is clear. Only a post-disinfection sample does that.
When chlorination beats thermal disinfection
Chlorination has clear advantages in certain systems, and choosing it over a heat treatment is often the right call.
- It treats hot and cold equally. Thermal disinfection works only on the hot side, because you cannot heat the cold-water system. A cold-water storage and distribution problem needs chemical treatment.
- No scald hazard. A thermal disinfection runs outlets at 60°C or above for several minutes, which is a real burns risk in occupied buildings, care settings especially. Chlorination avoids that.
- It does not depend on the calorifier. If the plant cannot reliably reach and hold disinfection temperature, a heat treatment is not an option, but a chemical one still is.
Chlorination reaches the cold side and avoids the scald risk of a heat treatment, but a disinfection certificate is not proof the water is clear. Only the follow-up sample is.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
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Where chlorination falls short
The method is not a cure-all, and an honest assessment records its limits.
- Biofilm. Free chlorine kills the bacteria suspended in the water quickly, but it struggles to penetrate an established biofilm on the pipe wall. Legionella sheltered inside that biofilm can survive a shock dose and reseed the system, which is why disinfection alone rarely fixes a persistent problem.
- Materials. A 50 ppm dose is aggressive. It can pit some grades of stainless steel, degrade rubber and elastomer washers and seals, and attack galvanised pipe. Repeated chlorination shortens the life of some fittings.
- Trade effluent. The flush water leaves the system at a high chlorine concentration and cannot lawfully go straight to drain. It has to be neutralised or dechlorinated first, and discharging it often needs trade-effluent consent from the water company.
- It is a snapshot. Chlorination cleans the system on the day. It does nothing about the underlying cause, whether that is a dead leg, a stagnant branch, or water sitting outside the temperature regime. Fix the cause or the problem returns.
Why chlorination is a contractor job
It is worth being blunt about this, because the procedure can read like something a competent maintenance person could manage. It is not a DIY step. It involves handling a hazardous oxidising chemical under COSHH controls, dosing accurately, measuring the free chlorine residual through the contact period, then neutralising the flush water and discharging it lawfully. Get the dose wrong and you either fail to disinfect or damage the system; get the discharge wrong and you have a trade-effluent breach. This is why chlorination is commissioned from a competent water treatment company, ideally one holding Legionella Control Association membership, who carry out the work and issue the certificate.
Chlorination and the guidance
Disinfection by chlorination sits inside the wider duty to control legionella risk. HSG274 Part 2 covers disinfection of hot and cold water systems and describes when a system should be disinfected, including after a positive result or significant work on the system. The Approved Code of Practice L8 sets the duty above it: assess the risk, put a written scheme of control in place, and keep records. A chlorination is one action within that scheme, and the certificate and the follow-up sample results belong in your records. If you are here because a sample came back positive, read what to do if legionella is detected first, so the disinfection is part of a considered response rather than a reflex.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
Frequently asked questions
What chlorine level is needed to disinfect a water system?
The standard shock disinfection dose is around 50 ppm (50 mg per litre) of free residual chlorine, held for at least one hour. The free chlorine residual is checked at the end of the contact period, and if fouling has consumed it the system is re-dosed and the hour restarts. Lower concentrations over longer times can achieve the same result, for example about 20 ppm held for two hours, because it is the combination of concentration and contact time that does the work. This is very different from continuous background chlorination, which runs at a far lower level of around 0.5 to 1 ppm kept in the system permanently.
How long does chlorination take?
The chemical contact period itself is at least one hour at a 50 ppm dose. The whole job takes considerably longer once you add draining and cleaning, dosing, drawing the chlorine through to every outlet, the hold, and then flushing the system clear again, which typically fills most of a working day on site. On top of that, the confirmation samples are sent to a laboratory, and legionella culture results take up to around ten days to come back, so the disinfection is not fully closed out until those results arrive.
Is chlorination better than thermal disinfection?
Neither method is universally better; they suit different systems. Chlorination treats hot and cold water equally and avoids the scald hazard of running outlets at 60°C, which makes it the right choice for cold-water systems and occupied buildings. Thermal disinfection needs no chemicals, no effluent consent, and no material damage risk, so it suits hot-water systems where the calorifier can reach temperature. Chlorination is also weaker against established biofilm, so the risk assessment and the type of system should decide which one is used.
Can I chlorinate my own water system?
For a managed building water system used for legionella control, realistically no. The procedure means handling a hazardous oxidising chemical under COSHH, dosing and measuring the free chlorine residual accurately, and neutralising the high-chlorine flush water before it goes to drain, which usually requires trade-effluent consent. It is a specialist activity that should be commissioned from a competent water treatment contractor who carries out the work and issues a disinfection certificate. Doing it yourself risks an ineffective disinfection, damaged fittings, or an unlawful discharge.
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