Can legionella live in chlorinated water?

Yes, legionella can live in chlorinated water. The chlorine residual that water companies maintain through the distribution network keeps legionella numbers very low, which is why mains water arrives safe, but that residual does not eliminate the organism, and it does not reach the bacteria sheltering inside biofilm. Chlorinated is not the same as sterile.

This is the answer to a question duty holders reasonably ask: if the water company chlorinates the supply, why is legionella my problem? The answer has two halves. The chlorine keeps the organism suppressed in the main, but the residual decays once the water enters your building, and wherever legionella has established itself inside a biofilm, chlorine at normal levels barely touches it anyway.

Why the supplier's chlorine does not protect your building

Chlorine is consumed as it works. It reacts with organic matter, with corrosion products, and with the pipe walls themselves, and it decays faster in warm water. Water that leaves the treatment works with a healthy residual arrives at your boundary with less, sits overnight in your cold water storage tank with less still, and by the time it has warmed up in a long pipe run there may be no effective residual at all. Every building water system is, in practice, a low-chlorine or chlorine-free environment within hours of the water arriving. The supplier's disinfectant protects the journey to you; it does not protect the storage and distribution that happen after.

The biofilm shield

Even where chlorine is present, it has limited reach. Most of the legionella in an established system is not floating free in the water; it is embedded in biofilm, the slime layer that coats the inside of tanks and pipework, and it also lives within other micro-organisms in that layer that shelter it further. The biofilm matrix physically and chemically protects the bacteria inside it, so a chlorine level that kills free-floating cells in minutes may leave the biofilm population largely intact. This is why a system can test positive again shortly after treatment, and why disinfection procedures pair chemical dosing with cleaning and descaling: you have to strip the habitat, not just dose the water.

Where chlorine does earn its place

None of this makes chlorine useless; it defines its role. At the supply level, the residual is what keeps environmental legionella at trace numbers. At the building level, chlorination, dosing the system to a deliberately raised concentration, holding it for a set contact time, then flushing through every outlet, is a standard disinfection procedure after commissioning, after a shutdown, or when sampling finds a colony. The guide to chlorination of water systems sets out when and how it is used, and legionella treatment methods compares it with thermal disinfection.

What chlorination cannot do is substitute for the routine. A disinfected system left warm, stagnant and scaled will be recolonised from the trace numbers arriving in the mains, sometimes within weeks. Disinfection resets the system; temperature control, flushing and cleaning keep it reset.

The day-to-day control in most buildings therefore remains the unglamorous one: hot water stored at 60°C and delivered at 50°C or above, cold water below 20°C, little-used outlets flushed weekly, tanks and calorifiers inspected and cleaned on schedule, and every check logged in a log book kept for at least five years. The legionella risk assessment tool helps you structure the assessment that decides whether your system needs more than that, and HSG274 is the guidance that defines what good control looks like. Chlorine has its place in that framework, but the place is supporting actor, not lead.