Does chlorine kill legionella?

Yes — chlorine kills legionella, provided the concentration is high enough and the contact time long enough. That is why chlorination is one of the standard methods for disinfecting building water systems. The honest answer has two qualifiers, though: legionella hiding inside biofilm can survive chlorine that kills free-floating bacteria, and the trace chlorine in ordinary mains water is nowhere near a disinfecting dose by the time it has stood in your pipework.

Concentration times time

Disinfection is a product of dose and contact: a higher concentration for a shorter period, or a lower one held longer. The standard chlorination procedure for building water systems, set out in the guidance and standards that govern disinfection, doses the system to around 50 milligrams per litre for one hour — or 20 milligrams per litre for two hours — held throughout every branch and outlet, then flushes the chemical through and verifies the system before it returns to use. Compare that with the mains residual, a fraction of a milligram per litre: enough to keep bacteria suppressed in the supply network, orders of magnitude too weak to disinfect a building. Our guide to chlorination of water systems covers the full procedure, and legionella treatment methods compares it with the alternatives.

Why the mains supply does not protect your building

A common assumption is that chlorinated tap water means a protected system. Two things defeat it. First, chlorine decays: it is consumed by organic matter, scale and rust, and it dissipates faster in warm water, so water that has stood for days in a warm branch, a loft tank or a dead leg carries little or no residual. Second, dose: even fresh mains water is dosed to suppress, not to sterilise. Mains chlorination is a background defence that keeps the network clean up to your boundary — inside the building, control is your responsibility, and it rests on temperature, turnover and cleaning rather than residual chlorine.

The biofilm problem

The single most important qualifier on "chlorine kills legionella" is biofilm. The slime layer that coats wet pipe surfaces shields the bacteria living in it from chemical attack, and legionella embedded in an established biofilm can survive a shock dose that wipes out everything free-floating. This is why a proper disinfection is never just a dose: strainers, shower heads and tap inserts are removed and cleaned, storage tanks are physically cleaned of sediment, and the chemical is given the best chance of reaching surfaces. It is also why a system can pass a sample taken immediately after chlorination and test positive again weeks later — the colony in the biofilm survived and re-seeded the water.

Chlorine versus heat — and why neither is a regime

Thermal disinfection — raising the hot water system so outlets run well above the temperatures legionella survives — treats the hot side without chemicals but leaves no residual effect; chlorination reaches the cold side and systems heat cannot, but struggles with biofilm and introduces its own handling and verification demands. Persistent contamination is often treated with both, plus physical cleaning. Whichever is chosen, disinfection is a one-off reset: unless the underlying condition that let legionella establish — lukewarm water, stagnation, scale, dead legs — is corrected and held by routine monitoring, the bacteria return on the growth clock. After any treatment, verification sampling confirms the reset worked; the action levels and response steps are in sampling and action levels and what to do if legionella is detected. Long-term prevention is set out in how to prevent legionella, and every treatment, result and follow-up belongs in the log book as part of the record.