Shock chlorination means dosing the whole water system to a high free-chlorine concentration, holding it for a set contact time so it reaches every outlet, then neutralising and flushing it out before anyone uses the water. Done properly it is a reset button after a positive Legionella result or significant works. Done in a rush it leaves a system that smells of bleach, a discharge you should not have made, and a disinfection you cannot prove worked.

This is the order that matters: prepare, dose, prove the residual everywhere, hold, neutralise, flush, sample, recommission. Skip the “prove it everywhere” and “neutralise” steps and you have the two failures I see most often.

Before you start: prerequisites

Hyperchlorination is a competent-person job, not a “borrow a drum of chlorine” job. The duty under ACoP L8 and the technical method in HSG274 both assume the work is planned, supervised and recorded by someone with the right training [1][2]. Confirm the following before a single tap is touched.

  • A reason and a method statement. A positive result, new pipework, an opened tank, or a system back from a long shutdown. If you are here off the back of a positive sample, work through What to do if you get a positive Legionella test first so the chlorination sits inside a wider response, not instead of one.
  • A clean system. Chlorine oxidises organic matter, so scale, sludge and biofilm soak up your dose and shield bacteria underneath. Clean first, disinfect second — the ordering in Cleaning and disinfection after remedial work: thermal and chemical approaches. Disinfecting a dirty tank is mostly wasted chlorine.
  • Isolation, signage and cover. The system out of use, every outlet labelled “do not use”, building users told. Anyone drinking or showering mid-procedure is exposed to a concentration far above potable water.
  • Discharge sorted. Know where the neutralised water goes and that you are allowed to put it there. Strong chlorine solution to a watercourse or surface drain is an environmental offence, not a detail.
  • Test kit and PPE. A DPD free-chlorine test (high range), a neutralising agent such as sodium thiosulphate, gloves and eye protection. You cannot run this blind; you measure at every stage.

The disinfection sequence, in order

This is the spine of the job. Work through it as written — each step has a point at which it is genuinely done, and a way it tends to go wrong.

  1. Dose the storage and draw it through. Add your chlorine source to the cold water storage tank (or dosing point) to reach the target free-chlorine concentration. A figure commonly cited for whole-system shock dosing is around 50 mg/l (50 ppm) free chlorine, though your method statement and the relevant guidance set the actual number for the system in front of you [2]. Done when: the tank itself reads at target on the DPD test. Common failure: dosing to the tank and assuming the rest of the system is now dosed — it is not, until you pull the chlorinated water out to the outlets.
  2. Pull the residual to every outlet. Open each outlet in turn, working from nearest to furthest, until the DPD test at that tap shows the target residual, then close it. This is the step people short-cut. Dead legs, the disabled wing, the rarely-used cleaner’s tap — if chlorinated water never reaches them, they were never disinfected. Done when: every outlet, including the awkward ones, has registered target free chlorine. Common failure: topping the tank back up with unchlorinated water as you draw off, diluting the system below target.
  3. Hold the contact time. Leave the system standing at the residual for the specified contact period — concentration and time are a trade-off, so a higher residual allows a shorter hold, and guidance commonly pairs a strong dose with roughly an hour, a weaker one with longer [2]. Confirm the figures against your method statement; do not improvise them. Done when: the contact time has fully elapsed and a re-check still shows an adequate residual. Common failure: the residual has decayed below target during the hold because the system was dirtier than assumed — if so, re-dose and start the clock again.
  4. Re-check the residual at the extremities. Before you neutralise, test the furthest and least-used outlets again. They are where disinfection fails quietly. Done when: the far points still hold the target residual at the end of the contact time. Common failure: a comfortable reading at the plant room masking a low reading three floors up.
  5. Neutralise, then flush to waste. Do not dump 50 ppm of chlorine into the drains. Neutralise the bulk water with a reducing agent, or flush in a controlled way to an agreed discharge point, until outlets read down to a level consistent with the incoming supply [2]. Done when: free chlorine at the outlets is back to normal supply levels and the discharge has gone where it was authorised to go. Common failure: treating neutralisation as optional — the single most common compliance and environmental slip in the whole procedure.
  6. Restore temperatures and clear-sample. Bring the hot side back up to its design temperature and confirm cold runs return below the multiplication range before sign-off [3]. Arrange post-disinfection clearance sampling to a recognised method, with the laboratory and sampling points agreed beforehand, and ideally taken a few days later so you are testing recovery, not the chlorine [4][5]. Done when: temperatures are back in regime, the system is back in normal use, and clearance results confirm the disinfection held. Common failure: handing the system back the same hour and sampling immediately, so the residual gives a falsely clean result.

The recommissioning end of this — restoring temperatures, refilling and returning outlets to service — overlaps with ordinary post-works restart, structured in Legionella flushing after system repairs: restart checklist.

Record it as you go, not afterwards

Every reading is evidence: the dose, the time you reached target at each outlet, the contact period, the residual before and after neutralisation, the discharge point, the clearance results. A disinfection with no record is, for audit purposes, a disinfection that did not happen. Paper sheets in a damp plant room get lost or filled in from memory at the van; logging each residual and timestamp into a digital logbook as you take it gives you a defensible, time-stamped trail an assessor can follow.

A note on competence and limits

This is general procedure, not a site-specific method statement, and chlorine at shock concentration is a genuine hazard to people and to the environment. The concentrations, contact times and discharge route for your system must come from a competent person working to your risk assessment and your local discharge consent — use a service provider working to the Legionella Control Association code of conduct if you are contracting it out [6]. Nothing here replaces that assessment, and nothing here is design or legal advice.

FAQ

What if the free-chlorine residual won’t hold during the contact time?

A residual that decays below target mid-hold usually means the system was dirtier than assumed — scale, sludge or biofilm consuming the chlorine. Stop, re-dose to target, and restart the contact clock. If it keeps collapsing, the system needs cleaning before it can be disinfected, not more chlorine.

Can I shock-chlorinate just the section I worked on?

You can isolate and disinfect a defined section, but only if you can genuinely isolate it and prove the residual reached every part of it. The risk is the join: water either side of an isolation valve mixes, and an untreated branch downstream undoes the work. Where isolation is uncertain, treat the whole system.

Why not just use thermal disinfection instead?

Heat is often simpler on a hot-only job, but it cannot reach cold storage, cold distribution or far cold outlets, and you cannot reliably push disinfection temperature everywhere on a mixed system. Chemical shock dosing is the route when the cold side is involved. The full thermal-versus-chemical decision is in Cleaning and disinfection after remedial work: thermal and chemical approaches.

What to do today

Before you book the chlorination, write the discharge plan and the outlet list. Walk the system, list every outlet you will draw the residual to, mark the dead legs and low-use taps, and confirm in writing where the neutralised water goes and that you are permitted to put it there. Get those two straight and the procedure itself becomes the easy part.

Sources

[1] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - ACoP 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] BSI, “BS 7592:2022 - Sampling for Legionella bacteria in water systems. Code of practice”. https://knowledge.bsigroup.com/products/bs-7592-sampling-for-i-legionella-i-bacteria-in-water-systems-code-of-practice-1 [6] Legionella Control Association, “Code of Conduct for Service Providers”. https://www.legionellacontrol.org.uk/