Chlorination can expose workers and building users to hazardous chemicals, damage materials, and create an unlawful discharge if it is poorly planned. This page does not provide a dose, contact time or neutraliser quantity. Those decisions belong in a substance-specific COSHH assessment and a site method statement prepared by a competent water-treatment specialist.
The Responsible Person still has an important job. Commissioning a contractor does not mean accepting a one-line proposal that says “chlorinate and flush.” You need enough evidence to know what is being treated, how people will be protected, how the treatment will be verified, and what must be true before the water system returns to use.
Start with the reason for treatment
Define the event that triggered the work. It might be significant new pipework, contamination introduced during an intervention, a system returning from a long shutdown, or a response to microbiological results. A positive result needs a wider incident decision before treatment is selected; the sequence in What to do if you get a positive Legionella test helps separate immediate exposure controls from the eventual remedial method.
Write the objective in observable terms. “Disinfect the building” is too vague. The scope should identify the systems, tanks, branches and outlets included, the parts deliberately excluded, the reason for each exclusion, and the evidence that will demonstrate completion. If the contractor cannot map that boundary, the quotation is not ready to become a method statement.
The competence boundary
Ask who designed the treatment and who will supervise it on site. Their competence needs to cover the actual system and chemical, not merely general water hygiene. A person who can take temperature readings is not automatically competent to specify a chemical treatment, manage exposure controls, assess compatibility or authorise discharge.
The method statement should be read alongside the current safety data sheet and COSHH assessment. Check that product identity is unambiguous, incompatible substances are identified, storage and transfer are controlled, suitable test equipment is specified, and emergency arrangements match the site. HSE’s ACoP L8 places competence and management responsibility inside the control system, rather than treating them as contractor paperwork [1].
Assurance checklist before work begins
Use this checklist in the pre-start meeting. A “no” or “not known” is a reason to pause and resolve the gap, not an invitation to fill in an assumption.
- Scope and schematic: the treated boundary and every relevant outlet are identified on a current schematic or outlet schedule.
- Site conditions: cleanliness, scale, sediment, biofilm, materials, temperature and likely chlorine demand have been considered rather than hidden behind a simple volume calculation.
- People and isolation: building users, vulnerable occupants and workers are protected; isolation, signage, access control and communication responsibilities are explicit.
- Treatment design: the competent person has set the target, verification method and acceptance criteria for this system. The proposal does not rely on an uncited generic internet figure.
- Measuring equipment: the method specifies suitable-range equipment, calibration or verification status, sampling points and who records each result.
- Distribution evidence: the plan explains how the team will demonstrate that treatment reached the defined system boundary and how unexpected loss of residual will be handled.
- Materials and assets: compatibility risks for metals, elastomers, coatings, heaters, filters and connected equipment have been assessed.
- Discharge route: neutralisation, measurement, discharge destination, relevant consent and environmental controls are agreed before treated water is released.
- Handback: flushing, normal operating conditions, any sampling, acceptance authority and user communication are defined before the system is isolated.
- Contingency: the team knows who can stop the work, what triggers escalation, and what happens if the planned verification cannot be achieved.
The related guide to cleaning and disinfection after remedial work helps frame why physical cleaning and chemical treatment solve different problems. A chemical cannot reliably compensate for an inaccessible dirty system.
What the evidence pack should contain
The final record needs to let another competent person reconstruct what happened without relying on memory. At minimum, expect the approved scope, named responsible people, risk assessments, method statement revision, product and safety information, isolation record, instrument details, timestamped measurements by location, deviations, corrective decisions, neutralisation and discharge evidence, handback authorisation, and any follow-up sampling plan.
Do not accept a certificate as a substitute for those records. A certificate can summarise completion, but it cannot show whether the treatment reached a remote branch, whether an instrument was suitable, or why a deviation was accepted. The same principle applies to digital records: timestamps are useful only when the underlying measurement, location and decision are clear.
Safe handback is a separate decision
Finishing the contractor’s chemical activity is not the same as making the water system available. The handback criteria should confirm that chemical controls have been removed safely, normal temperature and operating regimes are restored, water quality acceptance criteria are met, affected assets are functioning, and required stakeholders have authorised reoccupation or use.
Where sampling is part of the assurance plan, agree its purpose, locations, timing, bottles, neutralisers, transport and laboratory method before work begins. BS 7592 is a voluntary code of practice for representative Legionella sampling; it should be obtained from BSI and applied by competent people rather than paraphrased from a checklist [2].
Questions to ask the contractor
Can the site’s water volume be used to choose the product amount?
Volume is only one input. Starting residual, water demand, pH, temperature, organic load, product formulation, density, system mixing and measured verification all affect the treatment. A volume-only conversion is not a safe method statement.
Who approves a change when verification does not meet the method statement?
The method should name the competent decision-maker and the stop/escalate criteria. Site staff should not improvise a stronger treatment or extend it from memory when a result is unexpected.
Is a completion certificate enough for handback?
No. Ask for the underlying measurements, locations, times, deviations, discharge evidence and named handback authority. The certificate is an index to the evidence, not the evidence itself.
What to do today
Before accepting a chlorination quotation, ask for the treatment boundary, COSHH assessment, method statement, discharge plan and handback criteria. Compare the five documents for gaps and conflicting responsibilities. If the proposal provides a product amount but cannot show who set the target, how distribution will be proved, or where treated water will go, return it for competent revision.
Sources
- HSE, ACoP L8 (2013), “Managing the risk: management responsibilities, training and competence”, p.14. https://www.hse.gov.uk/pubns/books/l8.htm
- 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