Frequently Asked Questions
Short, direct answers to the questions we are asked most often about chlorination certificates, Legionella risk assessments, water sampling and cold water storage tanks.
- Chlorination, Legionella and water tank questions answered in one place
- Work carried out to BS8558, BS EN 806-4, BS 7592, ACOP L8 and HSG274
- Samples analysed by a UKAS accredited laboratory
Chlorination and certificates
Certificates, water company sign off, timings and who is allowed to sign them.
What is a chlorination certificate and when do I need one?
A chlorination certificate is the document issued once a water system has been disinfected, flushed and sampled, confirming the water meets the required bacteriological standard. You need one for new or altered pipework, before a new mains connection is approved, after a contamination event, and after a system has stood unused.
It should record the system treated, the method, the contact period, the flushing and the laboratory results. See our guide to the chlorination certificate.
Do I need a chlorination certificate before a Thames Water connection?
Yes. Thames Water will not approve a new connection until the new pipework has been chlorinated, flushed and sampled, with the analysis carried out by a UKAS accredited laboratory. A certificate with no accredited analysis behind it can be rejected, which means repeating the work.
We work to the Thames Water requirements and plan the job backwards from the date the certificate is needed.
How long does chlorination take?
Most systems are dosed, held for the contact period, flushed and sampled in a single visit. A domestic supply pipe can take a few hours. Larger or more complex pipework takes longer, and the laboratory analysis follows the visit, so the certificate is issued once the results are in.
Same day certification is available and we do take last minute bookings.
Is water safe to drink after chlorination?
Yes, once the system has been neutralised, flushed until the chlorine residual is back to normal background levels, and sampled. The water is only signed off as fit for use when the laboratory analysis confirms it, which is why the certificate follows the results rather than the site visit.
Who can issue a chlorination certificate?
The competent contractor who carried out the disinfection, working to BS8558 and BS EN 806-4, with samples analysed by a UKAS accredited laboratory. It should not be self certified by anyone without the training, the equipment and accredited laboratory access, because a certificate is only as good as the analysis behind it.
Legionella
Where the legal duty comes from, and what landlords are expected to do.
Is a Legionella risk assessment a legal requirement?
In practice, yes. There is no Act called the Legionella Regulations. The duty comes from the Health and Safety at Work etc. Act 1974 and COSHH 2002, and ACOP L8 requires a suitable and sufficient assessment of the risk of exposure to legionella bacteria from water systems and work activities.
It is an Approved Code of Practice rather than legislation, but it carries a special legal status. The duty holder is whoever controls the premises, which includes a landlord who keeps responsibility for maintenance. See Legionella risk assessments in London.
How often should a Legionella risk assessment be reviewed?
ACOP L8 sets no fixed interval. It requires the assessment to be reviewed regularly and updated whenever there is reason to believe it is no longer valid. We recommend a review every two years as a baseline, and sooner where the system is higher risk or the building has changed.
Do landlords need to test the water?
Not automatically. A landlord must assess and control the Legionella risk in a let property, but routine water sampling is not a default requirement. Sampling is one control measure, used where the risk assessment indicates it, for example where water is stored or the system has been out of use.
How long do Legionella test results take?
Legionella samples are analysed by culture, so the laboratory has to incubate them before a final result can be confirmed. Results are not same day. We send the laboratory report as soon as it is issued, with a plain explanation of what it means and any action it triggers.
Same day Legionella risk assessments are available. Read what Legionella testing involves.
Water tanks
Cold water storage tanks: inspection, cleaning, disinfection and replacement.
How often should a cold water storage tank be cleaned?
Inspect at least annually, then clean when the inspection shows it is needed rather than on a fixed date. Measurable sediment, visible biofilm, rust staining, insect or vermin debris, a damaged lid or screen, or a stored water temperature that has crept up all trigger a clean and disinfection.
That follows the inspection led logic of HSG274 Part 2 and BS 7592. See water tank cleaning.
How long does it take to clean a water tank?
A small domestic loft cistern is often a few hours. Most single commercial tanks are drained, cleaned, disinfected, refilled and sampled inside a working day. Large sectional tanks, or twin tanks cleaned one side at a time so the building stays in supply, take longer.
What is the difference between tank cleaning and tank disinfection?
Cleaning is the physical work: draining the tank, removing sediment, scale and debris, and scrubbing the surfaces and fittings. Disinfection is the chemical stage that follows, treating the cleaned surfaces and the refilled water. Cleaning without disinfection leaves organisms behind, so the two are normally done in one visit.
Water tank disinfection explains the chemical stage and the sampling that confirms it worked.
When does a tank need replacing rather than cleaning?
When the structure itself has failed. Corroded galvanised steel, split or distorted plastic, leaking sectional joints, a tank that cannot be properly lidded, screened or insulated, or one so oversized that the water sits and stagnates. Cleaning a failing tank buys a few months, not a solution.
See water tank replacement for the GRP, plastic and sectional options.
Working with us
Coverage, timings and how to get a price.
What areas do you cover?
London and the surrounding areas, from our base at Q West, 1110 Great West Road in Brentford. We work on residential and commercial property, from a single let flat or a loft cistern through to the pipework and cold water storage tanks of an office, school or block of flats.
Can you work out of hours?
Tell us the access window and we will tell you what is possible. Water hygiene work usually has to fit around an occupied building, its tenants, its kitchens and its shift patterns, so timing is part of the quote rather than an afterthought. We also take last minute bookings.
How do I get a quote?
Call 020 4532 7773, email info@londonwatertreatment.com, or send the enquiry form. Tell us the property type, the system involved, and the date any certificate is needed by. That is usually enough to price the work, or to say what we need to see first.
For anything unusual a site visit is quicker than an email thread. Contact our team and we will arrange one.
Still have a question?
Ask it. Most water hygiene questions have a short answer once someone understands the system, and a five minute call usually settles whether you need a chlorination certificate, a risk assessment, a tank clean or a replacement.
Call 020 4532 7773 or email info@londonwatertreatment.com. If your question relates to a deadline or a water company connection, say so, because it changes how we plan the work.
Ask us your question
Chlorination, Legionella control and water tank work across London and the surrounding areas.