A new or returning cooling tower should not pass cooling water until it has been cleaned and disinfected, the work certified, and the record filed. Skip that and you are circulating whatever sat in the pack, sump and pipework while the plant was built or idle, then blowing it into the air as breathable aerosol.
That single rule sits behind four situations a responsible person has to recognise. HSG274 Part 1 expects an evaporative system to be inspected, disinfected and, if required, cleaned before it is first commissioned, on a fouling-led schedule during normal running, and again after shutdown or disruptive work that could have disturbed the system [1]. The principle is the same each time; the trigger is what changes.
This is a guide to spotting which situation you are in, knowing roughly what the contractor should be doing, and judging from the paperwork whether it was actually done. It is not a method statement, the chemicals, doses and contact times belong to a competent contractor’s site-specific procedure.
The four situations that trigger a clean-and-disinfect
Before first commissioning. A brand-new tower, or one that has been replaced or substantially altered, gets cleaned and disinfected before it carries operational cooling water. Construction leaves swarf, flux, dust and debris in the pack and pipework, and the system has typically stood with stagnant water during testing. Commissioning disinfection clears that starting load so the treatment programme begins from a controlled baseline rather than playing catch-up. The wider commissioning of a new water system follows the same logic across the whole building.
On the fouling-based schedule. Even a well-run tower fouls. Scale, corrosion products, biofilm and airborne dirt scrubbed out of the cooling air accumulate in the pack and sump faster than continuous dosing alone can manage. HSG274 Part 1 treats cleaning frequency as risk-assessment-led: dirty environments may need cleaning at least every three months, comfort-cooling systems are commonly twice yearly, and clean industrial applications may be annual, with inspection history and fouling rate deciding the actual interval [1]. The point is not the calendar; it is cleaning before deposits become a control problem.
After a shutdown. A tower that has been off for a meaningful period, seasonal lay-up, a long plant outage, an extended low-load spell, should be cleaned and disinfected before it returns to service, not simply switched back on. Standing water and warming conditions during an idle period are exactly what favours growth, so restart without disinfection is a recognised failure mode.
After disruptive work. Any work that disturbs the system or its deposits, repairs, pack replacement, work on associated pipework, or anything that stirs up sediment, can mobilise contamination. A disinfection after disruptive work resets the system before normal aerosol release resumes.
In practice these overlap: a tower coming out of winter lay-up after a pack change is both “after a shutdown” and “after disruptive work”, and the disinfection covers both.
What the process actually involves
You do not need to run the job, but you should recognise its shape so you can tell a thorough clean from a quick splash of biocide. A proper clean-and-disinfect of an evaporative system generally moves through the same stages.
First a pre-clean disinfection: the system is dosed with biocide and circulated, often with a biodispersant to lift biofilm off surfaces, before anyone opens it up. This kills and loosens the bulk of the microbial load so the physical cleaning is not spreading live, heavily contaminated material. The dose and contact time are set by HSG274 Part 1 and the chemical manufacturer, not chosen on the day [1].
Then the system is drained and physically cleaned: sump, pack/fill, drift eliminators, distribution deck and accessible surfaces are cleared of sludge, scale and debris. Disinfection cannot reach bacteria sheltering under deposits, so the physical clean is what makes the chemistry effective, a tower that is only dosed and never opened is not being cleaned.
Next a disinfection stage brings the refilled system to a defined biocide residual and holds it for a contact period, again to the HSG274/manufacturer regime, so surfaces and the full water volume are treated [1]. Finally the system is drained, neutralised where required and refilled, the treatment programme is re-established, and the tower is returned to service with its normal biocide, conductivity and inhibitor control resumed, the ongoing chemistry that the planned clean exists to support.
Throughout, this is hazardous work: confined or awkward access, oxidising chemicals, and the COSHH duty to assess and control exposure to substances such as sodium hypochlorite and biodispersants all apply, and it should only be done by competent people working to a safe system of work.
Checklist: judging a cooling tower clean-and-disinfect
Use this to brief a contractor before the visit and to check the paperwork after. It tests the evidence, not your ability to do the work yourself.
Before the visit
- Confirm which trigger applies (pre-commission, scheduled, post-shutdown or post-disruptive-work) so the scope matches the reason.
- Check the contractor is competent for evaporative systems and works to a recognised service-provider code of conduct [3].
- Confirm a COSHH assessment and safe system of work are in place for the chemicals and access.
- Make sure affected occupants and neighbours are considered before aerosol-generating work and refill.
During / what should happen
- A pre-clean biocide dose with biodispersant, circulated for the stated contact time.
- The system drained and physically cleaned, sump, pack, drift eliminators and distribution surfaces, not just a dose.
- A disinfection stage to a defined residual held for a defined period.
- Drift eliminators inspected and refitted correctly before refill.
After / the evidence to keep
- A certificate of disinfection showing the date, the biocide and concentration used, contact times achieved and the engineer’s details.
- The before/after condition noted and any defects flagged for follow-up.
- The treatment programme confirmed back in control (biocide residual, conductivity) before return to service.
- The record filed in the cooling-tower logbook and the risk assessment updated if the system changed.
A certificate that records what was actually done, concentrations, contact times, condition, is the difference between evidence and a receipt. A line saying only “tower disinfected” proves nothing.
A note on getting it judged properly
Knowing the shape of the job lets you supervise it, but it does not make you the competent person. The frequency for your specific tower, the chemicals and doses, the contact regime, and whether post-disinfection sampling is warranted are decisions for a competent contractor working from a site-specific risk assessment of your system, its duty and its water quality. Treat this article as the questions to ask, not the answers to sign off yourself. Nothing here is legal, design or chemical-handling advice.
One more administrative point that catches new operators: a notifiable cooling tower or evaporative condenser must also be registered with the local authority. In Great Britain that sits under the 1992 notification duty; in Northern Ireland it sits under the 1994 Northern Ireland notification regulations and HSENI guidance [4]. Either way, it is a separate obligation from disinfection, but one that often surfaces at commissioning.
FAQ
Is a cooling tower clean-and-disinfect the same as disinfecting the building’s water system?
No. Shock-chlorinating a domestic hot and cold system is a different job with different chemistry and access. A cooling tower clean-and-disinfect targets the evaporative system, sump, pack, drift eliminators and condenser water circuit, and is built around physical cleaning plus biocide because that is where the fouling and aerosol risk live. Don’t accept a generic water-system disinfection certificate as evidence the tower was done.
How do I check the contractor actually cleaned it rather than just dosing biocide?
Ask for the certificate and read it. A real clean-and-disinfect records a pre-clean dose, the system being opened and physically cleaned, and a disinfection stage with stated concentrations and contact times. Photographs of sump and pack condition before and after, and any defects raised, are good signs. A bare “disinfection carried out” with no detail, or a visit too short to have drained and cleaned the system, should prompt questions.
How do I know if my tower needs cleaning more often?
Your risk assessment sets it. The fouling rate, the cooling duty, make-up water quality, the system’s history of control results and any past Legionella detections all push the frequency up. HSG274 gives typical examples rather than a single floor: dirty environments may need cleaning at least every three months, comfort-cooling systems are commonly twice yearly, and clean industrial applications may be annual. A competent contractor reviewing your control data should advise.
Should we take a Legionella sample after the clean-and-disinfect?
Sometimes. Routine post-clean clearance sampling is not automatic for every visit, but it is commonly advised after commissioning, after a return from shutdown, or where there has been a loss of control or a suspected case. Whether and when to sample, and how to read the result, is a judgement for your contractor and risk assessment rather than a fixed rule.
What to do next
Find out which of the four triggers your tower last met, then pull the certificate for that event. If it shows concentrations, contact times and the system being physically cleaned, file it and note the next due date. If it shows only that “the tower was disinfected”, ask your contractor for the detail behind it before the next aerosol season, and book the clean now if the system is about to be commissioned, restarted, or has just had disruptive work.
Sources
- HSE, HSG274 Part 1 (2024), “Inspection, cleaning and disinfection procedures”, p.27. https://www.hse.gov.uk/pubns/books/hsg274.htm
- HSE, ACoP L8 (2013), “Carrying out a risk assessment”, p.12. https://www.hse.gov.uk/pubns/books/l8.htm
- Legionella Control Association, “Code of Conduct for Service Providers”. https://www.legionellacontrolassociation.co.uk/
- Health and Safety Executive for Northern Ireland, “Legionella”. https://www.hseni.gov.uk/topics/legionella