A cooling tower is the highest risk device in this subject. It moves large volumes of warm water through the open air and creates drift, which is exactly the aerosol that carries the bacteria, and it can affect people who never enter the building. That is why cooling towers carry duties that hot and cold water systems do not, starting with telling the local authority they exist.
Notification, which is separate from the risk assessment
Cooling towers and evaporative condensers must be notified to the local authority under the Notification of Cooling Towers and Evaporative Condensers Regulations 1992. The notification names the premises and the device, and it has to be updated when the device is taken out of use. It exists so that when a cluster of cases appears in an area, the authority already knows where to look.
What routine control and legionella inspection look like
Continuous or dosed biocide treatment with the levels monitored and recorded, because temperature control is not available on a device designed to cool water. Regular inspection of the pack, the pond and the drift eliminators. Cleaning and disinfection on a defined schedule. And routine legionella sampling, which for cooling towers is genuinely part of the regime rather than the exception it is for domestic systems.
Where side stream filtration comes in
A side stream filter continuously draws a proportion of the circulating water, removes suspended solids and returns it. Solids feed biofilm and shield bacteria from biocide, so removing them makes the chemical treatment work better and reduces how often the tower has to be cleaned. It is a supporting control rather than a replacement for treatment or monitoring.
Why the duty reaches further than the site boundary
Section 3 of the Health and Safety at Work etc. Act 1974 covers people who are not your employees, and cooling tower outbreaks have reached people walking past. That is the reason the enforcement interest in towers is disproportionate to their number, and the reason a tower nobody has looked at for a year is the single most serious finding in this subject.
Does chlorine kill legionella, and at what concentration
Yes, and the useful detail is the dose and the contact time rather than the fact. A shock chlorination to disinfect a system is done at a stated concentration held for a stated period, typically fifty parts per million for an hour or a lower concentration for longer, followed by flushing every outlet until the residual has cleared, and the standard water industry guidance sets both figures. Continuous low level chlorination is used in some systems as a control rather than a disinfection. Three limits are worth knowing. Chlorine does not penetrate biofilm or scale well, so cleaning has to come first or the disinfection reaches only the water and not the surfaces the organism lives on. It is consumed by organic matter, so a dirty system uses up the dose. And it corrodes some materials, which is why the choice of biocide on a cooling system is an engineering decision rather than a purchase.
How to test legionella in water, and who does the analysis
The sample is taken by somebody trained, to a protocol that matches the question, and analysed by a laboratory with accreditation for that test. A pre flush sample tells you what is sitting in that outlet; a post flush sample tells you what the system is delivering; a swab of a showerhead or a tap outlet tells you about the biofilm there. The reference method is culture, which takes up to ten days because the organism is slow growing, and rapid methods are used as a screen rather than as the answer. Two practical points. Sampling on a cooling tower is routine and scheduled because the system is high risk by design, while in a building's water services it is used where the assessment calls for it rather than instead of temperature monitoring. And a result only ever speaks for that outlet on that day, which is why the monitoring regime rather than the certificate is what evidences control.
Questions people ask about cooling tower maintenance
Do cooling towers have to be registered?
Yes. Cooling towers and evaporative condensers must be notified to the local authority under the Notification of Cooling Towers and Evaporative Condensers Regulations 1992.
Is legionella sampling required on a cooling tower?
Routine sampling is part of the normal regime for cooling towers, unlike domestic hot and cold water systems where HSE states it is not usually required.
What is side stream filtration?
Continuous filtration of a proportion of the circulating water to remove suspended solids, which improves biocide performance and reduces fouling. It supports the treatment regime rather than replacing it.
Who is responsible for a cooling tower?
The duty-holder in control of the premises. The duty extends to people who are not employees, including members of the public near the site.
What do the legionella regulations actually require?
They are not a single set of regulations: the duty comes from health and safety law and the practical standard from the approved code of practice. What that asks for is a risk assessment, a written scheme of control, somebody named to manage it, and records.
How often is legionnaires water testing done on a cooling tower?
On a cooling tower or evaporative condenser, sampling is one of the few places where it is genuinely routine rather than exceptional. The approved code asks for regular microbiological monitoring on these systems because they aerosolise water by design, the results are read as a trend rather than as a single pass or fail, and an out of range count triggers the action levels written into the scheme of control: review, clean and disinfect, resample. That is the opposite of a domestic hot and cold system, where the control is temperature and sampling is indicated only when the controls cannot be relied on.