How often should a cooling tower be cleaned and disinfected?

It's one of the first questions we get asked when we take on a new site, usually followed by "and when was ours last done?" The short answer is at least twice a year. The longer answer is that some towers need it more often, and a clean done badly can do more harm than good.

November's a good time to think about it. The leaves are down, the summer's heat load is behind you, and plenty of towers are sitting on six months' worth of whatever the air has thrown at them. Here's what the guidance says, what a proper clean involves and how to tell if yours is overdue.

What the guidance actually says

The technical guidance for evaporative cooling is HSG274 Part 1, which sits alongside the Approved Code of Practice, ACOP L8. HSG274 Part 1 says cooling towers should be cleaned and disinfected at least twice a year. That's a minimum, not a target.

Your own frequency should come from your Legionella risk assessment and be written into your written scheme. If the assessment says your tower needs cleaning every four months, that's what you're expected to do, and an inspector will check your records against it.

For evaporative condensers, the frequency should also be set by the risk assessment, taking account of the guidance. They're often smaller and more enclosed than open towers, but they still collect sludge and biofilm, and they still produce aerosol.

Why chemistry alone isn't enough

A good treatment programme does a lot of heavy lifting. Inhibitors keep scale in solution, dispersants stop dirt settling, and biocides keep the bacterial count down. But no chemical programme removes what has already settled out.

Cooling towers are very good air scrubbers. Over a few months they pull in dust, pollen, insects, leaves and whatever's blowing off the building site next door. That ends up as sludge in the pond and deposits on the fill. Biofilm builds up in the corners where water moves slowly. All of it soaks up biocide and gives bacteria, including Legionella, somewhere to hide. Physical cleaning is the only way to get it out.

What a proper clean and disinfection involves

HSG274 Part 1 describes a sequence, and the order matters:

1. Pre-clean disinfection. Before anyone opens the tower up, the water is disinfected, usually by chlorinating the system and holding a set level for a set contact time. This cuts the bacterial load so the cleaning itself doesn't spray contaminated water around.
2. Drain down and physical clean. The pond and sump are emptied and cleaned out. Strainers, distribution troughs or spray nozzles, fill and drift eliminators are cleaned, and debris is removed rather than washed into the drain.
3. Inspection. While it's open, it's the best chance you'll get to look at the condition of the fill, eliminators, casing and fan. Damaged eliminators, for example, let more drift escape.
4. Post-clean disinfection. The system is refilled and disinfected again, then flushed and brought back to normal treatment levels before going back into routine service.
5. Records. What was done, chemical levels and contact times, and what was found all go in the log book.

A quick hose-down of the pond doesn't count. Neither does skipping the pre-clean disinfection to save a few hours. If the person doing the work isn't following that sequence, it's worth asking why.

When twice a year isn't enough

Plenty of towers need cleaning more often than the minimum. Your risk assessment should consider things like:

• Dirty surroundings. Nearby construction, demolition, busy roads, trees or industrial processes all increase the amount of debris the tower pulls in.
• Process contamination. Food, oil or other process leaks into the cooling water feed bacteria and foul the system fast.
• Monitoring trends. If dip slide counts keep creeping up between cleans, or biocide demand keeps rising, the system's telling you something.
• History. A system that's had positive Legionella results before deserves closer attention.
• Who's nearby. Towers close to hospitals, care homes, air intakes or busy public areas carry a higher consequence if things go wrong.

Signs your tower is overdue

You don't need to be a water treatment engineer to spot most of these during a walk-round:

• Visible sludge or debris in the pond
• Slime or algae on the fill, eliminators or the inside of the casing
• White or brown scale on wetted surfaces
• Dip slide results drifting upwards over several weeks
• Biocide or inhibitor reserves dropping faster than usual
• A cleaning certificate in the log book that's more than six months old

Any one of these is worth a conversation with whoever looks after your treatment. A few together usually means a clean is due now, not at the next scheduled date.

Planning a clean on a site that can't switch off

The most common reason cleans slip is that the cooling can't easily be turned off. Data centres, hospitals and process sites all have this problem. It's solvable, though:

• Multi-cell towers can often be cleaned one cell at a time with the others carrying the load.
• Sites with N+1 redundancy can take one tower out while the standby runs.
• Spring and autumn cleans can be timed for lower heat loads, when there's more spare capacity.

It just needs planning early, rather than squeezing it in when the certificate's already out of date. Our cleaning and disinfection page explains how we approach this, and if a tower has been sitting idle, take a look at our notes on shutdown and start-up too.

The bottom line

At least twice a year is the floor. Your risk assessment sets the real frequency, your monitoring results tell you whether it's working, and your records prove it was done. If you can't remember when your tower was last cleaned, or you're not sure the job's being done properly, that's the moment to get someone to take a look.