Restarting a cooling tower after shutdown or typhoon

Why an idle tower is dangerous

Legionella needs three things to multiply: warm water (roughly 20–45°C is the growth window), still water, and food — sludge, scale, rust, organic debris, and above all biofilm on wet surfaces. A shut-down cooling tower in a Hong Kong summer supplies all three at once. The basin water sits warm and unmoving, the biocide residual decays day by day, and the biofilm keeps growing with nothing to disturb it.

A running, treated tower is constantly fighting this. An idle tower is not. That is why the danger point is not the shutdown — it is the restart, when the fan switches on and the accumulated water becomes aerosol that people breathe.

What the Code of Practice says about shutdowns

The EMSD Code of Practice for Fresh Water Cooling Towers (2023 edition, Part 2) gives two acceptable ways to handle a system that will be out of use for more than a week:

• Keep it wet, keep it treated. Leave the system full of treated water, and check and circulate it once a week; or
• Drain it completely. Fully drain the system, dry it with a mechanical fan, and cover and shut off the inlet and outlet pipes.

Either way, the tower must be isolated from the main system during the shutdown — and full recommissioning, including cleaning and disinfection, is required before it goes back into service. There is no third option in which the tower simply sits as it was and gets switched back on. We cover the Code in full in the EMSD Code of Practice explained.

The restart sequence

This is the order of work for bringing a tower back safely. The order matters: disinfection before restart, verification before normal service.

1. Inspect. Look at the tower before touching anything. Check the basin, fill pack, drift eliminators, strainers and pipework for sludge, scale, debris and visible biofilm. Check what state the water is in — or whether the system was drained properly.
2. Clean if fouled. If there is sludge, slime or debris, the tower needs a physical clean and desludge before anything else. Disinfecting dirty water wastes chemicals: the fouling shelters the bacteria you are trying to kill.
3. Fill. Fill or refill the system with fresh make-up water. Do not restart on the water that sat through the shutdown.
4. Disinfect. Dose the disinfectant and circulate it through the whole system — tower, pipework and heat exchangers — so it reaches everywhere the water does. This happens before the tower returns to normal duty.
5. Verify the biocide residual. Measure that the disinfectant actually reached and held its working level. A dose without a verified residual is a hope, not a treatment.
6. Sample. Take water samples for laboratory testing, including Legionella, to confirm the system is starting from a clean baseline.
7. Document. Record what was found, what was done, the readings and the dates. The Code requires records to be kept for at least 2 years — and after an incident, the restart record is the first thing anyone asks for. Log it alongside the routine duties on the cooling tower compliance checklist.

After a typhoon

A typhoon deserves its own mention because it can compromise a tower without any shutdown decision being made. Three things happen in a severe storm:

• Debris loading. Wind drives leaves, dirt, construction dust and organic matter into the tower. That material is nutrient — food for bacterial growth — and it can arrive in hours rather than months.
• Contaminated water. Wind-driven rain and splashed-in runoff can carry contamination straight into the basin.
• Power interruption. If the storm knocked out power or the plant was shut down for safety, the tower may have sat with warm, untreated, unmoving water — a short unplanned shutdown nobody logged.

After a severe typhoon, the practical answer is a post-storm check: inspect the tower, clear debris, check the water condition and biocide level, and disinfect before returning to normal service. If the system was out of action for more than a week, treat it as a full recommissioning under the Code. A typhoon check is not a statutory requirement in itself — it is simply what a competent operator does, because the storm has done to your tower in one day what neglect does in a season.

The seasonal picture

Hong Kong's hot, humid season is when all of this matters most. Warm weather pushes cooling water further into the Legionella growth window, towers work hardest, and the typhoon season arrives in the same months. A tower idled in winter and restarted in spring — or knocked out by a storm in August — is meeting the highest-risk conditions of the year. Restarts in the warm months deserve the most care, not the least. Clean before the cooling season, not after it; see how often a cooling tower should be cleaned.

Common questions