Fresh water tanks after a typhoon or flooding

How a storm gets into a “sealed” tank

A building's fresh water tank is protected by passive things: a cover, a gasket, an insect screen on the vent and overflow, and the assumption that rain falls down and drains away. A typhoon attacks every one of those at once.

Roof tanks take the direct hit. Wind can shift or damage covers and access hatches, tear vent and overflow screens, and drive rain horizontally through gaps that vertical rain never finds. Everything loose on the roof — leaves, dirt, construction dust, debris — is picked up and washed across the slab, and some of it arrives at the tank. A displaced cover in a rainstorm is an open funnel.

Sumps and low-level break tanks face the opposite problem: water rising from below. If the pump room or tank room floods, floodwater can stand above the tank's covers and manholes and work past gaskets that were only ever designed to keep splashes out. Overflow and drain pipes normally carry water away — but when their outfalls are submerged, they become a route for contaminated water to reach the tank. Floodwater in a Hong Kong street or basement is not rain: it has been over roads, drains and soil, and it carries what it touched.

The mains interruption is the third route. If the storm damaged supply pipework, or repairs opened the main, the first water through can carry disturbed sediment and whatever entered the pipe while it was open. This is the same reason newly repaired plumbing is disinfected before use — a repair introduces exactly the contamination the closed system was designed to exclude.

Warning signs

Three things you can see, smell or find, and one thing you cannot:

• Discolouration. Brown or yellow water points to silt, soil or rust carried in or stirred up; grey or cloudy water to fine suspended matter. Either way the water changed, and something changed it.
• Odour. An earthy, musty or drain-like smell from the tank or the taps is a strong signal of organic material or floodwater ingress. Drinking water should smell of nothing.
• Sediment and debris. Grit, leaves or insects on the tank floor, in strainers or at outlets mean the physical barriers were breached — whatever else came in with them is the real concern.
• Nothing at all. This is the trap. Bacteria carried in with storm water are invisible and often tasteless at concentrations that make people ill. Clear water is not evidence of safe water — only a laboratory result is.

The decision path

The sequence below is the drinking-water version of the same logic we apply to cooling towers after a typhoon: look before you act, disinfect before you rely, and verify before you relax.

1. Inspect. Walk the installation: tank covers and hatches, vent and overflow screens, the roof around a roof tank, the floor level in a pump or tank room, the condition of visible pipework. The question you are answering is simple — could storm water have got in?
2. Test. If ingress was possible but nothing is visibly wrong, sample the water before doing anything else. Samples are analysed by a HOKLAS-accredited independent laboratory; bacteriological indicators are the classic markers of ingress, and the result turns “probably fine” into a document.
3. Clean and disinfect. If ingress is evident — or the test confirms it — the tank comes out of drinking service. Drain it, physically remove sediment and debris from the floor and walls, then disinfect the tank and connected pipework at the correct concentration and contact time, and flush the system through. Disinfecting a dirty tank wastes chemicals: sediment shelters the bacteria you are trying to kill.
4. Re-test. Sample again after the clean and flush, and hold the tank out of drinking service until the result confirms it is safe. The confirmation sample is the point of the whole exercise — cleaning without it is an assertion, not evidence.
5. Document. A dated record of what was found, what was done, and both sets of results. In an estate, hotel or hospital, this record is the answer to the question residents and management will ask: how do you know the water is safe?

Why disinfection comes before use

The tempting shortcut after a storm is to put the tank straight back to work and test afterwards. That order is backwards for the same reason it is backwards on a cooling tower — but with a harder edge, because nobody chooses whether to drink today. Bacteriological analysis takes time to culture; a building draws on its tank every hour. If contaminated water is in the tank, the days between sampling and result are days of exposure.

This is why disinfection after a suspected contamination — like disinfection after a plumbing repair, and for the same reason — happens before the water is used, with the test confirming success rather than discovering failure. The WHO's technical guidance on water storage tanks takes the same line: a tank that has been polluted or out of use is emptied, scrubbed clean, flushed, chlorinated and held for the contact time, then flushed and refilled — in that order — before it stores drinking water again.

What the authorities expect anyway

Post-storm checks sit on top of a routine, not instead of one. Once water passes the connection point, the inside service — pipes, tanks and fittings — is the owner's responsibility, and the Water Supplies Department recommends cleaning water storage tanks at least once every three months, because stored water accumulates dirt and impurities over time. The Centre for Health Protection's drinking-water guidance gives building users the same figure. A tank on that routine, with records to show it, starts a typhoon season in good condition; a tank nobody has opened in years starts it with sediment already on the floor — and the storm stirs, feeds and adds to it.

If the post-storm inspection is clean and the water is normal, the practical answer may simply be to bring the next routine clean forward. If the inspection is not clean, the decision path above applies in full.

Common questions