How Often Should a Yacht’s Fresh-Water Tanks Be Inspected and Disinfected?

The WHO Guide to Ship Sanitation says potable water tanks should be inspected, cleaned and disinfected at every dry dock or wet dock, or every two years, whichever comes first. Disinfection means raising free residual halogen to at least 50 mg/l for 4 hours, then flushing with potable water.

Why tanks deserve their own routine

Every tap, shower and washing machine on a yacht is fed from the same place. Water sits in the tank between fills, and anything that gets in there, such as sediment, slime or a contaminated fill, travels to every outlet.

That is why the tank has its own inspection and disinfection routine, separate from the pipework. For the pipework side, including hot-water temperatures and Legionella, see our guide to Legionella on yachts.

What the WHO guide says

The clearest written standard for vessels is the WHO Guide to Ship Sanitation, chapter 2 on potable water. It is written for ships, so a yacht’s own flag, class and water safety plan decide the final details. Its main points on tanks are:

• Inspect, clean and disinfect tanks at each dry dock or wet dock, or every two years, whichever comes first.
• Disinfect by raising free residual halogen to at least 50 mg/l for 4 hours, or by another procedure recognized by WHO.
• After contamination or repair, the tank should be cleaned, disinfected and flushed with potable water before it goes back into service.
• Build and place tanks so they are protected from contamination from inside or outside, ideally in rooms without sources of heat or dirt.
• Fit sample cocks on each tank, pointing downwards.
• Keep a chlorine residual, typically around 0.5–1 mg/l free chlorine as water enters the distribution or storage system.

The three-step tank clean

A tank clean follows the same order every time: look, disinfect, flush. Only after that does the tank go back into service.

The WHO adds that a procedure other than 50 mg/l for 4 hours can be used if WHO recognizes it. The right product also depends on the tank material, so it should be chosen by a qualified technician.

A sample routine

Built on the WHO guidance above. Adjust it to the vessel and its water safety plan.

• At every dry dock or wet dock, and at least every two years: inspect, clean and disinfect the tanks.
• After any contamination or repair: clean, disinfect and flush before use.
• At every fill: use the marked potable-water filling line and check the chlorine residual of the incoming water.
• If in doubt: take a sample from the tank sample cock and have it tested by a laboratory.

Warning signs

• Cloudy water, or a glass that clears from the bottom up.
• A smell or taste that was not there before.
• Sediment or slime seen through the hatch.
• A tank that has been open, repaired or left partly empty for a long time.

Our pages on tank contamination, cloudy water and water smells explain the likely causes.

FAQ

How often should yacht fresh-water tanks be cleaned and disinfected?

The WHO Guide to Ship Sanitation says at each dry dock or wet dock, or every two years, whichever comes first.

What disinfectant strength does the WHO guide describe?

At least 50 mg/l of free residual halogen held for 4 hours, or another procedure recognized by WHO, followed by flushing with potable water. A qualified technician should choose the product to suit the tank material.

Should a tank have a sample point?

Yes. The WHO asks for sample cocks directly on each tank, pointing downwards to avoid contamination.

What chlorine level should the incoming water have?

The WHO guide gives a typical figure of around 0.5–1 mg/l of free chlorine, or 1 mg/l for chloramines, as water enters the distribution or storage system.

Where should a tank be located?

It should be built and placed so it is protected from contamination from inside or outside, ideally in a room with no heat sources or dirt.

Is cleaning the tank enough?

No. The tank is only the start. Hot-water temperatures, dead legs and outlets matter too. See our guide to Legionella on yachts.

Sources

• World Health Organization — Guide to Ship Sanitation, chapter 2: Potable water (section 2.2.3 and guidance notes): tank inspection interval, disinfection at 50 mg/l for 4 hours, flushing, tank location, sample cocks, filling line, chlorine residual.

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