Decontamination a Tainted Water Tank

Those of you who missed our report on how a weak winterizing solution can create a Sandals Beach Resort for waterborne bacteria you may be noticing a pungent odor coming from your galley tap. Regardless of the cause of your water woes, our favorite chemist-sailor Drew Frye has spent most of the winter coming up with some simple steps to ensuring you have fresh-tasting water as good as any bottled variety on board this spring. Here he offers a step-by-step guide to decontaminating that foul-smelling tank.

With careful monitoring and pre-filtering of dockside water and maintenance of tank water, this procedure should only be required when contamination is suspected, or to start off fresh with a clean tank. Regular inspections, filtration at the dock, and maintenance doses of chlorine or treatment tablets (which we will compare in Part II of this series) when needed will prevent future problems. Using a proper mix of glycol if you winterize will prevent one of the common causes of contamination.

First the tank needs to be clean. Look inside with a flashlight; is there any sediment on the bottom or scum on the walls? Feel the walls; are they slick, evidence of healthy bacterial growth? It all must go. Hopefully there is reasonable access, for there is no substitute for a good hand scrubbing and rinse-down with a high powered hose. Machine dishwasher detergent works well as do long handled brushes. A power washer can help, but some angle fittings will be needed and it won’t do the job by itself. Once you’ve taken care of any growth, the next step is sanitizing.

Where’s all that gunk coming from? Here’s what one of our test pre-filter looked like after just 50 gallons from a dockside hose.

filterThere is a standard sanitizing procedure for recreational vehicles (ANSI A119.2 section 10.8) that works just as well for boats. We’ve added a few details, but the bones of it come straight from the code and have been reviewed and accepted by the U.S. Public Health Service.

Turn off the hot water heater until finished.

Remove any carbon canisters or micron rated filters. Remove any
faucet aerator screens. Wire mesh pump protection strainers should stay in place. The
plumbing will very likely slough off a layer of bacteria during later flushing steps.

Clean and remove the vent screen and flush the vent hose.

Use either following methods to determine the amount of common household bleach needed to sanitize the tank.

Multiply gallons of tank capacity by 0.13; the result is the ounces of bleach needed to sanitize the tank. This is 1/8 cup of plain bleach (no fragrance) per 10 gallons.

Multiply liters of tank capacity by 1.0; the result is the milliliters of bleach needed to sanitize the tank.

Mix the proper amount of bleach within a 1-gallon container of water. This will provide better mixing and reduce spot corrosion of aluminum tanks.

Pour the solution (water/bleach) into the tank and fill the tank with potable water.

If possible, allow some solution to escape though the vent. (If the vent is exterior, prevent any spillage into local waters.) This will sanitize the vent line.

Open all faucets (hot and cold) allowing the water to run until all air is purged and the distinct odor of chlorine is detected. Leave the pressure pump on.1

The standard solution must have four hours of contact time to disinfect completely.

Doubling the solution concentration reduces the contact time to one hour.

When the contact time is completed, drain the tank. Refill with potable water and purge the plumbing of all sanitizing solution. Repeat until bleach is no longer detectable.

If the smell of bleach persists after two refill and drain cycles, add a teaspoon of hydrogen peroxide per 20 gallons and mix. The peroxide will oxidize the hypochlorite to chloride (salt) and oxygen, neutralizing the bleach. Any excess peroxide will be harmless to drink and will have no taste. Peroxides are common ingredients in commercially available water freshening preparations like those we tested. Don’t use vinegar, which can ferment, undoing all of your hard work.

Replace all filters and the vent screen.

Note for aluminum tanks: Some sailors are afraid of using bleach in aluminum tanks for fear of rapid corrosion. This shouldn’t be a concern for infrequent cleaning when the recommended dosage and time is observed. As an alternative, we found PuriClean to be an effective sanitizer, and it was non-corrosive toward aluminum.

As a side note, at the end of the season I pump the water tank as dry as I can. After that, since I have installed an inspection port, I vacuum any remaining water with a dry/wet vacuum. The result is no misguided “stuff” sprouts their yucky messes.

water tank

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