Decontaminating 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 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 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.

There 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.
  1. 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 (4-6 percent sodium hypochlorite, no fragrance) per 10 gallons.
  2. 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.
  • 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.

Liveaboard sailors who care about water quality will appreciate our new four-volume eBook Onboard Water Storage, Treatment and Production, which covers everything from tank selection to filtration to cheap and easy water testing. With minimal investment in filters, we drink pure fresh-tasting water from our own tanks following the process described in this series. You can follow Frye’s other experiments and sailing adventures at Sail Delmarva.

Drew Frye
Drew Frye, Practical Sailor’s technical editor, has used his background in chemistry and engineering to help guide Practical Sailor toward some of the most important topics covered during the past 10 years. His in-depth reporting on everything from anchors to safety tethers to fuel additives have netted multiple awards from Boating Writers International. With more than three decades of experience as a refinery engineer and a sailor, he has a knack for discovering money-saving “home-brew” products or “hacks” that make boating affordable for almost anyone. He has conducted dozens of tests for Practical Sailor and published over 200 articles on sailing equipment. His rigorous testing has prompted the improvement and introduction of several marine products that might not exist without his input. His book “Rigging Modern Anchors” has won wide praise for introducing the use of modern materials and novel techniques to solve an array of anchoring challenges. 


  1. We follow a similar regiment, we filter all water as it comes on board, have a small filter in the galley for drinking water. We use AquaFresh at each filling. If the witnerizing is done well, we “bomb each tank with a lot of AquaFresh before running thru our system before filling tanks for use.
    This past year, the yard did not winterize well and we had a serious issue. This was solved using swimming pool grade chlorine many times stronger than bleach added it ad filled alltanks leaving it for a few days, we emptied the tanks with a sump pump to prevent anything from contaminating the fresh water system. then we applied hydrogen peroxide to remove any chlorine. and pumped that throught the system. We then filtered the fill up, used aquafresh and have pristine potable water tanks.

  2. Good description of procedure; one math nitpick. Your dilution ratios (for both metric and gallons) come out to a 1/1000 dilution. “1/8 cup per 10 gallons” falls a bit short of that, being 1 oz / 1280 ounces. Should have been more like 1/8 cup per 8 gallons.

  3. Thank you Drew for this article. I have a situation not covered by your article. Last week I inadvertently added 2 oz of FPPF Marine diesel Fuel Treatment to an empty 21 gallon plastic freshwater tank. (The filler caps for diesel and water are way too close together!!). I immediately flushed out the tank three times with fresh water. There was till some residual odour. With winter approaching, it’s soon time to anti-freeze the plumbing systems.
    Question: what more should I do now or in the spring to ensure safe potable water for next season?
    Thanks! – Chris