Water Quality in Tanks

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Senior Member
Oct 6, 2007
I had a trawler neighbor some time ago that told me one day she had never used the water from her "new" Grand Banks water tank because she did not know if it was safe. She always brought water from home in large bottles. I thought this was strange.

About a year later, I found that my marina has a well contaminated with ecoli. God knows how long we had been filling our water tanks with "bad" water and drinking and bathing with it? I had to do something quick.

I found a water purification system online and ordered it but what could I do with the 300 gallons still in the tank? Draining would help but I'd still have the contamination. A check in the marine catolog found some high priced additives. A bit more research led me to the Chlorox and NOAA websites if I recall.

I found that one gallon of regular unscented bleach equals about 3,000 gallons of clean drinking water when used to treat water. NOAA recommends this method be used to treat water following hurricanes at the National Hurricane Center website. You could use the same ratio if you had a disaster as well.

For treating my water tank, I added bleach at the ratio of 1/2 teaspoon bleach to 5 gallons of water then waited 30 min. The water should have a slight bleach odor when you turn on a tap. If not, repeat dose. Wait 15 minutes again. I doubled the amount the first treatment of the existing water in the tanks and let stand an hour before adding additional water afterwards at the normal ratio.

The water will have a bleach smell to it and the taste will be a little acidic but it will not make you sick.

After I recieved my purification system, I stopped adding bleach to the water and it all cleared out the next fill up.

It's always an environmentally sound choice too. After its purification is done, bleach (sodium hypochlorite) breaks down to little more than salt and water.

So if you have some concerns about the quality of water in the tanks of that trawler you are looking at*, treat it with Chlorox to be safe.
**** As a rain water collector we regularly add 1 teaspoon of bleach per 50 gallons of water. ½ teaspoon per 5 gallons seems like a lot, but considering that this procedure is recommended for really bad water it must be fine. Once a year or so we shock the system with bleach, kinda like what you do with a well system on land. For shocking we put the bleach in the tank, open up all faucets until you smell it, let it sit for about eight hours and flush it all out. It kills all the critters that might be lingering in the system.
*** We installed a charcoal filter in the salon JUST for drinking water. Using the 1 teaspoon per 50 gallon method, there is no chlorine taste in the drinking water. The shower water etc just smells clean. Works for us.
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