% bleach to sanitize fresh water tanks

rvshrinker wrote:
First time doing this chore.

Most sources say to use household bleach 1/4 cup for every 15 gallons of fresh water capacity. Mix required bleach amount in some fresh water, pour into your tanks, fill with potable water, let sit, drain, rinse.

However other sources say 1/4 cup for 10 gallons.

For a 60 gallon freshwater tank, this is either 1 cup or 1.5 cups of bleach.

But according to this fact sheet from the Northeastern University Dept of Environmental Health Sciences, bleach should be concentrated at 5000 - 20,000 ppm to disinfect. 5% bleach is 50,000 ppm, 10% bleach 100,000 ppm, etc. This would imply diluting 5% bleach no more than 10:1 to get to a concentration of 5000 ppm, the minimum for disinfecting. Then the 60 gallon tank needs ~5 gallons of 5% bleach and 55 gallons of water to be diluted properly for disinfection.

That’s a big difference.

https://www.northeastern.edu/ehs/wp-content/uploads/2014/12/Bleach-Fact-Sheet-Draft.ejc2_.pdf

And this Idaho DEQ website says dilute 1000:1:

https://www.deq.idaho.gov/media/517665-disinfecting-water-wells-storage-tanks-fact-sheet-0612.pdf

What is the correct concentration of bleach to disinfect the water tanks?

In a municipal water system, if chlorine is used as a disinfectant, it is introduced to the mains at around 1 to 2 PPM. The idea is to chlorinate at a level that maintains a trace level at the end of the system, so maybe up to 4 PPM is required to maintain a trace amount. A large system may have more than one disinfection station depending on system size and water source quality.

The American Water Works Association (AWWA) has been around since before the 1900's, and has been the accepted source for benchmark water testing and recommendations for many utility standards used by the US, Canada, and other countries around the world.

AWWA has procedures which detail solutions to disinfect public water mains and storage facilities (tanks) using several different methods and products including bleach which you mention above. In my 40 years of experience with building, testing and disinfecting municipal water systems, I have used three different methods depending on the situation involved-could be part of new pipeline construction, or maybe a main break. Their standards are in ANSI/AWWA C652-19.

https://www.awwa.org/Portals/0/Awwa/Publishing/Standards/C652-19LookInside.pdf?ver=2020-02-25-153628-643

OK, after all that, the disinfection standard for domestic water supply is 25 parts per million (PPM) chlorine to water which I had always exceeded just to be sure. I used 50 PPM. After the solution is introduced into the pipe or tank, it must (according to AWWA) be left for 24 hours, and then tested for a residual. A sample is then taken to a lab to test for fecal and total coliform counts. After another 24 hours have passed, another sample is taken and tested. If both samples are negative for coliform, the water is deemed to be safe to drink.

If the same pipe/tank was filled with 50 PPM, and the flow was regulated so that there was 3 hours minimum contact time throughout the system/tank, then the same testing procedure is followed immediately after flushing the system.

I realize that the above is lengthy, but the standards are the same for any domestic water application. Drinking water from a travel trailer tank is no different than drinking from a domestic water supply in a home. I have read in this forum in the last number of years posts from people who are not sure what the proper procedure should be. If you are OK with making up a solution at 25PPM, you should be good to go.

I would hazard a guess that most people do not do this because they really don't know much about it. For me at the first of the season, I chlorinate my RV system with 50 PPM, more or less, leave it for 24 hours, flush and use. After that, if the trailer sits unused for a month for example, I will put a couple of ounces in the filler hose and then fill the tank and flush the system a couple of times or whatever it takes to get rid of the chlorine taste.

I'm not saying that it's OK to do that, only that that's what I do.

Hope this is helpful.