[From an article in Practical Sailor--Subscribe!!]
https://www.practical-sailor.com/
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The RV code (ANSI A119.2 section 10.
has a standard procedure that is oft quoted and works very well. We’ve added a few details, but the bones of it come straight from the code and have been reviewed and accepted by the US Public Health Service. This is typically done annually, or when the boat has been unused for several months. This is based on household bleach.
1. Turn off the hot water heater until finished.
2. 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.
3. Clean and remove the vent screen and flush the vent hose.
4. Use the following methods to determine the amount of common household bleach needed to sanitize the tank.
1. A) 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.
1. B) Multiply “Liters of tank capacity” by 1.0; the result is the milliliters of bleach needed to sanitize the
tank.
5. 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.
6. Pour the solution (water/bleach) into the tank and fill the tank with potable water.
7. Allow some solution to escape though the vent, if safe and applicable (some boats use the vent as an over flow, while in some cases the vent is in the interior). This will sanitize the vent line.
8. 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.
9. The standard solution must have four (4) hours of contact time to disinfect completely. Doubling the solution concentration allows for contact time of one (1) hour.
10. 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.
11. 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. Others suggest vinegar, but vinegar at long dilutions can ferment, undoing all of your hard work.
12. Replace all filters and the vent screen.
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About 10x less bleach is used for routine treatment, and normally is not needed all if the source water is chlorinated. If you are chlorinating, use either bleach or Aqua Mega tabs (better for aluminum tanks), and test the water with aquarium or pool test tapes (~ 0.5 ppm residual after 1 hour).
Really, you want to use a particle filter during filling, not carbon--you DON'T want to remove the just chlorine yet. Just a 1-5 micron filter to keep the dirt out. Better yet, use a Baja water filter:
https://www.practical-sailor.com/issues ... 660-1.html
A granulated carbon filter will do NOTHING to remove microorganisms. This is a myth. In fact, they usually grow them. It will only remove chlorine, and perhaps traces of a few more things, depending on additives and doping. A carbon block filter with an NSF 53 rating--this is a vital and specific distinction--will actually remove gardia cysts etc. However, these do reduce the flow a little, and since they remove all of the chlorine, should only be used AFTER the pump.
Teamwork: Was that 0.5 micron carbon block, or something else? A 0.05 micron filter is not an easy thing to find. Just wondered if that might have been a typo. If not a typo, I would really love to know the specifics!!