How do I perform a shock treatment?

How do I perform a shock treatment?

Sometimes when water tanks and plumbing have not been treated, you can get a build up of biofilm.  This biofilm will create the rotten egg smell and bad taste to the water.  Because the tanks are vented to the outside, it is not a sealed system and accordingly, even the cleanest municipal or "RO" water can still contract biofilm growth due to outside contamination.  When that happens, Aquatabs Marine will not be powerful enough to eliminate the growth and you have to step up the shock treatment.

The most common method is a measured dose of Clorox.  Unlike Aquatabs Marine, Clorox is a high base solution and is corrosive to some materials found in marine plumbing and accordingly should only be used when necessary.  A one time shock, followed by ongoing treatment with Aquatabs Marine is the recommended procedure.

So if you need to perform a shock treatment, here is how you should go about it:

Clorox Performance Bleach contains 7.55% sodium hypochlorite, the normal Clorox formula includes 6.05%.  The version you choose will impact how much you add to get to the desired concentration of free chlorine.

To achieve the recommended 50 parts per million (50ppm or m/l) with the 7.55% concentration you need a ratio of  Clorox to water of 1:1,437 (1 oz. of Clorox to 11.25 gallons of water).  With the 6.05% concentration you need a ratio of Clorox to water of ratio of 1:1,151 (1 oz. of Clorox to 9 gallons of water)

Check the bottle of Clorox for the sodium hypochlorite concentration and also for the expiration date.

You will want a high chlorine test strip that measures over 50ppm (parts per million or mg/l) of Free Chlorine, (not Total Chlorine) in order to monitor your progress.  (

Different test strips measure different components of Chlorine. Total Chlorine is made up of Combined Chlorine, which is Chlorine which has attached to other compounds in the water, and Free Chlorine which is unattached. It is the Free Chlorine which provides the pathogen killing power, accordingly, this is the Chlorine we want to measure. The process may take up to 24 hours to complete.

Here is the process:
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 and biofilm during later flushing steps.

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

4. 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.

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

6. 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.

7. 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 and test the Free Chlorine level, it should be between 40-80 ppm (mg/l). Note the Free Chlorine level measured.

8. After about 6 to 8 hours of contact time, test the free chlorine again to evaluate the consumption. If the Free Chlorine has fallen to 10-20 ppm, you should re-treat to bring the level back up to 40-80ppm. Let the mixture set over night. Test again the next morning. If the Free Chlorine has again fallen to 10-20ppm, you should re-treat again. Continue this cycle until the treatment holds at 40-80ppm after the 6-to-8-hour contact time. This will ensure that the tank is fully sanitized.

9. When the contact time is completed, drain the tank from all of the faucets and showers. Make sure the shower heads and faucet screens have been removed as the plumbing will likely slough off a layer of bacteria and biofilm as noted above. Refill with potable water and purge the plumbing of all sanitizing solution. Repeat until free chlorine measured is below 6ppm.

10. If the concentration of bleach exceeds 6ppm 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. Don’t use vinegar, which can ferment, undoing all of your hard work.

11. Replace all filters and the vent screen.

12. You can now start your normal maintenance with Aquatabs Marine.

Enjoy your clean water tanks!

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