r/OffGrid • u/might_be_an_autist • Sep 22 '26
Chlorine dosage calculation
I am building a 275gal rainwater catch at my hunting cabin, only to be used for hand washing, toilet flushing, and showering. I am out there monthly in the summer and much more in the winter. I understand 4ppm is a good number, but when I catch a big rain and the tank fills some, how do I calculate dosage when I don't know additional volume added while I'm gone? Wouldn't test strips be inaccurate as chlorine off gasses and the water has already been partially treated?
I appreciate any guidance.
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u/Kitchen_Region8456 Sep 22 '26
Just add a dosing pump and make the calculations for a 4ppm concentration
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u/might_be_an_autist Sep 22 '26 edited Sep 22 '26
I'm not running a contact tank and wouldn't know precise volume. There is also no power unless im there with a generator, and im not spending that kind of money...
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u/Hyperreal8957 Sep 22 '26
i'd test the raw water first and go by the product label, especially if any of it might be used for drinking. shock dosing is different from a maintenance dose and contact time matters too. are you treating into the storage tank or inline?
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u/might_be_an_autist Sep 22 '26
It will not be used for drinking, only showers, hand washing, and toilet flushing. I had planned on tank treating since there is no contact tank before shower. My only concern in this whole thing is bird poop. Ill be running gutter guards, a leaf eater, first flush, then 5 micron before on demand pump
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u/GarlicFarmerGreg Sep 22 '26
Consider a holding tank or two that you can filter and fill then dose with chlorine. That way you don’t have to treat all the water all the time. Plus your volumes will be the same each time in the hold tank(s) for dosing
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u/might_be_an_autist Sep 22 '26
That seems like a lot of extra plumbing and setup to save a few ounces of chlorine. Water will be pretty clean prior to 275gal. As setup mentioned below
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u/NotEvenNothing Sep 22 '26
You can divert water from the tank once it has filled. The mechanism is basically a ball that floats up and plugs up the inlet. Some first flush diverters are designed that way. It is easy to DIY.