cistern water - can I do this?

just got an idea, which is kind of inspired by my grandparents' old house... at that house, there was a cistern dug in the ground by the barn, which was uphill from the house. It was plumbed into the basement of the house, and due to the fact that the hill was fairly steep and far away, it was possible for the water to reach all the way to the second floor of the house. So my grandfather had gotten creative with the pipes and valves, and had things set up so that the cold water supply to the bathroom could be changed from the cistern to the well, and vice versa. This way they weren't using valuable well water to flush the toilet, etc...
I have a 550 gallon plastic cistern, above ground, at my new house. It's behind the house, and is so located that this trick wouldn't work for anything but the deep sink. However, I was wondering if it were possible/advisable to set it up to feed the cold water to the deep sink and washing machine, so I wouldn't have to use the city water for that. (yes, I still tend to be somewhat ecologically minded, so long as it's no inconvenience for me.) The question is, is the setup with valves etc. as I described above acceptable per current plumbing codes, or is this a no-no? I suspect that it may not, as if both valves were opened at the same time, there's a slim possibility of cistern water contaminating the potable water supply, but I personally would know what was going on enough to avoid having that happen.
thanks,
nate
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You can add a pump and seperate supply lines to toilets and your all set.
todays toilets may not work by low gravity, but should work fine with a shallow well pump.
If you really want to do it right add a single hose that can be swapped location wise to prevent cross contamination.
a good friend drilled a well he has city water but uses the well for toilets, and washing machines
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It is legal if you use the proper backflow prevention valves. They are expensive, so the idea may not be economical. Check with your local building department and water department to see what they would require. Hopefully they don't have contradictory requirements. If the systems are completely separate, so contamination can't occur, it is none of their business.
A hotel I stayed at in Saint Thomas used seawater in the toilets.
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It is not permissible under plumbing codes nor is it a good idea to interconnect the city water supply and any well or cistern water. There is more than "a slim possibility of contamination". It is practically a certainty. Untreated water is an excellent breeding ground for viruses and bacteria. The only questions are how much contamination, what with, and how serious/dangerous/deadly is it?
Don't do it. Use completely separate piping and faucets with the potable water faucets high enough to prevent any contact by the cistern water.
Don Young
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