Can I use rainwater with a greywater system with an underground cistern?
I personally wouldn't, since I think the soaps and chemical residues are pollutants. Others may disagree. I suppose it depends on the need. I guess I could see that being practical and perhaps necessary in areas like Nevada or California, where people have created a virtually unsustainable system composed of far too many people and far too little fresh water. In areas like the great lakes, where we may not have balmy weather year around and tons of sun for solar farms, we do have plenty of water, and it makes much better sense to treat both "black" and "grey" water, rather than polluting our farmland and urban gardens with tons of detergent and fabric softener. I would hope than any such use of a cistern would be regulated by the government, so that a potential buyer of the property in future years would be well aware that the cistern has become toxic.
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Point taken. We're on septic system so we've gone much greener than we were when we had a city sewer that went to a full treatment plant. So our water is maybe only slightly "tinted" rather than fully grey. I tend to answer from my limited perspective unless I really give it some thought.
No offense intended. Just an observation...Circumstance often shapes what we accept locally.
I'm not sure what you're asking. You can put rainwater in a cistern and drink it and water the garden with it if the cistern is clean. You can put greywater in a cistern but you can't drink it but you can water the garden with it.
In some areas, there is an explicit recommendation against drinking rainwater due to concerns about both pollution and sterility. In my own neighborhood, for example, there are pretty explicit "fine for gardening, not for drinking" warnings. But adding rainwater to your greywater cistern system shouldn't cause any problems, at least not until you overload the cistern. (If you're doing the greywater thing, you should also be using biodegradable soaps and such. Which is a partial answer to Sean's concerns.)