Several things come to mind:

How will you keep the septic tank from contaminating the cistern?

What zoning codes do you need to follow?

How will you keep the water IN the cistern? What kind of lining will you use?

How will you cope with heavy rainfall ... or even non-drought conditions?

Stupid question here but what exactly is a cistern?
Thanks,
Nancy/VA

Nancy -
Large storage tank that collects rainwater for domestic use. they are VERY common in places with no ground water and ample rainwater, like Bermuda.

Lazygardens - I think you mistook what he said. It will be a new septic tank, used as a cistern.
Joe -
I went looking for a cistern in early summer. I finally found out yep a new septic tank is the way to go. I have friends that put one in when they built, so all the downspouts go into the cistern. They wanted a 1000 gallon one, but price was prohibative, so they went with a 500 gallon one. I think they said the pump was $50, not bad!
I ended up with 2 - 85 gallon rain barrels, but I haven't given up on the cistern! Let me know what you do.

Webegardnr,
As you say, new is the way to go...I was thinking of one of those 2K gal units....but my question to a septic tank is..
... isn't there a top half & a bottom half....there is a mechanical seal of some type to keep "most of the solids in at the split" ... I want to keep in all the liquid I can ...

Are there any plumbers out there that could answer my question of "top & bottom half" & sealing????

I'm not worried about zoning, I live in the country....

Hey, my fall crop of half runner greenbeans are already 1/2" long...3 rows 40 ft long, on horse fencing, staked at 5 ft. tall....
If I get a heavy rain fall, so it will overflow to the creek somewhere down hill from me....atleast I'll try to catch all I can on my property for my usage....If I don't use it & don't need it, I can always divert it away from the cistren...I think I'll be serving two masters, the earth by stopping erosion from taking place due to the over abundance of fast rains, and not using too much resources like county water....I can only try....

Ok got it now. Thought maybe that was the idea. We boxed in a spring and put a holding tank in about a month ago. We used a 1000 septic tank for the holding tank. Its sunk down into the ground about 6 or 8 feet. with an over flow pipe coming out the side. it discharges into the creek. And about a foot diameter hole in the top. Ours was a 2 piece thing I think, not exactly how they sealed it, but I know it fills all the way up to the overflow pipe. I'll ask DH if that tank was a 2 piece or not.
Nancy/VA