I will be adding a water cistern to my house soon. It will be filled via gutter downspouts as shown in Joel's _Secure Home_ and _HS Shelter_ books. I have no experience with water storage, but I have seen molded plastic tanks for sale locally. I could drop one of those in the hole I dig, or I could cast my own cistern out of cement.
I plan to dig the cistern hole within two feet of my house foundation walls. While I'm digging, I may widen the hole for an emergency exit of the shelter I plan to build on the other side of the wall.
Does anyone have suggestions?
Regards,
True North
Some rainwater collection systems use a diversion system that lets the initial rainfall flush the dirt off the roof, then later diverts the follow-on rainfall to the cistern. Joel's cistern plans do not use this flushing technique, but more simply, just collect all rainwater from the gutter. How important is an initial roof flush? I doubt bird poop is toxic, but radioactive or chemical-laced dust is a different matter. See http://saferain.hypermart.net/ for details.
Buried cisterns or water tanks have been know to "float" up out of the ground if the tank is empty and the ground is saturated with water. A cistern which will be filled only by roof run-off may be empty a good bit of the time. The weight of a concrete tank will naturally help to minimize this problem. I can't help but think that a concrete tank will also be a lot more rugged and long lasting.
I have a concrete cistern of 1500 gallons, kept filled by pumping from a well. The well driller who installed the cistern told me about the above "floating" problem since he had personally had it happen to one of his own installations.
If I remember correctly, Joel's roof water collection system filters it through sand before it enters the tank. This takes the place of the diversionary system you mention. The diversionary system is only meant to eliminate solid material anyway, not dissoved material.
Thank you for your comments, Alonzo. I have decided to build a concrete cistern, since the cement mixer and other gear I'll need to get will be useful for other projects I'm planning. I intend to build a shed on top of the cistern. The shed provides a ready reason for the construction project (inquisitive neighbors) and will help weigh the cistern down somewhat. I have built a french drain next to the cistern location to divert water that flows across my lawn toward the cistern. That will reduce the degree of soil saturation.
Can anyone suggest an inert waterproofing coating that I should "paint" the inside of the cistern with once the concrete has set? Is a coating needed?
True North:
Why not just buy a ready-made cistern? It's exactly the same item used for a septic tank, they come pre-made and delivered to your site, ready for the back-hoe to place in the hole after excavation. They are very reasonably priced. The whole job can be done for you by the backhoe operator in just a few hours. That's the way to go. They don't really need to be coated with anything.
Thanks for the info on your cistern, Alonzo. I am planning to do something similar. Are you using a float switch to control the filling of the cistern, or one of the newer electronic sensing probes? How long have you operated your cistern? Have you had to clean the inside of it at all? Thanks ahead of time for your help!
I've had the cistern for about 4 years. The well pump which fills the cistern is on a timer and runs for about 1 minute each 15 minutes or half hour. This timer is adjustable as to pumping duration and frequency. There is no float switch, just an overflow outlet which is piped away into the woods.
I have cleaned it once, which was accomplished by pumping the cistern down as far as possible, then using a portable sump pump to get the last of the dirty water out of the bottom. We climbed right down into the cistern with a ladder to do the cleaning.
As an update, I recently installed a +-1000 gallon plastic cistern. It's filled with water pumped out of my excavated crawl space. (I'm currently capturing about 300 gallons per day with a sump pump.)
Later I might add fittings for capturing roof runoff. I also might hook the cistern up to my lawn sprinkler system, since water here gets expensive during the summer.