“A cistern liner is a flexible membrane, sometimes called a geomembrane. You can get them in PVC or EPDM.
Here’s a link to where we get our potable liners:
Here’s some examples of a EPDM liner in a simple wire mesh to make a tank (do a google image search for tanques de geomembrana):
These are the cheapest tanks available. They usually run about 8 cents a gallon, installed. You can get EPDM liners with 20 and 30 year warranties.
Building directions (refer to drawing above):
Here is a super easy way to make water tanks. Dig a hole slightly larger than the size of the tank. Put posts around the perimeter of the hole, then place the tarp on the posts. Back fill against the tarp and tamp good. Put 6″ of sand to level the floor. Now, just drop in a HDPE liner in the hole, and do a simple roof (maybe latex cement dome). That would be one of the cheapest water tanks around, and the greater the diameter, the cheaper it would be per unit of volume. I just bought a liner for potable water, 6500 gallons. It cost about $800, shipped.
For the top cover, you have a few options. The fastest would be a silo top, which you could probably find locally. Another option would be an earthbag dome. Or you could do ferrocement.
I would probably do acrylic cement. Take some shade cloth, attach around the edges of the hole (maybe to the posts), then put a post in the center to make a cone shape. Brush on your layers of latex cement, let cure then remove the center post.
Alternatively, you could PVC or something similar as support members to make a frame, and then cover with the shade cloth or netting to make it.”