If your willing to wing it a little tanks are not a big problem.

One of the easiest, and most cost-effective, is to head down to the discount store and get a kiddie pool or two. A 6' diameter kiddie pool 12" deep holds about 200 gallons.

While there pick up some nylon scrim from the fabrics section and a large rubber ball. Drape the scrim over it and use the rubber ball to keep it above the water so bugs don't take up housekeeping.

I've seen someone mount kiddie pols rim-to-rim to make an nice tank. The key was arranging a piece of PVC pipe under both rims and screwing/bolting it together every few inches. I helped them bend the pipe. They used a urethane adhesive caulk after cleaning the plastic surfaces vigorously. Stacked you get about 400 gallons but joining struck me a lot of trouble to get the same capacity as two siting side-by-side.

Another alternative is to use 2bys joined at the ends with cheap galvanized steel angles to form a frame sitting on the ground. This can then be lined with sheep plastic to form a tank. If you can a safety liner intended for a waterbed you get pretty corners.

The main bladder for a waterbed will also work but bladders can be tricky. Sealed you avoid insects and contamination. Flexible they don't form vacuum. But cleaning and inspecting can be problematic.

Sheet polyethylene will work for simple open-frame tanks but exposure to sunlight degrades it. The black last longer in sunlight but all of it is pretty long lived if kept in the shade. It comes in various thicknesses. Thinner is cheaper but more prone to puncture.

Even simpler, but more labor, is to simply dig a hole and line it to create a pond. Most home centers will sell you a 'pond liner' which is heavy, typically black, PVC.

I would advise that if you try plastic lined tanks start with cheap Visqueen, sheet plastic, to assemble the project and get it all up and running. If you are still pleased with the design after a few months you can reline it with the more expensive liner materials.

The "salt" was actually some kind of potassium that's supposedly safer than the salt. I tossed it.

Can you post a picture of this? I'm having difficulting figuring out what this device looks like and if it would work with my rainwater harvesting system.
Thanks,
-Blast

Here's a really bad drawing:


In written out terms, starting from in the tank.

At the start of the system is a foot valve (keeps the water in the pipe) that's connected to 1" plastic piping with up and out of the tank and down to the pump, which is at ground level. I didn't want to pop a hole into the tank, so the up and over method was used.

The pump has 1" piping, at the discharge, I put in a check valve so that the water can only flow out of the pump. With a small nipple fitting, I then attached a 1" "T" fitting. The left side of the T fitting was fitted with a reducing bushing to get me to a 1/4" NPT female threads into which I placed one of these.

From that barb fitting, I ran 1/4" tubing back up and into the tank.

The other side of the "T" fitting connected to the existing 4 outlet hose manifold I had built some time ago. It's a simple array of hose bibs along a piece of copper tubing that I had found in the basement. Each is sweat-fitted.

When I run the pump, a small amount of water flows up the 1/4" tubing and back into the tank. This has almost no effect on the overall flow through the system, and actually serves as a "bypass mode" for the pump in the event that all of the hose bibs are closed, so it can actually extend the system life.

When I shut down the pump, the check valve on the pump closes, holding prime in the pump, and then air is drawn back into the 1/4" tubing, breaking the siphon quickly. Obviously, the end of the 1/4" tubing needs to be held above the water line at all times, otherwise it will just be a second path for the water siphon.

Total cost in materials was under $50 (not counting the pump, which cost about $300 - it's a commercial grade, continuous duty device.

With the system I have in place, I have 350 gallons of rainwater that is collected, it's amazing how little rainfall you need to fill that barrel.

I also have will over 250,000 gallons of water in my pond, which would not require ANY storage tanks, siphon breakers or much else, but I prefer to not use that for garden irrigation due to the preponderance of tiny squiggly wiggly things in that water and I'd rather not spray them onto my Strawberries and Lettuce!

Thanks, I get it now. Nice and simple. My rainwater system is all gravity-fed, no pumps are used. Currently it consists of six 55-gal plastic drums (four plumbed together and two independent) with soaker hoses running through the garden beds. I have to manually turn on and off the water flow. I've forgot on occasion and drained all my water.
I was hoping the thing you and Art were describing would prevent that but I guess not.

-Blast

Blast, this is rough drawing of what I'm talking about.

The loop rises above the maximum water level and it alone, where it not for the unintended creation of a siphon, would stop all flow away from the tank through the pump. The addition of a vacuum-break, a tiny hole or valve they opens when the pressure goes negative, prevents the creation of the siphon. The loop and vacuum break work together to prevent all flow once the pump is turned off.

If you go with a small hole it only needs to be big enough to allow in air and to not clog. A 1/16", or even smaller, will usually work as long as the water is not too chunky.

Some people advocate inserting a thin copper wire into the vent hole. This is bent to keep it from being drawn in or blown out and extends a bit into the water flow so it wiggles and vibrates when the pump is pushing water past. The vibrating wire tends to keep the hole from clogging up from fine particulates or calcium.

Alas, that system requires a pump, which I don't have.
Turns out I left one of the spigots open on my system last night and ended up draining the four hooked-up barrels. D'oh!!

-Blast

Blast - sorry

Sounds like what you need is a watering timer. These are small self-contained, battery operated, solenoid valves that are activated by a digital timer. These can often be set up to activate for a established watering time on certain days of the week. I've never had one but a neighbor ran one for years.

It was a smallish unit that screwed onto the hose-bib that took a nine volt battery to operate.

There is also a simple mechanical one that allows you to manually turn the water on and set the valve to close in some number of minutes. The one I saw allowed a run time of up to four hours. This is essentially an egg timer that works a valve to turn off the valve when it goes off so you don't have to remember , or bother, to turn off the water.

Most hardware stores have them and they run $15 for the mechanical unit and $40 and up for the electronic one.

Ok guys, here's the system that I ended up building. Works great. I did away with the return tube.

Overview:


Detail:

Nice setup, Martin. Glad it's working well.

Though I wonder if such a perfectly utilitarian setup is really appropriate for Blast, our resident mad scientist. His system should have an infusion of Rube Goldberg elegance. Consider the materials at hand: water, a garden in containers, and a cat; wild plants, paracord, and things that go boom; a house full of survival gear and a tolerant DW. Surely we can add some thoughtful suggestions as to the design?