When we looked at options, underground plastic tanks come in at about $1/gallon, and pre-cast septic tanks come in at around 60 cents a gallon.  When we asked our concrete guy how much it would cost for him to build us a single 5000 gallon tank, his price was similar to the cost of the pre-cast septic tanks, so we decided to go with that option.  As an added twist, we were going to build a patio on top of the tank anyhow, so we decided to put the top of the tank at the surface, and make it the finished patio, saving us the cost of building a patio.

Since the cost of a custom tank is mostly labor, making the tank a little bigger doesn't cost much, so we decided to increase the size of patio we wanted, trying to stay with even increments of the concrete forms (they are 8 feet long- this makes their job a little easier), which resulted in an interior dimension of ten feet by sixteen feet, and an exterior dimension of eleven by seventeen, with the tank being about 6'6" deep (tall enough so the workers could walk around in there to take the forms back out).  This gave us a tank that is more like 7000 gallons, where the exact number is determine by the height of the overflow drain (which is fact about 6" off the top of the tank).

Because concrete is somewhat porous, we decided we needed to seal the inside of the tank to prevent the slow loss of water from the tank.  After reviewing various options, we settled on Aquaseal, a cement based sealant that brushes on the concrete (very big stiff brush!), but needs to go on a rough surface and unfortunately we have fairly smooth walls (so we had to rough them up).

The tank has two 3" holes for input, one 2" hole for overflow and one 1" hole for the pump feed.  There is also a 2'x2' access hatch which is covered with a metal plate which is painted with rustoleum "hammered finish" paint (Its toxic, but it sticks to rusty metal).  To get in the tank you need a 6' ladder.

As of Feb 2006, we are now using our second pump setup, which was installed by Mike Broili of Living Systems Design.  The old system and reasons for change are described below under "Old system".  A schematic of our current system is shown below.  The intake for the pump is a floating screen filter (1) which sits below any floating debris, but well above any that sink to the bottom.  This is fed thru a 1" CPVC pipe to a check valve (2)  (which apparently is to help the pump keep its prime) and a shutoff value (3) that has no obvious purpose  A variable speed motor (4) keeps the output line to whatever pressure is set by the motor controller (10).  The motor is a 240V unit which requires it's own circuit.  The motor controller has an external pressure sensor in the output line (indicated by the thin black line).  A small pressure relief tank (6) helps keep the system pressure stable when the pump starts up and shuts off.  Connected to that same line is a valve & treaded input (11) that serves as a way to prime the pump.  The pump output has two other tee connections: the first goes directly to the irrigation controller (for more on this see landscaping), and the other goes to the filter (5), that removes most of coarse suspended solids so that the water in the toilets is mostly clear (the filter potentially keep the toilet valves from collecting crud--I don't really know if this is an issue of not).

For toilet water, we can either use our rainwater or we can switch them to use city water.   Making this easy was the stumbling block with the county health department, and the original reason we couldn't get a permit.  Finally after some degree of negotiation, the health department gave us a permit to use a reduced-pressure backflow prevention device to keep the rainwater from mixing (and contaminating) the city water.  However unlikely this event seems, apparently it happens somewhere in the US multiple times a year, so cities work really hard to prevent it.  The tradeoff for having the convenience of the backflow protector is that it has to be inspected every year.  In the diagram, city water is fed thru a shutoff valve and the backflow protector, where it is then joined with the pressurize rainwater.  One of the shutoff valves (7 or 8) is always off, so that the toilets only have one feed.  If by accident they were both open, the backflow device ensures that the rainwater can't flow back into the city water.  The shutoff for the rainwater side (7) is actually duplicated because the toilet connection is in a different room than the pump.  I've used purple to denote the pressurized rainwater pipe, because these pipes have to be actually be purple by code, so that everyone knows they contain non-potable water.

Actual rainwater pump equipment.  The black pipes are ABS drain pipe from the kitchen, while the small vertical white pipe is a floor drain prime line.  These are not part of the system.

Mike Broili of Living Systems Design (right), standing inside our rainwater tank, holding up the floating intake. The bright spots on the ceiling are water drops of condensation.  The old float value (gray object on floor) is no longer used.  The city water backup connection (above) shows the backflow protector and the separate toilet manifold.

Below are pictures of tank construction. For pictures of the completed tank, see the tour here and here.

We excavated out the side yard and set the bottom of the tank so that the top would end up at the finished grade level (left). Adding color to the top (center) then stamping (right).  The dark gray is mostly washed away.

Old System

The pump burned out in Aug 2005 and although we're not exactly sure why, we decided to try to fix all the possible reasons why.  First, we changed the tank inlet so that rather than splashing into the tank, the water diffuses into the tank.  Second, we changed the pump intake to a floating one because the cleanest water is about 6 inches or so below the surface.  We replaced the pump/pressure tank with a pump that is made to cycle on and off, since that's what our previous pump did, and apparently they don't like that.  The old intake also had a loop in it where the water went up out of the tank and then back down, which we already found our makes the pump hard to prime, but may also have caused it to cavitate (develop air bubbles), so the new pump is installed higher on the wall.

The following diagram shows how the system is set up. The tank feed line is held up off the floor by an inch or so, and is caped with a screen filter (1).  The water then enters the basement, where there is a "Y" type strainer (2), and then a shallow well pump (4).  Connected to this feed line, there is a "tee" to a stub which has ball values and a female hose thread on the end (3).  To prime the pump, you connect a garden hose to this stub, open the valve, turn on the water and then start the pump.

The output of the pump is then fed to a pressure tank (6) which keeps the pump from running every time water is used.  The pressurized water is fed and also to a drip irrigation manifold and to a manifold that all the toilets are hooked up to.  On the toilet line we have a particulate (clarity) filter (5) to try to make the water as clear as possible (but still not drinkable).  While this isn't necessary, most people expect that when the flush the toilet, the water will be clear.  As a backup for when the tank is dry (or the system is broken), we can switch our toilets to city water by changing which of the two valves (7,8) is on.  As further protection against contamination of city water by rainwater, we have a reduced pressure backflow protection setup (9).  The float switch (11) and motor check (10) prevent the motor from running if there is no water or it loses its prime.

The photo (below) is the actual rainwater system. Some of the rainwater equipment (most extraneous plumbing etc has been edited out for clarity).  The inset is of the backflow protector and toilet manifold system (which is 15' away from the pump).