Rainwater Harvesting

Since most of our rainfall occurs in large storm events, the ability to store collected rainwater is paramount.  Storage ranges from small barrels of 50 gallons that can be placed at the end of storm gutters up to large tanks that can store 100,000 gallons.  All of the components for rainwater harvesting can be found in the plumbing section of area retailers.  Small rainbarrels can usually be found in garden stores.

A typical system replaces metal downspouts with plastic piping.  By making the downspouts watertight, water can be carried by gravity to a storage tank.  Several downspouts can be joined together into one larger main pipe leading to the tank.  This main pipe is usually buried below ground and breaks the surface again at the side of the tank.

An inlet to the tank is installed as high as possible to maximize storage capacity.  The inlet can be on the side or the top of the tank.  Just remember, the piping system (downspouts) at the building must be at least 6 inches (preferable 18 inches) above the highest piping at the tank.  This will allow the tank to fill, as the pressure of the water will work like a “P” trap under a sink.  The water will equalize and flow into the tank.  This “P” trap part of the system also must have an outlet installed to allow water to drain out for maintenance and to prevent freezing.

Keeping Rainwater Clean – Leaves, twigs, dust and bird droppings can be screened out of your system in several ways.  The simplest method is a first-flush filter.  The “first flush” of rainwater is sacrificed to make the rest of the water cleaner.  Each downspout can direct the water to a plugged pipe that looks much like the typical downspout, except it has a sewer clean-out plug at the bottom.

When the plug is in, the water fills the downspout until it reaches the top.  From here, the water is directed down to the collection pipe, which goes into the tank.  Each of these first-flush filters should be drained of debris after each rain to be ready for the next rain event.  If your tank is full, just open the sewer clean-out plug and the first-flush filter works like a normal downspout.

Other devices can be used to filter the water going into a tank.  Some are large, fiberglass boxes with 30-micron filters inside to screen out the particles.  Since these filters require cleaning and maintenance, using the first-flush filter ahead of them increases the time between cleaning or replacement.

Practical Use – A gravity system uses the water from the tank via water hoses, buckets, and soaker hoses.  It has very little natural pressure.  Even a drip-irrigation system needs at least 15 pounds per square inch.  You may eventually want to pressurize your system using the same components you see used on water wells: pump, pressure tank, and fine mesh filter.  This creates a more usable system, but adds to the overall cost and maintenance.

Rainwater for Drinking – A drinking water quality rainwater system costs about as much as the typical well.  With 20,000 to 30,000 gallons of storage, a family of four can be comfortable without a well or piped water service.  But families using rainwater as their sole source of potable water seldom have lush landscapes.

Much of the rain in Austin comes in downpours and your tank may be full and not able to capture all of a rainfall.  The time between rains can be long, so rainwater harvesting goes hand-in-hand with water conservation.

To make rainwater safe to drink, additional filtration is needed, plus perhaps an ozone system, reverse osmosis and/or ultraviolet irradiation.  The health of your family and guests depends on the design and safe operation of your drinking water system.

Making Rainwater Cost Effective

Despite the many good qualities that rainwater systems have, they are quite expensive for small installations.  While a homeowner might justify cisterns in the country where they compete cost wise with well drilling, they almost never compete with municipal water supplies.  The one exception I have found is in, of all places, Boerne Texas, a small town about 30 miles north of San Antonio.

Boerne Champion High School built its unique system in 2006.²⁴  It has a traditional metal cistern to catch run-off from the roof, but it also catches run-off from the 120-acre school site and stores it in underground pipes.  The water is aerated to keep it from turning septic, and is primarily used for landscape irrigation.  Add to this condensate from the school’s air conditioning system and natural spring infiltration, and the system can capture about 6 million gallons in an average year to meet the school’s need for landscaping.

Savings will pay for the $265,000 cost in about 6 years, even after considering operation and maintenance, making it cost effective compared to the cost of new water sources in some Texas cities.

Granted that this strategy will only work on large school or office complexes, it still points the way to a viable alternative that can probably work at dozens of sites in Central Texas.

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