Cisterns

Cisterns 06/30/2009

                              Beyond the Rain Barrel
                                      By Giles Blunden 2008

As the drought lingers on in North Carolina, there is much discussion about how to more effectively use our water. The reality is that even in a drought year the Piedmont still had 32 inches of rain. In many parts of the country this is would be considered a huge amount. Los Angeles for instance averages about 8” of rain per year and a drought is 4”. By conscientiously watering our gardens with rainwater and storing more of it, we can reduce the impact of our drought to some degree. Last summer it was disheartening to see my flowers and vegetables wilting in the 100 degree heat. Fifteen hundred gallons of stored water provided by a cistern would have made my garden a much happier place. Catching the relatively clean water from roofs is a time-tested tradition and local use of rainwater cisterns has grown steadily over the past few years both in new homes and schools. There is a lot more water available than you might think.

In the North Carolina Piedmont, an inch of rain is a common occurrence; an average thunderstorm will produce as much. One inch of rain can produce 62 gallons of rainwater for each 100 square feet of roof, enough to fill up an average rain barrel. A modest 1,500 square foot roof can fill up 15 rain barrels with that one inch of rain --- that’s almost a thousand gallons of precious water. The next step up from a rain barrel is a cistern.

We don’t have to search far to find local examples of homes using cisterns. Historically in the South, cisterns were buried brick vaults with a hand pump. When I arrived in rural Tennessee in 1960, I lived in a pre-civil war home that had an underground cistern with a hand pump out side and another in the kitchen. Last fall, I visited Ocracoke and noticed the remains of many old brick cisterns. Cisterns went out of fashion when deep wells with electric pumps and municipal water systems became the norm. Droughts and increases in population have begun to stress our municipal systems and we have come to realize that ground water is a limited resource. Once again, we look to the cistern as perhaps the best solution for increasing our personal supply of water.

Cisterns come in all shapes and sizes, manufactured from plastic, metal or concrete. They are installed above and below ground, fully automatic and pressurized or a simple hand pump or gravity system.   The old home place cisterns were put in the ground for good reasons. Gravity is perhaps the best reason; water flows down hill and getting it from the roof to an underground tank is pretty straightforward. The downspout from the roof to the cistern provides a place to install filters and debris ejectors. Temperature is the other great advantage. The average underground temperature below 3 feet is 55 degrees Fahrenheit year round. Storing water at a cool 55 degrees solves the problem of freezing, it also reduces algae, which needs light and warm temperatures to grow.

Pre-cast concrete tanks are ideal for underground installation and readily available in the 1,000 to the 1,500-gallon range. Molded plastic tanks are also an option however the concrete tank has distinct advantages. Empty plastic underground tanks have a tendency to “float” out of the ground when the earth is saturated and concrete tanks are considerably heavier. Instructions for plastic tanks of 1,000 gallons or more recommend the tank maintain 25% capacity to act as a ballast to prevents the tank from popping out of the ground. As a result, with a plastic tank, only 750 gallons of water is available out of 1,000 gallons of stored water. Many plastic tank manufacturers also recommend a mix of sand and fine stone as backfill since the tank is more delicate, an additional expense. Lastly, the polyvinylchloride many plastic tanks are made of poses environmental liabilities during the manufacturing process. I recommend a concrete tank.   Similar to a septic tank, it is mass-produced and takes relatively light equipment to install. A 1,500-gallon tank can be purchased and installed for about $1,800 assuming the site can easily be accessed by truck.

The one disadvantage of an under ground tank is the need for a pump and a hose. For garden irrigation or outdoor water play, a small submersible pump will suffice. These pumps are readily available at your local building supply and are often used as sump pumps in basements or portable pumps on construction sites.   Many pumps are made with a hose connection. Place the pump in the tank and plug it into the nearest outside GFI outlet. There are many ways to improve this system with switches, timers, underground wiring, and pressure tanks; however plugging and unplugging the pump when you need water is the easiest and most trouble free.

Keeping debris out of the tank is important to maintain high water quality and to keep tank cleaning to a minimum. The best filtering medium is sand; it blocks large debris like leaves and catches the finer contaminants from your roof. The sand develops an active micro organic layer that neutralizes the bacteria and many of the pollutants and contaminants.

Besides gardening The North Carolina plumbing code allows cistern water to be used for toilet flushing, clothes washing and generally anything except personal hygiene and food preparation. Only fifty percent of water use in the average household for personal hygiene and food preparation so a lot could be provided for with a cistern. For health and safety reasons there are very strict rules about keeping potable and non-potable water separated and requirements for clear signage warning that the cistern water is not drinkable. Using rainwater in this way is a possibility but it involves replumbing your house at a sizable investment. If you choose this option, to stay safe and within the law, hire a licensed plumber.

Remember in North Carolina, even a drought is 32 inches of rain per year. This much rain on a 1,500 square foot roof produces about 30,000 gallons of water a year; enough to fill a 1,500 gallon cistern roughly twice per month! That’s a lot of tomatoes.

With a lot of help from Mark Moldenhaur and his mini-ex operator Billie my neighbor Randy I just installed one of these wonderful tanks. The installation had one surprise benefit. Once I had all of that red dirt from the hole piled up and spread around it rained for 5 days, just like washing your car only better, or maybe worse, that red dirt is now red mud to shovel back over the tank. I feel that I may have single handedly ended the restricted water use. It rained so much I kept looking out the window worrying that even that heavy concrete tank was going to float up out of that hole, I had no idea how to get it back down if it did, the boom truck was long gone. We diverted some roof rainwater into it with a quickly rigged piping system and it stayed right where it was. After a bit more shoveling I am going to kick back and just wait for those happy tomatoes.

For more information, the Internet is your best resource. One of the most informed sites is Oasis Design in Santa Barbara, CA (www.oasisdesign.net). For a diagram of a simple cistern system and resources for local conditions see www.blundenstudio.com or www.braewater.com. Another good resource for specialized hardware is Stark Environmental at www.starkenvironmental.com.

***
Giles Blunden is an architect and president of Blunden Studio, PA. He is a LEED (Leadership in Energy and Environmental Design) certified professional and has been designing green homes in the Triangle for over 30 years.

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