Water Supply

This chapter describes the water supply for the Florida Solar Cracker House. One of the most basic needs of man is a source of clean water. Although most of us take for granted our water supply, it is more and more commonplace to hear or read about water shortages, depleted aquifers, salt water intrusion, contaminated wells, etc. If you ask most people where their household water comes from, they reply "from the tap, of course". The location of human settlements and villages at one time was primarily determined by the proximity of a water supply, such as a river or lake. These days, water is piped long distances or pumped from deep wells to allow man to settle in "inhospitable" areas, a "triumph" of human technology.

A consequence of this technology mindset is apparent in Florida; each new household that is set up in a rural area pierces the Floridan aquifer with a well for its water supply. This naturally purified water is used to wash cars, flush toilets and water lawns. These numerous wells are direct links to the aquifer, and can be sources of contamination to the aquifer. Another source of naturally purified water is readily available to anyone in Florida with a roof - rainwater. Use of rainwater for household water needs causes far less disturbance to natural systems than does a well. This is because we simply "borrow" the rain that falls on our roof, cycle it through our house, then deposit it back into the earth, usually very close to place it would have fallen in the first place. Except for the contaminants we add to it (which we can try to minimize), the natural water cycle is hardly disturbed. Some of our used water will filter back through the earth to replenish the aquifer, and some will evaporate, and, repurified, fall again on someone else's roof.

This is the motivation for the water supply system developed for our house. It is hardly new. Cisterns were the norm for old homesteads and native peoples inhabiting Florida, and many other areas in this country, prior to the "improvements" brought by technology. This chapter is divided into five sections. Determining need describes how to estimate your household water needs. Cistern capacity describes what to consider in deciding how large a cistern to build or purchase. Cistern structure reviews the types of cisterns commonly used. Filtration and purification describes the kinds of filtration systems and purification methods that are available, and considerations for drinking water. And, finally, Emergency needs discusses requirements for fire protection.

Determining need

The first step in sizing a cistern for your home is estimating the total water consumption per day for your family. As should be true for every consumable in a responsible home, the first consideration is conservation. The average family of four consumes 6,300 gallons of water per month for indoor use. This works out to 53 gallons per person per day. By eliminating water use in toilets (see the chapter on Wastewater system for a discussion of composting toilets), which accounts for up to 45% of household water consumption, and installing low flow showerheads (showers are the single largest users of household water), one can easily cut this number down to 30 gallons per day per person. Recall that we are using a separate cistern for outdoor (garden and irrigation) use. Thus, we will take 60 gallons per day as an estimate of indoor water use for our family of two. Below we show an itemized breakdown of the estimated use.

ActivityRate of Water UseGallons per day
Shower1.5 gallons/minute30
Laundry28 gallons/load12
Sinks6 gallons/sink18

One way to cut down on water use is to use an efficient clothes washing machine, such as the Staber Industries System 2000 washing machine pictured here. It features a top loading, horizontal axis tub, which uses only 21 gallons of water, 25% less detergent and half the electric power (165 Watt-hours) per load.

Cistern capacity

A number of factors enter into the estimation of the cistern capacity required for your needs. The major factors are water consumption (discussed above), rain collection area, average rainfall in your area, longest dry period for your region and availability of other sources of water for emergencies. There is no exact formula to plug into once the factors above are determined, but there are rules of thumb and guidelines. All of the factors above need to be balanced against cost per gallon for the cistern and space restrictions. We will go through the calculations and rationale for the choice of our cistern size, and hope that it will provide a general guide for your system.

Ultimately, the issue of cistern size comes down to how fast you empty it versus how fast you fill it, and how much of a buffer you need. The rate of water loss is easiest to predict, and is just the 60 gallons/day estimated above. Water is produced at a rate of 0.6 gallons per square foot of roof area per inch of rain. Our roof is about 2300 square feet, which comes to 1,380 gallons/inch of rain, or about 1240 gallons/inch of rain considering 10% losses due to gutter overflow, roof washing, etc. The average rainfall in our region is 1 inch per week, so that rain on our roof produces about 3 times the amount of water that we need on average. Now comes the trickier question of how much of a buffer do we need? This depends primarily on the expected length of dry spells and how difficult it is to get water from another source if you run out of water in the cistern. If you have no other source of water, you need to prepare for at least a 100 year occurrence drought. This, of course, will require a much larger and more costly cistern than you need for the average year.

In our case, we have access to a well that is 600 feet from the cistern. In an emergency, we could link garden hoses and run water from the well into the cistern. We also have access to a lake about 200 feet from the house. Thus, we decided to build a cistern capable of carrying us through a 20 year occurrence drought. Monthly precipitation data are available for Tallahassee, FL from 1885 until the present through The Florida Climate Center. Although Tallahassee is somewhat wetter than Interlachen, we took this data as a guide. In reviewing the data, we found on average one or two months a year where the rainfall amount did not keep up with the expected rate of use of water. This deficit occurred in at least two consecutive months 28 times in 110 years, in three consecutive months 5 times in 110 years and in four consecutive months once in 110 years. It was further found that a cistern with a drawdown capacity of 3000 gallons would have run out of water only twice in the last 100 years. Thus we settled on a cistern capacity of 3,000 gallons as being reasonable. This is a small cistern by National (or World) standards. For example, the Ohio Department of Health Private Water System Rules states that "No new cistern of less than 5,000 gallons per dweling shall be installed". However, as in all aspects of design, the particular requirements and needs of the local region should be given the highest consideration.

Cistern structure

What is the most efficient way to build a 3,000 gallon water tank? One of the first ideas that many people think of is a raised tank. once the water is pumped to the tank, the water pressure is provided by the height of the water. Unfortunately, a tank 70 feet high would be required to provide 30 PSI of pressure, a minimal value for domestic water supply.

To be continued . . .

Filtration and purification

Emergency needs