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.
| Activity | Rate of Water Use | Gallons per day |
|---|---|---|
| Shower | 1.5 gallons/minute | 30 |
| Laundry | 28 gallons/load | 12 |
| Sinks | 6 gallons/sink | 18 |
| Total | 60 | |
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
