It is easy to overlook the fact that the Earth contains the same amount of water now as when dinosaurs roamed and Homo sapiens were just beginning to make their presence known. However, of all the existing water, 99.7 percent is locked in oceans, ice, and the atmosphere. Even much of the remaining 0.3 percent, comprising groundwater, lakes, and rivers, is too deep to easily access or increasingly becoming polluted from poor sanitation.
Due to increasing demand from worldwide population growth and the decreasing supply of accessible water sources, the outlook for potable fresh water availability looks grim. Clearly, extending the supply of potable fresh water is one of the most critical challenges facing plumbing engineers today. However, many traditional methods of obtaining fresh water are becoming increasingly impracticable. Wells are limited by the water supply depth and substrata composition.
Evaporation farms require large amounts of space and base infrastructure construction—space that is often not available and infrastructure that is too costly to build. Reverse osmosis systems, while very successful for creating potable water by desalinating seawater, require high energy amounts and maintenance. Also, reverse osmosis systems cannot be used with polluted water without seriously fouling critical components.
Furthermore, in less developed countries and even in remote locations in the United States, such systems’ construction and maintenance costs are not affordable. Yet there is one cost-effective and energy efficient option that reinvents a thousand year-old methodology: the harvesting of natural rainfall. Rainwater catchment is widely applicable, environmentally friendly, and renewable. It encourages water conservation, as individual users are responsible for operation and maintenance. Plus, the concept requires only three simple components: a surface where rain can be captured and collected, a storage device, or cistern, to store the water, and piping for moving the collected water. The key challenge for engineers designing rainwater catchment systems is matching water demand with the amount of stored rain.
Demand: How Much Water Does a Person Need?
The average US citizen uses an estimated 100 gallons (378 liters) per day for drinking, bathing, waste removal (i.e. toilets), and washing clothes. By comparison, United Kingdom citizens use 87 gallons (329 liters) per day; Asians use 22 gallons (83 liters) per day; and Africans use 12 gallons (45 liters) per day.
At a minimum, we need about 2?3 gallons (2.4 liters) of water per day to survive, which represents a vast gulf between water needs and water usage.
How much water is the correct amount to provide?
The engineer’s challenge is to calculate a water demand estimate that integrates a cost/benefit analysis with a facility’s minimum water needs. With a rainwater catchment system, you can exercise some control over water usage.
Rather than depending on standardized tables that imply an unlimited water supply, you can perform your own demand calculations that reflect the use of low-flow fixtures and your specific application’s usage patterns. This begins by understanding the facility’s usage and occupancy.
Using a commercial building as an example, you can eliminate drinking water from the calculation if it comes from a bottled-water provider. The statistical inclusion of ultra-low-flush water closet fixtures, urinals with electronic flush valves that control flush frequency and amount, and kitchen water needs (if applicable) creates a more manageable estimated load than provided by traditional calculation techniques.
Special-purpose facilities such as hotels add some complexity to the calculations due to the inclusion of showers in the water demand load. When you add laundry needs, spas, swimming pools, landscape irrigation, and other recreational options, accurate water demand calculations become more challenging. However, remember that these latter loads can use lower-quality water and can be separated from the facility’s domestic water system. It also is important to remember that water demand must be matched with the corresponding seasonal rain pattern.
It is not crucial to collect water for 100 percent of demand if replacement water from an alternative source (well, water delivery service, utility) is available. The actual safety factor is a judgment call that depends on replacement water availability and the consequences of running out of fresh water. Due to a number of factors, it may not be feasible to provide for 100 percent of demand without water rationing during periods of low rainfall.
Estimating the Rainwater Supply
The next step is to determine the amount of rainwater that potentially will be available to supply a catchment system. Two excellent sources of rainwater data are the National Oceanic and Atmospheric Administration’s National Climatic Data Center (www.ncdc.noaa.gov/oa/ncdc.html) and National Weather Service (www.nws.noaa.gov).
As a rule of thumb, you should analyze at least seven years of monthly rainfall records at a location close to the project site. While average and median rainfall amounts provide some guidance, rainwater availability extremes are most important.
After determining the available rainwater density, you can calculate the water that can be collected by multiplying rainfall density (in inches) by the collection surface area (in square feet).
A 35 percent buffer commonly is added to this amount to account for evaporation and system leakage.
Using this simple equation, you can estimate the expected amount of water supply per month. Since the rain is assumed to be a given, the only variable under your control is the collection surface area. The calculation to size the collection surface is:
Collection surface area (feet 2) = 2.2(conversion factor) × [G ÷ R] where 2.2 = (12 inches/foot) × (1 cubic foot ÷ 7.48 gallons) × 1.35 (leakage/evaporation factor)
G = Water to be harvested (gallons)
R = Precipitation density (inches)
Sizing the Storage Tank
You then must size the storage tank, or cistern. The tank’s size is calculated as:
V = D – G + L where
V = Tank volume (gallons)
D = Water demand out
G = Available rainwater in
L = Leakage of tank
The maximum storage tank size depends on the consequences of the cistern running dry. As previously mentioned, other available water sources are a factor. For example, if the facility is near a utility water loop to which the cistern can be connected, you can use an automatic float valve to maintain a minimum water level. If no nearby water supply is available, trucking in water to supplement a dry cistern may be an option. This water is often the same rainwater you are trying to collect, billed at the local utility rate with freight added. If no alternative sources exist, the tank size must be increased accordingly.
However, if the tank is oversized, the water can become stagnant if not regularly replaced. Balancing these two extremes—running out or causing stagnation—is the art in the engineering.
In some applications, multiple storage tanks enable the use of different quality water. For example, in tropical resort hotels, separate cisterns hold overflow water from swimming pools. This chemically treated water is then available to supplement pool evaporation losses. Reclaimed water from parking lots or other less sanitary areas possibly can be reused as landscape irrigation water by first passing it through an oil separator.
Bob Boulware is president of Design-Aire Engineering.