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Building Principles: Water, Water Everywhere . . .
Water from the tap or showerhead is something many of us take for granted. Until, of course, it’s not there on demand. Because of their inherent remoteness, camps and conference centers are particularly vulnerable to the problems caused by a water outage. Most camp facilities’ staff have a basic understanding of their particular system, but may not be especially well versed on how small changes in the system can have far reaching effects. Similarly, many executives and owners are asked to make decisions about their systems without really understanding how the choices can affect operations in the long run.
Let’s look at several of the basic design and operating parameters for small water systems including estimated daily demand, the sources, and types of supplies and methods for supplying pressure.
How much do we need?
Most states have adopted conservative design figures between forty and eighty gallons per day per person at resident camps for estimating both the required fresh water supply volume as well as the volume of wastewater expected to require treatment. In most areas, if your organization can provide historical water usage, those “real” figures can be used when designing new components of either system. For those reasons, we strongly recommend that all water systems be metered and monitored. Without site-specific data, we normally assume conservatively that each person will consume eighty gallons per day. This usage includes bathing, food preparation, and regular consumption. For example, if your camp capacity is 300, we would estimate your average daily demand at 300 people X 80 gallons per person = 24,000 gallons per day.
For the camp that is being approached (or in some cases, has been directed) to connect to a public supply network, there are often concerns about whether the amount charged is commensurate with the volume actually used. In some cases, the camp has negotiated a flat fee for which it receives water for its operating season.
In light of recent drought conditions in many regions, this type of arrangement has become increasingly rare. For the organizations that still operate this way, we suggest that the days are numbered. At the very least, the camp should expect to see fairly large increases in the fee. This is the method by which the seller of the water will encourage conservation measures. Most users of public water are metered, with the customer paying a regular fee per thousand gallons of water used.
Operating a “nontransient, community water supply” has certain monitoring requirements, which are imposed by state health agencies and enforced/monitored by local ones. This normally involves very basic periodic sampling for indicator organisms such as coliforms. Systems which operate under the influence of surface waters (as described above) often have additional testing requirements to ascertain whether other contaminants have entered the well.
Today, most camps’ water supplies are underground in the form of wells. The depth and construction of these sources varies by subsurface geology, but in general, the operating principles are the same. A hole is either drilled or dug through the surface layers of earth until a water bearing layer (or strata) is encountered. The amount of water that enters the hole will be dependent on the size of the hole, the physical size of the “underground pond” (or aquifer), and the characteristics of the material through which the water flows.
Water In = Water Out
It’s important to note that most regulating agencies will not permit a well to be operated continuously. For calculations and permitting, the well is typically assumed to be operating only 12 hours a day and recovering for 12 hours. Keep this in mind when considering whether a certain well has sufficient capacity to supply your organization. Using our example earlier, our average daily demand is 24,000 gallons. Your well(s) should be capable of supplying that amount in 12 hours or 24,000 gallons/12 hours = 2,000 gallons per hour. Dividing that by 60 (2,000 gallons per hour/60 minutes per hour = 33.3 gallons per minute. Therefore, your well would need a specific capacity of about 34 gallons per minute to meet the estimated demand.
As a final point on wells, if your records do not indicate the capacity of your wells, or if the tests are more than ten years old, you may want to have the tests run. Aquifers and their behavior change over time as do the characteristics of the well. In bedrock wells, the cracks which supply the water can fill with sediment, reducing the water available to the well pump. In screened wells (found in gravelly or sandy soils), the well screens which hold the hole open can become clogged with biomass, silt or corrosion, also reducing the net capacity of the well. Finally, development of the properties surrounding camp can be tapping into the same aquifer as the one from which your well draws. This testing also provides a great opportunity to inspect your well pump for wear of its moving parts as well as the electrical system.
More than just a great classic Rock hit, this is the property user’s notice first about your water system. If it takes several minutes to fill a bucket of water or a washing machine, the system operator is likely to hear about it. There are only a few methods to supply pressure to a system.
The first and simplest method of supplying pressure is gravity. The well pump moves the water from underground to a storage tank whose height has been set to deliver a regular, reliable system pressure. For each 1 pound of pressure required, the water needs to be raised about 0.4’. Put another way, raising the water 1’ increases the pressure 2.31 pounds per square inch (psi). Average system pressure range at a faucet or showerhead should be between 30 and 50 psi.
This means that the lowest water level above the highest faucet has to be at least 30 * 0.43 = 12.9¢ above the highest faucet.
The upper end of the operating range is calculated the same way: 50 * 0.43 = 21.5¢ above the highest faucet.
Another way of looking at this is that the fluctuating height of the water in the tank (“operating range”) is about 21.5¢ – 12.9¢ = 8.6 feet.
A pressure sensitive switch turns the well pump on when the pressure falls below the 30 psi and turns the pump off when the pressure reaches 50 psi.
You can see, then, that by combining sufficient storage (about one day’s supply) with a tank at the right elevation, the system can operate without electricity or the pump for a period of time with no interruption in service.
Looking back at our example, we could estimate a cylindrical tank size as follows:
There is a second way that pressure can be supplied to the network, and it uses a principle with which we’re all familiar: the aerosol can. This system is referred to as “hydro-pneumatic.” Again, a pump moves water into a system connected to a closed tank that contains air at a certain pressure. A pressure switch again turns the pump on and off as the pressure reaches certain preset levels. This system has the advantage of replacing physical height with air pressure, so giant columns of water aren’t required. However, the main disadvantage is that air, being highly compressible, takes up the majority of the space in the tanks. In fact, the effective storage of most hydro tanks is about 18 percent of the total volume.
Using our same 24,000 gallon example, and assuming that we get the 18 percent storage, we would calculate that the required hydropneumatic storage tank would have a total volume of 24,000 / 0.18 = 133,333 gallons! You can see that storage of a day’s supply in hydro tanks is not very common, so most often, the tanks are very undersized for the application. The frequent off/on cycles of the well pump to repressurize the system significantly shortens its service life.
The third way to supply pressure to a network is through a pump which runs constantly, often referred to as a jockey pump. Most often, this pressurizing method is for extraordinary circumstances such as accommodating fire demand at a hydrant. While it is possible to supply normal operating pressure for a small system, its absolute dependence on electricity makes it a fairly unattractive option.
The simplest water supply and storage systems can be extraordinarily complex. But when compared to the potential liability and inconvenience of a contaminated system or interrupted service, the investment in doing it right the first time is really very small. In our next installment, we will discuss some of the other aspects of water systems including water and system disinfection, taste and odor, network configurations, and the effect of friction in water systems.
Rick Stryker is a professional engineer with Camp Facilities Consulting providing study, design, permitting, and construction consultation services to the camp and conference center community. Camp personnel may contact him at 570-296-2765 or by e-mail at email@example.com.
Originally published in the 2002 September/October issue of Camping Magazine.