Examples

Design of Small Water Systems

Small water systems, serving less than 10,000 households, supplied about 42 million people in the United States (1980 statistics). Many of these systems are marginally designed and poorly operated and maintained due to insufficient budgets, very low water rates, poorly paid and trained operators, and uninformed management. Such systems are frequently inadequately monitored and fail to meet drinking water standards. Some are too small to provide sufficient revenue to support proper operation, maintenance, and management. Very often, small water systems are the only alternative for small isolated communities and developments. A partial answer, where feasible, is the consolidation of small water systems or connection to a large municipal system. Other alternatives include regional management of several small systems, including professional supervision, administration, and technical and financial assistance. Rural water associations, local water works associations, and regulatory agencies can, and in many areas do, provide training programs, seminars, and speakers to meet some of the needs. Compliance with drinking water standards, operational problems, and maintenance can be discussed. The opportunity to share experiences is provided and made accessible to the small water system operator.

Experiences in new subdivisions show that peak water demands of 6 to 10 times the average daily consumption rate are not unusual. Lawn-sprinkling demand has made necessary sprinkling controls, metering, or the installation of larger distribution and storage facilities and, in some instances, ground storage and booster stations. As previously stated, every effort should be made to serve a subdivision from an existing public water supply. Such supplies can afford to employ competent personnel and are in the business of supplying water, whereas a subdivider is basically in the business of developing land and does not wish to become involved in operating a public utility.

In general, when it is necessary to develop a central water system to serve the average subdivision, consideration should first be given to a drilled well-water supply. Infiltration galleries or special shallow wells may also be practical sources of water if their supply is adequate and protected. Such water systems usually require a minimum of supervision and can be developed to produce a known quantity of water of a satisfactory sanitary quality. Simple chlorination treatment will normally provide the desired factor of safety. Test wells and sampling will indicate the most probable dependable yield and the chemical and bacterial quality of the water. Well logs should be kept in duplicate.

Where a clean, clear lake supply or stream is available, chlorination and slow sand filtration can provide reliable treatment with daily supervision for the small development. The turbidity of the water to be treated should not exceed 30 NTU. Preliminary settling may be indicated in some cases.

Other more elaborate types of treatment plants, such as rapid sand filters, are not recommended for small water systems unless specially trained operating personnel can be assured. Pressure filters have limitations, as explained earlier in this chapter.

The design of small slow sand filter and well-water systems is explained and illustrated earlier in Figures 2.2 and 2.3 and shown in Figure 2.22.

An example (Figure 2.20) will serve to illustrate the design bases previously discussed. The design population at a development consisting of 100 two-bedroom dwellings, at two persons per bedroom, is 400. The average water use at 75 gallons per person or 150 gallons per bedroom is 30,000 gpd for the development. From Figure 2.10, the peak demand can vary from 100 to 320 gpm. An average conservative maximum or peak demand would be 210 gpm. Adjustment should be made for local conditions. This design provides no fire protection.

Examples showing calculations to determine pipe diameters, pumping head, pump capacity, and motor size follow.

In one instance, assume that water is pumped from a lake at an elevation of 658 feet to a slow sand filter and reservoir at an elevation of 922 ft. See Figure 2.20. The pump house is at an elevation of 665 feet and the intake is 125 feet long. The reservoir is 2,000 feet from the pump. All water is automatically

FIGURE 2.20 Water system flow diagram.

chlorinated as it is pumped. The average water consumption is 30,000 gpd. With the reservoir at an elevation of 922 feet, a pressure of at least 15 lb/in.2 is to be provided at the highest fixture. Find the size of the intake and discharge pipes, the total pumping head, the size pump, and motor. The longest known power failure is 14 hours and repairs can be made locally. Assume that the pump capacity is sufficient to pump 30,000 gallons in 10 hours, or 50 gpm. Provide one 50-gpm pump and one 30-gpm standby, both multistage centrifugal pumps, one to operate at any one time and one generator.

From the above, with a flow of 50 gpm, a 2-inch pipeline to the storage tank is indicated.

The head losses, using Tables 2.14 and Table 2.15, are as follows: Intake, 125 ft of 2-in. pipe(3.3 x 1.25) = 4.1ft

TABLE 2.14 Friction Due to Water Flowing in Pipe

Friction Head Loss by Pipe Diameter"__

Capacity (gpm) 1 3/4 1 1-1/4 1-1/2 2 2-1/2 3 4 5 6 8 10 12 14

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