Page 335 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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Problems/Questions 295
The average daily water consumption is 106 gpcd (400 Lpcd) Storage
and the maximum daily consumption is 125% of the daily aver- Top water surface
3
age. Determine the storage volume (in m ) necessary to balance 100 ft
supply and demand in each of the following cases:
(a) If water is supplied into the reservoir at a constant Water supply Pumping station
100 ft
rate over the 24-h period. 80 ft 10 in.
(b) If water is supplied into the reservoir at a constant 36 in.
rate, but only for a period of 12 h from 6 a.m. to 6 p.m.
A B C
8.20 A residential complex consists of 60 duplex villas (two
housing units in each). Each housing unit can accommodate a Figure 8.16 Water Supply System for Problem 8.22
maximum of 10 persons. The expected maximum daily water Conversion factors: 1 in. 25.4 mm; 1 ft 0.3048 m.
consumption is 106 gal/capita (400 L/capita). Water is supplied
from the municipal network at a uniform rate throughout the station at an elevation of 80 ft (24.38 m) at point B, water storage
day and is collected in a ground storage reservoir. Water is then tank at an elevation of 200 ft (60.96 m) at point C, 500 ft (152.4 m)
pumped to an elevated reservoir at a constant rate but only for a of suction transmission line between the reservoir and the pump-
period of 9 hours from 6 a.m. to 3 p.m. Water then flows from ing station, and 10,000 ft (3,048 m) of 10-in. (250-mm) pressure
the elevated reservoir to the various villas by gravity to satisfy transmission line between the pumping station and the water
the following daily demands:
storage tank. The water at the water storage tank site is treated
by UV for disinfection and then discharged to the water distribu-
Period % of Daily Demand tion system.
A proposal has been made to replace the current pumps
Midnight to 3 a.m. 5
with two centrifugal pumps, construct 10,000 ft (3,048 m) of
3 a.m. to 6 a.m. 5
16-in. (400-mm) transmission main to parallel the current 10-in.
6 a.m. to 9 a.m. 25
(250-mm) water main and to provide additional water storage at
9 a.m. to noon 15
an elevation of 200 ft (60.96 m) at the current water storage site.
Noon to 3 p.m. 15
The following conditions are assumed: (a) C factor for all pipes
3 p.m. to 6 p.m. 10
120; (b) fire flow requirements in the city 2,000 gpm (126.2 L/s)
6 p.m. to 9 p.m. 15
for 10 h; (c) average daily demand 3 MGD 131 L/s; (d) max-
9 p.m. to midnight 10
imum daily demand 5 MGD 219 L/s; (e) peak hourly de-
mand 10 MGD 438 L/s; (f) pumping station head losses
In order to balance supply and demand, determine: 500 ft of 16-in. pipe (or 152.4 m of 400 mm pipe) equivalent; and
(a) Required volume of the ground reservoir. (g) the field-measured total dynamic head delivered by the pump
from the raw water supply reservoir to the pump station, then to
(b) Required volume of the elevated reservoir.
the water storage tank 150 ft (45.72 m).
8.21 A small water system in a rural area has a population of
(a) What should the pump rating (gpm or L/min) be for
800, and its daily per capita usage is estimated to be 100 gpcd
each of the two new pumps?
(379 Lpcd). The required fire flow determined by the village
engineer is 500 gpm (32 L/s) for a duration of 2 h. There is no (b) What should the brake horsepower (BHP or BMP)
particular hazard to the water supply works. Determine the re- be for pump selection, and what should the motor
quired water storage to be provided for the village using the horsepower (MHP or MMP) be for motor selection?
Recommended Standards for Water Works, 2007 Edition. Visit (c) Will the two selected pumps be connected in parallel
the website or contact the local health department for the latest or in series?
edition of these Ten-States Standards for water works. 8.23 Using the same given data in Problem 8.22, explain why
8.22 A city is planning to improve its water supply system the pipe line is designed based on the maximum daily water de-
(Fig. 8.16). At present the city has a surface water supply reser- mand, and determine the total storage capacity of a new water
voir at an elevation of 100 ft (30.48 m) at point A, a pumping storage tank.

BIBLIOGRAPHY

DiGiano, F. A., Zhang, W., and Travaglia, A., Calculation of the Fair, G. M., Geyer, J. C., and Okun, D. A., Elements of Water
Mean Residence Times in Distribution Systems from Supply and Wastewater Disposal, John Wiley & Sons,
Tracer Studies and Models, Aqua, Journal of Water Supply New York, 1971.
Research and Technology, 54, 1, 1–14, February 2005.