Page 60 - Water and wastewater engineering

GENERAL WATER SUPPLY DESIGN CONSIDERATIONS 2-3

TABLE 2-1
Design periods for water works
Type of facility Characteristics Design period, y Life expectancy, y
Difficult and expensive
Large dams and pipelines to enlarge 40–60 100
Wells Easy to refurbish/replace 15–25 25
Treatment plants
Fixed facilities Difficult and expensive to 20–25 50
enlarge/replace
Equipment Easy to refurbish/replace 10–15 10–20
Distribution systems
Replacement is expensive
Mains 60 cm and difficult 20–25 60
Laterals and
mains 30 cm Easy to refurbish/replace To full development a 40–50

a
Full development (also called build-out) means that the land area being serviced is completely occupied by houses and/or
commercial and institutional facilities.
Unit Water Use
When the proposed project is in a community with an existing community supply, the community’s
historic records provide the best estimate of water use. Conversion of total demand to per capita
demand (liters per capita per day, Lpcd) allows for the separation of population growth from the
growth in unit consumption. If the proposed project is to improve the water quality, consideration
should be given to the likelihood that unit demand will increase because of the improved water
quality. In the absence of existing data for the client community, nearby communities with similar
demographics are a good alternative source. When the demographics differ in some particular aspect
such as a higher or lower density of commercial facilities or a major industrial component, adjust-
ment in the total demand will be appropriate. Although they were developed for wastewater flow
rates, Tables 2-2 and 2-3 can provide a basis for adjustment of commercial and institutional users.
Likewise, flow rates for recreational facilities may be estimated using Table 2-4 on page 2-6.
Community adoption of the use of one or more flow-reduction devices such as flow-limiting
shower heads and low-flush toilets may have a substantial impact on per capita consumption.
Typical results are shown in Table 2-5 on page 2-7. The implementation of requirements for water
saving devices conserves both water resources and energy. These aspects should be addressed in
strategic planning for community development as well as requirements for new or expanded
facilities.
Gross estimates of unit demand may be made using statewide data. Hutson et al. (2001) have
estimated water use by state and the U. S. Bureau of Census (Census, 2006) maintains a popula-
tion database by state. Great care should be used in making estimates from generalized data. Due
consideration must be given to the following local factors that modify gross estimates:
1. Climate

2. Industrial activity
3. Meterage