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4.1 Design Period 119
4. The area to be served, or design area, and the allowances to be made for population
density and areal water consumption as well as water supply and wastewater re-
lease from residential, commercial, and industrial districts
5. The rates of rainfall and runoff, or design hydrology, for storm and combined systems

4.1 DESIGN PERIOD
New water and wastewater works are normally made large enough to meet the needs and
wants of growing communities for an economically justifiable number of years in the fu-
ture. Choice of a relevant design period is generally based on:
1. The useful life of component structures and equipment, taking into account obso-
lescence as well as wear and tear
2. The ease or difficulty of enlarging contemplated works, including consideration of
their location
3. The anticipated rate of population growth and water use by the community and its
industries
4. The going rate of interest on bonded indebtedness
5. The performance of contemplated works during their early years when they are
expected to be under minimum load

Design periods often employed in practice are shown in Table 4.1.

Table 4.1 Design Periods for Water and Wastewater Structures
Design Period,
Type of Structure Special Characteristics Years

Water supply
Large dams and conduits Hard and costly to enlarge 25–50
Wells, distribution systems, Easy to extend
and filter plants When growth and interest 20–25
rates are low 1
When growth and interest 10–15
rates are high 1
Pipes more than 12 in. Replacement of smaller pipes 20–25
in diameter is more costly in long run
Laterals and secondary mains Requirements may change Full development
less than 12 in. in diameter fast in limited areas

Sewerage
Laterals and submains less than Requirements may change Full development
15 in. in diameter fast in limited areas
Main sewers, outfalls, and Hard and costly to enlarge 40–50
interceptors
Treatment works When growth and interest 20–25
rates are low 1
When growth and interest 10–15
rates and high 1

Conversion factor: 1 in. 25.4 mm.
1
The dividing line is in the vicinity of 3% per annum.