Page 173 - Water and Wastewater Engineering Design Principles and Practice

4-28 WATER AND WASTEWATER ENGINEERING

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Example 4-5. A very small village has an average day design demand of 190 m /d and a
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maximum day design demand of 380 m /d. They will have a distribution system. Compare the
number of wells and the capacity of each well for a system that includes the wells and one
elevated storage tank and a system that does not have an elevated storage tank.

Solution. To meet regulatory redundancy requirements there must be a minimum of two
wells. Each well must be capable of meeting the maximum day design demand with one
pump out of service.
Some alternative selections are:

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• Two wells, each well rated at 380 m /d. For the average day, one well would pump for
12 hours, that is
⎛ 12 h ⎞ 3 3
⎝ 24 h ⎠ ( 380m /d) 190m /d

• Two wells, each well capable of providing the average day demand in two hours to take
advantage of a higher efficiency pump. For the maximum day, the pump would operate for
four hours. Each well would have a rated capacity of

⎛ 24 h ⎞ 3 3
4 560m /d
⎝ 2 h ⎠ ( 380m /d) ,

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Thus, the provision of an elevated storage system gives a range of pumping capacity from 190 m /d
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to 4,560 m /d. Without the storage tank, the pump capacity is restricted to 190 m /d.

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Example 4-6. A community well system is to provide 11,450 m /d for the average day at its
design life. The minimum demand at the beginning of the design life of the well field is estimated
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to be 3,800 m /d. A hydraulic analysis of three wells operating at a maximum day demand of
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22,900 m /d sustained for a 10-day period will lower the piezometric surface to the bottom of the
confining layer of the artesian aquifer. The distribution system has storage capacity for one day
at the maximum demand.
Recommend a well system (number of wells and pumping rate) for this community.

Solution. The three-well system is not satisfactory for two reasons. First it does not provide
the redundancy requirement of one well out of service at the time of the maximum demand.
Second, it provides no margin of safety to protect the aquifer from overpumping. Even if the
demand fell to the average day demand after the sustained maximum demand, continued pump-
ing would lower the piezometric surface below the aquiclude. More likely, pumping to meet the
average day demand prior to the 10 days of maximum demand would have lowered the piezo-
metric surface sufficiently so the aquifer would be dewatered.
One alternative solution is to provide six wells with a capability of meeting the maximum day
requirement with only three wells operating. This would meet the regulatory requirement to have
one spare well available at the time of the maximum demand. The six wells would have to be