o_19f3m1n7k170b14e9u0oqipgu4a.pdf

l/s) at pressures of 120 psi (830 kPa). However, if the
golf course has the ability to store and repump irrigation
water, as is often the case, reclaimed water can be delivered
at atmospheric pressure to a pond at approximately
one-third the instantaneous demand. Where frostsensitive
crops are served, an agricultural customer may
wish to provide freeze protection through the irrigation
system. Accommodating this may increase peak flows
by an order of magnitude. Where customers that have no
history of usage on the potable system are to be served
with reclaimed water, detailed investigations are warranted
to ensure that the service provided would be compatible
with the user needs. These investigations should include
an interview with the system operator as well as an inspection
of the existing facilities.
Figure 3-14 provides a schematic of the multiple reuse
conveyance and distribution systems that may be encountered.
The actual requirements of a system will be
dictated by the final customer base and are discussed
in Chapter 2. The remainder of this section discusses
issues pertinent to all reclaimed water conveyance and
distribution systems.
A concentration or cluster of users results in lower customer
costs for both capital and O&M expenses than a
delivery system to dispersed users. Initially, a primary
skeletal system is generally designed to serve large institutional
users who are clustered and closest to the
treatment plant. A second phase may then expand the
system to more scattered and smaller users, which receive
nonpotable water from the central arteries of the
nonpotable system. Such an approach was successfully
implemented in the City of St. Petersburg, Florida.
The initial customers were institutional (e.g., schools,
golf courses, urban green space, and commercial). However,
the lines were sized to make allowance for future
service to residential customers.
As illustrated in St. Petersburg and elsewhere, once reclaimed
water is made available to large users, a secondary
customer base of smaller users often request
service. To ensure that expansion can occur to the projected
future markets, the initial system design should
model sizing of pipes to satisfy future customers within
any given zone within the service area. At points in the
system, where a future network of connections is anticipated,
such as a neighborhood, turnouts should be installed.
Pump stations and other major facilities involved
in conveyance should be designed to allow for planned
expansion. Space should be provided for additional pumps,
or the capacities of the pumps may be expanded by
changes to impellers and/or motor size. Increasing a pipe
diameter by one size is economically justified since over
half the initial cost of installing a pipeline is for excavation,
backfill, and pavement.
A potable water supply system is designed to provide
round-the-clock, “on-demand” service. Some nonpotable
systems allow for unrestricted use, while others place
limits on the hours when service is available. A decision
on how the system will be operated will significantly affect
system design. Restricted hours for irrigation (i.e.,
only evening hours) may shift peak demand and require
greater pumping capacity than if the water was used over
an entire day or may necessitate a programmed irrigation
cycle to reduce peak demand. The Irvine Ranch Water
District, California, though it is an “on-demand” system,
restricts landscape irrigation to the hours of 9 p.m. to 6
a.m. to limit public exposure. Due to the automatic timing
used in most applications, the peak hour demand
was found to be 6 times the average daily demand and
triple that of the domestic water distribution system (Young
et al., 1987). The San Antonio Water System (Texas)
established a requirement for onsite storage for all users
with a demand greater than 100 acre-feet per year as a
means of managing peak demands. As noted previously,
attributes such as freeze protection may result in similar
increases in peak demands of agricultural systems.
System pressure should be adequate to meet the user’s
needs within the reliability limits specified in a user agreement
or by local ordinance. The Irvine Ranch Water District,
California runs its system at a minimum of 90 psi
(600 kPa). The City of St. Petersburg, Florida currently
operates its system at a minimum pressure of 60 psi
(400 kPa). However, the City of St. Petersburg is recommending
that users install low-pressure irrigation devices,
which operate at 50 psi (340 kPa) as a way of transferring
to a lower pressure system in the future to reduce
operating costs. The City of Orlando, Florida is designing
a regional urban reuse system with a target minimum
pressure in the transmission main of 50 psi (350 KPa) at
peak hour conditions (CDM, 2001).
When significant differences in elevations exist within
the service area, the system should be divided into pressure
zones. Within each zone, a maximum and minimum
delivery pressure is established. Minimum delivery
pressures may be as low as 10 psi (70 kPa) and maximum
delivery pressures may be as high as 150 psi (1,000
kPa), depending on the primary uses of the water.
Several existing guidelines recommend operating the
nonpotable system at pressures lower than the potable
system (i.e., 10 psi, 70 kPa lower) in order to mitigate
any cross-connections. However, experience in the field
indicates that this is difficult to achieve at all times
throughout the distribution system.
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