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13.34 CHAPTER THIRTEEN

membrane performance have not been determined. The following variables are required
to predict system performance:
• Design feedwater composition (including temperature)
• Process train capacity and desired recovery
• System array, number of elements per vessel, and membrane module type
• Fouling characteristics of feedwater
• Membrane age (number of operating years)

Other Design Considerations. Depending on the type of membrane process and the spe-
cific application, the following additional items need to be considered.
Energy Recovery. RO and NF systems commonly include feedwater pumps with ad-
justable-frequency (variable-frequency) drives to minimize energy usage. For many ap-
plications, it is cost-effective to include energy recovery devices in the design, especially
when the waste concentrate pressure is high and the system recovery is relatively low.
Such devices are often included when the potential for energy savings using a particular
energy recovery device exceeds associated capital and maintenance costs for the device.
The selection of a specific type of energy recovery depends on the flow rates and vari-
ability, recovery available concentrate pressure, costs, and other factors. Some commonly
used energy recovery devices are
• Impulse turbine
• Integrated turbopump
• Turbocharger
• Pressure exchanger
Automatic Flushing Systems. The membrane system design should include a means
of automatically flushing membrane modules after planned or unplanned shutdown of the
membrane train. The flushing system is used to remove from the modules concentrate that
would otherwise remain if flushing were not performed. Flushing is particularly impor-
tant when concentrate contains sparingly soluble constituents in excess of their theoreti-
cal solubility (supersaturated) that may precipitate and scale the modules. A scale inhib-
itor may be used to prevent this under normal operating conditions. However, in many
cases the inhibitor will not permanently prevent scaling if concentrate remains in the mod-
ules for long periods. Flushing displaces the sparingly soluble constituents to eliminate
this scaling potential.
Permeate is often used for flushing because it is generally the best-quality water (low-
est in foulants and scalants) available on-site, although in many cases membrane feedwa-
ter is acceptable or possibly even more desirable. For example, many systems treating
groundwaters with hydrogen sulfide have been designed with feedwater flushing because
of the potential for membrane fouling when flushed with permeate containing colloidal sul-
fur formed while in storage. If permeate is used, design should include provisions for stor-
ing a sufficient amount of permeate to flush the system and to displace the concentrated
water. If chlorine-intolerant composite membranes are used, only unchlorinated permeate
should be stored and used for flushing. If cellulosic membrane modules are used, low-chlo-
rine permeate is often preferred because of its ability to inhibit bacterial action during the
downtime period. If multiple membrane trains are provided, a portion of total permeate
flow can be diverted to flush an individual train and limit the amount of storage needed.
Permeate Drawback Tank. Hollow-fiber seawater RO systems commonly have the
permeate drawback tank piped such that, after feed pump shutdown, an immediate back-