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MEMBRANE PROCESSES 13.25

Temperature Control. Water temperature significantly affects membrane systems in sev-
eral ways:
• It alters membrane material characteristics and membrane life.
• Water viscosity and density affect membrane hydraulic performance and required mem-
brane area.
• It changes solubility of sparingly soluble salts and silica, which limits the design re-
covery of membrane desalting processes.
Membrane systems must operate within the manufacturer's temperature guidelines to
maximize membrane life. The various types of cellulose acetate membranes deteriorate
from hydrolysis at increasing rates as temperatures rise. Flux decline caused by com-
paction of RO and NF membranes also is greater at higher temperatures.
The maximum temperature limit for the various brackish water cellulose acetate RO
and NF membranes ranges from 35 ° to 40 ° C, depending on the membrane. The various
types of polyamide RO membranes typically are rated at maximum temperatures ranging
from 40 ° to 50 ° C, but their useful life can be significantly reduced at these elevated
temperatures.
ED/EDR membranes commonly can operate at temperatures up to 45 ° C. Typically,
PVDF, polysulfone, polypropylene, and several other types of MF and UF membranes
tolerate a temperature up to at least 40 ° C. Inorganic (ceramic) MF and UF membranes
can tolerate temperatures well over 100 ° C.
Viscosity and density of water increase at colder temperatures, and for pressure-driven
systems greater membrane area is required to produce a specified product flow at a given
feed pressure; or for a given system and feed pressure, the production rate declines at
lower temperatures.
Temperature affects membrane system performance not only directly by influencing
product water flow rates or salt removal, but also indirectly by affecting solubility of com-
pounds, which can precipitate and foul the system. Some compounds, such as calcium
carbonate, have a greater tendency to scale at higher temperatures. However, most com-
pounds normally encountered in natural waters have improved solubilities as the temper-
ature rises.

MEMBRANE UNIT DESIGN

Once treatment objectives have been identified, design criteria can be developed for the
membrane process. The design considerations are grouped for

• RO and NF units
• UF and MF units
• ED and EDR units

Reverse Osmosis and Nanofiltration Unit Design

Because of the high solute rejection of RO and NF membranes, which concentrate inor-
ganic ions among other constituents, there are many special considerations in the treat-
ment unit design.