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MEMBRANE PROCESSES 13.29

The degree to which fouling would be lessened, if any, for a given membrane element
type and application, should be determined with pilot testing.
Temperature. Water temperature is a factor that must be considered in all membrane
systems. The important effects of temperature on membrane system design are these:

• Membrane operations using high-temperature waters require lower operating pressures
to achieve a given flux, compared with operation on low-temperature waters.
• Membrane permeate quality degrades as water temperature increases and membrane
flux is held constant (water flow through the membrane is constant and solute flow in-
creases). Minimum temperature should be used to determine maximum anticipated feed
pressure (and thus pump pressure and motor horsepower) at the design flux rate be-
cause the net driving pressure (NDP) is less at a higher temperature.
• If the design flux rate is held constant, maximum temperature dictates the worst per-
meate quality condition because solute passage is greater at higher temperatures.

Assuming all other factors (including membrane area and feed pressure) remain con-
stant, the permeate productivity of a pressure-driven membrane system is about 30% to
40% less at 15 ° C than it is at 30 ° C. For RO and NF systems, an approximation of per-
meate flow at any temperature relative to flow at 25 ° C, assuming all other factors are
constant, is as follows:
Qp -- Qp(25 ° c) x 1.03 (r - 25~
where Qp = permeate flow at temperature T
Qp(25 ° c) = permeate flow at 25 ° C
T = water temperature, °C
Typical membrane system design necessitates evaluation of performance at minimum
and maximum temperatures assuming new (initial start-up) and "used" (often 1, 3, or even
5 years of operation) membranes. A maximum design flux or feed pressure is assumed,
and the minimum temperature is used to determine the required membrane area for "used"
membranes. Given the membrane area, the performance at maximum temperature is then
evaluated to verify that product water quality goals are also met.
All polymeric membranes have maximum operating temperatures. The use of high-
temperature waters may exclude the use of cellulose acetate membrane, and if tempera-
tures are too high, membrane feedwater cooling will be required for all types of mem-
branes.

Feed Pressure Requirement. The required feed pressure depends on the following:
• Membrane type
• Flux
• Recovery
• Osmotic pressure
• Temperature
• Permeate pressure
• Changes over time
All the above points have been discussed previously except changes over time. To en-
sure that a system continues to produce the desired quantity and quality of permeate over