Page 439 -

13.36 CHAPTER THIRTEEN

coveries exceeding 98% can be attained with backwash recycling. The maximum recov-
ery can be identified by pilot testing.
Solute and Particle Rejection. In MF and UF systems, solute or particle rejection is pri-
marily a function of membrane type and pore size rating. However, previously rejected
substances remaining on the membrane surface also can act as a "dynamic membrane"
and affect solute removal, especially for UF systems. Typical MF membrane pore size
ratings range from 0.1 to 0.5/xm. Commonly used UF membranes range from about 2,000-
to 150,000-dalton molecular-weight-cutoff (MWCO).
Flux. The permeate or filtrate flux through UF and MF membranes depends signifi-
cantly on transmembrane pressure and water temperature. Design flux rate used for a sys-
tem is also strongly defined by membrane feedwater quality because the rate of plugging
and frequency of backwashing and cleaning are affected by the operating flux rate. De-
sign flux rate for a specific application is often determined by bench or pilot testing. Typ-
ical flux rates for UF and MF membranes vary widely, depending on membrane product
and specific application, and range from about 20 to 100 gpd/ft 2 (0.034 to 0.170 m/h).
Temperature. Temperature affects the required driving pressure for a UF or MF system
because of changes in feedwater viscosity. Figure 13.22 shows the viscosity of water as
a function of temperature. At 20 ° C, the absolute viscosity is approximately 1.00 cP. Be-
cause the filtrate/permeate flux through clean membrane is inversely proportional to vis-
cosity, about 50% more TMP is needed to maintain a constant flux at 5 ° C than at 20 °
C. Assuming a constant TMP and membrane area, the product flow at 5 ° C would be ap-
proximately 35% less than at 20 ° C.
For UF and MF systems, an approximation of permeate flow at any temperature rel-
ative to flow at 20 ° C is as follows:
Qp(20 ° C)
QpT- e_0.0239(T_20 )

where
apT = permeate flow at temperature T
Qp(20 ° c) = permeate flow at 20 ° C
T = water temperature, °C
e = 2.71828
In the design process, it is critical to define the desired production rate at a specific
water temperature. In many cases, less production is needed in the colder winter months
because of decreased water demands from the utility customers. The MF or UF system
could then be designed to produce the rated capacity at a warmer temperature, thus re-
ducing the required membrane area and saving capital costs.

.~ 2.0
~'~ 1.5
~..
1.o
N~ o.5
< 0.0
0 10 20 30 40 50 FIGURE 13.22 Absolute vis-
cosity of water versus tempera-
Temperature (°C)
ture.