Page 493 - Water and wastewater engineering

12-10 WATER AND WASTEWATER ENGINEERING

FIGURE 12-5
Hollow fiber membranes

The hollow fiber membranes may be configured in one of four modes: (1) outside-in,
(2) outside-in (cross-flow), (3) inside-out (dead-end), or (4) inside-out (cross-flow). For the out-
side-in system the flow pattern is against the outside of fiber and the permeate is in the lumen or
inside the fiber. The inside-out arrangement may be either in the dead-end mode or the cross-flow
mode. The advantages and disadvantages of each mode are summarized in Table 12-4 . Positive
pressure systems use either outside-in or inside-out modes. Vacuum systems use only the outside-
in mode.

Temperature and Pressure Effects
Flux is inversely proportional to the viscosity. Viscosity changes due to changes in water
temperature are an important design consideration in the treatment of surface water where
the water temperature may range from 1 C to 20 C over the course of a year. The influence
of viscosity may be estimated using tables such those found in Appendix A, or the follow-
ing second-order polynomial that is valid over the temperature range 0 C to 35 C (AWWA,
2005):

1 777. 0 052. T 6 25. 10 4 T () 2 (12-11)

where dynamic viscosity of water, mPa · s
T temperature, C
Example 12-1 illustrates the effect of temperature change on permeate flux.