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MEMBRANE PROCESSES 13.35

flow of permeate water is available to flow back through the membranes by means of the
natural osmotic pressure differential across the membranes. The water prevents potential
damage from this "osmotic drawback."

Ultrafiltration and Microfiltration
UF and MF membrane water treatment system design should involve communication with
manufacturers and others familiar with the products. Many systems have proprietary de-
sign features that the designer should know. MF and UF systems are relatively new to the
municipal water treatment marketplace, and new products continue to become available.
Facility design requirements can vary greatly between the commercially available MF and
UF systems. The following design parameters, however, are applicable to most, if not all,
membrane filtration systems.

Design Equations. Based on Darcy's law, the flux through a membrane can be deter-
mined as follows:
j ~ Ptm
lz(Rm q- Rt)
where J = filtrate or permeate flux (flow rate/membrane area)
Ptm ----- pressure across the membrane (transmembrane pressure TMP)
= absolute viscosity
R m = clean membrane resistance (reciprocal of length)
Rt = total resistance from foulants (reciprocal of length)
The flux is directly proportional to the transmembrane pressure (TMP). For a given
membrane area, the filtrate or permeate flow rate is also directly proportional to the TMP.
The flux is inversely proportional to the water viscosity. Therefore, as the water temper-
ature decreases, MF and UF systems must increase the TMP to maintain production (see
Temperature below).
The clean membrane resistance varies from one membrane type to another. The re-
sistance from foulants can be from materials deposited on the membrane surface (some-
times called "cake" resistance), concentration polarization, blocking of the pores, and ad-
sorbing to the membrane. The "cake" can improve the removal rates of the system until
it is removed by backwashing and re-formed in the next cycle.

Configuration. There are two basic types of UF and MF processes used in water treat-
ment: pressure-type systems where the membranes are housed in pressure vessels and
vacuum-type systems where the membranes are submerged or immersed in nonpressur-
ized tanks. There are significant differences in the facilities to be designed for these two
configurations; therefore, it is common to select the configuration (and even the specific
membrane product) before detailed design is started on the facility. The specific mem-
brane product is often determined from a competitive proposal process considering ini-
tial and life-cycle costs based on design criteria determined during pilot testing.

Recovery. The recovery of a UF or MF system, typically called feedwater recovery, is
the system's final product volume over a given time period divided by the feedwater flow
volume. The quantity of source water or permeate/filtrate used for backwashing and flush-
ing is considered in calculating feedwater recovery. Typical recoveries for UF and MF
systems range from 85% to greater than 95%. Overall recovery of a system can some-
times be improved by recycling backwash water after solids removal. In some cases, re-