Membranes - Principles of Chemical Separations with Environmental Applications - page 270

Permeate Permeate

Su spension Sus pension

Permeate Permeate

Backflushing

Figure 9.18 The principle of backflushing [2]. Reproduced with kind permission
of Kluwer Academic Publishers.

and

6 b

γ = ˙ (for rectangular channels) (9.56)

v = feed velocity

d = tube diameter

b = channel height.

Correlations are available in Appendix B to determine k for various configurations and
operating conditions.
Pretreatment is often used to reduce fouling. Methods include heating, pH adjustment,
chlorination, activated-carbon sorption, or chemical precipitation. Other factors such as
membrane pore-size distribution, hydrophilicity/hydrophobicity, or surface charge can
also reduce the effects of fouling. Methods which reduce concentration polarization, such
as using higher axial flow velocities, lower flux membranes, or turbulence promoters, also
help to reduce fouling.
Periodic changes in operation can help to reduce accumulation of foulants on the mem-
brane surface. This can be accomplished with alternate pressurization and depressuriza-
tion, changing the flow direction at a given frequency, or backflushing (Figure 9.18).
Backflushing can remove the fouling layer both at the membrane surface and within
the membrane. The forward filtration time and the duration of the backpulse need to be
optimized since permeate is lost to the feed side during the backpulse. A schematic is
shown in Figure 9.19. See Ref. [17] for additional details.
Cleaning with chemical agents can be used. They need to be compatible with the
membrane to avoid damaging the membrane structure. Some common cleaning agents
are: acids (strong or weak), caustic (NaOH), detergents (alkaline, non-ionic), enzymes,
complexers, and disinfectants.

9.12 Geomembranes

Some membrane applications are focused as barriers as opposed to separators. The clear
film that covers meat products in the supermarket serve as barriers to reduce oxygen