The MBR Book: Principles and Applications of Membrane

34 The MBR Book
Feed
Convective flow
Solute concentration
Back diffusion
Concentration
boundary layer
Gel
layer
Membrane
Figure 2.12 Concentration polarisation
Permeate
membrane surface itself the liquid velocity must be zero. This implies that the only
mode of transport within this layer is diffusion, which is around two orders of magnitude
slower than convective transport in the bulk liquid region. However, it has
been demonstrated (Romero and Davis, 1991) that transport away from the membrane
surface is much greater than that governed by Brownian diffusion and is actually
determined by the amount of shear imparted at the boundary layer; such
transport is referred to as “shear-induced diffusion”.
Rejected materials nonetheless build up in the region adjacent to membrane,
increasing their concentration over the bulk value, at a rate which increases exponentially
with increasing flux. The thickness of the boundary layer, on the other hand, is
determined entirely by the system hydrodynamics, decreasing in thickness when turbulence
is promoted. For crossflow processes, the greater the flux, the greater the buildup
of solute at the interface; the greater the solute build-up, the higher the
concentration gradient; the steeper the concentration gradient, the faster the diffusion.
Under normal steady-state operating conditions, there is a balance between those
forces transporting the water and constituents within the boundary layer towards,
through and away from the membrane. This balance is determined by CP.
2.1.4.5 Fouling control
In MBRs, as with many other membrane filtration processes, it is the balance
between the flux, physical and chemical cleaning protocol and, when relevant, the
control of CP which ultimately determines the extent to which fouling is successfully
suppressed. Ultimately, CP-related fouling can be reduced by two methods: (i) promoting
turbulence and (ii) reducing flux. For sidestream MBRs (sMBRs, Fig. 1.6a),
turbulence can be promoted simply by increasing the crossflow velocity (CFV), whereas
for an immersed system (iMBR, Fig. 1.6b) this can only reasonably be achieved by
increasing the membrane aeration. Whereas pumped flow of liquid along a tubular or
parallel plate channel, as with sidestream systems, allows estimation of the degree of
turbulence through calculation of the Reynolds number (density � velocity � tube
diameter/viscosity), determination of turbulence for an immersed aerated membrane
is more challenging (Section 2.3.7.1).