The MBR Book: Principles and Applications of Membrane

30 The MBR Book
The membrane resistance is governed by the membrane material itself, and mainly
the pore size, the surface porosity (percentage of the surface area covered by the
pores) and the membrane thickness. The fouling layer resistance is associated with
the filtration mechanism, which is then dependent on the membrane and filtered
solids characteristics. The membrane:solution interfacial region resistance is associated
with concentration polarisation (CP) (Section 2.1.4.4) which, for the more
perm-selective processes such as RO, produces a solution osmotic pressure at the
membrane surface which is higher than that in the bulk solution. The resistance
offered by foulants is often further delineated into generic types according to their
characteristics, behaviour and origin (Sections 2.1.4.2 and 2.3.6.8). However, in
general, the membrane resistance only dominates when fouling is either absent (i.e.
the feedwater is almost free of fouling materials) or is suppressed by operating under
specific conditions (Sections 2.1.4.5 and 2.3.9).
2.1.4.2 Dead-end and crossflow operation
Conventional pressure-driven membrane processes with liquid permeation can
operate in one of two modes. If there is no retentate stream then operation is termed
“dead-end” or “full-flow”; if retentate continuously flows from the module outlet
then the operation is termed crossflow (Fig. 2.7). Crossflow implies that, for a single
passage of feedwater across the membrane, only a fraction is converted to permeate
product. This parameter is termed the “conversion” or “recovery”. The recovery is
reduced further if product permeate is used for maintaining process operation, usually
for membrane cleaning.
Filtration always leads to an increase in the resistance to flow. In the case of a
dead-end filtration process, the resistance increases according to the thickness of the
cake formed on the membrane, which would be expected to be roughly proportional
to the total volume of filtrate passed. Rapid permeability decay then results, at a rate
proportional to the solids concentration and flux, demanding periodic cleaning
(Fig. 2.8). For crossflow processes, this deposition continues until the adhesive forces
binding the cake to the membrane are balanced by the scouring forces of the fluid
(either liquid or a combination of air and liquid) passing over the membrane. All
other things being equal, a crossflow filtration process would be expected to attain
steady-state conditions determined by the degree of CP (Section 2.1.4.4). In practice,
Feed
Permeate
or filtrate
Filter cake
membrane
or septum
(a) (b)
Feed Concentrate
Permeate
Figure 2.7 (a) Dead-end and (b) crossflow filtration
Membrane
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