The MBR Book: Principles and Applications of Membrane

only pseudo-steady-state (or stabilised) conditions are attained to do the unavoidable
deposition or adsorption of fouling material.
Filtration proceeds according to a number of widely recognised mechanisms, which
have their origins in early filtration studies (Grace, 1956), comprising (Fig. 2.9):
● complete blocking
● standard blocking
● intermediate blocking
● cake filtration
Fundamentals 31
All models imply a dependence of flux decline on the ratio of the particle size to the
pore diameter. The standard blocking and cake filtration models appear most suited
to predicting initial flux decline during colloid filtration (Visvanathan and Ben Aim,
1989) or protein filtration (Bowen et al., 1995). All of the models rely on empirically
derived information and some have been refined to incorporate other key determinants.
On the other hand, a number of empirical and largely heuristic expressions
Flux
300
250
200
150
100
50
0
Membrane backflush
Time
Flux
50
40
30
20
10
(a) (b)
0
Time
Figure 2.8 Flux transients for: (a) dead-end and (b) crossflow filtration for constant pressure operation
(a)
(b)
Figure 2.9 Fouling mechanisms: (a) complete blocking, (b) standard blocking, (c) Intermediate blocking,
(d) cake filtration
(c)
(d)