The MBR Book: Principles and Applications of Membrane

66 The MBR Book
to provide a hydraulic resistance of around twice that of a UF membrane (Choi et al.,
2005b). Interestingly, the DOC rejection of both membranes was similar following
2 h of operation, indicating the dynamic membrane layer formed on the membranes
to have provided the perm-selectivity rather than the membrane substrate itself.
Conventional wisdom considers smaller pores to afford greater protection of the
membrane by rejecting a wider range of materials, with reference to their size, thus
increasing cake (or fouling layer) resistance. Compared to that formed on membranes
having larger pores, the layer is more readily removed and less likely to leave
residual pore plugging or surface adsorption. It is the latter and related phenomena
which cause irreversible and irrecoverable fouling. However, when testing membranes
with pores ranging from 0.4 to 5 �m, Gander et al. (2000) conversely observed greater
initial fouling for the larger pore-size membranes and significant flux decline when
smaller pore-size membrane were used over an extended period of time, though these
authors used isotropic membranes without surface hydrophilicisation.
Characterisation of the distribution of MW compounds present in the supernatant
of MBRs operated with membranes of four pore sizes (ranging from 0.1 to
0.8 �m) has also been presented (Lee et al., 2005). Although providing a lower fouling
rate, the 0.8 �m pore-size MBR nonetheless had a slightly higher supernatant
concentration of most of the macromolecules. According to these results, it seems
unlikely for the small differences in MW distribution to cause significant variation in
fouling rates observed between the four MBR systems. In another study based on
short-term experiments, sub-critical fouling resistance and fouling rate increased
linearly with membrane resistance ranging from 0.4 to 3.5 � 10 9 m �1 , corresponding
to membrane pore size from 1 down to 0.01 �m (Le-Clech et al., 2003c). These
results suggest that a dynamic layer is created of greater overall resistance for the
more selective membranes operating under sub-critical conditions, and supports the
notion that larger pores decrease deposition onto the membrane at the expense of
internal adsorption. Long-term trials have revealed that progressive internal deposition
eventually leads to catastrophic increase in resistance (Cho and Fane, 2002; Le-Clech
et al., 2003b; Ognier et al., 2002a), as discussed in Section 2.1.4.6. Tests conducted
using a very porous support for the formation of a dynamic membrane have yielded
reasonable removal efficiencies and permeabilities (Wu et al., 2004), and full-scale
installations now exist based on this approach (Section 5.2.5).
Porosity/pore size distribution/roughness Membrane roughness and porosity were
identified as possible causes of differing fouling behaviour observed when four MF
membranes with nominal pore sizes between 0.20 and 0.22 �m were tested in parallel
(Fang and Shi, 2005). The track-etched membrane, with its dense structure
and small but uniform cylindrical pores, provided the lowest resistance due to its
high surface isoporosity whereas the other three membranes were more prone to
pore fouling due to their highly porous network. Although all membranes were of
similar nominal pore size, the PVDF, mixed cellulose esters (MCE) and PES membranes
resulted in relative pore resistance of 2, 11 and 86% of the total hydraulic
resistance respectively. It was suggested that membrane microstructure, material
and pore openings all affected MBR fouling significantly (Fang and Shi, 2005). Comparison
between two microporous membranes prepared by stretching demonstrated