The MBR Book: Principles and Applications of Membrane

70 The MBR Book
hydrophobic rather than hydrophilic membranes (Chang et al., 1999; Madaeni et al.,
1999; Yu et al., 2005a; Yu et al., 2005b). In the literature, changes in membrane
hydrophobicity are often linked with other membrane modifications such as pore size
and morphology, which make the correlation between membrane hydrophobicity
and fouling more difficult to assess. In a recent anMBR study, for example, the contact
angle measurement demonstrated that the apparent hydrophobicity of PES
membranes decreased (from 55 to 47°) with increasing MWCO (from 20 to 70 kDa
membranes, respectively) (He et al., 2005). The effect of membrane hydrophobicity
in an aerobic MBR, from a comparison of two UF membranes of otherwise similar
characteristics, revealed greater solute rejection and fouling and higher cake resistance
for the hydrophobic membrane (Chang et al., 2001a). It was concluded that the
solute rejection was mainly due to the adsorption onto or sieving by the cake deposited
on the membrane, and, to a lesser extent, direct adsorption into membrane pores and
at the membrane surface. It has also been suggested (Fang and Shi, 2005) that
membranes of greater hydrophilicity are more vulnerable to deposition of foulants
of hydrophilic nature, though in this study the most hydrophilic membrane was also
the most porous and this can also enhance fouling (Section 2.3.5.1).
Although providing superior chemical, thermal and hydraulic resistance, the use
of ceramic membranes in MBR technologies is limited by their high cost to niche
applications such as treatment of high-strength industrial waste (Luonsi et al.,
2002; Scott et al., 1998) and anaerobic biodegradation (Fan et al., 1996) in sMBRs.
A direct comparison of a 0.1 �m ceramic and 0.03 �m polymeric multi-channel
membrane modules operated in sidestream air-lift mode showed the former to operate
without fouling up to at least 60 LMH, the highest flux tested, whereas for the
latter criticality was indicated at �36 LMH (Judd et al., 2004). Novel stainless steel
membrane modules have recently been shown to provide good hydraulic performance
and fouling recovery when used in an anaerobic MBR (Zhang et al., 2005).
Since fouling is expected to be more severe at higher hydrophobicities, efforts have
naturally been focused on increasing membrane hydrophilicity by chemical surface
modification. Recent examples of MBR membrane modification include NH 3 and
CO 2 plasma treatment of PP HFs (Yu et al., 2005a; Yu et al., 2005b) to functionalise
the surface with polar groups. In both cases, membrane hydrophilicity significantly
increased and the new membranes yielded better filtration performance and flux
recovery than those of unmodified membranes. In another study, addition of TiO 2
nanoparticles to the casting solution and direct pre-filtration of TiO 2 allowed the
preparation of two types of TiO 2-immobilised UF membrane, respectively comprising
entrapped and deposited particles, which were used in MBR systems (Bae and
Tak, 2005). A lower flux decline was reported for the TiO 2-containing membranes
compared to the unmodified materials, the surface-coated material providing the
greatest fouling mitigation. When MBR membranes were precoated with ferric
hydroxide flocs and compared to an unmodified MBR, both effluent quality and productivity
were found to increase (Zhang et al., 2004).
Whilst many of the scientific studies of MBR membrane surface characterisation
and/or modification relate to fouling by EPS, it appears that in practice both the
choice of membrane material and the nominal membrane pore size are limited.
Commercially-available membranes and MBR systems are reviewed in Chapter 4
and their characteristics summarised in Annex 3 and Table 4.5.