Nanomembranes - Nanotechnologies for Water Environment Applications - page 375

FA was found to be retained more effectively than either HA or NOM. It is somewhat
surprising as HA is larger than FA. It might because that HA is more polydisperse than
FA and that the low molecular weight compounds, which are present to a similar extent
in both samples, determine NF rejection. Further, FA has a significant higher carboxylic
group content than HA, which indicates some charge effects. The increase of FA
concentration caused a moderate increase in DOC and UV rejection, as well as calcium
rejection. The results for pH effects confirmed that charge interactions are important for
organics rejection. While the humics and hydrolysates are retained by size exclusion
effects, the smaller compounds demonstrate pH effects.

100

pH 4.5
pH 8
pH 10

90

80

70

60

50

40

30

20

10

0

FA HA NOM

Figure 12.4 DOC rejection in the presence of different organic types for the TFC-SR
membrane (0.5 mM CaCl 2 , 1 mM NaHCO 3 , 20 mM NaCl).

Apart from the solute-solute interactions, the deposition of foulants on the
membrane can alter rejection. The problem of membrane fouling by NOM is commonly
experienced in membrane filtration of natural waters for drinking water treatment. Many
studies use polysulphone membranes, which have a tendency to adsorptive fouling. But
other membrane materials show almost no NOM fouling at moderate fluxes (Dal-Cin et
al., 1996; Thorsen, 1999). For example, some CA membranes give little adsorptive
fouling and are also cheaper and easier to clean than polysulphones (Thorsen et al.,
1993). It, therefore, makes sense to avoid this type of fouling in filtration by using
suitable membrane materials. Using high fluxes and various membrane types in
experiments with NF would result in organic fouling that might be caused by both
physical accumulation and adsorptive fouling on the membrane. In summary, the most
important organic fouling mechanisms are initial adsorption, precipitation and gel
formation, and the interaction with multivalent cations. Large organics appear to foul
membrane most, and for mixtures fouling is consistently worse, indicating solute-solute