Nanomembranes - Nanotechnologies for Water Environment Applications - page 395

membranes in the experiments of Fersi et al. (2005) and Qin et al. (2006) experienced
declination of flux during the operation. Fersi et al. (2005) reported a flux reduction by
83.5% after 5 hours of operation at a constant pressure and with recycling of the
retentate to the feed tank at a temperature of 30 o C. For the study of Qin et al., the
normalized flux was only about 58-63% and 54% of the normalized flux of the pure
water in Desal-5 and NE-70, respectively. The NE-70 membrane was only able to attain
63% of the normalized pure water flux after cleaning-in-place (CIP) with 2% citric acid
for 1 hour followed by flushing with deionized (DI) water. Further cleaning by soaking
overnight with 0.5% EDTA solution and flushing with DI water allowed it to regain
70% of the pure water flux. This reduction of flux was due to the combined effect of
concentration polarization, adsorption and/or pore blocking caused by high COD and
salt concentrations.
UF and MF would be suitable options to remove colloidal dyes such as disperse
and vat dyes, while NF could be used to remove soluble and ionic dyes such as acid,
basic, direct and reactive dyes (Buckley, 1992). This phenomenon explains the poor
color removal when MF and UF were used as these membranes were unable to retain the
soluble and ionic dyes present in the textile effluent. NF would be a more suitable
membrane for treating textile effluent to achieve a better overall performance. However,
the rapid flux declination in NF membranes would be a major challenge for its long term
operation and cost reduction.
Contrary results have been reported whereby an increased flux rate was observed
with increasing salt concentration. Similar to that discussed earlier in Section 12.3.1 ,
pore enlargement had also increased the flux rate in NF membranes treating dye
effluent. By determining reduced membrane resistance during pure water filtration
before and after solution filtration, pore enlargement was noted on a thin-film composite
membrane with a polyamide top layer on a polysulphone base following filtration of 20
g/L sodium sulfate (Na 2 SO 4 ) and dye solution (Denyer et al., 2007). This is contrary to
the assumption that clogging or adsorption of salts/dyes on the membrane will cause flux
to decline. The pore diameter was noted to increase by about 9.5% following filtration of
20 and 40 g/L Na 2 SO 4 . Significantly higher pore enlargement of 56-95% was noted in
the presence of dye in salt solution containing 10 and 15 g/L of Na 2 SO 4 . The force
provided by the transmembrane pressure could cause the divalent anion, SO 4 2- , to push
through the membrane. This caused pore enlargement due to the decrease in Donnan
effect in high ionic strength solution. Some pore enlargement was hypothesized to be
reversible, but it was believed that the deformation of the membrane would be
irreversible after reaching a saturation point or number of passages by a divalent anion.
The results indicated that pore enlargement could possibly be avoid by operating the NF
membrane at a suitable operating conditions, such as an optimum operating pressure and
selection of suitable membrane materials to ensure long term membrane integrity and
permeate quality.