Fig. 2. Factors influencing rejection performance of NF membranes.
produced per unit feed will cause a decrease in
rejection [78]. The influence of pH variation on
feed solution discussed above shows the influence
of pH towards the charge of the membrane
as well as the compounds to be removed. On the
other hand, temperature of the water feed matrix
also affects the rejection performance. An increase
in solution temperature causes an increase
in diffusivity and convective flux of compounds.
This produces high permeability and water, flux
which at the same time reduces retention [86]. A
study by Kimura et al. [82] also found that concentration
range of solutes (i.e. contaminants)
might influence the rejection efficiency of a
membrane because they observed that at the low
concentration of ng/L range the rejection efficiency
was lower compared to the concentration
at μg/L range. Degree of salinity in the feed
matrix will cause the effective pore radius of a
charged pore of NF membranes to increase as the
ionic strength of the surrounding liquid increases
[85]. Therefore, the rejection of monovalent ions
will decrease as their concentration in the feed
solution increases. The rejection of divalent ions
will be affected to a lower extent.