The design and operation of membrane nanofiltration processes requires
the use of good numerical predictive methods and prior characterisati
on of membrane properties. The best methods currently available are ba
sed on the use of the extended Nernst-Planck equation and a characteri
sation of the membrane in terms of the effective pore radius (r(p)), t
he effective charge density (X-d) and the ratio of effective membrane
thickness to membrane porosity (Delta x/A(k)). When the ranges of such
parameters for commercially available nanofiltration membranes are kn
own, it becomes possible to use the numerical predictive method to cho
ose the membrane best suited to a particular process requirement. The
present paper considers such an analysis for the processing of dye/sal
t/water streams. It is shown that if the requirement is the removal of
salt through diafiltration as part of the production process, then it
is best to choose a membrane with a moderate value of r(p) (matching
the hydrodynamic radius of the dye), a low value of X-d and a low valu
e of Delta x/A(k). However, if the requirement is the removal of colou
r from a waste rinse stream, then the best results will be obtained by
choosing a membrane with a high value of r(p) and a high value of X-d
.