Page 54 - Membranes for Industrial Wastewater Recovery and Re-Use

34 Membranes for Industrial Wastewater Recovery and Re-use

Figure 2.1 5 Membrane module mass balance

Rejection
The permselective property of the membrane is normally quantified as the
rejection where:
R = 100 % (1-Cp/C) (2.4)

It is possible to have negative rejection values if the membrane is selective for
specific contaminants, as would be the case for an extractive membrane system.

2.2.2 The driving force
The driving force for the process may be a transmembrane pressure gradient, as
with filtration and reverse osmosis (Fig. 2.1 6), a concentration gradient, as with
dialysis, or electromotive, as with electrodialysis. In almost all pressure-driven
membrane processes applied to water treatment the desired permeate is water,
such that the retained or rejected material (the retentate) is concentrated. In
extractive and electrodialytic operations the permeate is the dissolved solute and
the retentate the product water. For extractive systems the driving force is a
concentration gradient, whereas for electrodialysis an applied potential
difference is employed to move dissolved ions through electromigration.
Since the flux and driving force are interrelated, either one can be fixed for
design purposes. It is usual to fix the value of the flux and then determine the
appropriate value for the transmembrane pressure for pressure-driven processes.
An analogous situation arises in the determination of operational parameter
values for electrodialysis, where the appropriate operating current density is
normally fixed and the voltage determined accordingly. The actual flux or
current density value chosen depends upon the desired operating regime.

2.2.3 Factors opposing the driving force
The overall resistance at the membrane-solution interface is increased by a
number of factors which each place a constraint on the design and operation of
membrane process plant: