Page 55 - Membranes for Industrial Wastewater Recovery and Re-Use

Membrane technology 3 5

Pressure (bar)
100

10
U It raf i Itratio n
Microfiltration
1
MWCOIO~ 103 104 105 1 06
Particle Size 0.Olpm 0.lpm lpm 10pm

Figure 2.16 Normal ranges of transmembrane pressure values for membrane processes

0 the concentration of rejected solute, as in RO and UF, or permeated ions, as
in ED, near the membrane surface,
0 the depletion of ions near the membrane surface, as with ED,
0 the precipitation of sparingly soluble macromolecular species (gel
layer formation, as in UF) or salts (scaling, as in RO) at the membrane
surface, and
0 the accumulation of retained solids on the membrane (cake layer
formation, as in MF).

All of the above contribute to membrane fouling. Fouling can take place
through a number of physicochemical and biological mechanisms which all
relate to increased deposition of solid material onto the membrane surface (also
referred to as blinding) and within the membrane structure (pore restriction or
pore plugging/occlusion). This is to be distinguished from clogging, which is the
filling of the membrane channels with solids due to poor hydrodynamic
performance. Fouling may be both temporary (removed by washing) and
permanent (removed only by use of chemicals). Since it is intimately related to
concentration polarisation, under certain limiting conditions its effects can be
determined from a simple theoretical approach (Section 2.3.2). However, fouling
by individual components tends to be specific to membrane material and
application.
The membrane resistance is fixed, unless its overall permeability is reduced by
components in the feed water permanently adsorbing onto or into the
membrane. The resistance imparted by the interfacial region is, on the other
hand, dependent upon the total amount of fouling material residing in the
region. This in turn depends upon both the thickness of the interface,
the feedwater composition (and specifically its foulant content) and the flux
through the membrane. The feedwater matrix and the process operating
conditions thus largely determine process performance.