Environmental Engineering Reference
In-Depth Information
polarization. To understand the methods used for concentration polarization reduction, it
is useful to investigate the concentration polarization modulus, c io / c ib , equation:
J
D
v
δ
c
c
exp
i
io
ib
=
( 2 7. 3)
J
D
v
δ
1
+
E
exp
1
i
o
where c io /c ib is the concentration ratio at the membrane surface over the concentration in the
bulk, which is referred to as the polarization modulus; J v is the volume lux through the
membrane; δ is the boundary layer thickness; D i is the diffusion coeficient of the solute in
the luid; and E o is the membrane enrichment, or c ip / c io or the ratio of concentration at the
membrane surface over the concentration on the permeate side. From this equation, as lux
or boundary layer thickness increases, concentration polarization or c io / c ib also increases.
In many systems, the variable that is most readily manipulated is δ, the boundary layer
thickness [47]. The maximum lux through a membrane is the lux of deionized water
through that membrane at a given driving force, i.e., no salt rejection causing the lux to
reduce.
Mechanical methods of CP reduction tend to focus on increasing turbulence in the low
at the membrane surface in order to decrease δ, the boundary layer thickness [47]. Methods
of achieving this goal include increasing the luid velocity at the membrane surface, add-
ing membrane spacers to disturb the low, generating ultrasonic waves near the mem-
brane, oscillating the membrane, pulsing the feed low in the system, or some combination
of these methods [48-54]. The disadvantage of most of these methods is that they require
addition of external energy to the system. Therefore, the addition of CP mitigation meth-
ods is very dependent on membrane process and feed solution. Despite a higher energy
operating budget, mechanical methods such as feed spacers remain popular as they are
easy to implement in most commercial systems.
Chemical methods of polarization reduction focus on either membrane modiication
during membrane development, modiication of the surface via surface treatment, or
modiication of the feed solution with the addition of a chemical such as an antiscalant.
Membrane modiication has been affected by luorination of RO membranes and has been
shown to increase lux by six times versus an untreated membrane for FT-30 membranes
(FilmTec Corp., Minneapolis, MN) while maintaining rejection and membrane lifespan.
The mechanism behind this was hypothesized to be due to a thinning of the polymer
strands of the interfacial rejection layer, as shown in Figure 27.12 [55]. Membrane pretreat-
ment has been performed [56] on the basis of membrane material and polyethylene glycol
solution concentration without a decrease in membrane selectivity [56]. However, after
the initial lux enhancement experienced during membrane operation, membrane perfor-
mance was still hindered by concentration polarization, and as the separation continued,
the low decreased back below pretreatment conditions [56].
Electrical methods offer yet another alternative for reducing concentration polarization,
including applying a DC ield across or to the membrane, which has been demonstrated
by many researchers [57-60]. A commercial company, Graham Tek (Singapore), utilizes
both mechanical and electrical methods in their RO systems. The electrical component
consists of three coils 120° out of phase energized at an alternating ield of 2 kHz that
are wrapped around the membrane sandwich structure. The mechanical component is an
“integrated low distributor,” which creates 1-2 mm air bubbles that are allowed to low
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