TM 5-813-8
CHAPTER 7
MEMBRANE TECHNIQUES
c. Electrodialysis reversal.
One important
Electrodialysis . The ions in a water solution
7-1.
improvement now used in electrodialysis installations is
can be made to migrate by applying an electric field to
to reverse the polarity periodically and move the ions in
the solution. By arranging various barriers to the flow of
the opposite direction. This returns anions across the
ions, it is possible to directly desalinate water with
anionic membranes and helps break up scale formed on
Such barriers are called ion-exchange
the concentrating face of the membranes. Water will
membranes. Membranes that allow a reasonable flow
flow osmotically across both membranes from the dilute
of cations, but block or reduce the flow of anions, are
product stream to the concentrated brine stream in an
called cationic-exchange membranes. Membranes that
electrodialysis-reversal stack. This osmotic product
allow a reasonable flow of anions, but block or reduce
water loss concentrates uncharged material, such as
the flow of cations, are called anion-exchange
turbidity and bacteria. This concentration effect must be
membranes. Membranes that pass both anions and
considered during the design to ensure meeting water
cations are called neutral membranes.
turbidity and product water bacterial count requirements.
a. Theory. In solutions containing dissolved ions,
Most electrodialysis membranes are not tolerant of
electric currents are carried by movement of the ions.
chlorine.
When possible, water desalinated by
Positive ions migrate in the direction of the current flow,
electrodialysis reversal should be disinfected after
and negative ions migrate against the current direction.
desalination is completed. The membranes should be
When the anions are blocked by a cationic-exchange
protected by a 10-micron cartridge filter.
membrane, they stop and form a localized charge at the
membrane face. This accumulated negative charge is
Electrodialysis-reversal design .
When
neutralized by the flow of cations across the cationic
7-2.
electrodialysis reversal can be shown to be the most
membrane. This generates a concentrated solution on
economical process for desalination, then only an
the side of a cationic-exchange membrane that faces
electrodialysis-reversal system will be designed. When
the negative electrode. It also generates a dilute
the process selection does not yield a specific process,
solution on the side of the cationic membrane that faces
then designs must be prepared for more than one
the positive electrode as shown in figure 7-1.
process.
b. Electrodialysis stack. If both a cationic and
a. Identification of work. The design engineer will
anionic membrane are placed across a current flow in
be made aware of the base site and construction
an electrolyte solution, the side of the cationic
schedule. The location and time schedule will be
membrane facing the positive electrode and the side of
considered in the design; this includes the date the
the anionic membrane facing the negative electrode will
system must be online. The minimum number and
become less saline. If the cationic membrane is closer
minimum capacity of the modules will be determined.
to the negative electrode and the anionic membrane is
Any restrictions that storage will place on maximum
closer to the positive electrode, the solution between the
allowable downtime will also be considered.
A
membranes will become less saline as the ions migrate
maximum allowable output conductivity in the product
in their respective directions. Any number of pairs of
water will be determined, based on the worst possible
cationic and anionic membranes can be placed across a
feed water.
current-carrying solution, such that the cationic
b. Existing on planned facilities.
When
membrane is closest to the negative electrode, and the
solution between will be diluted (fig. 7-1). A battery of
electrodialysis reversal is being designed, it is essential
several such membrane pairs is called an electrodialysis
to determine the types of available electrical power.
stack. Several variations of the standard electrodialysis
The voltage, phase, frequency, and available amperage
stack have been developed, but none have been proven
of all electrical power sources, planned or existing, must
superior to this standard stack of alternating cationic-
be considered in the design.
and anionic-exchange membranes to desalinate natural
c. Raw water information.
One of two
brackish water.
circumstances will limit the quantity of raw water
consumed.
Both of these limitations must be
considered in the design:
7-1
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