Membrane and Desalination Technologies - TCE Moodle Website

Desalination of Seawater by Thermal Distillation and Electrodialysis Technologies 545
3.3.3.1 HOMOGENEOUS MEMBRANE
Charged groups are uniformly distributed through the membrane matrix. The cation-selective
membranes are often made up of crosslinked polystyrene (with divinyl benzene) that has been
sulfonated to produce sulfonate that is attached to the polymer. The anion-selective membranes
are made of crosslinked polystyrene containing quaternary ammonium groups.
3.3.3.2 HETEROGENEOUS MEMBRANE
The IX groups are contained in small domains distributed throughout an inert support
matrix that provides the mechanical support. The simplest form has very finely powdered
cation or anion exchange particles uniformly dispersed in polypropylene. A much finer
heterogeneous dispersion of IX can be made from polyvinylchloride plastisol.
3.3.4. Solution Pumping System
The low-pressure pump helps to circulate the feed water through the stack. The pump must
have enough power to overcome the resistance of water because it passes through the narrow
passages. The energy required for the solution pumping system increases as the average salt
concentration of the feed decreases. This can be significant when the feed concentration drops
to a very low value. The pressure drop per stack varies from 15 to 30 psi for flow cells and
70 to 90 psi for tortuous cells. Depending on the extent of separation required, the water is
pumped through 2–4 cells in series. Therefore, interstage pumps may be required.
3.3.5. Posttreatment
Posttreatment includes stabilizing the water and preparing it for distribution. Removal of
unwanted gases such as hydrogen sulfide and adjustment of the pH are normally conducted. If
the salt content in the water is still too high, other processes such as IX and RO can be used.
3.4. Electrodialysis Reversal
As the membranes used in the electrodialysis easily become fouled, efforts have been
made to overcome the problem. EDR process was developed to help eliminate membrane
fouling. In the early 1970s, an American company commercially introduced the EDR process
for ED. EDR is a continuous self-cleaning ED (17, 18).
EDR was developed with ED technology as the basis. In the EDR process, the membrane
polarity is reversed several times an hour as shown in Fig. 12.14. The brine stream becomes
the product water stream and vice versa. As a result, the ions are now attracted in the reverse
direction across the membrane stack. Immediately following the reversal of polarity and flow,
the product water is dumped until the stack and lines are flushed out, and the desired water
quality is restored. This flush takes about 1 or 2 min, and then the unit can resume producing
water. The reversal process is useful in breaking up and flushing out scales, slimes, and other
deposits in the cells before they can build up and create a problem. Flushing allows the unit to
operate with fewer pretreatment chemicals and minimizes membrane fouling. The EDR
systems operate with higher concentrations in the brine or concentrate the streams with less
flow to waste (19).