Civil Engineering Reference
In-Depth Information
Fig. 15-18.
A schematic of typical electrodialysis membrane (Courtesy of Ionics)
efficiency increasing with increasing temperature. Ion-selective membranes in com-
mercial electrodialysis equipment are commonly guaranteed for as long as 5 years,
and experience has demonstrated an effective life of over 10 years.
Since 1974, virtually all ED plants constructed have been the EDR type. Until the
EDR process was developed, all ED plants (and RO as well) operated with the transfer
of water or salts in one direction. Membranes had a brine side and a product side.
Films of scale, slime, and other deposits generally formed on one side of the membrane
only.
Partially successful attempts were made to control the precipitation of insoluble
salts such as calcium carbonate and calcium sulfate on the brine side of unidirectional
membrane processes by the addition of acids, polyphosphates, or similar agents. These
chemical additions can control calcium carbonate scale effectively, and calcium sulfate
scale less effectively, when precise dosing is maintained. However, when errors, fail-
ures, or upsets occur, scale starts to form at an accelerating rate.
The symmetrical nature of ED membranes allowed the development of the EDR
process. The EDR system utilizes a standard electrodialysis array of alternating anion
membranes, separated by alternating product and brine compartments, as shown in
Figure 15-16. The array is operated in the standard ED manner for a fixed period of
time—for example, 20 minutes—and then the process is reversed by an automatic
timing circuit in the following sequence:
1. The direction of the DC field is reversed by reversing the polarity of the elec-
trodes. This polarity reversal immediately begins converting the product compartments
into brine compartments, and the brine compartments into product compartments, by
reversing the direction of flow of the ions.
2. Automatic valves interchange the feed to and discharge from the product and
brine compartments.
3. There is a one- to two-minute period immediately following the polarity reversal
when the water from both sets of compartments is of lower quality, and both streams
are automatically diverted to waste. This ''purge'' of the brine and product compart-
ments every 20 minutes breaks up polarization films, carries off loose scale, and re-
duces the tendency of deposits to build up.
