Membrane Treatment - Public Water Systems - page 513

Fig. 15-17. Electrodialysis membrane stack arrangement (Courtesy of Ionics)
When a membrane is placed between two salt solutions and subjected to the passage
of a direct electric current, most of the current will be carried through the membrane
by ions; hence, the membrane is said to be ion-selective. Typical selectivities are
greater than 90 percent. When the passage of current is continued for a sufficient length
of time, the solution on the side of the membrane that is furnishing the ions becomes
more concentrated. These desalting and concentrating phenomena occur in thin layers
of solution immediately adjacent to the membrane, resulting in the desalting of the
bulk of the solution. Figure 15-18 is an illustration of typical membrane reactions.
Passage of water between the membranes of a single stack, or stage, usually requires
10 to 20 seconds, during which time the entering minerals in the feedwater are re-
moved. The actual percentage removal that is achieved varies with water temperature,
type and amounts of ions present, flow rate of the water, and stack design. Typical
removals per stage range from 25 to 40 percent; systems employ one to six stages.
An ED system will operate at temperatures up to 100 F (43 C), with the removal