How can electric fields move salt out of one water stream and into another?

Dissolved salt contains positively and negatively charged ions. Electrodialysis separates many of those ions from a water stream by applying an electric field across a stack of channels and ion-selective membranes.

Positive ions move toward the negative electrode, while negative ions move toward the positive electrode. A cation-selective membrane admits mainly positive ions; an anion-selective membrane admits mainly negative ions. Their alternating arrangement determines which channels lose salt and which gain it.

In a diluting channel, the two membrane types allow opposite kinds of ions to leave through opposite sides. The adjacent channels collect ions and become more concentrated. Separate flowing streams carry away the diluted water and the concentrate.

The salt is relocated rather than destroyed. Energy is needed to drive the process, and the membranes and concentrated stream require management. Selectivity, fouling and water chemistry affect performance, so electrodialysis is a designed separation step rather than an automatic guarantee of fully purified water.