How can ions be removed from the water

Electrodialysis (ED)


Electrodialysis is a separation process in which ions are transported through a semipermeable membrane under the influence of an electrical potential. The membranes used are only permeable to positively or negatively charged ions, i.e. they work in a cation- or anion-selective manner. Cation-selective membranes are polyelectrolytes with negatively charged matter that hold back negatively charged ions and are permeable to positively charged ions.

The arrangement of a large number of chambers, which are alternately delimited by anion- and cation-selective membranes, allows ions to be removed from the raw water. The chambers are located between the surface electrodes anode and cathode. Due to the arrangement, the ions of the raw water are concentrated in some chambers, while they can leave the raw water in other chambers. The concentrated raw water is circulated to a final concentration and then knocked off. This method is particularly suitable for removing ions from water. However, uncharged particles cannot be eliminated.

Cation-selective membranes are made of sulfonated polystyrene, while anion-selective membranes are made of polystyrene with quaternary amines. In some cases, water pre-treatment is necessary before the electrodialysis process. Suspended solids with a diameter of more than 10 ┬Ám must be removed beforehand in order to avoid clogging of the membrane pores. In addition, certain substances can neutralize the membrane charge, such as larger organic anions, colloids, iron and manganese oxides. These lead to a disruption of the selective membrane properties.

Possible pretreatment steps to protect the membranes are filtration through activated carbon (removal of organic matter), flocculation (removal of colloids) and pre-filtration stages.