Electrodialysis method and apparatus for passivating scaling species
In one embodiment, this invention relates to an electrodialysis device comprising an inlet for directing a feed stream into a plurality of first feed paths and a plurality of second feed paths; the feed stream is comprised of a first anionic scaling species and a first cationic scaling species; the first cationic scaling species is transferred from the second feed paths to the first feed paths through a first membrane group, the first anionic scaling species is transferred from the first feed paths to the second feed paths through the first membrane group. In another embodiment, this invention relates to a method for passivating scaling species in an electrodialysis device.
1. An electrodialysis device comprising
a) a vessel
b) a cathode and an anode in said vessel in operative association with a source of direct current;
c) a first feed path;
d) a second feed path;
e) a feed inlet providing an aqueous feed stream flow from an upstream direction to a downstream direction through both said first feed path and said second flow path, said feedstream comprising anionic scale imparting species (ASIS) present in a first (ASIS) concentration, cationic scale imparting species (CSIS) present in a first (CSIS) concentration, anionic non scale imparting species (ANSIS) and cationic non scale imparting species (CNSIS);
f) a first membrane group separating said first feed path and said second feed path, said first membrane group comprising at least two elements i) and ii); wherein: i) is an anion selective membrane permitting transfer of (ASIS) from said first feed path to said second feed path, ii) is a cation selective membrane permitting transfer of (CSIS) from said second feed path to said first feed path;
g) a second membrane in fluid communication with said second feed path, said second membrane permitting passage of (ANSIS) therethrough and out of said second feed path;
h) a third membrane in fluid communication with said first feed path, said third membrane permitting passage of (CNSIS) therethrough and out of said first feed path;
i) a first feed path outlet downstream from said first feed path adapted to carry a first effluent therein having a reduced (ASIS) concentration compared with said first (ASIS) concentration;
j) a second feed path outlet downstream from said second feed path adapted to carry a second effluent therein having a reduced (CSIS) concentration compared to said first (CSIS) concentration.
2. The electrodialysis device of claim 1 , wherein said first membrane group elements f) i) are selected from the group consisting of a monovalent selective anion exchange membranes, and divalent selective anion exchange membranes, and said first membrane group elements f) ii) are selected from monovalent selective cation exchange membranes and divalent selective cation exchange membranes.
3. The electrodialysis device of claim 1 , wherein said second membrane is comprised of at least one anion exchange membrane element.
4. The electrodialysis device of claim 1 , wherein said third membrane is comprised of at least one cation exchange membrane element.
5. The electrodialysis device of claim 1 , wherein said (CSIS) is selected from the group consisting of Mg 2+ , Ca 2+ , Sr 2+ , and Ba 2+ , and said (ASIS) is selected from the group consisting of SO 4 2− , HCO 3 − , CO 3 2+ , OH − , F − , PO 4 3− , HPO 4 2− , and H 2 PO 4 − .
6. An electrodialysis device as recited in claim 1 in combination with a desalination unit (DSU), said DSU having a DSU feed stream inlet wherein either but not both of said first feed path outlet and said second feed path outlet are in fluid communication with said DSU feed stream inlet.