Reverse osmosis is the process of water solution separation by means of their filtration through semipermeable membranes at pressure exceeding osmotic one. This method has significant advantages in comparison with other demineralization methods: lower power consumption, simple design, assembly and operation, small dimensions, etc.As the result of reverse osmosis, water and dissolved substances are separated at molecular level, thus almost demineralized water is accumulated on one side of membrane and all impurities are on the other side thereof.
Therefore, water with initial salt content of 3000 mg/l and more may be demineralized at reverse osmosis stage one to 10–200 mg/l.
High-selective membranes with working pressure 40-60 bars are used for sea water demineralization. Reverse osmosis membrane performance and permeate output are lower and, hence, they require more power consumption.
Composite roll-type polyamide membranes and acetate-cellulose membranes have been widely used in reverse osmosis processes.
Nanofiltration is used for brackish water demineralization with the use of membrane elements characterized be lower selective ability (50-80%).
Membrane element bodies are made of epoxy resin and fiberglass and stainless steel.
Electrodialysis:
Electrodialysis represents a process of demineralization of water and other fluids containing ionic form impurities using a constant electric field and selective ion-exchange membranes. Salt ions move along field orientation in constant electric field. Thus the principle of solution electroneutrality is met at any field point. Ion-exchange membranes represent polymer films, have fixed ionogenic groups trapping and exchanging ions (counterions). Membranes in electrolytic solutions are electron conduction-free, i.e. they act as electrolytic conductors, not bipolar electrodes.The principle of multichamber electrodialysis is concluded in placement of a set of separated alternating cation-exchange and anion exchange membranes between two electrodes. So, demineralized and concentrated solutions flow in the formed chambers. The transfer process in such a system is repeated for many times that allows to reduce dramatically relative losses of energy on electrodes. Appplication of the principle of multichamber electrodialysis made it possible to design compact electrodialyzers with a big surface of membrances.
Efficiency and quality of used membranes is of utmost priority in electrodialysis process. In addition to chemical stability, mechanical strength and stable size ion-exchange membrane shall have a high selective ability, good current conductivity, revers diffusion small rate and low osmotic permeability within the working range of compositions and solution concentrations.
.