DESALTING OF WATER — a way of processing of highly mineralized water for the purpose of decrease or full removal of the salts dissolved in it.
The lack of sweet water is already now felt in many countries of the world. In the USSR the lack of sweet water is felt in the territory of Turkmenistan, Kazakhstan, Uzbekistan, Azerbaijan, in a number of districts of the North Caucasus, Ukraine, Western and Southern Siberia, the Urals, the Volga region where it can be considerably covered due to desalting of the considerable resources of highly mineralized underground or sea waters which are available in these districts.
The lake of century is carried out or by department actually of water molecules, or removal of ions of salts from solution. Proceeding from these features, methods of desalting are divided into two groups: with change and without change of an aggregate (phase) state of water. To the first group thermal desalting (distillation) and processes treat with use of cold (winterizing), to the second — chemical, membrane, extraction and the adsorptive, and also biol, methods.
The most widespread methods O. of century are distillation, winterizing, electrodialysis, reverse osmosis and ion exchange.
Method distillations (see) it is based that during the heating of water of a molecule it due to thermal and oscillating motion gain the energy sufficient for overcoming forces of an intermolecular attraction, and the formed steam at the subsequent condensation gives a desalinated water. On the nature of boiling and design features distinguish the desalters boiling, adiabatic (low-temperature boiling in vacuum), thin-film, hygroscopic, hydrophobic and thermal diffusion. Use of distillation is most economic during the desalting of salt (sea) waters for receiving a significant amount of sweet water.
Desalting by method of winterizing (natural and artificial) is based on use of a difference of temperatures of freezing fresh (0 °) and saltish waters with a mineralization to 10 g/l (— 1,6 °). The method of natural winterizing is used in districts with a frigid climate. Artificial winterizing provides cooling of water with introduction of a hydrophobic coolant (buds, freon) directly in the desalted water, evaporation of cooled water in vacuum, use as a coolant of water with lower temperature. Theoretical calculations show that the method of winterizing can be one of the most economic. However tekhnol, difficulties, big metal consumption of desalters and a big expense of a desalinated water in technological process of such desalting detain industrial implementation of this method.
The method of electrodialysis (electrochemical) is based on transfer of ions of the salts dissolved in water in electric field between the electrodes shipped in the desalted water; at the same time cations move to the cathode, and anions to the anode. For prevention of back reaction near electrodes establish katio-no-and anionoselektivny membranes, to-rye do not pass opposite loaded ions. The method of electrodialysis is most reasonable for desalting of saltish waters (with a mineralization to 10 g/l).
The method of a reverse osmosis is based on creation in system with porous synthetic membranes of pressure exceeding osmotic; at the same time there is a filtering of water molecules through membranes and a delay of salts.
The method of ion exchange is based on consecutive filtering of water via the filters loaded ka-tiono-and anion-exchange resins — ionites (see). Process of desalting takes place in two consecutive stages: decationizing and anions. The method is most perspective for desalting of low-mineralized waters (with a mineralization to 2,5 g/l).
Depending on a method of desalting are established specific a gigabyte. the conditions of use and the modes of operation of desalination installations including methods of pre-treatment of initial water, its additional cleaning, correction of composition of salt in solution, disinfecting and conditioning of a desalinated water, and also a condition of use constructive and tekhnol, materials and reagents.
The water received by various methods of desalting shall conform to the operating state standard on drinking water (see. Water, sanitary and hygienic requirements ), and also to be physiologically full. The use for the drinking purposes of pure distillate reflects adversely on a state went. - kish. path and water salt metabolism of the person. According to «Methodical indications on hygienic control of design, construction and operation of group systems of agricultural water supply» No. 2058 — 79, to the approved M3 of the USSR, minimum necessary level of a mineralization (100 mg/l) and optimum levels of a mineralization of desalinated drinking water hloridno-sul-fatnogo (200 — 400 mg/l) and hydrocarbonate (250 — 500 mg/l) classes are accepted. Also minimum level of calcium (1,5 mg —), as much as possible is regulated (6,5 mg — ekv/l) and it is minimum (0,5 mg — ekv/l) tolerance levels of alkalinity, minimum necessary level of rigidity (1,5 mg — ekv/l), the most admissible concentration of boron (0,5 mg/l) and bromine (0,2 mg/l).
Bibliography: Apeltsin I. E. and Klyachko V. A. Desalting of water, M., 1968; Rakhmanin Yu. A., etc. Experimental and kliniko-physiological materials for justification of lower limits of a mineralization of desalinated drinking water, Gigabyte. and dignity., No. 7, page 16, 1975; Sidorenko G. I. and Rakhmanin Yu. A. Desalting as a hygienic problem in the USSR, in the same place, No. 12, page 14, 1977.
G. I. Sidorenko.