Introduction. Natural water is an aqueous solution of substances existing in different aggregate states. It has a complex chemical composition. Water contains ions of inorganic compounds (Na⁺, K⁺, Ca²⁺, Mg²⁺, Fe³⁺, Fe²⁺, Al³⁺, NH₄⁺, Cl^-, HCO₃^-, SO₄^2-, NO₃^-, NO^2-, F^-, SiO₃^2-, HS^- , CO3^2- etc.), dissolved gases (O₂, CO₂, N₂, H₂S), tiny particles of solid admixtures, organic substances of natural and artificial origin, microorganisms and products of their vital activity. The size of particles found in water may vary from several ten-thousandth fractions of micrometer to 100 and over micrometers, i.e. one particle may be more than ten million times different from another. Table 1 [1] demonstrates mean sizes of particles of different nature, which may be present in water, depending on their weight. It also contains size ranges of porous channels of various filtering elements (membranes), classified according to the types of filtration processes (reverse osmosis, nano-, ultra- and microfiltration). It should be noted that the pore sizes given in Table 1 are tentative. They are of a so-called mean size, determined on the basis of known substance density and particle weight, provided that the particles are approximately spherical in shape.

  Table 1. Types and sizes of drinking water admixtures

  In reality, most organic compounds, many microorganisms, their waste products and individual fragments of microorganism structure are linear, and dependant on their orientation, are capable of penetrating through the filtering material, whose pore size is arbitrarily believed to make it impermeable. For instance, antibacterial filters used in medicine for drug filtration have pores of 0.22 micrometer mean size. However, such filters easily let through flagella and bacteria pili, whose diameters range between 25 and 7 nanometers (0.025-0.007 µm). Reverse-osmotic filtration devices with membranes of no more than 0.0001 µm pore size are capable of producing almost ideally pure water. However, such water, if used for drinking, has an adverse effect on human body, since it is devoid of such essential components as ions of sodium, calcium, magnesium, fluorine, chlorine, sulfates, carbonates and many others, microquantities of which should necessarily be present in drinking water. In the course of many hundred thousand years of evolutionary development, the human body has adapted itself to drinking natural fresh clear water, whose mineralization ranges between 0.2-1.0 gram per liter, which contains a small amount of natural organic substances and has no pathogenic or conditionally-pathogenic microorganisms. As a result of technogenous activity of man, practically all fresh water of surface and underground springs on the Earth has become polluted with xenobiotic substances that are foreign and unnecessary for human body (detergents, herbicides, pesticides, oil derivatives and so on). In the process of biotransformation these substances are often decomposed into compounds that are more toxic than the initial ones. Xenobiotic substances adversely affect human immune system, cause various organic and functional disturbances. Besides, today's water contains more ions, producing a toxic action on the human body in any, even the lowest, concentrations (Hg²⁺, Pb²⁺, Cd²⁺), and concentrations of ions, although beneficiary when they are present in microquantities (Fe³⁺, Fe²⁺ , Cu²⁺, Zn²⁺, Ni²⁺ and others, see Table 2 [2]), have risen up to a dangerous level.

  Table 2. Metal ions necessary for man

  Natural fresh water also contains more microorganisms, adapted to the presence of xenobiotic substances and heavy metal ions. Their new forms have appeared, which are much more harmful for man than the already known and well investigated species because of their higher resistance to routinely used antiseptic means.

  The assumption that in the process of water purification, with the help of filtration or sorbing devices, it is possible to retain all harmful substances and let pass the useful ones is wrong. It is practically impossible to group dozens of thousands of various dissolved substances according to their usefulness with the help of filtration or sorption methods, either employed individually or in any possible combinations. Besides, concentration of useful or harmful substances contained in water on the filtering membrane surface, in the sorbent pores or on the surface of ion-exchange material leads primarily to microorganism retention, their accelerated reproduction and enhanced elimination of microbial toxins into water. This process is accompanied by a dramatic decrease of filtering, sorbing or ion-exchange ability of the active elements of a water-purifying device.

  The methods of water purification based on the introduction of biocidal substances (iodine, bromine and silver) bear the risk of their adverse effect on the human body. Ultraviolet or ozone treatment of water does not guarantee its efficient decontamination if it contains colloidal particles or organic compounds of natural or artificial origin. In the "Izumrud" devices, water purification is founded on oxidation and reduction processes which help destroy and neutralize all toxic substances in nature. In the "Izumrud" devices, the natural processes of normal oxidation-reduction destruction and neutralization of toxic substances are many times accelerated due to direct electrochemical reactions, as well as to the participation in the purification processes of the salts of highly-reactive reagents, such as ozone, atomic oxygen, peroxide compounds, chlorine dioxide and short-living free radicals, which have been electrochemically synthesized from purified water and dissolved in it. Water purification processes in the "Izumrud" devices consist of several stages divided in space and time, which differ in their action on water and the admixtures it contains. It makes the process of water purification in the "Izumrud" devices highly efficient and ecologically safe as compared to other methods employed.