Pollution entering a body of water causes a disturbance in its natural balance. The ability of a reservoir to resist this disturbance, to free itself from introduced contaminants, is the essence of the self-purification process.

Self-purification of water systems is due to many natural and sometimes man-made factors. These factors include various hydrological, hydrochemical and hydrobiological processes. Conventionally, three types of self-purification can be distinguished: physical, chemical, biological.

Among physical processes, dilution (mixing) is of paramount importance. Good mixing and a decrease in the concentration of suspended particles is ensured by the intense flow of rivers. The self-purification of reservoirs is facilitated by the settling of polluted waters and the settling of insoluble sediments to the bottom, the sorption of pollutants by suspended particles and bottom sediments. For volatile substances important process is evaporation.

Among chemical factors self-purification of reservoirs main role plays the oxidation of organic and inorganic substances. Oxidation occurs in water with the participation of oxygen dissolved in it, so the higher its content, the faster and better the process of mineralization of organic residues and self-purification of the reservoir. When a reservoir is heavily polluted, the reserves of dissolved oxygen are quickly consumed, and its accumulation due to physical processes of gas exchange with the atmosphere proceeds slowly, causing self-purification to slow down. Self-purification of water can also occur as a result of some other reactions in which poorly soluble, volatile or non-toxic substances are formed, for example, hydrolysis of pesticides, neutralization reactions, etc. Calcium and magnesium carbonates and bicarbonates contained in natural water neutralize acids, and carbonic acid dissolved in water neutralizes alkalis.

Under the influence of ultraviolet radiation from the sun in surface layers of the reservoir, photodecomposition of some chemicals, for example DDT, and water disinfection - death pathogenic bacteria. The bactericidal effect of ultraviolet rays is explained by their influence on the protoplasm and enzymes of microbial cells, which causes their death. Ultraviolet rays have a detrimental effect on vegetative forms of bacteria, fungal spores, protozoan cysts, and viruses.

Each body of water is a complex living system inhabited by bacteria, algae, higher aquatic plants, and various invertebrate animals. The processes of metabolism, bioconcentration, and biodegradation lead to changes in the concentration of pollutants. Biological factors of self-purification of a reservoir also include algae, mold and yeast, but in some cases the massive development of blue-green algae in artificial reservoirs can be considered a process of self-pollution. Representatives of the animal world can also contribute to the self-purification of water bodies from bacteria and viruses. Thus, oysters and some amoebas adsorb intestinal and other viruses. Each mollusk filters more than 30 liters of water per day. Common reed, angustifolia cattail, lake reed and other macrophytes are capable of absorbing not only relatively inert compounds from water, but also physiologically active substances such as phenols, poisonous salts heavy metals.

The process of biological water purification is related to the oxygen content in it. When there is sufficient oxygen, activity occurs aerobic microorganisms that feed on organic matter. When organic substances are broken down, carbon dioxide and water are formed, as well as nitrates, sulfates, and phosphates. Biological self-purification is the main link of the process and is considered as one of the manifestations of the biotic cycle in a reservoir.

The contribution of individual processes to the ability of natural aquatic environment to self-purification depends on the nature of the pollutant. For so-called conservative substances that do not decompose or decompose very slowly (metal ions, mineral salts, persistent organochlorine pesticides, radionuclides, etc.), self-purification is of an apparent nature, since only the redistribution and dispersion of the pollutant in the environment occurs, and contamination of adjacent objects occurs. A decrease in their concentration in water occurs due to dilution, removal, sorption, and bioaccumulation. With regard to nutrients, biochemical processes are the most important. For water-soluble substances not involved in biological cycle, the reactions of their chemical and microbiological transformation are important.

For most organic compounds and some inorganic substances, microbiological transformation is considered one of the main ways of self-purification of the natural aquatic environment. Microbiological biochemical processes include several types of reactions. These are reactions involving redox and hydrolytic enzymes (oxidases, oxygenases, dehydrogenases, hydrolases, etc.). Biochemical self-purification water bodies depends on many factors, the most important of which are temperature, active reaction environment (pH) and nitrogen and phosphorus content. The optimal temperature for biodegradation processes to occur is 25-30ºС. Great value for the life of microorganisms has a reaction of the environment, which affects the course of enzymatic processes in the cell, as well as changes in the degree of penetration of nutrients into the cell. For most bacteria, a neutral or slightly alkaline reaction environment is favorable. At pH<6 развитие и жизнедеятельность микробов чаще всего снижается, при рН <4 в некоторых случаях их жизнедеятельность прекращается. То же самое наблюдается при повышении щелочности среды до рН>9,5.

Cleaning processes include: mechanical sedimentation of suspended matter, biological or chemical oxidation of organic and other pollutants by their mineralization and precipitation; chemical processes involving oxygen, neutralization of heavy metals and similar pollutants; absorption of various pollutants by bottom sediments and aquatic vegetation and other similar processes.

The process of self-purification from non-conservative pollutants is accompanied by the consumption of oxygen for the mineralization of organic substances and the dissolution of oxygen coming from the surface of the water surface, the so-called reaeration.

The process of oxygen consumption is characterized by the equation

Lg(VA,) = ~*it, (1.9)

WhereL-a- BOD total at the initial moment of the oxygen consumption process, mg/l;L,-BODtotal over time{, mg/l;To\- oxygen consumption constant (BOD) at a given water temperature;t-time during which the processes of oxygen consumption and reaeration take place, days.

The solubility of oxygen in water is relatively limited, therefore, due to its low content in water, the intensity of oxidative processes decreases. Also, the intensity of oxidative processes is influenced by the initial oxygen content in water and the intensity of its replenishment from the air through the water surface as it is spent on oxidation.

The process of oxygen dissolution is characterized by the equation Lg(D t /DJ = -k 2 t, (1.10)

WhereD. a- deficit of dissolved oxygen at the initial moment of observation, mg/l;D t -the same after time /, mg/l; /с 2 - oxygen reaeration constant at a given water temperature.

Taking into account the simultaneous occurrence of both processes in mutually opposite direction, the final rate of change in oxygen deficiency over time t can be expressed by the equation

4=AA(South‘"-102- a)/(* 2 -TO )+ A- 1<¥ й. (1.11)

Equating to zero the first derivative of equation (1.11) with respect to tCan get expression for tKp, corresponding to the minimum oxygen content in water:

"cr = log((*2/*i))