The ability of surface water to self-purify. Patterns of self-purification of water in water bodies

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-cleaning water systems caused by 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, evaporation is an important process.

Among the chemical factors of 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, photodecomposition of certain chemicals, such as DDT, occurs in the surface layers of the reservoir, and water is disinfected - the death of pathogenic bacteria. Bactericidal effect ultraviolet rays 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.

Every body of water is complex living system, where bacteria, algae, higher aquatic plants, and various invertebrate animals live. 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 of heavy metals.

The process of biological water purification is related to the oxygen content in it. With a sufficient amount of oxygen, the activity of aerobic microorganisms that feed on organic substances manifests itself. When organic substances break down, they form carbon dioxide and water, as well as nitrates, sulfates, 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 occurs in environment, contamination of adjacent objects with it. 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 that are not involved in the 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, among which the most important are temperature, active reaction of the environment (pH) and the content of nitrogen and phosphorus. Optimal temperature for biodegradation processes to occur is 25-30ºС. Great importance 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.

Negative natural factors include the presence of steep slopes and flooded areas that are unstable to additional man-made load. Negative technogenic factors should be considered high clutter in certain areas, the influence of polluted and insufficiently treated wastewater from residential areas, industrial zones and enterprises, affecting the quality of water bodies. Consequently, the condition of the reservoirs does not meet the requirements for cultural and community facilities. In addition, excess air pollution along highways is typical for almost the entire territory.

II. Water bodies, being natural and natural-technogenic elements of landscape-geochemical systems, in most cases are the final link in the runoff accumulation of most of the mobile technogenic substances. In landscape-geochemical systems, substances from higher levels to lower hypsometric levels are transported with surface and underground runoff, and vice versa (from low to higher levels) by atmospheric flows and only in some cases by flows of living matter (for example, during a massive escape from reservoirs of insects after completion of the larval stage of development taking place in water, etc.).

Landscape elements representing the initial, most highly located links (occupying, for example, local watershed surfaces) are geochemically autonomous and the intake of pollutants into them is limited, with the exception of their entry from the atmosphere. Landscape elements that form lower stages of the geochemical system (located on slopes and in depressions of the relief) are geochemically subordinate or heteronomous elements that, along with the input of pollutants from the atmosphere, receive part of the pollutants that come with surface and groundwater from higher-lying parts of the landscape -geochemical cascade. In this regard, pollutants formed in the catchment due to migration into natural environment sooner or later they enter water bodies mainly with surface and groundwater runoff, gradually accumulating in them.

5 Basic processes of self-purification of water in a water body

Self-purification of water in reservoirs is a set of interconnected hydrodynamic, physico-chemical, microbiological and hydrobiological processes leading to the restoration of the original state of a water body.

Among the physical factors, dilution, dissolution and mixing of incoming contaminants are of paramount importance. Good mixing and reduction of suspended particle concentrations is ensured fast current rec. The self-purification of reservoirs is facilitated by the settling of insoluble sediments to the bottom, as well as the settling of polluted waters. In zones with a temperate climate, the river cleans itself after 200-300 km from the place of pollution, and in the Far North – after 2 thousand km.

Water disinfection occurs under the influence of ultraviolet radiation from the sun. The disinfection effect is achieved by the direct destructive effect of ultraviolet rays on protein colloids and enzymes of the protoplasm of microbial cells, as well as spore organisms and viruses.

Among the chemical factors of self-purification of reservoirs, oxidation of organic and inorganic substances should be noted. The self-purification of a reservoir is often assessed in relation to easily oxidized organic matter or by the total content of organic matter.

The sanitary regime of a reservoir is characterized primarily by the amount of oxygen dissolved in it. It should be at least 4 mg per 1 liter of water at any time of the year for reservoirs of the first and second types. The first type includes reservoirs used for drinking water supply to enterprises, the second type includes those used for swimming, sporting events, as well as those located within settlements.

Biological factors of self-purification of a reservoir include algae, mold and yeast. However, phytoplankton does not always have a positive effect on self-purification processes: 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, the oyster and some other amoebas adsorb intestinal and other viruses. Each mollusk filters more than 30 liters of water per day.

The cleanliness of water bodies is unthinkable without protecting their vegetation. Only on the basis of deep knowledge of the ecology of each reservoir and effective control over the development of the various living organisms inhabiting it can positive results be achieved, transparency and high biological productivity of rivers, lakes and reservoirs ensured.

Other factors also adversely affect the self-purification processes of water bodies. Chemical pollution of water bodies with industrial wastewater, nutrients (nitrogen, phosphorus, etc.) inhibits natural oxidative processes and kills microorganisms. The same applies to the descent of thermal Wastewater thermal power plants.

A multi-stage process, sometimes extending over a long time, is self-purification of oil. Under natural conditions, the complex of physical processes of self-purification of water from oil consists of a number of components: evaporation; settling of lumps, especially those overloaded with sediment and dust; sticking together of lumps suspended in the water column; floating of lumps forming a film with inclusions of water and air; reducing the concentrations of suspended and dissolved oil due to settling, floating and mixing with clean water. The intensity of these processes depends on the properties of a particular type of oil (density, viscosity, coefficient of thermal expansion), the presence of colloids, suspended and transportable plankton particles, etc. in water, air temperature and solar illumination.

6 Measures to intensify the processes of self-purification of a water body

Self-purification of water is an indispensable link in the water cycle in nature. Pollution of any type during self-purification of water bodies ultimately turns out to be concentrated in the form of waste products and dead bodies of microorganisms, plants and animals that feed on them, which accumulate in the silt mass at the bottom. Water bodies in which the natural environment can no longer cope with incoming pollutants are degraded, and this occurs mainly due to changes in the composition of biota and disruptions in food chains, primarily the microbial population of the water body. Self-cleaning processes in such water bodies are minimal or stop completely.

Such changes can only be stopped by purposefully influencing factors that contribute to reducing the generation of waste and reducing pollution emissions.

This task can be solved only by implementing a system of organizational measures and engineering and reclamation work aimed at restoring the natural environment of water bodies.

When restoring water bodies, it is advisable to begin the implementation of a system of organizational measures and engineering and reclamation work with the arrangement of the catchment area, and then carry out the cleaning of the water body, followed by the development of coastal and floodplain areas.

The main objective of the ongoing environmental protection measures and engineering and reclamation work in the catchment area is to reduce the generation of waste and prevent unauthorized discharge of pollutants onto the topography of the catchment area, for which the following activities are carried out: implementation of a system for regulating waste generation; organization of environmental control in the system of production and consumption waste management; conducting an inventory of facilities and locations for production and consumption waste; reclamation of disturbed lands and their improvement; tightening of fees for unauthorized discharge of pollutants onto the terrain; introduction of low-waste and non-waste technologies and recycling water supply systems.

Environmental protection measures and work carried out in coastal and floodplain areas include work on leveling the surface, leveling or terracing slopes; construction of hydraulic engineering and recreational structures, strengthening of banks and restoration of stable grass cover and tree and shrub vegetation, which subsequently prevent erosion processes. Landscaping work is carried out to restore the natural complex of a water body and transfer most of the surface runoff into the underground horizon for the purpose of its purification, using rocks of the coastal zone and floodplain lands as a hydrochemical barrier.

The banks of many water bodies are littered, and the waters are polluted with chemicals, heavy metals, petroleum products, floating debris, and some of them are eutrophicated and silted. It is impossible to stabilize or activate self-purification processes in such water bodies without special engineering and reclamation intervention.

The goal of carrying out engineering and reclamation measures and environmental protection work is to create conditions in water bodies that ensure the effective functioning of various water purification structures, and to carry out work to eliminate or reduce the negative impact of sources of distribution of pollutants of both off-channel and river-bed origin.

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))