Self-Purification of Water Bodies

SELF-PURIFICATION OF WATER BODIES. Water bodies possess the property of gradually purifying themselves from contaminants that have entered them under the influence of natural factors: suspended particles, bacteria, dissolved organic and inorganic substances. The mechanism of self-purification of water bodies from organic pollution consists of: 1) sorting of solid particles by their specific weight (their settling to the bottom), 2) distribution of pollution in the mass of water body water, which leads to closer contact of the pollution with dissolved O₂ in the water, which is one of the essential agents in the process of mineralization of organic matter, 3) biochemical processes of destruction of organic substances as a result of the vital activity of bacteria and other representatives of the flora and fauna of the water body, mainly their lower forms, and 4) chemical processes of exchange and oxidation of decomposition products of organic substances. As a result of biochemical processes of decomposition, organic matter is destroyed and gives a series of final compounds-free carbonic acid and its salts, nitrogenous, sulfuric, and phosphoric compounds, which are subsequently involved in the cycle of substances by the plant population and microbes of the water body. Factors that reduce the bacterial content in water include: 1) sedimentation of them during the settling of suspended particles in water to the bottom; 2) dilution of water by incoming masses of cleaner water; 3) death of bacteria under the influence of direct sunlight on them; 4) general decrease in water of organic substances that are nutritious for bacteria; and 5) consumption of bacteria by Protozoa. For example, the survival of cholera vibrio in river water ranges from 5 to 20 days, Bact. coli-from 6 to 18 days. To determine the degree of water self-purification, from bacteriological methods, determination of the total number of microbes and the titer of colon bacillus, as well as Bact. proteus vulgaris and Streptococcus as companions of the colon bacillus are used. The role of Protozoa in bacterial self-purification of water is evidenced by the experiments of Shepilevsky, noting the most intensive clarification of bacterial suspensions during their reproduction in Protozoa. Among Protozoa, colorless Flagellata play a significant role in the bacterial self-purification of water, the development curve of which in rivers, after the introduction of pollutants into them, repeats the development curve of bacteria with a shift of it downstream, as can be seen from the data on the study of the Oka River below the mouth of the Nara River (Kononov). In the Oka River, along its left bank, from the side of which the Nara River flows into the Oka River, receiving industrial and domestic waters of the city of Serpukhov, the content of bacteria and colorless Flagellata is expressed in the following quantities in 1 cm³ of water. Sampling location Number of bacteria Number of colorless Flagellata Oka River, 4 km above the mouth of the Nara River .......... Oka River, 0.5 km below the mouth of the Nara River......... 6,200-8,942 8,400 5,992 5,532 3,981 3,021 189 38-189 2 2U 2,740 2,098 1,021 Oka River, 6.0 km below the mouth of the Nara River Oka River, 13.0 km below the mouth of the Nara River Oka River, 22.5 km below the mouth of the Nara River Oka River, 30.5 km below the mouth of the Nara River......... The role of colorless Flagellata in the process of bacterial self-purification is also confirmed by the observation made by Kononov on the water of the Moscow River canal (Fig. 2) and Horowitz's experiments with broth cultures of Bact. coli with and without the presence of Flagellata in them. Protozoa most intensively consume those bacterial species that are not normal inhabitants of water, namely pathogenic microorganisms and among them cholera vibrio, typhoid, colon, blue pus bacilli, etc. When a water body is polluted, the composition of water in it changes, the flora and fauna of the water body also change, but subsequently, as a result of self-purification processes, the normal picture of the water body gradually restores itself. In the zone of maximum pollution (polysaprobic zone), the river is characterized by a high content of fresh organic substances. This zone is poor in dissolved O₂. It is inhabited by heterotrophic organisms (feeding on dissolved and suspended organic substances in water). Downstream, in the mesosaprobic zone, there is already a smaller amount of fresh organic substances compared to the polysaprobic zone, due to their partial decomposition. In this zone, a number of intermediate products of organic matter decomposition are present. Along with heterotrophic organisms, it is also inhabited by mixotrophic organisms (capable of utilizing both organic substances and nitrogenous products of their decomposition) and autotrophic (organisms with mineral nutrition). The mesosaprobic zone in turn is divided into α-mesosaprobic and β-mesosaprobic zones, of which the first approaches the polysaprobic zone in terms of pollution level, the second-to the subsequent oligosaprobic zone. The β-mesosaprobic zone is characterized by a high content of mineral nitrogenous compounds and, as a result of this, differs in the lush development of plant plankton with autotrophic nutrition. It is characteristic for the β-mesosaprobic zone in summer during daylight hours a sharp increase in the content of dissolved O₂ due to the processes of photosynthesis of free carbonic acid. Even further downstream, in the oligosaprobic zone, the river is already freed from organic nitrogen-containing substances; its plant plankton is represented by organisms with autotrophic nutrition, which, due to the already small content in water of nitrogenous products of organic matter decomposition, consumed in the previous β-mesosaprobic zone by plant plankton, give here a smaller production compared to the β-mesosaprobic zone. The content of dissolved O₂ in this zone corresponds to its absorption by water from the air. Fig. 1 shows an example of the course of river purification processes. The effectiveness of natural self-purification processes is directly dependent on the dissolved O₂ in water and the degree of settlement of the water body by plant and animal organisms, including microbes. When discharging wastewater into public water bodies with the expectation of their natural self-purification, it is necessary to consider the capacity of the water body, the ratio of the quantity and quality of water in the water body with the quantity and quality of the discharged wastewater, especially the need for oxygen for the biochemical breakdown of organic substances in the wastewater and the content of dissolved O₂ in the water body. Based on the minimum allowable reduction in dissolved O₂ in water by 4 mg per 1 liter of water according to NKZdr. standards, when calculating the discharge of wastewater into a river, the formula _Q(g-4) is adopted, where L is the biochemical oxygen demand of wastewater, Q is the water flow in the river, g is the flow of wastewater discharged into the river, a is the content of dissolved O₂ in the river water. When considering the question of the point where the self-purification of the river can be expected to end after the discharge of pollutants into it, it is always necessary to consider, along with other factors, also the flow rate of the river, with which the mechanical transport of pollution downstream and the process of mixing the wastewater stream with river water are associated. The study of this issue showed that in individual rivers, when wastewater is discharged into them or when tributaries flow into them, the observed heterogeneity of the river water composition below the discharge or tributary can persist over a long distance. Thus, the mixing of the stream of heavily polluted Yauza River with the water of Moscow River occurs 2-3 km below the mouth of the Yauza River along the course of Moscow River; the mixing of the stream of heavily polluted Tmaka River (in the city of Kalinin, former Tver) with the water of the Volga River ends only 11 km below the mouth of the Tmaka River along the course of the Volga River; the mixing of industrial-fecal wastewater of the city of Orekhovo-Zuevo with the water of the Klyazma River ends in the Klyazma River 10 km below the wastewater; the Volga River after the confluence of the Oka River into it for 180 km from the city of Gorky to the city of Vasily-Sursk has a heterogeneous water composition due to the incomplete mixing of the Oka River stream with the water of the Volga River. The seasons of the year have a great influence on the processes of natural self-purification.

In the winter season, when life in the water body comes to a standstill, biochemical processes manifest themselves to a weak degree; in this same season of the year, due to the presence of an ice cover, the reaeration of the water body is disrupted-all this has as its consequence the pollution of rivers when organic pollutants are discharged into them in the winter season over a greater extent compared to the summer.