Creation of Artificial Aeration System to Improve Water Quality in Reservoirs

Abstract

Hydroelectric power plants are widely used around the world, particularly in the countries of Central and South America. In Russia, there are more than 15 large hydroelectric power plants, which form the backbone of the country’s energy sector, providing about 20% of its energy needs. The construction and operation of these plants take a long time, and it is important to plan carefully and minimize environmental damage during their use. The most significant factors affecting the environmental condition of reservoirs is the low oxygen content and the impossibility of water self-purification due to low water turbulence in deep layers. Coastal erosion caused by large hydroelectric dams can lead to increased land and population destruction, as well as sedimentation in reservoirs. The objective of this review was to select a method that would enhance the quality of water in the reservoirs of hydroelectric power stations. The technical solution that has been proposed is the implementation of the aeration of the reservoir and the cleaning of the aquatorium from sediments, with the aim of compensating for the damage caused by the construction of the dam.

1. Introduction

In order to provide energy for the operation and development of many mining and metallurgical enterprises [1,2], such as aluminum plants, chemical plants and mining and processing facilities, hydroelectric power plants (HPPs) were built in the 1920s and 1930s.

Large hydroelectric power plants play a significant role in providing electricity [3]. In the European Union, hydroelectricity is considered an environmentally friendly form of energy and has a virtually carbon-neutral footprint [4]. However, the construction and commissioning of hydroelectric power plants are associated with a number of environmental problems, such as the flooding of territories for reservoir construction, changes in the biodiversity of adjacent ecological niches, impacts on the microclimate of the area, and others [5,6,7]. Therefore, it is necessary to take into account that the use of hydroelectric power plants cannot be considered a completely environmentally friendly method of energy generation [8].

Riverbed and dammed hydroelectric power plants are the main types of hydroelectric power plants [9,10]. For their construction, it is necessary to create reservoirs that block off the natural flow of the river with dams, upstream of which the artificial flooding of arable lands and other land types is conducted. These two categories of hydropower plants are the most frequently employed for large rivers. The configuration of a run-of-river hydropower plant entails the presence of a hydropower building and a spillway structure, in addition to the dam. In the case of a channel HPP, the building with its hydraulic units serves as an extension of the dam, and together with it creates the pressure front. The head height of channel HPPs typically ranges from 30 to 40 m. In the case of large, flat rivers, the main channel is usually overtopped by an earthen dam, adjoined with a concrete spillway dam, on which the HPP building is constructed [11].

The construction of dams and hydroelectric power plants also has a significant impact on water quality, as it changes the natural flow of the river [12].

The preparation of woodlands as a future location of a reservoir is one of the factors that allows for relatively improved water quality compared to unplanned flooding. Since pre-training requires capital financial expenditures, the work is not always performed to the required extent. When land is flooded, the main factor affecting the water quality in the resulting reservoir is organic matter—forests, peatlands and the quality and composition of soil cover [13]. Peatlands are a specific source of sediment formation in the bottom layers of the reservoir. When territories are flooded, peat slurry is washed out of the soil and participates in the formation of sediments, as well as serves as a source of anaerobic microorganisms and as their nutrient medium. In bottom sediments, the proportion of microorganisms in reservoirs of the temperate climate zone is 1–6% of the total amount of sediments [14].

The alteration in the biological condition of water is attributed to the nascent processes of the decomposition and disintegration of polysaccharides and organic acids. The existence of these substances in water presents a conducive environment for the vigorous proliferation and thriving of microorganisms, resulting in an increase in biomass within the ecosystem. Among the prominent contributors to this phenomenon are blue-green algae, which contribute to the acceleration of the eutrophication process in water bodies through their production of organic matter [15,16,17].

It is also necessary to take into account the change in water flow, compared with the unchanged river flow; the flow of a river without obstacles allows one to control the bottom sediments of organic matter and keep them at a constant level [18,19]. However, during the construction of a dam, their content increases markedly, especially in the wall layer of the dam, where the pressure of the water layers increases, and, due to the laminar flow and low turbulence of the flows in these layers, oxygen exchange is disrupted and the ability to self-purify decreases due to the constant mixing of water masses. As indicated by the oxygen quantification reports in routine analyses conducted by environmental departments, a satisfactory freshwater saturation level is defined as an oxygen content of 5 ppm or higher, as observed for natural inhabitants. On average, depending on the conditions for life support, levels between 3 and 5 ppm are recorded. A low oxygen level is defined as a level below this range [20,21]. The oxygen content in such places can decrease from 5–9 mg/L to 3.5 mg/L or less, which affects the condition of fish populations and other inhabitants living in rivers. Oxygen deficiency and an increase in the population of blue-green algae are also observed in the Kuibyshev reservoir, despite active use, the flow of water and the discharge of water streams [22,23].

During the operation of reservoirs of large hydroelectric power plants, an urgent problem is the erosion of their shores, due to the fact that dispersed rocks are eroded under the influence of floods and seasonal changes in river flows. This leads to the destruction of animal habitats and the inability to use these places for agriculture, and forestry and nature conservation lands are submerged under water. The above-mentioned problem exists in all reservoirs, the shores of which are not provided with the necessary reinforcement, for example, the Nizhnekamsk reservoir of the local hydroelectric power station [24].

Bottom sediments not only affect the water quality but also reduce the volume of a reservoir. The formation of silts occurs due to causes of biogenic origin, as a result of the vital activity of organisms, the death of higher plants from the surface of the water area, original plantations and vegetation, and peat deposits located on territories of the reservoir that had previously been flooded. A reservoir’s sediment sources are mainly surface runoff from the riverbed and rocks brought in as a result of the erosion of the banks and bottom. For example, the sedimentation rate of the Kuibyshev reservoir is 4 mm/year, and this rate is 7 mm/year in the Zain reservoir [25]. About 97% of the suspended solids entering a reservoir are deposited on its bottom, forming an underwater relief and reducing the water volume in the water area.

Since reservoirs are designed in such a way that the maximum depth of the water column is at the dam wall, and the minimum is along the banks of the riverbed, the heat exchange over its entire surface is characterized by unevenness. During periods of maximum heat exchange between the reservoir surface and the atmosphere, the surface heats up rapidly, reducing its density. The surface with a lower density is located on top of the water column, which becomes colder and denser with depth. This prevents vertical mixing in the reservoir and causes a clear separation between the layers in the depths of the water area. Such a difference in densities aggravates the deposition of suspended particles and reduces the mixing capacity of water flows, which reduces the degree of self-purification of the reservoir in the spillway (Figure 1) [26].

The operation of a reservoir is closely related to changes in the hydrological regime of the water column and its ecological state. The inevitable siltation and deposition of sediments has a particularly pronounced effect on water quality and waterlogging in the shallow waters of a reservoir or at medium and small hydroelectric power plants. In stagnant conditions, anaerobic processes in the absence of oxygen occur with the release of methane, acetylene and other compounds that partially dissolve in water. At the same time, sulfides and sodium carbonates are formed, which leads to a change in the pH of the water, increasing it to 8–8.5, negatively affecting the fertility of soils irrigated with water from this source. Artificial reservoirs are actively susceptible to “blooming”—the spread of microalgae in the volume, due to a violation of the biocenosis in the riverbed. The active vital activity of bacteria that eat algae organic substances leads to oxygen deficiency in water, as it is consumed for oxidation processes, and, at the same time, the release of phenols, indoles and other waste products [27].

The changed chemical composition leads to the death of local river fauna—fish, crustaceans and mollusks [28,29]. Water quality is gradually deteriorating, and the eutrophication of reservoirs is worsening.

The aim of this research was to identify a method for enhancing water quality and preserving the hydrological equilibrium of river channels in major watercourses during the construction phase of hydroelectric power plants.

2. Materials and Methods

The research methodology involved the selection and examination of the relevant literature by using specific keywords and search filters (Figure 2). Sources were identified through various search engines, including Lens.org, Semantic Scholar and Google Scholar.

The scientometric data used in this study were obtained from Scopus, the largest interdisciplinary database of peer-reviewed scientific research, widely recognized and frequently used for scientometric analysis [30,31], and also Web of Science, PubMed and eLibrary. Using the topical search queries “aeration” OR “water turbulence” OR “reservoir”, 1,198,044 documents from 1850 to 2025 were retrieved. And using the topical search queries “aeration” AND “water turbulence” AND “reservoir”, 607 documents were retrieved for a more specific selection of suitable research sources.

For data representativeness, the sample was limited to article, book chapter, report and review publication types over the last 10 years.

In summary, the first data sample consisted of 475,727 publications, each of which was downloaded from the Scopus database and consisted of many different variables (e.g., authors, publication title, abstract, keywords, citations, references, etc.) (Figure 3). The second data sample consisted of 294 publications (Figure 3).

In addition to the traditional analysis of quantitative data of publications and citations, new, more comprehensive and informative formats for assessing scientific research are now becoming available that allow for the processing and visualization of very large amounts of data, such as advanced research mapping. Such maps are typically two- or three-dimensional visualizations of the scientific landscape, consisting of topics and disciplines united by cited publications and common terminology. The degree of similarity of documents determines how far apart they are on the map, and differences in the density of publications lead to the formation of different elements on the map, such as “mountains” and “islands” of knowledge. Mapping allows analysts to see individual scientists, organizations, grantors and journals on a map, and to assess the extent to which an organization is engaged in research in particular areas and how that engagement has changed over time. In this way, these maps allow for a better understanding of the current situation, as well as the identification of key players and the most in-demand or promising new research topics [30]. To create maps based on network data and to visualize and explore these maps, the software tool VOSviewer version 1.6.20 was used (Figure 4).

Keyword-based coincidence analysis can help in understanding the development of hydroelectric power station research.

3. Existing Technical Solutions

Among the existing methods of combating the siltation of reservoirs, streams, basins and channels, there are hydrological methods related to the operation of the reservoir (

Table 1. Comparative analysis of hydrological methods for controlling reservoir siltation [32,33,34].

Method	Cost	Duration	Effectiveness	Implementation Location
The maintenance of the operating modes of the dam, which create the possibility of the maximum transit of incoming solid runoff	Low	Ongoing	Moderate	Dam spillway and surrounding areas
The operation of channels during periods in which high-turbidity water enters them in a near constant mode with high water consumption	Moderate	During high turbidity	Moderate	Channels and associated infrastructure
The flushing of streams, reservoirs and water intake thresholds and the clarification of water in settling tanks	Moderate	Periodic (days to weeks)	High	Streams, reservoirs, water intakes and settling tanks
The use of shore-protecting and nano-supporting devices	High	Long term (years)	High	Shoreline and reservoir banks
The removal of sediments by mechanical means	High	Periodic (days to months)	High	Reservoir bottom, channels and water intakes
The daily actuation of currents to the lowest possible level (for reservoirs with daily regulation)	Low	Daily	Low	Reservoir outlet and associated control structures

) [32,33,34]:

• The maintenance of the operating modes of the dam, which create the possibility of the maximum transit of incoming solid runoff;
• The operation of channels during periods in which high-turbidity water enters them in a near constant mode with high water consumption;
• The flushing of streams, reservoirs and water intake thresholds and the clarification of water in settling tanks;
• The use of shore-protecting and nano-supporting devices;
• The removal of sediments by mechanical means;
• The daily actuation of currents to the lowest possible level (for reservoirs with daily regulation).

Sediment flushing is a widely known technique that has been used to clean reservoirs since the last century. It consists of lowering the water level to a dead volume level acceptable for normal operation. By lowering the level, the area of the living section of the riverbed decreases, as a result of which the flow rate increases, which leads to turbulence and the effective flushing of sediments and their transportation to the lower reaches. The main disadvantage of this method is the low efficiency of flushing already deposited sediments and the preservation of sediments and low degree of turbulence in the bottom layers near the dam wall, with flushing not improving water quality in this volume. This method allows for the getting rid of freshly deposited sediments in shallow parts of the water area [35,36]. There are modern variations of this method that allow for the discharge of water of an acceptable turbidity due to a stepwise gradual lowering of the level, in order to get rid of blockages of the lower stream and the high turbidity of the water downstream.

The mechanical cleaning of bottom sediments, which affect the waterlogging of a reservoir and reduce its volume, is used in Switzerland [37]. The use of installations with a complex design makes it possible to transport pulp by an underwater device to the level of the upper stream and carry out its subsequent flushing. Among the disadvantages are the bulkiness of the design, low productivity and high capital costs, as a result of which the system is not widely used.

When designing new hydroelectric power plant reservoirs, the idea of transferring the flow and suspended solids in it may be applied in order to allow the bulk of the solid runoff to be deposited in a specially designed two-chamber sump [38]. The sump chamber is mounted in the reservoir bed, and it is necessary to establish a channel with a length of 600–800 m for the rectilinear movement of the bulk of suspended solids in the stream [39] while preventing sediment from spreading across the width of the reservoir bowl in the area of its useful volume, as a result of which the useful volume of the reservoir bowl is protected from sediments. All of this leads to an increase in the service life of the reservoir and the prevention of waterlogging in the bottom layers of the water column.

4. Application of Aerators

Among the factors considered that affect the ecosystems of rivers and reservoirs built for the operation of hydroelectric power plants, the state of the water quality and the gradual waterlogging of water areas are particularly important, since it is not possible to change the design features of already built facilities at the moment.

One of the most promising solutions is the method of increasing aeration in the reservoir to create and maintain the required oxygen level in the water [40,41]. Aerators of various types can be used for such purposes [42,43,44]. Hydroelectric power plants use aeration devices for the intake and exhaust of air in pressure ducts and in pipes, slots or valves, and they are necessary to prevent a shock wave in the turbine duct. Such devices do not ensure the penetration of oxygen into the wall-mounted water layers of the dam.

Aerators are currently used in fisheries, but they could also be used in reservoirs for the same purpose [45,46]. Their implementation would require materials and structures that can withstand high-pressure loads, since they are most needed in the bottom layers of the reservoir. The passage of oxygen through the laminar layers will contribute to the turbulence of the flow, which makes it possible to increase the degree of self-purification of water bodies by increasing the degree of mixing [47]. The presence of a higher oxygen concentration in bottom sediments will reduce the rate of putrefaction processes and hence the accumulation of organic nutrients, which in turn contribute to an increase in blue-green algae populations and the rate of waterlogging.

Due to the excess of electricity generated at HPPs, it becomes possible to efficiently use power pumps (compressors) in aerators. An increase in oxygen concentration in water due to the use of aerators would also affect populations of migrating fish, and therefore, fish passages and fish screens at hydroelectric power plants are currently mainly used across the world to maintain their natural migration paths.

A variety of techniques are currently employed for the process of aeration in sizable water bodies: Tailwater Aeration [48], Hypolimnetic Aeration [49], Surface Water Aeration [50] and Selective Withdrawal. Reservoir aeration solutions can exist in several forms, such as pressure-free and mechanical aerators. At the moment, aeration solutions for small reservoirs, ponds and reservoirs of fishing grounds are on the market, so the main difficulty is scaling such devices for use in large hydroelectric power plants.

To install a pressure-free aerator, a complete set of the following is required: a water lifting pipe, an air pipe and a discharge pipe. The water lifting pipe at the lower end has a water intake nozzle and a confuser with a guide pipe at the upper end. An air pipe connects the perforation to the atmosphere. The air from the atmosphere is ejected by the high-speed flow of water and mixed with it. The air–water mixture is raised above the water level in the reservoir by vacuuming. The raised water–air mixture is sent through a discharge pipe under the level of the free surface of the water in the reservoir. This method and device allow the aeration of the bottom layers of the reservoir due to the energy of the environment (Figure 5). Since the average height of the dam wall can reach up to 300 m at its lowest part, where aeration is especially necessary, unpressurized aerators will not be able to provide such a maximum suction height, even with an installed water lifting pipe of the required size. Therefore, such installations can only be considered for use in shallow water areas of the reservoir, near the shores, where the processes of waterlogging and the deterioration of water quality are proceeding at a high rate.

Mechanical aerators are able to cope with the task of oxygenation in conditions of deep immersion at the base of the dam [51,52]. The proposed device is a complex system that includes a pontoon mounted on the surface of the reservoir and a discharge pipe lowered into the bottom layers of the reservoir, through which atmospheric air will be supplied to the depth of the dam. An air compressor is installed on the pontoon. Through the outlet pipe, the air passes to a depth where, through a switchgear with a swirl and distributor, the air is distributed into the thickness of the stream. In order for the proposed system to function properly, the design requires the inclusion of a compressor station that is powered by the hydropower plant itself. The installation of such a station is undoubtedly challenging due to the limited availability of utility infrastructure [53].

The described device allows oxygen to quickly dissolve in water due to the formation of small bubbles to a concentration of 5–7 mg/L, which is necessary for the optimal functioning of the reservoir ecosystem and maintaining the vital activity of local fauna.

The necessary models of aerators must be made of corrosion-resistant materials that can withstand high loads. Since every 10 m of the water column has a pressure equal to one atmosphere, the pressure at the bottom of the reservoir can be up to 25 atmospheres. Furthermore, during aeration in the lower layers, sand particles, corrosion products, carbonate deposits and algae, as well as silt, can disrupt the surface film during water flow and contribute to a two- to three-fold increase in metal degradation. For carbon steels, there exist critical concentrations of suspended matter (ranging from 150 to 600 mg/L) at elevated flow rates, beyond which metal resistance in the water is drastically reduced.

For example, to produce a model that provides optimal performance and withstands such loads, 12X18H10T steel can be used, which is resistant to various types of corrosion, has high mechanical strength, has high abrasive resistance and is easily amenable to any type of welding. According to international standards, this grade corresponds to several foreign designations: American AISI321 and AISI321H, German X10CrNiTi18-10 and X12CrNiTi18-9, French X6CrNiTi18-10 and Z10CNT18-10, Japanese SUS321, and Chinese 0Cr18Ni10Ti and 0Cr18Ni11Ti, among others. When examining the properties of steel options and their equivalents, the differences between AISI 321 and 12X18H10T become apparent. For instance, the tensile strength of AISI 321 is 515 MPa, whereas 12X18H10T exhibits superior resistance to mechanical loads with a tensile strength of 530 MPa. Additionally, the yield strength of the Russian steel is 235 MPa, compared to 205 MPa for the American option. Furthermore, AISI 321 has a relative elongation of up to 40%, while 12X18H10T demonstrates slightly lower ductility at 38% [54,55]. The switchgear mechanism is shown in Figure 6.

The outlet pipe for air supply to the depth must be made of a durable flexible material, for example, a dense layer of rubber, equipped with stiffeners made of a metal spring and forming its frame.

To ensure the aeration of the reservoir, it is necessary to select compressors that are optimal in terms of oxygen supply performance. With a useful reservoir volume of 30 km³ and the necessary increase in oxygen content by 4 g/m³, an effective dissolution of only 40% oxygen and a 21% oxygen content in the air, it would be necessary to pass 1.5 km³ of air per hour [56,57], which is practically impossible and unnecessary. When using aerators, the task is not to fully compensate for the consumption part of the oxygen balance of the water storage.

An approximate calculation of the oxygen balance for the entire reservoir over a long period of time can be carried out using the following formula [58]:

O₂ = (C₁ − C₂)⋅V/(N⋅24)

(1)

where O₂ is the oxygen consumption of the entire reservoir on average, g per 1 h;

• C₁ is the average oxygen content in the reservoir at the beginning of the billing period, g/m³;
• C₂ is the average oxygen content in the reservoir at the end of the billing period, g/m³;
• V is the volume of the reservoir, m³;
• N is the duration of the period in days (the time interval between measurement C₁ and C₂);
• 24—conversion from days to hours.

When the data are substituted into the calculation and it is assumed that the death of the inhabitants begins at an oxygen content of 1.0 mg/L, 1.5 mg/L already guarantees the survival of the fish and the prevention of waterlogging. It is also necessary to take into account that the turbulence of flows will allow for improved hydrological regimes and self-cleaning in the lower layers of the reservoir. We will obtain the required capacity of about 437 thousand m³/h of oxygen for compressors in the entire volume of the reservoir. Given the need for mechanical aerators to operate only near the dam wall in the bottom layers (we believe that the volume around the perimeter will be 15%), it is necessary to install industrial screw compressors with a capacity of 30,000 L/min in an amount of 25 units around the perimeter of the dam, since each aerator covers its diameter with an area of 400 m². The pressure developed by the compressor is 8 bar, and the power is 185 kW. The compressor is powered by the electrical network of the HPP transformer station, since it generates twice as much electricity as is used by consumers.

The capital cost of aeration systems in dams can vary significantly depending on several factors. These factors include the type of aeration system [59], the size of the dam and the specific needs of the project. In the proposed solution, some of the major costs will be associated with the purchase of equipment and the construction of a facility for installation. Other sources of costs will include the power supply for compressors, and the installation and collection of the system, as well as annual maintenance, inspections and repairs. Additionally, there may be costs associated with corrosion inhibitors. A factor that can influence the decision to install an aeration system at a dam is the availability of low-cost electricity generated locally to power the hydropower plant.

Overall, while the capital cost of installing an aeration system may be significant, careful consideration of the long-term benefits in terms of water quality management can lead to a more cost-effective solution.

5. Discussion

The use of aerators in reservoirs has a significant impact on various levels of the ecosystem, including the hydrological regime and the chemical composition of water and sediments, as well as on the biological diversity of aquatic organisms. This section discusses the potential effect of applying the proposed solution on each of these levels [60].

(1) The molecular level (the biogeochemical level focuses on changes in the cycles of substances (carbon, nitrogen, phosphorus)).

Disturbances in stability during the construction of reservoirs, resulting in the formation of standing water, have been shown to affect the microbiological level of the ecosystem. Consequently, it is imperative to maintain a balance between aerobic and anaerobic microorganisms. The alteration in the level of biomatter, and consequently the oxygen content, has been demonstrated to affect the functioning of destructive processes. Consequently, during the summer months, there is a substantial increase in the prevalence of anaerobic processes within water bodies, thereby regulating carbon cycling within these systems and facilitating the decomposition of organic matter through anaerobic respiration, utilizing sulfates or nitrates for this purpose [61,62]. Anaerobic bacteria play a pivotal role in maintaining the balance of the humification of organic matter by regulating the degradation of sludge to methane gas and decomposing complex organic molecules, such as proteins and lipids. The transformation process of suspended organic particles can be represented by the following equation [63]:

$${\mathrm{C}}_{\mathrm{x}}{\mathrm{H}}_{\mathrm{y}}{\mathrm{O}}_{\mathrm{z}}⟶\mathrm{C}{\mathrm{H}}_4+\mathrm{C}{\mathrm{O}}_2+\mathrm{bacterial} \mathrm{biomass}$$

(2)

As can be seen, as a result of these processes, methane and carbon dioxide are released, exacerbating the greenhouse effect. In this case, the resulting carbon dioxide dissolves in water to form carbonic acid, reacting with sodium sulfides (Na₂S), leading to the formation of sodium carbonates and bicarbonates, which increase the pH of water. An increase in the alkalinity of water (i.e., the formation of OH groups), a phenomenon that occurs as a result of the hydrolysis of sodium carbonate, leads to the disruption of the ecosystem of the reservoir. And the release of hydrogen sulfide, H₂S, not only creates an unpleasant odor in wetlands, but is also one of the causes of and a source of sulfur for the formation of acid rain.

$$\mathrm{N}{\mathrm{a}}_2\mathrm{S}+{\mathrm{H}}_2\mathrm{C}{\mathrm{O}}_3\to \mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{S}\uparrow ,$$

(3)

$${\mathrm{N}\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O}\to \mathrm{N}{\mathrm{a}}^{+}+\mathrm{O}{\mathrm{H}}^{-}+\mathrm{N}\mathrm{a}\mathrm{H}\mathrm{C}{\mathrm{O}}_3,$$

(4)

With a sufficient level of oxygen and aeration of water, the inhabitants of the fauna, who receive oxygen during respiration through the surface of the skin or gills, are provided with energy metabolism processes.

Aerobic microorganisms exist only in an oxygen-containing environment and completely break down organic matter to CO₂ and H₂O, while synthesizing their own biomass. The formula for this process is as follows:

$${\mathrm{C}}_{\mathrm{x}}{\mathrm{H}}_{\mathrm{y}}{\mathrm{O}}_{\mathrm{z}}+{\mathrm{O}}_2⟶{\mathrm{H}}_2\mathrm{O}+\mathrm{C}{\mathrm{O}}_2+\mathrm{bacterial} \mathrm{biomass},$$

(5)

When applying the proposed solution, which is primarily responsible for the oxygenation requirements of water bodies, oxygen-free rotting processes, which are suppliers of hydrogen sulfide and methane to the atmosphere, are reduced.

(2) Biotope (impact on local flora and fauna; landscape changes).

Sediments and the deposition of biogenic products, silts, at the bottom of the reservoir form a relief at its bottom, gradually reducing the useful volume of the reservoir. The thermocline caused by the low mixing of streams and the different densities of the heated upper layer of water and the cooled lower layer reduces the survival of fauna inhabitants. Cold-water fish such as salmon and trout are sensitive to changes in water temperature. Extreme temperature variations can be fatal for their populations [64]. Warm water tends to contain less dissolved oxygen, which is crucial for the health of the aquatic environment.

Due to the hydromorphism of soils near the shores of the reservoir, higher plants spread most actively over the surface of the water area, increasing the degree of waterlogging due to a constant cycle of dying and growth and an increase in biomass in these places.

The solution of using reservoir aeration ensures a uniform temperature gradient over the entire height of the reservoir, which allows its inhabitants to be in optimal living conditions (Figure 7) [65]. The water quality improves and the oxygen content in it increases, creating favorable conditions for the inhabitants of the fauna of the riverbed.

(3) Ecosystem of the catchment area as a whole (impact on biodiversity, migration of species).

The disturbance of the biocenosis of the riverbed during the construction of the reservoir primarily affects the microalgae population and the “blooming” of the reservoir [66]. The dominance of microalgae in the ecosystem disrupts the natural biodiversity of the reservoir—a recurring cycle of dying and overgrowth of the algae population makes it possible to develop a population of anaerobic bacteria in the bottom layers and aerobic microorganisms in shallow water. This kind of separation occurs due to the accumulation of nutrients in the layers of the reservoir. The phenomenon of competition for food sources in the upper layers, where the oxygen level is initially higher, gives an advantage to aerobic bacteria. Conversely, in the bottom layer, the decrease in oxygen prevents them from actively spreading, allowing anaerobic bacteria to occupy this niche, leading to waterlogging and a decrease in biodiversity in the riverbed [67].

The proposed solution ensures the lifting of suspended matter from the bottom of the reservoir and hinders the growth of microalgae and colonies of microorganisms, ensuring the competitiveness and survival of various species.

It is also worth noting that in certain instances, the resuspension of sediment during aeration predominantly exerts deleterious effects on hydrobionts. For instance, suspended particulate matter has been observed to occlude the gills of fish and the filtration apparatuses of invertebrates, thereby impeding respiration and feeding [68]. Nevertheless, this effect also has its positive aspects. For instance, some hydrobionts, such as detritivores, rely on organic matter present in bottom sediments as their primary source of food. Consequently, disturbance may temporarily enhance the availability of this nutrition source for these organisms [69,70].

Moreover, one frequently observed adverse consequence of aeration is Gas Bubble Trauma, which has been documented to affect both fish and local fauna. This phenomenon can further be exacerbated by bubble disease, characterized by the formation of gas bubbles within the tissues and bloodstream of fish, potentially leading to fatal outcomes. Consequently, in order to prevent oxygen oversaturation in water (concentrations exceeding 8 mg/L), it is essential to conduct calculations prior to installing aeration systems. The proposed solution does not aim to fully aerate reservoirs; it rather focuses on coastal areas using non-pressurized aerators, which are safe for use in fisheries. Additionally, the solution includes the aeration of bottom layers near dam walls [71,72].

(4) Hydrological level (analysis of changes in the hydrology of the catchment area, including changes in flow regimes, water levels, erosion, sedimentation and river flow).

When aerators are installed at the bottom of the reservoir bed, the hydrodynamics of the catchment basin change, and high-pressure flows that occur at the depth of the dam instead occur laminarly at low velocity at the base of the dam. This state and mode of fluid movement does not allow water masses to move, but only exacerbates the deposition and sedimentation of sediment and nutrients at the bottom of the reservoir. Aerators, especially mechanical ones, by swirling the air flow and bubbling it through the water column, turbulate the flow, not only saturating it with oxygen but also swelling the water flow, preventing precipitation from settling but keeping it in the upper layers and allowing the bulk of the solid phase to be carried downstream.

(5) The climatic level (changes in precipitation, air and water temperature and wind patterns).

The miscibility of water masses increases, and the temperature gradient along the height of the liquid layer decreases, as a result of which the upper layers at the surface of the reservoir’s water do not have time to heat up to the maximum temperature, the color of the water remains transparent, unlike the color of the water in waterlogging conditions, and heat absorption decreases [73]. On a climatic temperature map, such a reservoir would not be distinguished by an abnormal temperature compared to the river upstream.

(6) Regional level (socio-economic changes impact on the sustainable development of the region) [74].

First, the opportunity and creation of jobs in the modernization of the hydropower industry should be considered. Among the negative aspects, it is possible to single out the investment of capital costs for the development of necessary devices, capital costs for maintenance and the maintenance of devices [75]. On the other hand, the use of new equipment opens up the possibility of new patents, with uses not only within a single power plant but also distributed throughout the country. One of the conditions for the successful implementation of this project is the high-quality performance of the equipment and the fulfillment of proper standards for the content of oxygen and nutrients in the water. The social development of the region can be ensured by a new wave of tourists and the recognition of the reservoir in the UNESCO Heritage list, for example, the State Power Station No. 1 named after P.G. Smidovich.

6. Conclusions and Future Directions

The findings outlined in this article substantiate the feasibility of incorporating aerators within reservoirs. The proposed solution is capable of preventing the siltation of reservoirs during the operation of hydroelectric power plants, thereby enhancing the quality of water in the reservoir.

The main conclusions of this study and recommendations depend on several factors at once and are given as follows:

• The use of aerators as a solution for waterlogging and improving water quality in reservoirs is innovative and practical, as is any solution for integration into existing systems that requires capital investment. The possibility of using aerators is also ensured by the availability and cheapness of electricity generated at the site of the hydroelectric power station and directed to the needs of the reservoir itself;
• The proposed solution provides the necessary level of turbulence in the wall layers of water along the hydroelectric dam and allows for the saturation of the water with oxygen to a level at which the optimal mode of life of the inhabitants of flora and fauna is maintained (5 ppm or higher) and there is no waterlogging, siltation and uncontrolled development of populations of green algae;
• According to the results of the calculation of the necessary equipment for the implementation of the proposed solution at a large hydroelectric power plant, it is necessary to install 25 aeration units along the length of the dam per 1 km. To ensure the required air supply, air compressors with a capacity of 30,000 L/min have been selected. The durable corrosion-resistant steel 12X18H10T has been selected for the material design of the structural parts;
• While aerators are not the sole solution to the problems of reservoirs, they allow for improvement in the condition of the system in a comprehensive manner, together with pressure-free aerators and hydraulic sediment flushing, to ensure the preservation of the useful volume of the reservoir.

Further investigations are scheduled to conduct pilot studies and validate the efficacy of aeration systems in reservoirs at hydroelectric power plants. At the initial stage, the establishment of a laboratory facility for testing is to be planned. The measurement of the following indicators is to be conducted: dissolved oxygen concentration, water transparency, and temperature in the lower layers of the reservoir. The second stage is planned to be the translation of the experiment into hydroelectric power plants.