Wastewater treatment of energy enterprises. Waste water from energy enterprises. Wastewater treatment methods for enterprises

wastewater mechanical treatment

Wastewater discharged from the territory of industrial enterprises can be divided into three types according to its composition:

industrial - used in the technological process of production or obtained during the extraction of minerals (coal, oil, ores, etc.);

household - from sanitary facilities of industrial and non-industrial buildings and buildings;

atmospheric - rain and from melting snow.

Contaminated industrial wastewater contains various impurities and is divided into three groups:

polluted mainly with mineral impurities (enterprises of the metallurgical, machine-building, ore and coal mining industries);

contaminated mainly with organic impurities (meat, fish, dairy and food, chemical and microbiological industries, plastics and rubber plants);

contaminated with mineral and organic impurities (oil producing, oil refining, petrochemical, textile, light, pharmaceutical industries).

By concentration pollutants, industrial wastewater is divided into four groups:

1 - 500 mg/l;
500 - 5000 mg/l;
5000 - 30,000 mg/l;

more than 30,000 mg/l.

Industrial wastewater may vary on physical properties polluting their organic products (for example, by boiling point: less than 120, 120 - 250 and more than 250 ° C).

According to the degree of aggressiveness these waters are divided into slightly aggressive (slightly acidic with pH=6h6.5 and slightly alkaline pH=8h9), highly aggressive (strongly acidic with pH6 and strongly alkaline with pH>9) and non-aggressive (with pH=6.5h8).

Uncontaminated industrial wastewater comes from refrigeration, compressor and heat exchangers. In addition, they are formed during the cooling of the main production equipment and products.

At different enterprises, even with the same technological processes, the composition of industrial wastewater is very different.

To develop a rational scheme for water disposal and assess the possibility of reusing industrial wastewater, their composition and mode of water disposal are studied. At the same time, the physico-chemical indicators of wastewater and the mode of entry into the sewer network of not only the general runoff of an industrial enterprise, but also wastewater from individual workshops, and, if necessary, from individual devices, are analyzed.

In the analyzed wastewater, the content of components specific to this type of production should be determined.

The operation of thermal power plants is associated with the use of natural water and the formation of liquid waste, some of which, after processing, is sent to the cycle again, but the main amount of water consumed is removed in the form of effluents, which include:

Waste water from cooling systems;

Sludge, regeneration and washing waters of water treatment plants and condensate treatment plants;

Waste water from hydraulic ash removal systems (GZU);

Waters polluted with oil products;

Waste solutions after cleaning of stationary equipment and its conservation;

Water from washing convective surfaces of thermal power plants burning fuel oil;

Water from hydraulic cleaning of premises;

Rain and melt water from the territory of the power facility;

Waste water from dewatering systems.

The compositions and quantities of the listed effluents are different. They depend on the type and capacity of the main equipment of the TPP, the type of fuel used, the quality of the source water, the methods of water treatment, the perfection of operating methods, etc. Getting into watercourses and water bodies, wastewater impurities can change the salt composition, oxygen concentration, pH value, temperature, and others. water indicators that hinder the processes of self-purification of water bodies and affect the viability of aquatic fauna and flora. To minimize the impact of waste water impurities on the quality of surface natural waters, standards for maximum permissible discharges of harmful substances have been established, based on the conditions for not exceeding the maximum allowable concentrations harmful substances in the control section of the reservoir.

All listed types of wastewater from TPPs are divided into two groups. The first group includes effluents from the circulating cooling system (RCS), WLU and hydraulic ash removal (HZU) from operating thermal power plants, which are characterized by either large volumes or high concentrations of harmful substances that can affect the water quality of water bodies. Therefore, these effluents are subject to mandatory control. The remaining six types of TPP wastewater must be reused after treatment within the TPP or by agreement at other enterprises, or they may be pumped into underground layers, etc.

The water supply system has a significant impact on the amount and composition of industrial wastewater: the more water is used in the recycling cycle for technological needs in the same or other operations of a given or neighboring enterprise, the lower the absolute amount of wastewater and large quantity they contain contaminants.

The amount of industrial wastewater is determined depending on the productivity of the enterprise according to the aggregated norms of water consumption and water disposal for various industries.

During the operation of the TLU, wastewater is generated in the amount of 5–20% of the flow rate of the treated water, which usually contains sludge consisting of calcium and magnesium carbonates, magnesium, iron and aluminum hydroxide, organic substances, sand, and various salts of sulfuric and hydrochloric acids. Taking into account the known MPCs of harmful substances in water bodies, the effluents of the WLU should be properly treated before they are discharged.

State environment directly depends on the degree of purification of industrial wastewater from nearby enterprises. Recently, environmental issues have become very acute. Over the past 10 years, many new effective technologies for industrial wastewater treatment have been developed.

Treatment of industrial wastewater from different facilities can occur in one system. Representatives of the enterprise can agree with public utilities on the discharge of their wastewater into a common centralized sewer locality where it is located. To make this possible, a chemical analysis of effluents is preliminarily carried out. If they have an acceptable degree of pollution, then industrial wastewater will be discharged together with domestic wastewater. It is possible to pre-treat wastewater from enterprises with specialized equipment for the elimination of pollution of a certain category.

Standards for the composition of industrial effluents for discharge into the sewer

Industrial waste waters may contain substances that will destroy sewer lines and city treatment plants. If they get into water bodies, they will negatively affect the mode of water use and life in it. For example, if the MPC is exceeded, toxic substances will harm surrounding water bodies and, possibly, humans.

To avoid such problems, before cleaning, the maximum permissible concentrations of various chemical and biological substances are checked. Such actions are preventive measures for the proper operation of the sewer pipeline, the functioning of treatment facilities and environmental ecology.

Effluent requirements are taken into account during the design of the installation or reconstruction of all industrial facilities.

Factories should strive to operate on technologies with little or no waste. Water must be reused.

Wastewater discharged into the central sewer system must comply with the following standards:

BOD 20 must be less than the allowable value of the design documentation of the sewerage treatment plant;
drains should not cause failures or stop the operation of the sewerage and treatment plant;
wastewater should not have a temperature above 40 degrees and a pH of 6.5-9.0;
waste water should not contain abrasive materials, sand and chips, which can form sediment in sewerage elements;
there should be no impurities that clog pipes and grates;
drains should not have aggressive components that lead to the destruction of pipes and other elements of treatment stations;
wastewater should not contain explosive components; non-biodegradable impurities; radioactive, viral, bacterial and toxic substances;
COD should be less than BOD 5 by 2.5 times.

If the discharged water does not meet the specified criteria, then local wastewater pre-treatment will be organized. An example would be the treatment of wastewater from the galvanizing industry. The quality of cleaning must be agreed by the installer with the municipal authorities.

Types of industrial wastewater pollution

Water treatment should remove environmentally harmful substances. The technologies used must neutralize and dispose of the components. As can be seen, treatment methods must take into account the initial composition of the effluent. In addition to toxic substances, water hardness, its oxidizability, etc. should be controlled.

Each harmful factor (HF) has its own set of characteristics. Sometimes one indicator can indicate the existence of several WFs. All WFs are divided into classes and groups that have their own cleaning methods:

coarsely dispersed suspended impurities (suspended impurities with a fraction of more than 0.5 mm) - screening, sedimentation, filtration;
coarse emulsified particles - separation, filtration, flotation;
microparticles - filtration, coagulation, flocculation, pressure flotation;
stable emulsions - thin-layer sedimentation, pressure flotation, electroflotation;
colloidal particles - microfiltration, electroflotation;
oils - separation, flotation, electroflotation;
phenols - biological treatment, ozonation, activated carbon sorption, flotation, coagulation;
organic impurities - biological treatment, ozonation, activated carbon sorption;
heavy metals - electroflotation, settling, electrocoagulation, electrodialysis, ultrafiltration, ion exchange;
cyanides - chemical oxidation, electroflotation, electrochemical oxidation;
tetravalent chromium - chemical reduction, electroflotation, electrocoagulation;
trivalent chromium - electroflotation, ion exchange, precipitation and filtration;
sulfates - settling with reagents and subsequent filtration, reverse osmosis;
chlorides - reverse osmosis, vacuum evaporation, electrodialysis;
salts - nanofiltration, reverse osmosis, electrodialysis, vacuum evaporation;
Surfactants - activated carbon sorption, flotation, ozonation, ultrafiltration.

Types of wastewater

Effluent pollution is:

mechanical;
chemical - organic and not organic matter;
biological;
thermal;
radioactive.

In every industry, the composition of wastewater is different. There are three classes that contain:

inorganic pollution, including toxic ones;
organics;
inorganic impurities and organic matter.

The first type of pollution is present in soda, nitrogen, sulfate enterprises that work with various ores with acids, heavy metals and alkalis.

The second type is characteristic of oil industry enterprises, factories organic synthesis etc. There is a lot of ammonia, phenols, resins and other substances in the water. Impurities during oxidation lead to a decrease in oxygen concentration and a decrease in organoleptic qualities.

The third type is obtained in the process of electroplating. There are a lot of alkalis, acids, heavy metals, dyes, etc. in the drains.

Wastewater treatment methods for enterprises

Classical cleaning can occur using various methods:

removal of impurities without changing their chemical composition;
modification of the chemical composition of impurities;
biological cleaning methods.

Removal of impurities without changing their chemical composition includes:

mechanical cleaning using mechanical filters, settling, filtering, flotation, etc.;
at a constant chemical composition, the phase changes: evaporation, degassing, extraction, crystallization, sorption, etc.

The local wastewater treatment system is based on many treatment methods. They are selected for a certain type of wastewater:

suspended particles are removed in hydrocyclones;
fine impurities and sediment are removed in continuous or batch centrifuges;
flotation plants are effective in removing fats, resins, heavy metals;
gaseous impurities are removed by degassers.

Wastewater treatment with a change in the chemical composition of impurities is also divided into several groups:

transition to sparingly soluble electrolytes;
the formation of fine or complex compounds;
decay and synthesis;
thermolysis;
redox reactions;
electrochemical processes.

The effectiveness of biological treatment methods depends on the types of impurities in the effluent, which can accelerate or slow down the destruction of waste:

the presence of toxic impurities;
increased concentration of minerals;
biomass nutrition;
structure of impurities;
biogenic elements;
environment activity.

In order for industrial wastewater treatment to be effective, a number of conditions must be met:

Existing impurities must be biodegradable. Chemical composition wastewater affects the rate of biochemical processes. For example, primary alcohols oxidize faster than secondary ones. With an increase in oxygen concentration, biochemical reactions proceed faster and better.
The content of toxic substances should not adversely affect the operation of the biological installation and treatment technology.
PKD 6 also should not disrupt the vital activity of microorganisms and the process of biological oxidation.

Stages of wastewater treatment of industrial enterprises

Wastewater treatment takes place in several stages using different methods and technologies. This is explained quite simply. It is impossible to carry out fine purification if coarse substances are present in the effluents. In many methods, limiting concentrations are provided for the content of certain substances. Thus, wastewater must be pre-treated before the main treatment method. The combination of several methods is the most economical in industrial enterprises.

Each production has a certain number of stages. It depends on the type of treatment plant, treatment methods and composition of wastewater.

The most appropriate way is a four-stage water treatment.

Removal of large particles and oils, neutralization of toxins. If the wastewater does not contain this type of impurities, then the first stage is skipped. It is a pre-cleaner. It includes coagulation, flocculation, mixing, settling, screening.
Removal of all mechanical impurities and preparation of water for the third stage. It is the primary stage of purification and may consist of settling, flotation, separation, filtration, demulsification.
Removal of contaminants up to a certain predetermined threshold. Secondary processing includes chemical oxidation, neutralization, biochemistry, electrocoagulation, electroflotation, electrolysis, membrane cleaning.
Removal of soluble substances. It is a deep cleaning - activated carbon sorption, reverse osmosis, ion exchange.

Chemical and physical composition defines a set of methods at each stage. It is allowed to exclude some stages in the absence of certain contaminants. However, the second and third stages are mandatory in the treatment of industrial wastewater.

If you comply with the listed requirements, then the disposal of wastewater from enterprises will not harm the ecological situation of the environment.

The operation of thermal power plants is associated with the use of large amounts of water. The main part of the water (more than 90%) is consumed in the cooling systems of various devices: turbine condensers, oil and air coolers, moving mechanisms, etc.

Wastewater is any stream of water that is removed from a power plant cycle.

Waste or waste water, in addition to water from cooling systems, includes: waste water from hydraulic ash recovery systems (GZU), spent solutions after chemical washing of thermal power equipment or its conservation: regeneration and sludge water from water treatment (water treatment) plants: oil-contaminated effluents, solutions and suspensions, arising from washing of external heating surfaces, mainly air heaters and water economizers of boilers burning sulfurous fuel oil.

The compositions of the listed effluents are different and are determined by the type of thermal power plant and the main equipment, its power, type of fuel, composition of the source water, the method of water treatment in the main production and, of course, the level of operation.

The water after cooling the condensers of turbines and air coolers, as a rule, carries only the so-called thermal pollution, since their temperature is 8 ... 10 С higher than the temperature of the water in the water source. In some cases, cooling waters can also introduce foreign matter into natural water bodies. This is due to the fact that the cooling system also includes oil coolers, the violation of the density of which can lead to the penetration of petroleum products (oils) into the cooling water. Oil-fired thermal power plants generate wastewater containing fuel oil.

Oils can also get into wastewater from the main building, garages, open switchgears, and oil farms.

The amount of water in the cooling systems is determined mainly by the amount of exhaust steam entering the turbine condensers. Consequently, most of these waters are at condensing thermal power plants (CPPs) and nuclear power plants, where the amount of water (t/h) cooling the turbine condensers can be found by the formula Q=KW where W- plant power, MW; To-coefficient, for TPP To= 100...150: for NPP 150...200.

In power plants using solid fuels, the removal of significant amounts of ash and slag is usually carried out hydraulically, which requires a large amount of water. Up to 4000 t/h of this fuel is burned at the TPP with a capacity of 4000 MW, operating on Ekibastuz coal, and about 1600...1700 t/h of ash is formed. To evacuate this quantity from the station, at least 8000 m 3 /h of water is required. Therefore, the main direction in this area is the creation of circulating gas storage systems, when the clarified water freed from ash and slag is sent back to the thermal power plant to the gas storage system.

Discharge waters of the GZU are significantly polluted with suspended solids, have increased mineralization and, in most cases, increased alkalinity. In addition, they may contain compounds of fluorine, arsenic, mercury, vanadium.

Effluent after chemical washing or conservation of thermal power equipment is very diverse in composition due to the abundance of washing solutions. For washing, hydrochloric, sulfuric, hydrofluoric, sulfamic mineral acids, as well as organic acids: citric, orthophthalic, adipic, oxalic, formic, acetic, etc. are used. Along with them, trilon B, various corrosion inhibitors, surfactants, thiourea, hydrazine, nitrite, ammonia.

As a result chemical reactions various organic and inorganic acids, alkalis, nitrates, ammonium, iron, copper salts, Trilon B, inhibitors, hydrazine, fluorine, urotropine, captax, etc. can be discharged during washing or conservation of equipment. Such a variety of chemicals requires an individual neutralization solution and disposal of toxic waste from chemical washings.

Water from washing the external heating surfaces is formed only at thermal power plants using sulfurous fuel oil as the main fuel. It should be borne in mind that the neutralization of these washing solutions is accompanied by the production of sludge containing valuable substances - vanadium and nickel compounds.

During the operation of water treatment of demineralized water at thermal power plants and nuclear power plants, effluents from the warehouse of reagents, washings of mechanical filters, removal of sludge water from clarifiers, and regeneration of ion-exchange filters arise. These waters carry a significant amount of calcium, magnesium, sodium, aluminum, and iron salts. For example, at a thermal power plant with a chemical water treatment capacity of 2000 t/h, salts are discharged up to 2.5 t/h.

From the pre-treatment (mechanical filters and clarifiers), non-toxic sediments are discharged - calcium carbonate, iron and aluminum hydroxide, silicic acid, organic substances, clay particles.

And, finally, at power plants using fire-resistant liquids such as Ivviol or OMTI in the lubrication and control systems of steam turbines, a small amount of wastewater contaminated with this substance is generated.

The main regulatory document establishing the system for the protection of surface waters is the "Rules for the protection of surface waters (standard provision)" (M.: Goskompriroda, 1991).

The water reserves on the planet are colossal - about 1.5 billion km3, however, the volume of fresh water is slightly > 2%, while 97% of them are represented by glaciers in the mountains, polar ice Arctic and Antarctic, which are not available for use. The volume of fresh water suitable for use is 0.3% of the total hydrosphere reserve. At present, the population of the world daily consumes 7 billion tons. water, which corresponds to the amount of minerals mined by mankind per year.

Every year water consumption increases dramatically. On the territory of industrial enterprises, wastewater of 3 types is formed: domestic, surface, industrial.

Household wastewater - generated during the operation of showers, toilets, laundries and canteens on the territory of enterprises. The company is not responsible for the amount of wastewater data and sends them to the city's treatment plants.

Surface sewage is formed as a result of washing off impurities accumulated on the territory, roofs and walls of industrial buildings with rain irrigation water. The main impurities of these waters are solid particles (sand, stone, shavings and sawdust, dust, soot, remains of plants, trees, etc.); petroleum products (oils, gasoline and kerosene) used in vehicle engines, as well as organic and mineral fertilizers used in factory squares and flower beds. Each enterprise is responsible for the pollution of water bodies, so it is necessary to know the volume of wastewater of this type.

Surface wastewater consumption is calculated in accordance with SN and P2.04.03-85 “Design standards. Sewerage. External networks and structures” according to the method of maximum intensity. For each section of the drain, the estimated flow rate is determined by the formula:

where is a parameter characterizing the intensity of precipitation depending on the climatic features of the area where the enterprise is located;

Estimated runoff area.

Enterprise area

Coefficient depending on the area;

Runoff coefficient, which determines V depending on the permeability of the surface;

The runoff coefficient, which takes into account the features of the processes of collecting surface wastewater and their movement in flumes and collectors.

Industrial wastewater is generated as a result of the use of water in technological processes. Their quantity, composition, concentration of impurities is determined by the type of enterprise, its capacity, types of technological processes used. To cover the needs of water consumption, the enterprises of the region take water from surface sources by enterprises of industry and heat power engineering, agricultural water use facilities, mainly for irrigation purposes.

The economy of the Republic of Belarus uses the water resources of the rivers: Dnieper, Berezina, Sozh, Pripyat, Ubort, Sluch, Ptich, Ut, Nemylnya, Teryukha, Uza, Visha.

Approximately 210 million m3/year is taken from artesian wells, and all this water is drinking water.

The total volume of wastewater forms about 500 million m3 per year. About 15% of effluents are polluted (insufficiently treated). About 30 rivers and rivers are polluted in the Gomel region.

Special types of industrial pollution of water bodies:

1) thermal pollution caused by the release of thermal water from various power plants. The heat supplied with heated waste waters to rivers, lakes and artificial reservoirs has a significant impact on the thermal and biological regime of water bodies.

The intensity of the influence of thermal pollution depends on t of water heating. For summer, the following sequence of the impact of water temperature on the biocenosis of lakes and artificial reservoirs was revealed:

at t up to 26 0С no harmful effects are observed

over 300С - harmful effect on the biocenosis;

at 34-36 0C, lethal conditions arise for fish and other organisms.

The creation of various cooling devices for the discharge of water from thermal power plants with a huge consumption of these waters leads to a significant increase in the cost of building and operating thermal power plants. In this regard, much attention is paid to the study of the effect of thermal pollution. (Vladimirov D.M., Lyakhin Yu.I., Environmental protection art. 172-174);

2) oil and oil products (film) - decompose in 100-150 days under favorable conditions;

3) synthetic detergents - difficult to remove from wastewater, increase the content of phosphates, which leads to an increase in vegetation, flowering of water bodies, depletion of oxygen in the water mass;

4) reset of Zu and Cu - they are not completely removed, but the forms of the compound and the migration rate change. Only by dilution can the concentration be reduced.

The harmful impact of mechanical engineering on surface water is due to high water consumption (about 10% of the total water consumption in industry) and significant wastewater pollution, which are divided into five groups:

with mechanical impurities, including metal hydroxides; with petroleum products and emulsions stabilized with ionic emulsifiers; with volatile oil products; with cleaning solutions and emulsions stabilized with non-ionic emulsifiers; with dissolved toxic compounds of organic and mineral origin.

The first group accounts for 75% of the volume of wastewater, the second, third and fourth - another 20%, the fifth group - 5% of the volume.

The main direction in the rational use of water resources is circulating water supply.

Wastewater from machine-building enterprises

Foundries. Water is used in the operations of hydraulic core knocking, transportation and washing of molding earth to regeneration departments, transportation of burnt earth waste, irrigation of gas cleaning equipment, and equipment cooling.

Wastewater is polluted with clay, sand, bottom ash from the burnt part of the sand cores and binding additives of the sand. The concentration of these substances can reach 5 kg/m3.

Forging and pressing and rolling shops. The main impurities of wastewater used for cooling process equipment, forgings, hydrodescaling of metal scale and treatment of the premises are particles of dust, scale and oil.

Mechanical shops. Water used for the preparation of cutting fluids, washing of painted products, for hydraulic testing and processing of the premises. The main impurities are dust, metal and abrasive particles, soda, oils, solvents, soaps, paints. The amount of sludge from one machine for rough grinding is 71.4 kg/h, for finishing - 0.6 kg/h.

Thermal sections: For the preparation of technological solutions used for hardening, tempering and annealing of parts, as well as for washing parts and baths after the discharge of waste solutions, water is used. Wastewater impurities - mineral origin, metal scale, heavy oils and alkalis.

Etching and galvanizing areas. Water used for the preparation of technological solutions, used in the pickling of materials and applying coatings to them, for washing parts and baths after the discharge of waste solutions and processing the premises. The main impurities are dust, metal scale, emulsions, alkalis and acids, heavy oils.

In welding, assembly, assembly shops of machine-building enterprises, wastewater contains metal impurities, oil products, acids, etc. in much smaller quantities than in the considered workshops.

The degree of pollution of wastewater is characterized by the following main physical and chemical indicators:

the amount of suspended solids, mg/l;

biochemical oxygen demand, mg/l O2/l; (BOD)

Chemical oxygen demand, mg/l (COD)

Organoleptic indicators (color, smell)

Active reaction medium, pH.

The energy industry is the largest consumer of water. TPP with a capacity of 2,400 MW consumes about 300 t/h of water only for desalination plants.
During the operation of power plants, a large amount of wastewater of various compositions is generated. Industrial waste is divided into categories and subjected to local treatment.
In the energy industry, the following categories of waste and waste water are distinguished: "hot" drains - water obtained after equipment cooling; wastewater containing elevated concentrations of inorganic salts; oil and oil-containing effluents; waste solutions of complex composition containing inorganic and organic impurities.
Let us examine in more detail the methods of purification and disposal of various categories of wastewater.
Cleaning and disposal of "hot" drains. Such drains do not have mechanical or chemical pollutants, but their temperature is 8-10 °C higher than the water temperature in a natural reservoir.
The capacity of the largest power plants in Russia ranges from 2,400 to 6,400 MW. The average consumption of cooling water and the amount of heat removed with this water per 1,000 MW of installed capacity is 30 m3/h and 4,500 GJ/h for TPPs (for NPPs, respectively, 50 m3/h and 7,300 GJ/h) .
When such an amount of water is discharged into natural reservoirs, the temperature in them rises, which leads to a decrease in the concentration of dissolved oxygen. In reservoirs, the processes of self-purification of water are disrupted, which leads to the death of fish.
According to regulatory documents Russian Federation, when hot water is discharged into reservoirs, the temperature in them should not rise by more than 3 K compared to the water temperature of the hottest month of the year. Additionally, an upper limit of the permissible temperature is set. The maximum water temperature in natural reservoirs should not exceed 28 °C. In reservoirs with cold-loving fish (salmon and whitefish), the temperature should not exceed 20 ° C in summer and 8 ° C in winter.
Similar prohibitions apply in Western countries. Thus, in the United States, the allowable heating of water in natural water bodies should not exceed 1.5 K. According to US federal law, the maximum temperature of waste water should not exceed 34 ° C for water bodies with heat-loving fish and 20 ° C - for water bodies with cold-loving fish.
In many countries, there is an upper limit on the discharge water temperature. In Western European countries, the maximum water temperature when discharged into the river should not be higher than 28 - 33 °C.
To prevent harmful thermal effect two ways are used on natural water bodies: separate flow-through reservoirs are built into which warm water is discharged, ensuring intensive mixing of waste water with the bulk of cold water; circulating circulation systems with intermediate cooling of heated water are used.
On fig. 7.1 shows a diagram of once-through cooling of water with its discharge into water bodies in summer and winter time.
Water after the turbine 1 enters the condenser 2 and from there it is sent to the device for cooling water 4 (usually a cooling tower). Then, through the intermediate tank, the water enters the water supply source.
On fig. 7.2 shows a circuit for circulating water cooling, a distinctive feature of which is the organization of a closed water circulation circuit. After cooling in the cooling tower 5, the water is again supplied to the condenser by pump 4. If necessary, water intake from a natural source is provided by pump 3. Circulating water supply systems with evaporative cooling of circulating water make it possible to reduce the needs of power plants in fresh water from external sources by 40 - 50 times.
Treatment of wastewater containing salt impurities. Such wastewater is generated during the operation of demineralized water treatment plants (DWT), as well as in hydraulic ash removal systems (HZU).
Waste water in WLU systems. During the operation of water treatment plants at power plants, effluents are formed from the washing of mechanical filters, the removal of sludge water from clarifiers, and as a result of the regeneration of ion-exchange filters. Wash water

Rice. 7.2. Reverse water cooling scheme:

contain non-toxic impurities - calcium carbonate, magnesium, iron and aluminum hydroxides, silicic acid, humic substances, clay particles. Salt concentrations are low. Since all these impurities are not toxic, after clarification, the water is returned to the head of the water treatment and used in the water treatment process.
Regeneration effluents containing significant amounts of calcium, magnesium and sodium salts are treated in plants using electrodialysis. Schemes of such installations were given earlier (see Fig. 5.19 and 5.23). After electrochemical treatment, purified water and a small volume of highly concentrated salt solution are obtained.
Utilization of wastewater from hydraulic ash removal systems (GZU). Most power plants use hydrotransport to remove ash and slag waste. The degree of mineralization of water in GZU systems is quite high. For example, when removing ash obtained from the combustion of fuels such as shale, peat and some types of coal, water is saturated with Ca (OH) 2 to a concentration of 2 - 3 g / l and has a pH gt; 12.
The discharge of water from the GZU systems is many times greater than the total volume of all other polluted liquid effluents from TPPs. The organization of a closed water circulation of wastewater in the GZU systems can significantly reduce the amount of waste water. In this case, the water clarified at the ash dump is returned to the power plant.
solution for reuse. In Russia, since 1970, all solid fuel power plants under construction have been equipped with a system of closed circulation cycles that take water from the GZU installations.
The complexity of the operation of these systems is due to the formation of deposits in pipelines and equipment. The most dangerous from this point of view are deposits of CaC03, CaS04, Ca(OH)2 and CaS03. They form in clarified water lines at pH gt; 11 and slurry pipelines during the hydrotransport of ash containing more than 1.4% free calcium oxide.
The main measures to prevent deposits are aimed at removing the supersaturation of clarified water. The water is kept in the ash dump pool for 200 - 300 hours. In this case, some of the salts precipitate. After sedimentation, the water from the pools is taken for reuse.
Treatment of wastewater contaminated with oil products. Water pollution with oil products at thermal power plants occurs during the repair of the fuel oil facilities, as well as due to oil leakage from the oil systems of turbines and generators.
On average, the content of oil products is 10 - 20 mg/l. Many streams have much less pollution - 1 - 3 mg/l. But there are also short-term discharges of water with oil and oil content up to 100 - 500 mg/l.
Treatment plants are similar to those used in oil refineries (see Figure 9.11). Effluent is collected in receiving tanks, where they are kept for 3-5 hours, and then sent to a two-section oil trap, which is a horizontal settling tank equipped with a scraper conveyor. In the sump for 2 hours, the separation of contaminants takes place - light particles float to the surface and are removed, while heavy particles settle to the bottom.
The effluent then passes through a flotation plant. Flotation is carried out using air supplied to the apparatus at a pressure of 0.35 - 0.4 MPa. The efficiency of removing oil products in the flotator is 30 - 40%. After the flotator, the water enters a two-stage pressure filter unit. The first stage is two-chamber filters loaded with crushed anthracite with a grain size of 0.8-1.2 mm. The filtration rate during the passage of these filters is 9-11 m/h. The water purification effect reaches 40%. The second stage is activated carbon filters DAK or BAU-20 (filtration rate 5.5-6.5 m/h; degree of purification - up to 50%).
Research recent years good adsorption of oil products by ash particles obtained at thermal power plants during coal combustion has been established. So, with an initial concentration of oil products in water of 100 mg/l, their residual content after contact with ash does not exceed 3–5 mg/l. With an initial concentration of oil products of 10 - 20 mg/l, which occurs most often during the operation of thermal power plants, their residual content is not higher than 1 -2 mg/l.
Thus, when wastewater comes into contact with ash, the same effect is practically achieved as when using expensive treatment plants. The discovered effect served as the basis for a number of design developments for the treatment of wastewater contaminated with oil. It is proposed to organize closed cycles for the use of oil and oil-containing wastewater in gas storage systems without their preliminary treatment.
Purification of wastewater of complex composition after conservation and washing of thermal power equipment. Waste water obtained after washing and conservation of equipment has a diverse composition. They include mineral (hydrochloric, sulfuric, hydrofluoric) and organic (citric, acetic, oxalic, adipic, formic) acids. Branch waters pass complexing agents - trilon and corrosion inhibitors.
According to their influence on the sanitary regime of water bodies, impurities in these waters are divided into three groups: inorganic substances, the content of which in wastewater is close to the MPC, - sulfates and chlorides of calcium, sodium and magnesium; substances, the content of which significantly exceeds the MPC, - salts of iron, copper, zinc, fluorine-containing compounds, hydrazine, arsenic. These substances cannot be processed biologically into harmless products; all organic substances, as well as ammonium salts, nitrites and sulfides. What all these substances have in common is that they can be oxidized biologically to harmless products.
Based on the composition of wastewater, their purification is carried out in three stages.
Initially, the water is sent to an equalizer. In this apparatus, the solution is adjusted for pH. When creating an alkaline environment, metal hydroxides are formed, which should precipitate. However complex composition wastewater creates difficulties in the formation of precipitation. For example, the conditions for the precipitation of iron are determined by the form of its existence in solution. If the water does not contain trilon (complexing agent), then the precipitation of iron occurs at pH 10.5-11.0. At the same pH values, trilonate complexes of ferric Fe3+ will be destroyed. In the case of the presence of a complex of ferrous iron Fe2+ in solutions, the latter begins to decompose only at pH 13. Trilonate complexes of copper and zinc remain stable at any pH value of the medium.
Thus, in order to isolate metals from wastes containing trilon, it is necessary to oxidize Fe2+ to Fe3+ and add alkali to pH 11.5-12.0. For citrate solutions, it is sufficient to add alkali to pH 11.0-11.5.
For the precipitation of copper and zinc from citrate and complexate solutions, alkalization is ineffective. Precipitation can only be carried out by adding sodium sulfide. In this case, copper and zinc sulfides are formed and copper can be precipitated at almost any pH value. Zinc requires a pH value above 2.5. Iron can be precipitated as iron sulfide at pH gt; 5.7. A sufficiently high degree of precipitation for all three metals can be obtained only with a certain excess of sodium sulfide.
The technology of wastewater treatment from fluorine consists in treating them with lime with sulfuric acid alumina. At least 2 mg of A1203 must be added per 1 mg of fluorine. Under these conditions, the residual concentration of fluorine in the solution will not exceed 1.4-1.6 mg/l.
Hydrazine (NH2)2 is highly toxic (see Table 5.20). It is present in wastewater only for a few days, since hydrazine is oxidized and destroyed over time.
Most of the organic compounds present in wastewater are destroyed during biological treatment. For wastewater containing inorganic substances, this method can be applied to the oxidation of sulfides, nitrites, ammonium compounds. Organic acids and formaldehyde respond well to biological treatment. "Hard" compounds that are not biochemically oxidized are Trilon, OP-Yu, and a number of inhibitors.
At the final stage of treatment, wastewater is sent to the municipal sewage system. At the same time, most pollutants are oxidized, and those substances that have not changed their composition, when diluted with domestic water, will have a value below the MPC. Such a decision is legitimized by sanitary norms and rules, which specify the conditions for receiving industrial effluents from thermal power plants to the treatment facilities.
Thus, the technology for treating wastewater with a complex composition is carried out in the following sequence.
The water is collected in a container, to which alkali is added to a predetermined pH value. The precipitation of sulfides and hydroxides occurs slowly; therefore, after adding the reagents, the liquid is kept in the reactor for several days. During this time, there is complete oxidation hydrazine with atmospheric oxygen.
Then a clear liquid containing only organic matter and an excess of precipitating reagents is pumped into the domestic sewage line.
At TPPs with hydraulic ash removal, effluents after chemical cleaning of equipment can be discharged into the slurry pipeline. Ash particles have a high adsorption capacity for impurities. After settling, such water is sent to the GZU system.