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An aeration tank is a rectangular structure through which wastewater mixed with activated sludge flows. Biochemical wastewater treatment is carried out in this tank. The aeration tank must be equipped with an aerator (mechanical or pneumatic). Thanks to the aeration system, the activated sludge effluent is saturated with oxygen, which is vital for aerobic microorganisms. This biological treatment scheme is implemented only under conditions of sufficient saturation of effluents with activated sludge, as well as with a continuous supply of oxygen. Only under such conditions active biochemical oxidation of organic matter is ensured, which guarantees high efficiency of biological treatment facilities.

There are several types of aeration tanks, depending on what technological cleaning schemes are provided in them. So, the following types of biological treatment facilities are distinguished:

1. Displacers. These are structures whose operation scheme is based on the supply of wastewater from one side and the outlet of treated wastewater from the opposite side.
2. Mixers. In these structures, the supply of wastewater and the outlet of the treated liquid are carried out simultaneously.
3. Constructions in which dispersed water infusion occurs... At the same time, the scheme provides that the polluted medium enters the structure from several points, is collected in one tank and, after cleaning, comes out through one hole.
4. Aeration tank with uneven dispersion of liquid... In such structures, the entry of contaminated water occurs from several points. After a certain time after cleaning, the liquid is also discharged into the ground through several outlet pipes.

The photo below shows the main types of aeration tanks: the first diagram shows displacers, the second - mixers, the third diagram in the photo shows the principle of operation of structures with dispersion.

Efficiency

As you already understood, active sludge is needed for efficient operation of the aeration tank. Its formation, viability, as well as the level of biological treatment are significantly influenced by temperature, the presence of a nutrient medium, the concentration of oxygen in the sludge mass, the acidity of the medium, and the presence of toxins. Also, for satisfactory operation, the technological mode in which the aeration tank works is important, namely:

• It is necessary to observe the basic relationships between the degree of pollution of wastewater and the amount of activated sludge. If the dose of sludge is less, then the load increases and the quality of purification decreases. If the dose of sludge is more than necessary, then the process of separating sludge from water in the secondary clarifier becomes more complicated.
• Another basic condition that must be strictly observed is the contact time of the contaminated liquid with the sludge, that is, the residence time in the sump.
• It is equally important that the amount of oxygen in the system is sufficient.

Important: the load on the sludge is the amount of pollution that the sludge must be processed in wastewater. The oxidizing capacity of sludge depends on the dose of dry matter in one liter of liquid. Different sludge dosages are used in different designs of aeration tanks. Usually it is 1-20 gr. per liter.

Features and differences from a septic tank

As you already understood, an aeration tank is a biological treatment facility that needs a continuous supply of air. Thanks to this, the oxidation of organic components of wastewater is faster and better. When using such a purification scheme, purified water is formed, which can be used for watering the garden, as well as for various technical purposes. In addition, activated sludge is successfully used to fertilize fields and vegetable gardens. The collection of treated waste water takes place in the second settling tank.

Do not confuse a conventional septic tank equipped with a biofilter and an aeration tank. The main differences between them are as follows:

• To pump air into the aeration tank, you need a compressor that runs on electricity. Therefore, this type of structure can be called volatile.
• Wastewater enters the biofilter in small portions, and the aeration tank is filled with wastewater to the entire volume.
• The scheme of purification of polluted water in a biofilter is very similar to the principles of biological purification in soil. However, in a septic tank, wastewater is treated faster and in smaller areas. The aeration tank uses the same cleaning scheme, but the speed of all processes is much higher. This high rate of biological treatment is achieved through the use of an aerator and oxygenation.

Principle of operation

The basic principles of aeration tank operation differ from a septic tank and are as follows:

1. Contaminated waste water flows into the central part of the structure. This is a primary sump, which closely resembles the sump used in a two-chamber septic tank.
2. After partial purification of waste water, they are pumped by airlift into the aeration tank. Here they are mixed with activated sludge, which is already present in this chamber. Activated sludge is a special substance consisting of plant residues, bacterial colonies, which are involved in the processing of organic components of wastewater. As a rule, aerobic microorganisms live in activated sludge, which need oxygen during their life. Oxygen access is provided by forced aeration.

Important: a compressor is used to pump air, and an air duct system is used to distribute it along the aeration tank. At the same time, the oxygen concentration in the treated water at the exit from the structure is at least 2 mg / l. Sometimes, to measure the oxygen level, built-in automation is used, which itself increases the oxygen supply when its concentration in the liquid at the outlet decreases.

1. After staying in the aeration tank, the effluent enters the secondary clarifier. In this case, microorganisms and activated sludge that have settled to the bottom return to the aeration tank. The residence time of the sludge in the secondary clarifier is limited, since a special pump is used for pumping back.
2. In the secondary sedimentation tank, the water stays for a sufficient time in order to pass the final stage of purification.

Since they constantly multiply during the life of bacteria, their number does not decrease over time, but only increases. This contributes to the fact that the cleaning efficiency only increases during the operation of the aeration tank.

Biological treatment facilities can be made in the form of a single container, which is internally divided into separate compartments, or in the form of a multi-chamber structure from separate blocks. Usually, when using a multi-chamber design, secondary sedimentation tanks are equipped to collect sludge with the subsequent discharge of the treated water into drainage ditches or storage tanks, from where the liquid will be used for watering the garden. At the same time, the volume of water entering the secondary sedimentation tank should not exceed 8-10 liters per second.

Aeration tanks, which consist of three structures in the form of a primary sedimentation tank, aeration tank and a secondary sedimentation tank, provide better water purification. However, such designs require difficult maintenance.

The following resources are needed to operate the aeration tank:

• Electricity with a voltage of 220 V. Depending on the modification, it can be consumed from 80 W. For the efficient operation of the structure, there must be no power outages.
• Aerobic microorganisms.

Advantages and disadvantages

The advantages of aeration tanks include the following points:

1. The whole design is very compact, which allows installation even in a small area.
2. Since gases are not emitted during the life of aerobes, there is absolutely no unpleasant odor from the structure.
3. Such a structure does not need to be insulated for the winter, since a large amount of energy is released during the processing of organic waste, which allows maintaining the required temperature inside the structure even in winter.

However, such products also have their drawbacks:

1. Without electricity, a sufficient level of cleaning cannot be ensured. Since the compressor will not work, bacteria and activated sludge will die.
2. High price for factory products.
3. The complex equipment used in the operation of the aeration tank needs constant monitoring.
4. If you do not use the sewer for a long time, then there will be no breeding ground for bacteria, and they will die.

Important: when the compressor is running and there is no wastewater flow, the activated sludge retains its viability for 3 months. If the electricity is turned off, the sludge will die in three months.

To prevent the death of activated sludge, a mixture of dry activated sludge with water is poured into the structure of the aeration tank. This should be done once a month. If, for some reason, the sludge died, then the aeration tank will have to be restarted. To do this, do the following:

• Free the aerotank from dead silt. To do this, rinse it with water.
• Live activated sludge can be taken in another aeration tank. To avoid problems with this, it is necessary to sign a maintenance contract for the aeration tank when purchasing it.

Installation

Usually, the installation of the aeration tank is carried out by specialists of the company where you purchased the equipment. Since the installation requirements may vary slightly from model to model, before installing the product, you must carefully read the instructions, which provide detailed instructions for installation.

The installation of a factory product is usually carried out in several stages:

1. The pit is dug, based on the dimensions of the product. Usually its dimensions are 180x180x260 cm.
2. A sand pillow 15 cm high is made at the bottom of the pit.
3. We lower the structure into the pit.
4. Before backfilling, water is poured into the aeration tank. In this case, the water is poured gradually as you fall asleep. The water level should always be 15-20 cm above the filling level. This is to ensure that the soil pressure does not damage the walls of the structure. We do the filling up to the level of the location of the pipes for fastening communications.
5. We connect communications to the aerotank.
6. We carry out the installation of the compressor.
7. We connect electricity.
8. We complete the backfill and ram the soil.

This article presents the technological parameters of the biological treatment in aeration tanks. Features of the technological scheme are described: high concentration of sludge mixture, high coefficient of activated sludge recycling. To eliminate the disadvantages inherent in the aeration tanks-displacers, structural changes were made to the aeration tanks: a new aeration system was installed, a longitudinal recycle of the sludge mixture in the aeration tanks was arranged using airlifts.

Photo 1. Ciliates Epistylis plicatilis

Photo 2. Colony of rotifers

Photo 3. Rotifer in a layer of silt

Control over the process of biological treatment in aerotanks is carried out in physicochemical and hydrobiological laboratories using modern methods of analysis using video cameras and computers to accumulate information about the state of the biocenosis and all its changes.

As a result of the reconstruction, high results of wastewater treatment were achieved. The concentration of organic pollutants after biological treatment does not exceed 3 mg / dm3. The total concentration of mineral nitrogen does not exceed 10 mg / dm 3, the cleaning efficiency for heavy metals is 94-96%, for oil products - 92-96%. The results achieved (in terms of the quality of treatment and indicators of energy efficiency) allow us to conclude that it is advisable to use the biological treatment process in aeration tanks with low loads to achieve high quality treatment at low costs for the reconstruction of biological treatment facilities. The renovation costs pay off within two to three years.

In the shop of NIOSV of JSC "Minudobreniya", wastewater treatment is carried out in two cities near Moscow - Yegoryevsk and Voskresensk. The volume of wastewater is on average 60-80 thousand m 3 / day. The nature of the incoming pollution is household. Wastewater has a concentration of suspended solids in the range of 150-180 mg / dm 3, for BOD-5 - up to 160 mg / dm 3, for COD - 250-350 mg / dm 3. The treatment facilities are made according to the classical biological treatment scheme. The resulting sediment, after special treatment, is used in full for the reclamation of an industrial landfill. The structures built 40 years ago have been reconstructed many times. In the last decade, the reconstruction of biological treatment in aerotanks has been completed in order to improve the quality of wastewater treatment and the energy efficiency of the treatment process.

Traditional biological treatment schemes (using microorganisms in suspension in aerotanks with their subsequent sedimentation in secondary sedimentation tanks) do not provide effective and reliable wastewater treatment up to strict established norms of permissible discharge. Especially great difficulties arise when reaching the permissible discharge norms for fishery water bodies.

To solve the problem of deep purification of wastewater from organic and biogenic compounds in the world practice, several fundamental technological processes have been developed: SBR technology (with variable-action reactors); technology of sequential alternation of aerobic, anoxic and anaerobic biological treatment zones in the aeration tank; biomass concentration technology by combining suspended and attached forms of microorganisms in reactors; technology for concentrating the biomass of suspended forms of microorganisms with their subsequent retention by special membranes.

The concentration of organic pollutants after biological treatment does not exceed 3 mg / dm3. The total concentration of mineral nitrogen does not exceed 10 mg / dm3, the cleaning efficiency for heavy metals is 94-96%, for oil products - 92-96%

SBR technology assumes sequential carrying out in one reactor in a periodic mode, during its operation, alternating aerobic and anaerobic processes. This technology is very expensive and requires a complex system of actuators to execute the commands of the process control system. The cyclic-variable oxygen conditions of the life of microorganisms in such a reactor, due to the adaptation factor, inhibit the rate of biochemical reactions and increase the time required for the reaction to proceed. This increases the size of the reactor.

The technology of sequential alternation of anaerobic, anoxic and aerobic zones during reconstruction reduces the productivity of biological treatment by 30-40%. The multivariate system of recycle of activated sludge and effluent from different processing zones significantly complicates the control over the technological process and its controllability. The number of actuators installed in inaccessible places increases, the volume of pumped activated sludge increases significantly.

The technology of biomass concentration using suspended forms of microorganisms attached to inert carriers is associated with the costs of purchasing carriers, installing these carriers in bioreactors and significant difficulties in the repair of aeration systems. The appearance of biofilm from inert carriers in the sludge mixture requires an increase in the settling time of the sludge mixture, that is, an increase in the size of the settling tanks. The technology for concentrating the biomass of suspended forms of microorganisms in a reactor (with subsequent separation on polymer membranes) is associated with the expense of reagents for membrane regeneration and with the complexity of operation.

However, it is necessary to reconstruct the existing biological treatment facilities with an increase in the efficiency of treatment to reduce the discharge of organic pollutants and biogenic elements into water bodies. This is possible when using propellant aeration tanks in the extended aeration mode.

The process of wastewater treatment in an aeration tank can be represented as follows. When clarified effluent enters the aeration tank, the effluent is mixed with the return sludge pops. Sorption of undissolved and colloidal contaminants, which come with clarified wastewater, occurs on the surface of zoogles, which make up the cotton of the silt. Located on the surface of zoogles, which are covered with polysaccharide helium, bacteria, in the presence of oxygen, release enzymes to oxidize impurities. Part of the dissolved impurities enters the body of bacteria, where they are oxidized with the help of enzymes. When contaminants are oxidized by bacterial enzymes, it is possible to use both oxygen dissolved in the sludge mixture and nitrates. The compounds obtained as a result of enzymatic oxidation are used by bacteria for reproduction, that is, growth in numbers.

The development of bacteria in the aerotank can be conditionally divided into three phases. The first is the logarithmic growth phase. In this phase, there is an increase in the number and mass of bacteria by the amount of contaminants contained in the incoming wastewater, minus the mass used by the bacteria themselves to obtain energy for life.

In the second phase (developed biocenosis of activated sludge), there is a rapid development of predatory microorganisms, which use the mass of bacteria and the remaining pollution as food and for subsequent reproduction. Depletion of reserves of easily oxidized organic matter transforms the activated sludge biocenosis into the phase of endogenous respiration or autotrophic oxidation. In this phase, the source of energy for the life and reproduction of microorganisms is the mass of microorganisms of the active sludge itself. The number of bacteria sharply decreases, the number of predatory microorganisms is determined by the rate of self-oxidation of microorganisms in the sludge.

In the third phase, the oxidation of inorganic nitrogen compounds resulting from the oxidation begins - a nitrification reaction takes place using a large amount of oxygen from the sludge mixture. In the phase of endogenous respiration of microorganisms, the following processes take place: the formation of large dense cotton sludge from zooglya bacteria, filamentous bacteria, fungi, actinomycetes; the process of oxidation of organic matter continues - the matter of organisms of the activated sludge biocenosis; oxidation of inorganic forms of nitrogen occurs in the presence of oxygen - nitrification, reduction in the presence of nitrates - denitrification.

The biocenosis of activated sludge in corridor aeration tanks operating under low loads, with deep nitrification and denitrification, is characterized by a large species diversity (over 30 species of protozoa), however, without a numerical predominance of any species

To carry out these opposite reactions with respect to oxygen, it is necessary to create conditions for each of them. This is only possible by creating different zones: anaerobic, aerobic and anoxidic. Sludge cotton can be considered as a spherical or ellipsoidal formation with zones inside it, where dissolved oxygen from the sludge mixture does not enter, even with a significant oxygen concentration (4-6 mg / dm 3) in waste water.

To carry out the process of purifying wastewater from incoming pollution, it is necessary to deeply oxidize the organic matter contained in the clarified water, deeply oxidize the substance of activated sludge bacteria. The resulting nitrogen-containing substances are oxidized to nitrates and reduced to gaseous nitrogen. To increase the speed of the reduction (denitrification) reaction, it is necessary to increase the anoxic and anaerobic zones in the aeration tank.

The increase is carried out in two ways:

• due to an increase in the number of sludge pops, which leads to an increase in sludge concentration up to 5-6 mg / dm 3;
• due to an increase in the size of sludge flocks, which leads to a decrease in the BOD load to 35-50 mg per gram of dry matter per day, maintaining microorganisms in the phase of endogenous respiration.

At the same time, low BOD loads on the aerotank allow deep oxidation of organic matter up to 3.5 mg / dm 3, almost up to theoretically achievable 2.5 mg / dm 3. On the basis of the above theoretical provisions in the aeration tanks of the NiOPSV workshop, a mode of operation was organized with the following values ​​of technological parameters: BOD load - 35-50 mg per gram of BOD dry matter per day; aeration time - 8-12 hours; sludge dose - 5-6 g / dm 3; dissolved oxygen concentration - 4-6 mg / dm 3; recirculation coefficient - 0.8-1.0; electrode potential within -200 ...- 250 mV; sludge index - 90-130; ash content of sludge - 35-40%; specific air consumption for aeration - 6-7 m 3 per 1 m 3 of effluent; specific power consumption for aeration - 0.35-0.4 kWh per 1000 m 3.

At the same time, it is necessary to note the disadvantages of corridor aeration tanks:

• uneven load on activated sludge along the length of the structures, which worsens its technological performance;
• lack of dissolved oxygen at the beginning of the first corridor and excess in the second half of the second corridor.

To eliminate these disadvantages, a longitudinal recycle of the sludge mixture was arranged in the aeration tanks. The diagram is shown in Fig. 1. The recirculation unit is made in the form of a water-air pump-airlift, which pumps the sludge mixture from the end of the second corridor to the beginning of the first. The value of the recycling ratio is 2.1-2.5. As a result of a longer stay of activated sludge in aerobic conditions and acceleration of biomass turnover: the oxidizing capacity of activated sludge biomass increases due to an increase in the level of enzymatic activity; macroturbulence in the aerotank increases - the size of stagnant zones decreases; the specific load on activated sludge is reduced; the oxygen regime of the structure is improved, without reducing the average path length of the treated wastewater, which excludes the "slip" of unoxidized contaminants.

This made it possible to achieve the following: increase the mineralization of activated sludge and reduce the amount of excess activated sludge to a minimum value; to increase the stability of the biocenosis of activated sludge when discharges of difficultly oxidized industrial effluents enter, the state of sludge was monitored according to the method of bioestimation; stabilize the oxygen regime in the sludge mixture during the repair of the blowers.

The biocenosis of activated sludge of corridor aerotanks operating under low loads, with deep nitrification and denitrification, is characterized by a large species diversity (over 30 species of protozoa) without a numerical predominance of any species. The number of filamentous bacteria, small colorless flagellates, small forms of naked and shell amoebas is insignificant. Of the ciliates, gastric and attached forms predominate.

Photo 1 shows a colony of Epistylis plicatilis. The presence of predators has a positive effect on the degree of water purification from organic pollution due to the intensification of biological processes in the bacterial environment due to the intake of substances released from microfauna fragments during their destruction in aerotanks in the phase of endogenous respiration. Active silt always contains rotifers (photo 2-3), sucking ciliates, predatory mushrooms, various worms, tardigrades.

For BOD5, a value of 3 mg / dm 3 was reached, corresponding to the maximum permissible discharge (MPD) for fishery water bodies (Fig. 2). The COD value is 30 mg / dm 3. For mineral nitrogen - 10 mg / dm 3 (Fig. 3), which corresponds to the recommendations of the Helsinki Commission (Helcom) for cities with a population of more than 100 thousand inhabitants. The efficiency of cleaning for iron was 90-92%, of cleaning for heavy metals - 94-96%, efficiency for oil products - 92-96%.

When aeration tanks operate at low loads with a longitudinal recycle coefficient of 2-3:

• high quality of wastewater treatment is achieved, in accordance with the recommendations of Helcom, without increasing the cost of electricity during operation;
• high quality cleaning does not require large expenditures of raw materials, materials;
• the process is easy to maintain and monitor;
• reconstruction of corridor aeration tanks into aeration tanks operating in the extended aeration mode requires minimal costs (for the reconstruction of the aeration system, increasing the productivity of pumps for return sludge, installation of airlifts for longitudinal recycle);
• the costs of paying to the budget for the discharge of pollution with treated wastewater are reduced;
• the amount of excess activated sludge is significantly reduced - the costs for its dehydration and disposal are reduced;
• the technological process is not complicated (no costs are required for complex control devices, executive regulating mechanisms, the requirements for the qualifications of service personnel are not increased).

Such reconstruction is a real way to improve the quality of treatment of most wastewater treatment plants of district significance. The costs of further improving the quality of nitrogen and phosphorus purification (until the established MPD standards for fishery water bodies are reached) are too high, for example, for the budget of a settlement with a population of less than 250-300 thousand people.

1. Belyaeva N.A., Gunther L.I. On the characterization of activated sludge biocenoses in highly loaded aeration tanks and aeration tanks with a long period of aeration // Biological Sciences, No. 7/1969.
2. Zhmur N.S. Process control and control of the result of wastewater treatment. - M .: Luch, 1997.
3. Zhmur N.S. Methodical guidelines for hydrobiological and bacteriological control of the biological treatment process at facilities with aeration tanks. - M .: LLC "Aquareos", 1996.
4. Nikitina O.G. Bioestimation: control and regulation of the processes of biological purification and self-purification of water. Abstract of thesis. for a job. uch. Art. Doctor of Biological Sciences - M., 2012.
5. G.V. Kapitonova Methodical recommendations for the hydrobiological control of wastewater treatment with activated sludge. - M., 2012.

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SEWERAGE - OUTDOOR NETWORKS AND STRUCTURES - SNiP 2-04-03-85 (approved by the Decree of the State Construction Committee of the USSR from 21-05-85 71) (revised from 20-05-86) ... Actual in 2018

Aeration plants for complete oxidation (aeration tanks with extended aeration)

6.166. Aeration plants for complete oxidation should be used for biological wastewater treatment.

Before supplying waste water to the installation, it is necessary to provide for the retention of large mechanical impurities.

6.167. The duration of aeration in aeration tanks for complete oxidation should be determined by the formula (48), while it should be taken:

р - average rate of oxidation according to BOD_full - 6 mg / (g x h);

a_i - sludge dose - 3 - 4 g / l;

s - ash content of sludge - 0.35.

The specific air consumption should be determined by the formula (61), while it should be taken:

q_O - specific oxygen consumption, mg / mg of removed BOD_full, - 1.25;

K_1, K_2, K_T, K_3, C_a - according to the data given in clause 6.157.

6.168. The duration of the wastewater stay in the sedimentation zone at the maximum inflow should be at least 1.5 hours.

6.169. The amount of excess activated sludge should be taken as 0.35 kg per 1 kg of BOD_full. Removal of excess sludge is allowed to be provided both from the settling tank and from the aeration tank when the sludge dose reaches 5-6 g / l.

The moisture content of the sludge removed from the settler is 98%, from the aeration tank 99.4%.

6.170. The load on the sludge pads should be taken as for sediments fermented in mesophilic conditions.

Aeration tanks-sedimentation tanks are designed in the form of rectangular tanks of structures that combine aeration tanks with extended aeration (aeration part) and secondary sedimentation tanks of a vertical type (settling part). Both structures are interconnected by overflow windows, which ensure the flow of the sludge mixture from the aeration zone to the settling zone.

The extended aeration mode, which is also called the complete oxidation method, is distinguished by a significantly longer duration of the wastewater stay in the aeration tanks. The duration of aeration of wastewater in the extended mode is 1-3 days. depending on the initial concentration of wastewater by BOD. Aeration tanks with extended aeration operate at doses of activated sludge on a dry matter basis of 3-6 g / l per day.

Aeration tanks operating in full oxidation mode can be operated with or without excess activated sludge removal. In the latter case, excess activated sludge is removed from the secondary clarifier, which reduces the quality of treatment. Therefore, for a higher degree of purification, the project provides for the removal of excess sludge from the system, especially since its low increase allows this operation to be performed at significant intervals.

The use of the extended aeration mode is due to a slight increase in activated sludge and a high degree of its mineralization, ease of operation, stability of operation in modes of uneven flow of wastewater flow.

Figure 4.4 Aeration tanks-settling tanks: 1 - aeration tank, 2 - sedimentation tank, 3 - sewage supply pipeline for treatment, 4 - purified water discharge pipeline, 5 - aeration system, 6 - circulating sludge pipeline, 7 - excess sludge discharge pipeline, 8 - air duct, 9 - airlift, 10 - chute, 11 - toothed weir

Table 4.2 Initial data for calculating aeration tanks-settling tanks

Aeration tank

The calculation of aeration tanks is made according to the extended aeration mode. The duration of aeration is:

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