An aeration tank is a rectangular structure through which wastewater mixed with activated sludge flows. Biochemical purification is carried out in this tank Wastewater. The aerotank-sump must be equipped with an aerator (mechanical or pneumatic). Thanks to the aeration system, activated sludge effluents are saturated with oxygen, which is vital for aerobic microorganisms. This scheme of biological treatment is implemented only under conditions of sufficient saturation of wastewater 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 the high efficiency of biological treatment facilities.

Aerotanks are of several types, depending on which technological schemes cleaning is included. So, the following types of biological treatment facilities are distinguished:

1. Displacers. These are facilities, the operation scheme of which is based on the supply of wastewater from one side and the exit of treated effluents from the opposite side.
2. Faucets. In these facilities, the supply of wastewater and the exit of the treated liquid are carried out simultaneously.
3. Structures in which there is a dispersed infusion of water. 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, exits through one opening.
4. Aerotank with non-uniform distribution of liquid. In such structures, polluted water enters from several points. After a certain time after purification, the liquid is also discharged into the ground through several outlet pipes.

The photo below shows the main types of aerotanks: the first diagram shows displacers, the second diagram shows mixers, the third diagram in the photo shows the principle of operation of structures with dispersal.

Efficiency

As you already understood, for effective work the aeration tank needs activated sludge. Its formation, viability, as well as the level of biological treatment are significantly affected 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 aerotank-settler operates is important, namely:

• It is necessary to observe the basic relationship between the degree of contamination 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 sump is complicated.
• Another basic condition that must be strictly observed is the contact time of the contaminated liquid with sludge, that is, the time spent 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 process in the wastewater. The oxidative capacity of sludge depends on the dose of dry matter in one liter of liquid. IN different designs Aerotanks use a different dosage of sludge. Usually it is 1-20 gr. per litre.

Features and differences from a septic tank

As you already understood, the aerotank-settler is a biological treatment facility that needs a continuous supply of air. Due 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 gardens. The collection of treated wastewater takes place in the second settling tank.

Do not confuse an ordinary 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 structures can be called energy-dependent.
• Wastewater enters the biofilter in small portions, and the aeration tank is filled with wastewater for the entire volume.
• The scheme of purification of polluted waters in a biofilter is very similar to the principles of biological purification in the 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. Such high speed biological treatment is achieved through the use of an aerator and oxygen saturation.

Operation principle

The basic principles of operation of the aerotank differ from the septic tank and are as follows:

1. Polluted wastewater enters the central part of the structure. This is a primary septic tank, which is very similar to the sump used in a two-chamber septic tank.
2. After partial purification of wastewater, they are pumped by an airlift to the aerotank. Here they are mixed with activated sludge, which is already present in this chamber. Activated sludge is a special substance consisting of plant remains, bacterial colonies that are involved in the processing of organic components of wastewater. As a rule, activated sludge is inhabited by aerobic microorganisms that 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 throughout the aeration tank. At the same time, the oxygen concentration in the purified water at the facility outlet is at least 2 mg/l. Sometimes built-in automation is used to measure the oxygen level, which itself increases the oxygen supply when its concentration in the outlet liquid decreases.

1. After being in the aeration tank, the effluents enter the secondary sump. At the same time, microorganisms and activated sludge that have settled to the bottom are returned to the aerotank. 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 sump, water is sufficient time to pass the final stage of purification.

Since bacteria constantly multiply during the life of bacteria, their number does not decrease for some time, but only increases. This contributes to the fact that the cleaning efficiency during the operation of the aerotank only increases.

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

Aerotanks, which consist of three structures in the form of a primary settling tank, an aerotank and a secondary settling tank, provide better water purification. However, such structures require complex maintenance.

For the operation of the aeration tank, the following resources are needed:

• Electricity with a voltage of 220 V. Depending on the modification, it can be consumed from 80 watts. For the efficient operation of the structure, there should be no interruptions in the supply of electricity.
• Aerobic microorganisms.

Advantages and disadvantages

The advantages of aerotanks include the following points:

1. The whole structure is very compact, which allows installation even in a small area.
2. Since no gases are released during the life of aerobes, there is absolutely no unpleasant smell 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 makes it possible to maintain the desired 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 aerotank needs constant monitoring.
4. If you do not use the sewer for a long time, then there will be no nutrient medium for bacteria, and they will die.

Important: when the compressor is running and there is no sewage flow, activated sludge remains viable for 3 months. If the electricity is also turned off, then in three months the silt will die.

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

• The aeration tank is freed from dead sludge. To do this, it must be washed 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 aerotank when buying it.

Installation

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

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

1. A pit is digging, based on the dimensions of the product. Usually its dimensions are 180x180x260 cm.
2. At the bottom of the pit, a sand cushion 15 cm high is made.
3. We lower the structure into the pit.
4. Before backfilling, water is poured into the aeration tank. At the same time, the filling of water is done gradually as it falls asleep. The water level should always be 15-20 cm above the backfill level. This is necessary so that the ground pressure does not damage the walls of the structure. Backfilling is done to the level of the location of the nozzles for fastening communications.
5. We connect communications to the aeration tank.
6. We carry out the installation of the compressor.
7. We connect electricity.
8. We complete the backfill and ram the soil.

Aerotanks of this type, as a rule, are in-line with separate settling tanks (Fig. 7). In this case, the aerotank is divided into sections operating in parallel, which include two or more longitudinal corridors.

The displacement mode is ensured when the ratio of the length of the corridor to its width is more than 30. If the indicated ratio is 30 or less, sectioning of the corridors with longitudinal partitions spaced from the transverse walls by 2 ... 5 m, by 5 ... 6 cells should be provided.

The treated wastewater is mixed with activated sludge and fed to the aeration tank through the channel, then it enters the sectional channels, from which it also enters the corridors through the channels. The treated water is collected by catchment trays and discharged through the canal and pipeline to the secondary settling tanks.

The duration of the aeration period, h,

where: φ is the coefficient of inhibition by the decomposition products of organic substances of activated sludge, l/g, (Table 9);

a i is the dose of activated sludge in terms of dry matter, g/l, (Table 10);

P max - maximum speed oxidation of organic substances, mg/(g h) (Table 9);

C 0 - the concentration of dissolved oxygen, equal to 1 ... 2 mg / l;

s is the ash content of activated sludge, fractions of a unit (Table 9);

K 0 is a constant characterizing the effect of oxygen, mg O 2 /l (Table 9);

L cm is the BOD total value, determined taking into account the dilution of wastewater by the recirculation flow rate of the return activated sludge, mg/l;

L t is the value of BOD full of treated wastewater, mg/l;

K l is a constant characterizing the properties of organic substances, mg BOD total / l (Table 9);

L 0 is the value of BOD full of wastewater entering the aerotank, mg/l;

K p - coefficient taking into account the effect of longitudinal mixing on the wastewater treatment process: K p \u003d 1.5 when treating wastewater up to L t \u003d 15 mg / l and K p \u003d 1.25 - at L t > 30 mg / l.

The BOD total value, taking into account the dilution of wastewater by the recirculation flow rate of the return activated sludge, mg/l,

(19)

here: r i is the activated sludge recirculation coefficient, fractions of a unit, determined from fig. 2, depending on the dose of activated sludge for the ashless substance a h i and the value of the sludge index i or according to the formula:

Note: 1. Formula (18) is valid for i<175 см 3 /г и а i £5 г/л;

2. The value of r i must be at least 0.3 for sedimentation tanks with

sludge pumps, 0.4 - with sludge scrapers, 0.6 - with gravity removal of activated sludge.

Dose of activated sludge for ashless substance, g/l,

The value of the silt index should be determined experimentally. In the absence of experimental data, it is allowed to take according to Table. 11 depending on the load on BOD full per 1 g of ashless substance of activated sludge per day R a , mg / (g. day), equal to:

(22)

where t p is the duration of the aeration period, taking into account the temperature of the waste water, h,

or calculate using the formula:

(23)

Fig.7. design scheme of the corridor of the aerotank-displacer

Table 9

Basic design data for the characteristics of the wastewater process in aerotanks

Note. For other industries, the specified parameters should be taken according to the data of research organizations.

Table 10

Main technological characteristics of aerotanks

Pollutant loading regime	Structures	Aeration duration, h	Dose of activated sludge by dry matter, g/l	Silt index, cm 3 /g
Low	Aerotanks of prolonged aeration	10…30	3…12	40…80
Medium	Aerotanks are ordinary	6...8	2…4	50…100
Aerotanks with regenerators	5…6	2…4	50…100
High performance aerotanks	3..5	3,5…8	50…100
High	Aerotanks highly loaded	0,4…4	1,5…10	80…200

Aeration period, taking into account the wastewater temperature, h,

where: T is the average annual wastewater temperature, 0 С.

Return activated sludge concentration, g/l,

(25)

Table 11

Silt index value

The concentration of activated sludge in the sludge mixture, taking into account the concentration of return sludge and the recirculation coefficient, g/l,

(26)

where: Свв is the concentration of suspended solids in wastewater entering the aerotank, g/l;

K and \u003d 0.80 ... 0.85.

The duration of the aeration period, taking into account the recirculation of the return activated sludge, h,

The working volume of the aeration tank, m 3,

(28)

Here: q is the estimated wastewater flow rate, m 3 / h, taken depending on the value of the coefficient of wastewater inflow non-uniformity:

· with coefficient of non-uniformity not more than 1.25 – q is equal to the average hourly flow of wastewater;

· with a non-uniformity coefficient of more than 1.25 - q is equal to the average flow rate during the hours of maximum wastewater inflow;

N is the number of aerotanks.

The working volume of the aeration tank section, m 3,

moreover, N c is the number of sections in the aerotank, N c ³2.

Note: The number of sections in the aerotank is approximately recommended for stations with a capacity of up to 50,000 m 3 /day equal to 4 ... 6, for stations with a higher productivity - 8 ... 10.

Corridor width, m,

where: K b = 1…2;

h 1 - working depth of the aerotank, h 1 = 3 ... 6.

Aerotank section width, m,

here n is the number of corridors in the section, n= 2…4.

Length of aeration tank corridors (working length of aeration tank), m,

Note: Since structures with large overall dimensions are usually made of precast concrete, the length of the corridors should be a multiple of 6 m and be 36 ... 114 m. If this condition is not met, then it is necessary to adjust the width of the corridors, their number, number of sections or the number aerotanks.

The total number of sections in the aerotank:

(33)

here N c.p. - the number of redundant sections, determined from the condition that their throughput should be at least 50% of the performance of the working sections, i.e.

(34)

Aerotank width, m,

Total depth of the aerotank, m,

where h 2 is the height of the sides of the aerotank, h 2 = 0.3 ... 0.5 m.

Diameter of the main pipeline for supplying wastewater to aerotanks, m,

(37)

here v sv is the speed of water movement in the pipeline, m / s, equal to 3 m / s for pressure movement, 0.8 ... 1.0 m / s for non-pressure movement.

Highly loaded, high-performance and extended aeration aerotanks

from "Treatment of industrial wastewater in aerotanks"

One of the possible ways to intensify the work of aeration facilities in order to increase their capacity is to increase the load on activated sludge. Highly loaded aerotanks are structures in which the biological treatment process takes place in 0.5-2 hours (urban wastewater), as a result of which the hydraulic loads are more than 20 m / day per 1 m of the structure and the daily load on the sludge according to BODtotal is more than 0.8 kg/kg with a cleaning effect of 70-95%.
Increasing the ratio of nutrients to the number of active microorganisms in highly loaded aeration tanks leads to a more intense oxidation process than in aeration tanks with a low load or sludge mineralization, where the process is inhibited by a lack of nutrition for microorganisms. The result of supplying excess nutrition to aerotanks is the predominance of the logarithmic growth phase of microorganisms, while ammonia nitrogen dominates in the treated water and contains a minimum amount of its oxidized forms.
As can be seen from Table. U.1, which presents the load ranges for all types of aeration structures, according to domestic and foreign studies, highly loaded structures make it possible to significantly increase the efficiency of the use of a unit volume of the aeration tank.
The second option - while maintaining the same load on the sludge, the concentration of activated sludge in the system is increased, which leads to the creation of aeration structures, which, unlike highly loaded ones, are highly productive. As you know, the rate of oxidation of the waste liquid - the source of nutrition and energy of microorganisms - is the greater, the greater the number of microorganisms functioning in the system. This position is in good agreement with the data obtained by I. S. Postnikov et al. for wastewater from a number of Moscow aeration stations. Interesting results confirming the effect of high-performance aerotanks are given by V. Emde (Table V.2). As can be seen from this table, the dose of sludge in the facilities did not fall below 3.6 g l, and in some cases reached 10.2-11.2 g l, which, even with relatively low loads on activated sludge, provided an oxidizing power in terms of BODtotal more than 5 kg - day.
To ensure the required high degree of circulation without additional costs for pumping the circulation flow of sludge, it is necessary to block the aerotank with a secondary clarifier.
The factor limiting the increase in the working dose of activated sludge to more than 7-10 g/l is a sharp deterioration in the sedimentation separation of concentrated sludge mixtures in secondary clarifiers. Department of sewerage MISI them. V. V. Kuibyshev put forward an original idea of ​​filtering the sludge mixture of aeration tanks with doses of sludge up to 25 g l through mesh filters in such a way that no more than 3-4 g l of suspended solids enter the secondary settling tanks. The technological scheme of the facility, called the filter tank, with a capacity of 37500 m / day is shown in fig. U.Yu.
When treating wastewater with a BODtotal value of more than 1,500 mg lu1 in a filter tank, the content of ether-soluble substances was about 1000 mg l, the purified water had a BODtotal equal to 20-Shmg l, with a residual content of ether-soluble substances of 7-9 mg l. The duration of the aeration of the waste liquid is 3-4 hours, which corresponds to the oxidizing power according to BODtotal 8000 - 12 000 g-day or 400-600 lay down per 1 g of sludge per day. At the same time, the height of the activated sludge layer in front of the mesh nozzle is 1-1.5 m, the period of filtering through it is 40-60 seconds, the period of reverse blowing of the mesh nozzle is 8-12 seconds at an air supply intensity of 80-120 m/h.
Technical and economic calculations show that the filter tank, which provides high oxidizing power at relatively low loads on activated sludge, makes it possible to achieve 12-15% savings on the cost of cleaning 1 waste liquid, while saving on capital costs during the construction period is 35-40%. Given the above, a high-performance aerotank of this design should be recognized as a progressive treatment facility, especially for the treatment of highly concentrated industrial wastewater, as well as for the treatment of wastewater that forms hard-setting activated sludge.
The study of the main design parameters of high-performance aerotanks was carried out by the authors in 1966-1968. on laboratory models with pneumomechanical aeration system. The cycle of observations was carried out on a synthetic waste liquid, and peptone was chosen as the main nutrient, and various concentrations of aliphatic amines, which are present in wastewater from many industries, were introduced as an industrial additive. During the experiment, the working dose of activated sludge was maintained at the level of 4-8 g l with the amount of circulating sludge 100-500% and the flow rate of the supplied air, depending on the load, 40-80 per 1 liter of purified liquid.
The possibility of intensifying wastewater treatment by increasing the working dose of activated sludge, and on the other hand, the unsuitability of traditional aeration stations for reliable operation in this mode determine one of the main directions in the development of high-performance aerotank designs.

Without regeneration (SF and return sludge are fed into corridor 1, biological treatment is carried out by corridor 4)

With 25% regeneration

With 50% regeneration

With 75% regeneration

Aerotanks - settling tanks

A characteristic feature of these structures is the structural combination of the aeration reservoir and the secondary sump in one structure.

The part of the structure in which the sludge mixture is aerated is called the aeration zone, and the other part is called the settling zone.

Both zones are interconnected by holes, windows and slots. Providing the flow of the sludge mixture from the aeration zone to the settling zone and the return of sludge from the settling zone to the aeration zone without the use of additional equipment.

Aerotank - settling tank "Oxycompact"

1. coolant inlet

2. Discharge of treated water

3. settling area

4. removal of excess sludge

5. air supply

SJ after the primary settling tanks is supplied to the aeration zone located in the center of a rectangular reservoir, on both sides of the central aeration zone there are settling zones, which are separated by partitions and have overflow windows in the upper part and slots in the lower. These holes serve to circulate the sludge.

Excess sludge is discharged from the lower part of the settling zone through special pipelines located at a certain distance from each other. Air is supplied through cap aerators mounted in the bottom plate, blocking the air channels, or into air ducts laid along the bottom along the aeration zone. The depth of the structure is assumed to be about 4 m, the length is from 15 to 70 m (depending on performance).

The advantage of a structure of this type is the recycling of activated sludge without auxiliary devices, as well as an increase in the dose of sludge to the aerotanks.

Aerotank of prolonged aeration

The aeration time in aerotanks of this type can reach 20 hours or more, which significantly exceeds the aeration time in conventional aerotanks (from 2 to 8 hours). During this time in the aerotank, not only the biological purification of the SF is carried out, but also the oxidation of activated sludge in the phase of endogenous respiration. This is due to the fact that activated sludge is under conditions of low organic load, and microorganisms are in the stage of starvation, as a result of which the cells of microorganisms undergo autoxidation. The return activated sludge after extended aeration aerotanks does not require regeneration, and the excess sludge does not need additional treatment and can immediately be sent for dehydration.

This scheme is a combined installation that combines an extended aeration aerotank and a secondary settling tank. From the aeration zone, the sludge mixture enters the degassing zone through a special window, where air bubbles are separated from the sludge flakes.

In the settling zone, the separation of the purified liquid and activated sludge is carried out, while the water in the sump moves from bottom to top, passing through a layer of suspended sediment, which intensifies the clarification process. The residence time in the settling zone is from 2 to 4 hours. The separated sludge is removed outside the facility under hydrostatic pressure and fed into the sludge PS. The INS returns part of the sludge to the aeration tank, and the excess is fed or pumped for dehydration.

2015-03-15

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

Photo 1. Epistylis plicatilis ciliates

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 have been achieved. The concentration of organic contaminants after biological treatment does not exceed 3 mg/dm3. The total concentration of mineral nitrogen does not exceed 10 mg / dm3, the purification efficiency for heavy metals is 94-96%, for oil products - 92-96%o. The results achieved (in terms of cleaning quality and energy efficiency indicators) allow us to conclude that it is expedient to use the biological treatment process in aerotanks with low loads to achieve high treatment quality at low costs for the reconstruction of biological treatment facilities. Reconstruction costs pay off within two to three years.

In the workshop of NiOPSV of Minudobreniya JSC, 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 concentrations of suspended solids in the range of 150-180 mg/dm 3 , according to BOD-5 - up to 160 mg/dm 3 , according to COD - 250-350 mg/dm 3 . Treatment facilities are made according to the classical scheme of biological treatment. The resulting sludge, after special treatment, is fully used for the reclamation of an industrial landfill. Buildings built 40 years ago have been reconstructed many times. In the last decade, the reconstruction of biological treatment in aerotanks was completed in order to improve the quality of wastewater treatment and the energy efficiency of the treatment process.

Traditional schemes of biological treatment (with the use of microorganisms in suspension in aerotanks with their subsequent sedimentation in secondary settling tanks) do not provide efficient and reliable wastewater treatment to the strict established norms of permissible discharge. Particularly great difficulties arise when reaching the norms of permissible discharge for reservoirs of fishery importance.

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

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

The SBR technology assumes successive carrying out in one reactor in a periodic mode, during its operation, alternation of aerobic and anaerobic processes. This technology is very expensive and requires a complex system of actuators to execute commands from the process control system. The cyclically variable oxygen conditions of vital activity of microorganisms in such a reactor, due to the adaptive factor, slow down 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 multivariant system of recycles of activated sludge and effluents from various treatment zones greatly complicates the control of the technological process and its controllability. The number of actuators installed in inaccessible places is increasing, the volume of pumped activated sludge is significantly increasing.

The technology of biomass concentration using forms of microorganisms suspended and attached on inert carriers is associated with the costs of acquiring carriers, installing these carriers in bioreactors, and significant difficulties in repairing aeration systems. The appearance of a 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 sedimentation tanks. The technology of concentrating the biomass of suspended forms of microorganisms in the reactor (with subsequent separation on polymeric membranes) is associated with the expense of funds for 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 in order to reduce the discharge of organic pollutants and biogenic elements into water bodies. This is possible when using aerotanks-displacers in the extended aeration mode.

The process of wastewater treatment in the aeration tank can be represented as follows. When clarified effluents enter the aeration tank, the effluents are mixed with return sludge. Sorption of undissolved and colloidal contaminants, which come with clarified effluents, occurs on the surface of zoogles that make up silt floes. Located on the surface of zoogles, which are covered with polysaccharide helium, bacteria in the presence of oxygen secrete enzymes to oxidize contaminants. Part of the dissolved contaminants enters the body of bacteria, where they are oxidized with the help of enzymes. During the oxidation of contaminants 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 process of bacterial development in the aerotank can be conditionally divided into three phases. The first of these 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 predator microorganisms that use a lot of bacteria and the remaining pollution as food and for subsequent reproduction. The depletion of easily oxidized organic matter transfers 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 activated sludge itself. The number of bacteria is sharply reduced, the number of predatory microorganisms is determined by the rate of self-oxidation of sludge microorganisms.

In the third phase, the oxidation of the resulting inorganic nitrogen compounds begins - the nitrification reaction occurs using a large amount of oxygen from the sludge mixture. In the phase of endogenous respiration of microorganisms, the following processes occur: the formation of large dense cotton silt from zoogles of bacteria, filamentous bacteria, fungi, actinomycetes; the process of oxidation of organic matter continues - the substances 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 activated sludge biocenosis of in-line aerotanks operating in the low-load mode, with deep nitrification and denitrification, is characterized by a large species diversity (over 30 species of protozoa), but without the 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 possible only by creating different zones: anaerobic, aerobic and anoxic. Sludge cotton can be considered as a spherical or ellipsoid formation with zones inside it where dissolved oxygen from the sludge mixture does not enter, even at a significant oxygen concentration (4-6 mg / dm 3) in wastewater.

To carry out the process of cleaning wastewater from the incoming pollution, it is necessary to carry out a deep oxidation of the organic matter contained in clarified water, a deep oxidation of the activated sludge bacteria substance. The obtained nitrogen-containing substances are oxidized to nitrates and reduced to gaseous nitrogen. To increase the rate of the reduction reaction (denitrification), it is necessary to increase the anoxic and anaerobic zones in the aerotank.

The increase is carried out in two ways:

• by increasing the amount of sludge pops, which leads to an increase in the concentration of sludge up to 5-6 mg/dm3;
• due to the increase in the size of the sludge cotton, which leads to a decrease in the BOD load to 35-50 mg per gram of dry matter per day, maintaining microorganisms in the endogenous respiration phase.

At the same time, low BOD loads on the aerotank allow for deep oxidation of organic matter up to 3.5 mg/dm 3 , almost to the theoretically achievable 2.5 mg/dm 3 . On the basis of the above theoretical provisions in the aerotanks of the NiOPSV shop, an operating mode 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; silt dose - 5-6 g / dm 3; the concentration of dissolved oxygen - 4-6 mg / dm 3; recirculation coefficient - 0.8-1.0; electrode potential within -200...-250 mV; silt index - 90-130; ash content of sludge - 35-40%; specific air consumption for aeration - 6-7 m 3 per 1 m 3 of wastewater; specific electricity consumption for aeration - 0.35-0.4 kWh per 1000 m 3.

At the same time, it is necessary to note the shortcomings of in-line aerotanks:

• uneven load on activated sludge along the length of 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 shortcomings, a longitudinal recycling of the sludge mixture was arranged in the aerotanks. The scheme 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 coefficient is 2.1-2.5. As a result of a longer stay of activated sludge in aerobic conditions and an acceleration of biomass turnover: the oxidizing ability 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 run length, of the treated wastewater, which eliminates the “breakthrough” of non-oxidized 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 difficult-to-oxidize industrial effluents are received, the control of the state of the sludge was carried out according to the method of bioestimation; to stabilize the oxygen regime in the sludge mixture during the repair of blowers.

The activated sludge biocenosis of in-line aerotanks operating in the low-load mode, with deep nitrification and denitrification, is characterized by a large species diversity (over 30 species of protozoa) without the numerical predominance of any species. The number of filamentous bacteria, small colorless flagella, small forms of naked and testate amoebas is insignificant. Of ciliates, ventral 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 entry into it of substances released from microfauna fragments during their destruction in aerotanks in the endogenous respiration phase. Activated sludge always contains rotifers (photo 2-3), sucking ciliates, predatory fungi, various worms, and tardigrades.

For BOD5, a value of 3 mg/dm 3 was reached, corresponding to the maximum allowable discharges (MPD) for fishery reservoirs (Fig. 2). In terms of COD - 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%, cleaning for heavy metals - 94-96%, efficiency for oil products - 92-96%.

During the operation of aerotanks in the low load mode with the value of the longitudinal recycle coefficient of 2-3:

• a high quality of wastewater treatment is achieved in accordance with Helcom recommendations without increasing the cost of electricity during operation;
• high quality cleaning does not require large expenditures of raw materials;
• the process is easy to maintain and control;
• reconstruction of in-line aerotanks into aerotanks operating in the extended aeration mode requires minimal costs (for the reconstruction of the aeration system, an increase in the productivity of pumps for return sludge, the 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 of its dehydration and disposal are reduced;
• the technological process does not become more complicated (there are no costs for complex control devices, executive regulatory mechanisms, the requirements for the qualification of maintenance personnel do not increase).

Such a reconstruction is a real way to improve the quality of treatment of most treatment facilities of regional significance. The costs of further improving the quality of purification in terms of nitrogen and phosphorus (until the established MPD standards for fishery reservoirs are reached) turn out to be 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 characteristics of activated sludge biocenoses in highly loaded aerotanks and aerotanks with a long aeration period // 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. Methodological guide for hydrobiological and bacteriological control of the biological treatment process at facilities with aerotanks. - M.: Aquaros LLC, 1996.
4. Nikitina O.G. Bioestimation: control and regulation of the processes of biological purification and self-purification of water. Abstract for the competition uch. Art. d.b.n. - M., 2012.
5. Kapitonova G.V. Guidelines for conducting hydrobiological control of wastewater treatment with activated sludge. - M., 2012.