JP3384951B2 - Biological water treatment method and equipment - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a biological water treatment method and equipment for biologically treating municipal wastewater, industrial wastewater, raw water for tap water, and the like, and in particular, for stably removing phosphorus in inflow water. Of biological water treatment method and equipment.

[0002]

2. Description of the Related Art In recent years, eutrophication has progressed in closed water areas such as lakes and marshes. The cause of this is the inflow of nitrogen and phosphorus into closed water areas, and it is desirable to immediately reduce this inflow. At the sewage treatment plant, sewage such as domestic wastewater and factory wastewater is mainly removed of organic substances in the sewage by a microbial treatment called an activated sludge method. However, the inflowing sewage contains, in addition to organic matter, nitrogen and phosphorus, which are pollution sources in the discharge area. Of these, phosphorus in sewage is mainly orthophosphoric acid (PO4-P).
As, nitrogen flows in as ammoniacal nitrogen. If these phosphorus and nitrogen cannot be removed at the sewage treatment plant and are discharged as they are, algae that use these as nutrient sources will proliferate abnormally, leading to water pollution. Therefore, at the sewage treatment plant, phosphorus must be removed as much as possible so that phosphorus does not flow out. For this reason, the number of sewage treatment plants introducing a system that can remove phosphorus and nitrogen in addition to organic substances is increasing. A flocculation method is a physicochemical method for removing phosphorus. The coagulation method is a method in which a coagulant is added to raw water or precipitation water to coagulate and precipitate phosphorus, and efficient phosphorus removal is possible. But,
In order to apply the coagulation method, a coagulation-sedimentation facility must be newly installed in addition to the conventional activated sludge facility. Therefore, there are many problems that must be solved before actual operation, such as an increase in construction cost, difficulty in securing a site, and an increase in running cost for adding a coagulant. As described above, in the treatment plant during actual operation, it is required to use the phosphorus and nitrogen removal method utilizing the conventional activated sludge method facility. The conventional activated sludge method only ingests a small amount of phosphorus in the sewage during the growth of the activated sludge cells, and cannot remove phosphorus for the purpose of preventing eutrophication. The coagulant-added activated sludge method is a method for injecting a coagulant into the outlet of an anaerobic tank, which is one of the facilities of this activated sludge process, to simultaneously remove organic matter and phosphorus. The coagulant-added activated sludge method has the advantage that existing facilities can be used, but the coagulant must be continuously injected in order to constantly remove the phosphorus in the effluent water, which significantly increases the running cost. Also, when coagulant is injected into activated sludge, the amount of sludge generated increases,
Sludge treatment costs also increase. Furthermore, the addition of a coagulant to activated sludge poses the problems of changing the biota and lowering the pH of discharged water.

On the other hand, as a biological method which does not use a flocculant, a biological reaction tank which is one facility of an activated sludge process is remodeled into an aerobic region and an anaerobic region, and a microbial reaction in which these regions are effectively combined. There is a biological phosphorus removal method that enables phosphorus removal as a tank. Activated sludge releases polyphosphoric acid accumulated in the cells as orthophosphoric acid in an anaerobic state without oxygen, and when the activated sludge is made aerobic,
Ingesting more orthophosphoric acid released in the previous step and accumulating as polyphosphoric acid in the cells, phosphorus decreases at the aerobic tank outlet than influent water. This biological phosphorus removal method includes anaerobic-aerobic method (AO method), anaerobic-anoxic-aerobic method (A2O).
Method), but with a reaction that releases phosphorus in an anaerobic state,
It has to go through two steps: the reaction of ingesting phosphorus in the aerobic tank. Therefore, in order to prevent water pollution caused by phosphorus, it is necessary to release orthophosphoric acid from the activated sludge well in the anaerobic tank and efficiently ingest the orthophosphoric acid in the aerobic tank.

As a conventional phosphorus removing method, Japanese Patent Laid-Open No. 63-
As disclosed in Japanese Patent No. 242392, a method of injecting a flocculant based on phosphorus in inflow water, JP-A-3-2788
As disclosed in Japanese Patent Publication No. 93, there is a proposal such as a method of adding based on the oxidation-reduction potential (ORP) of a reaction tank.

[0005]

However, in the above-mentioned conventional biological phosphorus removal method, phosphorus can be removed if the biological state is normal even if the phosphorus concentration in the inflowing water is high, and the phosphorus removal ability by the organism is high. The coagulant should be injected complementally only when

Further, in Japanese Patent Laid-Open No. 63-242392, which determines based on the phosphorus in the inflow water, the phosphorus in the inflow water is measured and the coagulant is injected, but from the value of the phosphorus in the inflow water, Unpredictable effluent phosphorus after biological phosphorus removal process treatment. For example, even if the phosphorus in the inflow water is directly measured and the coagulant is injected with the phosphorus, the coagulant may be injected even though the phosphorus in the discharge water is sufficiently removed. In this case, the living matter of the activated sludge is affected, and there is a possibility that the normal living state is deteriorated. Even if the phosphorus content in the inflow water is low, the phosphorus content in the released water may be higher than that in the inflow water if the balance between phosphorus release and intake is upset. Since the behavior of the organism that releases phosphorus from the activated sludge well in the anaerobic tank and then efficiently ingests the phosphorus in the aerobic tank is not considered, the decision is made based on the phosphorus in the inflow water. Proper injection is impossible by the method of the gazette.

Further, the method of Japanese Patent Laid-Open No. 3-278893, in which a flocculant is added based on the ORP of the reaction tank, is the ORP.
ORP because it is a method to maintain the anaerobic state by the value
There is no correlation between the measured value of and the amount of released phosphorus, and even if the anaerobic state can be maintained, as a result, the biological phosphorus removal ability cannot be maintained. The sewage that flows into the sewage treatment plant is not only the water quality of organic matter, phosphorus, nitrogen, etc., but also DO (di
The ssolved oxygen) and pH are also changing every moment. OR
P is an unstable value whose value changes under the influence of pH, water temperature, and DO. In addition to fluctuation of inflow sewage, air flow control,
Naturally, it also varies depending on operating conditions such as control of returned sludge. In addition, long-term data fluctuations due to electrode deterioration and instantaneous fluctuations due to the adhesion of foreign matter also occur. in this way,
ORP measurements do not provide insight into the biological phosphorus removal process. Furthermore, it cannot be applied to the optimization of process operation management. As described above, in the operation of the biological phosphorus removal method, it is premised that phosphorus is sufficiently released in the anaerobic tank, and it is very important to operate while understanding the phosphorus release and intake state. In the past, there was no response.

An object of the present invention is to provide a biological water treatment method and equipment capable of improving phosphorus removal efficiency.

[0009]

In order to achieve the above object, the present invention is an organism in which soluble phosphorus in influent water is released in an anaerobic tank and is excessively ingested in a subsequent aerobic tank to remove phosphorus. In the biological water treatment process, the amount of water and the organic matter concentration of the inflow water flowing into the anaerobic tank are measured, the phosphorus release rate in the anaerobic tank is calculated based on the measurement result, and the phosphorus release rate is calculated based on the phosphorus release rate. The biological water treatment method is to calculate the anaerobic tank residence time of the inflow water in the anaerobic tank and adjust the volume of the anaerobic tank based on the anaerobic tank residence time. The biological phosphorus removal method requires that phosphorus is sufficiently released in the anaerobic tank, and this phosphorus release amount is related to the residence time of inflow water in the anaerobic tank, and it is necessary to change this residence time, that is, the anaerobic tank. Focusing on the fact that the amount of phosphorus released can be controlled by adjusting the volume of, and that the phosphorus release rate is proportional to the concentration of organic matter in the inflow water,
The phosphorus release rate of the anaerobic tank was calculated from the amount of water flowing into the anaerobic tank and the concentration of organic matter, and the volume of the anaerobic tank was changed so that the phosphorus release rate was maintained based on the anaerobic tank residence time calculated from this phosphorus release rate. is doing. As a result, phosphorus is sufficiently released in the anaerobic tank, the biological phosphorus removal method can be operated without any trouble, and the quality of discharged water can be improved.

Further, the above-mentioned object is to release the soluble phosphorus in the inflowing water in the anaerobic tank and to ingest it excessively in the subsequent aerobic tank to remove phosphorus, and the inflowing water is Inflow water amount measuring means for measuring the amount of water flowing into the anaerobic tank, organic matter measuring means for measuring the concentration of organic matter in the inflow water flowing into the anaerobic tank, and phosphorus release in the anaerobic tank from the measurement result of the organic matter measuring means Phosphorus release rate calculation means for calculating the velocity, retention time calculation means for calculating the retention time of the anaerobic tank based on the calculation result by the phosphorus release rate calculation means, and based on the calculation result of the retention time calculation means It is also achieved by a biological water treatment facility provided with an anaerobic tank volume adjusting means for expanding or contracting the volume of the anaerobic tank. According to this configuration, the inflow water amount measuring means and the organic matter measuring means obtain the measurement contents necessary for the calculation of the phosphorus release rate in the anaerobic tank, and the residence time calculating means is based on the phosphorus release rate obtained by the phosphorus release rate calculating means. The residence time of the inflow water in the anaerobic tank is calculated by. The volume of the anaerobic tank is changed and controlled by the anaerobic tank volume adjusting means according to the size of the residence time. This ensures a sufficient residence time to release phosphorus in the anaerobic tank and improves the phosphorus removal efficiency.As a result, the operation of the biological phosphorus removal method can be performed without any problems, and high quality discharged water can be obtained. Like

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. Most of the phosphorus in the inflowing sewage is orthophosphoric acid. An aerobic tank is always installed in the sewage treatment plant, and when the anterior stage of this aerobic tank is an anaerobic tank without oxygen, the polyphosphoric acid accumulated in the activated sludge is released from the cells as orthophosphoric acid. Anaerobic conditions must be maintained for orthophosphoric acid release. On the other hand, in the aerobic tank, orthophosphoric acid is ingested by activated sludge in an amount more than the amount released, and is accumulated again in the microbial cells as polyphosphoric acid. Therefore, ingesting orthophosphoric acid, it is essential to cause sufficient phosphorus release in the preceding anaerobic tank. If the release is insufficient, the intake will also be insufficient, resulting in a high concentration of phosphorus in the discharged water. In this way, phosphorus can be biologically removed by the combination of the anaerobic tank and the aerobic tank. As a result, it is possible to prevent the discharge of the phosphorus source that is indispensable for cell synthesis, suppress the growth of plankton and the like in the public water level area, and suppress the water pollution. Therefore,
It is important for operation management to grasp the deterioration and recovery of biological phosphorus removal capacity at an early stage.

The present inventors have experimentally found that the inflowing sewage organic matter concentration, the phosphorus release amount in the anaerobic tank, and the phosphorus intake amount are closely related to each other, and that the phosphorus release amount can be expressed by the inflowing organic matter concentration. Came to. That is, the amount of released phosphorus is proportional to the volume of the anaerobic tank and the retention time of the inflow water in the anaerobic tank, while the retention time changes in proportion to the volume of the anaerobic tank. Therefore, the phosphorus release rate can be obtained based on the concentration of organic matter in the inflow water, and the anaerobic tank residence time can be obtained from this phosphorus release rate. By changing the anaerobic tank volume from the anaerobic tank retention time, stable phosphorus release can be maintained because the phosphorus release amount can be controlled, and biological phosphorus removal ability can be maintained in combination with the excessive intake in the subsequent stage. The details will be described below.

FIG. 1 shows a sewage activated sludge treatment facility to which the biological water treatment method of the present invention is applied. Here, a biological phosphorus removal facility consisting of an anaerobic tank and an aerobic tank is shown. In the figure, 11 is a first settling tank to which the inflow sewage 10 is supplied, 12 is a first anaerobic tank to which sewage is supplied from the first settling tank 11 via a water supply pipe 13, and 14 is adjacent to the first anaerobic tank 12. The second anaerobic tank provided at 15 is an aerobic tank provided adjacent to the second anaerobic tank 14. The first anaerobic tank 12, the second anaerobic tank 14, and the aerobic tank 15 are integrally configured as one tank as a whole. Then, a mechanical stirring facility 16 (first mechanical stirring facility) is installed in the first anaerobic tank 12,
A mechanical stirring facility 17 (second mechanical stirring facility) and an air diffuser 18 are installed in the second anaerobic tank 14, and an air diffuser 19 is installed in the second anaerobic tank 15. To the aerobic tank 15, a final settling tank 20 to which the treated sewage is supplied is connected. A sludge return pipe 21 is laid between the final settling tank 20 and the first anaerobic tank 12, and a return sludge facility 22 is provided in the middle thereof. A sludge discharge pipe 23 is connected so as to branch from the sludge return pipe 21, and a surplus sludge facility 24 is provided in the middle thereof.

An air pipe 33 is provided for sending air to the air diffusers 18 and 19, and a blower 25 for sending air under pressure is provided on the inlet side thereof. A valve 26 for adjusting the air supply amount is arranged between the outlet side of the blower 25 and the air diffuser 18. An organic matter concentration meter 27 and a flow meter 28 are attached to the water supply pipe 13, and the aerobic tank 15 has M
An LSS meter 29 is attached. This MLSS total 2
9, a phosphorus release rate calculation means 30 is connected to each of the organic substance concentration meter 27 and the flow meter 28, and each detection signal is fetched. A retention time calculation means 31 is connected to the phosphorus release rate calculation means 30, and an air volume control means 32 is connected to the retention time calculation means 31. The blower 25 and the valve 26 are controlled by the air volume control means 32.

Next, the operation of the water treatment facility having the above structure will be described. The inflow sewage 10 discharged from homes and factories is first settled in a settling tank 11 to remove foreign matters such as coarse dust and sand. The sewage that first flows out from the settling tank 11 is guided to the first anaerobic tank 12 via the water supply pipe 13. Further, in the first anaerobic tank 12, sludge returned from the final settling tank 20 through a sludge return pipe 21 (a group of microorganisms called activated sludge).
Is supplied. In the first anaerobic tank 12, the inflow sewage 10 and the returned sludge are agitated and mixed by the mechanical agitation equipment 16. Air for aeration is not sent to the first anaerobic tank 12, and the first anaerobic tank 12 is in an anoxic anaerobic state. In this anaerobic state, activated sludge hydrolyzes polyphosphoric acid accumulated in the body, and releases it as orthophosphoric acid (PO4-P) into sewage. or,
Activated sludge adsorbs organic matter at the same time as orthophosphoric acid is released and accumulates in the cells. By this biological reaction, phosphorus increases and organic matter decreases in the first anaerobic tank 12. In the second anaerobic tank 14, when the valve 26 is closed, it is operated as an anaerobic tank, and when the valve 26 is open, the blower 25 and the air pipe 3 are operated.
When the air sent through 3 is diffused by the diffuser pipe 43, it is operated as an aerobic tank. Second anaerobic tank 14
Effluent from the water is led to the aerobic tank 15. In the aerobic tank 15, the air sent from the blower 25 through the air pipe 33 diffuses from the air diffuser 19 to stir the mixed liquid of the sewage and the activated sludge in the aerobic tank 15 and generate oxygen. Supply. The returned sludge, that is, activated sludge, is a lump (floc) having a particle size of about 0.1 to 1.0 mm in which microorganisms are aggregated, and contains several tens of kinds of microorganisms. The pollutants of the mixed liquid in the aerobic tank 15 are processed by the action of activated sludge activated by oxygen supply. For example, activated sludge adsorbs organic matter, absorbs oxygen in the supplied air, oxidatively decomposes organic matter, and decomposes it into carbon dioxide gas and water. In addition, activated sludge ingests orthophosphoric acid in sewage and accumulates in the cells as polyphosphoric acid, and ammoniacal nitrogen (NH4-N) is oxidized to nitric acid (NO3-N) or nitrite nitrogen (NO2-N). To be done. In addition, some of these pollutants such as organic matter, phosphorus, and ammoniacal nitrogen are also utilized for the growth of activated sludge. The mixed liquid of activated sludge and sewage is guided to the final settling tank 20, where the activated sludge settles by gravity. The supernatant of the final settling tank 20 is usually sterilized by chlorine and then discharged into a river or the sea. On the other hand, most of the sedimented high-concentration activated sludge is returned to the aerobic tank 15 as return sludge by the sludge return facility 22, and a part of the activated sludge corresponding to the microbial growth is excess sludge from the sludge discharge pipe 23. As a result, the sludge is discharged to the outside of the system by the sludge discharge facility 24, and processed through a sludge treatment process such as dehydration and incineration. In this activated sludge process, the water discharged from the final settling tank 20 is
In addition to consuming dissolved oxygen in the effluent area, the purpose is to have a water quality that does not promote pollution.To secure this water quality, it is necessary to remove organic pollutants and remove nutrients such as phosphorus. is important.

In the first anaerobic tank 12, activated sludge releases orthophosphoric acid in an anaerobic state, so that phosphorus is higher at the outlet than at the first anaerobic tank 12 inlet. If the phosphorus release amount is sufficient, the second anaerobic tank 14 in the next step is operated as an aerobic tank.
In this case, the activated sludge in the second anaerobic tank 14 and the aerobic tank 15 is aerobically discharged, and in order to ingest more orthophosphoric acid than the amount released in the first anaerobic tank 12 into the microbial cells, it is discharged more than the inflowing sewage. Less orthophosphoric acid in water. Such a phosphorus removal method by releasing activated sludge and excessive intake is called a biological phosphorus removal method. This biological phosphorus removal method is a process on the assumption that phosphorus is sufficiently released under the anaerobic conditions in the first anaerobic tank 12, and if phosphorus release is insufficient, the phosphorus is aerobic. Overdose does not progress. In other words, there is a possibility that the phosphorus removal rate may decrease, and that the released phosphorus may be released as it is and the phosphorus may increase. As a factor that inhibits phosphorus release, there is a decrease in organic matter. Organic matter in the inflowing sewage fluctuates depending on the natural world and living environment, and when the organic matter decreases, phosphorus release from the first anaerobic tank 12 does not proceed,
There is no overdose of phosphorus.

The organic matter concentration meter 27 is first installed at the exit of the sedimentation tank 11 or the entrance of the anaerobic tank 12, and the biological oxygen consumption (BOD) or the chemical oxygen consumption (COD) before the microbial reaction is measured as the organic matter concentration. To measure. Generally, the organic matter concentration of the inflowing sewage is about 80 to 150 mg / l. The phosphorus release rate calculation means 30 calculates the anaerobic tank phosphorus release rate and the phosphorus removal rate from the organic matter concentration measured by the organic matter concentration meter 27. A major inhibitor of phosphorus release is the organic matter concentration of the influent sewage. The phosphorus release rate calculation means 30 calculates the phosphorus release rate from the relational expression between the concentration of inflowing sewage organic matter and the phosphorus release rate as shown in, for example, [Equation 1]. The calculation result is sent to the residence time calculation means 31.

## EQU1 ## Pv = A.10 ** S (where Pv is the phosphorus release rate, S is the organic matter concentration, and A is the coefficient) The residence time calculation means 31 is the phosphorus release rate, the flowmeter 2
Based on each of the inflow flow rate measured in 8 and the sludge concentration measured by the MLSS meter 29, the anaerobic tank residence time for making the phosphorus release amount equal to or more than a predetermined value is calculated. Air volume control means 3
When the retention time can be secured in the first anaerobic tank 12, the valve 2 is opened to send air to the second anaerobic tank 14 to generate aerobic conditions, and the retention time cannot be secured in the first anaerobic tank 12. In this case, the valve 26 is closed to generate the anaerobic condition in the second anaerobic tank 14. As a result, the anaerobic tank volume is increased, the residence time is increased, and the phosphorus release amount can be secured.

FIG. 2 is a characteristic diagram showing the relationship between organic matter concentration and phosphorus release, and shows the relationship between influent sewage organic matter concentration and phosphorus release rate in the biological phosphorus removal method. It can be seen that when the phosphorus release rate is small, it takes a lot of time to release phosphorus sufficiently. Based on this fact, the present inventors have found that there is a close relationship between the influent sewage organic matter concentration, the anaerobic tank phosphorus release amount, and the phosphorus intake amount, and it is possible to express the phosphorus release amount by the influent organic matter concentration. Found experimentally. That is, it was found that the higher the concentration of organic matter in the inflowing sewage, the higher the phosphorus release rate in the first anaerobic tank 12, and there is a correlation between the two.

FIG. 3 shows the relationship between phosphorus release and intake. As is clear from FIG. 3, the correlation between the phosphorus release amount and the intake amount is high, and the intake amount increases as the phosphorus release amount increases.

FIG. 4 shows the relationship between phosphorus release and excess intake. In the anaerobic tank, if the phosphorus release amount is a predetermined value (5 mg / L in this example) or more, excessive intake proceeds and the phosphorus removal rate increases. When the amount of phosphorus released in the anaerobic tank is small, intake does not proceed even if the next step is aerobic, and the phosphorus removal rate decreases. As described above, in the biological phosphorus removal method, it is important to keep the amount of released phosphorus at a predetermined value or more.

The relational expression between the concentration of inflowing sewage organic matter and the phosphorus release rate may be obtained in advance by experiments. Also, for example,
A display unit such as a display may be provided to display data or the like on the screen as numerical values or graphs. Alternatively, the display color of the graphic symbol may be changed depending on the numerical level. When the amount of released phosphorus is less than or equal to a predetermined value, an alarm sound or voice may be emitted.

[0022]

According to the present invention, the amount of water flowing into the anaerobic tank and the organic matter concentration are measured, the phosphorus release rate in the anaerobic tank is calculated based on the measurement result, and the phosphorus release rate is calculated based on the phosphorus release rate. The anaerobic tank residence time of the inflow water in the anaerobic tank was calculated, and the volume of the anaerobic tank was adjusted based on the anaerobic tank residence time. Therefore, the quality of the discharged water in a water treatment plant having an activated sludge treatment facility was calculated. As a result of the early understanding of the rise in phosphorus in water, the biological phosphorus removal process can be effectively controlled, and a high quality of discharged water can be obtained. Also, the anaerobic tank is changed to 2 tanks, and the second
By setting the anaerobic tank under the anaerobic condition or the aerobic condition according to the calculation result of the residence time, the anaerobic tank can be expanded or contracted, and the phosphorus release amount can be maintained at a predetermined value.

[Brief description of drawings]

FIG. 1 shows a sewage activated sludge treatment facility to which the biological water treatment method of the present invention is applied.

FIG. 2 is a characteristic diagram showing the relationship between inflowing sewage organic matter and phosphorus release rate.

FIG. 3 is a characteristic diagram showing a relationship between phosphorus release concentration and phosphorus intake concentration.

FIG. 4 is a characteristic diagram showing the relationship between phosphorus release concentration and phosphorus excessive intake concentration.

[Explanation of symbols]

11 First settling tank 12 First Anaerobic Tank 14 Second Anaerobic Tank 15 aerobic tank 16,17 Mechanical stirring equipment 18,19 Air diffuser 25 Blower 26 valves 27 Organic matter concentration meter 28 Flowmeter 29 MLSS meter 30 Phosphorus release rate calculation means 31 Residence time calculation means 32 Air volume control means 33 air tube

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-9-228319 (JP, A) JP-A-8-117793 (JP, A) JP-A-8-323393 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) C02F 3/12 C02F 3/28-3/34

Claims (5)

(57) [Claims] 1. A biological water treatment process in which soluble phosphorus in influent water is released in an anaerobic tank and is excessively ingested in a subsequent aerobic tank to remove phosphorus, and the influent water flowing into the anaerobic tank is discharged. The amount of water and the concentration of organic matter are measured, the phosphorus release rate in the anaerobic tank is calculated based on the measurement result, and the anaerobic tank residence time of the inflow water in the anaerobic tank is calculated based on the phosphorus release rate. A biological water treatment method comprising adjusting the volume of the anaerobic tank based on the residence time. 2. The volume of the anaerobic tank is adjusted so that the volume of the anaerobic tank is increased when the retention time of the anaerobic tank cannot be secured in a state where the anaerobic tank is set to a small volume. Item 2. The biological water treatment method according to Item 1. 3. A biological water treatment facility in which soluble phosphorus in influent water is released in an anaerobic tank and is excessively ingested in a subsequent aerobic tank to remove phosphorus, and the influent water flows into the anaerobic tank. Inflow water amount measuring means for measuring water amount, organic matter measuring means for measuring concentration of organic matter of inflow water flowing into the anaerobic tank, and phosphorus for calculating phosphorus release rate in the anaerobic tank from the measurement result of the organic matter measuring means. The release rate calculation means, the retention time calculation means for calculating the retention time of the anaerobic tank based on the calculation result by the phosphorus release rate calculation means, and the volume of the anaerobic tank based on the calculation result of the retention time calculation means A biological water treatment facility comprising an anaerobic tank volume adjusting means for expanding and contracting. 4. The anaerobic tank volume adjusting means sends the air to an air diffuser provided in the anaerobic tank, and the air volume control for controlling the anaerobic tank aeration means based on the residence time. The biological water treatment facility according to claim 3, further comprising means. 5. The anaerobic tank is configured by continuously arranging a first anaerobic tank equipped with a first mechanical stirring facility and a second anaerobic tank equipped with a second mechanical stirring facility and an air diffusing pipe. When the residence time can be secured only by the first anaerobic tank, air is supplied to the air diffuser pipe of the second anaerobic tank by the air volume control means, and when the residence time cannot be secured, the air volume control means is used. The biological water treatment facility according to claim 4, wherein the air supply to the second anaerobic tank is stopped.