JP6893546B2 - Sewage treatment system and sewage treatment method - Google Patents

Description

The present invention relates to a sewage treatment system and a sewage treatment method.

In recent sewage treatment, in addition to the removal (oxidation) of organic substances by the activated sludge method, the treatment of ammoniacal nitrogen is also performed. Ammonia nitrogen is nitrified (oxidized to nitrate nitrogen) by the action of nitrifying bacteria in, for example, an aerobic tank. Nitrate nitrogen may further denitrify (reduce to nitrogen gas) by causing denitrifying bacteria to act.

Patent Document 1 describes an aeration air volume control device for a sewage treatment plant including a plurality of series of sewage treatment processes of the same treatment method. At this sewage treatment plant, organic matter, phosphorus compounds, and nitrogen compounds are removed by an anaerobic-anoxic-aerobic method. In each series, an anaerobic tank, an oxygen-free tank, an aerobic tank, and a final settling basin are connected in order from the first settling basin. Aeration treatment is performed in each aerobic tank to oxidize organic matter and nitrify ammoniacal nitrogen. Each aeration tank has an aeration device that operates according to an aeration air volume target value. The aeration air volume control device includes an inflow pump that controls the flow of sewage flowing into all series so that the flow rates are equal, and an ammonia meter that measures the ammonia nitrogen concentration in the aerobic tank of any one of the multiple series. A dissolved oxygen concentration meter that measures the dissolved oxygen concentration of all the aerobic tanks of multiple series, a No. 1 control target value setting machine that sets the ammonia nitrogen concentration target value of the treated water, and an ammonia meter are installed. No. 2 control that sets the target value of the dissolved oxygen concentration of the other series of aerobic tanks without an ammonia meter based on the measured value of the dissolved oxygen concentration of the aerobic tank of the series It is equipped with a target value setting machine. In addition, the aeration device of the aerobic tank of the series in which the ammonia meter is installed uses the No. 1 ammonia controller that calculates the aeration air volume target value that brings the measured value of the ammonia nitrogen concentration closer to the ammonia nitrogen concentration target value. Each aeration device in the aerobic tank in which is not installed is equipped with a No. 2 DO controller that calculates an aeration air volume target value that brings the measured value of the dissolved oxygen concentration closer to the dissolved oxygen concentration target value. This aeration air volume control device performs aeration control based on the ammonia nitrogen concentration in the aerobic tank without using a large number of expensive ammonia meters.

Japanese Unexamined Patent Publication No. 2005-199115

According to the aeration air volume control device described in Patent Document 1, the aeration air volume target value of the aeration device of the aeration tank in which the ammonia meter is not installed is set in the aeration tank of the series in which the ammonia meter is installed. It is determined based on the dissolved oxygen concentration measured by the dissolved oxygen concentration meter. That is, after the dissolved oxygen concentration in the aerobic tank in which the ammonia meter is installed fluctuates due to the aeration treatment based on the ammonia nitrogen concentration target value, the dissolved oxygen concentration measurement value fluctuates according to the fluctuation. , The dissolved oxygen concentration in the aerobic tank of the series without an ammonia meter also fluctuates. When the dissolved oxygen concentration in the aerobic tank without an ammonia meter is hunted due to this fluctuation and the dissolved oxygen concentration is insufficient, and sewage purification such as oxidation of organic substances and nitrification of ammonia nitrogen cannot be performed sufficiently. There is. In order to prevent this, it is necessary to set the aeration air volume target value of the aeration tank of the series without the ammonia meter after the dissolved oxygen concentration of the aerobic tank of the series with the ammonia meter is stable. There is. As a result, there was a problem that the control of the dissolved oxygen concentration in the aerobic tank of the series in which the ammonia meter was not installed was delayed as compared with the control of the dissolved oxygen concentration in the aerobic tank of the series in which the ammonia meter was installed. .. If the control of the dissolved oxygen concentration is delayed, the amount of aerated air becomes excessive and energy is wasted. Therefore, it is desired to provide a sewage treatment system and a sewage treatment method capable of appropriately controlling the aeration air volume of each aeration tank in a plurality of series of sewage treatment processes and appropriately controlling the dissolved oxygen concentration.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a sewage treatment system and a sewage treatment method capable of appropriately controlling the dissolved oxygen concentration.

The characteristic configuration of the sewage treatment system according to the present invention for achieving the above object is a plurality of treatment series having an aerobic treatment unit that aerates the inflowing sewage, and ammonia provided in at least one of the treatment series. It has an ammonia measuring unit that has a meter and acquires the ammoniacal nitrogen concentration of the treatment series, and a dissolved oxygen concentration meter provided in each of the aerobic treatment units in the plurality of the treatment series. The control unit includes a dissolved oxygen concentration measuring unit that acquires the dissolved oxygen concentration of the air treatment unit and a control unit that controls the aeration of each of the aeration treatment units. The control unit is the dissolved oxygen concentration of the aerobic treatment unit. It has a first control unit that sets a target value and a second control unit that controls the aeration air volume of the aerobic treatment unit, and the first control unit has each based on the ammoniacal nitrogen concentration. The dissolved oxygen concentration target value of the aerobic treatment unit is set, and the second control unit is based on the dissolved oxygen concentration target value and each dissolved oxygen concentration acquired by the dissolved oxygen concentration measuring unit. The point is to control the aeration air volume of each of the aerobic treatment units.

According to the above configuration, in a sewage treatment system having a plurality of treatment series of the same treatment method, the ammonia property acquired by the ammonia measurement unit in a specific treatment series in which the ammonia meter is installed without using many expensive ammonia meters. The first control unit sets the dissolved oxygen concentration target value of the aerobic treatment unit based on the nitrogen concentration, and the second control unit sets the dissolved oxygen concentration target value of the aerobic treatment unit and the dissolved oxygen concentration of each aerobic treatment unit. The aeration air volume can be controlled. For example, the first treatment series is based on the ammonia nitrogen concentration of the treatment series in which at least one of the treatment series is provided with an ammonia meter and at least one of the other treatment series is not provided with an ammonia meter. The control unit can set the dissolved oxygen concentration target value of each aerobic treatment unit. Further, the second control unit can control the aeration air volume of each aerobic treatment unit based on the dissolved oxygen concentration target value and the dissolved oxygen concentration of each aerobic treatment unit.

Further, according to the above configuration, the dissolved oxygen concentration target value is set based on the ammonia nitrogen concentration acquired by the ammonia measuring unit, and the aeration air volume of each aerobic treatment unit is controlled. Therefore, the second control unit can appropriately and quickly control the aeration air volume in each aerobic treatment unit. Thereby, the dissolved oxygen concentration can be appropriately controlled. For example, the first control (for example, the control of the dissolved oxygen concentration of the aerobic treatment section of the treatment series provided with the ammonia meter) is performed based on the ammonia nitrogen concentration, and the control result (the favor of the treatment series provided with the ammonia meter) is performed. Compared to the case where the second control (for example, the control of the dissolved oxygen concentration in the aerobic treatment unit without an ammonia meter) is performed based on the controlled dissolved oxygen concentration in the air treatment unit. Control delays and hunting can be avoided.

A further characteristic configuration of the sewage treatment system according to the present invention is that the ammonia meter is provided in the aerobic treatment section.

According to the above configuration, the dissolved oxygen concentration target value of the aerobic treatment unit is first controlled based on the ammonia nitrogen concentration acquired by the ammonia measurement unit in the aerobic treatment unit in the specific treatment series in which the ammonia meter is installed. The unit sets, and the second control unit can control the aeration air volume based on the dissolved oxygen concentration target value and the dissolved oxygen concentration of each aerobic treatment unit. For example, an aerobic treatment unit of a treatment series in which an ammonia meter is provided in an aerobic treatment unit in at least one of the treatment series, an ammonia meter is not provided in at least one of the other treatment series, and an ammonia meter is provided. Based on the ammoniacal nitrogen concentration, the first control unit can set the dissolved oxygen concentration target value of each aerobic treatment unit. Further, the second control unit can control the aeration air volume of each aerobic treatment unit based on the dissolved oxygen concentration target value and the dissolved oxygen concentration of each aerobic treatment unit. The aerobic treatment unit includes the concept of a treatment tank (so-called aerobic tank) in which oxygen is supplied by aeration and a treatment category in which oxygen is supplied in the treatment tank.

A further characteristic configuration of the sewage treatment system according to the present invention further includes an anaerobic treatment unit or an oxygen treatment unit upstream of the aerobic treatment unit, and the ammonia meter is the anaerobic treatment unit or the oxygen treatment unit. It is in the point provided in.

According to the above configuration, when an anaerobic treatment unit or an oxygen-free treatment unit is provided in each treatment series, an ammonia measurement unit is provided in the anaerobic treatment unit or the oxygen-free treatment unit in a specific treatment series in which an ammonia meter is installed. The first control unit sets the dissolved oxygen concentration target value of the aerobic treatment unit based on the ammonia nitrogen concentration acquired by the first control unit, and based on the dissolved oxygen concentration target value and the dissolved oxygen concentration of each aerobic treatment unit. The second control unit can control the aerated air volume. For example, an ammonia meter is provided in an anaerobic treatment unit or an oxygen-free treatment unit in at least one of the treatment series, and an ammonia meter is not provided in at least one of the other treatment series, and an ammonia meter is provided in the treatment series. Based on the ammoniacal nitrogen concentration of the treatment unit or the anoxic treatment unit, the first control unit can set the dissolved oxygen concentration target value of each aerobic treatment unit. Further, the second control unit can control the aeration air volume of each aerobic treatment unit based on the dissolved oxygen concentration target value and the dissolved oxygen concentration of each aerobic treatment unit.

A further characteristic configuration of the sewage treatment system according to the present invention is a storage unit that stores an ammoniacal nitrogen concentration target value, and sends out control information based on the ammoniacal nitrogen concentration and the ammoniacal nitrogen concentration target value. A control unit is further provided, and the first control unit sets a target value of the dissolved oxygen concentration of each of the aerobic treatment units based on the control information.

According to the above configuration, the third control unit sends out control information based on the ammonia nitrogen concentration acquired by the ammonia measurement unit and the ammonia nitrogen concentration target value stored in the storage unit. The first control unit sets the dissolved oxygen concentration target value based on this control information, and sets the dissolved oxygen concentration target value based on the ammoniacal nitrogen concentration and the ammoniacal nitrogen concentration target value acquired by the ammonia measurement unit. Can be set.

The control information includes information corresponding to the degree of deviation between the ammoniacal nitrogen concentration and the ammoniacal nitrogen concentration target value (hereinafter referred to as the degree of deviation), a predetermined signal derived from the ammoniacal nitrogen concentration, and the like. Can be mentioned. For example, if the ammoniacal nitrogen concentration deviates within a predetermined range from the ammoniacal nitrogen concentration target value, the third control unit sends out the degree of dissociation as a control signal. When, for example, the difference between the ammoniacal nitrogen concentration and the ammoniacal nitrogen concentration target value is used as the degree of deviation, the first control unit sets the dissolved oxygen concentration target value based on the positive / negative of the degree of deviation and the magnitude relationship thereof. If the degree of deviation is positive, the first control unit sets a larger dissolved oxygen concentration target value as the degree of deviation becomes positive. If the degree of divergence is negative, the first control unit sets a smaller dissolved oxygen concentration target value as the degree of divergence becomes negative. This makes it possible to set a dissolved oxygen concentration target value at which the ammoniacal nitrogen concentration can be appropriately controlled. Further, for example, if the ammoniacal nitrogen concentration exceeds the predetermined range and is smaller than the ammoniacal nitrogen concentration target value due to changes in the amount and quality of sewage due to changes in the time zone, weather, and season, the above-mentioned predetermined signal can be used. An instruction signal that ignores the ammoniacal nitrogen concentration is sent, or a corrected deviation degree with a predetermined correction is sent instead of the above deviation degree. Based on this predetermined signal, the first control unit can set an appropriate dissolved oxygen concentration target value in consideration of (priority) oxygen consumption for oxidation of organic matter. As described above, according to the above configuration, the dissolved oxygen concentration target value is set in consideration of the control of the ammoniacal nitrogen concentration, and the dissolved oxygen concentration target is considered in consideration of the oxygen consumption for oxidation of organic substances. You can set the value.

A further characteristic configuration of the sewage treatment system according to the present invention is that the storage unit stores the ammoniacal nitrogen concentration target value in the aerobic treatment unit.

According to the above configuration, the third control unit sends out control information based on the ammonia nitrogen concentration acquired by the ammonia measurement unit and the ammonia nitrogen concentration target value in the aerobic treatment unit. The first control unit sets the dissolved oxygen concentration target value based on this control information, and dissolves based on the ammonia nitrogen concentration acquired by the ammonia measurement unit and the ammonia nitrogen concentration target value in the aerobic treatment unit. Oxygen concentration target value can be set.

A further characteristic configuration of the sewage treatment system according to the present invention is that the second control unit PID controls the aeration air volume.

According to the above configuration, the second control unit can PID control the aerated air volume based on the dissolved oxygen concentration target value and each dissolved oxygen concentration acquired by the dissolved oxygen concentration measuring unit. As a result, the dissolved oxygen concentration can be appropriately maintained at the dissolved oxygen concentration target value.

A further characteristic configuration of the sewage treatment system according to the present invention is that the first control unit sets the dissolved oxygen concentration target value by PID control.

According to the above configuration, the first control unit sets the dissolved oxygen concentration target value by PID control based on the ammonia nitrogen concentration, so that the amount and quality of sewage change due to changes in time zone, weather and season. It is possible to set a dissolved oxygen concentration target value corresponding to.

A further characteristic configuration of the sewage treatment system according to the present invention is that it has a distribution unit for distributing the sewage so that the residence time of the sewage flowing into the treatment series in the treatment series is equal.

According to the above configuration, the sewage is distributed by the distribution unit so that the residence time of the sewage flowing into each aerobic tank is equal among the aerobic tanks, so that each aerobic tank is in an even state. Can be squeezed. As a result, when the dissolved oxygen concentration target value of each aerobic tank is set and the aeration air volume is controlled based on the ammonia nitrogen concentration of the specific aerobic tank in which the ammonia meter is installed, each favor is obtained. The dissolved oxygen concentration in the air tank can be controlled more appropriately to the dissolved oxygen concentration target value.

The characteristic configuration of the sewage treatment method according to the present invention for achieving the above object is an aeration step in which the inflowing sewage is distributed to a plurality of treatment series and aerated in each aerobic treatment unit, and at least one of the above. An ammonia measurement step for acquiring the aeration nitrogen concentration of the treatment series, a dissolved oxygen concentration measurement step for acquiring the dissolved oxygen concentration of each of the aeration treatment units, and each of the aeration based on the aeration nitrogen concentration. Aeration of each of the aerobic treatment units based on the first control step of setting the dissolved oxygen concentration target value of the treatment unit, the dissolved oxygen concentration target value, and the dissolved oxygen concentration of each of the aerobic treatment units. It has a second control step for controlling the air volume.

According to the above configuration, the same effect as that of the above-mentioned sewage treatment system can be obtained.

Explanatory drawing of sewage treatment system of 1st Embodiment Explanatory drawing of air volume control method in 1st Embodiment Explanatory drawing of modification of 1st Embodiment

The sewage treatment system and the sewage treatment method according to the embodiment of the present invention will be described with reference to FIGS. 1 to 3.

[First Embodiment]
[Explanation of overall configuration]
FIG. 1 illustrates a case where the sewage treatment system 100 has three series (plural series) of treatment series (series A, series B, and series C) parallel to the inflowing sewage. .. Each series of series A, series B, and series C has the same processing method. Each series of series A, series B, and series C has an aerobic tank 1a, an aerobic tank 1b, and an aerobic tank 1c as an aerobic tank 1 (an example of an aerobic treatment unit). These plurality of aerobic tanks 1 are provided in parallel with the inflowing sewage. In the following, the mixed solution of the sewage and activated sludge in the aerobic tank 1 will be simply referred to as the mixed solution.

In the series A, the series B, and the series C, an ammonia meter 20 for measuring the ammoniacal nitrogen concentration in the tank is attached to the aerobic tank 1a of the series A, and the aerobic tanks of the series B and the series C are attached. The tank 1b and the aerobic tank 1c are different in that the ammonia meter 20 is not attached, and the other configurations are the same. Series B and series C have the same configuration.

As shown in FIG. 1, the sewage treatment system 100 obtains the dissolved oxygen concentration of each aerobic tank 1 by the dissolved oxygen concentration meter 30 (an example of the dissolved oxygen concentration measuring step), and the dissolved oxygen concentration measuring unit 3 is favorable. This is a sewage treatment facility having an ammonia measuring unit 2 for acquiring the ammonia-dissolved nitrogen concentration in the air tank 1a with an ammonia meter 20. Each operation of the sewage treatment system 100 is performed based on the ammoniacal nitrogen concentration target value stored in the storage unit 8 and the command of the control unit 9.

In the sewage treatment system 100, the first settling basin 51 and the aerobic tank 1 are arranged in order from the upstream side where the sewage flows into each series downstream of the distribution unit 5 which evenly distributes the sewage to the series A, the series B, and the series C. , Has a final settling basin 59. Purified water is discharged from the final settling basin 59 to the outside of the sewage treatment system 100. Initially, the settling basin 51 and the aerobic tank 1 are connected by waterways in order from the upstream. By the equal distribution of sewage by the distribution unit 5, the residence time of each series of sewage purified by series A, series B, and series C becomes equal. In addition, the residence time in the aerobic tank 1 of each series is also the same.

In the following, the common parts of the series A, the series B, and the series C will be described by exemplifying the series A, and the description of the other series will be omitted. In particular, since the matters described for the series B are the same for the series C, the description for the series C will be omitted. Regarding the intersections of the series A, the series B, and the series C, for example, the aerobic tank 1 of the series A is represented by the aerobic tank 1a, and the aerobic tank 1 of the series B is represented by the aerobic tank 1b. The symbols a, b and c corresponding to the series A, the series B, and the series C shall be added to the code notation of the common parts, and the description for each series may be omitted.

[Explanation of each part]
The distribution unit 5 is a flow path device that evenly distributes the sewage flowing into the sewage treatment system 100 to the series A, the series B, and the series C. The distribution unit 5 of the present embodiment is a movable weir with an adjustable opening degree provided in a flow path for guiding sewage to the sewage treatment system 100. Each flow path of the flow path 5a, the flow path 5b, and the flow path 5c connected to the series A, the series B, and the series C is branched from the distribution unit 5. Sewage is evenly distributed to each flow path by adjusting the height of the weir of the distribution unit 5.

The first settling basin 51 is a tank for settling and removing a part of sludge contained in the inflowing sewage. The sewage containing sludge that could not be settled and removed in the first settling basin 51 flows into the aerobic tank 1.

The aerobic tank 1 has an aeration device 6 for performing aeration to supply air to the mixed solution (an example of an aeration step), and is equipped with a dissolved oxygen concentration meter 30 for measuring the dissolved oxygen concentration in the tank. It is a tank. The aerobic tank 1 is an aeration tank that performs microbial decomposition (oxidation) treatment of organic matter by an activated sludge method and nitrification of ammoniacal nitrogen by microbial treatment. Series A, Series B, and Series C have an aerobic tank 1a, an aerobic tank 1b, and an aerobic tank 1c, respectively, as the aerobic tank 1. An ammonia meter 20 for measuring the concentration of ammoniacal nitrogen in the tank (an example of the ammonia measurement step) is attached to the aerobic tank 1a.

The aeration device 6 is an air supply device that includes a blower 60 such as a fan or a blower, sucks outside air, and supplies air to the mixed liquid of the aerobic tank 1. In the following, the amount of air per unit time supplied by the aeration device 6 into the aeration tank 1 will be referred to as an air volume (an example of the aeration air volume). In the following, the oxidation treatment of organic substances and the nitrification of ammoniacal nitrogen are collectively referred to as treatment. The aeration device 6 maintains a predetermined air volume by adjusting the rotation speed of the blower 60 based on the operation command of the control unit 9.

In the aerobic tank 1, oxygen in the air supplied from the aeration device 6 is consumed by the oxidation of organic matter and the nitrification of ammoniacal nitrogen. In the aerobic tank 1, organic matter is decomposed into carbon dioxide and water by oxidation. In the aerobic tank 1, ammoniacal nitrogen is nitrified and converted to nitrate nitrogen.

In the aerobic tank 1, if the amount of air supplied from the aeration device 6 is insufficient, the oxygen required for the treatment is insufficient, and an appropriate (sufficient) treatment cannot be performed. In this case, undecomposed organic matter and ammoniacal nitrogen flow out to the outside of the sewage treatment system 100. If the amount of air supplied from the aeration device 6 is excessive, energy such as electric power consumed by the blower 60 or the like is wasted.

The final settling basin 59 is a settling basin that receives the mixed liquid flowing out of the aerobic tank 1 and setstles the activated sludge and the like in the aerobic tank 1. The activated sludge settled in the final settling basin 59 is discharged to the outside of the sewage treatment system 100 by a discharger (not shown) or the like. A part of the activated sludge settled in the final settling basin 59 may be returned to the aerobic tank 1 (so-called returned sludge).

The ammonia measuring unit 2 is a processing circuit including an ammonia meter 20 and transmitting the measured value of the ammoniacal nitrogen concentration acquired by the ammonia meter 20 to the control unit 9.

The dissolved oxygen concentration measuring unit 3 includes a dissolved oxygen concentration meter 30 attached to each aerobic tank 1, and transmits the acquired measured value of the dissolved oxygen concentration for each aerobic tank 1 to the control unit 9. It is a circuit.

As shown in FIGS. 1 and 2, the control unit 9 is a central control mechanism that controls each operation of the sewage treatment system 100. The control unit 9 controls the aeration air volume in each aerobic tank 1 based on the ammonia nitrogen concentration received from the ammonia measurement unit 2. The control unit 9 includes a first control unit 91, a second control unit 92, and a third control unit 93, which are functional units realized by executing a software program stored in the storage unit 8.

As shown in FIG. 2, the third control unit 93 sets the ammonia nitrogen concentration received from the ammonia measurement unit 2 and the ammonia nitrogen concentration target value DB 83, which is a database (so-called DB) constructed in the storage unit 8, in advance. This is a functional unit that sends control information to the first control unit 91 based on the stored ammoniacal nitrogen concentration target value. In the present embodiment, the storage unit 8 stores the target value in the aerobic tank 1a as the target value of the ammoniacal nitrogen concentration.

As control information, the third control unit 93 gives a signal including the degree of deviation between the ammoniacal nitrogen concentration and the ammoniacal nitrogen concentration target value (hereinafter referred to as a deviation degree signal) and an instruction to ignore the ammoniacal nitrogen concentration. A signal including the signal (hereinafter referred to as an ignore signal) can be transmitted. In the present embodiment, the third control unit 93 sends a dissociation degree signal when the ammoniacal nitrogen concentration deviates within a predetermined range from the ammoniacal nitrogen concentration target value. The predetermined range is, for example, a range exceeding a predetermined lower limit value smaller than the target value of ammoniacal nitrogen concentration. Further, the third control unit 93 sends an ignore signal when the ammoniacal nitrogen concentration is equal to or less than a predetermined lower limit value.

In the present embodiment, the third control unit 93 receives the ammoniacal nitrogen concentration from the ammonia measurement unit 2 in real time, and outputs control information in real time in response to the fluctuation of the ammoniacal nitrogen concentration.

The first control unit 91 individually determines or updates the dissolved oxygen concentration target value in each aerobic tank 1 based on the control information received from the third control unit 93 (an example of the first control step). It is a functional part. In the following, both the determination and the operation of updating the determined value are collectively referred to as a setting. The first control unit 91 has DO setting units 91a to DO setting units 91c, which are functional units for setting the dissolved oxygen concentration target values of the aerobic tank 1a, the aerobic tank 1b, and the aerobic tank 1c, respectively.

When the dissolved oxygen concentration target value is set, the first control unit 91 stores the dissolved oxygen concentration target value in the DO target value DB81 (DO target value DB81a to DO target value DB81c) in the storage unit 8. In the present embodiment, the first control unit 91 updates the dissolved oxygen concentration target value in real time in response to the control information transmitted by the third control unit 93. The dissolved oxygen concentration target value set individually may be the same value, or for each aerobic tank 1 in consideration of the arrangement of each series, the equipment configuration, the variation in oxygen dissolution efficiency, and the like. May be different values.

When the first control unit 91 receives the deviation degree information as the control information, the first control unit 91 sets the dissolved oxygen concentration target value by PID control so that the deviation degree becomes zero. When the first control unit 91 receives the ignore signal as control information, the first control unit 91 sets a predetermined dissolved oxygen concentration target value. As a result, the first control unit 91 sets the dissolved oxygen concentration target value in consideration of the control of the ammoniacal nitrogen concentration, and the dissolved oxygen concentration target value in consideration of the oxygen consumption for oxidation of the organic substance. It can be switched appropriately with the setting of.

The second control unit 92 has the dissolved oxygen concentration target value for each aerobic tank 1 stored in the DO target value DB 81 of the storage unit 8 and each aerobic tank 1 received from the dissolved oxygen concentration measuring unit 3. It is a functional unit that controls the aeration air volume in each aerobic tank 1 based on the dissolved oxygen concentration of (an example of the second control step). In the present embodiment, the second control unit 92 controls the rotation speed of the blower 60 of the aeration device 6 as the control of the aeration air volume. The second control unit 92 includes an aeration device controller 92a to an aeration device controller 92c, which are functional units that control the rotation speeds of the blowers 60a to 60c of the aerobic tank 1a, the aerobic tank 1b, and the aerobic tank 1c, respectively. ..

The second control unit 92 increases the rotation speed of the blower 60 to increase the air volume, for example, when the dissolved oxygen concentration is smaller than the dissolved oxygen concentration target value. This increases the dissolved oxygen concentration of the mixture. When the dissolved oxygen concentration exceeds the dissolved oxygen concentration target value, the rotation speed of the blower 60 is reduced to reduce the air volume. This reduces the dissolved oxygen concentration in the mixture. When the dissolved oxygen concentration is equal to the dissolved oxygen concentration target value, the rotation speed control for maintaining the rotation speed of the blower 60 is performed. The rotation speed control is performed by, for example, PID control. In this way, the second control unit 92 controls the aeration air volume based on the dissolved oxygen concentration target value set by the first control unit 91 based on the control information, thereby nitrifying the ammoniacal nitrogen in the aerobic tank 1. Aeration operation (air volume control) suitable for both oxidation of organic matter is realized.

[Modified example of the first embodiment]
In the first embodiment, the case where each series of the sewage treatment system 100 has the first settling basin 51, the aerobic tank 1, and the final settling basin 59 arranged in order from the upstream side where the sewage flows in has been described. However, as shown in FIG. 3, each series of the sewage treatment system 100 has an anaerobic tank 52 (anaerobic treatment unit) that dephosphorizes activated sludge in addition to the first settling basin 51, the aerobic tank 1, and the final settling basin 59. An example), the sewage treatment system 100 executes an anaerobic-anoxic-aerobic method having an anoxic tank 53 (an example of an anoxic treatment unit) that reduces nitrate nitrogen to nitrogen by activated sludge. It may be a sewage treatment facility. In this case, since the operations of the aerobic tank 1 and the control unit 9 are the same as those in the first embodiment, they will be omitted in the following description. The anaerobic tank 52 and the oxygen-free tank 53 are tanks.

When the sewage treatment system 100 has an anaerobic tank 52 and an anoxic tank 53, each series of the sewage treatment system 100 has the first settling basin 51, the anaerobic tank 52, the anoxic tank 53, and the like, in order from the upstream side where the sewage flows. The air tank 1 and the final settling basin 59 are arranged in this order.

A part of the activated sludge settled in the final settling basin 59 is returned to the anaerobic tank 52. Phosphorus is released (dephosphorus) by placing the returned activated sludge in an oxygen-deficient state in the anaerobic tank 52. The activated sludge transferred to the aerobic tank 1 after dephosphorization absorbs an excessively large amount of phosphorus with respect to the phosphorus released in the anaerobic tank 52 under the aerobic state of the aerobic tank 1. As a result, the sewage treatment system 100 removes phosphorus from the sewage.

A part of the mixed solution is returned from the aerobic tank 1 to the oxygen-free tank 53. Nitrite nitrogen contained in the mixed solution returned from the aerobic tank 1 is reduced to nitrogen by utilizing nitric acid respiration or nitrite respiration of the denitrifying bacterium. Denitrifying bacteria also consume organic matter contained in sewage when performing nitrate respiration or nitrite respiration. As a result, the sewage treatment system 100 removes nitrate nitrogen from the sewage. In addition, organic matter contained in sewage is removed along with the removal of nitrate nitrogen.

As described above, the sewage treatment system and the sewage treatment method can appropriately control the dissolved oxygen concentration.

[Another Embodiment]
(1) In the above embodiment, each series of the sewage treatment system 100 has a first settling basin 51, an aerobic tank 1, and a final settling basin 59 arranged in this order from the upstream side where the sewage flows in, and sewage. The case where the first settling basin 51, the anaerobic tank 52, the anoxic tank 53, the aerobic tank 1, and the final settling basin 59 are arranged in this order from the upstream side into which the sewage flows in has been described. However, each series of the sewage treatment system 100 is not limited to the case where neither the anaerobic tank 52 nor the anoxic tank 53 is provided, or the case where both the anaerobic tank 52 and the anoxic tank 53 are provided. Each series of the sewage treatment system 100 may have either an anaerobic tank 52 or an oxygen-free tank 53. Further, there is also a case of a so-called step inflow type multi-stage nitrification denitrification method in which a combination of the oxygen-free tank 53 and the aerobic tank 1 is connected in series in two or three stages or four or more stages. In this case, the ammonia meter 20 and the dissolved oxygen concentration meter 30 are attached to the aerobic tank 1 in the final stage.

When each series of the sewage treatment system 100 has the first settling basin 51, the anaerobic tank 52, the aerobic tank 1, and the final settling basin 59 in this order from the upstream side where the sewage flows in, the sewage treatment system 100 has. The so-called anaerobic-aerobic method can remove organic matter and phosphorus.

When each series of the sewage treatment system 100 has the first settling basin 51, the oxygen-free tank 53, the aerobic tank 1, and the final settling basin 59 in this order from the upstream side where the sewage flows in, the sewage treatment system 100 Nitrogen can be removed by the so-called anoxic-aerobic method.

(2) In the above embodiment, the control unit 9 has a first control unit 91 and a third control unit 93, and the third control unit 93 has the ammoniacal nitrogen concentration received from the ammonia measurement unit 2 and the storage unit 8. The case where the control information is transmitted to the first control unit 91 based on the ammoniacal nitrogen concentration target value stored in advance in the ammoniacal nitrogen concentration target value DB83 in the above has been described. Further, the first control unit 91 sets the dissolved oxygen concentration target values of the aerobic tank 1a, the aerobic tank 1b, and the aerobic tank 1c based on the control information transmitted by the third control unit 93, and sets the dissolved oxygen concentration target value. The case where the dissolved oxygen concentration target value is stored in the DO target value DB 81 of the storage unit 8 has been described. However, the control unit 9 may not have the third control unit 93. In this case, for example, the aerobic tank 1a, based on the ammonia nitrogen concentration received from the ammonia measurement unit 2 by the first control unit 91 and the ammonia nitrogen concentration target value stored in advance in the ammonia nitrogen concentration target value DB83, The dissolved oxygen concentration target values of the aerobic tank 1b and the aerobic tank 1c may be set, and the dissolved oxygen concentration target value may be stored in the DO target value DB 81 of the storage unit 8. In this case, the first control unit 91 may set the dissolved oxygen concentration target value by PID control so that the ammoniacal nitrogen concentration approaches the ammoniacal nitrogen concentration target value.

(3) In the above embodiment, the case where the ammoniacal nitrogen concentration target value is stored in advance in the ammoniacal nitrogen concentration target value DB83 of the storage unit 8 has been described. However, the ammoniacal nitrogen concentration target value is not limited to the case where it is simply stored as a numerical value.

For example, the relational information (hereinafter referred to as the first relational information) that can refer to the ammoniacal nitrogen concentration target value corresponding to the value of the ammoniacal nitrogen concentration is stored in the storage unit 8 as a relational information database or a predetermined function. deep. Then, for example, the third control unit 93 can select or determine the ammoniacal nitrogen concentration target value based on the ammoniacal nitrogen concentration and the first relational information.

The above first-related information is used to learn in advance the target value of ammonia nitrogen concentration corresponding to the value of ammonia nitrogen concentration when there is a change in the amount or quality of sewage due to changes in time zone, weather or season. Can be memorized and used. In this case, the sewage treatment system 100 is further provided with an input unit (not shown) for change information such as the amount and quality of sewage due to changes in time zone, weather and season, and the third control unit 93 provides the change information. Based on the ammonia nitrogen concentration received from the ammonia measuring unit 2, the dissolved oxygen concentration target value set based on the ammonia nitrogen concentration target value, and the dissolved oxygen concentration received from the dissolved oxygen concentration measuring unit 3. It is also possible to learn the first relational information that the dissolved oxygen concentration quickly converges to the dissolved oxygen concentration target value, and to update the relational information database related to the ammoniacal nitrogen concentration target value corresponding to the ammoniacal nitrogen concentration value. ..

(4) In the above embodiment, the first control unit 91 sets (updates) the dissolved oxygen concentration target value in real time by PID control in response to the fluctuation of the ammoniacal nitrogen concentration target value updated by the third control unit 93. ) Was explained. However, the ammoniacal nitrogen concentration target value is not limited to the case where it is set by PID control.

For example, the relational information (hereinafter referred to as the second relational information) that can refer to the dissolved oxygen concentration target value corresponding to the value of the ammoniacal nitrogen concentration target value is stored in the storage unit 8 as a relational information database or a predetermined function. Keep it. Then, the third control unit 93 can also set the dissolved oxygen concentration target value based on the ammoniacal nitrogen concentration and the second relational information.

As the second relational information, it is possible to use information in which the dissolved oxygen concentration target value when there is a change in the amount or quality of sewage due to a change in time zone, weather or season is learned in advance. In this case, the sewage treatment system 100 is further provided with an input unit (not shown) for change information such as the amount and quality of sewage due to changes in time zone, weather, and season, and the first control unit 91 is provided with the change information. Based on the ammonia nitrogen concentration target value, the dissolved oxygen concentration target value, and the dissolved oxygen concentration received from the dissolved oxygen concentration measuring unit 3, the dissolved oxygen concentration quickly converges to the dissolved oxygen concentration target value. (Ii) It is also possible to learn relational information and update the relational information database.

(5) In the above embodiment, when the ammoniacal nitrogen concentration deviates within a predetermined range from the ammoniacal nitrogen concentration target value, the third control unit 93 provides control information of the ammoniacal nitrogen concentration and the ammoniacal nitrogen concentration. The case where a deviation degree signal including the deviation degree from the target value is transmitted and an ignore signal including an instruction to ignore the ammoniacal nitrogen concentration is transmitted as control information when the ammoniacal nitrogen concentration is equal to or less than a predetermined lower limit value has been described. .. Further, when the first control unit 91 receives the deviation degree information as the control information, the dissolved oxygen concentration target value is set based on the deviation degree information, and when the ignore signal is received as the control information, it is predetermined. The case of setting the dissolved oxygen concentration target value has been described. However, instead of the ignore signal, the third control unit 93 uses, as control information, a correction deviation degree signal including a correction deviation degree obtained by adding a predetermined correction to the deviation degree between the ammoniacal nitrogen concentration and the ammoniacal nitrogen concentration target value. It can also be sent. As the correction degree of divergence, for example, the degree of divergence when the ammoniacal nitrogen concentration is a predetermined value (however, the value is smaller than the ammoniacal nitrogen concentration target value and equal to or more than the above-mentioned predetermined lower limit value) can be adopted.

In this case, the first control unit 91 sets the dissolved oxygen concentration target value based on the deviation degree information or the correction deviation degree information received as the control information.

(6) In the above embodiment, the case where the distribution unit 5 is a movable weir with an adjustable opening degree provided in the flow path for guiding the sewage to the sewage treatment system 100 has been described, but the distribution unit 5 is limited to the movable weir. I can't. The distribution unit 5 may be a valve, a pump, or a combination thereof, in addition to the movable weir. For example, a branch flow path that branches the distribution unit 5 from a flow path that guides sewage to the sewage treatment system 100 to each flow path of the flow path 5a, the flow path 5b, and the flow path 5c, and a valve provided in each of these flow paths. Or a pump, or a combination of a pump and a valve. If it is a valve, the opening degree of the valve may be adjusted for each flow path to evenly distribute the sewage. If it is a pump, the output of the pump may be adjusted for each flow path, or a valve is provided in the flow path on the downstream side of the pump, and the opening of the valve is adjusted so that the output of the pump is constant to evenly distribute the sewage. You may.

(7) In the above embodiment, the aeration device 6 for each aeration tank 1 has a blower 60 (blower 60a, etc.), and the second control unit 92 (aeration device controller 92a, etc.) is aerobic for controlling the aeration air volume. The case where the rotation speed of the blower 60 is controlled for each tank 1 has been described. However, the configuration and control of the blower 60 and the second control unit 92 are not limited to this example. Instead of providing a blower 60 for each aerobic tank 1, one blower may be provided for a plurality of aerobic tanks 1. Then, the blower pipe or the like may be branched from the one blower to blow air into each aerobic tank 1. The above-mentioned blower is a device having one or more blowers and the like. In this case, an air volume adjusting valve can be provided for each pipe to adjust the air volume for each aerobic tank 1, and the second control unit 92 (aeration device controller 92a, etc.) controls the rotation speed of the blower 60. Instead, the opening degree of the air volume adjusting valve may be adjusted.

(8) In the above embodiment, the aerobic tank 1 which is a tank has been illustrated and described as an example of the aerobic treatment unit. Further, in particular, in the description of the modified example of the first embodiment, the anaerobic tank 52, which is a tank, is exemplified as an example of the anaerobic treatment unit, and the oxygen-free tank 53, which is a tank, is exemplified as an example of the oxygen-free treatment unit. .. However, the aerobic treatment unit, the anaerobic treatment unit, or the anoxic treatment unit is not always limited to a single tank such as the aerobic tank 1, the anaerobic tank 52, or the oxygen-free tank 53. For example, when one tank is divided by providing a weir or the like in the tank as an anaerobic classification corresponding to the anaerobic treatment unit, an oxygen-free classification corresponding to the anoxic treatment unit, and an aerobic classification corresponding to the aerobic treatment unit. There is also. The same applies to the first settling basin 51 and the final settling basin 59.

(9) In the description of the modified example of the first embodiment, the case where the ammonia meter 20 for measuring the ammonia nitrogen concentration in the tank is attached to the aerobic tank 1a of the series A is illustrated. Further, the case where the control unit 9 controls the aeration air volume in each aerobic tank 1 based on the ammonia nitrogen concentration received from the ammonia measurement unit 2 having the ammonia meter 20 has been described. Further, the storage unit 8 has described the case where the target value in the aerobic tank 1a is stored as the target value for the ammonia nitrogen concentration. Further, the control unit 9 has a first control unit 91, a second control unit 92, and a third control unit 93, and first controls control information based on the ammoniacal nitrogen concentration and the ammoniacal nitrogen concentration target value. The case of sending to the unit 91 has been described. However, in some cases, an ammonia meter 20 for measuring the concentration of ammoniacal nitrogen in these tanks may be attached to the anaerobic tank 52 or the oxygen-free tank 53 of the series A. In this case as well, the control unit 9 may control the aeration air volume in each aerobic tank 1 based on the ammonia nitrogen concentration and the ammonia nitrogen concentration target value received from the ammonia measurement unit 2.

In this case, the storage unit 8 has related information (for example, an anaerobic tank 52 or an oxygen-free tank) for predicting the ammonia nitrogen concentration in the aerobic tank 1 based on the ammonia nitrogen concentration received from the ammonia measuring unit 2. It is preferable to further store the function that outputs the predicted value of the ammoniacal nitrogen concentration in the aerobic tank 1 with the ammoniacal nitrogen concentration in 53 as an input variable (hereinafter referred to as prediction information). By using the prediction information in this way, the control unit 9 predicts the ammoniacal nitrogen concentration in the aerobic tank 1a based on the ammoniacal nitrogen concentration in the anaerobic tank 52 and the anoxic tank 53, and the predicted ammoniacality. The amount of exposed air in each aerobic tank 1 can be controlled based on the nitrogen concentration and the target value of the ammoniacal nitrogen concentration in the aerobic tank 1a.

The ammonia meter 20 is preferably attached to the oxygen-free tank 53 immediately before the aerobic tank 1.

(10) In the description of the modified example of the first embodiment, the case where the ammonia meter 20 for measuring the ammonia nitrogen concentration in the tank is attached to the aerobic tank 1a of the series A is illustrated. Further, the case where the control unit 9 controls the aeration air volume in each aerobic tank 1 based on the ammonia nitrogen concentration received from the ammonia measurement unit 2 having the ammonia meter 20 has been described. Further, the storage unit 8 has described the case where the target value in the aerobic tank 1a is stored as the target value for the ammonia nitrogen concentration. Further, the control unit 9 has a first control unit 91, a second control unit 92, and a third control unit 93, and first controls control information based on the ammoniacal nitrogen concentration and the ammoniacal nitrogen concentration target value. The case of sending to the unit 91 has been described. However, the anaerobic tank 52 or the oxygen-free tank 53 may be provided with an ammonia meter 20 for measuring the concentration of ammoniacal nitrogen in these tanks. Then, the storage unit 8 may store the target value in the anaerobic tank 52 or the anoxic tank 53 as the target value for the ammonia nitrogen concentration.

The configuration disclosed in the above embodiment (including another embodiment, the same shall apply hereinafter) can be applied in combination with the configuration disclosed in other embodiments as long as there is no contradiction. The embodiments disclosed in the present specification are examples, and the embodiments of the present invention are not limited thereto, and can be appropriately modified without departing from the object of the present invention.

The present invention can be applied to sewage treatment systems and sewage treatment methods in which the dissolved oxygen concentration can be appropriately controlled.

1: Aerobic tank (aerobic treatment unit)
1a: Aerobic tank (aerobic treatment unit)
1b: Aerobic tank (aerobic treatment unit)
1c: Aerobic tank (aerobic treatment unit)
2: Ammonia measuring unit 3: Dissolved oxygen concentration measuring unit 5: Distributing unit 6: Aeration device 9: Control unit 20: Ammonia meter 30: Dissolved oxygen concentration meter 52: Anaerobic tank (anaerobic treatment unit)
53: Anoxic tank (anoxic treatment section)
81: DO target value DB (dissolved oxygen concentration target value)
83: Ammonia nitrogen concentration target value DB (ammonia nitrogen concentration target value)
91: First control unit 92: Second control unit 93: Third control unit 100: Sewage treatment system A: Series (treatment series)
B: Series (processing series)
C: Series (processing series)

Claims (9)

A plurality of treatment sequences having an aerobic treatment unit that aerates the inflowing sewage and an anaerobic treatment unit or an oxygen-free treatment unit upstream of the aerobic treatment unit.
An ammonia measuring unit having an ammonia meter provided in the anaerobic treatment unit or the anoxic treatment unit in at least one of the treatment series and acquiring the ammoniacal nitrogen concentration of the anaerobic treatment unit or the anoxic treatment unit. When,
A dissolved oxygen concentration measuring unit having a dissolved oxygen concentration meter provided in each of the aerobic treatment units in the plurality of processing series and acquiring the dissolved oxygen concentration of each of the aerobic treatment units.
A control unit that controls the aeration of each of the aerobic treatment units is provided.
The control unit
The first control unit that sets the dissolved oxygen concentration target value of the aerobic treatment unit, and
It has a second control unit that controls the aeration air volume of the aerobic treatment unit.
The first control unit sets the dissolved oxygen concentration target value of each of the aerobic treatment units based on the ammoniacal nitrogen concentration.
The second control unit is a sewage treatment system that controls the aeration air volume of each of the aerobic treatment units based on the dissolved oxygen concentration target value and the respective dissolved oxygen concentrations acquired by the dissolved oxygen concentration measurement unit. .. A plurality of treatment sequences having an aerobic treatment unit that aerates the inflowing sewage,
An ammonia measuring unit having an ammonia meter provided in at least one of the treatment series and acquiring the ammonia nitrogen concentration of the treatment series.
A dissolved oxygen concentration measuring unit having a dissolved oxygen concentration meter provided in each of the aerobic treatment units in the plurality of processing series and acquiring the dissolved oxygen concentration of each of the aerobic treatment units.
A control unit that controls the aeration of each of the aerobic treatment units,
It is equipped with a storage unit that stores the target value of ammonia nitrogen concentration.
The control unit
The first control unit that sets the dissolved oxygen concentration target value of the aerobic treatment unit, and
A second control unit that controls the aeration air volume of the aerobic treatment unit, and
It has a third control unit that sends out control information including the degree of deviation between the ammoniacal nitrogen concentration and the ammoniacal nitrogen concentration target value.
When the ammoniacal nitrogen concentration exceeds a predetermined lower limit value, the first control unit sets the dissolved oxygen concentration target value of each of the aerobic treatment units by PID control based on the control information, and the ammoniacality. When the nitrogen concentration is equal to or less than the lower limit value, a predetermined value is set as the dissolved oxygen concentration target value.
The second control unit is a sewage treatment system that controls the aeration air volume of each of the aerobic treatment units based on the dissolved oxygen concentration target value and the respective dissolved oxygen concentrations acquired by the dissolved oxygen concentration measurement unit. .. A plurality of treatment sequences having an aerobic treatment unit that aerates the inflowing sewage,
An ammonia measuring unit having an ammonia meter provided in at least one of the treatment series and acquiring the ammonia nitrogen concentration of the treatment series.
A dissolved oxygen concentration measuring unit having a dissolved oxygen concentration meter provided in each of the aerobic treatment units in the plurality of processing series and acquiring the dissolved oxygen concentration of each of the aerobic treatment units.
A control unit that controls the aeration of each of the aerobic treatment units,
It is provided with an input unit for acquiring change information including a change in the amount or quality of the sewage.
The control unit
The first control unit that sets the dissolved oxygen concentration target value of the aerobic treatment unit, and
It has a second control unit that controls the aeration air volume of the aerobic treatment unit.
The first control unit sets the dissolved oxygen concentration target value of each of the aerobic treatment units based on the ammoniacal nitrogen concentration and the change information.
The second control unit is a sewage treatment system that controls the aeration air volume of each of the aerobic treatment units based on the dissolved oxygen concentration target value and the respective dissolved oxygen concentrations acquired by the dissolved oxygen concentration measurement unit. .. The sewage treatment system according to claim 2 or 3 , wherein the ammonia meter is provided in the aerobic treatment unit. The sewage treatment system according to any one of claims 1 to 4 , wherein the second control unit controls the aeration air volume by PID. The sewage treatment system according to any one of claims 1 to 5 , further comprising a distribution unit that distributes the sewage so that the residence time of the sewage flowing into the treatment series in the treatment series is equal. An aeration step in which the inflowing sewage is distributed to multiple treatment series and aerated in each aerobic treatment unit.
In at least one of the treatment series, an ammonia measurement step for acquiring the ammoniacal nitrogen concentration of the anaerobic treatment section or the anoxic treatment section provided upstream of the aerobic treatment section, and
A dissolved oxygen concentration measurement step for acquiring the dissolved oxygen concentration of each of the aerobic treatment units, and
The first control step of setting the dissolved oxygen concentration target value of each of the aerobic treatment units based on the ammoniacal nitrogen concentration,
A sewage treatment method having a second control step of controlling the aeration air volume of each of the aerobic treatment units based on the dissolved oxygen concentration target value and the dissolved oxygen concentration of each of the aerobic treatment units. An aeration step in which the inflowing sewage is distributed to multiple treatment series and aerated in each aerobic treatment unit.
Ammonia measurement step to obtain the ammoniacal nitrogen concentration of at least one of the above treatment series, and
A memory step to memorize the ammonia nitrogen concentration target value,
A dissolved oxygen concentration measurement step for acquiring the dissolved oxygen concentration of each of the aerobic treatment units, and
A transmission step for transmitting control information including the degree of deviation between the ammoniacal nitrogen concentration and the ammoniacal nitrogen concentration target value, and a transmission step.
When the ammoniacal nitrogen concentration exceeds a predetermined lower limit value, the dissolved oxygen concentration target value of each of the aerobic treatment units is set by PID control based on the control information, and the ammoniacal nitrogen concentration is equal to or less than the lower limit value. In the case, the first control step in which a predetermined value is set as the dissolved oxygen concentration target value, and
A sewage treatment method having a second control step of controlling the aeration air volume of each of the aerobic treatment units based on the dissolved oxygen concentration target value and the dissolved oxygen concentration of each of the aerobic treatment units. An aeration step in which the inflowing sewage is distributed to multiple treatment series and aerated in each aerobic treatment unit.
Ammonia measurement step to obtain the ammoniacal nitrogen concentration of at least one of the above treatment series, and
A dissolved oxygen concentration measurement step for acquiring the dissolved oxygen concentration of each of the aerobic treatment units, and
The acquisition step of acquiring change information including the change in the amount or quality of the sewage, and
A first control step for setting a dissolved oxygen concentration target value for each of the aerobic treatment units based on the ammoniacal nitrogen concentration and the change information.
A sewage treatment method having a second control step of controlling the aeration air volume of each of the aerobic treatment units based on the dissolved oxygen concentration target value and the dissolved oxygen concentration of each of the aerobic treatment units.

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