Patent Publication Number: US-2022227647-A1

Title: Water treatment system, water treatment method, and recording medium

Description:
TECHNICAL FIELD 
     The present disclosure relates to a water treatment system, a water treatment method, and a recording medium. 
     BACKGROUND OF THE INVENTION 
     As a water treatment system for treating water to be treated such as domestic wastewater or industrial wastewater, a system that performs biological treatment on water to be treated is available. In such a water treatment system, while flowing water to be treated into a tank, aeration treatment of supplying air to aerobic microorganisms existing in the tank is performed. Organic matter contained in the water to be treated in the tank is decomposed by the aerobic microorganisms, and as a result stable treated water quality is obtained. 
     SUMMARY OF THE INVENTION 
     A water treatment system according to one of the disclosed embodiments includes: a plurality of tanks; a plurality of blow systems connected to tanks respectively; a blower unit configured to supply air to tanks through the air blow systems; and a control device configured to: calculate a pressure loss in each of the air blow systems; and control supply of water to be treated to each of the tanks, according to the calculated pressure losses. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
         FIG. 1  is a diagram illustrating an example of the structure of a water treatment system according to one of the disclosed embodiments. 
         FIG. 2  is a block diagram illustrating an example of the structure of a control unit illustrated in  FIG. 1 . 
         FIG. 3  is a flowchart illustrating an example of the operation of the water treatment system illustrated in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     One method of supplying air from a blower unit to each of a plurality of tanks through an air blow pipe in the foregoing water treatment system is a method (first method) of calculating expected maximum pressure loss and supplying air to each of a plurality of tanks at the air blow pressure corresponding to the calculated pressure loss. Another method is a method (second method) of calculating, based on the water quality of water to be treated in each of a plurality of tanks, the amount of air necessary for treating the water to be treated and the pressure loss of an air blow pipe and the like and supplying air to each of the plurality of tanks at the air blow pressure corresponding to the maximum pressure loss out of the calculated pressure losses (see JP 2018-167249 A). 
     With the foregoing first method, air is supplied to each tank at the air blow pressure corresponding to the expected maximum pressure loss, and thus there is a possibility that air is supplied to each tank at excessive pressure. With the foregoing second method, air is supplied to each tank at the air blow pressure corresponding to the maximum pressure loss calculated based on the water quality, and thus there is a possibility that air is supplied to each tank other than the tank corresponding to the calculated maximum pressure loss at excessive pressure. Accordingly, with the first and second methods, the electricity (air blow electricity) consumed by the blower unit for blowing air is wasted, and efficient use of electricity in water treatment cannot be achieved. 
     There is thus a need to reduce wasted air blow electricity of a blower unit and achieve efficient use of electricity in water treatment. 
     One of the disclosed embodiments will be described in detail below, with reference to the drawings. In the drawings, the same reference signs represent the same or equivalent components. 
       FIG. 1  is a diagram illustrating an example of the structure of a water treatment system  1  according to one of the disclosed embodiments. The water treatment system  1  according to this embodiment is a system that performs aeration treatment on water to be treated. The water to be treated is any of various types of water subjected to aeration treatment. Non-limiting examples of the water to be treated include domestic wastewater, industrial wastewater, rainwater, human waste, supernatant liquor after sludge dewatering process in sewerage treatment plants, and wastewater such as leachate in landfills. 
     The water treatment system  1  illustrated in  FIG. 1  includes tanks  10 A,  10 B, and  10 C, a blower unit  20 , an air blow pipe  30  as an air blow system, and a control apparatus  40 . In the water treatment system  1 , the control apparatus  40  controls the amount of air supplied from the blower unit  20  to each of the tanks  10 A,  10 B, and  10 C and the supply of water to be treated to each of the tanks  10 A,  10 B, and  10 C to perform biological treatment on the water to be treated in each of the tanks  10 A,  10 B, and  10 C. Hereafter, the tanks  10 A,  10 B, and  10 C are collectively referred to as “tank  10 ” when not distinguished from one another. 
     Each tank  10  is a tank that has an air diffuser  12  inside and in which activated sludge is stored. The water to be treated is flown (supplied) into the tank  10  through a water pump  13 . The air diffuser  12  aerates the activated sludge stored in the tank  10  with the air supplied from the blower unit  20 . The tank  10  biologically treats the water to be treated in the tank  10  with the aerated activated sludge, and discharges the treated water after the biological treatment. 
     The water to be treated is supplied to the tanks  10 A,  10 B, and  10 C in parallel. Although this embodiment describes an example in which the water treatment system  1  includes three tanks  10 A,  10 B, and  10 C, the presently disclosed techniques are not limited to such. The water treatment system  1  includes a plurality of tanks  10 . Hence, the water treatment system  1  may include two tanks  10  or four or more tanks  10 . 
     The blower unit  20  includes air blowers  22 A,  22 B,  22 C, and  22 D. The air blowers  22 A,  22 B,  22 C, and  22 D are blowers having the same function. The blower unit  20  supplies air for biological treatment to the plurality of tanks  10 A,  10 B, and  10 C through the air blow pipe  30  as an air blow system. Hereafter, the air blowers  22 A,  22 B,  22 C, and  22 D are collectively referred to as “air blower  22 ” when not distinguished from one another. 
     Each air blower  22  is a blower that introduces air from the outside and discharges the introduced air by a rotating blade portion. Non-limiting examples of the blower  22  include an inlet vane type blower, an inverter type blower, and a gear type blower. The air blowers  22  are connected to the air blow pipe  30  in parallel with one another on the side where air is discharged from the blade portion, and discharge air to the air blow pipe  30 . Although this embodiment describes an example in which the blower unit  20  includes four air blowers  22 A,  22 B,  22 C, and  22 D, the presently disclosed techniques are not limited to such. The blower unit  20  may include any number of air blowers  22 . That is, the blower unit  20  includes one or more air blowers  22 . 
     The air blow pipe  30  is a pipe that conducts air inside. The air blow pipe  30  is connected to the tanks  10 A,  10 B, and  10 C. The air blow pipe  30  includes an introduction pipe  31 , a header pipe  32 , and branch pipes  34 A,  34 B, and  34 C. The air blow systems corresponding to the tanks  10 A,  10 B, and  10 C respectively include the branch pipes  34 A,  34 B, and  34 C respectively, and the air blow systems corresponding to the tanks  10 A,  10 B, and  10 C respectively further include air diffusers  12  respectively. The air blow systems is connected to the header pipe  32 . The introduction pipe  31  is a pipe that has one end branched and connected to the air blowers  22 A,  22 B,  22 C, and  22 D and is supplied with air from each air blower  22 . The introduction pipe  31  has the other end connected to the header pipe  32 , and merges the air supplied from the air blowers  22  and introduces the merged air into the header pipe  32 . The header pipe  32  has one end connected to the introduction pipe  31 , and the other end connected to the branch pipes  34 A,  34 B, and  34 C. 
     The branch pipe  34 A is a pipe that has one end connected to the header pipe  32  and the other end connected to the air diffuser  12  in the tank  10 A. The branch pipe  34 A supplies part of the air supplied from the header pipe  32 , to the tank  10 A. The branch pipe  34 B is a pipe that has one end connected to the header pipe  32  and the other end connected to the air diffuser  12  in the tank  10 B. The branch pipe  34 B supplies part of the air supplied from the header pipe  32 , to the tank  10 B. The branch pipe  34 C is a pipe that has one end connected to the header pipe  32  and the other end connected to the air diffuser  12  in the tank  10 C. The branch pipe  34 C supplies part of the air supplied from the header pipe  32 , to the tank  10 C. Hereafter, the branch pipes  34 A,  34 B, and  34 C are collectively referred to as “branch pipe  34 ” when not distinguished from one another. 
     Each branch pipe  34  is provided with an introduction valve  36 . The introduction valve  36  is a valve that is opened and closed by the control apparatus  40 . The introduction valve  36  adjusts the amount of air supplied from the branch pipe  34  to the tank  10  by adjusting the degree of opening. 
     The control apparatus  40  is a device that controls the amount of air supplied to each tank  10 . The control apparatus  40  also controls the supply of the water to be treated to each tank  10  through the water pump  13 . The control apparatus  40  includes a nitric acid meter  41 , an ammonia meter  42 , an intake air measurement unit  43 , a header pipe internal pressure measurement unit  44 , a branch pipe air volume measurement unit  45 , and a control unit  50 . 
     In the tank  10 , ammoniacal nitrogen in the water to be treated is nitrified into nitrite nitrogen and nitrate nitrogen by nitrifying bacteria which are aerobic microorganisms in activated sludge under aerobic conditions. Meanwhile, in a region where the amount of oxygen in the water to be treated is low in the tank  10 , denitrification reaction by denitrifying bacteria occurs. By supplying a carbon source sufficient for denitrification reaction, the denitrification reaction can progress sufficiently. Consequently, in the region where the denitrification reaction occurs, nitrogen can be removed by decomposing nitrous oxide gas generated due to insufficient nitrification or reducing nitrite and decomposing it into nitrogen and carbon dioxide without generating nitrous oxide. 
     The nitric acid meter  41  is a sensor that is provided in each tank  10  and measures the nitric acid concentration in the water to be treated in the tank  10  to detect the degree of progress of denitrification reaction, i.e. the degree of decomposition of nitric acid. Herein, nitric acid in the water to be treated represents a concept that includes nitric acid (HNO 3 ), nitrite (HNO 2 ), nitrate nitrogen (NO 3 —N), nitrite nitrogen (NO 2 —N), an assembly of nitrate nitrogen and nitrite nitrogen, and NO x . 
     The ammonia meter  42  is a sensor that is provided in each tank  10  and measures the ammonia concentration in the water to be treated in the tank  10  to detect the degree of progress of nitrification reaction, i.e. the degree of decomposition of ammonia. Herein, ammonia in the water to be treated represents a concept that includes ammonia and ammoniacal nitrogen. 
     The intake air measurement unit  43  is an airflow meter that is provided on the intake side of each air blower  22  and measures the amount of air taken in by the air blower  22 . 
     The header pipe internal pressure measurement unit  44  is a pressure gauge that is attached to the header pipe  32  and measures the internal pressure of the header pipe  32 , i.e. the pressure of air from the blower unit  20 . 
     The branch pipe air volume measurement unit  45  is provided in each branch pipe  34 . Specifically, the branch pipe air volume measurement unit  45  is an airflow meter that is provided in the branch pipe  34  between the introduction valve  36  and the header pipe  32  and measures the amount of air supplied from the branch pipe  34  to the tank  10 . In the case where the branch pipe air volume measurement unit  45  as an airflow meter is provided in each branch pipe  34 , a pressure gauge may be provided in the branch pipe  34  instead of the header pipe internal pressure measurement unit  44 . 
     The control unit  50  controls the amount of air supplied to each tank  10  based on the measurement results of the foregoing components. The control unit  50  also controls the supply of the water to be treated to each of the plurality of tanks  10  according to the calculation result of the pressure loss in the air blow system of each of the plurality of tanks  10 . Specifically, the control unit  50  calculates the pressure loss in each of the plurality of air blow systems, and controls the supply of the water to be treated to each of the plurality of tanks  10  according to the calculated respective pressure losses in the plurality of air blow systems. 
       FIG. 2  is a block diagram illustrating an example of the structure of the control unit  50 . 
     The control unit  50  (control device) illustrated in  FIG. 2  includes a control circuit  501 , a memory  502 , and a bus  503 . The control circuit  501  can access the memory  502  via the bus  503 . The control circuit  501  is an electric circuit. For example, the control circuit  501  may be any of a processor, a central processing unit (CPU), an application specific integrated circuit (ASIC), an application specific instruction-set processor (ASIP), a field-programmable gate array (FPGA), and a system-on-a-chip (SoC). The memory  502  is any of various types of recording media. The memory  502  is, for example, an electric circuit. For example, the memory  502  may be a random access memory (RAM), a read only memory (ROM), a hard disk drive (HDD), or a solid state drive (SDD), or may be a non-transitory computer-readable medium storing instructions executable by one or more control circuits. The memory  502  stores data  502   a . For example, the data  502   a  may be data (such as the below-described water quality-air amount relationship) accessed during execution by the control circuit  501 , or a program according to this embodiment executed by the control circuit  501 . 
     The control circuit  501  includes an acquisition unit  51 , a required air amount calculation unit  52 , a target pipe internal pressure calculation unit  53 , an air blow control unit  54 , an introduced air control unit  55 , and a water-to-be-treated supply control unit  56 . For example, the control unit  50  can be implemented by a computer (e.g. a personal computer) including a CPU and a memory. In the case where the control unit  50  is implemented by the computer, the foregoing components in the control unit  50  are implemented by the CPU reading a program according to this embodiment stored in the memory and executing it. 
     The acquisition unit  51  acquires the measurement results of the nitric acid meter  41 , the ammonia meter  42 , the intake air measurement unit  43 , the header pipe internal pressure measurement unit  44 , and the branch pipe air volume measurement unit  45 . The acquisition unit  51  outputs the measurement results of the nitric acid meter  41 , the ammonia meter  42 , and the branch pipe air volume measurement unit  45  to the required air amount calculation unit  52 . The acquisition unit  51  outputs the measurement results of the intake air measurement unit  43  and the header pipe internal pressure measurement unit  44  to the air blow control unit  54 . The acquisition unit  51  outputs the measurement result of the branch pipe air volume measurement unit  45  to the introduced air control unit  55 . 
     The required air amount calculation unit  52  calculates, for each tank  10 , the amount of air (required air amount) necessary for adjusting the water quality of the water to be treated in the tank  10  to predetermined target water quality, based on accumulated data from past to present of the state of the water to be treated in the tank  10  (the nitric acid concentration and the ammonia concentration of the water to be treated) and the measurement result of the branch pipe air volume measurement unit  45  output from the acquisition unit  51 . 
     For example, the required air amount calculation unit  52  stores a predetermined water quality-air amount relationship, and calculates the required air amount based on the water quality-air amount relationship. The water quality-air amount relationship is the relationship between the amount of air supplied to the tank  10  and the change in water quality in the tank  10  in the case where the amount of air is supplied. From the predetermined water quality-air amount relationship, the required air amount calculation unit  52  calculates, as the required air amount, such an amount of air with which the nitric acid concentration of the water to be treated measured by the nitric acid meter  41  and the ammonia concentration of the water to be treated measured by the ammonia meter  42  match the respective target concentrations. Although this embodiment describes a method of calculating the required air amount based on the measurement results of the nitric acid meter  41 , the ammonia meter  42 , and the branch pipe air volume measurement unit  45 , the presently disclosed techniques are not limited to such. Any method that can calculate the amount of air necessary for bringing the water to be treated to the predetermined target water quality may be used. 
     The required air amount calculation unit  52  outputs the calculation result of the required air amount for each tank  10  to the target pipe internal pressure calculation unit  53  and the introduced air control unit  55 . 
     The target pipe internal pressure calculation unit  53  calculates a target value (target pipe internal pressure) of the pressure of air in the air blow pipe  30 , based on the required air amount for each tank  10  calculated by the required air amount calculation unit  52 . The target pipe internal pressure is pressure set as the target pressure of the header pipe internal pressure measurement unit  44  necessary to supply air of the required air amount to each tank  10 . 
     The target pipe internal pressure calculation unit  53  calculates pipe pressure loss H P  which is the pressure of air lost due to pressure loss in the air blow pipe  30  in the case where air of the target air amount calculated by the required air amount calculation unit  52  is supplied to the tank  10 . 
     The pipe pressure loss H of a pipe is typically calculated based on the following Formulas (1) and (2): 
         H= 4· f   1 ·(1/ d )·(γ· v   2 /2)  Formula (1)
 
         H=f   2 ·(γ· v   2 /2)  Formula (2).
 
     Formula (1) is a formula for calculating the pipe pressure loss H in the case where the pipe is a straight pipe. Formula (2) is a formula for calculating the pipe pressure loss H in the case where the pipe is a deformed pipe other than a straight pipe. f 1  and f 2  are loss coefficients, which are predetermined constants. l is the pipe length (m) of the straight pipe. d is the inner diameter (m) of the straight pipe. The pipe length l and the pipe inner diameter d are constants that depend on the shape of the pipe. γ is the air density (kg/m 3 ), which is a predetermined constant. v is the flow velocity (m/s) of air. In Formulas (1) and (2), the flow velocity v is a variable. Hence, the pipe pressure loss H of the pipe changes according to the flow velocity v. The flow velocity v is proportional to the flow quantity Q of air as defined in the following Formula (3). In Formula (3), A is the flow path area, which is a constant that depends on the shape of the pipe: 
         Q=A·v   Formula (3).
 
     Thus, the pipe pressure loss H can be calculated based on the flow quantity Q of air, i.e. the required air amount. The target pipe internal pressure calculation unit  53  calculates the flow velocity v of air in the case where air of the required air amount is flown through the header pipe  32  and the branch pipe  34 , based on Formula (3). The target pipe internal pressure calculation unit  53  then calculates the pipe pressure loss H P  from Formulas (1) and (2), using the calculated flow velocity v and the foregoing constants. Specifically, the target pipe internal pressure calculation unit  53  calculates pipe pressure loss H PA  in the path from the blower unit  20  to the tank  10 A, pipe pressure loss H PB  in the path from the blower unit  20  to the tank  10 B, and pipe pressure loss H PC  in the path from the blower unit  20  to the tank  10 C. 
     Following this, the target pipe internal pressure calculation unit  53  calculates pressure loss H L  in the air blow system of each of the plurality of tanks  10 , based on the following Formula (4): 
         H   L   =h+H   P   +H   M   +H   A   Formula (4).
 
     In Formula (4), h is the water head pressure of the water to be treated in the tank  10 . H M  is the loss pressure (ventilation pressure loss) by the header pipe internal pressure measurement unit  44 , the branch pipe air volume measurement unit  45 , and the introduction valve  36 . H A  is the loss pressure (air diffuser pressure loss) by the air diffuser  12 . The water head pressure h is, for example, calculated from the volume of the tank  10  beforehand. A sensor for measuring the water level or the amount of water may be provided in the tank  10 , and the water head pressure h may be obtained from the measurement result of the sensor. In this embodiment, the same amount of treated water as the water to be treated flowing into the tank  10  flows out of the tank  10 . Hence, the water head pressure h is constant. The ventilation pressure loss H M  is a design value or a value measured beforehand. The air diffuser pressure loss H A  is pressure that depends on the pollutant load of the water to be treated in the tank  10 , which is fixed pressure or pressure proportional to the square of the supplied air volume depending on the device type of the air diffuser  12 . The pollutant load is expressed as the product of the amount of the water to be treated supplied to the tank  10  and the concentration (pollutant concentration such as biochemical oxygen demand (BOD), chemical oxygen demand (COD), or NH4) of the water to be treated supplied to the tank  10 . 
     In Formula (4), the pressure loss H L  in the air blow system is calculated as the sum of the water head pressure h of the water to be treated in the tank  10 , the pipe pressure loss H P , the ventilation pressure loss H M , and the air diffuser pressure loss H A . However, the pressure loss H L  in the air blow system is not limited to such. For example, in the case where the header pipe  32  is shared by the respective air blow systems of the plurality of tanks  10 , the pressure loss H L  in the air blow system may be at least one of the pressure loss (first pressure loss) in the branch pipe  34  included in the air blow system, the pressure loss (second pressure loss) corresponding to the water head pressure of the water to be treated in the tank  10  connected to the branch pipe  34 , and the pressure loss (third pressure loss) by the air diffuser  12  in the tank  10 . The pressure loss H L  may be the sum of at least two of the first pressure loss, the second pressure loss, the third pressure loss, and the pressure loss (fourth pressure loss) in the header pipe  32  included in the air blow system. An example of calculating the pressure loss H L  based on the foregoing Formula (4) will be described below. 
     The target pipe internal pressure calculation unit  53  calculates the pressure loss H L  in the air blow system of each of the plurality of tanks  10 . In detail, the target pipe internal pressure calculation unit  53  calculates the sum of the water head pressure h of the tank  10 A, the pipe pressure loss H PA , the ventilation pressure loss H MA  of the path from the blower unit  20  to the tank  10 A, and the air diffuser pressure loss H AA  by the air diffuser  12  in the tank  10 A, as the pressure loss H LA  in the air blow system of the tank  10 A. In the same manner, the target pipe internal pressure calculation unit  53  calculates the pressure loss H LB  in the air blow system of the tank  10 B and the pressure loss H LC  in the air blow system of the tank  10 C. The foregoing method of calculating the pressure loss H L  is merely an example, and any method that can calculate the pressure loss H L  in the air blow system of each of the plurality of tanks  10  may be used. 
     The target pipe internal pressure calculation unit  53  determines the maximum value out of the respective pressure losses H L  (pressure losses H LA , H LB , and H LC ) in the air blow systems of the plurality of tanks  10 , as the target pipe internal pressure. The target pipe internal pressure calculation unit  53  outputs the calculation result of the target pipe internal pressure to the air blow control unit  54 . The target pipe internal pressure calculation unit  53  also outputs the calculation result of the pressure loss H L  in the air blow system of each of the plurality of tanks  10  to the water-to-be-treated supply control unit  56 . 
     The air blow control unit  54  controls the supply of air from the blower unit  20  so that the pressure measured by the header pipe internal pressure measurement unit  44  will match the target pipe internal pressure calculated by the target pipe internal pressure calculation unit  53 . Specifically, the air blow control unit  54  controls the amount of air supplied from the blower unit  20  so that the internal pressure in the header pipe  32  measured by the header pipe internal pressure measurement unit  44  will match the target pipe internal pressure, based on the measurement result of the intake air measurement unit  43 . 
     The introduced air control unit  55  controls the degree of opening of the introduction valve  36  so that the amount of air supplied to the tank  10 , which is measured by the branch pipe air volume measurement unit  45 , will match the required air amount calculated by the required air amount calculation unit  52 . Specifically, the introduced air control unit  55  controls the degree of opening of the introduction valve  36  so that the amount of air supplied to the tank  10  will follow the target air amount, by PID (proportional integral differential) control using the target air amount as the target value and the measurement result of the branch pipe air volume measurement unit  45 . 
     The water-to-be-treated supply control unit  56  controls the supply of the water to be treated to each of the plurality of tanks  10  through the water pump  13 , according to the pressure loss H L  in the air blow system of each of the plurality of tanks  10  calculated by the target pipe internal pressure calculation unit  53 . Specifically, the water-to-be-treated supply control unit  56  controls the supply of the water to be treated to each of the plurality of tanks  10  so as to equalize the respective pressure losses H L  in the plurality of air blow systems. 
     By controlling the supply of the water to be treated to each tank  10 , the required air amount in each tank  10  changes, and the amount of air blown in the air blow system of each tank  10  changes. Therefore, by controlling the supply of the water to be treated to each of the plurality of tanks  10  so as to equalize the respective pressure losses H L  in the plurality of air blow system, the respective pressure losses in the air blow systems can be equalized. As a result of the respective pressure losses in the air blow systems being equalized, the wasted air blow electricity of the blower unit  20  caused by supplying air to the air blow system of each tank  10  at excessive pressure can be reduced, and efficient use of electricity in water treatment can be achieved. The control of the supply of the water to be treated to each tank  10  will be described in detail later. 
     The operation of the water treatment system  1  according to this embodiment will be described below.  FIG. 3  is a flowchart illustrating an example of the operation of the water treatment system  1  according to this embodiment, for describing a water treatment method in the water treatment system  1 . The operation relating to the control of the supply of water to be treated to each tank  10  will be mainly described below with reference to  FIG. 3 . 
     The target pipe internal pressure calculation unit  53  calculates the pressure loss H L  in the air blow system of each of the plurality of tanks  10  (step S 11 ). As mentioned above, the target pipe internal pressure calculation unit  53  calculates the sum of the water head pressure h of the tank  10 , the pipe pressure loss H P  in the air blow pipe  30 , the ventilation pressure loss H M , and the air diffuser pressure loss H A , as the pressure loss H L  in the air blow system. 
     Next, the water-to-be-treated supply control unit  56  determines whether the difference between the pressure loss H L  (maximum pressure loss) in the air blow system maximum in pressure loss H L  and the pressure loss H L  (minimum pressure loss) in the air blow system minimum in pressure loss H L  is greater than or equal to a predetermined threshold (step S 12 ). The threshold may be, for example, a numeric value (e.g. 0.5 kPa) set by an administrator of the water treatment system  1 . The threshold may be, for example, the ratio (e.g. 5 □) of the difference between the maximum pressure loss and the minimum pressure loss to the maximum pressure loss, which is set by the administrator of the water treatment system  1 . 
     In the case where the water-to-be-treated supply control unit  56  determines that the difference between the maximum pressure loss and the minimum pressure loss is less than the predetermined threshold (step S 12 : No), the water-to-be-treated supply control unit  56  ends the process. 
     In the case where the water-to-be-treated supply control unit  56  determines that the difference between the maximum pressure loss and the minimum pressure loss is greater than or equal to the predetermined threshold (step S 12 : Yes), the water-to-be-treated supply control unit  56  controls the supply of the water to be treated to each of the plurality of tanks  10  according to the pressure loss H L  in the air blow system of each of the plurality of tanks  10  (step S 13 ). Specifically, the water-to-be-treated supply control unit  56  controls the supply of the water to be treated to each of the plurality of tanks  10  so as to equalize the respective pressure losses H L  in the plurality of air blow systems. In the case where the water-to-be-treated supply control unit  56  controls the supply of the water to be treated to each of the plurality of tanks  10  according to the pressure loss H L  in the air blow system of each of the plurality of tanks  10 , the water-to-be-treated supply control unit  56  may notify the administrator of the water treatment system  1  of the control. 
     Thus, the water treatment method according to this embodiment includes: calculating the pressure loss H L  in the air blow system of each of the plurality of tanks  10 ; and controlling the supply of the water to be treated to each of the plurality of tanks  10  according to the pressure loss H L  in the air blow system of each of the plurality of tanks  10 . The pressure loss H L  in the air blow system of each of the plurality of tanks  10  may be calculated outside the water treatment system  1 . 
     For example, the water treatment system  1  performs the process described with reference to  FIG. 3  at a predetermined frequency (e.g. once a day). The water treatment system  1  may perform the process described with reference to  FIG. 3  in real time. In the case of performing the process described with reference to  FIG. 3  in real time, the water-to-be-treated supply control unit  56  controls the supply of the water to be treated to each tank  10  at a speed corresponding to the time until the water to be treated supplied to the tank  10  is subjected to biological treatment and the treated water is flown out of the tank  10 . 
     The operation of the water treatment system  1  according to this embodiment will be described in more detail below. For comparison, the operation in the case of applying each of the first and second methods described above to a water treatment system that includes the tanks  10  (the tanks  10 A,  10 B, and  10 C), the blower unit  20 , and the air blow pipe  30  illustrated in  FIG. 1  will be described first. It is assumed here that the amount of water to be treated in the tanks  10 A,  10 B, and  10 C is constant and the water head pressure h is 60 kPa. It is also assumed that the ventilation pressure loss H M  is a constant value, and its description is omitted. Suppose, in the first and second methods, the same amount of water to be treated is supplied to each tank  10 . For example, the water to be treated of 2000 m 3 /hr in water amount is supplied to each tank  10 . 
     The operation in the case of applying the first method will be described below. In the first method, the expected maximum pressure loss H L  is calculated, and air is supplied to the plurality of tanks  10  at the air blow pressure corresponding to the calculated pressure loss H L , as mentioned above. Suppose the pipe pressure loss H P  in the path of the air blow pipe  30  from the blower unit  20  to the tank  10 A is 5 kPa at the maximum, and the air diffuser pressure loss H A  in the air diffuser  12  is 3 kPa at the maximum. In this case, the sum of the water head pressure h (60 kPa), the expected maximum pipe pressure loss H P  (5 kPa), and the expected maximum air diffuser pressure loss H A  (3 kPa), i.e. 68 kPa, is set as the air blow pressure of the blower unit  20 . With the first method, air is supplied to the air blow system of each tank  10  at excessive pressure. Consequently, the air blow electricity of the blower unit  20  is wasted, and efficient use of electricity in water treatment cannot be achieved. 
     Next, the operation in the case of applying the second method will be described below. In the second method, the pressure loss in each air blow system is calculated based on the water quality of water to be treated in the corresponding tank  10 , and air is supplied to the plurality of tanks  10  at the air blow pressure corresponding to the calculated maximum pressure loss H L , as mentioned above. Suppose the pressure loss H L  in the air blow system of the tank  10 A is 4 kPa, the pressure loss H L  in the air blow system of the tank  10 B is 3 kPa, and the pressure loss H L  in the air blow system of the tank  10 C is 2 kPa. Also suppose the air diffuser pressure loss H A  is 2 kPa. In this case, the sum of the water head pressure h (60 kPa), the maximum pressure loss H L  (4 kPa), and the air diffuser pressure loss H A  (2 kPa), i.e. 66 kPa, is set as the air blow pressure of the blower unit  20 . With the second method, the air blow pressure of the blower unit  20  is set based on the actual pressure loss H L  in the air blow system of each tank  10  and the actual air diffuser pressure loss H A , so that the air blow pressure of the blower unit  20 , i.e. the air blow electricity of the blower unit  20 , can be reduced as compared with the first method. With the second method, however, air is blown at excessive pressure into the tanks  10 B and  10 C other than the tank  10  whose air blow system is maximum in pressure loss H L . Consequently, the air blow electricity of the blower unit  20  is wasted, and efficient use of electricity in water treatment cannot be achieved. 
     In this embodiment, the supply of the water to be treated to each of the plurality of tanks  10  is controlled according to the pressure loss H L  in the air blow system of each of the plurality of tanks  10  (so as to equalize the respective pressure losses H L  in the air blow systems of the plurality of reaction tanks  10 ), thus reducing the wasted air blow electricity of the blower unit  20  and achieving efficient use of electricity in water treatment. The details of the control of the supply of the water to be treated to each of the plurality of tanks  10  according to the pressure loss H L  in the air blow system of each of the plurality of tanks  10  in this embodiment will be described below. 
     The water-to-be-treated supply control unit  56  controls the pollutant load ratio or the pollutant load amount of the water to be treated supplied to each of the plurality of tanks  10  so as to reduce the difference between the respective pressure losses H L  in the air blow systems of the plurality of tanks  10 . The pollutant load ratio is the ratio of the pollutant load of the water to be treated in each tank  10  to the pollutant load of the water to be treated in all tanks  10 . 
     The case of controlling the pollutant load ratio of the water to be treated supplied to each of the plurality of tanks  10  will be described first. Suppose the concentration of the water to be treated supplied to each tank  10  is constant. Assuming that the total amount of the water to be treated supplied to the plurality of tanks  10  is constant, the water-to-be-treated supply control unit  56  controls the amount of the water to be treated supplied to each tank  10 . The pollutant load is expressed as the product of the amount of the water to be treated supplied to the tank  10  and the concentration of the water to be treated supplied to the tank  10 , as mentioned above. In the case where the concentration of the water to be treated is constant, the pollutant load ratio of the water to be treated supplied to each tank  10  is proportional to the amount of the water to be treated supplied to the tank  10 . 
     When the pollutant load ratio of the water to be treated supplied to each tank  10  changes, the required air amount in the tank  10  changes and also the pressure loss H L  in the air blow system of the tank  10  changes. Hence, by controlling the pollutant load ratio of the water to be treated supplied to each of the plurality of tanks  10  so as to reduce the difference between the respective pressure losses in the air blow systems of the plurality of tanks  10 , the respective pressure losses in the air blow systems can be equalized. As a result of the respective pressure losses in the air blow systems being equalized, the wasted air blow electricity of the blower unit  20  can be reduced, and efficient use of electricity in water treatment can be achieved. 
     An example of control of the pollutant load ratio of water to be treated supplied to each of the plurality of tanks  10  according to the pressure loss H L  in the air blow system of each of the plurality of tanks  10  by the water-to-be-treated supply control unit  56  will be described below. Suppose water to be treated of 2000 m 3 /hr is supplied to each of the tanks  10 A,  10 B, and  10 C, that is, the total amount of the water to be treated supplied to the tanks  10 A,  10 B, and  10 C is 6000 m 3 /hr. Also suppose the pressure loss H LA  in the air blow system of the tank  10 A is 4 kPa, the pressure loss H LB  in the air blow system of the tank  10 B is 3 kPa, the pressure loss H LC  in the air blow system of the tank  10 C is 2 kPa, and the air diffuser pressure loss H A  in each of the tanks  10 A,  10 B, and  10 C is 2 kPa. 
     While maintaining the total amount of the water to be treated supplied to the tanks  10 A,  10 B, and  10 C constant, the water-to-be-treated supply control unit  56  reduces the amount of the water to be treated supplied to the tank  10 A maximum in pressure loss H L  and increases the amount of the water to be treated supplied to the tank  10 C minimum in pressure loss H L . For example, the water-to-be-treated supply control unit  56  sets the amount of the water to be treated supplied to the tank  10 A to 1500 m 3 /hr, and the amount of the water to be treated supplied to the tank  10 C to 2500 m 3 /hr. Meanwhile, the water-to-be-treated supply control unit  56  maintains the amount of the water to be treated supplied to the tank  10 B at 2000 m 3 /hr. Thus, the water-to-be-treated supply control unit  56  calculates the pressure loss H L  (first calculated pressure loss) in the air blow system (first air blow system among the plurality of air blow systems) connected to the tank  10 A (first tank among the plurality of tanks  10 ) and the pressure loss H L  (second calculated pressure loss) in the air blow system (second air blow system among the plurality of air blow systems) connected to each of the tanks  10 B and  10 C (second tank among the plurality of tanks  10 ). Here, the second calculated pressure loss is less than the first calculated pressure loss. In this case, the water-to-be-treated supply control unit  56  reduces the amount of the water to be treated supplied to the tank  10 A. The water-to-be-treated supply control unit  56  calculates the pressure loss H L  (first calculated pressure loss) in the air blow system (first air blow system among the plurality of air blow systems) connected to each of the tanks  10 A and  10 B (first tank among the plurality of tanks  10 ) and the pressure loss H L  (second calculated pressure loss) in the air blow system (second air blow system in the plurality of air blow systems) connected to the tank  10 C (second tank among the plurality of tanks  10 ). Here, the second calculated pressure loss is less than the first calculated pressure loss. In this case, the water-to-be-treated supply control unit  56  increases the amount of the water to be treated supplied to the tank  10 C. That is, the water-to-be-treated supply control unit  56  specifies the first calculated pressure loss and the second calculated pressure loss less than the first calculated pressure loss among the pressure losses H L  calculated for the respective air blow systems of the plurality of tanks  10 . The water-to-be-treated supply control unit  56  also specifies the first tank connected to the air blow system having the first calculated pressure loss and the second tank connected to the air blow system having the second calculated pressure loss. The water-to-be-treated supply control unit  56  reduces the amount of the water to be treated supplied to the first tank. Moreover, the water-to-be-treated supply control unit  56  increases the amount of the water to be treated supplied to the second tank. 
     As a result of reducing the amount of the water to be treated supplied to the tank  10 A, for example, the air diffuser pressure loss H AA  in the air diffuser  12  in the tank  10 A is reduced to 1.8 kPa. Moreover, as a result of reducing the amount of the water to be treated supplied to the tank  10 A, the required air amount in the tank  10 A is reduced. As a result of the required air amount and the air diffuser pressure loss H AA  in the tank  10 A being reduced, the pipe pressure loss H PA  in the air blow system of the tank  10 A is reduced from before the control of the supply amount of the water to be treated. For example, while the pipe pressure loss H PA  before the control of the supply amount of the water to be treated is 4 kPa as mentioned above, the pipe pressure loss H PA  after the control of the supply amount of the water to be treated is reduced to 3.2 kPa. 
     As a result of increasing the amount of the water to be treated supplied to the tank  10 C, for example, the air diffuser pressure loss H AC  in the air diffuser  12  in the tank  10 C is increased to 2.2 kPa. Moreover, as a result of increasing the amount of the water to be treated supplied to the tank  10 C, the required air amount in the tank  10 C is increased. As a result of the required air amount and the air diffuser pressure loss H AC  in the tank  10 C being increased, the pipe pressure loss H PC  in the air blow system of the tank  10 C is increased from before the control of the supply amount of the water to be treated. For example, while the pipe pressure loss H PC  before the control of the supply amount of the water to be treated is 2 kPa, the pipe pressure loss H PC  after the control of the supply amount of the water to be treated is increased to 2.8 kPa. 
     Thus, the water-to-be-treated supply control unit  56  controls the supply amount of the water to be treated to each of the plurality of tanks  10  so as to equalize the respective pressure losses H L  in the air blow systems of the plurality of reaction tanks  10 . As a result of controlling the supply amount of the water to be treated, the pressure loss required in the air blow system of the tank  10 A is 65 kPa (=60 kPa+3.2 kPa+1.8 kPa), the pressure loss required in the air blow system of the tank  10 B is 65 kPa (=60 kPa+3 kPa+2 kPa), and the pressure loss required in the air blow system of the tank  10 C is 65 kPa (=60 kPa+2.8 kPa+2.2 kPa). Thus, the respective pressure losses in the air blow systems of the tanks  10  are equalized. Consequently, air is blown in proper quantity into each tank  10  with appropriate pressure loss. Moreover, the same amount of water to be treated can be treated at lower air blow pressure than in the first and second methods. Hence, the wasted air blow electricity of the blower unit  20  can be reduced, and efficient use of electricity in water treatment can be achieved. 
     The case of controlling the pollutant load amount of the water to be treated supplied to each of the plurality of tanks  10  will be described next. Suppose the water to be treated of 2000 m 3 /hr is supplied to each of the tanks  10 A,  10 B, and  10 C. Also suppose the pressure loss H LA  in the air blow system of the tank  10 A is 4 kPa, the pressure loss H LB  in the air blow system of the tank  10 B is 3 kPa, and the pressure loss H LC  in the air blow system of the tank  10 C is 2 kPa. 
     When the pressure loss H L  in the air blow system of one tank  10  is lower than the pressure loss H L  in the air blow system of another tank  10 , the water-to-be-treated supply control unit  56  increases the pollutant load amount of the water to be treated supplied to the tank  10  to be greater than that of the other tank  10 . Assuming that the concentration of the water to be treated is constant, the water-to-be-treated supply control unit  56  maintains the amount of the water to be treated supplied to the tank  10 A, and increases the amount of the water to be treated supplied to each of the tanks  10 B and  10 C. Here, the water-to-be-treated supply control unit  56  increases the amount of the water to be treated supplied to the tank  10 C by a greater amount than the amount of the water to be treated supplied to the tank  10 B. Specifically, for example, the water-to-be-treated supply control unit  56  maintains the amount of the water to be treated supplied to the tank  10 A at 2000 m 3 /hr, increases the amount of the water to be treated supplied to the tank  10 B to 2500 m 3 /hr, and increases the amount of the water to be treated supplied to the tank  10 C to 3000 m 3 /hr. 
     Thus, as a result of increasing the supply amount of the water to be treated to each of the tanks  10 B and  10 C, the air diffuser pressure loss H A  and the pipe pressure loss H P  are increased, and the pressure loss H L  is increased. Since the increase of the supply amount of the water to be treated is greater in the tank  10 C than in the tank  10 B, the increase of the pressure loss H LC  in the air blow system of the tank  10 C is greater than the increase of the pressure loss H LB  in the air blow system of the tank  10 B. Accordingly, the respective pressure losses H LB  and H LC  in the air blow systems of the tanks  10 B and  10 C approach the pressure loss H LA  in the air blow system of the tank  10 A, and thus the respective pressure losses in the air blow systems of the plurality of tanks  10  are equalized. Consequently, air is supplied in proper quantity to each tank  10  with appropriate pressure loss. Hence, the wasted air blow electricity of the blower unit  20  can be reduced, and efficient use of electricity in water treatment can be achieved. 
     Although the above describes an example in which, when the pressure loss H L  in the air blow system of one tank  10  is lower than the pressure loss H L  in the air blow system of another tank  10 , the water-to-be-treated supply control unit  56  increases the pollutant load amount of the water to be treated supplied to the tank  10  to be greater than that of the other tank  10 , the presently disclosed techniques are not limited to such. When the pressure loss H L  in the air blow system of one tank  10  is higher than the pressure loss H L  in the air blow system of another tank  10 , the water-to-be-treated supply control unit  56  may reduce the pollutant load amount of the water to be treated supplied to the tank  10  to be smaller than that of the other tank  10 . In this way, too, the respective pressure losses in the air blow systems of the plurality of tanks  10  are equalized, so that the wasted air blow electricity of the blower unit  20  can be reduced and efficient use of electricity in water treatment can be achieved. 
     Although the above describes an example in which the concentration of the water to be treated supplied to each tank  10  is constant, the presently disclosed techniques are not limited to such. As mentioned above, the pollutant load is expressed as the product of the amount of the water to be treated supplied to the tank  10  and the concentration of the water to be treated supplied to the tank  10 . Accordingly, the water-to-be-treated supply control unit  56  may control the pollutant load amount or the pollutant load ratio of the water to be treated supplied to each tank  10  by controlling the concentration of the water to be treated supplied to the tank  10 . 
     In the case of controlling the supply of the water to be treated to each of the plurality of tanks  10 , for example, the water-to-be-treated supply control unit  56  controls the water to be treated to each of the plurality of tanks  10  with any pollutant load ratio or pollutant load amount. If this control results in the difference between the respective pressure losses H L  in the air blow systems of the plurality of tanks  10  being within a predetermined range, the water-to-be-treated supply control unit  56  adopts the pollutant load ratio or pollutant load amount. If the difference between the respective pressure losses H L  in the air blow systems of the plurality of tanks  10  is outside the predetermined range, on the other hand, the water-to-be-treated supply control unit  56  changes the pollutant load ratio or pollutant load amount based on the degree of deviation from the redetermined range, and compares the respective pressure losses H L  in the air blow systems of the plurality of tanks  10  again. By repeating this process, the water-to-be-treated supply control unit  56  determines a pollutant load ratio or pollutant load amount with which the difference between the respective pressure losses H L  in the air blow systems of the plurality of tanks  10  is within the predetermined range. 
     As described above, in this embodiment, the water treatment system  1  includes: a plurality of tanks  10 ; a plurality of air blow systems (for example, branch pipes  34  ( 34 A,  34 B,  34 C)) connected to the respective plurality of tanks  10 ; a blower unit  20  configured to supply air to the plurality of tanks  10  through the air blow systems; and a control circuit  50  (control device). The control circuit  50  is configured to calculate a pressure loss H L  in an air blow system of each of the plurality of tanks, and control supply of water to be treated to each of the plurality of tanks  10 , according to the calculated pressure loss H L  in the air blow system of each of the plurality of tanks  10 . 
     By controlling the supply of the water to be treated to each of the plurality of tanks  10  according to the pressure loss H L  in the air blow system of each of the plurality of tanks  10 , the required air amount in each tank  10  changes, and the respective pressure losses in the air blow systems of the tanks  10  are equalized. This prevents supply of air to the air blow system of each tank  10  at excessive pressure, so that the wasted air blow electricity of the blower unit  20  can be reduced and efficient use of electricity in water treatment can be achieved. 
     While the presently disclosed techniques have been described by way of the drawings and embodiments, various changes and modifications may be easily made by those of ordinary skill in the art based on the present disclosure. Such changes and modifications are therefore included in the scope of the present disclosure. For example, the functions included in the means, steps, etc. may be rearranged without logical inconsistency, and a plurality of means, steps, etc. may be combined into one means, step, etc. and a means, step, etc. may be divided into a plurality of means, steps, etc. The above description merely relates to one of the disclosed embodiments, and various changes may be made within the scope of claims. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  water treatment system 
               10 ,  10 A,  10 B,  10 C tank 
               12  air diffuser 
               13  water pump 
               20  blower unit 
               22 ,  22 A,  22 B,  22 C,  22 D air blower 
               30  air blow pipe 
               31  introduction pipe 
               32  header pipe 
               34 ,  34 A,  34 B,  34 C branch pipe 
               36  introduction valve 
               40  control apparatus 
               41  nitric acid meter 
               42  ammonia meter 
               43  intake air measurement unit 
               44  header pipe internal pressure measurement unit 
               45  branch pipe air volume measurement unit 
               50  control unit 
               51  acquisition unit 
               52  required air amount calculation unit 
               53  target pipe internal pressure calculation unit (pressure loss calculation unit) 
               54  air blow control unit 
               55  introduced air control unit 
               56  water-to-be-treated supply control unit 
               501  control circuit (control device) 
               502  memory 
               503  bus