Treatment system and control method of the same

Abstract

To provide a treatment system for adding a coagulant agent to an object to be treated and performing solid-liquid separation, which maintains a coagulant agent addition rate in an appropriate range, suppresses deterioration of LCC by excessive addition of the coagulant agent and an overload in a solid-liquid separation part, improves a coagulant floc property in a coagulant by addition of a sufficient and adequate amount of the coagulant agent, and improves and stabilizes the coagulant in the solid-liquid separation part and performs solid-liquid separation.SOLUTION: A treatment system executes coagulant agent addition rate control of measuring liquid levels Li and Lo in storage parts which are liquid levels of a coagulant S2 or a concentrate S3 in the storage parts 20 and 23 which are installed on the inlet side or the outlet side of a solid-liquid separation part 10 and temporarily store the coagulant S2 or the concentrate S3, and controlling a coagulant agent addition rate x to an object S1 to be treated by a coagulant agent addition part 2 on the basis of the liquid levels Li and Lo in the storage part.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a treatment system including a flocculant addition unit that adds a flocculant to an object to be treated to form flocs, and a solid-liquid separation unit that separates the flocs into a concentrate by solid-liquid separation, and a control method thereof.

Background Art

[0002] As a treatment system for reducing the volume of objects to be treated such as sewage sludge and biomass, by adding a flocculant such as a polymer flocculant to the object to be treated, the solid content of the object to be treated contained in the object to be treated is aggregated to form flocculated flocs (or also referred to as "coarse flocs"), and by solid-liquid separating the flocs containing such flocculated flocs, a concentrate with a reduced water content is generated. In such a treatment system, when the addition rate of the flocculant to the object to be treated (also referred to as the "flocculant addition rate" in the present application) is low, the flocculant is insufficient in the flocs, the formation of flocculated flocs becomes insufficient, and the flocs cannot be appropriately solid-liquid separated in the solid-liquid separation unit, and the water content of the concentrate may not be reduced to the target value. Conversely, when the flocculant addition rate is high, the flocculant is excessively added to the flocs, and some of the flocculant is not used for aggregating the object to be treated and is discharged as a separated liquid in the solid-liquid separation unit without reaction, which deteriorates the LCC (life cycle cost), and there may be a problem of overload in the solid-liquid separation unit. Further, when the separated liquid containing the unreacted flocculant is returned to a biological treatment facility performing aerobic treatment such as the activated sludge method, it may have an adverse effect on the biological treatment facility. Therefore, in the treatment system, it is desirable to control the flocculant addition rate to the object to be treated within an appropriate range.

[0003] Therefore, a technique has been proposed in which the solid content concentration of the concentrate discharged from the solid-liquid separation unit is detected, and the flocculant addition rate is controlled based on the detection result (see, for example, Patent Document 1). In the treatment system described in Patent Document 1 above, the input amount and solid content concentration of the flocs, the flow rate and turbidity of the separated water discharged by solid-liquid separation, the discharge amount of the concentrate, etc. are measured, and the solid content concentration of the concentrate is calculated from these measurement results.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In a configuration where the solid content concentration of a concentrate is detected and the detection result is used to control the flocculant addition rate as in the above Patent Document 1, since it takes a certain amount of time to detect the solid content concentration of the concentrate, there is a delay in controlling the flocculant addition rate. Therefore, the flocculant addition rate cannot follow relatively rapid fluctuations in the solid content concentration of the concentrate, and there is a risk that the actual flocculant addition rate will deviate from an appropriate range. Furthermore, since the detection accuracy of the solid content concentration of the concentrate depends greatly on the measurement accuracy of each parameter used for the detection, it is not very high. Therefore, when controlling the flocculant addition rate based on a detection result with low accuracy, the actual flocculant addition rate may deviate from within an appropriate range. In view of this actual situation, the main problem of the present invention is to maintain the flocculant addition rate within an appropriate range in a treatment system that performs solid-liquid separation after adding a flocculant to an object to be treated, suppress deterioration of LCC due to excessive addition of the flocculant and overload in the solid-liquid separation section, improve the floc properties in the flocs by adding an appropriate amount of the flocculant without shortage, and provide a technique capable of stably and satisfactorily performing solid-liquid separation of the flocs in the solid-liquid separation section.

Means for Solving the Problems

[0006] A first characteristic configuration of the treatment system according to the present invention is a treatment system including a flocculant addition section that adds a flocculant to an object to be treated to form flocs, and a solid-liquid separation section that separates the flocs into a solid-liquid separation to obtain a concentrate, a storage section installed on the inlet side or the outlet side of the solid-liquid separation section for temporarily storing the flocs or the concentrate, A storage tank liquid level measurement unit that measures the liquid level in the storage tank, which is the liquid level of the aggregate or the concentrate in the storage tank; and a control unit that executes a flocculant addition rate control for controlling the flocculant addition rate to the object to be treated by the flocculant addition unit based on the liquid level in the storage tank. Further, a characteristic configuration of the control method of the treatment system according to the present invention is a control method of a treatment system including a flocculant addition unit that adds a flocculant to an object to be treated to form an aggregate, and a solid-liquid separation unit that separates the aggregate into a solid and a liquid to obtain a concentrate, measuring the liquid level in the storage tank, which is the liquid level of the aggregate or the concentrate in the storage tank that is installed on the inlet side or the outlet side of the solid-liquid separation unit and temporarily stores the aggregate or the concentrate, and executing a flocculant addition rate control for controlling the flocculant addition rate to the object to be treated by the flocculant addition unit based on the liquid level in the storage tank.

[0007] According to this configuration, when the flocculant addition rate to the object to be treated changes, the properties of the floc formed in the aggregate to which the flocculant is added, such as the viscosity, fineness, and strength (hereinafter referred to as "floc properties"), change, and the ease of engulfment of the aggregate in the solid-liquid separation unit and the ease of water separation change. As a result, the liquid level in the storage tank on the inlet side or the outlet side of the solid-liquid separation unit changes. Therefore, it is possible to determine the excess or deficiency of the flocculant addition rate from the state of the liquid level in the storage tank measured by the liquid level measurement unit in the storage tank itself or its change rate. Then, the control unit executes the flocculant addition rate control and controls the flocculant addition rate based on the liquid level in the storage tank, so that the liquid level in the storage tank can be maintained within an appropriate range as much as possible. Therefore, according to the present invention, in a treatment system in which a flocculant is added to an object to be treated and then solid-liquid separation is performed, the flocculant addition rate is maintained within an appropriate range, the deterioration of LCC due to excessive addition of the flocculant and the overload in the solid-liquid separation unit are suppressed, and the floc properties in the aggregate are improved by adding an appropriate amount of the flocculant without shortage, so that the aggregate can be solid-liquid separated well and stably in the solid-liquid separation unit. A technology can be provided.

[0008] The second characteristic configuration of the processing system according to the present invention is that, in the coagulant addition rate control, the control unit temporarily increases the coagulant addition rate in the addition rate temporary increase process from when the liquid level in the storage unit exceeds a predetermined determination value for temporary increase until the liquid level in the storage unit becomes a predetermined return determination value lower than the temporary increase determination value.

[0009] According to this configuration, when the liquid level in the storage unit exceeds the above-mentioned temporary increase determination value, it is determined that the coagulant is insufficient in the aggregate, and the execution of the above-mentioned addition rate temporary increase process is started, and the coagulant addition rate is increased. By this, deterioration of the coagulation floc properties such as a decrease in the strength and subdivision of the coagulation floc due to the shortage of the coagulant in the aggregate is suppressed, and an excessive increase in the liquid level in the storage unit due to the deterioration of the coagulation floc properties can be avoided. And after the execution of the above-mentioned addition rate temporary increase process is started, as the coagulant addition rate increases, the liquid level in the storage unit decreases, and when the liquid level in the storage unit decreases to the above-mentioned return determination value, the execution of the above-mentioned addition rate temporary increase process is terminated, and the coagulant addition rate is returned to the one before the increase. By this, deterioration of the LCC due to the increase in the coagulant addition rate can be suppressed as much as possible.

[0010] The third characteristic configuration of the processing system according to the present invention is that, in the coagulant addition rate control, when the liquid level in the storage unit exceeds a predetermined increase correction determination value that is equal to or higher than the temporary increase determination value, the control unit increases the coagulant addition rate by a predetermined increase correction width, and when the liquid level in the storage unit falls below a predetermined decrease correction determination value that is less than the return determination value, the control unit decreases the coagulant addition rate by a predetermined decrease correction width in the addition rate correction process.

[0011] According to this configuration, when the liquid level in the storage unit exceeds the increase correction determination value by executing the addition rate correction process, it is determined that the flocculant is insufficient in the aggregate, and the flocculant addition rate is increased by the increase correction width. Further, when the liquid level in the storage unit falls below the decrease correction determination value, it is determined that the flocculant is excessively added in the aggregate, and the flocculant addition rate is decreased by the decrease correction width. Thus, the flocculant addition rate can be appropriately corrected in a timely manner to be maintained within a range where the liquid level in the storage unit is less than or equal to the increase correction determination value and greater than or equal to the decrease correction determination value. Furthermore, the increase correction determination value, which is the criterion for determining the increase in the flocculant addition rate by the addition rate correction process, is set to be equal to or higher than the temporary increase determination value, which is the criterion for starting the execution of the above-described addition rate temporary increase process. Thus, in a case where the temporary increase in the flocculant addition rate due to the start of the execution of the addition rate temporary increase process does not eliminate the shortage of the flocculant and cannot contribute to avoiding the rise in the liquid level in the storage unit, the flocculant addition rate can be increased and corrected by the increase correction width by the addition rate correction process, thereby surely avoiding an excessive rise in the liquid level in the storage unit. Also, the decrease correction determination value, which is the criterion for determining the decrease in the flocculant addition rate by the addition rate correction process, is set to be less than the return determination value, which is the criterion for ending the execution of the above-described addition rate temporary increase process. Thus, in a case where the temporary decrease in the flocculant addition rate due to the end of the execution of the addition rate temporary increase process does not eliminate the excessive addition of the flocculant and cannot contribute to avoiding the drop in the liquid level in the storage unit, the flocculant addition rate can be decreased and corrected by the decrease correction width by the addition rate correction process, thereby surely avoiding an excessive drop in the liquid level in the storage unit.

[0012] A fourth characteristic configuration of the processing system according to the present invention includes, as the storage unit, an inlet-side storage unit that is installed on the inlet side of the solid-liquid separation unit and temporarily stores the aggregate immediately before being introduced into the solid-liquid separation unit. As the liquid level measurement unit in the storage unit, an inlet-side storage unit liquid level measurement unit that measures the liquid level in the inlet-side storage unit, which is the liquid level in the storage unit of the aggregate in the inlet-side storage unit, is provided. The control unit executes a first flocculant addition rate control for controlling the flocculant addition rate based on the liquid level in the inlet-side storage unit as the flocculant addition rate control.

[0013] According to this configuration, if the flocculant addition rate is too low, the floc properties in the flocs deteriorate, making it difficult for the flocs to be swallowed from the inlet-side storage unit of the solid-liquid separation unit. As a result, the liquid level in the inlet-side storage unit rises. Therefore, the control unit executes the first flocculant addition rate control to control the flocculant addition rate based on the liquid level in the inlet-side storage unit, thereby maintaining the liquid level in the inlet-side storage unit within an appropriate range as much as possible. By doing so, while suppressing the deterioration of LCC due to excessive addition of the flocculant and the overload in the solid-liquid separation unit, the floc properties in the flocs are improved by adding an appropriate amount of the flocculant without shortage, making it easier for the flocs to be swallowed from the inlet-side storage unit of the solid-liquid separation unit, and enabling good and stable solid-liquid separation of the flocs in the solid-liquid separation unit.

[0014] A fifth characteristic configuration of the treatment system according to the present invention includes an outlet-side storage unit installed on the outlet side of the solid-liquid separation unit for temporarily storing the concentrate immediately before it is discharged from the solid-liquid separation unit and introduced into the subsequent treatment unit as the storage unit, an outlet-side storage unit liquid level measurement unit for measuring the liquid level in the outlet-side storage unit, which is the liquid level in the storage unit of the concentrate in the outlet-side storage unit, as the storage unit liquid level measurement unit, and the control unit executes a second flocculant addition rate control for controlling the flocculant addition rate based on the liquid level in the outlet-side storage unit as the flocculant addition rate control.

[0015] According to this configuration, if the coagulant addition rate is too low, the properties of the coagulation flocs in the aggregates deteriorate, the ease of water separation in the solid-liquid separation section deteriorates, the solid content concentration of the concentrate decreases, and as a result, the liquid level in the outlet-side storage section rises. Therefore, the control unit executes the second coagulant addition rate control and controls the coagulant addition rate based on the liquid level in the outlet-side storage section, so that the liquid level in the outlet-side storage section can be maintained within an appropriate range as much as possible. By this, while suppressing the deterioration of LCC due to excessive addition of the coagulant and the overload in the solid-liquid separation section, the properties of the coagulation flocs in the aggregates are improved by adding an appropriate amount of the coagulant without shortage, the ease of water separation in the solid-liquid separation section is made good, the decrease in the solid content concentration of the concentrate is suppressed, and the aggregates can be solid-liquid separated well and stably in the solid-liquid separation section.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Modes for Carrying Out the Invention

[0017] Embodiments of the processing system and its control method according to the present invention will be described with reference to the drawings. As shown in FIG. 1, in the processing system 100 according to the present embodiment, a flocculant supply pump 2 (an example of a flocculant addition unit) that adds a flocculant F to organic waste S1 (an example of a material to be processed) containing sludge supplied from a front-stage processing unit 1 such as a sludge storage tank, a sludge supply pump, or a concentration device to form flocculated sludge S2 (an example of a flocculated product), and a solid-liquid separation unit 10 that separates the flocculated sludge S2 to which the flocculant F has been added into concentrated sludge S3 (an example of a concentrated product) are provided. In the present embodiment, the flocculant F is added to the organic waste S1 being stirred in the sludge flocculation tank 5, but it may also be added to the organic waste S1 before it is supplied to the sludge flocculation tank 5. Then, by stirring the organic waste S1 to which the flocculant F has been added in the sludge flocculation tank 5, the generation of flocculated flocs is promoted, and the flocculated sludge S2 containing the generated flocculated flocs is supplied from the sludge flocculation tank 5 to the subsequent solid-liquid separation unit 10. As the flocculant F, any known flocculant used for sludge flocculation, such as a cationic, amphoteric, or polyamidine polymer flocculant, can be used. Also, in the processing system 100 of the present embodiment, organic waste S1 containing sludge is used as the material to be processed, but biomass or the like may also be used as the material to be processed.

[0018] The solid-liquid separation unit 10 is configured to separate the flocculated sludge S2 supplied from the sludge flocculation tank 5 and discharge the concentrated sludge S3 obtained by separating moisture through the solid-liquid separation. Depending on the degree of concentration, it may be referred to as a concentration device or a dehydration device. Although there is no specific distinction between the concentration device and the dehydration device, generally, a device that gives a higher solid concentration of the processed material after treatment than the concentration device is called a dehydration device. For example, the solid-liquid separation unit 10 that increases the solid concentration of the solid content from about 1% to about 15% is called a concentration device, and a device that gives a higher solid concentration of the solid content than the concentration device is called a dehydration device. Also, the concentrated sludge S3 corresponds to the concentrated sludge S3 regardless of the solid concentration as long as it is separated by the solid-liquid separation unit 10. As the treatment system 100, there are those having only a concentration device as the solid-liquid separation unit 10, those having only a dehydration device, and those having these concentration device and dehydration device arranged in parallel in series. However, the characteristic configuration of the present invention is applicable to both or either of the concentration device and the dehydration device. Further, when the concentration device and the dehydration device are provided as the solid-liquid separation unit 10, a sludge transfer pump for transferring the sludge discharged from the previous-stage concentration device to the subsequent-stage dehydration device may be provided.

[0019] The solid-liquid separation unit 10 is configured as a screw press type in which a screw 12 housed in an outer cylinder screen 11 is rotationally driven by a drive motor 13, the flocculated sludge S2 introduced from an inlet-side hopper 20 is pressurized in a pressurization chamber 15 formed between the outer cylinder screen 11 and the screw 12 for solid-liquid separation, and the concentrated sludge S3 after the solid-liquid separation is discharged to an outlet-side hopper 23. In the present invention, the solid-liquid separation unit 10 may be of a type different from the above screw press type.

[0020] In the solid-liquid separation unit 10, an inlet-side hopper 20 (an example of an inlet-side storage unit) for temporarily storing the flocculated sludge S2 is provided on the inlet side where the flocculated sludge S2 is supplied, and an outlet-side hopper 23 (an example of an outlet-side storage unit) for temporarily storing the concentrated sludge S3 is provided on the outlet side where the concentrated sludge S3 is discharged. That is, the flocculated sludge S2 supplied from the sludge flocculation tank 5 is once stored in the inlet-side hopper 20, and the flocculated sludge S2 stored in the inlet-side hopper 20 is swallowed into the pressurization chamber 15 by the rotational drive of the screw 12. Further, the concentrated sludge S3 discharged from the pressurization chamber 15 is once stored in the outlet-side hopper 23, and the concentrated sludge S3 stored in the outlet-side hopper 23 is introduced into a subsequent treatment unit 30 such as a transfer pump or a dehydration device. The outlet-side hopper 23 is usually equipped in the solid-liquid separation unit 10 itself such as a concentration device, but may be an inlet-side hopper of the treatment unit 30 installed in the subsequent stage. Further, the structures of the inlet-side hopper 20 and the outlet-side hopper 23 are not limited as long as they can store the flocculated sludge S2 and the concentrated sludge S3, and they may be storage tanks placed separately from the solid-liquid separation unit 10.

[0021] Furthermore, the inlet-side hopper 20 is provided with a level sensor 21 that functions as an inlet-side hopper internal liquid level measurement unit (an example of an inlet-side storage unit internal liquid level measurement unit) for measuring the inlet-side hopper internal liquid level li (an example of the hopper internal liquid level of the agglomerated sludge S2 in the inlet-side hopper 20), which is the liquid level in the hopper of the agglomerated sludge S2 in the inlet-side hopper 20. On the other hand, the outlet-side hopper 23 is provided with a level sensor 24 that functions as an outlet-side hopper internal liquid level measurement unit (an example of an outlet-side storage unit internal liquid level measurement unit) for measuring the outlet-side hopper internal liquid level lo (an example of the hopper internal liquid level of the concentrated sludge S3 in the outlet-side hopper 23), which is the liquid level in the hopper of the concentrated sludge S3 in the outlet-side hopper 23.

[0022] The treatment system 100 is provided with a control device 50 that controls the operations of the flocculant supply pump 2, the drive motor 13 of the solid-liquid separation unit 10, and the like. And the control device 50 functions as a control unit that executes flocculant addition rate control for controlling the flocculant addition rate x, which is the addition rate of the flocculant F to the organic waste S1 by the flocculant supply pump 2, based on the hopper internal liquid levels li and lo measured by the level sensors 21 and 24 by executing a predetermined computer program. Note that the above flocculant addition rate is a value obtained by dividing the solid content amount of the flocculant by the solid content amount in the sludge, and generally can be obtained by the following calculation formula. Flocculant addition rate = (chemical solution dissolution concentration × chemical supply amount) / (sludge solid concentration × sludge supply amount)

[0023] That is, although details will be described later, as shown in FIG. 2, the control device 50 executes the first flocculant addition rate control (step #1) and the second flocculant addition rate control (step #2), and determines the flocculant addition rate basic value x0 that becomes the basic value of the flocculant addition rate x, and the variables of the increase width variables Δx1i, Δx2i, Δx1o, and Δx2o that become the increase width of the flocculant addition rate x, respectively. Then, in step #3, the flocculant addition rate x is determined as a value obtained by adding these increase width variables Δx1i, Δx2i, Δx1o, and Δx2o to the flocculant addition rate basic value x0. Then, the flocculant addition amount vh is determined by multiplying the thus determined flocculant addition rate x by the supply amount vs of the organic waste S1 from the previous treatment unit 1. The output of the flocculant supply pump 2 is set so as to add the flocculant F corresponding to the determined flocculant addition amount vh to the organic waste S1. In addition, in the present embodiment, the initial value of the flocculant addition rate basic value x0 that becomes the basic value of the flocculant addition rate x is set to a value of 0.1% to 2.5%, preferably about 1.8%, and the initial values of the increase width variables Δx1i, Δx2i, Δx1o, and Δx2o that become the increase width are set to 0 points. Hereinafter, details of the first flocculant addition rate control (step #1 in FIG. 2) and the second flocculant addition rate control (step #2 in FIG. 2) will be described in order.

[0024] 〔First flocculant addition rate control〕 As the above-described flocculant addition rate control (step #1 in FIG. 2), the control device 50 executes a first flocculant addition rate control for controlling the flocculant addition rate x based on the liquid level li in the inlet-side hopper measured by the level sensor 21 (see FIG. 1) as shown in FIG. 3. That is, referring also to FIG. 1, when the flocculant addition rate x is too low, the floc properties of the flocculated sludge S2 deteriorate, and the ease of ingestion of the flocculated sludge S2 from the inlet side hopper 20 in the solid-liquid separation section 10 deteriorates, resulting in an increase in the liquid level li in the inlet side hopper. Therefore, by performing the first flocculant addition rate control and controlling the flocculant addition rate x based on the liquid level li in the inlet side hopper, the liquid level li in the inlet side hopper is maintained within an appropriate range as much as possible. Thus, while suppressing the deterioration of LCC due to excessive addition of the flocculant F and the overload in the solid-liquid separation section 10, the floc properties of the flocculated sludge S2 are improved by adding an appropriate amount of the flocculant F without shortage, and the ingestion of the flocculated sludge S2 from the inlet side hopper 20 in the solid-liquid separation section 10 becomes good, and the flocculated sludge S2 is solid-liquid separated well and stably in the solid-liquid separation section 10.

[0025] The first flocculant addition rate control shown in FIG. 3 controls the flocculant addition rate x by comparing the liquid level li in the inlet side hopper itself with various determination values Li1, Li2, Li3, Li4, Li5. This first flocculant addition rate control includes a first addition rate temporary increase process for determining the first addition rate increase width variable Δx1i, a second addition rate temporary increase process for determining the second addition rate increase width variable Δx2i, and an addition rate correction process for determining the flocculant addition rate basic value x0. The details of each process will be described below. In each process, five determination values Li1, Li2, Li3, Li4, Li5 are set in order from the higher side as determination values for the liquid level li in the inlet side hopper. Also, the constants Ai, Bi, Ci, Di regarding the increase and decrease widths set for the variables x0, Δx1i, Δx2i constituting the flocculant addition rate x are all positive numbers greater than 0. Also, in the following description, the names of the determination values Li1, Li2, Li3, Li4, Li5 are made to match the process, but those with the same sign are determination values set to the same value.

[0026] (First addition rate temporary increase process) In the first coagulant addition rate control, the first addition rate temporary increase process is a process of temporarily increasing the coagulant addition rate x from when the liquid level li in the inlet side hopper exceeds a predetermined determination value Li1 for temporary increase until the liquid level li in the inlet side hopper becomes a predetermined return determination value Li3 lower than the temporary increase determination value Li1. This first addition rate temporary increase process is composed of step #1-03, step #1-04, step #1-06, step #1-07 shown in FIG. 3 and step #3 shown in FIG. 2.

[0027] That is, in this first addition rate temporary increase process, mainly as shown in FIG. 3, when it is determined (yes in step #1-03) that the liquid level li in the inlet side hopper exceeds the temporary increase determination value Li1, it is judged that the coagulant F is insufficient in the coagulated sludge S2, and the first addition rate increase width variable Δx1i is changed from 0 to the first temporary increase width Ai (for example, 0.05 to 0.50 points) (step #1-04). Then, in step #3 (see FIG. 2), the coagulant addition rate x is temporarily increased by the first temporary increase width Ai. By this, the deterioration of the coagulated floc properties such as the decrease in the strength and the subdivision of the coagulated flocs due to the shortage of the coagulant F in the coagulated sludge S2 is suppressed, and the excessive increase in the liquid level li in the inlet side hopper due to the deterioration of the coagulated floc properties is avoided. In addition, since the increase in the coagulant addition rate x (step #1-04, step #3 (see FIG. 2)) by the above first addition rate temporary increase process is highly urgent from the viewpoint of avoiding the overflow of the coagulated sludge S2 from the inlet side hopper 20, it is immediately executed when it is determined (yes in step #1-03) that the liquid level li in the inlet side hopper exceeds the temporary increase determination value Li1.

[0028] Furthermore, as the liquid level li in the inlet hopper decreases by the first temporary increase width Ai of the flocculant addition rate x and it is determined that the liquid level li in the inlet hopper has reached the return determination value Li3 (yes in step #1-06), the first addition rate increase width variable Δx1i is reset to 0 (step #1-07). Then, in step #3 (see Figure 2), the temporary increase by the first temporary increase width Ai of the flocculant addition rate x ends, and the flocculant addition rate x is returned to its value before the increase. This suppresses, as much as possible, the deterioration of LCC caused by the increase by the first temporary increase width Ai of the flocculant addition rate x. The decrease in the flocculant addition rate x due to the end of the first addition rate temporary increase process (step #1-07, step #3 (see Figure 2)) is immediately executed when it is determined that the liquid level li in the inlet hopper has reached the return determination value Li3 (yes in step #1-06). Incidentally, since the decrease in the flocculant addition rate x due to the end of the first addition rate temporary increase process (step #1-07, step #3 (see Figure 2)) is less urgent than during the increase, it can also be executed when it is determined that the liquid level li in the inlet hopper has remained below the return determination value Li3 for a set time (for example, 5 to 30 minutes). The above set time is set in consideration of the hydraulic retention time (HRT) of the sludge flocculation tank 5 and the time until the flocculated sludge S2 is supplied to the solid-liquid separation unit 10, and can be set, for example, to the sum of the hydraulic retention time (HRT) of the sludge flocculation tank 5 and the time until the flocculated sludge S2 is supplied to the solid-liquid separation unit 10.

[0029] (Second addition rate temporary increase process) In the second addition rate temporary increase process in the first flocculant addition rate control, similar to the first addition rate temporary increase process described above, the flocculant addition rate x is temporarily increased from when the liquid level li in the inlet hopper exceeds the predetermined temporary increase determination value Li2 until the liquid level li in the inlet hopper reaches a predetermined return determination value Li4 lower than the temporary increase determination value Li2. This second addition rate temporary increase process is composed of step #1-01, step #1-02, step #1-08, step #1-09 shown in Figure 3, and step #3 shown in Figure 2.

[0030] That is, in this second addition rate temporary increase process, mainly as shown in FIG. 3, when it is determined (yes in step #1-01) that the liquid level li in the inlet side hopper exceeds the determination value Li2 for temporary increase, it is judged that the flocculant F is insufficient in the flocculated sludge S2, and the second addition rate increase width variable Δx2i is changed from 0 to the second temporary increase width Bi (for example, 0.10 points) (step #1-02). Then, in step #3 (see FIG. 2), the flocculant addition rate x is temporarily increased by the second temporary increase width Bi. By this, the deterioration of the floc properties such as the decrease in the strength and subdivision of the floc due to the shortage of the flocculant F in the flocculated sludge S2 is suppressed, and the excessive increase in the liquid level li in the inlet side hopper due to the deterioration of the floc properties is avoided. Incidentally, the increase in the flocculant addition rate x by the above second addition rate temporary increase process (step #1-02, step #3 (see FIG. 2)) is highly urgent from the viewpoint of avoiding the overflow of the flocculated sludge S2 from the inlet side hopper 20, and thus is immediately executed when it is determined (yes in step #1-03) that the liquid level li in the inlet side hopper exceeds the determination value Li2 for temporary increase.

[0031] Furthermore, as the liquid level li in the inlet hopper decreases by the second temporary increase width Bi of the flocculant addition rate x, and when it is determined that the liquid level li in the inlet hopper has reached the return determination value Li4 (yes in step #1-08), the second addition rate increase width variable Δx2i is reset to 0 (step #1-09). Then, in step #3 (see Figure 2), the temporary increase by the second temporary increase width Bi of the flocculant addition rate x ends, and the flocculant addition rate x returns to its value before the increase. This can suppress the deterioration of LCC caused by the increase by the second temporary increase width Bi of the flocculant addition rate x as much as possible. The decrease in the flocculant addition rate x due to the end of the second addition rate temporary increase process (step #1-09, step #3 (see Figure 2)) is immediately executed when it is determined that the liquid level li in the inlet hopper has reached the return determination value Li4 (yes in step #1-08). Incidentally, since the decrease in the flocculant addition rate x due to the end of the second addition rate temporary increase process (step #1-09, step #3 (see Figure 2)) is less urgent than during the increase, it can also be executed when it is determined that the liquid level li in the inlet hopper has remained below the return determination value Li4 for a set time (e.g., 5 to 30 min). The set time is set in consideration of the hydraulic retention time (HRT) of the sludge flocculation tank 5 and the time until the flocculated sludge S2 is supplied to the solid-liquid separation unit 10, and can be set, for example, to the sum of the hydraulic retention time (HRT) of the sludge flocculation tank 5 and the time until the flocculated sludge S2 is supplied to the solid-liquid separation unit 10.

[0032] In the present embodiment, in order to gradually execute a temporary increase in the flocculant addition rate x, as the addition rate temporary increase process, two types of the above-described first addition rate temporary increase process and the above-described second addition rate temporary increase process with different temporary increase determination values Li1 and Li2 and return determination values Li3 and Li4 are executed. And, the temporary increase determination value Li1, which is the determination criterion for the liquid level li in the inlet-side hopper when starting the temporary increase in the flocculant addition rate x in the above-described first addition rate temporary increase process, is higher than the temporary increase determination value Li2, which is the determination criterion for the liquid level li in the inlet-side hopper when starting the temporary increase in the flocculant addition rate x in the above-described second addition rate temporary increase process. That is, when the liquid level li in the inlet-side hopper rises and exceeds the temporary increase determination value Li2, a temporary increase by one second temporary increase width Bi of the flocculant addition rate x by the above-described second addition rate temporary increase process is started. Further, when the liquid level li in the inlet-side hopper continues to rise and exceeds the temporary increase determination value Li1 even after the flocculant addition rate x is increased by the second temporary increase width Bi, a temporary increase by one first temporary increase width Ai of the flocculant addition rate x by the above-described first addition rate temporary increase process is also started. From this, in order to surely lower the liquid level li in the inlet-side hopper when the liquid level li in the inlet-side hopper exceeds the temporary increase determination value Li1, the first temporary increase width Ai (for example, 0.05 to 0.50 points) in the above-described first addition rate temporary increase process is preferably set larger than the second temporary increase width Bi (for example, 0.10 points) in the above-described second addition rate temporary increase process, and is preferably set to about twice the second temporary increase width Bi. Note that only one of the above-described first addition rate temporary increase process and the above-described second addition rate temporary increase process may be configured to be executed.

[0033] (Addition Rate Correction Process) In the addition rate correction process for the first flocculant addition rate control, when the liquid level li in the inlet hopper exceeds a predetermined increase correction determination value Li1, the flocculant addition rate x is increased by a predetermined increase correction width Ci, and when the liquid level li in the inlet hopper falls below a predetermined decrease correction determination value Li5 that is lower than the increase correction determination value Li1, the flocculant addition rate x is decreased by a predetermined decrease correction width Di. This addition rate correction process is composed of step #1-03, step #1-05, step #1-10, step #1-11, step #1-12 shown in FIG. 3 and step #3 shown in FIG. 2.

[0034] That is, in this addition rate correction process, mainly as shown in FIG. 3, when it is determined (yes in step #1-03) that the liquid level li in the inlet hopper exceeds the predetermined increase correction determination value Li1, it is judged that the flocculant F is insufficient in the flocculated sludge S2, and the basic value x0 of the flocculant addition rate is increased by the increase correction width Ci (for example, 0.05 points) (step #1-05). Then, in step #3 (see FIG. 2), the flocculant addition rate x is increased by the same increase correction width Ci as the increase in the basic value x0 of the flocculant addition rate. Incidentally, since the increase in the flocculant addition rate x by this temporary addition rate increase process (step #1-05, step #3 (see FIG. 2)) is highly urgent from the viewpoint of avoiding the overflow of the flocculated sludge S2 from the inlet hopper 20, it is immediately executed when it is determined (yes in step #1-03) that the liquid level li in the inlet hopper exceeds the increase correction determination value Li1.

[0035] Furthermore, when it is determined that the liquid level li in the inlet hopper has fallen below the predetermined determination value Li5 for reduction correction (yes in step #1-10), it is judged that the flocculant F has been excessively added to the flocculated sludge S2, and the basic value x0 of the flocculant addition rate is decreased by the reduction correction width Di (for example, 0.05 points) (step #1-12). Then, in step #3 (see FIG. 2), the flocculant addition rate x is decreased by the same reduction correction width Di as the reduction amount of the basic value x0 of the flocculant addition rate. Also, the decrease in the flocculant addition rate x due to the addition rate correction process (step #1-12, step #3 (see FIG. 2)) may be executed immediately. However, in this embodiment, since the urgency is lower than that during the increase, it is executed when it is determined that the liquid level li in the inlet hopper has continued (yes in step #1-11) for the set time T1 (for example, 5 to 30 minutes) and has fallen below the determination value Li5 for reduction correction (yes in step #1-10). Note that the above set time is set in consideration of the hydraulic retention time (HRT) of the sludge flocculation tank 5 and the time until the flocculated sludge S2 is supplied to the solid-liquid separation unit 10. For example, it can be set to the sum of the hydraulic retention time (HRT) of the sludge flocculation tank 5 and the time until the flocculated sludge S2 is supplied to the solid-liquid separation unit 10. Also, when it is determined that the liquid level li in the inlet hopper has fallen below the determination value Li5 for reduction correction (yes in step #1-10), the decrease in the flocculant addition rate x due to the addition rate correction process (step #1-12, step #3 (see FIG. 2)) may be immediately performed.

[0036] By executing the addition rate correction process as described above, the flocculant addition rate x is timely corrected to an appropriate value to be maintained within the range between the increase correction determination value Li1 on the highest side and the decrease correction determination value Li5 on the lowest side of the liquid level li in the inlet hopper. Further, in the present embodiment, the increase correction determination value Li1, which is the criterion for determining the increase in the flocculant addition rate x by the above addition rate correction process, is set to the same value as the temporary increase determination value Li1, which is the criterion for starting the execution of the above-described first addition rate temporary increase process. However, it may be set to another value. Further, it is desirable to set the increase correction determination value, which is the criterion for determining the increase in the flocculant addition rate x by the above addition rate correction process, to a value larger than the temporary increase determination value, which is the criterion for starting the execution of the above-described addition rate temporary increase process. By setting it in this way, when the temporary increase in the flocculant addition rate x due to the start of the execution of the above addition rate temporary increase process alone cannot eliminate the shortage of the flocculant F and cannot contribute to avoiding the increase in the liquid level li in the inlet side hopper, the flocculant addition rate x is increased and corrected by the increase correction width Ci by the above addition rate correction process, and an excessive increase in the liquid level li in the inlet side hopper is surely avoided.

[0037] Also, the decrease correction determination value Li5, which is the criterion for determining the decrease in the flocculant addition rate x by the above addition rate correction process, is set to be less than the return determination values Li3 and Li4, which are the criteria for ending the execution of the above-described addition rate temporary increase process. Thus, when the temporary decrease in the flocculant addition rate x due to the end of the execution of the above addition rate temporary increase process alone cannot eliminate the excessive addition of the flocculant F and cannot contribute to avoiding the decrease in the liquid level li in the inlet side hopper, the flocculant addition rate x is decreased and corrected by the decrease correction width Di by the above addition rate correction process, and an excessive decrease in the liquid level li in the inlet side hopper is surely avoided.

[0038] Note that the increase correction width Ci (for example, 0.05 points) and the decrease correction width Di (for example, 0.05 points) in the above addition correction process are different from the purpose of the temporary increase and decrease by the above addition rate temporary increase process. Since they increase and decrease the flocculant addition rate basic value x0 itself, which is the basis of the flocculant addition rate x, it is preferable to set them to be smaller than the first temporary increase width Ai (for example, 0.05 to 0.50 points) and the second temporary increase width Bi (for example, 0.10 points) in the above addition rate temporary increase process.

[0039] [Second Flocculant Addition Rate Control] As the flocculant addition rate control (step #2 in FIG. 2), the control device 50 executes a second flocculant addition rate control for controlling the flocculant addition rate x based on the liquid level lo in the outlet-side hopper measured by the level sensor 24 (see FIG. 1) as shown in FIG. 4. That is, referring also to FIG. 1, if the flocculant addition rate x is too low, the floc properties in the flocculated sludge S2 deteriorate, the ease of water separation in the solid-liquid separation section 10 deteriorates, and the solid content concentration of the concentrated sludge S3 decreases, resulting in an increase in the liquid level lo in the outlet-side hopper. Therefore, by executing the second flocculant addition rate control and controlling the flocculant addition rate x based on the liquid level lo in the outlet-side hopper, the liquid level lo in the outlet-side hopper is maintained within an appropriate range as much as possible. Thus, while suppressing the deterioration of LCC due to excessive addition of the flocculant F and the overload in the solid-liquid separation section 10, the floc properties in the flocculated sludge S2 are improved by adding an appropriate amount of the flocculant F without shortage, the ease of water separation in the solid-liquid separation section 10 becomes good, the decrease in the solid content concentration of the concentrated sludge S3 is suppressed, and the flocculated sludge S2 is solid-liquid separated well and stably in the solid-liquid separation section 10.

[0040] The second flocculant addition rate control shown in FIG. 4 controls the flocculant addition rate x by comparing the liquid level lo itself in the outlet-side hopper with various determination values Lo1, Lo2, Lo3, Lo4, Lo5. This second flocculant addition rate control includes a first addition rate temporary increase process for determining the first addition rate increase width variable Δx1o, a second addition rate temporary increase process for determining the second addition rate increase width variable Δx2o, and an addition rate correction process for determining the flocculant addition rate basic value x0. The details of each process will be described below. In each process, five determination values Lo1, Lo2, Lo3, Lo4, Lo5 are set in order from the higher side as determination values for the liquid level lo in the outlet-side hopper. Also, the constants Ao, Bo, Co, Do regarding the increase and decrease widths set for the variables x0, Δx1o, Δx2o constituting the flocculant addition rate x are all positive numbers greater than 0. Also, in the following description, the names of the determination values Lo1, Lo2, Lo3, Lo4, Lo5 are made to match the process, but those with the same sign are determination values set to the same value.

[0041] (First dosing rate temporary increase process) In the first dosing rate temporary increase process in the second flocculant dosing rate control, the flocculant dosing rate x is temporarily increased from when the liquid level lo in the outlet hopper exceeds a predetermined determination value Lo1 for temporary increase until the liquid level lo in the outlet hopper becomes a predetermined return determination value Lo3 lower than the determination value Lo1 for temporary increase. This first dosing rate temporary increase process is composed of step #2-03, step #2-04, step #2-06, step #2-07 shown in FIG. 4, and step #3 shown in FIG. 2.

[0042] That is, in this first dosing rate temporary increase process, mainly as shown in FIG. 4, when it is determined that the liquid level lo in the outlet hopper exceeds the determination value Lo1 for temporary increase (yes in step #2-03), it is judged that the flocculant F is insufficient in the flocculated sludge S2, and the first dosing rate increase width variable Δx1o is changed from 0 to the first temporary increase width Ao (for example, 0.05 to 0.50 points) (step #2-04). Then, in step #3 (see FIG. 2), the flocculant dosing rate x is temporarily increased by the first temporary increase width Ao. By this, the deterioration of the floc properties such as the decrease in the strength and the subdivision of the flocs due to the shortage of the flocculant F in the flocculated sludge S2 is suppressed, and the excessive rise of the liquid level lo in the outlet hopper due to the deterioration of the floc properties is avoided. In addition, since the increase in the flocculant dosing rate x by the above first dosing rate temporary increase process (step #2-04, step #3 (see FIG. 2)) is highly urgent from the viewpoint of avoiding the overflow of the flocculated sludge S2 from the outlet hopper 23, it is immediately executed when it is determined that the liquid level lo in the outlet hopper exceeds the determination value Lo1 for temporary increase (yes in step #2-03).

[0043] Furthermore, as the liquid level lo in the outlet-side hopper decreases by the amount Ao of the first temporary increase in the flocculant addition rate x and it is determined that the liquid level lo in the outlet-side hopper has reached the return determination value Lo3 (yes in step #2-06), the first addition rate increase width variable Δx1o is reset to 0 (step #2-07). Then, in step #3 (see Figure 2), the temporary increase in the flocculant addition rate x by the amount Ao is terminated, and the flocculant addition rate x is returned to its value before the increase. This suppresses, as much as possible, the deterioration of LCC caused by the increase in the first temporary increase width Ao of the flocculant addition rate x. The decrease in the flocculant addition rate x due to the end of the first addition rate temporary increase process (steps #2-07, step #3 (see Figure 2)) is immediately executed when it is determined that the liquid level lo in the outlet-side hopper has reached the return determination value Lo3 (yes in step #2-06). Incidentally, since the decrease in the flocculant addition rate x due to the end of the first addition rate temporary increase process (steps #2-07, step #3 (see Figure 2)) is less urgent than during the increase, it can also be executed when it is determined that the liquid level lo in the outlet-side hopper has remained below the return determination value Lo3 for a set time (e.g., 5 to 30 min). The set time is set in consideration of the hydraulic retention time (HRT) of the sludge flocculation tank 5 and the time until the flocculated sludge S2 is supplied to the solid-liquid separation unit 10, and can be set, for example, to the sum of the hydraulic retention time (HRT) of the sludge flocculation tank 5 and the time until the flocculated sludge S2 is supplied to the solid-liquid separation unit 10.

[0044] (Second addition rate temporary increase process) In the second addition rate temporary increase process in the second flocculant addition rate control, similar to the first addition rate temporary increase process described above, the flocculant addition rate x is temporarily increased from when the liquid level lo in the outlet-side hopper exceeds the predetermined temporary increase determination value Lo2 until the liquid level lo in the outlet-side hopper reaches a predetermined return determination value Lo4 lower than the temporary increase determination value Lo2. This second addition rate temporary increase process is composed of steps #2-01, #2-02, #2-08, #2-09 shown in Figure 4 and step #3 shown in Figure 2.

[0045] That is, in this second addition rate temporary increase process, mainly as shown in FIG. 4, when it is determined that the liquid level lo in the outlet side hopper exceeds the temporary increase determination value Lo2 (yes in step #2-01), it is determined that the flocculant F is insufficient in the flocculated sludge S2, and the second addition rate increase width variable Δx2o is changed from 0 to the second temporary increase width Bo (for example, 0.10 points) (step #2-02). Then, in step #3 (see FIG. 2), the flocculant addition rate x is temporarily increased by the second temporary increase width Bo. By this, the deterioration of the floc properties such as the decrease in the strength and the subdivision of the floc due to the shortage of the flocculant F in the flocculated sludge S2 is suppressed, and the excessive increase in the liquid level lo in the outlet side hopper due to the deterioration of the floc properties is avoided. In addition, since the increase in the flocculant addition rate x by the above second addition rate temporary increase process (step #2-02, step #3 (see FIG. 2)) is highly urgent from the viewpoint of avoiding the overflow of the flocculated sludge S2 from the outlet side hopper 23, it is immediately executed when it is determined that the liquid level lo in the outlet side hopper exceeds the temporary increase determination value Lo2 (yes in step #2-03).

[0046] Furthermore, as the liquid level lo in the outlet-side hopper decreases by the amount Bo of the second temporary increase in the flocculant addition rate x and it is determined that the liquid level lo in the outlet-side hopper has reached the return determination value Lo4 (yes in step #2-08), the second addition rate increase width variable Δx2o is reset to 0 (step #2-09). Then, in step #3 (see FIG. 2), the temporary increase in the flocculant addition rate x by the amount Bo of the second temporary increase ends, and the flocculant addition rate x is returned to its value before the increase. This suppresses as much as possible the deterioration of LCC caused by the increase in the amount Bo of the second temporary increase in the flocculant addition rate x. The decrease in the flocculant addition rate x due to the end of the second addition rate temporary increase process (steps #2-09, step #3 (see FIG. 2)) is immediately executed when it is determined that the liquid level lo in the outlet-side hopper has reached the return determination value Lo4 (yes in step #2-08). Incidentally, since the decrease in the flocculant addition rate x due to the end of the second addition rate temporary increase process (steps #2-09, step #3 (see FIG. 2)) is less urgent than during the increase, it can also be executed when it is determined that the liquid level lo in the outlet-side hopper has remained below the return determination value Lo4 for a set time (for example, 5 to 30 minutes). The set time is set in consideration of the hydraulic retention time (HRT) of the sludge flocculation tank 5 and the time until the flocculated sludge S2 is supplied to the solid-liquid separation unit 10, and can be set, for example, to the sum of the hydraulic retention time (HRT) of the sludge flocculation tank 5 and the time until the flocculated sludge S2 is supplied to the solid-liquid separation unit 10.

[0047] In the present embodiment, in order to gradually execute a temporary increase in the flocculant addition rate x, as the addition rate temporary increase process, two types of the above-described first addition rate temporary increase process and the above-described second addition rate temporary increase process with different temporary increase determination values Lo1 and Lo2 and return determination values Lo3 and Lo4 are executed. And, the temporary increase determination value Lo1, which is the determination criterion for the liquid level lo in the outlet side hopper when starting the temporary increase in the flocculant addition rate x in the first addition rate temporary increase process, is higher than the temporary increase determination value Lo2, which is the determination criterion for the liquid level lo in the outlet side hopper when starting the temporary increase in the flocculant addition rate x in the second addition rate temporary increase process. That is, when the liquid level lo in the outlet side hopper rises and exceeds the temporary increase determination value Lo2, a temporary increase by the second temporary increase width Bo of the flocculant addition rate x by the second addition rate temporary increase process is started. Further, when the liquid level lo in the outlet side hopper continues to rise and exceeds the temporary increase determination value Lo1 even after increasing the flocculant addition rate x by the second temporary increase width Bo, a temporary increase by the first temporary increase width Ao of the flocculant addition rate x by the first addition rate temporary increase process is also started. From this, in order to surely lower the liquid level lo in the outlet side hopper when the liquid level lo in the outlet side hopper exceeds the temporary increase determination value Lo1, the first temporary increase width Ao (for example, 0.05 to 0.50 points) in the first addition rate temporary increase process is preferably set larger than the second temporary increase width Bo (for example, 0.10 points) in the second addition rate temporary increase process, and is preferably set to about twice the second temporary increase width Bo, for example. Note that only one of the first addition rate temporary increase process and the second addition rate temporary increase process may be configured to be executed.

[0048] (Addition rate correction process) In the addition rate correction process for the second flocculant addition rate control, when the liquid level lo in the outlet side hopper exceeds a predetermined increase correction determination value Lo1, the flocculant addition rate x is increased by a predetermined increase correction width Co, and when the liquid level lo in the outlet side hopper falls below a predetermined decrease correction determination value Lo5 that is lower than the increase correction determination value Lo1, the flocculant addition rate x is decreased by a predetermined decrease correction width Do. This addition rate correction process is composed of step #2-03, step #2-05, step #2-10, step #2-11, step #2-12 shown in FIG. 4, and step #3 shown in FIG. 2.

[0049] That is, in this addition rate correction process, mainly as shown in FIG. 4, when it is determined (yes in step #2-03) that the liquid level lo in the outlet side hopper exceeds the predetermined increase correction determination value Lo1, it is determined that the flocculant F is insufficient in the flocculated sludge S2, and the basic value x0 of the flocculant addition rate is increased by the increase correction width Co (for example, 0.10 point) (step #2-05). Then, in step #3 (see FIG. 2), the flocculant addition rate x is increased by the same increase correction width Co as the increase in the basic value x0 of the flocculant addition rate. In addition, since the increase in the flocculant addition rate x due to this temporary addition rate increase process (step #2-05, step #3 (see FIG. 2)) is highly urgent from the viewpoint of avoiding the overflow of the flocculated sludge S2 from the outlet side hopper 23, it is immediately executed when it is determined (yes in step #2-03) that the liquid level lo in the outlet side hopper exceeds the increase correction determination value Lo1.

[0050] Furthermore, when it is determined that the liquid level lo in the outlet-side hopper has fallen below a predetermined determination value Lo5 for reduction correction (yes in step #2-10), it is judged that the flocculant F has been excessively added to the flocculated sludge S2, and the basic value x0 of the flocculant addition rate is decreased by a reduction correction width Do (for example, 0.10 point) (step #2-12). Then, in step #3 (see Figure 2), the flocculant addition rate x will be decreased by the same reduction correction width Do as the reduction amount of the basic value x0 of the flocculant addition rate. Also, the decrease in the flocculant addition rate x due to the addition rate correction process (step #2-12, step #3 (see Figure 2)) may be executed immediately, but in this embodiment, since the urgency is lower than when increasing, it is executed when it is determined that the liquid level lo in the outlet-side hopper has continued (yes in step #2-11) for a set time T2 (for example, 5 to 30 minutes) and has fallen below the determination value Lo5 for reduction correction (yes in step #2-10). Note that when it is determined that the liquid level lo in the outlet-side hopper has fallen below the determination value Lo5 for reduction correction (yes in step #2-10), the decrease in the flocculant addition rate x due to the addition rate correction process (step #2-12, step #3 (see Figure 2)) may be performed immediately.

[0051] Note that in this embodiment, the addition rate correction process in the second flocculant addition rate control has a role of emergency response when the correction of the flocculant addition rate x is insufficient only by the addition rate correction process in the first flocculant addition rate control described above. Therefore, the reduction process of the flocculant addition rate x (steps #2-10 to #2-12) with relatively low urgency in the addition rate correction process in the second flocculant addition rate control can also be omitted. Also, when the correction of the flocculant addition rate x is sufficient only by the addition rate correction process in the first flocculant addition rate control described above, or when the solid-liquid separation unit 10 is a dehydration device with few phenomena related to poor concentration that cause a sharp increase in the liquid level lo in the outlet-side hopper, etc., the addition rate correction process itself in the second flocculant addition rate control may be omitted.

[0052] By performing the addition rate correction process as described above, the flocculant addition rate x is timely corrected to an appropriate value to be maintained within the range between the increase correction determination value Lo1 on the side where the liquid level lo in the outlet hopper is the highest and the decrease correction determination value Lo5 on the lowest side. Furthermore, the increase correction determination value Lo1, which serves as the criterion for determining an increase in the flocculant addition rate x by the above addition rate correction process, is set to be equal to or higher than the temporary increase determination values Lo1 and Lo2, which serve as the criterion for starting the execution of the above-described addition rate temporary increase process. Thus, in the case where the temporary increase in the flocculant addition rate x due to the start of the execution of the above addition rate temporary increase process alone cannot eliminate the shortage of the flocculant F and cannot contribute to avoiding an increase in the liquid level lo in the outlet hopper, the flocculant addition rate x is increased and corrected by the increase correction width Co by the above addition rate correction process, and an excessive increase in the liquid level lo in the outlet hopper is surely avoided.

[0053] Also, the decrease correction determination value Lo5, which serves as the criterion for determining a decrease in the flocculant addition rate x by the above addition rate correction process, is set to be less than the return determination values Lo3 and Lo4, which serve as the criterion for ending the execution of the above-described addition rate temporary increase process. Thus, in the case where the temporary decrease in the flocculant addition rate x due to the end of the execution of the above addition rate temporary increase process alone cannot eliminate the excessive addition of the flocculant F and cannot contribute to avoiding a decrease in the liquid level lo in the outlet hopper, the flocculant addition rate x is decreased and corrected by the decrease correction width Do by the above addition rate correction process, and an excessive decrease in the liquid level lo in the outlet hopper is surely avoided.

[0054] Furthermore, the increase correction width Co (e.g., 0.10 point) and the decrease correction width Do (e.g., 0.10 point) in the above addition correction process are different from the temporary increase and decrease due to the above addition rate temporary increase process. Since they increase and decrease the coagulant addition rate basic value x0 itself, which is the basis of the coagulant addition rate x, it is preferable to set them smaller than the first temporary increase width Ao (e.g., 0.05 to 0.50 point) and the second temporary increase width Bo (e.g., 0.10 point) in the above addition rate temporary increase process. Also, the addition rate correction process in the second coagulant addition rate control has a role of emergency response when the correction of the coagulant addition rate x is insufficient only by the addition rate correction process in the above-described first coagulant addition rate control. Therefore, the increase correction width Co (e.g., 0.10 point) and the decrease correction width Do (e.g., 0.10 point) in the addition correction process in the second coagulant addition rate control are preferably slightly larger than the increase correction width Ci (e.g., 0.05 point) and the decrease correction width Di (e.g., 0.05 point) in the addition rate correction process in the above-described first coagulant addition rate control.

[0055] In the processing system 100 of the present embodiment, when controlling the coagulant addition rate x by the control device 50, the first coagulant addition rate control is preferentially executed based on the liquid level li in the inlet side hopper. As a follow-up when the coagulant addition rate x cannot be fully optimized by the first coagulant addition rate control, the second coagulant addition rate control is executed based on the liquid level lo in the outlet side hopper. In particular, when the concentrated sludge S3 in the outlet side hopper 23 is conveyed to the subsequent processing unit 30 by the conveying pump, the conveying pump is controlled so that the liquid level lo in the outlet side hopper is maintained within an appropriate range. Therefore, the coagulant addition rate x is preferentially controlled by the first coagulant addition rate control based on the liquid level li in the inlet side hopper, and the coagulant addition rate x is controlled by the second coagulant addition rate control based on the liquid level lo in the outlet side hopper only when the liquid level lo in the outlet side hopper cannot be maintained within an appropriate range by the control of the conveying pump. In the present embodiment, both the first coagulant addition rate control and the second coagulant addition rate control are configured to be executed, but it may be configured to execute only one of them.

[0056] 〔Alternative Control of the Second Flocculant Addition Rate〕 In the above-described control of the second flocculant addition rate (refer to FIG. 4), the liquid level lo in the outlet-side hopper itself is compared with the determination values Lo1, Lo2, Lo3, Lo4, Lo5 to control the flocculant addition rate x. However, instead of the liquid level lo in the outlet-side hopper itself, the flocculant addition rate x can be controlled using the state such as the change rate of the liquid level lo in the outlet-side hopper. For example, as shown in FIG. 5, the increase rate Δlo of the liquid level lo in the outlet-side hopper can be compared with the determination value ΔLo1 to control the flocculant addition rate x. In the alternative control of the second flocculant addition rate shown in FIG. 5, when it is determined (step #2-21) that the increase rate Δlo of the liquid level lo in the outlet-side hopper exceeds the predetermined increase correction determination value ΔLo1 in the case where poor concentration occurs in the solid-liquid separation unit 10, etc., the flocculant addition rate basic value x0 is increased by the increase correction width Eo (for example, 0.20 to 0.30 points) (step #2-22). Then, in step #3 (refer to FIG. 2), the flocculant addition rate x is increased by the same increase correction width Eo as the increase amount of the flocculant addition rate basic value x0.

[0057] In the processing system 100 of the present embodiment, for example, when the concentration device where the liquid level Lo in the outlet-side hopper may rapidly rise due to poor concentration is used as the solid-liquid separation unit 10, in addition to the second flocculant addition rate control shown in FIG. 4, the alternative second flocculant addition rate control shown in FIG. 5 is executed. However, it can also be configured to execute any one of these second flocculant addition rate controls. Further, when executing the second flocculant addition rate control shown in FIG. 4, instead of the addition rate correction processing (step #2-03, step #2-05, step #2-10, step #2-11, step #2-12) in the second flocculant addition rate control, the alternative second flocculant addition rate control shown in FIG. 5 can be incorporated.

Explanation of Reference Numerals

[0058] 1 Processing unit 2 Flocculant supply pump (flocculant addition unit) 5 Sludge flocculation tank 10 Solid-liquid separation section 20 Inlet side hopper (inlet side storage section) 21 Level sensor (liquid level measurement section in the inlet side storage section) 23 Outlet side hopper (outlet side storage section) 24 Level sensor (liquid level measurement section in the outlet side storage section) 30 Subsequent processing section 50 Control device (control section) 100 Processing system S1 Organic waste (object to be processed) S2 Coagulated sludge (coagulated matter) S3 Concentrated sludge (concentrated matter) F Coagulant li Liquid level in the inlet side hopper lo Liquid level in the outlet side hopper x Coagulant addition rate Ci Increase correction width Co Increase correction width Di Decrease correction width Do Decrease correction width Eo Increase correction width Li1 Increase correction determination value, temporary increase determination value Li2 Temporary increase determination value Li3 Return determination value Li4 Return determination value Li5 Decrease correction determination value Lo1 Increase correction determination value, temporary increase determination value Lo2 Temporary increase determination value Lo3 Return determination value Lo4 Return determination value Lo5 Decrease correction determination value ΔLo1 Increase correction determination value

Claims

1. A treatment system comprising a flocculant addition unit that adds a flocculant to a material to be treated to form flocs, and a solid-liquid separation unit that separates the flocs into a concentrate by solid-liquid separation, a storage unit installed on the inlet side or the outlet side of the solid-liquid separation unit to temporarily store the flocs or the concentrate, a storage unit internal liquid level measurement unit that measures the liquid level of the flocs or the concentrate in the storage unit, which is the liquid level in the storage unit, and a control unit that executes flocculant addition rate control to control the flocculant addition rate to the material to be treated by the flocculant addition unit based on the liquid level in the storage unit.

2. The treatment system according to claim 1, wherein in the flocculant addition rate control, the control unit temporarily increases the flocculant addition rate during a period from when the liquid level in the storage unit exceeds a predetermined temporary increase determination value until the liquid level in the storage unit becomes a predetermined return determination value lower than the temporary increase determination value, and executes an addition rate temporary increase process.

3. The treatment system according to claim 2, wherein in the flocculant addition rate control, when the liquid level in the storage unit exceeds a predetermined increase correction determination value that is equal to or higher than the temporary increase determination value, the control unit increases the flocculant addition rate by a predetermined increase correction width, and when the liquid level in the storage unit falls below a predetermined decrease correction determination value that is less than the return determination value, the control unit decreases the flocculant addition rate by a predetermined decrease correction width, and executes an addition rate correction process.

4. The treatment system according to claim 1 or 2, comprising an inlet side storage unit installed on the inlet side of the solid-liquid separation unit to temporarily store the flocs immediately before being introduced into the solid-liquid separation unit as the storage unit, an inlet side storage unit internal liquid level measurement unit that measures the liquid level in the inlet side storage unit, which is the liquid level of the flocs in the inlet side storage unit, as the storage unit internal liquid level measurement unit, and the control unit executes first flocculant addition rate control to control the flocculant addition rate based on the liquid level in the inlet side storage unit as the flocculant addition rate control.

5. The treatment system according to claim 1 or 2, comprising an outlet side storage unit installed on the outlet side of the solid-liquid separation unit to temporarily store the concentrate immediately before being discharged from the solid-liquid separation unit and introduced into a subsequent treatment unit as the storage unit, an outlet side storage unit internal liquid level measurement unit that measures the liquid level in the outlet side storage unit, which is the liquid level of the concentrate in the outlet side storage unit, as the storage unit internal liquid level measurement unit, The processing system according to claim 1 or 2, wherein the control unit executes second flocculant addition rate control for controlling the flocculant addition rate based on the liquid level in the outlet-side storage unit as the flocculant addition rate control.

6. A control method for a processing system including a flocculant addition unit that adds a flocculant to a material to be processed to form flocs, and a solid-liquid separation unit that separates the flocs into a concentrate by solid-liquid separation, measuring a liquid level in a storage unit that is installed on the inlet side or the outlet side of the solid-liquid separation unit and temporarily stores the flocs or the concentrate, and being a liquid level of the flocs or the concentrate in the storage unit, and executing flocculant addition rate control for controlling a flocculant addition rate to the material to be processed by the flocculant addition unit based on the liquid level in the storage unit.

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