Method for inhibiting scale formation in sludge dehydration equipment and sludge dehydration equipment

Abstract

To provide a method and a device for suppressing scale generation that can easily suppress a precipitation of MAP, calcium carbonate, and calcium sulfate without using a special device and without interrupting dehydration operation in an existing water treatment facility.SOLUTION: There is provided a method for suppressing scale generation in a sludge dewatering treatment device having a thickener and a dehydrator, wherein a cycle of alternately shifting dehydrated liquid to acidity and alkalinity is repeated one or more times by switching between (1) operation using a polymer flocculant alone and (2) operation using a combination of a polymer flocculant and an inorganic flocculant,.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for inhibiting scale formation in a dehydrator used in a water treatment facility, and more particularly to a method for inhibiting scale formation in a dehydrator that coagulates, thickens, and dehydrates sludge, and a dehydration treatment device capable of implementing the method.

[0002] Sewage treatment plants and sewage treatment facilities typically use two methods: a dehydration process in which a flocculant is added to flocculate sludge and then dehydrate it, or a concentration and dehydration process in which a flocculant is added to flocculate the sludge, then the sludge is concentrated and dehydrated. Polymer flocculants are typically used to flocculate the sludge. However, to improve dehydration efficiency, other methods are also used: a "pre-addition of inorganic flocculant" method in which an inorganic flocculant is added to the sludge and then a polymer flocculant is added; and a "post-addition of inorganic flocculant" method in which a polymer flocculant is added to the sludge to flocculate it, then the sludge is concentrated, and then an inorganic flocculant is added to the concentrated sludge. The post-addition of inorganic flocculant method is suitable for dehydration of organic sludge with a high M-alkali component concentration.

[0003] When polymer flocculants are added but inorganic flocculants are not, the pH of the sludge rises due to the physical decarbonation effect caused by stirring during sludge flocculation treatment, thinning of the sludge due to concentration, and internal pressure and shear force during dewatering, causing magnesium ammonium phosphate (MAP) and calcium carbonate to precipitate, resulting in the formation of scale.

[0004] When polymer coagulants and inorganic coagulants are added, sulfate ions from the inorganic coagulant combine with metal ions in the sludge to form sparingly soluble sulfates (such as CaSO4), causing scale to form.

[0005] Dehydration equipment is equipped with metallic filter media such as punched metal and piping for discharging the dehydrated separated liquid, and if scale builds up on these, it will reduce dehydration capacity and cause piping blockages. In order to remove scale that has formed or built up inside the dehydration equipment, it is necessary to stop the dehydration operation and add a large amount of chemicals to dissolve the scale, which reduces the efficiency of the dehydration process and increases the cost of chemicals.

[0006] MAP scale is likely to occur when the pH rises, so one method of adjusting the pH is to add acid and / or alkali chemicals, but this requires a stock of pH-adjusting chemicals, an addition device, and a pH measurement means.

[0007] As a method for preventing MAP scale from adhering to dehydrators and the inside of pipes, a sewage sludge treatment facility has been proposed that is equipped with a digestion tank for digesting sludge generated at a sewage treatment plant, and that the facility is equipped with an electromagnetic treatment means in the pipe downstream of the digestion tank through which at least one of the digested sludge, supernatant, and dehydrated filtrate flows, for negatively charging the magnesium ammonium phosphate in the fluid flowing through the pipe (Patent Publication No. 2019-13865).

[0008] In the inorganic coagulant post-addition method, an inorganic coagulant is added to concentrated coagulated flocs, but because the permeability and reactivity of the inorganic coagulant depend greatly on the size of the coagulated flocs, a method has been proposed in which the inorganic coagulant is added so that the pH of the coagulated flocs to which the inorganic coagulant has been added or the pH of the dewatered separated liquid is between 3 and 6 (JP 2012-45441 A). This publication describes that by controlling the pH of the coagulated flocs or the pH of the dewatered separated liquid to between 3 and 6, it is possible to suppress the generation of magnesium ammonium phosphate (MAP), thereby suppressing the buildup of scale on the filtering surfaces of the dehydrator and the piping through which the dewatered separated liquid passes.

[0009] In sewage treatment plants, when digested sludge is dehydrated using a polymer coagulant to form flocs, polyferric sulfate is added to the dehydrator, and the sludge is dehydrated using a screw press dehydrator, a large amount of scale precipitated from unreacted polyferric sulfate adheres to the screen inside the screw press dehydrator. To address this issue without controlling the pH of the flocculated flocs or the dehydrated filtrate, a method has been proposed in which a polymer coagulant is added to organic sludge, the resulting flocculated sludge is concentrated by adding an inorganic coagulant while being transported on a water-permeable endless belt of a belt-type concentrator, and then a polymer coagulant is further added to the concentrated flocculated sludge, and the resulting flocculated sludge is dehydrated using a dehydrator (Japanese Patent Laid-Open Publication No. 2015-174000). This publication describes that by reducing the amount of unreacted polyferric sulfate, the generation of scale precipitated from unreacted polyferric sulfate can be controlled.

[0010] However, conventional methods proposed to prevent scale formation have not actually sufficiently suppressed scale formation. Conventional methods using inorganic and polymer flocculants involve the constant addition of both polymer and inorganic flocculants, without alternating between operation with only polymer flocculants and operation with inorganic flocculants. In wastewater treatment facilities, the type and location of flocculant addition are determined at the design stage and are not changed during steady-state operation. Furthermore, when inorganic flocculants are used in combination to obtain low-moisture dehydrated cake, the addition of inorganic flocculants is never stopped. Furthermore, it was not known that the pH and type of scale in concentrated and dehydrated separated liquids differ depending on whether or not inorganic flocculants are added, and the location of their addition, i.e., pre- or post-addition. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] JP 2019-13865 A [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-45441 [Patent Document 3] JP 2015-174000 A Summary of the Invention [Problem to be solved by the invention]

[0012] In treatment methods that add polymer flocculants, MAP and calcium carbonate precipitate, causing scale formation. In the inorganic flocculant pre-addition method and inorganic flocculant post-addition method, which use a polymer flocculant and an inorganic flocculant in combination, scale such as calcium sulfate forms. Conventional scale prevention techniques have problems such as the use of special equipment such as electromagnetic treatment devices, pH control of flocs or dewatered separated liquid, or the use of belt-type concentrators, which complicate the equipment configuration and operation management. The present invention aims to provide a method and apparatus for suppressing scale formation that can easily suppress the precipitation of MAP, calcium carbonate, and calcium sulfate in existing water treatment facilities without using special equipment and without interrupting dewatering operations. [Means for solving the problem]

[0013] Dewatering in a typical water treatment facility involves a flocculation process in which a polymer flocculant is added to sludge and a dewatering process in which the flocculated flocs are dewatered. A concentration process, in which the flocculated flocs are separated into thickened flocs and a thickened separated liquid, may be included between the flocculation and dewatering processes. An inorganic flocculant is added to the thickened flocs and the dewatering process may then be performed. Alternatively, a reaction process in which an inorganic flocculant is added to the sludge before the flocculation process may be included. Whether a polymer flocculant is used alone or in combination with an inorganic flocculant, the pH is maintained within the optimal range for flocculating the sludge. Therefore, the pH of the separated liquid from the dewatering process and the concentrated separated liquid from the concentration process is fixed on the acidic or alkaline side. MAP or calcium carbonate precipitates on the alkaline side, while calcium sulfate precipitates on the acidic side, resulting in the formation of at least one type of scale: MAP, calcium carbonate, and calcium sulfate. The present inventors have found that the precipitation of MAP, calcium carbonate, and calcium sulfate can be suppressed by repeatedly shifting the pH of the dehydrated separated liquid from the dehydration step alternately between the acidic side and the alkaline side by changing the timing of addition of the polymer flocculant and the inorganic flocculant, and have completed the present invention.

[0014] According to the present invention, there are provided a method for suppressing scale generation in a dehydration treatment device and a dehydration treatment device having the following aspects. [1] A method for suppressing scale formation in a sludge dehydration treatment device having a concentrator that concentrates sludge to which a flocculant has been added and separates it into concentrated sludge and concentrated separated liquid, and a dehydrator that dehydrates the concentrated sludge and separates it into dehydrated cake and dehydrated separated liquid, (1) Polymer flocculant-only operation, in which a polymer flocculant is added to sludge, the flocs formed are concentrated, and the concentrated sludge is dewatered; (2) A method for inhibiting scale formation, characterized by repeating a cycle of alternately shifting the dehydrated separated liquid to acidity and alkalinity at least once by switching between operations using a polymer flocculant and an inorganic flocculant in combination. [2] The operation using the polymer flocculant and the inorganic flocculant in combination (2) is as follows (A) to (C): (A) Pre-addition of inorganic flocculant to sludge, followed by addition of polymer flocculant, and dewatering the thickened sludge formed by thickening the flocs. (B) A post-addition operation of inorganic flocculant in which a polymer flocculant is added to sludge, and the resulting flocs are concentrated, followed by adding an inorganic flocculant to the concentrated sludge and then dewatering the sludge; and (C) Addition of inorganic coagulant before and after the sludge is treated with polymer coagulant. The resulting coagulated flocs are concentrated and then the inorganic coagulant is added to the concentrated sludge, after which the sludge is dewatered. The method according to [1] above, characterized in that it is at least one embodiment selected from the following. [3] The method according to [1] or [2] above, characterized in that the switching between (1) the operation using only a polymer flocculant and (2) the operation using a polymer flocculant and an inorganic flocculant in combination is carried out when (I) the pH of the concentrated separated liquid or the dehydrated separated liquid shifts to the acidic or alkaline side more than expected due to the added flocculant, or / and (II) the flow rate of the concentrated separated liquid or the dehydrated separated liquid decreases by 20% or more from the normal flow rate value. [4] The method according to any one of [1] to [3] above, wherein the inorganic flocculant is at least one inorganic flocculant selected from polyferric sulfate, ferric chloride, polyaluminum chloride, and aluminum sulfate. [5] A sludge dehydration treatment device, an inorganic flocculant pre-addition means for adding a predetermined amount of inorganic flocculant to the sludge at a predetermined timing; a polymer flocculant adding means for adding a predetermined amount of polymer flocculant to the sludge at a predetermined timing; a thickening tank for thickening the sludge to which the polymer flocculant has been added and separating it into thickened sludge and thickened separated liquid; an inorganic flocculant post-addition means for adding a predetermined amount of inorganic flocculant to the concentrated sludge at a predetermined timing; a dehydrator that dehydrates the concentrated sludge and separates it into a dehydrated cake and a dehydrated separated liquid; Equipped with (1) Polymer flocculant-only operation, in which a polymer flocculant is added to sludge, the flocs formed are concentrated, and the concentrated sludge is dewatered; (2) An apparatus characterized by having a control unit that controls the inorganic flocculant pre-addition means, the polymer flocculant addition means, and the inorganic flocculant post-addition means so as to switch between operations using a polymer flocculant and an inorganic flocculant in combination, and to repeat a cycle of shifting the dehydrated separated liquid alternately between acidity and alkalinity at least once. [6] The control unit includes a detection means for concentrated separated liquid, a detection means for dehydrated separated liquid, and an electromagnetic valve for switching the timing of adding an inorganic flocculant; The device described in [5] above is characterized in that the opening and closing of the solenoid valve is controlled according to the flow rate or pH of the concentrated separated liquid detected by the concentrated separated liquid detection means and the flow rate or pH of the dehydrated separated liquid detected by the dehydrated separated liquid detection means when (I) the pH of the concentrated separated liquid or dehydrated separated liquid shifts to the acidic or alkaline side more than expected due to the added flocculant, or / and (II) the flow rate of the concentrated separated liquid or dehydrated separated liquid decreases by 20% or more from the normal flow rate value. [7] (2) Operation using a polymer flocculant and an inorganic flocculant in combination: (2) (A) to (C) below: (A) Pre-addition of inorganic flocculant to sludge, followed by addition of polymer flocculant, and dewatering the thickened sludge formed by thickening the flocs. (B) A post-addition operation of inorganic flocculant in which a polymer flocculant is added to sludge, and the resulting flocs are concentrated, followed by adding an inorganic flocculant to the concentrated sludge and then dewatering the sludge; and (C) Addition of inorganic coagulant before and after the sludge is treated with polymer coagulant. The resulting coagulated flocs are concentrated and then the inorganic coagulant is added to the concentrated sludge, after which the sludge is dewatered. The device according to [5] or [6] above, characterized in that it is at least one embodiment selected from the following: [Effects of the Invention]

[0015] According to the present invention, the precipitation of MAP, calcium carbonate, and calcium sulfate can be suppressed without requiring any special equipment or interrupting dehydration operations, thereby extending the life of dehydration treatment equipment. Furthermore, the method for preventing scale in a dehydration treatment equipment of the present invention does not reduce the dehydration performance of existing water treatment facilities, and can maintain dehydration performance equivalent to or better than that of existing facilities. Furthermore, the amount of chemicals, such as scale inhibitors and detergents, that were required in large quantities in conventional methods can be reduced, thereby reducing maintenance costs. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic explanatory diagram of a dehydration treatment device of the present invention, showing control when "operation of adding inorganic flocculant before and after" is performed. [Figure 2] FIG. 1 is a schematic explanatory diagram showing control when "polymer flocculant sole operation" is performed. [Figure 3] FIG. 10 is a schematic explanatory diagram showing control when performing "pre-addition operation of inorganic flocculant." [Figure 4] FIG. 10 is a schematic explanatory diagram showing control when performing "inorganic flocculant post-addition operation." [Figure 5] FIG. 2 is an explanatory diagram of the flocculant addition cycle in the control example and the example. Preferred Embodiments

[0017] The present invention provides a method for suppressing scale formation in a sludge dehydration treatment device having a concentrator that concentrates sludge to which a flocculant has been added and separates it into concentrated sludge and concentrated separated liquid, and a dehydrator that dehydrates the concentrated sludge and separates it into dehydrated cake and dehydrated separated liquid, comprising: Polymer flocculant-only operation involves adding polymer flocculant to sludge, concentrating the flocs that form, and dewatering the concentrated sludge. (A) Pre-addition of inorganic flocculant to sludge, followed by addition of polymer flocculant, and dewatering the thickened sludge formed by thickening the flocs. (B) A post-addition operation of inorganic flocculant in which a polymer flocculant is added to sludge, and the resulting flocs are concentrated, followed by adding an inorganic flocculant to the concentrated sludge and then dewatering the sludge; and (C) Addition of inorganic coagulant before and after the sludge is treated with polymer coagulant. The resulting coagulated flocs are concentrated and then the inorganic coagulant is added to the concentrated sludge, after which the sludge is dewatered. and an inorganic flocculant addition operation of at least one mode selected from the above, thereby shifting the dehydrated separated liquid from the dehydrator alternately to acidity and alkalinity, which constitutes one cycle, and the cycle is repeated one or more times.

[0018] A typical example of a sludge dewatering apparatus to which the method of the present invention can be applied is shown in FIG. The sludge dehydration treatment apparatus shown in Figure 1 comprises a sludge storage tank 4 for storing sludge from a digester tank 2, a pre-inorganic flocculant addition means 10 for adding a predetermined amount of inorganic flocculant to the sludge at a predetermined time, a mixing tank 6 for thoroughly mixing the inorganic flocculant and sludge, a polymer flocculant addition means 20 for adding a predetermined amount of polymer flocculant to the sludge from the mixing tank 6 at a predetermined time, a coagulation tank 8 for coagulating the sludge to which the polymer flocculant has been added, a thickening tank 30 for thickening the sludge containing flocs and separating it into thickened sludge and a thickened separated liquid, a post-inorganic flocculant addition means 40 for adding a predetermined amount of inorganic flocculant to the thickened sludge at a predetermined time, a dehydrator 50 for dehydrating the thickened sludge and separating it into a dehydrated cake and a dehydrated separated liquid, and a control unit 60 for controlling the pre-inorganic flocculant addition means 10, the polymer flocculant addition means 20, and the post-inorganic flocculant addition means 40.

[0019] The polymer flocculant addition means 20 includes a polymer flocculant storage tank 22, a polymer flocculant addition line 24 that supplies the polymer flocculant from the polymer flocculant storage tank 22, and a pump 26 provided in the polymer flocculant addition line 24.

[0020] The inorganic flocculant pre-addition means 10 is an inorganic flocculant piping 72 connected to an inorganic flocculant storage tank 70, and includes an inorganic flocculant pre-addition line 12 branching off from the inorganic flocculant piping 72 on which a pump 74 is provided, and a first solenoid valve 14 provided in the inorganic flocculant pre-addition line 12.

[0021] The inorganic coagulant post-addition means 40 is an inorganic coagulant piping 72 connected to the inorganic coagulant storage tank 70, and includes an inorganic coagulant post-addition line 42 branching off from the inorganic coagulant piping 72 on which a pump 74 is provided, and a second solenoid valve 44 provided in the inorganic coagulant post-addition line 42.

[0022] The control unit 60 is configured to receive an electrical signal from a concentrated separated liquid detection means 62 that measures the pH or flow rate of the concentrated separated liquid from the concentration tank 30, and an electrical signal from a dehydrated separated liquid detection means 64 that measures the pH or flow rate of the dehydrated separated liquid from the dehydrator 50, and to send a control signal to operate the first solenoid valve 14 provided in the inorganic flocculant pre-addition line 12 and the second solenoid valve 44 provided in the inorganic flocculant post-addition line 42.

[0023] The inorganic flocculant storage tank 70 is shown as a single tank common to the inorganic flocculant pre-addition means 10 and the inorganic flocculant post-addition means 40 for adding the same inorganic flocculant, but if different inorganic flocculants are to be added, the inorganic flocculant pre-addition means 10 and the inorganic flocculant post-addition means 40 may each be provided with separate inorganic flocculant storage tanks.

[0024] As shown in Figure 2, "polymer flocculant only operation," in which only polymer flocculant and no inorganic flocculant are added to sludge, can be performed by stopping pump 74 of inorganic flocculant storage tank 70 and operating only pump 26 of polymer flocculant storage tank 22, and adding polymer flocculant from polymer flocculant storage tank 22 to sludge via polymer flocculant addition line 24.

[0025] As shown in Figure 3, the "pre-inorganic flocculant addition operation" in which an inorganic flocculant is first added to sludge and then a polymer flocculant is added can be performed by operating the pump 74 of the inorganic flocculant storage tank 70, opening the first solenoid valve 14, closing the second solenoid valve 44, adding the inorganic flocculant to the sludge via the pre-inorganic flocculant addition line 12, and operating the pump 26 of the polymer flocculant storage tank 22 to add the polymer flocculant from the polymer flocculant storage tank 22 to the sludge via the polymer flocculant addition line 24.

[0026] As shown in Figure 4, the "post-inorganic coagulant addition operation" in which an inorganic coagulant is added after a polymer coagulant has been added to sludge can be performed by operating pump 26 of polymer coagulant storage tank 22 to add polymer coagulant from polymer coagulant storage tank 22 to sludge via polymer coagulant addition line 24, operating pump 74 of inorganic coagulant storage tank 70, closing first solenoid valve 14, opening second solenoid valve 44, and adding inorganic coagulant to sludge via inorganic coagulant post-addition line 42.

[0027] As shown in Figure 1, the "pre- and post-addition operation of inorganic coagulant" in which an inorganic coagulant is added to sludge, then a polymer coagulant is added, and then another inorganic coagulant is added can be performed by operating pump 74 of inorganic coagulant storage tank 70, opening first solenoid valve 14, closing second solenoid valve 44, adding inorganic coagulant to sludge via pre-addition line 12 of inorganic coagulant, operating pump 26 of polymer coagulant storage tank 22, adding polymer coagulant from polymer coagulant storage tank 22 to sludge via polymer coagulant addition line 24, then closing first solenoid valve, opening second solenoid valve, and adding inorganic coagulant to sludge via post-addition line 42 of inorganic coagulant.

[0028] Switching between "polymer flocculant only operation," "inorganic flocculant pre-addition operation," "inorganic flocculant post-addition operation," and "inorganic flocculant pre-addition operation" is controlled by the control unit 60 according to the flow rate or pH of the concentrated separated liquid and dehydrated separated liquid detected by the concentrated separated liquid detection means 62 and the dehydrated separated liquid detection means 64. If the flow rate of the concentrated separated liquid or dehydrated separated liquid is 20% or more lower than the normal value, it can be determined that scale has formed in the concentration tank or dehydrator, causing the filtration surface to become clogged. If the pH of the concentrated separated liquid or dehydrated separated liquid has shifted to the acidic or alkaline side from the pH value expected due to the added flocculant, it can be determined that the scale has dissolved.

[0029] During "polymer flocculant sole operation," if the concentrated separated liquid detection means 62 detects that the flow rate of the concentrated separated liquid has dropped by 20% or more from the normal value or that the pH is alkaline, the control unit 60 sends a command to open the first solenoid valve 14 and close the second solenoid valve 44, and switches to "inorganic flocculant pre-addition operation." If the concentrated separated liquid detection means 62 detects that the flow rate of the concentrated separated liquid has returned to the normal value or that the pH is acidic, the control unit 60 closes the first solenoid valve 14, stops the pump 72, and switches to "polymer flocculant sole operation."

[0030] If the dehydrated separated liquid detection means 64 detects that the flow rate of the dehydrated separated liquid is 20% or more lower than the normal value or that the pH is alkaline while the "polymer flocculant sole operation" is being performed, the control unit 60 sends a command to close the first solenoid valve 14 and open the second solenoid valve 44, and switches to the "inorganic flocculant post-addition operation." If the dehydrated separated liquid detection means 64 detects that the flow rate of the dehydrated separated liquid has returned to the normal value or that the pH is acidic, the control unit 60 closes the second solenoid valve 44, stops the pump 72, and switches to the "polymer flocculant sole operation."

[0031] During "polymer flocculant sole operation," if the concentrated separated liquid detection means 62 detects that the flow rate of the concentrated separated liquid is 20% or more lower than normal or that the pH is alkaline, and if the dehydrated separated liquid detection means 64 detects that the flow rate of the dehydrated separated liquid is 20% or more lower than normal or that the pH is alkaline, the control unit 60 sends a command to open the first solenoid valve 14 and the second solenoid valve 44, and switches to "inorganic flocculant before and after addition operation." If the concentrated separated liquid detection means 62 detects that the flow rate of the concentrated separated liquid has returned to normal or that the pH is acidic, and if the dehydrated separated liquid detection means 64 detects that the flow rate of the dehydrated separated liquid has returned to normal or that the pH is acidic, the control unit 60 closes the first solenoid valve 14 and the second solenoid valve 44, stops the pump 72, and switches to "polymer flocculant sole operation."

[0032] During the "inorganic flocculant pre-addition operation," if the concentrated separated liquid detection means 62 detects that the flow rate of the concentrated separated liquid is 20% or more lower than the normal value or that the pH is acidic, or if the dehydrated separated liquid detection means 64 detects that the flow rate of the dehydrated separated liquid is 20% or more lower than the normal value or that the pH is acidic, the control unit 60 sends an instruction to close the first solenoid valve 14 and stop the pump 72, and switches to "polymer flocculant only operation." When the concentrated separated liquid detection means 62 detects that the flow rate of the concentrated separated liquid has returned to its normal value or that the pH is alkaline, or when the dehydrated separated liquid detection means 64 detects that the flow rate of the dehydrated separated liquid has returned to its normal value or that the pH is alkaline, the control unit 60 operates the pump 72, opens the first solenoid valve 14, and switches to ``pre-addition operation of inorganic coagulant'' while keeping the second solenoid valve 44 closed, or opens the second solenoid valve 44 while keeping the first solenoid valve 14 closed and switches to ``post-addition operation of inorganic coagulant'', or opens the first solenoid valve 14 and the second solenoid valve 44 and switches to ``pre-addition operation of inorganic coagulant''.

[0033] If the dehydrated separated liquid detection means 64 detects that the flow rate of the dehydrated separated liquid is 20% or more lower than normal or that the pH is acidic during the "inorganic flocculant post-addition operation," the control unit 60 either closes the second solenoid valve 44 and stops the pump 72, switching to "polymer flocculant only operation," or closes the second solenoid valve 44 and opens the first solenoid valve 14, switching to "inorganic flocculant pre-addition operation." If the dehydrated separated liquid detection means 64 detects that the flow rate of the dehydrated separated liquid has returned to normal or that the pH is alkaline, the control unit 60 operates the pump 72, opens the first solenoid valve 14, and switches to "inorganic flocculant pre-addition operation" while keeping the second solenoid valve 44 closed, or opens the second solenoid valve 44 while keeping the first solenoid valve 14 closed, switching to "inorganic flocculant post-addition operation," or opens the first solenoid valve 14 and the second solenoid valve 44 to switch to "inorganic flocculant pre-addition operation."

[0034] During the "pre- and post-addition operation of inorganic flocculant," if the concentrated separated liquid detection means 62 detects that the flow rate of the concentrated separated liquid is 20% or more lower than the normal value or that the pH is acidic, and if the dehydrated separated liquid detection means 64 detects that the flow rate of the dehydrated separated liquid is 20% or more lower than the normal value or that the pH is acidic, the control unit 60 sends instructions to close the first solenoid valve 14, close the second solenoid valve 44, and stop the pump 72, and switches to "polymer flocculant only operation." When the concentrated separated liquid detection means 62 detects that the flow rate of the concentrated separated liquid has returned to its normal value or that the pH is alkaline, and when the dehydrated separated liquid detection means 64 detects that the flow rate of the dehydrated separated liquid has returned to its normal value or that the pH is alkaline, the control unit 60 operates the pump 72, opens the first solenoid valve 14, and switches to ``pre-addition operation of inorganic coagulant'' while keeping the second solenoid valve 44 closed, or opens the second solenoid valve 44 while keeping the first solenoid valve 14 closed and switches to ``post-addition operation of inorganic coagulant'', or opens the first solenoid valve 14 and the second solenoid valve 44 and switches to ``pre-addition operation of inorganic coagulant''.

[0035] In the present invention, the sludge to be dehydrated is not particularly limited as long as it is organic sludge containing phosphoric acid, ammonia nitrogen, and soluble magnesium. Preferred examples include biologically treated sludge, flocculated sludge, and sewage digested sludge generated in sewage treatment plants, sewage treatment plants, and private factories.

[0036] The present invention is characterized by a dehydration treatment that involves the addition of a polymer flocculant and an inorganic flocculant, and by combining operational modes that vary the type and timing of the flocculant added to the sludge to alternately shift the pH of the dehydrated separated liquid between acidic and alkaline. While sludge dehydration treatment devices have traditionally operated in a manner that adds only a polymer flocculant or in a manner that adds both a polymer flocculant and an inorganic flocculant, switching between the operation that adds only a polymer flocculant and the operation that adds both a polymer flocculant and an inorganic flocculant has not been practiced. In the present invention, alternating between a dehydration treatment operation that adds only a polymer flocculant and a dehydration treatment operation that uses both a polymer flocculant and an inorganic flocculant dissolves scale deposited on the alkaline side on the acidic side, and dissolves scale deposited on the acidic side on the alkaline side, thereby preventing deposits from accumulating on the filter surface of the dehydrator and clogging the filter surface. When the flow rate of the concentrated separated liquid or dehydrated separated liquid is 20% or more lower than normal, it can be determined that scale has formed in the thickening tank or dehydrator, causing clogging of the filter surface. If the pH of the concentrated separated liquid or dehydrated separated liquid is shifted to the acidic or alkaline side from the pH value expected due to the added flocculant, it can be determined that scale has dissolved.

[0037] The mode of flocculant addition operation in the present invention is to alternate between (1) polymer flocculant only operation and (2) at least one of inorganic flocculant pre-addition operation, inorganic flocculant post-addition operation, and inorganic flocculant pre-addition operation, thereby shifting the dehydrated separated liquid alternately between acidity and alkalinity in one cycle.

[0038] The "pre-addition operation of inorganic flocculant" is an operation mode in which an inorganic flocculant is added to sludge, then a polymer flocculant is added, and the resulting concentrated sludge is dewatered after concentrating the flocs formed. The "post-addition operation of inorganic flocculant" is an operation mode in which a polymer flocculant is added to sludge, and the resulting flocs are concentrated to form concentrated sludge, to which an inorganic flocculant is then added, followed by dewatering. "Operation with inorganic coagulant added before and after" refers to an operation in which an inorganic coagulant is added to sludge, then a polymer coagulant is added, and the resulting coagulated flocs are concentrated into concentrated sludge, to which the inorganic coagulant is then added before dewatering. "Polymer flocculant only operation" refers to an operation mode in which a polymer flocculant is added to sludge without adding an inorganic flocculant, and the resulting concentrated sludge is dewatered after concentrating the flocs formed.

[0039] An example of one cycle can be any combination of (1) "polymer flocculant only operation" and (2) one or more selected from "inorganic flocculant pre-addition operation", "inorganic flocculant post-addition operation", and "inorganic flocculant pre-addition operation". For example, the following combinations can be preferably mentioned. "Polymer flocculant only operation" → "Inorganic flocculant pre-addition operation" "Polymer flocculant only operation" → "Inorganic flocculant post-addition operation" "Polymer flocculant only operation" → "Inorganic flocculant before and after addition operation" "Inorganic flocculant pre-addition operation" → "Polymer flocculant sole operation" "Inorganic flocculant post-addition operation" → "Polymer flocculant sole operation" "Inorganic flocculant before and after addition operation" → "Polymer flocculant only operation" "Inorganic flocculant pre-addition operation" → "Polymer flocculant only operation" → "Inorganic flocculant post-addition operation" "Inorganic flocculant pre-addition operation" → "Polymer flocculant sole operation" → "Inorganic flocculant pre-addition operation" "Inorganic flocculant post-addition operation" → "Polymer flocculant only operation" → "Inorganic flocculant pre-addition operation" "Inorganic flocculant post-addition operation" → "Polymer flocculant only operation" → "Inorganic flocculant before and after addition operation" "Inorganic flocculant before and after addition operation" → "Polymer flocculant only operation" → "Inorganic flocculant before addition operation" "Inorganic flocculant before and after addition operation" → "Polymer flocculant only operation" → "Inorganic flocculant after addition operation"

[0040] Using the above combination as a basic form, any combination can be made, for example, "polymer flocculant only operation" → "inorganic flocculant pre-addition operation" → "polymer flocculant only operation" → "inorganic flocculant post-addition operation" → "polymer flocculant only operation" → "inorganic flocculant pre-addition operation." The combination of inorganic flocculant addition modes can be changed as appropriate to achieve appropriate treatment according to the properties of the sludge to be treated.

[0041] From the viewpoint of operation and maintenance of dewatering treatment equipment, it is desirable to obtain great effects by simple switching. For example, it is preferable to switch only once within one cycle, such as "polymer flocculant only operation" → "inorganic flocculant pre-addition operation", "polymer flocculant only operation" → "inorganic flocculant post-addition operation", "polymer flocculant only operation" → "inorganic flocculant before and after addition operation", "inorganic flocculant pre-addition operation" → "polymer flocculant only operation", "inorganic flocculant post-addition operation" → "polymer flocculant only operation", or "inorganic flocculant before and after addition operation" → "polymer flocculant only operation".

[0042] Furthermore, "inorganic flocculant pre-addition operation" → "polymer flocculant only operation", "inorganic flocculant post-addition operation" → "polymer flocculant only operation", or "inorganic flocculant before / after addition operation" → "polymer flocculant only operation" are more preferable because by performing the operation in combination with inorganic flocculant first, MAP precipitation and accumulation in the concentrator can be suppressed. Furthermore, "inorganic flocculant before / after addition operation" → "polymer flocculant only operation" is particularly preferable because it can increase dewatering efficiency.

[0043] The operating times of the "polymer flocculant only operation," "inorganic flocculant pre-addition operation," "inorganic flocculant post-addition operation," and "inorganic flocculant before and after addition operation" within one cycle are not particularly limited as long as they are times that allow the dehydrated separated liquid to be alternately shifted to acidity and alkalinity by switching. For example, a cycle in which "polymer flocculant only operation" is performed for one week, followed by "inorganic flocculant post-addition operation" for two weeks, a cycle in which "inorganic flocculant pre-addition operation" is performed for one week, followed by "polymer flocculant only operation" for one week, followed by "inorganic flocculant post-addition operation" for two weeks, a cycle in which "inorganic flocculant before and after addition operation" is performed for three weeks, followed by "polymer flocculant only operation" for one week, and the like can be preferably exemplified.

[0044] The timing of switching between "polymer flocculant-only operation," "inorganic flocculant pre-addition operation," "inorganic flocculant post-addition operation," and "inorganic flocculant pre-addition operation" may be when each operation has been performed for a predetermined period of time, or when the flow rate of the concentrated separated liquid or dehydrated separated liquid drops by 20% or more from the normal value, as described in detail above, or when the pH of the concentrated separated liquid or dehydrated separated liquid shifts to an acidic or alkaline state, or may be controlled by a combination of these. Switching is usually performed after a predetermined period of time, but it is preferable to switch the operating mode when the flow rate drops by 20% or more from the normal value even if the predetermined period has not yet elapsed, as this can more effectively suppress scale formation.

[0045] As the inorganic flocculant, one selected from polyferric sulfate, ferric chloride, polyaluminum chloride, and aluminum sulfate can be used alone or in combination of any two or more thereof.

[0046] The amount of inorganic coagulant added is preferably 10 to 30% per TS, more preferably 10 to 25%, and most preferably 10 to 20%. Adding an amount of inorganic coagulant of 10% or more can produce good flocs and reduce the moisture content of the dehydrated cake. It also lowers the pH and prevents the formation of scale derived from MAP. On the other hand, adding an amount of inorganic coagulant of 30% or less can prevent the formation of scale derived from the inorganic coagulant and also reduce the cost of the inorganic coagulant.

[0047] As the polymer flocculant, any of anionic polymer flocculants, nonionic polymer flocculants, cationic polymer flocculants, and amphoteric polymer flocculants can be used. When treating organic sludge, it is particularly desirable to use cationic polymer flocculants or amphoteric polymer flocculants. As the cationic polymer flocculant, acrylic acid-based, polyacrylic acid-based, methacrylic acid-based, and polymethacrylic acid-based can be preferably used.

[0048] The amount of polymer flocculant added is preferably 1.5 to 2.0% per TS, more preferably 1.5 to 2.5%, and most preferably 2.0 to 2.5%. By adding a polymer flocculant amount of 1.5 to 2.0%, good flocs with high strength can be obtained. On the other hand, by adding a polymer flocculant amount of 2.0 to 2.5%, it is possible to prevent a decrease in flocculation force due to shear force in the dehydrator and also suppress a deterioration in moisture content. [Example]

[0049] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0050] 1. Target sludge The target sludge used was digested sludge from a sewage treatment plant with the properties shown in Table 1.

[0051] [Table 1]

[0052] 2. Dehydration treatment test Digested sludge was dewatered using a sludge treatment system with the configuration shown in Figure 1. The sludge dewatering equipment included a mixing tank, a coagulation tank, a thickener, and a dehydrator. A screw press dehydrator was used as the dehydrator. Various combinations were used, as shown in Figure 2: "polymer coagulant addition," in which a polymer coagulant was added to the digested sludge before it entered the coagulation tank; "post-addition of inorganic coagulant," in which an inorganic coagulant was added to the concentrated sludge before it entered the dehydrator; and "pre-addition of inorganic coagulant," in which an inorganic coagulant was added to the digested sludge before it entered the mixing tank. The pH of the concentrated filtrate, the pH of the dehydrated filtrate, the degree of blockage of the thickener filter area, the degree of blockage of the dehydrator punching filter area, the main scale components, and the moisture content of the cake were measured and comprehensively evaluated. In Figure 2, "inorganic" refers to the addition of inorganic coagulant, and "polymer" refers to the addition of polymer coagulant. The dehydration operating conditions and results are shown in Table 2. Ferric polysulfate was used as the inorganic coagulant, and sodium polyacrylate, a cationic polymer coagulant, was used as the polymer coagulant. Each operation period was three months.

[0053] In the case of "polymer flocculant addition," the addition rate of the polymer flocculant was 1.9% of the total TS amount. In the case of "post-addition of inorganic flocculant" or "pre-addition of inorganic flocculant," the addition rate of the inorganic flocculant was 20% of the total TS amount. In the case of "post-addition of inorganic flocculant" and "pre-addition of inorganic flocculant," the inorganic flocculant was allocated to "post-addition of inorganic flocculant" and "pre-addition of inorganic flocculant" so that the total addition rate of the inorganic flocculant was 20% of the total TS amount.

[0054] The degree of clogging of the filtration area of ​​the concentrator is measured by counting the filtering surfaces with scale attached and calculating the degree of clogging relative to the total filtering surface. That is, the calculation is: degree of clogging of filtration area = (clogged filtering surface / total filtering surface) x 100. The degree of clogging of the filtration area of ​​the concentrator is rated as "◎" for 10% or less, "○" for over 10% to 20% or less, "△" for over 20% to 30% or less, and "×" for over 30%.

[0055] The degree of blockage of the dehydrator punching filtration area was calculated by counting the number of punching holes with scale attached and calculating the degree of blockage relative to the total number of punching holes. That is, the degree of blockage of the punching filtration area was calculated as follows: degree of blockage of punching filtration area = (blocked punching holes / total punching holes) x 100. A degree of blockage of the dehydrator punching filtration area of ​​20% or less was marked as "Good," and a degree of blockage exceeding 20% ​​was marked as "Poor."

[0056] The moisture content of the dehydrated cake was determined in accordance with the sewage test method from the mass of water evaporated when the dehydrated cake was evaporated to dryness at 105 to 110°C. A moisture content of less than 80% was marked as "Good" and a moisture content of 80% or more was marked as "Poor."

[0057] The overall evaluation was made as follows: if there was an "x" in the evaluation of the degree of clogging of the filtration area of ​​the concentrator, the degree of clogging of the punched hole area of ​​the dehydrator, and the cake moisture content, the overall evaluation was made as "x"; if there was a "△" the overall evaluation was made as "△"; if there was a "◎" the evaluation was made as "◎"; otherwise the evaluation was made as "○".

[0058] In Control Example 1, a polymer flocculant was added to digested sludge, and the resulting flocs were concentrated and the resulting thickened sludge was dewatered (referred to as "polymer flocculant-only operation"), which was carried out continuously for three months. The concentrated filtrate was neutral to slightly alkaline (pH = 7.9), and the dewatered filtrate was alkaline (pH = 8.2). After three months, MAP scale was observed in the thickener and dehydrator. The thickener filtration area blockage was 30%, the dehydrator punching filtration area blockage was 35%, and the average moisture content of the dehydrated cake was 80.5%.

[0059] In Control Example 2, a polymer coagulant was added to digested sludge, and the resulting coagulated flocs were concentrated. The thickened sludge was then thickened and dewatered with an inorganic coagulant (referred to as "post-inorganic coagulant addition operation"). This operation was carried out continuously for three months. The concentrated sludge was neutral to slightly alkaline (pH = 7.9), and the dewatered sludge was acidic (pH = 5.5). After three months, calcium sulfate scale was observed in the thickener separator and dehydrator. The degree of clogging of the thickener's filtration area was 30%, the degree of clogging of the dehydrator's punched filtration area was 44%, and the average moisture content of the dehydrated cake was 78.5%.

[0060] In Control Example 3, an inorganic coagulant was added to digested sludge, followed by the addition of a polymer coagulant. The resulting coagulated flocs were concentrated and the resulting thickened sludge was dewatered (referred to as "pre-inorganic coagulant addition operation"), and this operation was carried out continuously for three months. The concentrated sludge was acidic (pH = 6.1), and the dewatered sludge was neutral (pH = 7.1). After three months, almost no scale was observed in the thickener, but calcium sulfate scale was observed in the dehydrator. The thickener filtration area blockage rate was 6%, the dehydrator punching filtration area blockage rate was 28%, and the average moisture content of the dehydrated cake was 79.3%.

[0061] In Example 1, four cycles were repeated over three months, each cycle consisting of one week of "polymer flocculant-only operation" followed by two weeks of "inorganic flocculant post-addition operation." That is, one week of operation was performed in which a polymer flocculant was added to digested sludge to thicken the flocs formed, and the resulting thickened sludge was dewatered. After that, the addition of the polymer flocculant was continued, and then the operation was switched to another operation in which an inorganic flocculant was added to the thickened sludge and then dewatered, and this was repeated for two weeks.

[0062] During "polymer flocculant-only operation," the concentrated filtrate was neutral to slightly alkaline (pH = 7.9), while the dehydrated filtrate was alkaline (pH = 8.2). When subsequently switched to "inorganic flocculant post-addition operation," the concentrated filtrate remained neutral to slightly alkaline (pH = 7.9), while the dehydrated filtrate shifted to acidic (pH = 5.5). Repeated cycles of switching from "polymer flocculant-only operation" to "inorganic flocculant post-addition operation" confirmed that the dehydrated filtrate alternated from alkaline to acidic to alkaline to acidic. After three months, scaling was observed in the concentrator, but was reduced compared with Controls 1 and 2. A small amount of calcium sulfate scaling was observed in the dehydrator, but was significantly reduced compared with Controls 1 to 3. The concentrator filter area blockage was 27%, the dehydrator punching filter area blockage was 19%, and the average moisture content of the dehydrated cake was 78.8%. By switching from "polymer flocculant only operation" to "inorganic flocculant post-addition operation," the concentrated separated liquid is maintained at a neutral or weakly alkaline pH, and the dehydrated separated liquid shifts from alkaline to acidic, which dissolves the MAP and calcium carbonate that precipitated during "polymer flocculant only operation," suppressing the accumulation of scale, and the eluted phosphate ions bind and fix with the Fe ions derived from the inorganic flocculant, suppressing the formation of new MAP. By switching to "polymer flocculant only operation" in the next cycle, the concentrated separated liquid is maintained at a neutral or weakly alkaline pH, and the dehydrated separated liquid shifts from acidic to alkaline, which dissolves the calcium sulfate that precipitated during "inorganic flocculant post-addition operation," suppressing the accumulation of scale, which is thought to have suppressed the formation of scale after the entire operating period.

[0063] In Example 2, three cycles were repeated over three months, each cycle consisting of one week of "inorganic flocculant pre-addition operation," one week of "polymer flocculant only operation," and then two weeks of "inorganic flocculant post-addition operation." That is, an operation was performed for one week in which an inorganic flocculant was added to the digested sludge, followed by the addition of a polymer flocculant, to concentrate the formed flocs and dewater the concentrated sludge. Then, the addition of the inorganic flocculant was stopped, and the operation was switched to another week in which a polymer flocculant was added to the digested sludge, to concentrate the formed flocs and dewater the concentrated sludge. Then, the operation was switched to another week in which the addition of the polymer flocculant was continued, and the operation was switched to another week in which an inorganic flocculant was added to the concentrated sludge and dewatered. This was then repeated for two weeks.

[0064] During "pre-addition of inorganic flocculant" operation, the concentrated effluent was acidic (pH = 6.2) and the dehydrated effluent was neutral (pH = 7.2). After switching to "polymer flocculant-only operation," the concentrated effluent shifted to neutral (pH = 7.7) and alkaline (pH = 8.2). After switching to "post-addition of inorganic flocculant," the concentrated effluent shifted to neutral or weakly alkaline (pH = 7.9) and acidic (pH = 5.5). Repeating the cycle of switching from "pre-addition of inorganic flocculant" to "polymer flocculant-only operation" to "post-addition of inorganic flocculant" confirmed that the concentrated effluent shifted from acidic to neutral to neutral or weakly alkaline, and the dehydrated effluent shifted from neutral to alkaline to acidic. After three months, scaling was observed on the concentrator, but was reduced compared to Controls 1 and 2. A small amount of calcium sulfate scaling was observed on the dehydrator, but was significantly reduced compared to Controls 1-3. The concentrator filter area blockage was 20%, the dewaterer punching filter area blockage was 14%, and the average moisture content of the dewatered cake was 79%. By switching from "pre-addition inorganic coagulant operation" to "polymer coagulant only operation" to "post-addition inorganic coagulant operation," the concentrated separated liquid was maintained from acidic to neutral or slightly alkaline, and the dewatered separated liquid shifted from neutral to alkaline to acidic. This resulted in the dissolution of MAP and calcium carbonate precipitated during "polymer coagulant only operation" during "post-addition inorganic coagulant operation," suppressing scale buildup. Furthermore, the dissolved phosphate ions bonded with and were fixed by the inorganic coagulant-derived Fe ions, suppressing the formation of new MAP. It is believed that the dissolution of calcium sulfate precipitated during "post-addition inorganic coagulant operation" during the next cycle of "polymer coagulant only operation," suppressing scale buildup, thereby suppressing scale formation throughout the entire operation period.

[0065] In Example 3, an operation was performed for three weeks in which an inorganic coagulant was added to digested sludge, followed by the addition of a polymer coagulant, and the inorganic coagulant was added to the concentrated sludge resulting from the thickened flocculation that formed (referred to as "operation before and after inorganic coagulant addition"), followed by one week of "polymer coagulant only operation." This cycle was repeated. That is, an operation in which half the amount of inorganic coagulant was added to digested sludge, followed by the addition of a polymer coagulant, followed by the addition of half the amount of inorganic coagulant to the concentrated sludge and dewatering was performed for three weeks, after which the addition of the inorganic coagulant was stopped, and the polymer coagulant was added to the digested sludge, and the operation was switched to an operation in which the polymer coagulant was added to the digested sludge and the thickened sludge resulting from the thickened flocculation that formed was dewatered, and this was repeated for one week.

[0066] During "pre- and post-addition of inorganic flocculant" operation, the concentrated filtrate was acidic (pH = 6.5), and the dehydrated filtrate was acidic (pH = 5.9). After the inorganic flocculant addition was discontinued and the operation switched to "polymer flocculant-only operation," the concentrated filtrate shifted to neutral or weakly alkaline (pH = 7.9), and the dehydrated filtrate shifted to alkaline (pH = 8.0). Repeated cycles of switching from "pre- and post-addition of inorganic flocculant" to "polymer flocculant-only operation" confirmed that the concentrated filtrate shifted from acidic to neutral or weakly alkaline, and the dehydrated filtrate shifted from acidic to alkaline. After three months, almost no scale was observed in the concentrator, and although a small amount of calcium sulfate scale was observed in the dehydrator, it was significantly less than in Controls 1-3. The concentrator filter area blockage was 8%, the dehydrator punching filter area blockage was 12%, and the average moisture content of the dehydrated cake was 78.8%. By alternating between "operation with before and after inorganic flocculant addition" and "operation with polymer flocculant only", the concentrated separated liquid is maintained from acidic to neutral or weakly alkaline, and the dehydrated separated liquid shifts from acidic to alkaline. Calcium sulfate that precipitates during "operation with before and after inorganic flocculant addition" dissolves during "operation with polymer flocculant only". The MAP and calcium carbonate that precipitate during "operation with polymer flocculant only" then dissolve during the next cycle of "operation with before and after inorganic flocculant addition", suppressing the accumulation of scale. At the same time, the eluted phosphate ions bind and become fixed with the Fe ions derived from the inorganic flocculant, suppressing the generation of new MAP. This is thought to have suppressed the generation of scale after the entire operating period.

[0067] The operation of Example 3, which alternates between "operation before and after adding inorganic coagulant" and "operation using polymer coagulant alone," not only reduces the degree of clogging of the filtration area of ​​the thickener and dehydrator, but also reduces the frequency of switching, making operation management easy. It is believed that the scale generation inhibition effect can be further improved by adjusting the ratio of inorganic coagulant added to the digested sludge and concentrated sludge and / or the ratio of the period of "operation before and after adding inorganic coagulant" to the period of "operation using polymer coagulant alone."

[0068] [Table 2]

Claims

1. A method for suppressing scale formation in a sludge dehydration treatment device having a concentrator that concentrates sludge to which a flocculant has been added and separates it into concentrated sludge and concentrated separated liquid, and a dehydrator that dehydrates the concentrated sludge and separates it into dehydrated cake and dehydrated separated liquid, comprising: (1) Polymer flocculant-only operation, in which a polymer flocculant is added to organic sludge containing phosphoric acid, ammonia nitrogen, and soluble magnesium, and the resulting flocs are concentrated and the concentrated sludge is dewatered; (2) A combined operation in which a polymer flocculant and an inorganic flocculant are added to the sludge; The method for inhibiting scale formation is characterized by repeating a cycle of alternately shifting the dehydrated separated liquid between acidity and alkalinity at least once by switching between the above two conditions, dissolving scale containing at least magnesium ammonium phosphate that has precipitated on the alkaline side on the acidic side, and dissolving scale that has precipitated on the acidic side on the alkaline side.

2. The operation (2) using a polymer flocculant and an inorganic flocculant in combination is as follows: (A) to (C): (A) A pre-addition operation of an inorganic flocculant in which an inorganic flocculant is added to sludge, then a polymer flocculant is added, and the resulting concentrated sludge is dewatered after concentrating the flocs formed. (B) A post-addition operation of inorganic flocculant in which a polymer flocculant is added to sludge, and an inorganic flocculant is added to the concentrated sludge formed by concentrating the flocculant flocs, and then the sludge is dewatered; and (C) Addition of inorganic flocculant before and after the operation in which an inorganic flocculant is added to the sludge, then a polymer flocculant is added, and the resulting flocs are concentrated and the sludge is dewatered after adding the inorganic flocculant. The method according to claim 1, characterized in that it is at least one embodiment selected from the following or any two or more embodiments:

3. The method described in claim 1, characterized in that the (2) operation using a polymer coagulant and an inorganic coagulant in combination is (C) an operation of adding an inorganic coagulant before and after the addition of the inorganic coagulant to the sludge, then adding a polymer coagulant, and concentrating the formed coagulated flocs to the concentrated sludge, which is then dewatered after adding the inorganic coagulant to the concentrated sludge.

4. The method according to any one of claims 1 to 3, characterized in that the switching between (1) the operation using only a polymer flocculant and (2) the operation using a polymer flocculant and an inorganic flocculant in combination is performed when (I) the pH of the concentrated separated liquid or the dehydrated separated liquid shifts to the alkaline side in (1) the operation using only a polymer flocculant, or (2) the pH of the concentrated separated liquid or the dehydrated separated liquid shifts to the acidic side in (2) the operation using a polymer flocculant and an inorganic flocculant in combination, or / and (II) the flow rate of the concentrated separated liquid or the dehydrated separated liquid decreases by 20% or more from the normal flow rate value.

5. The method according to any one of claims 1 to 4, wherein the inorganic flocculant is at least one inorganic flocculant selected from polyferric sulfate, ferric chloride, polyaluminum chloride, and aluminum sulfate.

6. A sludge dehydration treatment device, an inorganic flocculant pre-addition means for adding a predetermined amount of inorganic flocculant to the sludge at a predetermined timing; a polymer flocculant adding means for adding a predetermined amount of polymer flocculant to the sludge at a predetermined timing; a thickening tank for thickening the sludge to which the polymer flocculant has been added and separating it into thickened sludge and thickened separated liquid; an inorganic flocculant post-addition means for adding a predetermined amount of inorganic flocculant to the concentrated sludge at a predetermined timing; a dehydrator that dehydrates the concentrated sludge and separates it into a dehydrated cake and a dehydrated separated liquid; Equipped with (1) Polymer flocculant-only operation, in which a polymer flocculant is added to organic sludge containing phosphoric acid, ammonia nitrogen, and soluble magnesium, and the resulting flocs are concentrated and the concentrated sludge is dewatered; (2) An apparatus characterized by having a control unit that controls the inorganic flocculant pre-addition means, the polymer flocculant addition means, and the inorganic flocculant post-addition means so that the pH of the dehydrated separated liquid is alternately shifted to the acidic side and the alkaline side by switching between a combined operation in which a polymer flocculant and an inorganic flocculant are added to the sludge, and the cycle is repeated at least once, so that scale containing at least magnesium ammonium phosphate that has precipitated on the alkaline side is dissolved on the acidic side, and scale that has precipitated on the acidic side is dissolved on the alkaline side.

7. The control unit includes a concentrated separated liquid detection means, a dehydrated separated liquid detection means, and an electromagnetic valve for switching the timing of addition of the inorganic flocculant; The device described in claim 6, characterized in that the opening and closing of the solenoid valve is controlled according to the flow rate or pH of the concentrated separated liquid detected by the concentrated separated liquid detection means and the flow rate or pH of the dehydrated separated liquid detected by the dehydrated separated liquid detection means when: (I) (1) in the case of operation using only a polymer flocculant, the pH of the concentrated separated liquid or the dehydrated separated liquid shifts to the alkaline side; (2) in the case of operation using a polymer flocculant and an inorganic flocculant in combination, the pH of the concentrated separated liquid or the dehydrated separated liquid shifts to the acidic side; or (II) the flow rate of the concentrated separated liquid or the dehydrated separated liquid decreases by 20% or more from the normal flow rate value.

8. The operation (2) using a polymer flocculant and an inorganic flocculant in combination is as follows: (A) to (C): (A) A pre-addition operation of an inorganic flocculant in which an inorganic flocculant is added to sludge, then a polymer flocculant is added, and the resulting concentrated sludge is dewatered after concentrating the flocs formed. (B) A post-addition operation of inorganic flocculant in which a polymer flocculant is added to sludge, and an inorganic flocculant is added to the concentrated sludge formed by concentrating the flocculant flocs, and then the sludge is dewatered; and (C) Addition of inorganic flocculant before and after the operation in which an inorganic flocculant is added to the sludge, then a polymer flocculant is added, and the resulting flocs are concentrated and the sludge is dewatered after adding the inorganic flocculant.

8. The device according to claim 6, wherein the device is at least one aspect selected from the above or any two or more aspects.

9. The apparatus described in claim 6 or 7, characterized in that the (2) operation using a polymer coagulant and an inorganic coagulant in combination is (C) an inorganic coagulant pre-addition operation in which an inorganic coagulant is added to sludge, then a polymer coagulant is added, and the formed coagulated flocs are concentrated into concentrated sludge, to which the inorganic coagulant is added, and then the concentrated sludge is dewatered.

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