JP6573004B2 - Sludge dewatering method - Google Patents

Description

  The present invention relates to a sludge dewatering method in which an amphoteric polymer is added to sludge and then a cationic polymer having an amidine structural unit is added.

  Food-based sludge generated by microbial treatment of waste generated due to reasons such as raw material production for food, food production, seasoning production, food processing and food unsold goods, food leftovers, etc. Compared with sludge generated by chemical factory wastewater, it has a feature of high organic content.

  When flocs are formed by adding a polymer flocculant to the above-mentioned food sludge to form a floc, and solid-liquid separation with a dehydrator, the floc formed compared to sludge of other industries other than food such as chemical industry Since it was fine and soft, it was very difficult to dewater sludge.

  For example, when a food press sludge is dehydrated using a belt press dehydrator, the flock is fine and soft, so the filter cloth is frequently clogged, and it is necessary to frequently perform cleaning to eliminate clogging. was there. Moreover, since many solid substances leaked into the filtrate, the recovery rate of solid substances was deteriorated. Furthermore, since the strength of the formed floc was weak, when the pressure dehydration was performed, the cake leaked from the side of the filtration surface, and the moisture content of the cake after the dehydration was as high as about 90%.

  A multi-disk dehydrator is generally used for dewatering the food sludge because of its ease of operation. In the case of multiple disk type dehydrators used in food sludge, the moisture content of the cake after dehydration generated is higher than in the case of screw press dehydrators and belt press dehydrators commonly used in sewage sludge. It was.

  Various methods have been disclosed for reducing the moisture content of the cake after dehydration by improving the strength of the floc. For example, a sludge treatment method (Patent Document 1) in which a cationic polymer having an amidine structural unit and a specific amphoteric polymer are simultaneously added is known.

  However, the method of Patent Document 1 cannot be said to exhibit sufficient floc strength, and when applied to the food sludge, the moisture content of the cake after dehydration cannot be lowered. Dehydration was required.

JP-A-6-218399

  An object of the present invention is to provide a sludge dewatering method in which the moisture content of a cake after dehydration is low by increasing the strength of the flocs in mechanical depressing in the sludge dewatering process by increasing the strength of the flocs flocs. .

The present invention has the following aspects.
[1] A method for dewatering sludge, wherein an amphoteric polymer is added to sludge generated by treating food waste, and then a cationic polymer having an amidine structural unit is added.
[2] The sludge dewatering method according to [1], wherein the amidine structural unit is an amidine structural unit represented by the following general formula (1) and / or the following general formula (2).

[In General Formulas (1) and (2), R ^{1 to} R ⁴ are each independently a hydrogen atom or a methyl group. X ⁻ and Y ⁻ are each an anion and may be the same or different. ]
[3] Sludge dewatering according to [1] or [2], wherein the amphoteric polymer is a polymer having 1 to 70 mol% of (meth) acrylic acid structural units and / or (meth) acrylate structural units. Method.
[4] The sludge dewatering method according to any one of [1] to [3], wherein an inorganic flocculant is added before the addition of the amphoteric polymer.

  According to the dewatering method of the present invention, in the sludge dewatering treatment, the sludge dewatering method having a low moisture content of the cake after dewatering is provided by increasing the strength of the flocs in mechanical pressing by increasing the strength of the flocs flocs. be able to.

  The sludge dewatering method of the present invention can be suitably used for hardly dewatering sludge generated by microbial treatment of food waste, but for sludge generated by sewage treatment or chemical factory wastewater. Can also be used.

  Examples of the sludge used in the present invention include sludge generated by microbial treatment of food wastes aerobically or anaerobically. Specifically, mixed raw sludge (mixed sludge of primary sludge and surplus sludge), primary sediment sludge, surplus sludge, anaerobic digested sludge and the like can be mentioned.

  The sludge used in the present invention is a sludge having a large organic content.

  The absolute amount of sludge organic matter used in the present invention can be measured as the absolute amount of organic matter such as polysaccharides and proteins by measuring, for example, ignition loss value, fiber content, organic carbon concentration and the like.

  Examples of the fiber content include dietary fiber. Dietary fiber is an indigestible component contained in food that is not digested by human digestive enzymes. The above-mentioned dietary fiber is a component constituting the cell wall of vegetable, algal and fungal foods, and consists of polysaccharides among carbohydrates.

  The concentration of the organic matter in the sludge used in the present invention is preferably 30% by mass or more, more preferably 40% by mass or more, as an ignition loss value (hereinafter abbreviated as VTS). Moreover, 98 mass% or less is preferable and 85 mass% or less is more preferable.

  The concentration of the fiber content of sludge used in the present invention (hereinafter abbreviated as a crude suspended matter value) is preferably 0.1% by mass or more, and more preferably 2% by mass or more. Moreover, 85 mass% or less is preferable, and 65 mass% or less is more preferable.

  The total organic carbon concentration (hereinafter abbreviated as TOC value) of the sludge used in the present invention is preferably 1 mg / L or more, and more preferably 40 mg / L or more. Moreover, 2000 mg / L or less is preferable and 1000 mg / L or less is more preferable.

[Inorganic flocculant]
As the inorganic flocculant used in the present invention, known inorganic flocculants such as sulfate band (aluminum sulfate), ferric chloride, ferrous sulfate, polyferric sulfate and the like can be used. These can be obtained from commercial products. An inorganic flocculant may be used individually by 1 type, and may use 2 or more types together.

  As the polymer in the present invention, an amphoteric polymer and a cationic polymer having an amidine structural unit are used.

[Amphoteric polymer]
The amphoteric polymer used in the present invention comprises a polymer having a structural unit having an anionic group and a structural unit having a cationic group in the molecule. Furthermore, you may have the structural unit which has a nonionic group. As long as it has an anionic group and a cationic group, the structural unit may be a ternary system or a quaternary system.

  Examples of the anionic group include a carboxyl group and a sulfonic acid group.

  Examples of the cationic group include tertiary amines, neutralized salts thereof, and quaternary salts.

  The polymer is preferably a copolymer obtained by polymerizing an anionic monomer, a cationic monomer, and an optional nonionic monomer.

  Examples of the anionic monomer include acrylic acid and / or a salt thereof, maleic acid and / or a salt thereof, acrylamido-2-methylpropanesulfonic acid and / or a salt thereof, and the like. Of these, acrylic acid and / or its salts are preferred.

  Cationic monomers include dimethylaminoethyl (meth) acrylate, diethylaminoethyl (meth) acrylate, dimethylaminopropyl (meth) acrylamide, diethylaminopropyl (meth) acrylamide, allyl dimethylamine or neutralized salts thereof, quaternary Examples include salts. Of these, dimethylaminoethyl (meth) acrylate methyl chloride quaternary salt is preferred.

  Nonionic monomers include (meth) acrylamide, N, N-dimethyl (meth) acrylamide and the like. Of these, acrylamide is preferred.

  Here, “(meth) acryl” means “acryl” and / or “methacryl”.

Any of these monomers can be used alone or in combination of two or more.

  The molar percentage of the (meth) acrylic acid structural unit and / or the (meth) acrylate structural unit in the amphoteric polymer used in the present invention relative to all the structural units is preferably 1 mol% or more, more preferably 3 mol% or more. 7 mol% or more is more preferable. Moreover, 70 mol% or less is preferable, 59 mol% or less is more preferable, and 44 mol% or less is still more preferable. If it is in the said range, the moisture content of the cake after spin-drying | dehydration can be made low.

  One amphoteric polymer may be used alone, or a plurality of amphoteric polymers may be used in combination.

  The polymerization method of the amphoteric polymer is not particularly limited, and examples thereof include precipitation polymerization, bulk polymerization, dispersion polymerization, and aqueous solution polymerization.

  A solid acid may be added to the polymer for the purpose of improving the solubility of the polymer and improving the storage stability of the polymer solution. Examples of the solid acid include sulfamic acid and acidic sodium sulfite.

  The amphoteric polymer can be produced, for example, by the method described in JP 2010-264363 A. It can also be obtained as a commercial product.

[Cationic polymer having amidine structural unit]
The cationic polymer used in the present invention comprises a cationic polymer having an amidine structural unit represented by the following general formula (1) and / or the following general formula (2). The cationic polymer may have a structural unit other than the amidine structural unit. The content ratio of the amidine structural unit to the total structural unit of the cationic polymer is preferably 40 mol% or more, and more preferably 70 mol% or more. Moreover, 95 mol% or less is preferable and 80 mol% or less is more preferable.

  The cationic polymer having an amidine structural unit used in the present invention is obtained by copolymerizing N-vinylformamide and acrylonitrile, and subjecting the obtained copolymer to a hydrolysis reaction by heating under hydrochloric acid acidity. A polymer in which the primary amino group of the inner chain and the cyano group are cyclized intramolecularly to form an amidine ring is preferable. For example, a cationic polymer having an amidine structural unit can be produced by the method described in JP-A-8-243600.

  The intrinsic viscosity [η] of the polymer is preferably 1 dL / g or more, and more preferably 6 dL / g or more. Moreover, 20 dL / g or less is preferable and 12 dL / g or less is more preferable.

  Within the above range, the higher the [η], the easier it is to form a floc having a larger and stronger strength, and the better the dehydration process, so the moisture content of the cake after dehydration can be lowered.

[Dehydration method]
First, an amphoteric polymer is added to the sludge, and then a cationic polymer having an amidine structural unit is added. The time from the addition of the amphoteric polymer to the addition of the cationic polymer having an amidine structural unit is not particularly limited, but usually a floc is formed when the amphoteric polymer is added, and then a cation having an amidine structural unit. The addition of a functional polymer makes the flocs stronger and larger. It is also possible to add an inorganic flocculant first to the sludge, then add an amphoteric polymer, and then sequentially add a cationic polymer having an amidine structural unit. The time from the addition of the inorganic flocculant to the addition of the amphoteric polymer and the cationic polymer having an amidine structural unit is not particularly limited.

  In the present invention, a treatment object to which an inorganic flocculant, an amphoteric polymer, and a cationic polymer having an amidine structural unit are sequentially added is dehydrated by a known technique. The dehydration treatment can be performed using a known dehydrator such as a centrifugal dehydrator, a screw press dehydrator, a belt press dehydrator, or a multiple disk dehydrator.

  The time from the sequential addition of the inorganic flocculant, the amphoteric polymer and the cationic polymer having an amidine structural unit to the dehydration treatment is not particularly limited.

  According to the present invention, an amphoteric polymer is added to sludge, and the positive and negative ions of the amphoteric polymer molecular chain are entangled with the molecular chain while neutralizing the sludge particles to form a floc. Next, a cationic polymer having an amidine structural unit is added to combine with the amphoteric polymer and sludge negative ions to neutralize the charge. At the same time, a stronger coarsening floc can be formed by the synergistic effect of the hydrophobic interaction between the hydrophobic part in the sludge and the hydrophobic part derived from the amidine ring. Thereby, the dehydration process using a dehydrator can be performed more favorably. In addition, an inorganic flocculant is first added to the sludge, the amphoteric polymer is added after the sludge is microfloced in advance, and then the cationic polymer having an amidine structural unit is added. Can be made stronger and coarser frock.

  Therefore, the sludge can be efficiently dehydrated, and the moisture content of the cake after dehydration can be further reduced. Thereby, the incineration processing cost of sludge can be reduced significantly.

  EXAMPLES Hereinafter, the present invention will be specifically described with reference to examples. However, the present invention is not limited to the following examples unless it exceeds the gist.

[Measurement of intrinsic viscosity [η]]
The intrinsic viscosity [η] of each polymer was measured by sequentially performing the following methods (1) to (3).

(1) First, the viscosity of only the solvent was measured (blank test).
That is, an Ubbelohde viscometer is set in a thermostat adjusted to 30 ± 0.05 ° C., a 1N-sodium nitrate solution is placed in the viscometer, and after standing for 10 to 30 minutes, the solution is raised. The flow time required to pass between the upper and lower marked lines displayed on the viscometer when it was allowed to flow naturally was measured three times, and the average value was taken as the viscosity (t0) of the solvent.

  (2) Next, the reduced viscosity was calculated by the following method.

  The same operation as the above (1) was performed using each 1N-sodium nitrate solution having a sample concentration of 0.02 to 0.10% by mass. The average flow time required to pass between the upper and lower marked lines displayed on the viscometer of the sample solution was measured three times, and the average value was taken as the viscosity (t) of each sample solution. Next, the ratio t / t0 was calculated for each sample, and the relative viscosity ηr was obtained. The specific viscosity ηsp = ηr-1 was calculated from the relative viscosity ηr, and the reduced viscosity ηsp / C of each sample was calculated by dividing each specific viscosity ηsp = ηr-1 by the concentration of each sample.

  (3) Next, the intrinsic viscosity was calculated by the following method.

  The concentration (mass%) of the sample solution and the reduced viscosity ηsp / C were plotted, and the intrinsic viscosity [η] was calculated by extrapolation.

In the following Examples and Comparative Examples, various polymers described in Table 1 below were used by diluting them in a 0.3% by mass aqueous solution. In addition, the symbol of the raw material monomer as a structural unit of the polymer in Table 1 is as follows.
DME: dimethylaminoethyl acrylate / methyl chloride quaternary salt structural unit DMC: dimethylaminoethyl methacrylate / methyl chloride quaternary salt structural unit AAm: acrylamide structural unit AA: acrylic acid structural unit VAD: vinylamidine structural unit (general formula (1 ), R ¹ = hydrogen atom, R ² = hydrogen atom, X ⁻ = Cl ⁻ .
NVF: N-vinylformamide structural unit AN: Acrylonitrile structural unit VAM: Vinylamine structural unit

  Examples of the amphoteric polymer of the present invention include the polymers of types A to F in Table 1 above. Examples of the cationic polymer having an amidine structural unit of the present invention include polymers of type G in Table 1 above. For comparison, examples of the cationic polymer having no amidine structural unit are the types H and I in Table 1.

[Analytical measurement of sludge]
Sludge properties were measured. Measurement items for each characteristic were performed by the following methods.
-PH (hydrogen ion concentration) value: (1) An electrode of a pH measuring device was placed in a sludge sample. (2) The place where the indicated value of the pH meter was stabilized was measured as a pH value (edited by Japan Sewerage Association, “Sewerage Test Method, Vol. 1997, p. 296)”.

  TS (evaporation residue) value: (1) First, the sludge sample was evaporated to dryness. (2) The mass of the substance remaining when heated and dried at 105 to 110 ° C. for 2 hours was measured. (3) The mass percentage of the remaining substance with respect to the sludge sample was determined as a TS value (edited by the Japan Sewerage Association, “Sewage Test Method, Vol. 1997,” August 25, 1997, p. 296 297).

  VTS (ignition loss) value: (1) The above evaporation residue was subjected to ignition carbonization at 600 ± 25 ° C. for 1 hour to obtain an ignition residue. (2) The difference in ignition residue from the above evaporation residue was calculated as a percentage (edited by Japan Sewerage Association, “Sewerage Test Method, Vol. 1997, p297”).

  -Coarse suspended matter (fiber content) value: (1) Disperse the sludge sample in water in advance. (2) Transfer to a sieve and rinse with water. (3) After the residue was dried, it was ignited by ignition and the mass of the residue was measured. (4) The mass percentage of the residue with respect to the sludge sample was obtained, and this value was divided by the TS value to calculate the percentage (edited by Japan Sewerage Association, “Sewage Test Method, Vol. 1997, p. 298”).

  TOC (total organic carbon concentration) value: (1) The organic matter in the sludge sample was burned with hot air or hot oxygen. (2) The carbon dioxide concentration generated by combustion was measured with a gas analyzer, and the TOC value was measured. The unit of the TOC value was expressed in mg / L (edited by Japan Sewerage Association, “Sewerage Test Method, Vol. 1997 edition” p.157).

[Sludge dewatering test]
<Examples 1-6>
Using each polymer shown in Table 1 above, a dehydration test was performed on sludge generated by microbial treatment of waste generated from the production of food for livestock processing according to the following procedure. Example 1 using a sludge having a pH value of 6.5, a TS value of 1.9% by mass, a VTS value of 84.2% by mass, a crude suspended matter value of 1.1% by mass, and a TOC value of 320 mg / L. A sludge dewatering test was conducted using each polymer shown in Table 1 at -6. The results are shown in Table 2. A dehydration test was performed by sequentially performing the following procedures (1) to (7).
(1) 300 mL of the sludge was collected in a 500 mL beaker.
(2) Polymers A to F shown in Table 1 were added so as to have the addition ratio shown in Table 2, and stirred and mixed for 30 seconds at a rotation speed of 200 rpm using a spatula.
(3) Next, the polymer G shown in Table 1 was added so that it might become the addition rate shown in Table 2, and it stirred and mixed for 30 second at the rotation speed of 200 rpm using the spatula.
(4) The aggregated sludge was transferred to a 50 mesh nylon filter cloth, gravity filtered for 10 seconds, and the volume of the filtrate was measured. The results are shown in the column of 10 second filtration amount in Table 2.
(5) Next, one cup of concentrated sludge on the filter cloth was collected with a spatula, and the floc average particle diameter was measured.
(6) Subsequently, the sludge on the nylon filter cloth was rolled up to the left and right and gathered together at the center of the filter cloth to evaluate the concentrated floc strength.
(7) The concentrated sludge on the filter cloth was sandwiched between the filter cloths, and pressed and dehydrated at a pressure of 0.1 MPa for 1 minute, and the moisture content of the dehydrated cake was measured. The results are shown in Table 2. In addition, the addition rate of the inorganic flocculant and the polymer (1) and the polymer (2) is a value indicating the amount added per solid sludge as a percentage by mass. Moreover, the solid substance of the sludge was measured by drying the sludge to a constant weight in a dryer at 110 ° C.

[Sludge dewatering test evaluation items]
・ Flock average particle size: Sludge condensed by sludge dehydration test is filtered with a 50 mesh nylon filter cloth, and one cup of concentrated sludge on the filter cloth is collected with a spatula, and this is filled with a small amount of water. The flocs were transferred to the petri dish and broken into individual flocs so that the flocs were not crushed, the particle size of each floc was measured, and the average value was taken as the floc average particle size.

  -Filtration amount for 10 seconds: The sludge aggregated by the sludge dehydration test was poured onto a 50 mesh nylon filter cloth, filtered for 10 seconds, and the volume of the filtrate was measured.

Concentrated floc strength: Aggregated floc filtered and concentrated by sludge dehydration test was rolled 50 times on the filter cloth, and the strength of the aggregated floc was evaluated according to the following criteria.
(Double-circle): Water rolls off by rolling on a filter cloth, and agglomeration flocs become several dumplings.
◯: Water is cut off by rolling on the filter cloth, and the aggregated flocs become one lump.
(Triangle | delta): Although water cuts by rolling on a filter cloth, an aggregation floc collapses and it does not become a lump shape.
X: By rolling on a filter cloth, the coagulated sludge flows and becomes muddy.

  -Dehydrated cake moisture content: The sludge after concentration floc strength evaluation was sandwiched up and down with a polypropylene filter cloth, attached to a press machine, and press dehydrated at a pressure of 0.1 MPa to obtain a dehydrated cake. The dehydrated cake was evaporated to dryness and further dried by heating at 110 ° C. for 2 hours, and the moisture content was determined from the mass of the evaporated water (edited by Japan Sewerage Association, “Sewerage Test Method, Vol. 1997 edition”, p. 296-297).

<Comparative Examples 1-4>
A dehydration test was carried out under the same conditions as in Examples 1 to 6, except that an amphoteric polymer was added and then a polymer other than a cationic polymer having an amidine structural unit was added. The results are shown in Table 2.

<Comparative Example 5>
A dehydration test was performed under the same conditions as in Examples 1 to 6 except that the mixture was added as a mixture of an amphoteric polymer and a cationic polymer having an amidine structural unit. The results are shown in Table 2.

<Examples 7 to 12>
Using the sludge used in Examples 1 to 6, the sludge dewatering test was conducted using the polymers shown in Table 1. Before adding the polymers A to F shown in Table 1, polyferric sulfate is added as an inorganic flocculant so as to have the addition rate shown in Table 2, and stirred and mixed for 15 seconds at a rotation speed of 200 rpm using a spatula. A dehydration test was performed under the same conditions as in Examples 1 to 6 except that. The results are shown in Table 2.

<Comparative Examples 6-9>
A dehydration test was conducted under the same conditions as in Examples 7 to 12 except that the amphoteric polymer was added and then the polymer combination except that the cationic polymer having an amidine structural unit was added. The results are shown in Table 2.

<Comparative Example 10>
A dehydration test was performed under the same conditions as in Examples 7 to 12 except that the mixture was added as a mixture of an amphoteric polymer and a cationic polymer having an amidine structural unit. The results are shown in Table 2.

  As is clear from Table 2, in Examples 1 to 6, the floc average particle size of the formed floc floc is large, the filtration speed (filtration amount for 10 seconds) is large, the concentrated floc strength is strong, and the water content of the dehydrated cake is also high. The result was low and excellent. Moreover, Examples 7-12 which used polyferric sulfate together with the same sludge were the further excellent results. On the other hand, good results were not obtained in the dehydration tests of Comparative Examples 1 to 4 in which the combination of the polymers was different from the examples. In addition, good results were not obtained in the dehydration tests of Comparative Example 5 and Comparative Example 10 using the polymer D and the polymer G as a mixture.

<Examples 13 to 18>
Sludge generated by microbial treatment of waste generated from sugar raw material extraction and seasoning production (pH value: 6.6, TS value: 3.9% by mass, VTS value: 59.8% by mass, crude suspended matter value) : 9.9 mass%, TOC value: 575 mg / L) Using the polymers shown in Table 1, the same sludge dewatering test as in Examples 1 to 6 was performed. The results are shown in Table 3.

<Comparative Examples 11-14>
A dehydration test was conducted under the same conditions as in Examples 13 to 18 except that an amphoteric polymer was added and then a combination of polymers except that a cationic polymer having an amidine structural unit was added. The results are shown in Table 3.

<Comparative Example 15>
A dehydration test was performed under the same conditions as in Examples 13 to 18 except that the mixture was added as a mixture of an amphoteric polymer and a cationic polymer having an amidine structural unit. The results are shown in Table 3.

As is clear from Table 3, in Examples 13-18, the floc average particle size of the formed floc flocs is large, the filtration speed (filtration amount for 10 seconds) is large, the concentrated floc strength is strong, and the water content of the dehydrated cake is also high. The result was low and excellent. On the other hand, good results were not obtained in the dehydration tests of Comparative Examples 11 to 14 in which the combination of the polymers was different from that of the examples. In addition, the dehydration test of Comparative Example 15 using the polymer D and the polymer G as a mixture did not give good results.
<Examples 19 to 24>
Sludge generated by microbial treatment of waste generated from the sewage treatment process (pH value: 5.8, TS value: 2.3 mass%, VTS value: 86.2 mass%, crude suspended solid value: 18.5 Using the polymers shown in Table 1 for mass%, TOC value: 1320 mg / L), the same sludge dewatering test as in Examples 1 to 6 was performed. The results are shown in Table 4.

<Comparative Examples 16-19>
A dehydration test was carried out under the same conditions as in Examples 19 to 24 except that an amphoteric polymer was added and then a combination of polymers except that a cationic polymer having an amidine structural unit was added. The results are shown in Table 4.

<Comparative Example 20>
A dehydration test was performed under the same conditions as in Examples 19 to 24 except that the mixture was added as a mixture of an amphoteric polymer and a cationic polymer having an amidine structural unit. The results are shown in Table 4.

  As is apparent from Table 4, Examples 19 to 24 have a large floc average particle size of the formed floc flocs, a large filtration rate (filtered amount for 10 seconds), a strong concentrated floc strength, and a low dehydrated cake moisture content. It was a good result. In particular, the result of Example 23 was good. On the other hand, in the dehydration tests of Comparative Examples 16 to 19 having different polymer combinations from the examples, the filtration rate (filtered amount for 10 seconds) is low and the moisture content of the dehydrated cake tends to be high, and good results cannot be obtained. It was. Further, in the dehydration test of Comparative Example 20 using the polymer D and the polymer G as a mixture, the concentrated floc strength was particularly weak, and the moisture content of the dehydrated cake tended to be high, and good results were not obtained.

  The present invention is widely applicable as a method for dewatering sludge generated by microbial treatment of food waste.

Claims (4)

A method for dewatering sludge, wherein an amphoteric polymer is added to sludge generated by treating food waste, and then a cationic polymer having an amidine structural unit is added. The sludge dewatering method according to claim 1, wherein the amidine structural unit is an amidine structural unit represented by the following general formula (1) and / or the following general formula (2).

[In General Formulas (1) and (2), R ^{1 to} R ⁴ are each independently a hydrogen atom or a methyl group. X ⁻ and Y ⁻ are each an anion and may be the same or different. ]
  The method for dewatering sludge according to claim 1 or 2, wherein the amphoteric polymer is a polymer having 1 to 70 mol% of (meth) acrylic acid structural units and / or (meth) acrylate structural units.   The method for dewatering sludge according to any one of claims 1 to 3, wherein an inorganic flocculant is added before the addition of the amphoteric polymer.