JP7489062B2 - Dewatering method for high salt concentration sludge - Google Patents

Description

The present invention relates to a method for dehydrating high-salt sludge using a flocculating agent, and more specifically, to a method for dehydrating sludge in which a flocculating agent of a primary amino group-containing polymer having a specific structure composed of a water-in-oil emulsion is added to high-salt sludge having an electrical conductivity of 1000 mS/m or more.

In treatment plants for municipal sewage and the like, a sludge dehydration method is widely used in which a polyacrylamide (PAM)-based polymer or a polyamidine-based polymer is added as a flocculating agent to organic sludge, such as primary settled raw sludge settled from sewage, excess sludge settled from effluent from an activated sludge tank, or mixed raw sludge, and then the sludge is dehydrated.
However, some sludges have high salt concentrations, i.e., high electrical conductivity, and these general polymer flocculants may not be able to provide sufficient dehydration treatment effects. This is because, in high salt concentration solutions, the molecules of PAM-based polymers roll up like thread balls, and the binding strength of polymer droplets, called polymer bundles, in the cross-linking adsorption action decreases, which suppresses the formation of ionic complexes with anionic dissociation groups on the sludge particle surface, resulting in reduced flocculation properties. It is also known that amidine-based water-soluble polymers with primary amino groups also have reduced flocculation effects at high salt concentrations. For this reason, various flocculation treatment agents have been proposed for organic sludge with high salt concentrations.
For example, Patent Document 1 discloses a technique in which a dehydrating agent, which is a mixture of a polymer obtained by performing reverse phase emulsion polymerization of acryloyloxyethyl dimethylbenzyl ammonium chloride and a monomer containing a bifunctional monomer in the presence of a chain transfer agent, and a hydrophilic surfactant, is applied to organic sludge having an electric conductivity of 1000 mS/m or more. However, there are cases in which stable and efficient dehydration cannot be achieved.
Patent Document 2 discloses the addition of a flocculating treatment agent consisting of two types of agents: a water-in-oil emulsion of a cationic or amphoteric aqueous polymer obtained by emulsion polymerization of a specific monomer mixture aqueous solution as the dispersed phase and a water-immiscible organic liquid as the continuous phase using a surfactant, or a water-in-oil emulsion, to organic sludge with a high salt concentration and an electrical conductivity of 100 mS/m or more. However, it is difficult to achieve the desired effect with sludge with a high salt concentration and an electrical conductivity of 1000 mS/m or more.
Therefore, there is a demand for the development of a flocculating treatment agent that can stably and efficiently dewater sludge even with a high salt concentration of 1000 mS/m or more.

Japanese Patent Application Laid-Open No. 10-277600 JP 2015-100763 A

The objective of this invention is to develop a method for dehydrating sludge that enables efficient dehydration by adding a flocculating agent to organic sludge with high salt concentrations generated during the treatment of sewage, human waste, and industrial wastewater.

In order to solve the above problems, the inventors conducted extensive research and came up with the following invention. That is, the invention is a method for dehydrating sludge in which a primary amino group-containing polymer made of a water-in-oil emulsion is added to organic sludge with a high salt concentration and an electrical conductivity of 1000 mS/m or more.

The flocculating agent of the present invention is particularly effective for organic sludge with high salt concentrations, particularly for sludge with an electrical conductivity of 1000 mS/m or more, and can achieve a better dewatering effect than the addition of conventional flocculating agents.

The primary amino group-containing polymer in the present invention is a primary amino group-containing polymer obtained by polymerizing a monomer mixture essentially containing a cationic monomer represented by the following general formula (1). Examples of the cationic monomer represented by general formula (1) include salts of organic acids or inorganic acids such as 2-aminoethyl acrylate, 2-aminoethyl methacrylate, 3-aminopropyl acrylate, and 3-aminopropyl methacrylate. These monomers can usually only exist in the form of inorganic or organic acid salts, and examples of such salts include sulfates, hydrochlorides, phosphates, oxalates, methanesulfonates, paratoluenesulfonates, naphthalenesulfonates, methylphosphonic acid, and phenylphosphonic acid salts. Of these, sulfates, hydrochlorides, methanesulfonates, and paratoluenesulfonates are preferred. Preferred are 2-aminoethyl acrylate hydrochloride, 2-aminoethyl methacrylate hydrochloride, 2-aminoethyl acrylate sulfate, 2-aminoethyl methacrylate sulfate, 2-aminoethyl acrylate methanesulfonate, 2-aminoethyl methacrylate methanesulfonate, 2-aminoethyl acrylate paratoluenesulfonate, and 2-aminoethyl methacrylate paratoluenesulfonate.
General formula (1)
R ₁ represents hydrogen or a methyl group, A represents an oxygen atom or NH, B represents an alkylene group or an alkoxylene group having 2 to 3 carbon atoms, and X ⁻ represents an anion.

The cationic monomer represented by the general formula (1) may be polymerized alone or may be copolymerized with other monomers. For example, nonionic monomers such as (meth)acrylamide, N,N'-dimethylacrylamide, vinyl acetate, acrylonitrile, methyl acrylate, (meth)acrylate 2-hydroxyethyl, diacetone acrylamide, N-vinylpyrrolidone, N-vinylformamide, and N-vinylacetamide may be mentioned, and it is also possible to combine and copolymerize one or more of the nonionic monomers. The most preferred example of a nonionic monomer is acrylamide. It can also be copolymerized with anionic monomers such as vinylsulfonic acid, styrenesulfonic acid, acrylamido 2-methylpropanesulfonic acid, acrylic acid, methacrylic acid, itaconic acid, and maleic acid. It can also be copolymerized with tertiary amino group- or quaternary ammonium group-containing monomers. Examples of tertiary amino group-containing monomers include dimethylaminoethyl (meth)acrylate and dimethylaminopropyl (meth)acrylamide. Examples of quaternary ammonium group monomers include (meth)acryloyloxyethyl trimethyl ammonium chloride, (meth)acryloyloxy 2-hydroxypropyl trimethyl ammonium chloride, (meth)acryloylaminopropyl trimethyl ammonium chloride, (meth)acryloyloxyethyl dimethyl benzyl ammonium chloride, (meth)acryloyloxy 2-hydroxypropyl dimethyl benzyl ammonium chloride, (meth)acryloylaminopropyl dimethyl benzyl ammonium chloride, etc., which are quaternary products of the tertiary amino group-containing monomers with methyl chloride or benzyl chloride. They can also be copolymerized with diallyl dimethyl ammonium chloride, etc. Terpolymers consisting of these quaternary ammonium group-containing monomers, nonionic monomers, and the cationic monomer represented by the general formula (1) used in the present invention can also be used.

The mole percentage of the primary amino group-containing monomer represented by general formula (1) in these primary amino group-containing polymers is preferably 70 to 100 mole percent, more preferably 80 to 100 mole percent, in order to maximize the effect of the flocculating treatment agent in the present invention. In the case of amphoteric polymers, the mole percentage of the anionic monomer is 1 to 20 mole percent.

The primary amino group-containing polymer is produced by a water-in-oil emulsion polymerization method. For example, first, an aqueous solution containing the cationic monomer represented by the general formula (1) or, in the case of copolymerization, the monomer to be copolymerized is prepared, and the pH is adjusted to 2.0 to 6.0. After that, oxygen is removed from the reaction system by nitrogen substitution, and the polymer is produced by a conventional water-in-oil emulsion polymerization method. Polymerization is started by adding a radical polymerization initiator, and the polymer can be produced. Therefore, the polymerization concentration can be in the range of 5 to 60% by mass, and preferably 20 to 50% by mass. The reaction temperature can be in the range of 10 to 100°C.

The polymerization mechanism is a general radical polymerization using a radical polymerization initiator to produce a polymer. In other words, the polymerization can be carried out using any initiator, including azo, peroxide, and redox. Examples of oil-soluble azo initiators include 2,2'-azobisisobutyronitrile, 1,1'-azobis(cyclohexanecarbonitrile), 2,2'-azobis(2-methylbutyronitrile), and 2,2'-azobis(2-methylpropionate), which are dissolved in a water-miscible solvent and added. Examples of water-soluble azo initiators include 2,2'-azobis(amidinopropane) dihydrochloride, 2,2'-azobis[2-(5-methyl-2-imidazolin-2-yl)propane] dihydrochloride, and 4,4'-azobis(4-cyanovaleric acid). Examples of redox initiators include combinations of ammonium or potassium peroxydisulfate with sodium sulfite, sodium hydrogensulfite, trimethylamine, tetramethylethylenediamine, etc. Examples of peroxides include ammonium peroxodisulfate, hydrogen peroxide, benzoyl peroxide, lauroyl peroxide, octanoyl peroxide, succinic peroxide, t-butylperoxy 2-ethylhexanoate, etc. Among these, particularly preferred initiators are water-soluble azo initiators such as 2,2'-azobis(amidinopropane) dihydrochloride and 2,2'-azobis[2-(5-methyl-2-imidazolin-2-yl)propane] dihydrochloride.

A structural modifier, i.e., a crosslinking monomer that structurally modifies the polymer, may be used during polymerization. The crosslinking monomer is present in a range of 0.5 to 200 ppm by mass relative to the total amount of monomers. Examples of crosslinking monomers include N,N'-methylenebis(meth)acrylamide, triallylamine, ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, polyethylene glycol di(meth)acrylate, N-vinyl(meth)acrylamide, N-methylallylacrylamide, glycidyl acrylate, polyethylene glycol diglycidyl ether, acrolein, glyoxal, and vinyltrimethoxysilane. In this case, the crosslinking agent is more preferably a water-soluble polyvinyl compound, and most preferably N,N'-methylenebis(meth)acrylamide. In addition, the use of chain transfer agents such as sodium formate, isopropyl alcohol, and sodium methallylsulfonate is also an effective method for adjusting crosslinking. The addition rate is 0.001 to 1.0% by mass, preferably 0.01 to 0.1% by mass, based on the total amount of monomers.

The weight average molecular weight of the primary amino group-containing polymer in the present invention is 1 million to 6 million. If it is less than 1 million, the flocculation performance is insufficient, and if it is more than 6 million, the solution viscosity becomes too high and dispersibility decreases, which is not preferable. 2 million to 6 million is more preferable, and 2.5 million to 6 million is even more preferable. When completely dissolved in 4% by mass saline solution so that the polymer concentration becomes 0.5% by mass, the viscosity of the salt solution measured with a rotational viscometer at 25°C is in the range of 5 mPa·s to 70 mPa·s. 8 mPa·s to 70 mPa·s are preferable, and 10 mPa·s to 70 mPa·s are even more preferable.

In the present invention, the primary amino group-containing polymer is preferably such that the ratio of AQV/SLV, which is the viscosity of a 0.2% by weight aqueous solution at 25°C, and SLV, which is the viscosity of 0.5% by weight of the water-soluble polymer in a 4% by weight saline solution, is 10 or more. This value can be used to express the degree of crosslinking. In cases where branching has progressed or crosslinked ionic water-soluble polymers are crosslinked within the molecule, they have the property of being difficult to spread even in water, and should spread less in water compared to linear polymers, but as the degree of crosslinking increases, the viscosity increases when measured with a B-type viscometer (a type of rotational viscometer). The cause of this is presumed to be friction or entanglement between the rotor (rotor during measurement) of the B-type viscometer and the solution, but the exact cause is unknown. On the other hand, the viscosity of crosslinked ionic water-soluble polymers in salt water decreases as the degree of crosslinking increases. Since the molecules are contracted by crosslinking, it is thought that they are more significantly affected by the large amount of ions in salt water. For these reasons, the ratio of the two viscosity measurements, AQV/SLV, increases as the degree of crosslinking increases (this relationship does not hold if the crosslinking progresses further and the material becomes water insoluble). In the flocculation treatment agent of the present invention, this value is preferably in the range of 10 or more and less than 30. For linear water-soluble polymers, this value tends to be less than 10, and for water-soluble polymers with a high degree of crosslinking, it tends to be 30 or more. Note that AQV is a value measured with a Brookfield viscometer using a No. 2 rotor at 30 rpm (25°C), and SLV is a value measured with a No. 1 rotor at 60 rpm (25°C). For the Brookfield viscometer, a Tokyo Keiki B8M or the like is used.

The primary amino group-containing polymer of the present invention is applied to organic sludge with a high salt concentration having an electrical conductivity of 1000 mS/m or more. Electrical conductivity (JIS K0102) is generally used as an indicator of high salt concentration, but the effect of PAM-based polymer flocculants decreases in sludge with high electrical conductivity. On the other hand, the primary amino group-containing polymer of the present invention is effective even in sludge with an electrical conductivity of 1000 mS/m or more. The higher the electrical conductivity, the more the effect difference appears compared to conventional flocculants, so it is preferably 2000 mS/m or more. However, depending on the sludge properties, not only the electrical conductivity value but also the amount of anionic substances in the sludge (anionic degree) may inhibit the effect of the flocculant. Therefore, the flocculant of the present invention is preferably applied to sludge with an anionic degree of 0.1 meq/g (mass% relative to sludge substance suspended concentration; SS) or more. More preferably, it is 0.15 meq/g or more, and even more preferably, it is 0.2 meq/g or more. As a result of extensive research by the present applicant, it has been found that in sludge having an electrical conductivity of 1000 mS/m or more and an anionic degree of 0.1 meq/g or more, the difference in effectiveness of the coagulation treatment agent of the present invention compared to commonly used polyacrylamide-based water-soluble polymers and amidine-based water-soluble polymers becomes greater, making it preferable.
However, it is not preferable that the M-alkalinity, which is generally used as an index of sludge putrefaction and fermentation, is 1000 mg/L or more, since the effect of the coagulation treatment agent will be poor. Note that M-alkalinity is a value measured by the amount of hydrochloric acid consumed by alkaline components such as carbonates, hydrogen carbonates, or hydroxides contained in sludge, and is expressed by converting the amount of hydrochloric acid required to neutralize the sludge to pH 4.8 into the concentration (mg/L) of calcium carbonate (CaCO ₃ ) equivalent to this.

In the present invention, the anionic degree of sludge is measured according to the following colloid titration method.
(Measurement of anionic degree of sludge by colloid titration method)
(1) Collect 5 mL of sludge in a 300 mL beaker and add 175 mL of pure water.
(2) Add 20 mL of 1/400 N p-DADMAC (polydiallyldimethylammonium chloride) solution to (1) and stir at 500 rpm for 5 minutes.
(3) (2) is transferred to a centrifuge tube and centrifuged at 3,000 rpm for 10 minutes.
(4) Place 100 mL of the supernatant from (3) in a conical beaker, add a few drops of 0.1% toluidine blue solution, and slowly (at about 3 to 5 mL/min) add 1/400 N PVSK (potassium polyvinyl sulfate) solution dropwise while stirring.
(5) The endpoint of the titration is the point at which the color changes from blue to reddish purple (the point at which the color does not fade for several minutes), and this titer is designated as a (mL).
(6) Similarly, perform a blank test using distilled water, and record the titration amount as b (mL).
(7) Calculate the anionic degree using the following formula.
Anionic degree (meq/g)=-f(b-a)/(10×SS)
*f is the potency (factor) of the PVSK solution, and SS is the suspended solids concentration of the sludge (mass%).

The improvement of the dewaterability of the primary amino group-containing polymer of the present invention allows it to exert a good flocculating force even on sludge with a high salt concentration. In the case of commonly used acrylic polymer flocculants containing quaternary ammonium salts, in a high salt concentration solution, the cationic groups are neutralized by the counter ions in the salts, causing the polymer chains to become randomly coiled, resulting in poor contact with the sludge particles and reduced effectiveness. On the other hand, the primary amines contained in the primary amino group-containing polymer of the present invention have high hydrogen bonding strength and high adsorption to hydrophilic fine particles.
Furthermore, it is speculated that by optimizing the high cationicity, molecular weight adjustment, and polymer structure, it has become possible to achieve efficient dewatering treatment even for sludge with high salt concentrations.

The primary amino group-containing polymer of the present invention is dissolved in water and added to the target substance. The water in which the primary amino group-containing polymer is dissolved can be distilled water, ion-exchanged water, tap water, industrial water, etc. A mixture of these is also acceptable. The primary amino group-containing polymer of the present invention is dissolved in 0.01 to 1.0% by mass and added to the target substance. It is also acceptable to further dilute it a second time or a third time with water.

The sludge applicable as the primary amino group-containing polymer in the present invention is excess sludge generated during biological treatment of wastewater from papermaking, chemical industry, food industry, etc., or organic sludge generated during treatment of urban sewage, human waste, and industrial wastewater (so-called raw sludge, excess sludge, mixed raw sludge, digested sludge, precipitated/floated sludge, and mixtures thereof), and is diluted with water to any concentration and added to these sludges. The addition rate to the sludge varies depending on the type of sludge and the dehydration model, but is 1 to 1000 ppm relative to the amount of sludge liquid. The type of dehydrator used can be a belt press, centrifugal dehydrator, screw press, multiple disk type dehydrator, rotary press, filter press, etc. It may also be used in combination with inorganic coagulants such as ferric chloride, ferric sulfate, polyferric sulfate, PAC, and aluminum sulfate.

The present invention will be specifically explained below using examples, but the present invention is not limited to the following examples.

As the primary amino group-containing polymer of the present invention, Samples A and B were prepared by the usual method of water-in-oil emulsion polymerization. Their compositions and physical properties are shown in Table 1. In addition, polyacrylamide-based water-soluble polymer Samples 1 to 8 and polyamidine-based water-soluble polymer Sample 9, which are widely used as flocculation treatment agents, were prepared and prepared. Their compositions and physical properties are shown in Table 2.

形態：ＥＭ；油中水型エマルジョン
０．２質量％水溶液粘度（ｍＰａ・ｓ）；０．２質量％高分子水溶液をＢ型粘度計により測定（２５℃）。
０．５質量％塩水溶液粘度（ｍＰａ・ｓ）；０．５質量％高分子水溶液に４質量％塩化ナトリウムを添加、完全溶解後にＢ型粘度計により測定（２５℃）。
ＡＱＶ；０．２質量％水溶液粘度（ｍＰａ・ｓ）
ＳＬＶ；０．５質量％塩水溶液粘度（ｍＰａ・ｓ）
ＡＱＶ／ＳＬＶ；無次元 (Table 1)

Form: EM; water-in-oil emulsion Viscosity of 0.2% by mass aqueous solution (mPa·s); 0.2% by mass aqueous polymer solution was measured using a B-type viscometer (25° C.).
Viscosity of 0.5% by mass salt solution (mPa·s): 4% by mass sodium chloride was added to a 0.5% by mass polymer aqueous solution, and after complete dissolution, the viscosity was measured using a B-type viscometer (25° C.).
AQV: Viscosity of 0.2% by mass aqueous solution (mPa s)
SLV: Viscosity of 0.5% by mass salt solution (mPa·s)
AQV/SLV; dimensionless

ＤＭＱ；アクリロイルオキシエチルトリメチルアンモニウム塩化物
ＤＭＢＺ；アクリロイルオキシエチルジメチルベンジルアンモニウム塩化物
ＡＡＭ；アクリルアミド、ＡＡＣ；アクリル酸
形態：ＥＭ；油中水型エマルジョン、ＤＲ；塩水中分散液、Ｐ；粉末
０．２質量％水溶液粘度（ｍＰａ・ｓ）；０．２質量％高分子水溶液をＢ型粘度計により測定（２５℃）。
０．５質量％塩水溶液粘度（ｍＰａ・ｓ）；０．５質量％高分子水溶液に４質量％塩化ナトリウムを添加、完全溶解後にＢ型粘度計により測定（２５℃）。
ＡＱＶ；０．２質量％水溶液粘度（ｍＰａ・ｓ）
ＳＬＶ；０．５質量％塩水溶液粘度（ｍＰａ・ｓ）
ＡＱＶ／ＳＬＶ；無次元 (Table 2)

DMQ: acryloyloxyethyl trimethylammonium chloride DMBZ: acryloyloxyethyl dimethylbenzylammonium chloride AAM: acrylamide, AAC: acrylic acid Form: EM: water-in-oil emulsion, DR: dispersion in salt water, P: powder Viscosity of 0.2% by mass aqueous solution (mPa·s): 0.2% by mass aqueous polymer solution measured with a B-type viscometer (25°C).
Viscosity of 0.5% by mass salt solution (mPa·s): 4% by mass sodium chloride was added to a 0.5% by mass polymer aqueous solution, and after complete dissolution, the viscosity was measured using a B-type viscometer (25° C.).
AQV: Viscosity of 0.2% by mass aqueous solution (mPa s)
SLV: Viscosity of 0.5% by mass salt solution (mPa·s)
AQV/SLV; dimensionless

(Example 1) A sludge dehydration test was carried out using food surplus sludge (pH 6.5, SS content 15000 mg/L, electrical conductivity 2820 mS/m, VSS 66.7 mass%, VTS 29.6 mass%, M-alkalinity 249 mg/L, anion degree 0.8 meq/g). 200 mL of sludge was collected in a polybeaker, and 200 ppm or 250 ppm of the sludge liquid volume of the 0.2 mass% water solution of sample A in Table 1 was added, and the mixture was stirred for 30 seconds at a stirring speed of 1000 rpm in a CST measuring device, filtered through a nylon filter cloth (#202), and the amount of the filtrate after 60 seconds was measured. After the measurement, the sludge filtered for 60 seconds was dehydrated for 30 seconds at a press pressure of 3 kg/cm ² , and the cake moisture content (dried at 105 ° C for 20 hours) was measured. The results are shown in Table 3.

(Comparative Example 1) Using the same sludge as in Example 1, a similar sludge dewatering test was carried out using the coagulation treatment agent sample in Table 2 above. The results are shown in Table 3.

(Table 3)

When the primary amino group-containing polymer of the present invention was added, the dewatering effect was good. The same test was also conducted for sample B, and the same effect as sample A was obtained. On the other hand, in the comparative examples, most of the common sludge dewatering agents had poor coagulation and did not provide a good dewatering effect.

(Example 2) A sludge dehydration test was carried out using chemical excess sludge (pH 6.9, SS content 9872 mg/L, electrical conductivity 2340 mS/m, VSS 50.2 mass%, VTS 30.7 mass%, M-alkalinity 169 mg/L, anion degree 0.4 meq/g). 200 mL of sludge was collected in a polybeaker, and 150 ppm of the sludge liquid volume of 0.2 mass% water solution of sample A in Table 1 was added, and the mixture was stirred for 30 seconds at a stirring speed of 1000 rpm in a CST measuring device, filtered with a nylon filter cloth (#202), and the amount of filtrate after 60 seconds was measured. After the measurement, the sludge filtered for 60 seconds was dehydrated for 30 seconds at a press pressure of 3 kg/cm ² , and the cake moisture content (dried at 105 ° C for 20 hours) was measured. The results are shown in Table 4.

(Comparative Example 2) Using the same sludge as in Example 2, a similar sludge dewatering test was carried out using the flocculating treatment agent samples in Table 2. The flocculating treatment agent samples 8 and 9 in Table 2 were mixed at a mass ratio of 1:1 to obtain 0.2% aqueous solutions. The results are shown in Table 4.

Table 4

When the primary amino group-containing polymer of the present invention was added, the dehydration effect was better than that of the comparative example. The same test was also carried out for sample B, and the same effect as that of sample A was obtained.
In the examples of the present invention, the cake moisture content decreased, and the effect of the primary amino group-containing polymer of the present invention on high-salt sludge having an electric conductivity of 1000 mS/m or more was confirmed.

Claims (4)

A method for dewatering sludge, comprising the steps of: adding a primary amino group-containing polymer comprising a water-in-oil emulsion obtained by emulsion polymerization of a monomer or a monomer mixture containing one or more selected from organic acid and inorganic acid salts of 2-aminoethyl acrylate, 2-aminoethyl methacrylate, 3-aminopropyl acrylate, and 3-aminopropyl methacrylate as a cationic monomer represented by the following general formula (1), to high-salinity sludge having an electric conductivity of 1000 mS/m or more, thereby carrying out a flocculation treatment.
General formula (1)
R ₁ represents hydrogen or a methyl group, A represents an oxygen atom or NH, B represents an alkylene group or an alkoxylene group having 2 to 3 carbon atoms, and X ⁻ represents an anion. The sludge dewatering method according to claim 1, characterized in that the primary amino group-containing polymer is a water-in-oil emulsion obtained by emulsion polymerization of a monomer mixture containing 70 to 100 mol % of the cationic monomer represented by the general formula (1). 2. The method for dewatering sludge according to claim 1, wherein the anionic degree per gram of suspended solids in the sludge is 0.1 meq/ g or more . The viscosity of a 0.2% by mass aqueous solution of the primary amino group-containing polymer at 25° C. is defined as AQV, and the viscosity of a 0.5% by mass solution of the polymer in 4% by mass saline is defined as SLV. The ratio of AQV to SLV is 10≦AQV/SLV<30.
2. The method for dewatering sludge according to claim 1,