JPS61181600A - Aid for dehydrating sludge and its method - Google Patents

Abstract

PURPOSE:To increase remarkably the dehydration performance of sluge by adding both an organic fibrous substance wherein the bulk density is increased in the specified density and above by pressurizing and molding it and a high molecular flocculant to the sludge and regulating the flock diameter to the specified value and below. CONSTITUTION:An organic fibrous substance (e.g. crushed material of paper) A wherein the bulk density is increased in >=0.3g/cm<3> density by pressurizing and molding it and a high molecular floacculant (e.g. chitosan) are added to sludge and thereafter the diameter of the obtained flock is regulated to <=5mm and it is dehydrated. As the amount added to the sludge, 0.5-200wt% substance A and 0.05-3wt% flocculant B are ordinarily added to the solid material of sludge. Furthermore as a regulating method of the flock diameter, the flock which is once formed by the stirring or the like is strongly stirred and thereby it is destroyed to <=5mm flock diameter. By the above method, the dehydration performance (the decrease of water content of dehydrated cake and the increase of the release characteristics from a filter cloth) is remarkably increased.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to sludge dewatering aids and methods.

Inorganic flocculants such as ferric chloride, slaked lime, or polymer flocculants are used as dehydration aids to dehydrate sludge.The method of using inorganic flocculants requires a large amount of fuel to incinerate the dehydrated cake. There are problems such as high incineration ash content, -
The blade polymer flocculant has problems such as not being able to sufficiently lower the moisture content of the dehydrated cake and not having good releasability from the cloth, and none of the methods are necessarily satisfactory. There is also a method of simply mixing or using a polymer coagulant and fibers, etc., but in this case as well, 1) the moisture content of the dehydrated cake and the releasability from the P cloth are insufficient, and 1) the fibers are bulky, so storage , there were problems such as requiring large equipment for transportation, time and effort to use, and inability to obtain stable results.

The present inventors have made extensive studies aimed at solving the above-mentioned problems, and as a result, have arrived at the present invention. That is, the present invention is pressure-molded to a density of 0.3 g/cn! A sludge dewatering aid (first invention) made by using an organic fibrous material 4 whose bulk specific gravity has been increased in combination with a polymer flocculant (B) and a density o, ag of 77 or more by pressure molding. A method for dewatering sludge, which comprises adding an organic fibrous substance (A) with increased bulk specific gravity and a polymer flocculant (B) to sludge, and then adjusting the obtained floc diameter to 5 mm or less and dewatering. (Second invention).

In the present invention, the organic fibrous material is natural fiber (
Vegetable fibers include cellulose-based fibers such as paper, cotton, straw, sawdust, grass charcoal, and pulp; animal fibers include silk, wool, etc.), and man-made fibers (cellulose-based fibers such as rayon).

acetate, etc.) 9 synthetic fibers (polyamide, polyester, acrylic, etc.) and combination systems of two or more of these. Among these, preferred are fibers or vegetable fibrous substances that can become fibrous in water (hereinafter, organic fibrous substances are simply referred to as fibers), such as paper 2 crushed paper,
Cotton, pulp.

Such as grass charcoal. Among these, crushed paper is particularly preferred6.
Small pieces of unpulverized paper (for example, 1 to 50 m
In addition to pulverized paper, other fibers can be used in combination. The ratio of pulverized paper to other fibers is usually 100:0 to 1:99.
, preferably 100:0 to 50:50.

The fibers can be in the form of powder, such as finely pulverized or ground fibers, and fibers, such as cut single fibers (usually with a thickness of 100 denier or less), or bundles of multiple fibers. Examples include those treated with a suitable sizing agent and cut, other woven fabrics, non-woven two-knit fabrics, sheets (for example, paper), etc., cut or loosened. The length of the fiber is not particularly limited, but is usually 0.01 to 50 mm, preferably 0.01 to 50 mm.
01-5 Tomo.

The fibers are pressure-molded to a density o of 5g7cnt or more. In consideration of economical transportation and storage of this pressure-molded product, the density is preferably 0.5 g77 or more, more preferably 0.7 glcrd or more. Density is 0
．． If it is less than 3g/7, when it is added to sludge in combination with (B) for dehydration, the water content of the dehydrated cake will not be sufficiently reduced, and the cake will not be easily peeled off from the F cloth.

Pressure molding methods include pressure molding the fibers into pellets in a mold at room temperature, and press molding them into sheet, rod, or block shapes at room temperature, and then cutting or cutting them into appropriate sizes. One method is to crush it. In the above, the pressurization may be carried out under heating (for example, 20 to 150° C.) and humidification (for example, 60 to 100 degrees Celsius).

A preferred method of pressure molding the fibers is to partially reduce the density of the fibers to 0.
2g/crd or less, particularly preferably 0.01 to 0.1
In this method, the fibers are crushed to 59/C1d and pressure-molded in a dry state (moisture content is 20% by weight or less, preferably 15% by weight or less based on the fibers). Moisture content during pressure molding is 20% by weight
If the dehydration rate is exceeded, the fibers will become so entangled that the desired dehydration effect cannot be obtained.

The pressure during pressure molding is such that the density of the obtained pressure molded product is o, 3
The pressure can be selected as long as it is at least g/crd, and can be selected appropriately depending on the type, form, properties, etc. of the fiber, but it is usually 1 to 8000 kg/7, preferably 100 to 2.
000 kg/crd. Pressurization and molding can be carried out using, for example, a roll press machine, a hydraulic half-plate press machine, a screw press machine, or the like. Examples of roll press machines include compacting machines with corrugated rolls, calender machines, and briquette machines.

The shape of the obtained pressure-molded product may be arbitrary, such as a spherical shape, a cylindrical shape, a cubic shape, a rectangular parallelepiped shape, a conical shape, a pyramid shape, a rod shape, a sheet shape, and a roll shape of a rolled sheet. can give. The size is such that the shortest diameter of the pressure-molded product is usually 10C1rL or less, preferably 8cm or less. If the shortest diameter is longer than 10CrfL, it will take time to disperse the dehydration aid when added to sludge, making it difficult to obtain a sufficient dewatering effect in a short period of time. The longest diameter is not particularly limited, but is preferably 1 m or less and 1 mm or more (
In particular, the diameter is 2 mm or more).

The press-formed product thus obtained is added to the sludge and 1
Used as a dewatering aid when dewatering sludge.

As the polymer flocculant (B), cationic, anionic and nonionic polymer flocculants can be used, with cationic ones being preferred.

Suitable cationic polymeric flocculants include polymers and copolymers of at least one cationic monomer and at least one other ethylenically unsaturated monomer.

Examples of cationic monomers include cationic American monomers such as tertiary nitrogen-containing acrylates, tertiary nitrogen-containing acrylamides, quaternary nitrogen-containing acrylates, quaternary nitrogen-containing acrylamides; containing heterocycles or aromatic rings. Vinyl monomers such as vinylimidazolines, vinylbenzyl quaternary ammonium salts, and allylamines are mentioned.

The cationic acrylic monomer has the general formula (wherein A
is an oxygen atom; NHLB is an alkylene group having 1 to 4 carbon atoms, a hydroxyalkylene group having 2 to 4 carbon atoms, or a phenylene group; R4 is H
, an alkyl group or an aralkyl group 1xe indicates a counter anion).Specifically, dialkylaminoalkyl (meth)acrylates (dimethylamine ethyl (meth)acrylate,
Diethylaminoethyl (metallolylate, 3-dimethylamine-2-hydroquinepropyl (metallolylate, etc.), dialkylaminoalkyl (meth)acrylamides (dimethylaminoethyl (meth)acrylamide, 3
Inorganic acid salts (hydrochloride, sulfate, etc.) such as -dimethylamino-2-hydroxypropylacrylamide.

), organic acid salts (acetate, etc.) and quaternizing agents (methyl chloride, dimethyl sulfate, benzyl chloride, etc.).
Examples include grade ammonium salts.

Examples of vinylimidazolines include N-vinylimidazoline and its tertiary and quaternary salts; examples of vinylbenzyl quaternary ammonium salts include vinylbenzyl triC1-4 alkylammonium salts such as those described in British Patent No. 862.719; allylamine; Examples include (meth)allylamine (representing allylamine and methalylamine. The same expressions will be used hereinafter) and their mono- and di-C1-4 alkyl substituted products, and di(meth)allylamine.
Examples include allylamine, alkyl substituted products thereof, and quaternized products of these alkyl substituted products.

Other ethylenically unsaturated monomers that may be copolymerized with the cationic monomer if necessary include (meth)acrylamide, N
- Mono- or N, N-di C1-4 alkyl (meth)acrylamide, (meth)acrylonitrile + CI4 alkyl (meth)acrylate, styrene, vinyl esters (vinyl acetate).

Vinyl ethers (such as vinyl methyl ether).

Nonionic monomers such as vinyl halides (vinyl chloride, etc.); and unsaturated carboxylic acids [(meth)acrylic acid, maleic acid, etc.], unsaturated sulfonic acids [vinyl sulfonic acid, (meth)allylsulfonic acid, etc.] Aliphatic sulfonic acids: pinyltoluenesulfonic acid, styrenesulfonic acid, tonoaromatic vinylsulfonic acid; sulfoC2-4 alkyl (meth)acrylate, 2-hydroxy-3-(
Sulfo(meth)acrylates such as meth)acryloxypropanesulfonic acid: (meth)acrylamide C2-
4-alkanesulfonic acid, a-(meth)acrylamide-
sulfo(meth)acrylamide of sodium 2-hydroxypropanesulfonate] and their salts (alkali metal salts such as Na, ammonium salts, mono-, di- and triethanolamines, alkyl (C1-4)
Examples include anionic monomers such as amine salts such as amines.

The proportion of cationic monomer is usually at least 5 parts, preferably at least 10 parts, and particularly preferably at least 30 parts, based on the weight of total monomers. The proportion of (meth)acrylamide is usually 0 to 95%, preferably 0 to 70%. The proportion of anionic and hydrophobic monomers is usually 20
It is less than 10 inches, preferably less than 10 inches.

Specific examples of polymers and copolymers of such cationic monomers and optionally other monomers are described in U.S. Pat. Nos. 4,152,807 and 3,259,570. I can give you what is being said.

Other examples of cationic polymer flocculants include cationically modified polyacrylamide and chitosan.

Mention may be made of polyethyleneimine, shrimp halohydrin-amine condensate and cationized starch.

Cationically modified polyacrylamides include Mannich modifications of polyacrylamide (such as those described in U.S. Pat. No. 2,828,901) and Offmann modifications. Examples of polyethyleneimine include polyethyleneimine having a degree of polymerization of 400 or more, preferably 2000 or more, its N-methyl substituted products, and their octagrade and quaternary salts (for example, U.S. Pat. No. 3,259,5
(described in Specification No. 70). As an epihalohydrin-amine condensate, epichlorohydrin and 0
Examples include polycondensates of 2-6 with alkylene diamines. As cationized starch, Encyclopedia
Op Chemical Technology (2nd edition), Volume 18, 6
Examples include cationic starches described on pages 88-689.

Chitosan includes deacetylated chitin, which is found in the shells of crabs, shrimp, insects, etc., and is heated and hydrolyzed in concentrated alkali.

Furthermore, among the above cationic polymer flocculants, the intrinsic viscosity [η] dl/i (hereinafter expressed as [η]) and colloid 1 (, equivalent value Cmeq/& (hereinafter expressed as C) are expressed by the formula % Formula % (2) (wherein C)0.Cη)>0 and [η] is l at 80°C
(measured in an N-NaNOs aqueous solution) is preferable; if [η] is outside the above range, a sufficient dehydration effect will not be obtained when the sludge is added and mixed with the sludge for dehydration.

Examples of the anionic polymer flocculant and nonionic polymer flocculant include the aforementioned nonionic monomers [(meth)acrylamide, etc.], anionic monomers [(meth)acrylic acid,
(meth)acrylamide C2-4 alkanesulfonic acid,
vinyl sulfonic acid, etc. and their salts] or polymers and copolymers of two or more thereof, and anionic modified products thereof such as polyacrylamide and its hydrolyzate; polyethylene oxide; naturally occurring nonionic and anionic polymer flocculants For example, sodium alginate,
Guar gum, starch, carboxymethylcellulose
Examples include sodium salts.

Among the polymer flocculants, preferred are cationic polymer flocculants. Among the cationic polymer flocculants, preferred are 8th class nitrogen-containing (meth)acrylates or (
Salt (co)polymer of meth)acrylamide and acid and 4
nitrogen-containing (meth)acrylate or (meth)acrylamide (co)polymer, Mannich modified product of polyacrylamide.

Polyvinylimidazoline, chitosan and polydiallylamine are particularly preferred, and methachloroyloxyethyltrimethylammonium chloride polymer is particularly preferred.

In the present invention, the polymer flocculant (B) is preferably in powder form, but an aqueous solution or emulsion type flocculant (concentration usually 2% by weight or more, preferably 20% by weight or more) can also be used.

In the present invention, the fibers are used after being press-molded to increase the bulk specific gravity to a density of 0.397 d or more.

The density of the fiber before pressure molding is usually 0.2 fI/crA
Hereinafter, the bulk specific gravity is preferably increased by 1.5 times or more, preferably 2 to 800 times by pressure molding.

According to the invention, pressure molded to a density o, 3 g/crd
When adding the organic fibrous material (7!) and the polymer flocculant (B) that have increased the bulk specific gravity to the sludge,
B) may be added separately in any order. (A) and (
B) can be added at the same time, or (B) can be added after mixing 4 with the sludge.
) is preferably added.

Furthermore, when two or more types of (B) (for example, a cationic flocculant and an anionic flocculant) are used together, they can be used separately in any order (cation followed by anion).

or vice versa).

In the present invention, the dehydration aids may optionally contain other flocculants (basic aluminum chloride, ferrous and ferric sulfate, ferric chloride, aluminum sulfate).

sodium aluminate, inorganic flocculants such as activated silicic acid), known filter aids (slaked lime, pulverized coal, bentonite,
Diatomaceous earth, kaolin, celite.

activated clay, etc.), surfactants, inorganic salts, deodorants.

Antifoaming agents and the like can also be used together. Known anionic, cationic, amphoteric and nonionic surfactants can be used as the surfactant, but cationic and nonionic surfactants are preferred, such as primary to tertiary salts of island long chain alkyl amines or quaternary salts. A cationic surfactant such as a class ammonium salt is more preferred.The inorganic salt may be any arbitrary salt and includes, without particular limitation, common salt, sulfur salt, ammonium sulfate, and the like.

The amount of these added is usually 200% or less, preferably 100% or less, based on the total weight of fibers and flocculant. These may be added in advance by mixing with the auxiliary agent of the present invention, or may be added separately in any order.

The amount of the dewatering aid added to the sludge in the present invention is usually 0.5 to 200% by weight, preferably 1 to 100% by weight, particularly preferably 2 to 50% by weight, based on the solid content of the sludge. (B) is usually 0.05 to 8% by weight, preferably 0.
．． It is 1 to 2% by weight. If the amount added is less than 0.5% by weight, the dehydration effect will be insufficient, and if it exceeds 200%, it will become uneconomical.

Methods for adding the dewatering aid to sludge include a method in which it is added directly to the sludge, and a method in which the dewatering aid is mixed and dispersed with water to form a slurry and then added to the sludge. It is also possible to add inorganic salts, surfactants, etc. before or at the same time as adding the dewatering aid to the sludge (to further speed up the dispersion of the sludge).

In the present invention, a dewatering aid is added to the sludge, and then the floc diameter is adjusted to 51 tit or less. The floc diameter can be changed depending on the type of dehydrator. When a vacuum dehydrator or a filter press is used as the dehydrator, the floc diameter is adjusted to 1 mm or less, preferably 0.1 to 0.5 mm.

If the floc diameter is larger than the above, the cake content after dehydration will not be sufficiently reduced, and the amount of cake adhering to the F cloth will be small.
The objective of the present invention cannot be achieved because the sludge treatment speed is low.

One method for adjusting the floc diameter is to add and mix the dewatering aid directly to the sludge and mix it with normal stirring or gentle stirring (for example, 8
0 to less than 800 rpm for 10 to 1,000 seconds) to form flocs, and then the once formed flocs are stirred vigorously (e.g., 800 rpm or more, for 10 to 1,000 seconds) to form flocs.
(seconds) to break the flocs to a diameter of 5 mm or less (1 mm or less in the case of a vacuum dehydrator or filter press, the same applies hereinafter). Stirring (usually stirring).

The method of stirring (gentle stirring, 2-strong stirring, etc.) may be any method (method using a two-blade stirrer, a stirrer equipped with a chi-shaped stirring rod, a Satake type stirrer, etc.), and is not particularly limited. By doing so, the moisture content of the cake after dewatering is further reduced, the amount of cake adhering to the furnace cloth is increased, and the sludge treatment speed is increased. ) Also, the peelability of the cake from the oven cloth becomes better. The floc diameter can be adjusted by changing the stirring intensity and time according to the target sludge. Another way to adjust the floc diameter is to set the floc diameter to 5 mm or less (1 m
In order to generate flocs of less than 100 m (200 m), take the type of sludge in question and adjust the composition of the dewatering aid, the amount added, or the stirring conditions (stirring speed and stirring time) after adding it to the sludge. You can also do it.

After adjusting the floc diameter to 5 mm or less (1 mm or less), dehydration is performed. Dehydration is performed using a vacuum dehydrator, a pressure dehydrator, a centrifugal dehydrator, or the like.

Examples of vacuum dehydrators that can be used include Oliver filters, belt filters, drum filters, and Frecote filters.

The pressurized dehydrator may be any device that has a mechanism capable of dehydrating under pressure, such as a filter press, belt press, or capillary roll press.

The product may be dehydrated or lightly pre-dehydrated using an appropriate dehydrator and then subjected to the dehydrator.

The dehydrated cake is incinerated or the like by a known method. It is also extremely easy to convert it into fuel and compost (fertilizer).

When adding a dewatering aid to sludge, the target sludge may be any sludge, such as raw sludge such as sewage, human waste, and industrial wastewater, activated sludge, digested sludge, coagulated sedimentation sludge, or mixed sludge made of a combination thereof. However, sludge (activated sludge) is obtained through microbial treatment.

It is immediately effective for difficult-to-filter sludge containing digested sludge, etc., where the organic content in the sludge is 40% by weight or more based on the solid content, and the organic fiber content in the sludge is 20% by weight or less based on the solid content. be.

According to the present invention, an organic fibrous material (A) whose bulk specific gravity has been increased to a density o, sg/crd or higher by pressure molding and a polymer flocculant (B) are added to sludge) and a floc diameter of 5
By adjusting it to ml or less (1 dragon or less), the dehydration performance is greatly improved (reduction in the water content of the dehydrated cake and improvement in releasability from the furnace cloth).

Conventionally, sludge is treated with a flocculant and dehydrated using a vacuum filter or filter press, and an inorganic flocculant is used.In this method, if an organic polymer flocculant is used, the furnace The releasability from cloth was poor and the moisture content of the cake was not lowered sufficiently, so that it could not be put to practical use satisfactorily. However, according to the present invention, by press-molding the polymer flocculant to a density of O, S, the peelability and moisture content reduction are significantly improved. It was a success.

Furthermore, it is possible to obtain dewatering efficiency (reduction in water content of dehydrated cake, improvement in peelability from furnace cloth) that is superior to conventional vacuum dehydration methods using inorganic flocculants. Moreover, the serious drawbacks of the inorganic flocculant method are solved, the dehydrated cake is highly flammable;
Significantly less fuel is required during incineration, and the amount of incinerated ash produced is also significantly reduced. Therefore, the present invention has extremely high practical value.

The present invention will be further explained below with reference to Examples and Comparative Examples, but the present invention is not limited thereto.

Example 1. and Comparative Example 1 Various organic fibrous materials (bulk specific gravity 0.02 to 0.05, water content 3 to 10% by weight) were pressed at a linear pressure of 50k using a roll press machine.
g/at* to form a flat plate with a thickness of 5 mm (bulk specific gravity 0.5~
Create 1.2) and cut it with scissors to make it 1.5cm long.
IrL, pressed pellets with a width of 1.5 mm and a thickness of 5 mm were prepared.

Mixed raw sludge from A city sewage treatment plant collected in an aoo ml beaker 200.9 (solid content 2.9% by weight, organic content 6
The pellets prepared above were added to 9.2% by weight/sludge solid content, fiber content 4.5% by weight/sludge solid content, and the mixture was stirred at 250 rpm using a small experimental stirrer equipped with a chi-type stirring bar.
The mixture was stirred for 30 seconds. Next, methachloroyloxyethyltrimethylammonium chloride poly(hereinafter referred to as
Called METAC polymer, intrinsic viscosity 5.5 dI! /
El near ooid roar value 4.8 me q/ji)
0.6 mJ (METAC poly-
q-o, s weight i% / sludge solid content) plus 25O
It was stirred at rpm for 30 seconds to form some flocs, and then stirred at 500 rpm for 2 minutes to break up the previously formed flocs.

A small leaf with an inner diameter of 3.5 cIIL equipped with a commercially available F cloth for vacuum dehydration was immersed in the floc of the beaker prepared above, and vacuum dehydration was started under reduced pressure of 450 mmHg.

After measuring the time (seconds) required for 60 me of tear fluid to flow out, the leaf was immediately removed from the beaker, stood upside down, and then suctioned at -450 mrgHg for 2 minutes. The thickness of the cake adhered to the P cloth, the moisture content (wt%) of the cake, the releasability from the P cloth, and the clarity of the filtrate were measured. The test results are shown in Table 1.

As comparative examples, sludge was prepared using the same method as in Example 1, with nothing added to the sludge, pellets containing only fibers were processed into sludge, and M
ETAC polymer alone as a 0.8 wt% aqueous solution added to sludge, and 0 bulk specific gravity instead of compressed pellets.
A vacuum dehydration test was conducted using an organic fibrous material of .02 to 0.05 (water content 10% or less). The results are also listed in Table 1.

Table-1 *1. Good peelability of Zelkova after dehydration...The cake is almost completely peeled off and there is no clogging of the 1F cloth.

Fair: Peeling is insufficient and some cake remains on the F cloth.

Some F cloths are clogged. Ru.

Not possible...“After peeling off, a considerable amount of cake remains on the F cloth.F
Almost the entire area of the cloth is clogged.

*2. Clarity of P liquid Good: Hardly any cake leakage is observed.

Fair: A very small amount of cake leaks into the P solution.

Not possible...The cake has leaked into the F solution.

Claims (1)

[Claims] 1. Organic fibrous material (A) whose bulk specific gravity has been increased to a density of 0.3 g/cm^3 or more by pressure molding and a polymer flocculant (B) are used together. An aid for sludge dewatering. 2. The auxiliary agent according to claim 1, wherein the fibrous material is a fiber or a vegetable fibrous material that can become fibrous in water. 3. The auxiliary agent according to claim 2, wherein the vegetable fibrous material is pulverized paper. 4. The auxiliary agent according to any one of claims 1 to 3, wherein (B) is a cationic polymer flocculant. 5. The cationic polymer flocculant is (1) tertiary nitrogen-containing acrylates, tertiary nitrogen-containing acrylamides, quaternary nitrogen-containing acrylates, quaternary nitrogen-containing acrylamides,
At least one cationic monomer selected from the group consisting of vinylimidazolines and allylamines, or a polymer of this and at least one other ethylenically unsaturated monomer, (2) cation-modified polyacrylamide ,(
The auxiliary agent according to claim 4, which is at least one selected from the group consisting of 3) chitosan, (4) polyethyleneimine, (5) epihalohydrin-amine condensate, and (6) cationized starch. 6. The cationic monomer has a general formula ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ (1) (In the formula, A is an oxygen atom or NH, B is an alkylene group with 1 to 4 carbon atoms, and 2 to 4 carbon atoms. hydroxyalkylene group or phenylene group, R_1 is H or methyl group, R_
2, R_3 and R_4 represent H, an alkylene group or an aralkyl group, and X^■ represents a counter anion. ) The auxiliary agent according to claim 5, which is a monomer represented by: 7. The fibrous material has a density of 0.01 to 0.2 g/cm^3,
The auxiliary agent according to any one of claims 1 to 6, which has a fiber length of 0.01 to 5 mm. 8. Add organic fibrous material (A) whose bulk specific gravity has been increased to a density of 0.3 g/cm^3 or more by pressure molding and a polymer flocculant (B) to sludge, and then add the resulting floc diameter. 5
A method for dewatering sludge, which consists of adjusting the sludge to less than mm and dewatering it. 9. (A) and (B) at the same time or (A) and then (
9. The method according to claim 8, wherein B) is added. 10. The method according to claim 8 or 9, wherein the floc diameter is adjusted to 1 mm or less and dewatering is performed using a vacuum dehydrator. 11. The method according to claim 8 or 9, wherein the floc diameter is adjusted to 1 mm or less and dewatering is performed using a filter press.