TWI891644B - Sludge dehydrator - Google Patents

Abstract

The present invention provides a sludge dehydrating agent comprising a fibrous material (a) whose surface has been treated with a hydrophilic oil. The fibrous material (a) is characterized in that the fibrous material (a) is composed of a plurality of fibrous materials having different fiber lengths and/or fiber diameters. The sludge dehydrating agent can reduce the moisture content of sludge.

Description

Sludge dehydrator

The present invention relates to a sludge dehydrating agent for dehydrating sludge generated during biological treatment or chemical treatment of organic wastewater such as sewage or factory drainage.

In recent years, the industry has increasingly demanded environmental considerations, such as reducing greenhouse gas emissions, conserving energy, and reducing waste. Consequently, sewage treatment plants, purifiers, fecal matter treatment facilities, industrial wastewater treatment facilities, and other wastewater treatment facilities are shifting toward environmentally conscious treatment methods. Environmental considerations are also linked to cost reductions, and further technological advancements are expected in the future.

Sludge generated during the biological treatment of sewage or organic wastewater is dewatered and reduced in weight before being disposed of or reused. After disposal, sludge is often incinerated. In either process, reducing the weight of sludge by dewatering it is expected to reduce the amount of waste and improve transportation or handling efficiency. In particular, reducing the moisture content of sludge reduces the amount of auxiliary fuel used during incineration, thereby simultaneously reducing the environmental impact and lowering the operating costs of incineration.

However, due to changes in treatment methods or changes in sewage and drainage itself, sludge dewatering and volume reduction have become increasingly difficult in recent years, leading to an increase in sludge with low dewaterability, or high moisture content. Among these sludges, digested sludge, standard activated sludge, and residual sludge from oxidation ditch processes are known to be difficult to dewater.

Therefore, proposals include adding a coagulant to the sludge to be dehydrated, using fibers such as regenerated cellulose as a sludge dehydrating agent, to make the sludge easily dehydrated, and then using mechanical or gravity dehydration treatment (Patent Documents 1 and 2).

Patent Document 1: Japanese Patent No. 5,658,107

Patent Document 2: Japanese Patent No. 4,817,431

However, in the above-mentioned dewatering treatment method, it is desirable to further improve the dispersibility of the fibers in the sludge dewatering agent and further reduce the moisture content of the sludge dewatering filter blocks. In order to calculate the added sludge dewatering agent as part of the sludge weight after dehydration, it is desirable to reduce the amount of sludge dewatering agent added itself.

The present invention aims to provide a sludge dehydrator that can reduce the moisture content of sludge dehydration filter blocks by utilizing the capillary properties of fibers. Furthermore, the present invention aims to provide a sludge dehydrator that is more effective even with a smaller addition amount than existing sludge dehydrators. Furthermore, the present invention aims to provide a sludge dehydrator that minimizes clogging of the dehydration equipment during sludge dehydration.

The inventors discovered that by treating the surface with a hydrophilic oil along with a coagulant (b), and using multiple fibrous materials (a) of varying sizes as a dehydrating agent and dispersing them in the sludge, the dehydration efficiency of the sludge can be enhanced and the water content of the sludge can be reduced when dehydrated using a sludge dehydrator. This led to the completion of the present invention.

That is, the present invention is a sludge dehydrating agent, which is composed of a fibrous material (a) whose surface has been treated with a hydrophilic oil agent. The characteristic of the fibrous material (a) is that the fibrous material (a) is composed of a plurality of fibrous materials having different fiber lengths and/or fiber diameters.

The present invention also relates to a sludge dehydration composition, which is composed of the above-mentioned sludge dehydrating agent (a), coagulant (b) and sludge.

The present invention further provides a coagulated sludge composition for dewatering, which is obtained by dispersing the above-mentioned sludge dehydrating agent (a) in sludge, adding a coagulant (b) pre-dispersed in water thereto, and stirring.

Furthermore, the present invention provides a sludge dewatering method, which sequentially comprises (step 1) dispersing the above-described sludge dehydrating agent (a) in the sludge, (step 2) adding a coagulant (b) dispersed in water to the sludge and stirring the sludge to cause the sludge to coagulate, and (step 3) dewatering the coagulated sludge.

According to the present invention, by utilizing the capillary properties of fibers, a sludge dewatering agent can be provided that reduces the moisture content of sludge dewatering filter blocks. Furthermore, compared to existing sludge dewatering agents, a sludge dewatering agent is effective even with a smaller addition amount. Furthermore, a sludge dewatering agent can be provided that causes less clogging of the dewatering device during sludge dewatering.

The sludge dehydrating agent of the present invention is composed of a fibrous material (a) whose surface has been treated with a hydrophilic oil.

<Fiber-like material (a)>

Examples of the fiber-like material (a) include at least one selected from the group consisting of synthetic fibers, semi-synthetic fibers, regenerated fibers, and natural fibers.

Examples of synthetic fibers include nylon, polyethylene terephthalate, vinylon, carbon fiber, aramid fiber, polyvinyl chloride, acrylic acid, polyurethane, and polylactic acid fiber. Polyethylene terephthalate is preferred due to its low cost and ease of sludge incineration, while aliphatic polyesters such as polylactic acid, polyethylene succinate, and polybutylene succinate are preferred due to their ease of landfill disposal.

Examples of synthetic fibers include nylon, aromatic polyesters such as polyethylene terephthalate, polypropylene terephthalate, or polybutylene terephthalate, aliphatic polyesters such as polylactic acid, polyethylene succinate, or polybutylene succinate, vinylon, polyvinyl alcohol, ethylene vinyl alcohol copolymers, polyolefins such as polyethylene or polypropylene, carbon fiber, aramid fiber, polyvinyl chloride, acrylic, and polyurethane fiber.

Examples of semi-synthetic fibers include rayon yarn, cupro yarn, acetate, and regenerated cellulose fibers.

Examples of recycled fibers include waste paper, scraps of waste paper or paper, biomass such as rice stalks and reeds, fibrous materials derived from plants, cut woven fabrics, non-woven fabrics, and mesh fabrics, and defiberized woven fabrics, non-woven fabrics, and mesh fabrics.

Examples of natural fibers include wood pulp, hemp, cotton, flax, wool, silk, and other animal hairs.

The sludge dehydrator of this invention utilizes a fiber-like material whose surface has been treated with a hydrophilic oil. Without this surface treatment, the capillary properties of the fiber cannot be effectively utilized.

The fibrous material (a) of the present invention is composed of a plurality of fibrous materials having different fiber lengths and/or fiber diameters. In particular, it is preferred that the fibrous material (a) is composed of two types of fibrous materials having different fiber lengths and/or fiber diameters.

In this case, the fiber-like material (a) may be any of the following: (1) a fiber-like material (a1) and a fiber-like material (a2) having a longer fiber length than the fiber-like material (a1) and a thicker fiber diameter, (2) a fiber-like material (a1) and a fiber-like material (a2) having a thicker fiber diameter than the fiber-like material (a1) and having the same fiber length, or (3) a fiber-like material (a1) and a fiber-like material (a2) having a longer fiber length than the fiber-like material (a1) and having the same fiber diameter.

In the present invention, the term "fibrous material (a2) is longer than fibrous material (a1)" means that the fiber length of fibrous material (a2) is at least 20% longer than the fiber length of fibrous material (a1). Furthermore, the term "fibrous material (a2) is thicker than fibrous material (a1)" means that the fiber diameter of fibrous material (a2) is at least 20% larger than the fiber diameter of fibrous material (a1).

The term "equal fiber length" means that the fiber length of the fibrous object (a2) is at least 80% and less than 120% of the fiber length of the fibrous object (a1), preferably 90-110%. Furthermore, the term "equal fiber diameter" means that the fiber diameter of the fibrous object (a2) is at least 80% and less than 120% of the fiber diameter of the fibrous object (a1), preferably 90-110%.

When the fibrous material (a) is composed of two types of fibrous materials, the fibrous material (a) preferably does not contain fibrous materials other than the fibrous material (a1) and the fibrous material (a2). However, it is permissible to contain fibrous materials other than the fibrous material (a1) and the fibrous material (a2) in an amount of, for example, 30% by weight or less, further, for example, 20% by weight or less, and particularly, for example, 10% by weight or less of the total weight of the fibrous material (a).

The fibers constituting the fiber-like material (a1) are preferably fibers of the same fiber length and the same fiber diameter. Furthermore, the fibers constituting the fiber-like material (a2) are preferably fibers of the same fiber length and the same fiber diameter. Fiber lengths or fiber diameters are considered identical if the difference is, for example, less than 20%, or further, for example, within 10%.

The fiber length of the fibrous material (a) is preferably 1-50 mm, more preferably 3-20 mm. The fiber diameter of the fibrous material (a) is preferably 2-300 μm, more preferably 5-30 μm. The aspect ratio of the fibrous material (a) is preferably 3-25,000, more preferably 100-4,000. Here, the aspect ratio is the value of fiber length divided by fiber diameter. In other words, aspect ratio = fiber length / fiber diameter. If the aspect ratio is less than this, the shape is not fibrous but too spherical, and it cannot fully function as a water conduit and is therefore not ideal. If the pressure is higher than this, the fibers will become entangled, reducing dispersion and increasing the likelihood of entanglement in the dehydrator filter, which is undesirable. Furthermore, the dehydration filter blocks may become too hard, causing clogging and disrupting stable operation of the dehydrator.

The fiber length of the fibrous material (a1) is preferably 2-15 mm, more preferably 5-15 mm. If the fiber length is shorter than this, it cannot fully function as a water conduit to drain water from the fibrous material, which is undesirable. If it is longer, the fibers will become entangled with each other, reducing dispersion and increasing the possibility of problems such as entanglement in the dehydrator filter section, which is undesirable. Furthermore, if the fiber length is longer than this, the dehydration filter blocks may become too hard, causing clogging of the dehydrator and unstable operation, which is undesirable.

In the case of the above-mentioned aspect (2), the fiber length of the fibrous article (a2) is the same as that of the fibrous article (a1), but in the cases of the above-mentioned aspects (1) and (3), the fiber length of the fibrous article (a2) is longer than that of the fibrous article (a1) by more than 20%, preferably by 20% to 300%, more preferably by 20% to 200%, and particularly preferably by 50% to 150%. For example, if the fiber length of the fibrous article (a1) is 10 mm, the fiber length of the fibrous article (a2) is preferably 12 to 40 mm, more preferably 12 to 30 mm, and particularly preferably 15 to 25 mm. When the fiber length is shorter than the limit, the function of the fiber (a1) as a water conduit for discharging water from the sludge drawn out of the sludge is not fully exerted. When it is longer than the limit, the pressure flexibility of the fiber increases, which increases the possibility of causing problems such as entanglement in the dehydrator filter section, which is not ideal.

The fiber diameter of the fibrous material (a1) is preferably 3-100 μm, more preferably 4-50 μm, and particularly preferably 5-30 μm. A fiber diameter smaller than this value is less effective because the water retention of the fibrous material is increased, making it difficult to function as a dehydrating agent. A fiber diameter larger than this value is less effective because the capillary effect is reduced, preventing the water retained in the sludge from being drawn out by the fibrous material, thereby reducing its function as a sludge dehydrating agent and making it less effective.

In the above-mentioned aspect (3), the fiber diameter of the fibrous material (a2) is the same as that of the fibrous material (a1), but in the above-mentioned aspects (1) and (2), the fiber diameter of the fibrous material (a2) is preferably larger than the fiber diameter of the fibrous material (a1) by more than 20%, more preferably by 20-300%, more preferably by 50-200%, and particularly preferably by 50-150%.

More specifically, the fiber diameter of the fibrous material (a2) is preferably 12-40 μm, more preferably 15-30 μm, and particularly preferably 15-25 μm. The above range is particularly preferred when the fiber diameter of the fibrous material (a1) is 10 μm.

When the fiber diameter is smaller than the limit, the gaps in the fiber network become smaller, so the function of the fiber as a water conduit for draining the water in the sludge drawn by the fiber-like object (a1) out of the sludge is not fully exerted, and the function is poor. When the fiber diameter is larger than the limit, the surface area that can function as a water conduit becomes smaller, so the function of draining the water in the sludge drawn by the fiber-like object (a1) out of the sludge is lower and poor.

The weight ratio of fiber (a1) to fiber (a2) in the fibrous material (a) is preferably 0.1-4, more preferably 0.2-3, and particularly preferably 0.3-1, with the total weight of fiber (a1) being 1. When the weight ratio of fiber (a2) is smaller than this, the surface area for functioning as a water conduit is reduced, resulting in a lower function of draining water from the sludge drawn by the fiber (a1) out of the sludge, making it less desirable. On the other hand, when the weight ratio is greater than this, the effect of drawing water from the sludge is reduced due to the capillary phenomenon, and the function as a dehydrating agent is reduced, making it less desirable.

Examples of hydrophilic oils include polyether-polyester copolymers and ester-type non-ionic surfactants. Ester-type non-ionic surfactants are preferred. Examples of polyether-polyester copolymers include copolymers of terephthalic acid-alkylene glycol-polyalkylene glycol, terephthalic acid-isophthalic acid-alkylene glycol-polyalkylene glycol, terephthalic acid-alkylene glycol-polyalkylene glycol monoether, and terephthalic acid-isophthalic acid-alkylene glycol-polyalkylene glycol monoether.

Preferred lower alkyl glycols for producing the copolymers are ethylene glycol, propylene glycol, butylene glycol, and pentanediol. Preferred polyalkylene glycols are polyethylene glycol, polyethylene glycol-polypropylene glycol copolymers, and polypropylene glycol, typically with an average molecular weight of 400-12,000, preferably 600-6,000. Examples of polyalkylene glycol monoethers include monomethyl ether, monoethyl ether, and monophenyl ether of polyethylene glycol. Monoethers of polyethylene glycol are particularly preferred due to their improved dispersibility.

Furthermore, the above-mentioned block copolymer has a particularly good dispersibility when the molar ratio of terephthalic acid units to isophthalic acid units is within the range of 95:5 to 50:50. Furthermore, the dispersibility is particularly good when the molar ratio of terephthalic acid units + isophthalic acid units to polyalkylene glycol units is within the range of 3:1 to 10:1.

Furthermore, the average molecular weight of the block copolymer varies depending on the molecular weight of the polyol used, but is, for example, 1,000 to 20,000, preferably 3,000 to 10,000.

Examples of ester-type nonionic surfactants include compounds represented by the following general formula.

R-CO ₂ (CH ₂ CH ₂ O) _n -H

In the above formula, R refers to a long-chain alkyl group with 8 to 22 carbon atoms, and n refers to a natural number.

The amount of the hydrophilic oil is preferably 0.5 to 10,000 parts by weight, more preferably 1 to 2,500 parts by weight, relative to 100 parts by weight of the fibrous material (a).

Methods for imparting a hydrophilic oil to fibrous materials include immersing the fibrous material in a water bath or oil bath containing a hydrophilic oil, and spraying an aqueous solution containing a hydrophilic oil.

<Agglutinating agent (b)>

Examples of the coagulant (b) include at least one selected from the group consisting of inorganic coagulants and polymer coagulants.

Examples of inorganic coagulants include at least one selected from the group consisting of polyferric sulfate (PFS), polyaluminum chloride (PAC), ferric chloride, aluminum sulfate, slaked lime, and ferrous sulfide.

Examples of polymeric coagulants include at least one selected from the group consisting of polyacrylamide, acrylamide-sodium acrylate copolymer, acrylamide-acrylamide-sodium 2-methylpropane sulfonate copolymer, alkyl methacrylate quaternary salt compound, alkyl methacrylate quaternary salt compound-acrylamide copolymer, polyvinylamidine, chitosan, polyglutamate, alginic acid, pectin, starch, copolymers of acrylates and acrylamide, and methacrylate polymers.

<Dewatering Sludge Composition>

Examples of sludge include sludge from sewage treatment plants, agricultural settlement drainage facilities, purification tanks, fecal and urine treatment facilities, industrial wastewater treatment facilities, water purification plants, paper mills, and mines. The moisture content of sludge is generally 90-99.9% by weight, but the sludge dehydrator of the present invention preferably contains 0.3-10 parts by weight, and more preferably 0.6-6 parts by weight, per 100 parts by weight of sludge solids (TS).

The amount of coagulant (b) added is, for example, 0.1 to 3 parts by weight relative to 100 parts by weight of sludge TS.

The sludge dehydrator of the present invention can be prepared by dispersing the sludge dehydrator in sludge, adding a coagulant (b) pre-dispersed in water, and stirring the mixture to obtain a coagulated dehydrated sludge composition. This dehydrated sludge composition is suitable for dehydration using dehydrators such as centrifugal dehydrators, belt filter presses, screw filter presses, filter presses, multi-disc dehydrators, and double-drum pressure dehydrators.

The sludge dehydrator of the present invention can be used to dehydrate sludge by sequentially performing (step 1) dispersing the sludge dehydrator in sludge, (step 2) adding a coagulant dispersed in water to the sludge and stirring to agglomerate the sludge, and (step 3) dehydrating the agglomerated sludge.

Implementation Examples

The following examples and comparative examples illustrate the details of the present invention, with the results shown in Table 1. Example 1, Example 2, and Comparative Example 1 in the table represent the results of tests conducted using the same sludge, the same chemical, and the same environment. Example 3 represents the average of two tests conducted under the same conditions using different sludge and testing equipment but the same chemical. These are merely examples and, unless otherwise specified, do not specify specific conditions such as sludge type, dehydrating agent material or form, or method of use.

The following methods were used to measure and evaluate the performance of the Examples and Comparative Examples. "Sludge TS" refers to the total amount of solids (evaporation residue) contained in the sludge.

1. Moisture content

(1) Measure the empty weight of the aluminum cup. X (g)

(2) Move the dehydrated filter block to be measured into the aluminum cup and measure the weight Y (g).

(3) Place the dehydrated filter block together with the aluminum cup into the dryer and dry at 105°C overnight.

(4) After drying, measure the weight Z (g) of the dehydrated filter block together with the aluminum cup.

(5) Use the following formula to calculate the moisture content of the dewatering filter block.

Moisture content (weight %) = (Y-Z)/(Y-X) × 100

2. Sludge TS concentration

Calculate the sludge TS concentration using the following formula, using the weight values X (g), Y (g), and Z (g) obtained using the same measurement method, replacing the dewatering filter blocks listed in 1. Moisture Content with sludge.

Sludge TS concentration (weight %) = (Z-X)/(Y-X) × 100

3. Blockage

To check for blockage in the dehydrator, visually inspect the screen.

4. Dewatering filter block production reduction rate

(1) Add a polymer coagulant at a concentration of B (weight %) relative to the TS (total solid content) of the sludge to the sludge with a TS concentration of A (weight %). Dehydrate the sludge directly using a dehydrator to produce sludge filter cakes. Determine the moisture content _W0 (weight %) of the dehydrated filter cakes after dehydration.

(2) Using the following formula, calculate the amount of dehydrated filter cake produced (D ₀ (tons)) when the sludge volume is 100 tons and no dehydrating agent is added.

D ₀ =(100 tons × A (weight %) + 100 tons × A (weight %) × B (weight %))/(1-W ₀ (weight %))

(3) Add a dehydrating agent at a concentration of C (weight %) relative to the sludge TS (total solid content) to the sludge at a TS concentration of A (weight %) and stir. Then, add a polymer flocculant at a concentration of B (weight %) relative to the sludge TS and stir. Then, dehydrate the sludge in a dehydrator to produce a dehydrated filter cake. The water content of the dehydrated filter cake, _W1 (weight %), is measured.

(4) Using the following formula, calculate the amount of dehydrated filter cake produced (D ₁ (tons)) when a dehydrating agent is added to a sludge volume of 100 tons.

D ₁ =(100 tons × A (weight %) + 100 tons × A (weight %) × (B + C) (weight %))/(1-W ₁ (weight %))

(5) Calculate the reduction rate of dewatering filter block production using the following formula.

Dewatering filter cake production reduction rate = (1-D ₁ /D ₀ ) × 100 (weight %)

5. Length of fibrous material (fiber length)

Remove the fiber-like object and straighten it into a straight line. Measure its length with a vernier caliper. Measure three of the fiber-like objects, and use the average value as the fiber length.

6. Thickness of fibrous materials (fiber diameter)

The fiber diameter was measured from the side of the fiber using a digital microscope (VHX1000/VH-Z75) manufactured by KYENCE Co., Ltd. The measurement was performed on three fiber structures, and the average value was taken.

[Example 1]

Add 3.4 wt% of sludge dehydrating agent to the remaining sludge relative to the TS of the sludge and stir with a mixer until the sludge dehydrating agent is dispersed in the sludge. As sludge dehydrating agents, (a) a hydrophilic oil was applied to straight polyethylene terephthalate chopped fibers with a circular cross-section of 12.6 μm and a fiber length of 20 mm without curling, and a polyether-polyester copolymer was applied to straight polyethylene terephthalate chopped fibers with a circular cross-section of 7.5 μm and a fiber length of 10 mm without curling, in a weight ratio of (a):(b) = 1:2. To the sludge dispersed with the sludge dehydrating agent (the sludge dehydrating agent prepared in (a) and (b) above), 1.1% by weight of a coagulant (alkylaminoacrylate quaternary salt, acrylamide copolymer) was added and stirred to achieve coagulation, thereby obtaining a dehydration sludge composition. This sludge composition was then dehydrated in a double-drum pressure dehydrator at a rate of 10 ^m³ /h to obtain dehydrated filter cakes. The moisture content of the resulting dehydrated filter cakes was measured.

[Example 2]

Add 3.4 wt% of sludge dehydrating agent to the remaining sludge relative to the TS of the sludge and stir with a mixer until the sludge dehydrating agent is dispersed in the sludge. As a sludge dehydrating agent, the sludge dehydrating agent was used in a weight ratio of (c): (b) = 1:2. (c) was applied to straight polyethylene terephthalate chopped strands with a fiber diameter of 17.5 μm and a fiber length of 10 mm, which were not crimped and were treated with a polyether-polyester copolymer as a hydrophilic oil. (b) was applied to straight polyethylene terephthalate chopped strands with a fiber diameter of 7.5 μm and a fiber length of 10 mm, which were not crimped and were treated with a polyether-polyester copolymer as a hydrophilic oil.

To the sludge dispersed with the sludge dehydrating agent (the sludge dehydrating agent prepared in (c) and (b) above), 1.2% by weight of a coagulant (alkylaminoacrylate quaternary salt, acrylamide copolymer) was added and stirred to achieve coagulation, thereby obtaining a dehydration sludge composition. This sludge composition was then dehydrated in a double-drum pressure dehydrator at a rate of 10 ^m³ /h to obtain dehydrated filter cakes. The moisture content of the resulting dehydrated filter cakes was measured.

[Example 3]

A sludge dehydrator at 3.0% by weight relative to the TS of the sludge was added to the mixed raw sludge in which the raw sludge and the residual sludge were mixed at a weight ratio of 90:10. The mixture was stirred with a mixer until the sludge dehydrator was dispersed in the sludge. As a sludge dewatering agent, the following were used: (c) (b) = 2.5:1 by weight: (c) 17.5 μm diameter, 10 mm length, straight, uncurled polyethylene terephthalate chopped strands treated with a polyether-polyester copolymer as a hydrophilic oil; and (b) 7.5 μm diameter, 10 mm length, straight, uncurled polyethylene terephthalate chopped strands treated with a polyether-polyester copolymer as a hydrophilic oil.

To the sludge dispersed with the sludge dehydrating agent (the sludge dehydrating agent prepared by (c) and (b) above), 0.6% by weight of a coagulant (alkylaminoacrylate quaternary salt, acrylamide copolymer) was added and stirred to achieve coagulation, thereby obtaining a dehydration sludge composition. This sludge composition was dehydrated using a pressure dehydration LABOTEST apparatus at a pressure of 0.6 MPa for 5 minutes to obtain a dehydrated filter cake. The moisture content of the resulting dehydrated filter cake was measured.

Based on the amount of sludge dehydrating agent added and the moisture content after dehydration, the reduction rate of dewatering filter blocks is calculated to evaluate the sludge generation reduction effect.

[Comparative example 1]

The same procedures as in Example 1 were followed, except that the sludge dewatering agent used was a straight polyethylene terephthalate chopped fiber with a fiber diameter of 17.5 μm and a fiber length of 10 mm, which was not crimped and was treated with a polyether-polyester copolymer as the hydrophilic oil. The results are shown in Table 1.

[Industrial Availability]

The sludge dehydrating agent of this invention can be used on sludge generated from sewage treatment plants, purification tanks, fecal and urine treatment facilities, industrial wastewater treatment facilities, other wastewater treatment plant construction sites, sludge generated from water purification plants, sludge generated from paper mills, sludge generated from construction and civil engineering projects, and sludge from mining wastewater.

Claims (5)

A sludge dehydrating agent is a sludge dehydrating agent formed by a fibrous material (a) whose surface is treated with a hydrophilic oil agent of a polyether-polyester copolymer or an ester-type non-ionic surfactant, wherein the fibrous material (a) is formed by a plurality of fibrous materials having different fiber lengths and/or fiber diameters, and wherein the fibrous material (a) is any of the following: (1) a fibrous material (a1) and a fibrous material having a fiber length greater than that of the fibrous material (a1); (2) A state formed by a fiber-like material (a2) having a longer fiber diameter and a thicker fiber diameter, and (3) A state formed by a fiber-like material (a1) and a fiber-like material (a2) having a fiber diameter thicker than that of the fiber-like material (a1) and a fiber length equal to that of the fiber-like material (a1), wherein the weight ratio of the fiber-like material (a1) to the fiber-like material (a2) in the fiber-like material (a) is 1 relative to the fiber-like material (a1) and 0.1 to 4 relative to the fiber-like material (a2). The sludge dehydrating agent of claim 1, wherein the fiber length of the fibrous material (a1) is 2 to 15 mm, the fiber diameter is 3 to 100 μm, and the aspect ratio is 3 to 25,000. A sludge dehydration composition comprising the sludge dehydrating agent (a) as claimed in claim 1, a coagulant (b), and sludge. A coagulated sludge composition for dewatering is obtained by dispersing the sludge dewatering agent (a) as claimed in claim 1 in sludge, adding a coagulant (b) pre-dispersed in water thereto, and stirring. A method for dewatering sludge, comprising, in sequence, (step 1) dispersing the sludge dehydrating agent (a) according to claim 1 in the sludge, (step 2) adding a coagulant (b) dispersed in water to the sludge and stirring the sludge to cause the sludge to coagulate, and (step 3) dewatering the coagulated sludge.