AU2012215865C1

AU2012215865C1 - Method and apparatus for sludge flocculation

Info

Publication number: AU2012215865C1
Application number: AU2012215865A
Authority: AU
Inventors: Takao Hagino; Tomoyuki Morita; Ryoji TSUKUI
Original assignee: Swing Corp
Current assignee: Swing Corp
Priority date: 2011-02-10
Filing date: 2012-02-01
Publication date: 2015-10-01
Anticipated expiration: 2032-02-01
Also published as: JP2015057288A; JPWO2012108312A1; JP5728506B2; JP6143835B2; AU2012215865B2; AU2012215865A1; JP5878619B2; CN103347825B; WO2012108312A1; CN103347825A; JP2016034646A

Abstract

Provided is a novel sludge flocculation method that can reduce the volumes pumped of the polymer flocculants necessary for the sludge flocculation, reduce the water content of the dewatered cake, and reduce waste matter volume. A sludge flocculation method is proposed, the method comprising: a first stirring process of pumping a first polymer flocculant solution into the sludge and mixing the sludge with the first polymer flocculant solution by high speed stirring at 1000 rpm or more to prepare a mixed sludge; and a second stirring process of pumping a second polymer flocculant solution into the mixed sludge and mixing the mixed sludge with the second polymer flocculant solution by stirring at 10 - 500 rpm to form a flocculated floc.

Description

DESCRIPTION METHOD AND APPARATUS FOR SLUDGE FLOCCULATION TECHNICAL FIELD [0001] The present invention relates to a method for sludge flocculation and a apparatus for sludge flocculation. With regard to a dewatering treatment for reducing sludge discharged from wastewater treatment facilities or water purification facilities, for example, the invention relates to a flocculation method for sludge flocculation and a flocculation apparatus used therefor. BACKGROUND ART [0002] As lowering an environmental burden by reducing waste volume is required now more than ever, a dewatering treatment technique for reducing sludge discharged from wastewater treatment facilities, water purification facilities, or the like is considered to be very important, and thus a technique for more efficient dewatering treatment of sludge is waited for. [0003] The sludge dewatering treatment generally consists of a flocculation step for flocculating sludge by using a flocculant and a dewatering step for dewatering flocculated sludge by using a dewatering apparatus. Success of the sludge dewatering treatment is largely dependent on a method for efficient flocculation using a flocculant. [0004] As for the method for sludge flocculation using 1 a flocculant, there is a previously disclosed technique to be described below which is related to a technique for flocculating sludge by two mixing steps with different rotational speed during mixing. [00051 JP 2006-263514 A (Patent Document 1) discloses a method for flocculation and dewatering of muddy water which is characterized in that an inorganic flocculant A is added to muddy water and mixed by mixing, an organic flocculant B is added thereto and stirred at low speed to produce floc L, an inorganic flocculant C is added to the floc L and mixed by mixing, and the floc L is either fragmented or broken down and then subjected to a dewatering treatment. [00061 JP 11-57800 A (Patent Document 2) discloses a method for dewatering of sludge which is characterized in that an inorganic flocculant and an amphoteric polymer as a first polymer are added to organic sludge and stirred vigorously, an amphoteric polymer is added again as a second polymer and stirred slowly, and dewatering by pressure is carried out. [00071 JP 62-277200 A (Patent Document 3) discloses a method for flocculation of sludge which is characterized in that, for flocculating organic sludge by using an amphoteric polymer flocculant, sludge and part of the amphoteric polymer flocculant are brought into contact with each other under relatively strong mixing as a first treatment and the sludge after the first treatment and remainder of the amphoteric polymer flocculant are brought into contact with each other under relatively mild mixing as a second treatment. [00081 JP 57-130599 A (Patent Document 4) discloses a method for dewatering of sludge including performing a first mixing after adding to a sludge a first polymer flocculant having an opposite charge to the charge of sludge, performing a second mixing after adding a second polymer flocculant having an opposite charge to the charge of the first polymer flocculant, and dewatering the formed floc, which is characterized in that 2 the first mixing is strong mixing which does not allow generation of any floc or only allows floc with diameter of 2 mm or less. Citation List Patent Document [00091 Patent Document 1: JP 2006-263514 A Patent Document 2: JP 11-57800 A Patent Document 3: JP 62-277200 A Patent Document 4: JP 57-130599 A [0009a] The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application. [0009b] Where the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof. DISCLOSURE OF THE INVENTION [0010] An aspect of the present invention is to provide, in relation to a method for flocculation of sludge by two mixing steps with different rotational speed during mixing, a novel method for sludge flocculation that can reduce the addition amount of a flocculant necessary for the sludge flocculation and also reduce the water content of the cake (dewatered sludge) obtained after dewatering, and thus can reduce volume of waste, and a flocculation apparatus therefor. 3 [0010a] In another aspect, the present invention provides a sludge flocculation method comprising: a first mixing step of adding a first polymer flocculant solution into the sludge as an object to be treated and mixing the sludge with the first polymer flocculant solution by high speed mixing using a mixer having a rotational speed of 1,000 rpm or higher to prepare a mixed sludge; and a second mixing step of adding a second polymer flocculant solution into the mixed sludge and mixing the mixed sludge with the second polymer flocculant solution in a mixing tank to grow a flocculated sludge. [0010b] In yet another aspect, the present invention provides an apparatus for sludge flocculation comprising a first mixing tank and a second mixing tank, wherein the first mixing tank comprises a means for adding a first polymer flocculant solution into the sludge as an object to be treated and mixing the sludge with the first polymer flocculant solution by high speed mixing using a mixer having a rotational speed of 1,000 rpm or higher to prepare a mixed sludge; and the second mixing tank comprises a means for adding a second polymer flocculant solution into the mixed sludge and mixing the mixed sludge with the second polymer flocculant solution to grow a flocculated sludge. [0011] Provided by the invention is a sludge flocculation method including a first mixing step of adding a first polymer flocculant solution into the sludge as an object to be treated and mixing the sludge with the first polymer flocculant solution by high speed mixing using a mixer having a rotational speed of 1,000 rpm or higher to prepare a mixed sludge and a second mixing step of adding a second polymer flocculant 4 solution into the mixed sludge and mixing the mixed sludge with the second polymer flocculant solution to form a floc (flocculated sludge). [00121 Also provided by the invention is a apparatus for sludge flocculation comprising a first mixing tank and a second mixing tank, in which the first mixing tank is comprising a means for adding a first polymer flocculant solution into the sludge as an object to be treated and mixing the sludge with the first polymer flocculant solution by high speed mixing using a mixer having a rotational speed of 1,000 rpm or higher to prepare a mixed sludge and the second mixing tank is comprising a means for adding a second polymer flocculant solution into the mixed sludge and mixing the mixed sludge with the second polymer flocculant solution to form a floc (flocculated sludge). 4a EFFECT OF THE INVENTION [0013] According to the sludge flocculation method and sludge flocculation apparatus provided by the invention, the addition amount of a polymer flocculant used for flocculating sludge as an object for treatment can be reduced. Further, as the water content of cake (dewatered sludge) obtained after dewatering can be reduced, volume of waste can be reduced. In this regard, by suitably adjusting the ratio of addition amount of the first polymer flocculant and the second polymer flocculant in accordance with characteristics of each of the first polymer flocculant and the second polymer flocculant, not only the total addition amount of the polymer flocculant used for sludge flocculation is further reduced but also water content of cake (dewatered sludge) obtained after dewatering is further reduced, and thus the volume of waste can be reduced more. Further, due to high speed mixing with high speed rotation, a polymer flocculant solution containing polymer flocculant at high concentration can be used. And thus, the dissolution volume of the polymer flocculant can be further reduced. Accordingly, a solubilizing tank for polymer flocculant and peripheral facilities such as liquid transport pump for polymer flocculant solution can be minimized and energy related thereto can be saved. Furthermore, when separated liquid is returned to a water treatment system, returned water volume can be reduced, and thus the energy and space required for the entire water 5 treatment system can be saved. BRIEF DESCRIPTINO OF DRAWINGS [0014] Fig. 1 is a schematic drawing illustrating one embodiment of the flocculation apparatus of the invention; Fig. 2 is a schematic drawing illustrating one embodiment of the flocculation apparatus of the invention, which is different from the one described above; Fig. 3 is a schematic drawing illustrating one embodiment of the flocculation apparatus of the invention, which is different from the one described above; Fig. 4 is a schematic drawing illustrating one embodiment of the flocculation apparatus of the invention, which is different from the one described above; Fig. 5 is a schematic drawing illustrating one embodiment of the flocculation apparatus of the invention, which is different from the one described above; Fig. 6 is a graph illustrating the test result of Example 5, in which the relationship between the addition amount of the first polymer flocculant and the SS recovery ratio is illustrated; and Fig. 7 is a graph illustrating the test result of Example 6, in which the relationship between the addition ratio of the polymer flocculant and the water content in cake (dewatered sludge) is illustrated. BEST MODE(S) FOR CARRYING OUT THE INVENTION [0015] Next, the invention will be described on the basis of the exemplary embodiments for carrying out the 6 invention, but the invention is not limited to the following embodiments to be described below. [0016] <Present flocculation method> The sludge flocculation method according to the present embodiment (herein after, referred to as the "present flocculation method") is a sludge flocculation method including a first mixing step of adding a first polymer flocculant solution into the sludge and mixing the sludge with the first polymer flocculant solution by high speed mixing to prepare a mixed sludge and a second mixing step of adding a second polymer flocculant solution into the mixed sludge and mixing the mixed sludge with the second polymer flocculant solution by mixing at slower speed than above (also referred to as "normal mixing") to form a floc (flocculated sludge). [0017] <Sludge> The sludge as an object for treatment by the present flocculation method may be any one of organic sludge and inorganic sludge. [0018] Examples of the organic sludge include organic sludge generated from sewage treatment, night soil treatment, or various industrial wastewater treatments. More specific examples include sludge from a primary sedimentation tank, excess sludge, anaerobically-digested sludge, aerobically-digested sludge, septic tank sludge, and supernatant from a sludge digestion tank. The organic sludge may contain inorganic matters. [0019] Examples of the inorganic sludge include inorganic sludge generated from water purification 7 treatment, construction wastewater treatment, and various industrial wastewater treatments. As described herein, the sludge generated by water purification treatment includes sludge discharged from a clarifier, a sludge basin, and a thickener of facilities for water purification. The inorganic sludge may contain organic matters. [00201 As described above, both of the organic sludge and inorganic sludge may be an object to be treated in the invention. However, from the viewpoint of obtaining larger effect of the invention, the organic sludge is preferable and anaerobically-digested sludge, which has poor dewaterablility, is particularly preferable. [00211 <First mixing step> The first mixing step has a main aspect of performing simultaneously the neutralization of surface charge of sludge and flocculation according to adsorption or cross-linking effect of the polymer as a result of homogeneously dispersing a polymer flocculant in sludge by high speed mixing and allowing the flocculant to deeply distribute even into inner portion of the sludge. For a case in which an inorganic flocculant is added into sludge and mixed, only surface charge of the sludge is neutralized. However, according to the present flocculation method, a large floc (flocculated sludge) can be formed, and thus flocculated sludge with good filtering property can be yielded. For such reasons, compared to a case in which an inorganic flocculant is added into sludge and mixed, the sludge flocculated by the present 8 flocculation method can be quickly dewatered with high filtration rate. [0022] (First polymer flocculant) As a first polymer flocculant, any one of an anionic polymer flocculant, a non-ionic polymer flocculant, a cationic polymer flocculant, and an amphoteric polymer flocculant may be used. For treating organic sludge, it is particularly preferable to use a cationic polymer flocculant or an amphoteric polymer flocculant. [0023] Examples of the anionic polymer flocculant include sodium polyacrylate, copolymer of sodium acrylate and acrylamide, sodium polymethacrylate, and copolymer of sodium methacrylate and acrylamide. Examples of the non-ionic polymer flocculant include polyacrylamide and polyethylene oxide. Examples of the cationic polymer flocculant include an acrylate polymer flocculant (also referred to as "DAA polymer flocculant"), a methacrylate polymer flocculant (also referred to as "DAM polymer flocculant"), polyvinylamidine containing an amide group, a nitrile group, an amine hydrochloride salt, a formamide group or the like (also referred to as "amidine polymer flocculant"), and Mannich modified product of polyacrylamide. Examples of the DAA polymer flocculant include a polymerization product of quaternized dimethylamino ethyl acrylate, and a copolymer between quaternized dimethylamino ethyl acrylate and acrylamide. Examples of the DAM polymer flocculant include a polymerization product of quaternized dimethylamino ethyl methacrylate, and a copolymer between 9 quaternized dimethylamino ethyl methacrylate and acrylamide. Examples of the amphoteric polymer flocculant include a copolymer of quaternized dimethylamino methyl acrylate, acrylamide, and acrylic acid, and a copolymer of quaternized dimethylamino methyl methacrylate, acrylamide, and acrylic acid. However, those described above are only for exemplification and the invention is not limited to them. [0024] Molecular weight of the first polymer flocculant is preferably 4,500,000 or more. More preferably, the molecular weight is 5,000,000 or more. As described herein, the molecular weight indicates average molecular weight obtained by viscometry. It is due to the fact that, when molecular weight of the polymer flocculant is excessively low during flocculation by high speed mixing and molecular chain of the polymer flocculant is fragmented by high speed mixing, flocculating performance of the polymer flocculant is reduced. For such reasons, by using a polymer flocculant having molecular weight of 4,500,000 or more, it becomes possible to disperse the polymer flocculant homogeneously in sludge and distribute it deeply even into inner portion of the sludge without compromising the flocculating performance of the polymer flocculant. [0025] From the same viewpoint as the molecular weight, viscosity of the first polymer flocculant is preferably 150 mPa-s or higher, and particularly preferably 175 mPa-s or higher. Still, it is preferably 200 mPa-s or higher. The viscosity indicates a value measured by 10 dissolving the polymer flocculant at 2 g/L in pure water and performing measurement at rotational speed of 60 rpm and 25'C by using a B type viscometer. [0026] When the molecular weight of the first polymer flocculant is 4,500,000 or more, the addition amount of the first polymer flocculant is preferably adjusted such that it is 45 to 95% by mass of total addition amount of the first polymer flocculant and the second polymer flocculant. The addition amount of the first polymer flocculant is controlled to be preferably 50 to 95% by mass, in particular 55 to 90% by mass. When the ratio of the addition amount of the polymer flocculant is excessively high during the first mixing step, although it is possible to disperse the polymer flocculant homogeneously in sludge and distribute it deeply even into inner portion of the sludge, as the addition amount of the polymer flocculant during the second mixing is excessively low so that the floc (flocculated sludge) may not grow. As a result, filterability worsens during a thickening treatment or a dewatering treatment. On the other hand, when the ratio of the addition amount of the polymer flocculant is excessively low during the first mixing step, the ratio of the polymer flocculant which is homogeneously dispersed in the sludge is reduced so that the effectiveness of the high speed mixing is reduced. Thus, by controlling the addition amount of the first polymer flocculant to 45 to 95% by mass of total addition amount, not only the polymer flocculant can be homogeneously dispersed in the sludge but also floc (flocculated sludge) 11 can be grown. [0027] As for the solvent of the first polymer flocculant solution, pure water, tap water, industrial water, underground water, treated water obtained by treating various wastewater, and sea water can be exemplified. However, from the viewpoint of maximal exhibition of the flocculating ability of the polymer flocculant, pure water is preferable (ditto for the second polymer flocculant solution). Meanwhile, from the economical point of view, tap water, industrial water, underground water, or treated water obtained by treating various types of wastewater is preferable (ditto for the second polymer flocculant solution). The concentration of the polymer flocculant in the first polymer flocculant solution may be 1 to 3 g/L. However, it is preferably 3 g/L or higher, more preferably 5 g/L or higher and still more preferably 10 g/L or higher. For the sludge flocculation using a polymer flocculant, the polymer flocculant solution is generally prepared to be 1 to 3 g/L. Further, in a normal case, a polymer flocculant solution at 3 g/L or higher is not used. It is due to the fact that, when polymer flocculant concentration is 3 g/L or higher, the polymer flocculant solution is highly viscous, and thus it becomes difficult to homogeneously disperse the polymer flocculant in the sludge at rotational speed of a mixer conventionally used for a flocculation tank (that is, 10 to 500 rpm or so). Meanwhile, with high speed mixing, the polymer flocculant can be homogeneously dispersed even when a high 12 concentration solution at 3 g/L or higher is used. As a result, there may be a merit of reducing dissolution volume of the polymer flocculant. Another merit of using a high concentration polymer flocculant solution is that, since the concentration of the polymer flocculant in the sludge added with the polymer flocculant can be increased, the addition amount of the polymer flocculant can be reduced and the water content of cake (dewatered sludge) after dewatering treatment can be reduced. For example, when 200 mL of 2 g/L polymer flocculant solution is added to 1 L sludge (that is, 0.4 g as the polymer flocculant), the concentration of the polymer flocculant in sludge is 333 mg/L. Meanwhile, when 40 mL of 10 g/L polymer flocculant is added to 1 L sludge (that is, 0.4 g as the polymer flocculant), the concentration of the polymer flocculant in sludge is 385 mg/L. Accordingly, even for adding the same amount of 0.4 g of the polymer flocculant, use of 10 g/L polymer flocculant instead of 2 g/L polymer flocculant solution can yield higher polymer flocculant concentration in sludge so that the addition amount of the polymer flocculant can be reduced and the water content in the cake (dewatered sludge) after dewatering treatment can be reduced. [0028] (High speed mixing) It is important for the first mixing to have high speed mixing at 1,000 rpm or higher. The rotational speed is more preferably 2,000 rpm or higher. The rotational speed is still more preferably 3,000 rpm or higher. When the rotational speed is increased, the mixing 13 time can be shortened so that there is no specific upper limit for the rotational speed. However, it has been presently confirmed by an experiment that the speed up to 15,000 rpm is effective. When sludge flocculation is performed using a polymer flocculant, it is important to disperse the polymer flocculant homogeneously in the sludge and distribute it deeply even into inner portion of the sludge. By homogeneously dispersing the polymer flocculant in the sludge, an unnecessary polymer flocculant can be reduced and also the addition amount of the polymer flocculant can be reduced. In addition, by distributing it deeply even into inner portion of the sludge, more densely flocculated sludge can be obtained so that the water content in the cake (dewatered sludge) after dewatering treatment can be reduced. Since the polymer flocculant solution is liquid with highly viscosity, it is difficult to homogeneously disperse the polymer flocculant in the sludge at rotational speed of a mixer conventionally used for a flocculation tank (that is, 10 to 500 rpm or so), and also it is impossible to disperse the polymer flocculant homogeneously in the sludge and to distribute the polymer flocculant deeply even into inner portion of the sludge. For such reasons, an increased addition amount of the polymer flocculant or unfavorable water content in the cake (dewatered sludge) is caused. However, according to high speed mixing, it is possible to disperse the polymer flocculant homogeneously in the sludge and also distribute it deeply even into inner portion of the sludge. As a 14 result, the addition amount of the polymer flocculant can be reduced and the water content in the cake (dewatered sludge) can be reduced. [0029] Meanwhile, the rotational speed for high speed mixing is preferably adjusted to be 1,000 rpm or higher depending on the type of sludge, properties of sludge (for example, TS or the like), molecular weight of the polymer flocculant, and dissolution concentration of the polymer flocculant or the like. [0030] The mixing time for the first mixing step, that is, time of mixing and mixing the first polymer flocculant solution and sludge, is preferably 20 sec or less, more specifically 5 sec to 20 sec, and more preferably 5 sec to 15 sec, and still more preferably 5 sec to 10 sec. If the mixing time for high speed mixing is too long, the molecular chain of the polymer flocculant is broken to the extent that the flocculating performance of the polymer flocculant is weakened. Thus, by controlling the mixing time to 20 sec or less, it becomes possible to disperse the polymer flocculant homogeneously in sludge and distribute it deeply even into inner portion of the sludge without compromising the flocculating performance of the polymer flocculant. [0031] Examples of the means for high speed mixing which may be used include a high speed mixer consisting of a mixing blade, a shaft, and a motor, or a high speed mixer consisting of a rotor, a stator, and a motor. High speed mixing may be also performed by using an in-line mixer. The in-line mixer indicates a mixer installed inside 15 a pipe. Advantages of the in-line mixer include that, as the mixer is sealed, liquid can be transported to downstream if there are two pumps including a pump for sludge and a pump for polymer flocculant at upstream side. Meanwhile, when a mixer is installed on a tank, as the top part of the tank is open, the liquid cannot be transported to downstream if one additional pump or anything corresponding to a pump is not present other than a pump for sludge and a pump for polymer flocculant at upstream side. For such reasons, without installing any pump, the liquid is generally transported to downstream with an aid of height difference. [00321 <Second mixing step> A main aspect of the second mixing step is to form a floc (flocculated sludge). [00331 (Second polymer flocculant) As for the second polymer flocculant, the same polymer flocculant as those described in the section of the first polymer flocculant above can be used. In such case, the same polymer flocculant as the first polymer flocculant may be used as the second polymer flocculant or a different type of the polymer flocculant may be used. From the viewpoint of sharing a polymer flocculant solubilizing tank, the same polymer flocculant as the first polymer flocculant is preferably used as the second polymer flocculant. [00341 The concentration of the polymer flocculant in the second polymer flocculant solution may be 1 to 3 g/L. However, it is preferably 3 g/L or higher, more preferably 5 g/L or higher and still more preferably 10 g/L or higher. 16 [0035] (Normal mixing) The rotational speed for mixing of the second mixing step may be general rotational speed adopted for conventional apparatus for sludge flocculation, that is, 10 to 500 rpm. The reason is that, during the second mixing step, it is necessary to grow a floc (flocculated sludge) by moderately contacting the mixed sludge prepared by the first mixing step with the polymer flocculant. From this point of view, the rotational speed for mixing of the second mixing step is preferably between 20 rpm and 400 rpm, more preferably between 30 rpm and 300 rpm. [0036] Meanwhile, the rotational speed for mixing of the second mixing step is preferably adjusted within the range of 10 to 500 rpm according to the type of sludge, properties of sludge (for example, TS or the like), molecular weight of the polymer flocculant, and dissolution concentration of the polymer flocculant or the like. [0037] The mixing time for the second mixing step, that is, time of mixing and mixing the second polymer flocculant solution and sludge, is preferably between 1 min to 20 min. The reason is that it is necessary to grow a floc (flocculated sludge) by moderately contacting the mixed sludge prepared by the first mixing step with the polymer flocculant during the second mixing step. From this point of view, the mixing time for the mixing of the second mixing step is preferably between 2 min and 15 min, and more preferably between 3 min and 10 min. [0038] Examples of the means for mixing which may be used include a common mixer consisting of a mixing blade, a 17 shaft, and a motor, and its type is not specifically limited. [00391 <Dewatering> After forming a floc (flocculated sludge) by the second mixing step, by solid and liquid separation using a dewatering apparatus, a cake (dewatered sludge) and separated filtrate can be obtained as a solid and a liquid, respectively. As for the dewatering method, a means for dewatering under pressure is generally used, but it is not specifically limited thereof. For example, a dewatering apparatus conventionally used for sludge dewatering including a screw press, a belt filter press, a centrifuge, a vacuum filter, a filter press, and a multiple disc dewatering device may be used. [00401 <Sludge dilution step> According to the present flocculation method, a sludge dilution step may be performed before the first mixing step, if necessary. [ 00411 An aspect of the sludge dilution step is to enhance the flocculation effect of the polymer flocculant by diluting sludge as an object to be treated with water for dilution. The sludge having high concentration of soluble components such as salts has inhibited dissociation of ionic groups of the polymer flocculant and thus has lower solubility, so that it is difficult for the polymer flocculant to exhibit the flocculation effect. Thus, by diluting sludge and lowering the concentration of soluble components, the flocculation effect of the polymer 18 flocculant can be enhanced. [0042] The dilution ratio is preferably adjusted according to the type of sludge, properties of sludge (for example, TS, electrical conductivity, M alkalinity, concentration of soluble components, concentration of salt, or the like), molecular weight of the polymer flocculant, and dissolution concentration and solubility of the polymer flocculant or the like. For example, when the electrical conductivity of sludge serves as an indicator, the dilution ratio is preferably adjusted such that the electrical conductivity of the sludge after dilution is 15 mS/cm or less. More preferably, the dilution ratio is adjusted such that the electrical conductivity of the sludge after dilution is 10 mS/cm or less. When M alkalinity of the sludge serves as an indicator, the dilution ratio is preferably adjusted such that the M alkalinity of the sludge after dilution is 7000 mg-CaCO 3 /L or less. More preferably, the dilution ratio is adjusted such that the M alkalinity of the sludge after dilution is 5000 mg-CaCO 3 /L or less. [0043] As for the water for dilution, pure water, tap water, industrial water, underground water, treated water obtained by treating various wastewater, and sea water can be exemplified. However, from the viewpoint of maximal exhibition of the effect of dilution, pure water is preferable. Meanwhile, from the economical point of view, tap water, industrial water, ground water, and treated water obtained by treating various types of wastewater are 19 preferable. [0044] <Apparatus for sludge flocculation> Next, the apparatus for performing the sludge flocculation method suggested by the invention will be described. [0045] (First example of flocculation apparatus) Fig. 1 is a schematic drawing illustrating an exemplary flocculation apparatus for carrying out the first embodiment of the invention. The apparatus illustrated in Fig. 1 has a configuration in which the high speed mixing tank 6, the normal speed mixing tank 8, and the dewatering apparatus 10 are sequentially disposed in communication with the sludge storage tank 1, the first polymer flocculant solubilizing tank 2 is disposed in communication with the high speed mixing tank 6 via the first polymer flocculant pump 4, and the second polymer flocculant solubilizing tank 3 is disposed in communication with the normal speed mixing tank 8 via the second polymer flocculant pump 5. [0046] According to the apparatus, sludge is stored in the sludge storage tank 1 and the stored sludge is fed to the high speed mixing tank 6. The first polymer flocculant solution is fed from the first polymer flocculant solubilizing tank 2 to the high speed mixing tank 6 via the first polymer flocculant pump 4. In the high speed mixing tank 6, sludge and the first polymer flocculant solution are mixed with each other by the high speed mixer 7 to form mixed sludge. The formed mixed sludge is fed from the high speed mixing tank 6 to the normal speed mixing tank 8. The 20 second polymer flocculant solution is fed from the second polymer flocculant solubilizing tank 3 to the normal speed mixing tank 8 via the second polymer flocculant pump 5. In the normal speed mixing tank 8, the mixed sludge and the second polymer flocculant solution are mixed with each other by the normal speed mixer 9, and the mixed sludge is flocculated to form a floc (flocculated sludge) . The floc (flocculated sludge) is dewatered by the dewatering apparatus 10. [0047] (Second example of flocculation apparatus) Fig. 2 is a schematic drawing illustrating an exemplary flocculation apparatus which is different from the flocculation apparatus described above. The apparatus illustrated in Fig. 2 has a configuration in which the high speed mixing tank 6, the normal speed mixing tank 8, and the dewatering apparatus 10 are sequentially disposed in communication with the sludge storage tank 1, the first polymer flocculant solubilizing tank 2 is disposed in communication with the high speed mixing tank 6 via the first polymer flocculant pump 4, and the first polymer flocculant solubilizing tank 2 is disposed in communication with the normal speed mixing tank 8 via the second polymer flocculant pump 5. [0048] According to the apparatus, the second polymer flocculant solubilizing tank 3 of the apparatus of Fig. 1 is not provided, and the first polymer flocculant solution is supposed to be fed from the first polymer flocculant solubilizing tank 2 to the high speed mixing tank 6 via the first polymer flocculant pump 4. Further, the first 21 polymer flocculant solution is supposed to be fed from the first polymer flocculant solubilizing tank 2 to the normal speed mixing tank 8 via the second polymer flocculant pump 5. [0049] (Third example of flocculation apparatus) Fig. 3 is a schematic drawing illustrating an exemplary flocculation apparatus which is different from the flocculation apparatus described above. The apparatus illustrated in Fig. 3 has a configuration in which the high speed mixing tank 6, the normal speed mixing tank 8, and the dewatering apparatus 10 are sequentially disposed in communication with the sludge storage tank 1, the first polymer flocculant solubilizing tank 2 is disposed in communication with the high speed mixing tank 6 via the first polymer flocculant pump 4, and the first polymer flocculant solubilizing tank 2 is disposed in communication with the normal speed mixing tank 8 via the first polymer flocculant pump 4 and the polymer flocculant flow control valve 11. [0050] According to the apparatus, the second polymer flocculant pump 5 of the apparatus of Fig. 2 is not provided while the polymer flocculant flow control valve 11 is installed. Thus, the first polymer flocculant solution is fed from the first polymer flocculant solubilizing tank 2 to the high speed mixing tank 6 via the first polymer flocculant pump 4. Further, the first polymer flocculant solution is fed from the first polymer flocculant solubilizing tank 2 to the normal speed mixing tank 8 via the first polymer flocculant pump 4. The flow rate of the 22 first polymer flocculant solution to the normal speed mixing tank 8 is controlled by the polymer flocculant flow control valve 11. [0051] (Fourth example of flocculation apparatus) Fig. 4 is a schematic drawing illustrating an exemplary flocculation apparatus which is different from the flocculation apparatus described above. The apparatus illustrated in Fig. 4 has a configuration in which the in-line mixer 12, the normal speed mixing tank 8, and the dewatering apparatus 10 are sequentially disposed in communication with the sludge storage tank 1, the first polymer flocculant solubilizing tank 2 is disposed in communication with the inlet side of the in-line mixer 12 via the first polymer flocculant pump 4, and the second polymer flocculant solubilizing tank 3 is disposed in communication with the normal speed mixing tank 8 via the second polymer flocculant pump 5. [0052] According to the apparatus, the high speed mixing tank 6 and the high speed mixer 7 of Fig. 1 are not provided while the in-line mixer 12 is installed on a pipe which connects the sludge storage tank 1 to the normal mixing tank 8. Thus, the first polymer flocculant solution is fed from the first polymer flocculant solubilizing tank 2 to the pipe at inlet side of the in-line mixer 12 via the first polymer flocculant pump 4. [0053] (Fifth example of flocculation apparatus) Fig. 5 is a schematic drawing illustrating an exemplary flocculation apparatus which is different from the flocculation apparatus described above. 23 The apparatus illustrated in Fig. 5 has a configuration in which the in-line mixer 12, the normal speed mixing tank 8, and the dewatering apparatus 10 are sequentially disposed in communication with the sludge storage tank 1, the first polymer flocculant solubilizing tank 2 is disposed in communication with the inlet side of the in-line mixer 12 via the first polymer flocculant pump, the second polymer flocculant solubilizing tank 3 is disposed in communication with the normal speed mixing tank 8 via the second polymer flocculant pump 5, and the dilution water feed line 13 is disposed on a pipe which connects the sludge storage tank 1 to the in-line mixer 12. [0054] According to the apparatus, the dilution water feed line 13 is disposed on a pipe which connects the sludge storage tank 1 to the in-line mixer 12 of Fig. 4. Thus, the sludge is diluted with dilution water fed from the dilution water feed line 13, and then fed to the in line mixer. [0055] Meanwhile, although the dilution water feed line 13 is disposed on a pipe which connects the sludge storage tank 1 to the in-line mixer 12 in the apparatus illustrated in Fig. 5, it may be disposed on any location as long as it is at inlet side of the in-line mixer 12. It may be disposed on the sludge storage tank 12, for example. It may be also disposed at the inlet side of the sludge storage tank 12. [0056] The first to fifth examples of the flocculation apparatus described above are only for exemplification and the invention is not limited to them. 24 [0057] <Explanation of terminologies> As described herein, the expression "X to Y" (X and Y represent any number) includes the meanings of "between X and Y" and also "preferably higher than X" and "preferably smaller than Y", unless specifically described otherwise. Further, the expressions "X or higher" (X represents any number) and "Y or smaller" (Y represents any number) include the meanings of "preferably higher than X" and "preferably smaller than Y". EXAMPLES [0058] Hereinafter, the invention will be described in more detail in view of the following Examples and Comparative Examples. [0059] (Example 1) In this Example, for the step of obtaining a cake (dewatered sludge) by performing the first mixing by adding the first polymer flocculant solution into the sludge, performing the second mixing by adding the second polymer flocculant solution, and dewatering the resulting flocculated sludge by using a belt filter press, the relation with water content of cake (dewatered sludge) was examined in accordance with modification of the rotational speed during mixing of the first mixing. [0060] For the test, three types of sludge (A, B, and C) were used, which were all anaerobically-digested sludge collected from different wastewater treatment facilities. TSs of sludge A, B, and C were 12.0, 26.2, and 34.9 g/L, respectively. Meanwhile, TS indicates the 25 concentration of substances after evaporation, that is, substances obtained after evaporating and drying sludge at 105 to 110'C. The measurement is based on Wastewater Examination Method. For the test of sludge A, a cationic polymer flocculant a (DAM-based polymer flocculant, molecular weight of 3,000,000, viscosity of 114 mPa-s) was used both for the first and the second polymer flocculant. For the test of sludge B, a cationic polymer flocculant g was used for both the first and the second polymer flocculant. For the test of sludge C, a cationic polymer flocculant h (amidine-based polymer flocculant, molecular weight of 3,000,000, viscosity of 34 mPa-s) was used for both the first and the second polymer flocculant. Further, both the first polymer flocculant solution and the second polymer flocculant solution are an aqueous solution obtained by dissolving a polymer flocculant in water, and the concentration indicates the concentration of the polymer flocculant in an aqueous solution (ditto for the examples to be described below). [0061] The test procedure is as follows. To sludge (250 mL), a pre-determined amount of the first polymer flocculant solution (concentration of 10 g/L) was added, and by mixing the sludge and the polymer flocculant solution for 10 seconds using a high speed mixer, mixed sludge was prepared. Next, to the mixed sludge, a pre-determined amount of the second polymer flocculant solution (concentration of 10 g/L) was added, and by mixing 26 the sludge and the polymer flocculant for 2 minutes using a mixer at rotational speed of 150 rpm during mixing, the mixed sludge was flocculated to form a floc (flocculated sludge) . As a final step, the floc (flocculated sludge) was dewatered by using a belt filter press and the water content (%) in the obtained cake (dewatered sludge) was measured. [0062] Meanwhile, for the test of sludge A, the first polymer flocculant solution was added in an amount of 2 mL and the second polymer flocculant solution was added in an amount of 2 mL. For the test of sludge B, the first polymer flocculant solution was added in an amount of 4 mL and the second polymer flocculant solution was added in an amount of 5 mL. For the test of sludge C, the first polymer flocculant solution was added in an amount of 20 mL and the second polymer flocculant solution was added in an amount of 7 mL. [0063] Water content (%) in cake (dewatered sludge) was obtained from the mass of evaporated water when a cake (dewatered sludge) is evaporated and dried at 105 to 110'C. The measurement method is based on Wastewater Examination Method (ditto for the examples described below). The test results are listed in Table 1. Meanwhile, the symbol "-" in Table means that there is no data available. [0064] [Table 1] 27 Mixing speed Water content in cake (dewatered sludge) of high speed (%) mixer (rpm) Sludge A Sludge B Sludge C 500 83.8 84.3 No flocculation 1000 82.3 83.0 No flocculation 3000 82.8 83.1 75.9 5000 81.0 83.1 75.9 10000 81.8 74.9 11000 82.9 15000 76.1 [0065] For sludge A and B, water content in cake (dewatered sludge) can be reduced when the rotational speed of a high speed mixer is at least 1,000 rpm or so. For the sludge C, sludge can be flocculated and a cake (dewatered sludge) can be obtained when the rotational speed of a high speed mixer is at least 3,000 rpm or so. Based on these results, it was found that, by adjusting the rotational speed for mixing by high speed mixing preferably to 1,000 rpm or higher, more preferably to 2,000 rpm or higher, and still more preferably to 3,000 rpm or higher, the water content in cake (dewatered sludge) can be reduced, and also sludge can be flocculated and cake (dewatered sludge) can be obtained. [0066] (Example 2) In this Example, for the step of obtaining a cake (dewatered sludge) by performing the first mixing by adding the first polymer flocculant solution into the sludge, performing the second mixing by adding the second polymer 28 flocculant solution, and dewatering the resulting flocculated sludge by using a belt filter press, the relation with water content of cake (dewatered sludge) was examined in accordance with modification of the concentration of the first polymer flocculant solution in the range of from 2 to 20 g/L. [0067] For the test, three types of sludge (A, D, and E) were used, which are all anaerobically-digested sludge. Sludges A and D were collected from the same wastewater treatment facility, but had different sludge concentration. Sludge E was collected from a wastewater treatment facility which is different from the facility of sludges A and D. TSs of sludge A, D, and E were 12.0, 12.2, and 37.1 g/L, respectively. For the test of sludges A and D, a cationic polymer flocculant a (DAM-based polymer flocculant, molecular weight of 3,000,000, viscosity of 114 mPa-s) was used both for the first and the second polymer flocculant. For the test of sludge E, a cationic polymer flocculant h (amidine-based polymer flocculant, molecular weight of 3,000,000, viscosity of 34 mPa-s) was used for both the first and the second polymer flocculant. For the test, the first and the second polymer flocculant solutions were prepared to have the same concentration. For example, when the first polymer flocculant solution is prepared to be 2 g/L, the second polymer flocculant solution was also prepared to be 2 g/L. When the first polymer flocculant solution is prepared to be 20 g/L, the second polymer flocculant solution was also 29 prepared to be 20 g/L. [0068] The test procedure is as follows. To sludge (250 mL), a pre-determined amount of the first polymer flocculant solution (concentration of 2 to 20 g/L) was added, and by mixing the sludge and the polymer flocculant solution for 10 seconds using a high speed mixer set at rotational speed of 10,000 to 11,000 rpm, mixed sludge was prepared. Next, to the mixed sludge, a pre determined amount of the second polymer flocculant solution (concentration of 2 to 20 g/L) was added, and by mixing the sludge and the polymer flocculant for 2 minutes using a mixer set at rotational speed of 150 rpm, the mixed sludge was flocculated to form a floc (flocculated sludge). As a final step, the floc (flocculated sludge) was dewatered by using a belt filter press and the water content (%) in the obtained cake (dewatered sludge) was measured. Meanwhile, for the test of sludge A, the first polymer flocculant was added in an amount of 50% of the total addition volume and the second polymer flocculant was added in an amount of 50% of the total addition volume. For the test of sludge D, the first polymer flocculant was added in an amount of 57% of the total addition volume and the second polymer flocculant was added in an amount of 43% of the total addition volume. For the test of sludge E, the first polymer flocculant was added in an amount of 75% of the total addition volume and the second polymer flocculant was added in an amount of 25% of the total addition volume. 30 The test results are listed in Table 2. [0069] [Table 2] Concentration Water content in cake (dewatered sludge) of polymer (%) flocculant Sludge A Sludge D Sludge E (g/l) 2 82.9 82.5 76.4 3 82.6 81.7 75.2 5 82.4 81.7 74.8 10 81.8 81.8 74.5 20 82.4 82.0 75.6 [0070] For sludge A, water content in cake (dewatered sludge) can be reduced when the concentration of the first polymer flocculant was 3 g/L or higher, and the decrease in water content in cake (dewatered sludge) was largest at the concentration of 10 g/L or so. For sludge D, water content in cake (dewatered sludge) can be reduced when the concentration of the first polymer flocculant is 3 g/L or higher, and the decrease in water content in cake (dewatered sludge) was largest at the concentration of 3 g/L and 5 g/L. For sludge E, water content in cake (dewatered sludge) can be reduced when the concentration of the first polymer flocculant is 3 g/L or higher, and the decrease in water content in cake (dewatered sludge) was largest at the concentration of 10 g/L or so. Based on these results, it was found that, by adjusting the first polymer flocculant solution to have concentration of 31 preferably 3 g/L or higher, more preferably 5 g/L or higher, and still more preferably 10 g/L or higher, the water content in cake (dewatered sludge) can be reduced. [0071] (Example 3) In this Example, for the step of obtaining a cake (dewatered sludge) by performing the first mixing by adding the first polymer flocculant solution into the sludge, performing the second mixing by adding the second polymer flocculant solution, and dewatering the resulting flocculated sludge by using a belt filter press, the relation with water content of cake (dewatered sludge) was examined in accordance with modification of the mixing time of the first mixing in the range of 3 to 30 seconds. [0072] For the test, two types of sludge (F and G) were used, which are all anaerobically-digested sludge. Sludge F and G were collected from different wastewater treatment facilities. TSs of sludge F and G were 20.9 and 28.9 g/L, respectively. For the test of sludge F, a cationic polymer flocculant f (DAA-based polymer flocculant, molecular weight of 8,000,000, viscosity of 280 mPa-s) was used both for the first and the second polymer flocculant. In addition, the solution of the cationic polymer flocculant f was prepared to be 2 g/L. For the test of sludge G, a cationic polymer flocculant h (amidine-based polymer flocculant, molecular weight of 3,000,000, viscosity of 34 mPa-s) was used for both the first and the second polymer flocculant. In 32 addition, the solution of the cationic polymer flocculant h was prepared to be 10 g/L. [0073] The test procedure is as follows. To sludge (250 mL), a pre-determined amount of the first polymer flocculant solution was added, and by mixing the sludge and the polymer flocculant solution for 3 to 30 seconds using a high speed mixer set at rotational speed of 5,000 rpm, mixed sludge was prepared. Next, to the mixed sludge, a pre-determined amount of the second polymer flocculant solution was added, and by mixing the sludge and the polymer flocculant for 2 to 5 minutes using a mixer set at rotational speed of 100 to 200 rpm during mixing, the mixed sludge was flocculated to form a floc (flocculated sludge) . As a final step, the floc (flocculated sludge) was dewatered by using a belt filter press and the water content (%) in the obtained cake (dewatered sludge) was measured. [0074] For the test of sludge F, the first polymer flocculant solution was added in an amount of 35 mL and the second polymer flocculant solution was added in an amount of 5 mL. For mixing and mixing of the mixed sludge and the second polymer flocculant, the rotational speed for mixing was 100 rpm and the mixing time was 2 min. For the test of sludge G, the first polymer flocculant solution was added in an amount of 15 mL and the second polymer flocculant solution was added in an amount of 5 mL. For mixing of the mixed sludge and the second polymer flocculant, the rotational speed for mixing was 200 rpm and the mixing time was 5 min. 33 The test results are listed in Table 3. Meanwhile, the symbol "-" in Table means that there is no data available. [0075] [Table 3] Mixing time of Water content in cake (dewatered high speed mixing sludge) (seconds) Sludge F Sludge G 3 78.0 5 77.9 10 82.2 76.1 20 Poor flocculation 76.6 30 Poor flocculation [0076] For sludge F, poor flocculation was formed when the mixing time for high speed mixing was 20 sec or longer. For sludge G, water content in cake (dewatered sludge) can be reduced when the mixing time for high speed mixing is 5 sec or longer. When the mixing time for high speed mixing is 10 sec, the water content in cake (dewatered sludge) can be reduced most significantly. When the mixing time for high speed mixing is 20 sec, the water content in cake (dewatered sludge) is worsened compared to 10 sec. Based on these results, it was found that, by adjusting the mixing time of high speed mixing preferably to 5 to 20 sec, more preferably 5 to 15 sec, and still more preferably 5 to 10 sec, the water content in cake (dewatered sludge) can be reduced. [0077] 34 (Example 4) In this Example, for the step of obtaining a cake (dewatered sludge) by performing the first mixing by adding the first polymer flocculant solution into the sludge, performing the second mixing by adding the second polymer flocculant solution, and dewatering the resulting flocculated sludge by using a belt filter press, the relation with water content of cake (dewatered sludge) and SS recovery rate was examined in accordance with modification of the type of the first polymer flocculant. [0078] For the test, sludge F was used. Sludge F was anaerobically-digested sludge. TS of sludge F was 20.9 g/L. As for the polymer flocculant, polymer flocculants (a, b, c, d, e, and f) with different molecular weight were used. Polymer flocculants a, b, c, d, and f are a cationic polymer flocculant. Among them, polymer flocculant a and b are a DAM-based polymer flocculant and polymer flocculants c, d, and f are a DAA-based polymer flocculant. Polymer flocculant e is an amphoteric polymer flocculant. Molecular weights of polymer flocculants a, b, c, d, e, and f (viscosity in mPa-s) were 3,000,000 (114 mPa-s), 4,000,000 (143 mPa-s), 5,000,000 (147 mPa-s), 6,000,000 (225 mPa-s), 7,000,000 (238 mPa-s), and 8,000,000 (280 mPa-s), respectively. As described herein, the molecular weight indicates average molecular weight obtained by viscometry. Further, the viscosity indicates a value obtained by dissolving the polymer flocculant in water at concentration of 2 g/L and performing measurement at rotational speed of 60 rpm and 35 25'C by using a Type B viscometer. [0079] The test procedure is as follows. To sludge (250 mL), 31 mL of the first polymer flocculant solution (concentration of 2 g/L) was added, and by mixing the sludge and the polymer flocculant solution for 10 seconds using a high speed mixer set at rotational speed of 5,000 rpm, mixed sludge was prepared. Next, to the mixed sludge, 9 mL of the second polymer flocculant solution (concentration of 2 g/L) was added, and by mixing the sludge and the polymer flocculant for 5 minutes using a mixer set at rotational speed of 200 rpm, the mixed sludge was flocculated to form a floc (flocculated sludge). As a final step, the floc (flocculated sludge) was dewatered by using a belt filter press and the water content (%) in the obtained cake (dewatered sludge) was measured. SS recovery rate was also measured. Meanwhile, the SS recovery rate (%) was calculated as "(Dry mass of cake (dewatered sludge) obtained from 250 mL sludge) + (SS contained in 250 mL sludge) x 100 [ditto for the following Examples]. The results are listed in Table 4. [0080] [Table 4] Polymer flocculant Water content in Ratio of SS Molecular cake (dewatered recovery rate Type weight sludge) (%) (-) a 3,000,000 82.2 87 b 4,000,000 82.8 81 36 c 5,000,000 82.0 97 d 6,000,000 82.0 97 e 7,000,000 82.4 97 f 8,000,000 82.2 100 [0081] In Table 4, the ratio of SS recovery rate indicates the ratio when SS recovery rate obtained by using polymer flocculant f is 100. The ratio of SS recovery rate was 97 or higher when the molecular weight is 5,000,000 or more (that is, polymer flocculants c, d, e, and f) . However, the ratio of SS recovery rate was 81 to 87 when the molecular weight is 4,000,000 or less (that is, polymer flocculant a and b). Meanwhile, the water content in cake (dewatered sludge) was in the range of 82 to 83% for any polymer flocculant used. Based on these results, it was found that, by using a polymer flocculant with molecular weight of preferably 4,500,000 or more and more preferably 5,000,000 or more, the SS recovery rate can be increased. [0082] (Example 5) In this Example, for the step of obtaining a cake (dewatered sludge) by performing the first mixing by adding the first polymer flocculant solution into the sludge, performing the second mixing by adding the second polymer flocculant solution, and dewatering the resulting flocculated sludge by using a belt filter press, the relation with SS recovery rate was examined in accordance with modification of addition amount of the first polymer 37 flocculant. [0083] For the test, sludge F was used. Sludge F was anaerobically-digested sludge. TS of sludge F was 20.9 g/L. As for the polymer flocculant, polymer flocculants (c, d, e, and f) with different molecular weight were used. Polymer flocculants c, d, and f were a cationic polymer flocculant and polymer flocculant e was an amphoteric polymer flocculant. Molecular weights of polymer flocculants c, d, e, and f (viscosity in mPa-s) were 5,000,000 (147 mPa-s), 6,000,000 (225 mPa-s) , 7,000,000 (238 mPa-s) , and 8,000,000 (280 mPa-s) , respectively. As described herein, the molecular weight indicates average molecular weight obtained by viscometry. Further, the viscosity indicates a value obtained by dissolving the polymer flocculant in water at concentration of 2 g/L and performing measurement at rotational speed of 60 rpm and 25'C by using a Type B viscometer. For the test, the same type of the first and the second polymer flocculants was used, and the first and the second polymer flocculant solutions were prepared to have the same concentration. [0084] The test procedure is as follows. To sludge (250 mL), a pre-determined amount of the first polymer flocculant solution (concentration of 2 g/L) was added, and by mixing the sludge and the polymer flocculant solution for 10 seconds using a high speed mixer set at rotational speed of 5,000 rpm, mixed sludge was prepared. Next, to the mixed sludge, a pre-determined 38 amount of the second polymer flocculant solution (concentration of 2 g/L) was added, and by mixing the sludge and the polymer flocculant for 5 minutes using a mixer set at rotational speed of 200 rpm, the mixed sludge was flocculated to form a floc (flocculated sludge). As a final step, the floc (flocculated sludge) was dewatered by using a belt filter press to obtain a cake (dewatered sludge). SS recovery rate was then measured. [0085] For the test, the total addition amount of the first and the second polymer flocculant solutions was 40 mL while the first polymer flocculant solution was added in an amount varying in the range of 2.5 to 37.5 mL. The test results are illustrated in Fig. 6. [0086] Fig. 6 is a graph illustrating the relation between the amount of polymer flocculant added for high speed mixing and ratio of average SS recovery rate when polymer flocculants c, d, e, or f is used. In the figure, the ratio of the average SS recovery rate indicates a ratio when the average SS recovery rate at the time of having addition amount of 59% for the first polymer flocculant was 100. Based on these results, it was found that, by adjusting the addition amount of the polymer flocculant added during high speed mixing preferably 45 to 95%, more preferably 50 to 95%, and still more preferably 55 to 90% of the total addition amount, the SS recovery rate can be increased. [0087] (Example 6) 39 In this Example, for the step of obtaining a cake (dewatered sludge) by performing the first mixing by adding the first polymer flocculant solution into the sludge, performing the second mixing by adding the second polymer flocculant solution, and dewatering the resulting flocculated sludge by using a belt filter press, the relation with water content in cake (dewatered sludge) was examined in accordance with modification of total addition ratio of the first and the second polymer flocculant. [0088] For the test, sludge C was used. Sludge C was anaerobically-digested sludge. TS of sludge C was 34.9 g/L. For the test, a cationic polymer flocculant h (amidine-based polymer flocculant, molecular weight of 3,000,000, viscosity of 34 mPa-s) was used for both the first and the second polymer flocculant. In addition, the cationic polymer flocculant h was prepared to be 10 g/L. [0089] The test procedure is as follows. To sludge (250 mL), a pre-determined amount of the first polymer flocculant solution was added, and by mixing the sludge and the polymer flocculant solution for 10 seconds using a high speed mixer set at rotational speed of 11,000 rpm, mixed sludge was prepared. Next, to the mixed sludge, 5 mL of the second polymer flocculant solution was added, and by mixing the sludge and the polymer flocculant for 2 minutes using a mixer set at rotational speed of 150 rpm, the mixed sludge was flocculated to form a floc (flocculated sludge). As a final step, the floc (flocculated sludge) was dewatered by using a belt filter press and the water content (%) in the obtained cake 40 (dewatered sludge) was measured. For the test, the addition amount of the second polymer flocculant solution was kept constant (5 mL) but the addition amount of the first polymer flocculant solution was varied in the range of 12 to 19 mL in accordance with the total addition amount. [0090] As Comparative Example, sludge was flocculated from 2 g/L polymer flocculant solution by mixing at normal speed. The flocculated sludge was dewatered by using a belt filter press. The test procedure of Comparative Example is as follows. To sludge (250 mL), a pre-determined amount of the polymer flocculant solution (concentration of 2 g/L) was added, and by mixing the sludge and the polymer flocculant solution for 3 minutes using a mixer set at rotational speed of 150 rpm, sludge was flocculated to form a floc (flocculated sludge) . Thereafter, the floc (flocculated sludge) was dewatered by using a belt filter press and the water content (%) in the obtained cake (dewatered sludge) was measured. For the test of Comparative Example, the addition amount of the polymer flocculant solution was varied in the range of 85 to 125 mL in accordance with the total addition amount. The test results are illustrated in Fig. 7. [0091] Fig. 7 illustrates the relation between the addition ratio of polymer flocculant and the water content in cake (dewatered sludge). Based on the results, it is 41 found that, having high speed mixing and the 10 g/L polymer flocculant solution can reduce the water content in cake (dewatered sludge) by 2 to 3 percentage points or so compared to a case in which normal speed mixing and the 2 g/L polymer flocculant solution are used. It is also found that, having high speed mixing and the 10 g/L polymer flocculant solution can reduce addition ratio of the polymer flocculant by 20 to 30 % or so compared to a case in which normal speed mixing and the 2 g/L polymer flocculant solution are used. [0092] (Example 7) In this Example, for the step of obtaining a cake (dewatered sludge) by performing sludge dilution by diluting sludge with water for dilution, performing the first mixing by adding the first polymer flocculant solution into the sludge, performing the second mixing by adding the second polymer flocculant solution, and dewatering the resulting flocculated sludge by using a screw press, it was examined whether or not the water content in cake (dewatered sludge) can be reduced by performing sludge dilution. [0093] For the test, sludge H was used. Sludge H is anaerobically-digested sludge. TS of sludge H was 42.5 to 43.5 g/L during the test. The concentration of a soluble component in sludge H is very high. Electrical conductivity of sludge H was 19.9 to 21.1 mS/cm and M alkalinity was 7,600 to 9,000 mg-CaCO 3 /L during the test. For the test, a cationic polymer flocculant h 42 (amidine-based polymer flocculant, molecular weight of 3,000,000, viscosity of 34 mPa-s) was used for both the first and the second polymer flocculant. In addition, the cationic polymer flocculant h was prepared to be 2 g/L or 5 g/L. For dissolution of polymer flocculant, industrial water was used. As water for dilution, industrial water was also used. [0094] The test procedure was according to a continuous mode as follows. To sludge (sludge flow rate of 3.0 m 3 /h), water for dilution (dilution water flow rate of 1.5 m 3 /h) was added to prepare diluted sludge (x 1.5 dilution). To the diluted sludge, the first polymer flocculant solution was added in an amount of 70% of the total addition amount, and then the diluted sludge and the polymer flocculant solution were mixed and stirred using a high speed mixer (mixer part volume: 0.8 L) set at rotational speed of 3,000 rpm, thus mixed sludge was prepared. Next, to the mixed sludge, the second polymer flocculant solution was added in an amount of 30% of the total addition amount, and then the mixed sludge and the polymer flocculant were mixed and stirred using a mixer (mixer tank volume: 900 L) set at rotational speed of 33 rpm during mixing. As a result, the mixed sludge was flocculated to form a floc (flocculated sludge). As a final step, the floc (flocculated sludge) was dewatered using a screw press and the water content (%) in the obtained cake (dewatered sludge) was measured. [0095] As Comparative Example, a test without adding 43 water for dilution was also performed. The test procedure was the same as described above except that water for dilution was not used. The test results are listed in Table 5. [0096] [Table 5] Concentration Addition ratio Water content of dissolved Dilution of polymer in cake polymer step flocculant (% (dewatered flocculant to TS) sludge) (%) (g/l) No 2.7 81.9 2 Yes 2.5 81.4 No 2.8 82.8 5 Yes 2.6 80.4 [0097] When the concentration of the polymer flocculant solution is 2 g/L, the addition ratio of the polymer flocculant and the water content in cake (dewatered sludge) can be reduced by performing the dilution step. When the concentration of the polymer flocculant solution is 5 g/L, the addition ratio of the polymer flocculant and the water content in cake (dewatered sludge) can be also reduced by performing the dilution step. Based on the results, it was found that the addition ratio of the polymer flocculant and the water content in cake (dewatered sludge) can be reduced by introducing a sludge dilution step. [0098] While the tests of the above Examples 1 to 4 44 correspond to a batch mode, the test of the present Example (that is, Example 7) was performed in a continuous mode. In addition, as a result of performing the same test as Examples 1 to 4 in a continuous mode, no difference in the test result was observed between the batch mode test and the continuous mode test. EXPLANATION OF SYMBOLS [0099] 1 Sludge storage tank 2 First polymer flocculant solubilizing tank 3 Second polymer flocculant solubilizing tank 4 First polymer flocculant pump 5 Second polymer flocculant pump 6 High speed mixing tank 7 High speed mixer 8 Normal speed mixing tank 9 Normal speed mixer 10 Dewatering apparatus 11 Polymer flocculant flow control valve 12 In-line mixer 45 Editorial Note The claims that follow this Description are numbered as pages 28-30.

Claims

1. A sludge flocculation method comprising: a first mixing step of adding a first polymer flocculant solution into the sludge as an object to be treated and mixing the sludge with the first polymer flocculant solution by high speed mixing using a mixer having a rotational speed of 1,000 rpm or higher to prepare a mixed sludge; and a second mixing step of adding a second polymer flocculant solution into the mixed sludge and mixing the mixed sludge with the second polymer flocculant solution in a mixing tank to grow a flocculated sludge.

2. The sludge flocculation method according to claim 1, wherein the polymer flocculant in the first polymer flocculant solution has a concentration of 3 g/L or higher.

3. The sludge flocculation method according to claim 1 or 2, wherein the time for mixing the sludge as an object to be treated and the first polymer flocculant solution during the first mixing step is 20 sec or less.

4. The sludge flocculation method according to any one of claims 1 to 3, wherein the first polymer flocculant has a molecular weight of 4,500,000 or higher.

5. The sludge flocculation method according to any one of claims 1 to 4, wherein the first polymer flocculant has a viscosity of 150 mPa-s or higher that is obtained by the measurement to be described below: measurement of the viscosity: polymer flocculant is dissolved in pure water at a concentration of 2 g/L and viscosity is measured at rotational speed of 60 rpm and 25'C by 46 using a Type B viscometer.

6. The sludge flocculation method according to any one of claims 1 to 5, wherein the addition amount of the first polymer flocculant is 45 to 95% by mass of the total addition amount of the first polymer flocculant and the second polymer flocculant.

7. The sludge flocculation method according to any one of claims 1 to 6, wherein a step of diluting sludge for diluting the sludge as an object to be treated with water for dilution is performed before the first mixing step.

8. The sludge flocculation method according to any one of claims 1 to 7, wherein the sludge flocculation method described in any one of claims 1 to 7 is performed before a mechanical dewatering step.

9. The sludge flocculation method according to any one of claims 1 to 7, wherein the flocculated sludge is formed and dewatered in the second mixing step.

10. The sludge flocculation method according to claim 8 or 9, wherein the dewatering is centrifugal dewatering.

11. An apparatus for sludge flocculation comprising a first mixing tank and a second mixing tank, wherein the first mixing tank comprises a means for adding a first polymer flocculant solution into the sludge as an object to be treated and mixing the sludge with the first polymer flocculant solution by high speed mixing using a mixer having a rotational speed of 1,000 rpm or higher to prepare a mixed sludge; and the second mixing tank comprises a means for adding a second polymer flocculant solution into the mixed sludge and mixing the mixed sludge with the second polymer flocculant 47 solution to grow a flocculated sludge.

12. The apparatus for sludge flocculation according to claim 11, the apparatus being connected to an apparatus for mechanical dewatering.

13. The apparatus for sludge flocculation according to claim 11 or 12, wherein the flocculated sludge is formed and dewatered in the second mixing tank.

14. The apparatus for sludge flocculation according to claim 12 or 13, wherein the dewatering is centrifugal dewatering. 48