Method for producing highly dehydrated sludge cake

Abstract

To obtain a highly dewatered sludge cake which is sufficiently dewatered.SOLUTION: A production method of a highly dewatered sludge cake includes immersing an activated sludge cake having a water content of 90% or less in hyperosmotic liquid having an osmotic pressure of 5 MPa or more to obtain the highly dewatered sludge cake.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for producing highly dehydrated sludge cake.

Background Art

[0002] Patent Document 1 describes a method for dehydrating sludge by bringing the sludge into contact with a 5% saline solution with a permeable membrane interposed therebetween. Patent Document 2 describes an osmotic pressure dehydration method of a filter press having an osmotic pressure dehydration step of supplying brine having a salt concentration of 2 to 5% to a filtration chamber.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] The proportion of sludge in industrial waste is large, and reduction of sludge is required. However, with conventional methods, it has not been possible to sufficiently dehydrate the sludge. If the sludge can be sufficiently dehydrated, it can contribute to the reduction of sludge. In addition, the sufficiently dehydrated sludge cake is also expected to be used as solid fuel. The present invention has been made in view of the above circumstances, and an object thereof is to obtain a highly dehydrated sludge cake that is sufficiently dehydrated.

Means for Solving the Problems

[0005] The inventors focused on the water content inside the cells of the microorganisms that make up the sludge, and intensively studied the responsiveness and dewatering properties when pressure was applied to the microbial cells. As a result, they obtained a new finding that by applying an ultra-high osmotic pressure of 5 MPa or more, it is possible to dehydrate even types of microorganisms that are difficult to dehydrate by conventional methods, and thus developed the method of the present invention.

[0006] The method for producing a highly dehydrated sludge cake of the present invention comprises immersing an activated sludge cake with a water content of 90% or less in a high osmotic pressure liquid with an osmotic pressure of 5 MPa or more to obtain a highly dehydrated sludge cake.

[0007] According to the present invention, a highly dehydrated sludge cake that is sufficiently dehydrated can be obtained.

Brief Description of the Drawings

[0008]

Figure 1

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Embodiments for Carrying Out the Invention

[0009] Preferred embodiments of the present invention will be described. While the activated sludge cake is immersed, it is advisable to stir the high osmotic pressure liquid. By doing so, a highly dewatered sludge cake with a lower moisture content can be obtained in a shorter time.

[0010] It is advisable to dehydrate the activated sludge to obtain the activated sludge cake with a moisture content of 90% or less. By performing the step of dehydrating the activated sludge before applying the high osmotic pressure liquid, a highly dewatered sludge cake can be efficiently obtained.

[0011] It is advisable that the moisture content of the highly dewatered sludge cake is 74% or less. By doing so, the highly dewatered sludge cake can be self-ignited and used as solid fuel.

[0012] Hereinafter, embodiments embodying the present invention will be described. In this specification, for the description using "~" for a numerical range, unless otherwise specified, it includes the lower limit value and the upper limit value. For example, in the description of "10~20", both the lower limit value "10" and the upper limit value "20" are included. That is, "10~20" has the same meaning as "10 or more and 20 or less".

[0013] 1. Method for producing highly dewatered sludge cake The method for producing a highly dewatered sludge cake is a method of obtaining a highly dewatered sludge cake by immersing an activated sludge cake with a moisture content of 90% or less in a high osmotic pressure liquid with an osmotic pressure of 5 MPa or more.

[0014] (1) Activated sludge cake Activated sludge is, for example, sludge containing microorganisms discharged during the process of sewage treatment or wastewater treatment by the activated sludge method. The microorganisms contained in the activated sludge decompose organic substances contained in sewage or wastewater and biologically purify the sewage or wastewater. Examples of the microorganisms include bacteria such as Escherichia coli and lactic acid bacteria, and fungi such as yeast and mold.

[0015] In this application, "activated sludge cake" means solid sludge with a water content below a predetermined level. The activated sludge cake can be obtained, for example, by solid-liquid separation of activated sludge containing water, as the separated solid content. The method of solid-liquid separation is not particularly limited, but from the perspective of an energy-saving treatment method, the mechanical dehydration method described later is preferable. The size and shape of the activated sludge cake are not particularly limited. The activated sludge cake may be in a lump form or may be crushed to ensure a surface area.

[0016] (2) High osmotic pressure liquid The high osmotic pressure liquid is used to highly dehydrate the activated sludge cake by utilizing the osmotic pressure difference. The high osmotic pressure liquid can act to dehydrate the water inside the cells of the microorganisms contained in the activated sludge cake. The mode of dehydration is not particularly limited, but it is preferable that cell rupture of the microorganisms contained in the activated sludge cake occurs due to the action of the high osmotic pressure liquid. In addition, the high osmotic pressure liquid can also act to dehydrate the interstitial water of the activated sludge cake.

[0017] The type of the high osmotic pressure liquid is not particularly limited. Examples of the solute of the high osmotic pressure liquid can include sodium chloride (NaCl), calcium chloride (CaCl 2 ), glucose, etc. Also, from the perspective of cost, the high osmotic pressure liquid may adopt a solution obtained by concentrating seawater, etc.

[0018] The osmotic pressure of the high osmotic pressure liquid is 5 MPa or more, preferably 8 MPa or more, more preferably 12 MPa or more, and even more preferably 20 MPa or more. The upper limit of the osmotic pressure of the high osmotic pressure liquid is not particularly limited, but for example, in the case of an aqueous calcium chloride solution, etc., it is 28 MPa, and usually 25 MPa or less. The osmotic pressure P of the high osmotic pressure liquid can be obtained from the van't Hoff equation shown in the following formula (1). The temperature of the high osmotic pressure liquid is affected by the outside air temperature, the structure of the apparatus used in this method, etc., but can be, for example, 20 °C. P = c × R × T ···(1) c is the ionic molar concentration (mol / dm 3 ), R is the gas constant (atm·dm 3 / K·mol), and T is the absolute temperature (K).

[0019] From the perspective of being able to achieve a high osmotic pressure and being inexpensive, an aqueous sodium chloride solution is preferred as the high osmotic pressure liquid. The concentration of the aqueous sodium chloride solution is preferably 7% or more, more preferably 10% or more, still more preferably 15% or more, and even more preferably 20% or more. Note that the concentration of the aqueous sodium chloride solution is usually 26% or less, and from the perspective of ease of preparing the solution, 25% or less is preferred, and 22% or less is more preferred. In this specification, the concentration of the solution is expressed as the ratio of the mass of the solute to the mass of the entire solution.

[0020] (3) Method The method for preparing a highly dehydrated sludge cake includes, for example, the following steps I and II. Step I is an optional step. Also, the method for preparing a highly dehydrated sludge cake may further include any steps other than Step I. FIG. 1 shows an example of the process of treating activated sludge. Step I: A step of dehydrating activated sludge to obtain an activated sludge cake with a water content of 90% or less Step II: A step of immersing the activated sludge cake with a water content of 90% or less in a high osmotic pressure liquid with an osmotic pressure of 5 MPa or more to obtain a highly dehydrated sludge cake

[0021] The method for dehydrating activated sludge in Step I is not particularly limited, but a mechanical dehydration method is preferred because it is an energy-saving treatment method and is widely adopted in sewage treatment or wastewater treatment. Examples of the mechanical dehydration method include, in the vacuum type, a method using a vacuum dehydrator; in the pressure type, a method using a dehydrator such as a filter press, a belt press, or a screw press; and in the centrifugal type, a method using a centrifugal dehydrator. The water content of the activated sludge cake obtained by the mechanical dehydration method is usually 70% - 80%.

[0022] The water content of the activated sludge cake treated in Process II is 90% or less, preferably 85% or less, more preferably 80% or less, and even more preferably 75% or less. If the activated sludge cake has a water content within the above range, it may be obtained only by ordinary mechanical dewatering methods, or it may be obtained by performing additional consolidation treatment in addition to ordinary mechanical dewatering methods, drying treatment to partially evaporate the water in the activated sludge cake, etc. Even when drying treatment is performed, the water content can be sufficiently reduced in the subsequent Process II, so that the energy and time required for the drying treatment can be reduced compared to conventional methods. The lower limit of the water content of the activated sludge cake is not particularly limited, and may be, for example, 65% or more, 70% or more.

[0023] Process II is a process of further dewatering the activated sludge cake with a water content of 90% or less by the osmotic pressure dewatering method. Specifically, the activated sludge cake is put into a tank filled with a high osmotic pressure liquid. Then, due to the action of osmotic pressure, the activated sludge cake is dewatered to obtain a highly dewatered sludge cake. By performing Process II, it is possible to omit thermal drying treatments such as hot air drying and sun drying, which have been conventionally performed in sewage treatment or wastewater treatment. In other words, from the viewpoints of energy efficiency and shortening the time required for treatment, it is preferable that the method for producing the highly dewatered sludge cake of the present invention does not include a thermal drying process for evaporating the water in the activated sludge cake.

[0024] The time for immersing the activated sludge cake in the high osmotic pressure liquid (immersion time) is preferably 5 minutes or more, more preferably 10 minutes or more, and even more preferably 20 minutes or more from the viewpoint of sufficiently reducing the water content. The upper limit of the immersion time is not particularly limited, but it is preferably the time when the water content of the activated sludge cake immersed in the high osmotic pressure liquid stops decreasing or greater than that. The time when the water content stops decreasing can be experimentally calculated from the relationship between the immersion time in the high osmotic pressure liquid and the water content of the immersed activated sludge cake, as described in Experiment 5 of the examples described later. The upper limit of the immersion time can be, for example, 2 hours, or it can be 1 hour or even 30 minutes. The upper limit of the immersion time may be appropriately set in accordance with the required time for other steps of continuous sewage treatment or wastewater treatment. In other words, compared with the mechanical dewatering method, the dehydration method using the high osmotic pressure liquid can reduce the water content in a short time and is less likely to cause factors that delay sewage treatment or wastewater treatment.

[0025] In Step II, it is preferable to stir the high osmotic pressure liquid while the activated sludge cake is immersed. When stirring the high osmotic pressure liquid, the immersion time can be shortened, and the water content of the highly dehydrated sludge cake can be made even lower. The method of stirring the high osmotic pressure liquid is not particularly limited. Specifically, a method of rotating a rod, plate, propeller-shaped stirrer, etc. at a constant speed in one direction in the tank to stir and mix the high osmotic pressure liquid can be exemplified. The degree of stirring is not particularly limited, but a water flow may be generated so that the water discharged from the activated sludge cake does not stay near the activated sludge cake.

[0026] The high osmotic pressure liquid after immersing the activated sludge cake has its concentration decreased and its osmotic pressure also decreased due to the water discharged from the activated sludge cake. The high osmotic pressure liquid after immersing the activated sludge cake may be concentrated by reverse osmosis or evaporation of water, etc., and reused in Step II.

[0027] 2. Highly dehydrated sludge cake The highly dehydrated sludge cake obtained by Process II is a sludge cake with a lower water content than the activated sludge cake before being immersed in the high osmotic pressure liquid. The water content of the highly dehydrated sludge cake is preferably 74% or less, more preferably 70% or less, still more preferably 60% or less, and even more preferably 55% or less. The lower limit of the water content of the highly dehydrated sludge cake is not particularly limited, but is usually 40% or more. It is said that when the water content of the sludge cake is 74% or less, it can be self-ignited. The highly dehydrated sludge cake with the water content in the above range has a large calorific value due to its high organic matter content and can be self-ignited, so it can be expected to be used as a solid fuel.

[0028] 3. Estimation of the mechanism for obtaining a highly dehydrated sludge cake The mechanism for obtaining a highly dehydrated sludge cake is not clear, but is estimated as follows. However, the present invention is not to be construed as being limited in any way by this estimated mechanism. In the following description, the Escherichia coli cake is a model of the activated sludge cake containing Escherichia coli. The lactic acid bacteria cake is a model of the activated sludge cake containing lactic acid bacteria. The yeast cake is a model of the activated sludge cake containing yeast. Details of each cake will be described in the examples later.

[0029] The inventors have found that in the case of Escherichia coli cake and lactic acid bacteria cake, the water inside the cells is dehydrated by applying a pressure of 1000 kPa (1 MPa), but in the case of yeast cake, the water inside the cells is not dehydrated even when a pressure of 2000 kPa (2 MPa) is applied (see Experiment 1 in the Examples and Figure 2). As a reason, it is conceivable that the structure of the outer covering (cell membrane and the layer outside thereof) such as the glucan-mannan type cell wall of yeast contributes to the difficulty of dehydrating yeast. Subsequently, when the pressure dependence of the modified consolidation coefficient was confirmed in Escherichia coli cake and yeast cake, the following findings were obtained (see Experiment 2 in the Examples and Figure 3). The modified consolidation coefficient of Escherichia coli cake increased proportionally with pressure up to 700 kPa, and then became a constant value. The modified consolidation coefficient of yeast cake increased proportionally with pressure up to 5 MPa, and then became a constant value. From these findings, it was suggested that even for microorganisms having a structure of an outer covering that is difficult to dehydrate, the water inside the cells can be dehydrated by applying a pressure of 5 MPa or more. That is, it is presumed that according to the present invention, by applying a high osmotic pressure liquid having an osmotic pressure of 5 MPa or more, it is possible to dehydrate even types of microorganisms that are difficult to dehydrate by conventional methods in activated sludge cake, and a highly dehydrated sludge cake that is sufficiently dehydrated can be obtained.

[0030] 4. Effects of the Embodiment According to the present embodiment, a highly dehydrated sludge cake that is sufficiently dehydrated can be obtained by an energy-saving method using an osmotic pressure difference. For example, it is also possible to obtain a highly dehydrated sludge cake that is self-combustible without performing a drying process. When the highly dehydrated sludge cake is self-combustible, the sludge can be used as a solid fuel. Effective utilization of sludge is an important issue, and it is expected to promote the utilization of sewage or wastewater sludge as biomass. That is, according to the present embodiment, the technology for converting sludge into solid fuel can be put into practical use by an energy-saving method. Furthermore, according to the present embodiment, the time required for dehydrating the activated sludge cake can be significantly shortened as compared with mechanical dehydration methods such as consolidation.

Examples

[0031] The following will be described more specifically by way of examples.

[0032] 1. Experiment 1 (Relationship between pressure and type of microorganism) In order to confirm the relationship between the pressure during pressing and the type of microorganism, the following Experiment 1 was conducted. First, as microorganisms, Escherichia coli (Gram-negative bacterium), Lactococcus lactis (Gram-positive bacterium), and yeast were mass-cultured. The microorganisms were separated from the culture suspension by centrifugation, and extracellular components were removed by washing. Then, they were suspended in a 10 mM citric acid - 20 mM Na 2 HPO 4 buffer at pH 7.0 to prepare a sample solution with a predetermined concentration. For the pressing (filtration and consolidation) dehydration test, constant-pressure pressing of each microorganism was performed at a constant pressure, and the change over time in the filtrate volume v was measured. For Escherichia coli and lactic acid bacteria, a precision filtration membrane made of cellulose mixed ester with a pore size of 0.1 μm (manufactured by ADVANTEC) was used as the filter medium. For yeast, a precision filtration membrane made of cellulose mixed ester with a pore size of 0.45 μm (manufactured by ADVANTEC) was used as the filter medium. The pressures for constant-pressure pressing were 100 kPa, 500 kPa, 1000 kPa, and 2000 kPa. The Escherichia coli cake, Lactococcus lactis cake, and yeast cake (collectively also referred to as the microorganism cake) that had reached the consolidation equilibrium were taken out of the filter, and the water content of the microorganism cake was measured. In this example, the water content of the microorganism cake was measured using an infrared moisture meter (manufactured by Shimadzu Corporation, MOC-120H).

[0033] The upper part of Figure 2 is a graph showing pressure and the water content of Escherichia coli cake. The middle part of Figure 2 is a graph showing pressure and the water content of yeast cake. The lower part of Figure 2 is a graph showing pressure and the water content of lactic acid bacteria cake. In each graph, the horizontal axis represents the pressure of pressing (kPa), and the vertical axis represents the water content (%). The dashed line in the figure is the water content when only intracellular water exists. When the value shown by this dashed line is exceeded, it is considered that all the water between cells (interstitial water) has disappeared and the water inside the cells has been dehydrated. The water content of the Escherichia coli cake fell below the dashed line at a pressure of 1 MPa or more. The water content of the Escherichia coli cake fell below the dashed line at a pressure of 500 kPa or more. The water content of the lactic acid bacteria cake fell below the dashed line at a pressure of 1000 kPa (1 MPa) or more. On the other hand, even when a pressure of 2000 kPa (2 MPa) was applied to the yeast cake, the water content of the cake did not fall below the dashed line. From these results, it was found that the water inside the cells of Escherichia coli and lactic acid bacteria is dehydrated by applying a pressure of 1 MPa, but the water inside the cells of yeast is not dehydrated even when a pressure of 2 MPa is applied. As a reason, it is considered that the glucan-mannan type cell wall possessed by yeast may be affecting.

[0034] 2. Experiment 2 (Pressure Dependence of Modified Consolidation Coefficient) In Escherichia coli and yeast, the following Experiment 2 was conducted to confirm the pressure dependence of the modified consolidation coefficient. This modified consolidation coefficient serves as an index for the progress of primary consolidation. Escherichia coli and yeast were used as the microorganisms for the experimental samples. After separating the microorganisms from each culture suspension by centrifugation and removing extracellular components by washing, they were suspended in a 10 mM citric acid - 20 mM Na 2 HPO 4 buffer at pH 7.0, and the sample solution was adjusted to a predetermined concentration. For the compression permeation and consolidation tests, a cell cross-sectional area of 28.3 cm 2A compression permeability test cell of the type was used. A constant pressure was applied and the change in the compressed cake thickness L over time was measured. The pressure was set to a predetermined pressure in the range of 100 kPa to 15,000 kPa. For E. coli, a microfiltration membrane made of mixed cellulose ester with a pore size of 0.1 μm (manufactured by ADVANTEC) was used as the filter medium. For yeast, 4A filter paper (manufactured by ADVANTEC) was used as the filter medium. After the consolidation equilibrium was reached, the porosity ε was calculated from the cake thickness, and the flow resistance α was calculated by a water permeability test of the cake layer. The corrected consolidation coefficient was calculated by the method described in the literature "Iritani, E., T. Sato, N. Katagiri and KJ Hwang; J. Chem. Eng. Japan, 43, 140-149 (2010)".

[0035] The upper graph in Fig. 3 shows the relationship between pressure and the corrected compaction coefficient for E. coli cake. The lower graph in Fig. 3 shows the relationship between pressure and the corrected compaction coefficient for yeast cake. The horizontal axis is the pressure p 2 (E. coli cake: kPa, yeast cake: MPa). The vertical axis shows the corrected consolidation coefficient C e (cm 2 / s). The modified compaction coefficient of the E. coli cake increased in proportion to the pressure up to 700 kPa, and then became a constant value. It is assumed that the E. coli cells burst at 700 kPa in the E. coli cake. The modified compaction coefficient of the yeast cake increased in proportion to the pressure up to 5 MPa, and then became a constant value. It is assumed that the yeast cells burst at 5 MPa in the yeast cake. In other words, it was suggested that yeast can be effectively dehydrated by applying a pressure of 5 MPa or more.

[0036] 3. Experiment 3 (Effect of hyperosmotic fluid on yeast) To confirm the effect of the high-osmotic-pressure solution on yeast, the following Experiment 3 was conducted. Yeast was immersed in a buffer solution and a high-osmotic-pressure solution (4M NaCl aqueous solution, osmotic pressure 19.5 MPa) for 1 hour each, and the particle size distribution of the yeast cells was measured. The particle size distribution was measured using a laser diffraction particle size distribution analyzer (manufactured by Shimadzu Corporation, SALD-2200). Figure 4 is a graph showing the particle size distribution of the yeast cells after immersion in each solution. The horizontal axis represents the particle diameter, and the vertical axis represents the relative particle amount (%). The solid-line graph is for the yeast cells immersed in the high-osmotic-pressure solution, and the dotted-line graph is for the yeast cells immersed in the buffer solution. From this result, it was found that the yeast immersed in the high-osmotic-pressure solution had a smaller particle diameter than the yeast immersed in the buffer solution. It was confirmed that by immersing in the high-osmotic-pressure solution, the water in the yeast cells moved into the high-osmotic-pressure solution and the yeast was dehydrated.

[0037] 4. Experiment 4 (Effect of High-Osmotic-Pressure Solution on Activated Sludge Cake) Activated sludge cake was prepared as follows. Artificial sewage was prepared and cultured at 30°C in an open system. The microbial community that grew spontaneously was used as activated sludge in the sample. 60 g of an activated sludge suspension adjusted to a concentration of 10,000 mg / L was charged into a filter, and filtration and subsequent consolidation were performed at a constant pressure of 500 kPa. As the filter medium, a precision filtration membrane made of cellulose mixed ester with a pore diameter of 0.1 μm (manufactured by ADVANTEC) was used. Note that filtration and squeezing (filtration and subsequent consolidation) are examples of mechanical dewatering methods.

[0038] Figure 5 is a graph showing the filtration behavior of the sludge suspension. The horizontal axis represents the filtrate volume v (cm) per unit area, and the vertical axis represents the reciprocal of the filtration rate dθ / dv (s / cm). As filtration proceeded, sludge accumulated and the plots showed a linear relationship. However, a behavior was observed where the value of dθ / dv increased rapidly at approximately 5.5 cm. This indicates a sharp decrease in the filtrate volume, suggesting that filtration ended and the process shifted to consolidation. The water content of the activated sludge cake taken out immediately after filtration ended was 71.68%. After filtration ended, pressure was continuously applied, and the water content of the activated sludge cake that reached consolidation equilibrium was 65.56%. Figure 6 is a graph showing the consolidation behavior of the activated sludge cake. The horizontal axis represents the time elapsed θ - θ t (s) since the end of filtration, and the vertical axis represents the filtrate volume v - v t (cm) since the end of filtration. As shown in Figure 6, it took approximately 11 hours to reach the consolidation equilibrium where the dehydration process was completely finished.

[0039] Subsequently, in order to confirm the effect of the high osmotic pressure liquid on the activated sludge cake, the following experiment was conducted using the activated sludge cake obtained as described above. The activated sludge cake with a water content of 71.68% taken out immediately after filtration ended was immersed in the high osmotic pressure liquid (4M NaCl aqueous solution, osmotic pressure 19.5 MPa) for each membrane. Then, a highly dehydrated sludge cake with a water content of 56.65% was obtained. The filtration cake taken out immediately after filtration ended was soft and sticky, and it was difficult to peel it off the membrane. However, when immersed in the high osmotic pressure liquid, it instantly became a hard and dense highly dehydrated sludge cake and naturally peeled off the membrane. This is considered to be because water was removed from inside the activated sludge cake and at the interface between the membrane and the activated sludge cake due to the action of the high osmotic pressure liquid. Next, the activated sludge cake with a water content of 65.56% that reached consolidation equilibrium was immersed in the high osmotic pressure liquid (4M NaCl aqueous solution, osmotic pressure 19.5 MPa). Then, a highly dehydrated sludge cake with a water content of 46.95% was obtained.

[0040] 5. Experiment 5 (Investigation of Immersion Conditions) To examine the immersion conditions, the following Experiment 5 was conducted. Activated sludge cakes with a water content of 70% to 75% were prepared in the same manner as in Experiment 4, and the relationship between the immersion time and the water content of the cake was investigated with and without stirring the high osmotic pressure liquid. When the high osmotic pressure liquid was not stirred, the activated sludge cake together with the membrane was immersed in the high osmotic pressure liquid (4M NaCl aqueous solution, osmotic pressure 19.5 MPa) and allowed to stand. Then, the water content of the activated sludge cake was measured at predetermined times from 0 hours to 180 hours. When the high osmotic pressure liquid was stirred, the activated sludge cake together with the membrane was immersed in the high osmotic pressure liquid (4M NaCl aqueous solution, osmotic pressure 19.5 MPa), and a stirrer was rotated in the high osmotic pressure liquid to perform stirring and mixing. Then, the water content of the activated sludge cake was measured at predetermined times from 0 hours to 40 minutes.

[0041] Figure 7 is a graph showing the change in the water content of the activated sludge cake during immersion in the high osmotic pressure liquid. The plots of circles "〇" are the cases where the high osmotic pressure liquid was not stirred, and the plots of triangles "△" are the cases where the high osmotic pressure liquid was stirred. The horizontal axis indicates the immersion time (minutes), and the vertical axis indicates the water content (%) of the activated sludge cake. When the high osmotic pressure liquid was not stirred, the water content decreased until the immersion time reached 20 minutes, and then became almost constant. The water content at an immersion time of 20 minutes was 60.97%. From these results, it was suggested that when dehydrating the activated sludge cake by applying the high osmotic pressure liquid, even when the high osmotic pressure liquid was not stirred, i.e., only by the immersion operation, a sufficient dehydration effect could be obtained in a shorter time compared to mechanical dehydration methods such as filtration and consolidation. When the high osmotic pressure liquid was stirred, the water content decreased until the immersion time reached 10 minutes, and then no further decrease in the water content was confirmed. The water content at an immersion time of 10 minutes was 56.42%. It is considered that when the water in the activated sludge cake and inside the microbial cells moves into the high osmotic pressure liquid, the liquid near the cells is diluted and the osmotic pressure effect decreases. When the high osmotic pressure liquid was stirred, it is presumed that the decrease in the concentration of the high osmotic pressure liquid near the activated sludge cake was suppressed, and a lower water content could be achieved in a shorter time.

[0042] 6. Experiment 6 (Examination of the osmotic pressure of the high osmotic pressure liquid) In order to examine the conditions regarding the osmotic pressure of the hypertonic solution, the following Experiment 6 was conducted. Activated sludge cake A was prepared in the same manner as in Experiment 4. Also, the activated sludge collected at the sewage treatment plant was centrifuged to prepare activated sludge cakes B and C. The obtained activated sludge cakes A to C were immersed in an aqueous NaCl solution with a concentration of 0% to 25% for 20 minutes, and the water contents of the obtained highly dehydrated sludge cakes A to C were measured. The water content of the activated sludge cake A before treatment was 80%, the water content of the activated sludge cake B was 89%, and the water content of the activated sludge cake C was 83%. The aqueous NaCl solution was not stirred. The temperature of the aqueous NaCl solution was 20°C. The measured water contents of the highly dehydrated sludge cakes A to C are shown in Table 1. In Table 1, "-" indicates that the test was not conducted and the water content was not measured.

Table 1

[0043] As shown in Table 1, it was found that by immersing the activated sludge cakes A to C in an aqueous NaCl solution with a salt concentration of 7% or more, that is, an osmotic pressure difference from inside the microbial cells of 5 MPa or more, highly dehydrated sludge cakes A to C that were sufficiently dehydrated could be obtained. Also, as the osmotic pressure of the aqueous NaCl solution increased, the water content of the obtained highly dehydrated sludge cake decreased. In any of the activated sludge cakes A to C, ultimately, a highly dehydrated sludge cake with a water content of 74% or less could be obtained. In particular, when the activated sludge cake C with a water content of 83% was immersed in the aqueous NaCl solution, a highly dehydrated sludge cake with a water content of 74% or less was obtained under the condition of an osmotic pressure of 5 MPa.

[0044] 7. Summary By immersing the activated sludge cake with a water content of 90% or less in a hypertonic solution with an osmotic pressure of 5 MPa or more, a highly dehydrated sludge cake that was sufficiently dehydrated could be obtained.

[0045] The foregoing examples are for illustrative purposes only and are not to be construed as limiting the present invention. Although the present invention has been described by way of examples of typical embodiments, the language used in the description and illustration of the present invention is to be understood as explanatory and exemplary rather than restrictive. As detailed herein, changes can be made within the scope of the appended claims without departing from the scope or essence of the present invention in its form. Although specific structures, materials, and examples have been referred to in the detailed description of the present invention, it is not intended to limit the present invention to the disclosures herein. Rather, the present invention is intended to cover all functionally equivalent structures, methods, and uses within the scope of the appended claims. The present invention is not limited to the embodiments detailed above, and various modifications or changes are possible within the scope indicated in the claims of the present invention.

Claims

**Claim 1**: Activated sludge is dehydrated by a mechanical dehydration method to obtain an activated sludge cake with a water content of 90% or less. A method for producing a highly dehydrated sludge cake, comprising: immersing the activated sludge cake with a water content of 90% or less in a high osmotic pressure liquid with an osmotic pressure of 5 MPa or more to dehydrate the interstitial water of the activated sludge cake and dehydrate the water inside the cells of the microorganisms contained in the activated sludge cake to obtain a highly dehydrated sludge cake. **Claim 2** The method for producing a highly dehydrated sludge cake according to claim 1, wherein the high osmotic pressure liquid is stirred while the activated sludge cake is immersed. **Claim 3**: The method for producing a highly dehydrated sludge cake according to claim 1 or 2, wherein the microorganism contains at least yeast. **Claim 4** The method for producing a highly dehydrated sludge cake according to any one of claims 1 to 3, wherein the water content of the highly dehydrated sludge cake is 60% or less.

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