Excess sludge volume reduction method and excess sludge volume reduction system

Abstract

To provide a method for volume reduction of excess sludge and a system for volume reduction of excess sludge that reduce an amount of electricity required for volume reduction treatment of excess sludge and achieve a good rate of volume reduction of excess sludge.SOLUTION: A method for volume reduction of excess sludge includes: a first step of reducing a diameter of particles contained in the excess sludge removed from the settling tank under the influence of cavitation; a second step of bringing the excess sludge that has undergone the first step into contact with ozone gas to reduce the diameter of particles contained in the excess sludge; a third step of bringing the excess sludge that has undergone the second step into contact with proteolytic enzymes to reduce the diameter of particles contained in the excess sludge; and a fourth step of decomposing the excess sludge that has undergone the third step by aerobic bacteria to decompose the excess sludge. Also provided is a system for volume reduction of excess sludge in which the excess sludge is decomposed by the method.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for reducing the volume of excess sludge and a system for reducing the volume of excess sludge. [Background technology]

[0002] The activated sludge process is commonly used in purification facilities to break down organic matter contained in wastewater discharged from homes, factories, etc. In the activated sludge process, air or oxygen is aerated into the wastewater that flows into an aeration tank, causing microorganisms such as aerobic bacteria to grow and turn into activated sludge, which then decomposes the organic matter. When wastewater is supplied to the aeration tank, an amount of liquid equal to the amount of the inflowing wastewater flows into a settling tank located downstream of the aeration tank.

[0003] Because activated sludge has a greater specific gravity than water, it accumulates at the bottom of the settling tank, and the liquid from the decomposed organic matter accumulates on top of the activated sludge. The supernatant is then subjected to post-treatment such as filtration and disinfection as necessary before being released into the external environment.

[0004] As wastewater treatment continues, the volume of activated sludge increases, resulting in the generation of excess sludge. Excess sludge is disposed of by landfilling the solids remaining after removing the liquid, or by incineration. This treatment is costly and places a burden on the environment, so there is a need to reduce the amount of excess sludge discharged outside the system.

[0005] For example, Patent Document 1 below describes a method in which excess sludge extracted from a settling tank is irradiated with ultrasound, then treated with ozone, and the ozone-treated excess sludge is returned to a biological reactor. It is said that approximately 50-60% of the excess sludge is solubilized by treating 100 ml of activated sludge liquid with ultrasound at 100 W for 60 minutes. The ozone concentration is 20 g / Nm 3 from 300g / Nm 3 The reaction time of ozone is said to be 0.1 to 6 hours.

[0006] Furthermore, for example, Patent Document 2 below describes a method in which ultrasonic waves are applied to excess sludge extracted from a solid separation tank, a chemical solution is added to the ultrasonically treated excess sludge and stirred, and the excess sludge with the added chemical solution is returned to the biological treatment tank. It describes that when the power consumption of ultrasonic treatment is 5 kW / kg-SS, the particle size of sludge with a particle size of less than 1 μm increases.

[0007] Patent Document 3 also describes a method in which activated sludge extracted from a sludge separation tank is sprayed into a container from a nozzle. This method is said to reduce the amount of COD in the solution, break down the suspended matter constituting the activated sludge, and disperse the microorganisms without killing most of them. The dispersed activated sludge is then reacted with dissolved oxygen and other substances in a digestion tank to cause the activated sludge to self-oxidize and mineralize, resulting in a reduction in the volume of the activated sludge. The sludge volume reduction rate is said to reach a maximum of approximately 18% when the cavitation coefficient is in the range of 0.01 to 0.1. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-202484 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-95717 [Patent Document 3] Japanese Patent Application Laid-Open No. 2001-314887 Summary of the Invention [Problem to be solved by the invention]

[0009] The method in Patent Document 1 relates to a small-scale experiment, and it is said that sludge can be solubilized by subjecting only 100 ml of excess sludge liquid to ultrasonic treatment at an output of 100 W for 60 minutes. In actual wastewater treatment, hundreds of kg to several tons of excess sludge are treated. When treating wastewater on a large scale, ultrasonic treatment consumes a large amount of electricity. In addition, the ozone concentration is set to 20 g / Nm 3from 300g / Nm 3 The method of Patent Document 2 also involves ultrasonic treatment, and consumes a large amount of power.

[0010] The method of Patent Document 3 utilizes cavitation to reduce the volume of excess sludge, but the volume reduction rate is low, reaching a maximum of 18%.

[0011] The present invention aims to provide a method and system for reducing the volume of excess sludge that reduces the amount of electricity required for the volume reduction treatment of excess sludge and achieves a good excess sludge volume reduction rate. [Means for solving the problem]

[0012] The above-mentioned problems are solved by a method for reducing the volume of excess sludge generated in the treatment of wastewater using aerobic bacteria, which includes a first step of reducing the particle size of the excess sludge removed from a sludge settling tank under the influence of cavitation; a second step of contacting the excess sludge that has undergone the first step with ozone gas to reduce the particle size of the excess sludge; a third step of contacting the excess sludge that has undergone the second step with a proteolytic enzyme to reduce the particle size of the excess sludge; and a fourth step of decomposing the excess sludge that has undergone the third step with aerobic bacteria to decompose the excess sludge.

[0013] The above problems are solved by a system for reducing the volume of excess sludge, which includes a first treatment section using cavitation to reduce the particle size of excess sludge discharged from a sludge settling tank under the influence of cavitation, a second treatment section using ozone gas to bring the excess sludge discharged from the first treatment section into contact with ozone gas to reduce the particle size of the excess sludge, and a third treatment section using protease to bring the excess sludge discharged from the second treatment section into contact with protease to reduce the particle size of the excess sludge, and which returns the excess sludge discharged from the third treatment section to an activated sludge tank and decomposes the excess sludge.

[0014] The volume reduction method and system described above utilize a combination of cavitation treatment, ozone gas treatment, and proteolytic enzyme treatment. Cavitation treatment can be achieved, for example, by passing a liquid through a constriction such as an orifice or a venturi, or by rotating a propeller, and requires relatively little power. Proteolytic enzyme treatment utilizes enzymes and does not require power for the enzymatic treatment itself. Ozone gas treatment requires a relatively large amount of power to generate ozone. The present invention utilizes three different processes—cavitation treatment, enzyme treatment, and ozone gas—to reduce the amount of ozone gas used, thereby reducing power consumption and reducing the size of excess sludge particles, making them more easily assimilated by aerobic bacteria. This reduces power consumption and achieves a favorable excess sludge reduction rate.

[0015] In the above-mentioned method for reducing the volume of excess sludge, the total amount of ozone supplied when contacting the excess sludge with ozone gas in the second step can be 20 to 150 g. Similarly, in the above-mentioned system for reducing the volume of excess sludge, it is preferable to control the amount of ozone gas supplied to the second treatment unit so that the total amount of ozone supplied is 20 to 150 g.

[0016] In the above-mentioned method for reducing the volume of excess sludge, the pH of the system in which the excess sludge is brought into contact with the protease is preferably controlled to 5.0 to 8.0 in the third step. Similarly, in the above-mentioned system for reducing the volume of excess sludge, the pH of the system in which the excess sludge is brought into contact with the protease is preferably controlled to 5.0 to 8.0 in the third treatment section.

[0017] In the above-mentioned method for reducing the volume of excess sludge, the first step is preferably a step of subjecting the excess sludge to the influence of cavitation by passing the excess sludge through a narrowed section multiple times.Similarly, in the above-mentioned system for reducing the volume of excess sludge, the first treatment section is preferably a step of subjecting the excess sludge to the influence of cavitation by passing the excess sludge through a narrowed section multiple times, thereby reducing the particle size contained in the excess sludge.

[0018] In the above-mentioned method for reducing the volume of excess sludge, the first step preferably circulates the excess sludge, thereby subjecting the excess sludge that has been subjected to cavitation treatment once again to the influence of cavitation.Similarly, in the above-mentioned system for reducing the volume of excess sludge, the first treatment section preferably comprises a storage tank for excess sludge and a cavitation generator having a plurality of narrowed sections, and the excess sludge is preferably circulated between the storage tank and the generator, thereby subjecting the excess sludge that has been subjected to cavitation treatment once again to the generator and subjecting it to the influence of cavitation. [Effects of the Invention]

[0019] According to the present invention, it is possible to provide a method for reducing the volume of excess sludge and a system for reducing the volume of excess sludge, which reduce the amount of electricity required for the volume reduction treatment of excess sludge and achieve a good volume reduction rate of excess sludge. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is an explanatory diagram showing one embodiment of a system for reducing the volume of excess sludge. [Figure 2] FIG. 2 is a cross-sectional view of the cavitation generating device shown in FIG. [Figure 3] 1 is a graph showing the results of analyzing the particle size distribution of sludge settled at the bottom of a sludge settling tank using a laser scattering particle size distribution analyzer. [Figure 4] 10 is a graph showing the results of analyzing the particle size distribution of excess sludge after the cavitation treatment in the first treatment section using a laser scattering particle size distribution analyzer. [Figure 5]10 is a graph showing the results of analyzing, with a laser scattering particle size distribution analyzer, the particle size distribution of excess sludge that has been treated with ozone gas in the second treatment unit. [Figure 6] 10 is a graph showing the results of analyzing, with a laser scattering particle size distribution analyzer, the particle size distribution of excess sludge that has been treated with a protease in the third treatment section. [Figure 7] 1 is a graph showing the results of analyzing, using a laser scattering particle size distribution analyzer, the particle size distribution of a mixture immediately after mixing activated sludge collected from the activated sludge tank with excess sludge that has undergone protease treatment in the third treatment unit. [Figure 8] 1 is a graph showing the results of analyzing, using a laser scattering particle size distribution analyzer, the particle size distribution of a mixture obtained by mixing activated sludge collected from the activated sludge tank with excess sludge that had undergone protease treatment in the third treatment unit and aerating the mixture for 12 hours. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, an embodiment of the method and system for reducing the volume of excess sludge according to the present invention will be described with reference to the drawings. The embodiment described below is merely an example of the present invention, and the technical scope of the present invention is not limited to the illustrated embodiment.

[0022] 1 shows an excess sludge volume reduction system 1. The volume reduction system 1 of this embodiment includes a first cavitation treatment section 5 that reduces the particle size of the excess sludge discharged from a sludge settling tank 4 under the influence of cavitation, a second ozone gas treatment section 6 that brings the excess sludge discharged from the first treatment section 5 into contact with ozone gas to reduce the particle size of the excess sludge, and a third protease treatment section 7 that brings the excess sludge discharged from the second treatment section 6 into contact with a protease to reduce the particle size of the excess sludge.

[0023] The volume reduction system 1 in Figure 1 further includes a raw water tank 2 into which untreated wastewater is introduced and temporarily stored, an activated sludge tank 3 in which organic substances contained in the wastewater are decomposed by aeration using aerobic bacteria, and a sludge settling tank 4 in which the treated liquid discharged from the activated sludge tank 3 is separated into a supernatant and a sediment.

[0024] In the volume reduction system 1 of Figure 1, the raw water tank 2, activated sludge tank 3, and sludge settling tank 4 are existing facilities. The excess sludge volume reduction system 1 is constructed by adding a first treatment unit 5, a second treatment unit 6, and a third treatment unit 7 to the existing facilities. The volume reduction system 1 of this embodiment can utilize existing facilities, thereby reducing construction costs. Of course, the raw water tank 2, activated sludge tank 3, sludge settling tank 4, first treatment unit 5, second treatment unit 6, and third treatment unit 7 may be newly installed, and the entire volume reduction system 1 may be newly constructed.

[0025] In the volume reduction system 1 shown in Figure 1, the raw water tank 2, activated sludge tank 3, and sludge settling tank 4 have known configurations. The raw water tank 2, activated sludge tank 3, and sludge settling tank 4 are connected via pumps so that wastewater flows in the listed order. The pump used to transport the wastewater is not shown. The raw water tank 2 temporarily stores wastewater discharged from factories and households. The wastewater flowing into the raw water tank 2 may be filtered through a screen to remove solids. The activated sludge tank 3 releases air or oxygen supplied via a pump from an aeration pipe located near the bottom, bringing these gases into gas-liquid contact with the wastewater. The sludge settling tank 4 receives wastewater aerated in the activated sludge tank 3, and allows the wastewater to settle, separating it into a supernatant liquid and a sediment (solids). The supernatant contains organic matter decomposed by the activated sludge, and is released to the external environment after solids are removed and sterilized as necessary.

[0026] In the volume reduction system 1 of FIG. 1, the sludge settling tank 4 and the first treatment unit 5 are connected by a first pipeline 44. A second pipeline 45 branching from the first pipeline 44 is connected to the first pipeline 44. The downstream end of the second pipeline 45 is connected to the activated sludge tank 3. A first valve 42 is provided in the first pipeline 44 downstream of the branch point between the first pipeline 44 and the second pipeline 45. Similarly, a second valve 43 is provided in the second pipeline 45 downstream of the branch point between the first pipeline 44 and the second pipeline 45. A first pump 41 and a third valve 46 are provided in the first pipeline 44 upstream of the branch point between the first pipeline 44 and the second pipeline 45.

[0027] The first valve 42, the second valve 43, and the third valve 46 are solenoid valves, and their opening and closing is controlled by a control unit (not shown). Similarly, the flow rate of the first pump 41 is controlled by a control unit (not shown). When the first pump 41 is operated with the first valve 42 and the third valve 46 open, excess sludge removed from the sludge settling tank 4 is transported to the first treatment unit 5 via the first pipeline 44. When the first pump 41 is operated with the second valve 43 and the third valve 46 open, excess sludge removed from the sludge settling tank 4 is returned to the activated sludge tank 3 via the first pipeline 44 and the second pipeline 45. The first pump 41 may be operated with either the first valve 42 or the second valve 43 and the third valve 46 open, or may be operated with all of the first valve 42, the second valve 43, and the third valve 46 open.

[0028] The first treatment unit 5 is a section for carrying out a first step of reducing the particle size of the excess sludge removed from the settling tank under the influence of cavitation. In the volume reduction system 1, as shown in Fig. 1, the first treatment unit 5 includes a storage tank 55 into which the excess sludge transported through the first pipeline 44 flows, and a cavitation generator 51 into which the liquid stored in the storage tank 55 is introduced by a pump 57 to be subjected to the influence of cavitation, and the treated excess sludge is returned to the storage tank 55. The cavitation generator 51 forms a circulation path with the storage tank 55, and the excess sludge treated in the cavitation generator 51 is supplied again to the cavitation generator 51 for treatment by cavitation.

[0029] As shown in FIG. 2, the cavitation generator 51 has an inlet opening 52, a first volume 511, a second volume 512, a third volume 513, a fourth volume 514, and an outlet opening 53. Each volume is a closed space having a predetermined volume. The first volume 511 and the second volume 512 are connected to each other by a first narrowing section 54. The second volume 512 and the third volume 513 are connected to each other by a second narrowing section 55. The third volume 513 and the fourth volume 514 are connected to each other by a third narrowing section 56. The inlet opening 52 is connected to a pipeline that allows excess sludge to flow from a storage tank 55 into the cavitation generator 51. The outlet opening 53 is connected to a pipeline that discharges excess sludge from the cavitation generator 51 to the storage tank 55.

[0030] When the first narrowed section 54, the second volume section 512, the second narrowed section 55, the third volume section 513, and the third narrowed section 56 are considered to be one set of cavitation generating sections, the first treatment section 5 in Fig. 2 is provided with two sets of cavitation generating sections, one in an upper and one in a lower stage. This is configured to increase the flow rate of excess sludge. The upper and lower first narrowed sections 54 each communicate with the first volume section 511, and the upper and lower third narrowed sections 56 each communicate with the fourth volume section 514.

[0031] Excess sludge flows into the first volume 511 through the inlet opening 52, passes through the first narrowed section 54, the second narrowed section 512, the second narrowed section 55, the third narrowed section 513, and the third narrowed section 56, in that order, and then passes through the fourth volume 514 before being discharged through the outlet opening 53. As the excess sludge passes through the first narrowed section 54, the second narrowed section 55, and the third narrowed section 56, its pressure instantly drops, causing cavitation. Cavitation is a physical phenomenon in which a pressure difference in a liquid flow causes the rapid generation and disappearance of tiny bubbles. Even steel ship propellers can be eroded and perforated under the influence of cavitation. The cavitation generator 51 utilizes the physical effects of cavitation to reduce the particle size of the excess sludge. In other words, as excess sludge flows in through the inlet opening and is discharged through the outlet opening, the excess sludge is subjected to the effects of cavitation multiple times, which reduces the particle size of the excess sludge.

[0032] In the case of an ultrasonic irradiation device, a relatively large amount of power is required to send the excess sludge to an ultrasonic treatment tank using a pump and irradiate it with ultrasonic waves in the ultrasonic treatment tank.However, the cavitation generator 51 is driven by the liquid sent by the pump 57, so it is possible to reduce the size of the excess sludge particles with less power.

[0033] As shown in FIG. 2, the first narrowed portion 54 is a circular hole provided in the partition wall separating the first volume portion 511 and the second volume portion 512. As shown by the dashed-dotted line in FIG. 2, the first narrowed portion 54 is provided so as to be inclined obliquely upward or downward with respect to a horizontal line drawn so as to be tangent to the edge of the inlet side of the hole. The inclination angle θ1 of the first narrowed portion 54 is preferably set to 30 to 40°. The third narrowed portion 56 is similarly provided so as to be inclined in the partition wall separating the third volume portion 513 and the fourth volume portion. The first narrowed portion 54 and the third narrowed portion 56 are provided in the plate-shaped partition wall and have a shape similar to that of an orifice.

[0034] The second narrowed section 55 has a venturi shape. Specifically, the second narrowed section 55 has a conical portion with a hypotenuse formed on the inlet side, a conical portion with a hypotenuse formed on the outlet side, and a cylindrical pipe connecting the two.

[0035] The excess sludge is discharged from the second pump 53 provided inside the storage tank 55 of the first treatment unit 5 , passes through the second pipeline 45 and flows into the second treatment unit 6 .

[0036] The second treatment unit 6 includes a tank for storing excess sludge and an ozone gas supply unit that supplies ozone gas from the bottom of the tank and brings the excess sludge into gas-liquid contact with the ozone gas. The liquid level in the second treatment unit 6 is monitored by the aforementioned control unit. When a certain liquid level is reached, the second pump 53 is stopped, terminating the supply of excess sludge, and the ozone gas supply unit begins supplying ozone gas. The aforementioned control unit controls the total amount of ozone supplied to the tank to be 20 to 150 g. The total amount of ozone can be controlled with a simple configuration by supplying a constant amount of ozone per unit time, recording the time for ozone supply, and controlling the supply of ozone gas to stop after the specified time has elapsed. The ozone gas can be generated using a known ozone gas generating device (ozonizer). The ozonizer may be combined with an oxygen concentrator.

[0037] When gas-liquid contact between the ozone gas and the excess sludge in the second treatment unit 6 is completed, the control unit operates the third pump 61 to transport the excess sludge through the fourth pipeline 62 to the third treatment unit 7. The third treatment unit 7 has a tank for temporarily storing the excess sludge, an enzyme supply unit, an acid or alkali supply unit, and a stirrer. When the liquid level in the tank of the third treatment unit 7 reaches a predetermined value, the control unit stops the third pump 61.

[0038] In the third treatment unit 7, the pH of the excess sludge temporarily stored in the tank is measured, and the control unit monitors whether the measured pH is within the range of 5.0 to 8.0. If the pH of the excess sludge deviates from this range, the control unit controls the acid or alkali supply unit to supply acid or alkali to the excess sludge to adjust the pH. In the third treatment unit 7, the control unit detects when the liquid volume of the excess sludge reaches a predetermined value and controls the enzyme supply unit to supply a proteolytic enzyme. The enzyme supply unit is configured to measure and supply a quantity of enzyme to the tank according to the enzyme amount set by the control unit.

[0039] When a predetermined amount of enzyme has been supplied to the excess sludge stored in the tank of the third treatment unit 7, the control unit activates the stirrer to stir the excess sludge and enzyme, and after a predetermined time has passed, stops stirring and allows the mixture to stand. The control unit detects the temperature of the mixture of excess sludge and enzyme stored in the tank and maintains it at a predetermined temperature in the range of 5.0 to 40.0°C.

[0040] After the stirring process is completed and a predetermined time has elapsed, the control unit operates the fourth pump 71 installed in the tank of the third treatment unit 7 to return the enzymatically treated excess sludge to the activated sludge tank 3. The particle size of the excess sludge becomes smaller than before treatment due to the treatments in the first treatment unit 5, second treatment unit 6, and third treatment unit. The reduced particle size excess sludge is completely assimilated and decomposed by the aerobic bacteria, i.e., activated sludge, present in the activated sludge tank, thereby achieving a reduction in the volume of the excess sludge.

[0041] The control unit is equipped with an arithmetic unit, a memory device, input units such as a touch panel, keyboard, mouse, buttons, etc., and an output unit such as a display, and the memory device stores the programs necessary to control the above-mentioned excess sludge volume reduction system.

[0042] The above-described excess sludge volume reduction system is a batch type in which a given amount of excess sludge is supplied to the first treatment unit 5, the second treatment unit 6, and the third treatment unit 7, the supply of excess sludge is stopped temporarily, and the excess sludge is treated to reduce its particle size. When the amount of excess sludge increases, the excess sludge is again supplied to the first treatment unit 5, the second treatment unit 6, and the third treatment unit 7, and treated to reduce its particle size. The control unit controls the first pump 41, the second pump 53, the third pump 61, the fourth pump 71, the pump 57 of the first treatment unit 5, the first valve 42, the second valve 43, and the third valve 46 to switch between supplying and not supplying excess sludge. The amount of excess sludge withdrawn from the sludge settling tank in one batch is not particularly limited, but can be, for example, 500 to 1500 kg.

[0043] The configuration of the existing facility consisting of the raw water tank 2, activated sludge tank 3, and sludge settling tank 4 is not limited to the above example. It may also have other treatment tanks, such as an adjustment tank for adjusting the amount of wastewater flowing into the activated sludge tank, a tank for adjusting the pH of the wastewater, or a settling tank installed before the activated sludge tank for settling solids.

[0044] Downstream of the sludge settling tank 4, other equipment may be provided as needed, such as equipment for sterilizing the treated liquid with chlorine or the like, and equipment for filtering solids mixed in the treated liquid.

[0045] The arrangement of the pumps, valves, and pipelines is not limited to the above examples and can be changed as appropriate. The pH control mechanism and stirring device are not essential and may be omitted.

[0046] The first processing unit 5, the second processing unit 6, and the third processing unit 7 may each be provided with a temperature adjusting means such as a chiller, a heater, a heating jacket, or a cooling jacket. The temperature adjusting means can be controlled by a control unit based on the temperature value obtained by a temperature detecting means such as a thermocouple.

[0047] The process of reducing the particle size of excess sludge by subjecting it to the influence of cavitation is preferably carried out by passing the excess sludge through a constriction section multiple times. For example, the number of times the excess sludge is passed through the constriction section is preferably three or more. It is also preferable to circulate the excess sludge so that excess sludge that has already been subjected to cavitation treatment is again subjected to the influence of cavitation. This method reduces the particle size during the process of transporting the excess sludge using the pump 51, eliminating the need for large amounts of power, such as ultrasonic irradiation. By passing the excess sludge through the constriction section multiple times, the particle size is further reduced, thereby reducing the amount of ozone used in the subsequent gas-liquid contact of ozone gas and the power required to generate ozone gas.

[0048] In the third step, the pH of the system in which the excess sludge is brought into contact with the protease is preferably 5.0 to 8.0. This increases the activity of the protease in breaking down the excess sludge into smaller particles. Furthermore, in the third step, the temperature of the system in which the excess sludge is brought into contact with the protease may be room temperature, preferably 5.0 to 40.0°C, and more preferably 15.0 to 40.0°C. This increases the activity of the protease in breaking down the excess sludge into smaller particles. The reaction between the enzyme and the excess sludge itself does not require electricity. Compared to contacting ozone gas with the excess sludge for a long period of time, the method using the protease reduces power consumption.

[0049] In the second step of contacting ozone gas with excess sludge to reduce the particle diameter of the excess sludge, the total amount of ozone supplied can be 20 to 150 g. The time for which the excess sludge is in gas-liquid contact with the ozone gas is not particularly limited, but can be, for example, 5 to 60 minutes or 5 to 40 minutes.

[0050] The total power consumption in the first processing apparatus 5, the second processing apparatus 6, and the third processing apparatus 7, excluding the power consumption for operating the pump for transporting the excess sludge, can be, for example, 1.0 to 10 kWh, 2.0 to 8.0 kWh, or 3.0 to 6.0 kWh. Furthermore, for example, the power consumption in the first processing apparatus 5, excluding the power consumption for operating the pump for transporting the excess sludge to the second processing apparatus 6, can be 0.1 to 0.9 kWh. Furthermore, for example, the power consumption in the second processing apparatus 6, excluding the power consumption for operating the pump for transporting the excess sludge to the third processing apparatus 7, can be 0.1 to 1.2 kWh. Furthermore, for example, the power consumption in the third processing apparatus 7, excluding the power consumption for operating the pump for transporting the excess sludge to the activated sludge tank 3 and the power required for temperature adjustment, can be 0.1 to 0.9 kWh.

[0051] In the first treatment unit 5, the time for which the excess sludge is subjected to the influence of cavitation is not particularly limited, but can be, for example, 5 to 60 minutes. In the second treatment unit 6, the time for which the excess sludge is brought into gas-liquid contact with the ozone gas can be 10 to 60 minutes. In the third treatment unit 7, the time for which the excess sludge is stirred with the enzyme can be 1 to 40 minutes. The time for which the enzyme is brought into contact with the excess sludge can be 60 to 120 minutes. [Example]

[0052] The present invention will be specifically described below by way of an example, but the technical scope of the present invention is not limited to the following example.

[0053] 1,000 kg of excess sludge was extracted from the sludge settling tank, which constitutes part of the excess sludge volume reduction system shown in FIG. 1, by the first pump, and the excess sludge was flowed into the storage tank 55 of the first treatment unit 5 shown in FIG. 2 via the first pipe 44. When the liquid level in the storage tank 55 reached a certain level, the control unit stopped the first pump 41 and activated the pump 57 of the first treatment unit 5. The excess sludge was circulated between the treatment unit 51 and the cavitation generator 51, and repeatedly passed through the first constriction section 54, the second constriction section 55, and the third constriction section 56. This process was carried out for 30 minutes. After 30 minutes had elapsed, the control unit stopped the pump 57 and activated the second pump 53 to transport the excess sludge from the storage tank 2 to the tank of the second treatment unit.

[0054] When the liquid level in the tank of the second treatment unit 6 reached a certain level, the control unit stopped the second pump 53 and activated the oxygen concentrator and an ozonizer capable of generating 150 g of ozone per hour to supply ozone gas at a certain concentration from the bottom of the tank, and the supply of ozone gas was stopped after 30 minutes. As a result, a total of 75 g of ozone was supplied into the tank.

[0055] The control unit operated the third pump 61 to transport excess sludge from the tank of the second treatment unit 6 to the tank of the third treatment unit 7. In winter, the control unit adjusted the temperature of the excess sludge in the tank using a heater to keep it within the range of 30 to 40°C. The control unit also adjusted the pH in the tank to keep it within the range of 5.0 to 8.0. The control unit added an appropriate amount of commercially available protease to the excess sludge, stirred it for 15 minutes, and then allowed it to stand for 70 minutes. The control unit operated the fourth pump 71 to return the settled excess sludge to the activated sludge tank 3. The control unit stopped the fourth pump 71 when the liquid level in the tank of the third treatment unit 7 dropped.

[0056] In the above-described method for reducing the volume of excess sludge, the particle size distribution of the sludge that settled to the bottom of the sludge settling tank 4 was analyzed using a laser scattering particle size distribution analyzer (HORIBA, LA-960-S2). The results of the analysis are shown in Figure 3. As is clear from the results in Figure 3, the particle size range of the sludge before volume reduction was 1.0 to 20 μm.

[0057] In the above-described method for reducing the volume of excess sludge, the particle size distribution of the excess sludge after the cavitation treatment in the first treatment unit 5 was analyzed using a laser scattering particle size distribution analyzer (HORIBA, LA-960-S2). The results of the analysis are shown in Figure 4. As is clear from the results in Figure 4, the particle size range of the sludge before volume reduction was 0.5 to 10 μm, which is smaller than the particle size distribution range of the excess sludge before treatment in the first treatment unit 5.

[0058] In the above-described method for reducing the volume of excess sludge, the particle size distribution of the excess sludge that had been treated with ozone gas in the second treatment device 6 was analyzed using a laser scattering particle size distribution analyzer (HORIBA, LA-960-S2). The results of the analysis are shown in Figure 5. As is clear from the results in Figure 5, the particle size range of the sludge before volume reduction was 0.10 to 0.7 μm, which is smaller than the particle size distribution range of the excess sludge before treatment in the second treatment device 6.

[0059] In the above-described method for reducing the volume of excess sludge, the particle size distribution of the excess sludge that had been treated with the protease in the third treatment unit 7 was analyzed using a laser scattering particle size distribution analyzer (HORIBA, LA-960-S2). The results of the analysis are shown in Figure 6. As is clear from the results in Figure 6, the particle size range of the sludge before volume reduction was 0.060 to 0.40 μm, which is smaller than the particle size distribution range of the excess sludge before treatment in the third treatment unit 7.

[0060] [Volume reduction rate] Next, the rate at which the volume of excess sludge that had been treated with the protease in the third treatment unit 7 was reduced by aerobic bacteria present in the activated sludge tank was examined.

[0061] First, the liquid containing suspended activated sludge was collected from the activated sludge tank 3 and used as the first sample. Next, the excess sludge immediately after the protease treatment in the third treatment unit 7 was collected and used as the second sample. The first and second samples were mixed at a volume ratio of 4 parts first sample to 6 parts second sample. The particle size distribution of the liquid immediately after mixing was analyzed using a laser scattering particle size distribution analyzer (HORIBA, LA-960-S2). The analysis results are shown in Figure 7. As is clear from the results in Figure 7, the particle size distribution of the liquid immediately after mixing included relatively large particles in the range of 0.60 to 9.0 μm and relatively small particles in the range of 0.30 to 0.40 μm. The former particles were aerobic bacteria present in the activated sludge tank, and the latter were excess sludge that had been refined by the volume reduction treatment.

[0062] Next, the liquid containing suspended activated sludge was collected from the activated sludge tank 3 and used as the first sample. The excess sludge immediately after the protease treatment in the third treatment unit 7 was collected and used as the second sample. The first and second samples were mixed in a container equipped with an aeration device, with a volume ratio of 4:6. Oxygen was supplied from the aeration device located at the bottom of the container for 12 hours. The particle size distribution of the treated liquid after aeration was analyzed using a laser scattering particle size distribution analyzer (HORIBA, LA-960-S2). The analysis results are shown in Figure 8. As is clear from the results in Figure 8, after aeration, all particles in the 0.30-0.40 μm range had disappeared, while particles in the 1.0-11.0 μm range still remained. The former particles are excess sludge that has been refined by the volume reduction treatment, and the latter particles are aerobic bacteria present in the activated sludge tank. These results show that the excess sludge after volume reduction treatment has been assimilated and decomposed by the aerobic bacteria present in the activated sludge tank, resulting in a reduction in volume. Since all particles in the 0.30 to 0.40 μm range have disappeared, the volume reduction rate of the excess sludge is 100%.

[0063] In the above-mentioned Patent Document 3, the volume reduction rate of excess sludge when aeration treatment was performed without ultrasonic irradiation was 6%, and when aeration treatment was performed with ultrasonic irradiation, the volume reduction rate was 10%. Ta The volume reduction rate of excess sludge at this time is a maximum of 18%. It can be seen that the volume reduction rate of excess sludge in this example is extremely high.

[0064] After the gas-liquid contact treatment between the ozone gas and the excess sludge in the second treatment unit 6 was completed, the excess sludge was collected, and the liquid portion was removed by centrifugation, after which the components contained in the solid portion were analyzed using the method shown in Table 1 below. As shown in Table 1 below, it was found that the solid portion of the excess sludge that had been treated with ozone gas contained a large amount of lipids, carbohydrates, sugars, and proteins. From the composition of the components contained in the solid portion, it is presumed that the aerobic bacteria contained in the excess sludge had been disrupted, and lipids, sugars, and proteins (peptides) derived from peptidoglycans and the like that make up the cell walls of the aerobic bacteria were detected.

[0065] [Table 1]

[0066] The cavitation generator 51 in the first treatment unit 5 is driven by a single 0.56 kW pump. The power consumption when the pump is operated for 30 minutes is 0.28 kWh. A PSA ozone concentrator, compressor, and ozonizer are used to generate ozone in the second treatment unit, and these consume 8.0 kW of power. The power consumption when these devices are operated for 30 minutes is 4.0 kWh. A 0.4 kW agitator is used for enzyme treatment in the third treatment unit 7. The power consumption when the agitator is used for 15 minutes is 0.1 kWh. The total power consumption per excess sludge volume reduction treatment is 4.38 kWh, which is extremely low.

[0067] A pump is used to transport the excess sludge, and in the conventional method, a pump is also used to transport the excess sludge. Furthermore, the pump only operates for a very short time, about 3 minutes per pump. Therefore, the power consumption required to operate the pump is ignored. The power consumption of the pump is about 0.2 to 0.75 kW per pump. [Explanation of symbols]

[0068] 1. Excess sludge volume reduction system 3 Activated sludge tank 4 Sludge settling tank 5. First Processing Section 55 Reservoir 51 Cavitation generator 6. Second Processing Section 7 Third Processing Section

Claims

1. This is a method for reducing the volume of excess sludge generated in wastewater treatment using aerobic bacteria. a first step of reducing the particle size of excess sludge removed from the sludge settling tank under the influence of cavitation; a second step of contacting the excess sludge that has been subjected to the first step with ozone gas to reduce the diameter of particles contained in the excess sludge; a third step of contacting the excess sludge that has been subjected to the second step with a protease to reduce the diameter of particles contained in the excess sludge; a fourth step of decomposing the excess sludge that has been subjected to the third step using aerobic bacteria to decompose the excess sludge; The volume reduction of the excess sludge removed from the sludge settling tank is carried out in a batch manner, A method for reducing the volume of excess sludge, wherein the total power consumption in the first, second, and third steps is 1.0 to 10 kWh per batch, excluding the power consumption of a pump that transports the excess sludge.

2. 2. The method for reducing the volume of excess sludge according to claim 1, wherein the second step is a step of contacting the excess sludge with ozone gas in a second treatment unit having a tank for storing the excess sludge, and the total amount of ozone supplied to the tank is 20 to 150 g per batch.

3. 3. The method for reducing the volume of excess sludge according to claim 1, wherein in the third step, the pH of the system in which the excess sludge is brought into contact with the protease is 5.0 to 8.

0.

4. 3. The method for reducing the volume of excess sludge according to claim 1, wherein the temperature of the system in which the excess sludge is brought into contact with the protease in the third step is 5.0 to 40.0°C.

5. 5. A method for reducing the volume of excess sludge according to claim 1, wherein the first step is a step of passing the excess sludge through a constricted section a plurality of times to expose the excess sludge to the influence of cavitation.

6. 6. A method for reducing the volume of excess sludge according to claim 1, wherein the first step is a step of circulating the excess sludge and subjecting the excess sludge that has been subjected to cavitation treatment once again to the influence of cavitation.

7. a first treatment section using cavitation that reduces the particle size of the excess sludge discharged from the sludge settling tank under the influence of cavitation; a second treatment section using ozone gas, which brings the excess sludge discharged from the first treatment section into contact with ozone gas to reduce the diameter of particles contained in the excess sludge; a third treatment section using a protease to reduce the diameter of particles contained in the excess sludge by contacting the excess sludge discharged from the second treatment section with the protease; The excess sludge discharged from the third treatment section is returned to the activated sludge tank to decompose the excess sludge. The volume of excess sludge removed from the sludge settling tank is reduced in a batch process. A system for reducing the volume of excess sludge, wherein the total power consumption in the first treatment unit, the second treatment unit, and the third treatment unit is 1.0 to 10 kWh per batch, excluding the power consumption of a pump that transports the excess sludge.

8. an activated sludge tank that decomposes organic matter contained in wastewater while aerating it with aerobic bacteria; 8. The excess sludge volume reduction system according to claim 7, further comprising a sludge settling tank for separating the treated liquid discharged from the activated sludge tank into a supernatant and a sediment.

9. 9. The excess sludge volume reduction system according to claim 7 or 8, wherein the amount of ozone gas supplied to the tank for storing excess sludge in the second treatment unit is controlled so that the total amount of ozone supplied to the tank in one batch is 20 to 150 g.

10. 10. The system for reducing the volume of excess sludge according to claim 7, wherein the pH of the system in which the excess sludge is brought into contact with the protease in the third treatment section is controlled to be in the range of 5.0 to 8.

0.

11. 11. The excess sludge volume reduction system according to claim 7, wherein the temperature of the system in which the excess sludge is brought into contact with the protease in the third treatment section is controlled to 5.0 to 40.0°C.

12. A system for reducing the volume of excess sludge as described in any one of claims 7 to 11, wherein the first treatment unit reduces the particle size contained in the excess sludge by passing the excess sludge through a narrowed section multiple times and subjecting it to the influence of cavitation.

13. A system for reducing excess sludge volume as described in claims 7 to 12, wherein the first treatment unit comprises a storage tank for excess sludge and a cavitation generator having multiple narrowing sections, and the excess sludge is circulated between the storage tank and the generator, and the excess sludge that has been subjected to cavitation treatment is again flowed into the generator and subjected to the influence of cavitation.

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