JP2012200691A - Method and system for methane fermentation of sludge using hydrothermal reaction - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a methane fermentation method and system that can increase the amount of methane gas recovered from sludge pretreated by hydrothermal reaction.SOLUTION: After sludge S is concentrated to a predetermined moisture content by a concentration device 4, the concentrated sludge S is circulated between a hydrothermal reactor 11 and a gas-liquid separator 12 for a predetermined time to reduce the molecular weight thereof while heating the sludge to a hydrothermal reaction temperature T maximizing the amount of recovered methane gas per unit amount of the sludge resulting from methane fermentation, and the sludge S having being reduced in the molecular weight is held in a methane fermentation tank 20 for a predetermined time to recover methane gas G. Preferably, the hydrothermal reaction temperature T is a temperature in a range of 160-200°C, maximizing the amount of recovered methane gas per unit amount of the sludge resulting from methane fermentation, and the sludge S having being reduced in the molecular weight is held for 3-5 days in the methane fermentation tank 20 to recover the methane gas G.

Description

  TECHNICAL FIELD The present invention relates to a sludge hydrothermal reaction type methane fermentation treatment method and system, and in particular, a method and apparatus for methane fermentation treatment after sludge discharged from a sewage treatment plant, wastewater treatment plant, etc. is reduced in molecular weight by a hydrothermal reaction. About.

  In wastewater treatment plants such as sewage treatment plants, chemical factories, and food factories, the activated sludge method that microbially decomposes organic matter in sewage and wastewater is widely used, and a large amount containing solid matter such as undegraded organic matter and grown microorganisms. Raw sludge and surplus sludge (hereinafter referred to simply as sludge). Sludge must be disposed of as industrial waste, and many sludges have been disposed of in landfills after dehydration or incineration, but recently they have been reduced in volume and volume by avoiding incineration from the viewpoint of preventing global warming. As one of the methods, research and development of methane fermentation treatment technology that can reduce the volume and volume of sludge and at the same time recover methane gas as an energy resource is underway. Although it is possible to decompose sludge into methane gas using conventional methane fermentation treatment technology, the sludge contains a large amount of difficult-to-decompose solids derived from microorganisms, so the decomposition efficiency is low and it is decomposed into methane gas. It takes a very long time (for example, 15 to 30 days or more of digestion days), and there is a problem that the scale of the facility becomes large.

  In order to increase the efficiency of sludge decomposition treatment by methane fermentation treatment, it has been proposed that methane fermentation treatment is performed after sludge is depolymerized by pretreatment such as alkali treatment, ozone treatment, and ultrasonic treatment (for example, Patent Document 1). reference). However, these pretreatments have not yet reached an economic balance from the viewpoint of energy consumption. On the other hand, hydrothermal reaction (hydrolysis reaction with high-temperature and high-pressure water) has attracted attention as a pretreatment technology that can reduce sludge to a low molecular weight with relatively small energy consumption. Hydrothermal reaction is hydrolysis using high-temperature and high-pressure water (hereinafter sometimes referred to as hot water) having an ionic product nearly 300 times lower than the critical point of water (374 ° C., 22 MPa) but having a temperature and pressure lower than 300 times. While it is a reaction and can hydrolyze sludge containing solids to a low molecule that can be easily fermented to methane in a short time by hydrolyzing action of hot water, it has weaker decomposition power than supercritical water reaction, Since it can be taken out before being decomposed into inorganic substances, it is said to be suitable for pretreatment of sludge (see Patent Documents 2 and 3).

  For example, in Patent Document 2, as shown in FIG. 4, the sludge slurry stored in the supply tank 6 is supplied to the hydrothermal reaction device 10a by the supply device (high pressure pump or the like) 7 through the heater 11a having a predetermined temperature. A method for producing methane gas is disclosed in which, after molecularization, the aqueous phase separated from the low molecular weight treated product is introduced into the methane fermentation tank 20 by a metering pump 19a and subjected to methane fermentation. For example, the low molecular weight processed product after the hydrothermal reaction is temporarily stored in the raw material tank 19 and separated into three layers of an oil phase, an aqueous phase and a solid phase by a centrifuge (not shown), and the separated aqueous phase is converted into methane. Introduce into the fermenter 20. Methane gas generated in the methane fermentation tank 20 is collected in the gas tank 23 through a pipe. Although the illustrated example shows a continuous hydrothermal reaction apparatus 10a for continuously reducing the molecular weight of sludge slurry, a batch type hydrothermal reaction apparatus can also be used.

  Patent Document 3 discloses a sludge pretreatment method using a pulverization tank 8a, a supply tank 6, and a circulating hydrothermal reactor 10 as shown in FIG. The sludge stored in the pulverization tank 8a in the illustrated example is pulverized into a slurry while circulating through a pulverization pump 8b and a three-way valve 8c (switched to the pulverization tank 8a side), and then the three-way valve 8c (supply tank). 6 is switched to the supply side 6) and further supplied to the suction side of the circulation pump 14 of the hydrothermal reaction device 10 via the supply device (Mono pump or the like) 7. The circulation type hydrothermal reaction apparatus 10 in the illustrated example includes a hydrothermal reactor 11 including a heat exchanger, a gas-liquid separator 12 and a circulation pump 14, and the supplied sludge slurry is converted into the hydrothermal reactor 11. The molecular weight is reduced while circulating in the circulation path connecting the gas-liquid separator 12 and the circulation pump 14.

  In FIG. 5, the sludge slurry supplied to the hydrothermal reactor 10 is heated by the heat exchange with the heating medium H while moving up inside the thin tube group (heat exchanger) in the hydrothermal reactor 11 by the circulation pump 14. Thus, the molecular weight is reduced, and it is fed into the gas-liquid separator 12 through the upper communication pipe 11a. The pressure difference between the hydrothermal reactor 11 and the gas-liquid separator 12 is eliminated by the pressure equalizing tube 11b, and the liquid level of the gas-liquid separator 12 is maintained constant by the control device 15. Therefore, in response to the sludge being fed from the hydrothermal reactor 11, a part of the decomposition aqueous solution (sludge with reduced molecular weight) in the gas-liquid separator 12 reacts through the liquid level control valve 15a under its own pressure. Overflowing or withdrawing to the outside of the apparatus 10, the remaining decomposition aqueous solution is returned again into the hydrothermal reactor 11 by the circulation pump 14 and circulated. By making the internal volume of the circulation path coincide with the product (xy) of the supply amount x of the sludge slurry and the circulation time y of the decomposition aqueous solution, the circulation time y of the sludge slurry in the hydrothermal reactor 10 is kept constant, The decomposed aqueous solution output to the outside can have a uniform concentration. In the figure, reference numeral 16 denotes a thermometer of the gas-liquid separator 12, reference numeral 17 denotes a pressure gauge with a pressure relief valve 17a, and reference numeral 18 denotes a safety valve.

JP 2000-288594 A International Publication No. 2004/037731 Pamphlet JP 2008-296192 A JP 2000-167523 A

  However, the continuous (or batch) hydrothermal reactor 10a shown in FIG. 4 is capable of reducing the molecular weight of a sludge slurry having a relatively low concentration, but the sludge having a high solid content. There is a problem that is not suitable for lowering the molecular weight of the slurry. That is, when a high-concentration sludge slurry is supplied to the one-pass (through) type hydrothermal reaction apparatus 10a as shown in FIG. 4, solid-liquid separation occurs inside the apparatus, and the liquid component is discharged preferentially and solid. Since the water stays inside the device, the concentration of the sludge discharged from the device (sludge with reduced molecular weight) tends to vary. Further, there is a possibility that the solid content staying inside the apparatus may come into contact with the high temperature inner surface and be fixed or burnt. Therefore, the pretreatment using the hydrothermal reactor 10a in FIG. 4 requires that the sludge be heated at a low concentration containing a large amount of water, and therefore requires relatively large heating energy for the low molecular weight treatment. Issues remain.

  On the other hand, the circulation type hydrothermal reaction apparatus 10 as shown in FIG. 5 can also perform a low molecular weight treatment on a high-concentration sludge slurry containing a solid content (for example, a sludge slurry having a solid content rate of 30 to 70% by weight). is there. That is, in the hydrothermal reactor 10 of FIG. 5, the internal volume of the circulation path connecting the hydrothermal reactor 11 and the gas-liquid separator 12 is expressed as xy (= sludge slurry supply amount x and decomposition aqueous solution circulation time y. The slurry residence time in the circulation path can be kept constant even when the slurry is repeatedly circulated in the system, and the flow rate is faster than the terminal sedimentation rate of the solid content in the slurry (for example, the terminal). It is possible to avoid sedimentation and stagnation of solid content in the apparatus described above by generating a flow several times faster than the sedimentation speed. When the circulation type hydrothermal reactor 10 of FIG. 5 is combined with the methane fermentation tank 20 of FIG. 4 as a pretreatment, sludge is reduced in molecular weight with relatively small energy consumption, and the methane fermentation treatment efficiency of sludge is economically increased. I can expect that.

  However, when looking at the energy efficiency of the entire system that combines the hydrothermal reactor 10 and the methane fermentation tank 20, it is not sufficient that the energy consumption of the pretreatment for reducing the sludge to be a low molecular weight is sufficient. It is important that a sufficient amount of methane gas can be recovered, and that the energy consumed in the pretreatment can be supplemented or covered by the energy of the recovered methane gas. Patent Document 3 suggests that the optimum conditions of the hydrothermal reactor 10 can be set from the viewpoint of sludge decomposition characteristics in the pretreatment. From the viewpoint of the entire system combined with the methane fermentation tank 20, the methane fermentation efficiency is It is necessary to set the conditions of the pretreatment hydrothermal reactor 10 so that the recovered amount of methane gas is increased.

  Therefore, an object of the present invention is to provide a methane fermentation treatment method and system capable of increasing the amount of methane gas recovered from sludge pretreated by a hydrothermal reaction.

  As a result of research and development of methane fermentation treatment combined with hydrothermal reaction as a pretreatment, the present inventors have achieved sludge that has been reduced in molecular weight by the hydrothermal reaction in the pretreatment (hereinafter sometimes referred to as low molecular weight sludge). Even if the decomposition rate of the solid content is about the same, the amount of methane gas per unit sludge that can be recovered during methane fermentation treatment varies depending on the reaction temperature of the hydrothermal reaction (hereinafter sometimes referred to as hot water temperature) (See the graph of FIG. 3 described later). If the hot water temperature of the hydrothermal reaction in the pretreatment is set so as to be suitable for the methane fermentation treatment, it can be expected that the recovery amount of methane gas is increased and the efficiency of the entire system including the pretreatment is increased. The present invention has been completed as a result of further research and development based on this finding.

  Referring to the embodiment of FIG. 1, the sludge hydrothermal reaction type methane fermentation treatment method according to the present invention concentrates the sludge S to a predetermined moisture content (see the concentrator 4 in FIG. 1), and then concentrates the sludge S. Is circulated between the hydrothermal reactor 11 and the gas-liquid separator 12 for a predetermined time to reduce the molecular weight while heating to a hot water temperature T at which the amount of methane gas recovered per unit amount of sludge by methane fermentation treatment is maximized (see FIG. 1), the low molecular weight sludge S is retained in the methane fermentation tank 20 for a predetermined time and the methane gas G is recovered.

  Referring to the block diagram of FIG. 1, the sludge hydrothermal reaction type methane fermentation treatment system according to the present invention is a concentrator 4 for concentrating the sludge S to a predetermined moisture content, and the concentrated sludge S is converted into a hydrothermal reactor. A circulating hydrothermal reactor that circulates between the gas 11 and the gas-liquid separator 12 for a predetermined time and reduces the molecular weight while heating to a hot water temperature T at which the amount of methane gas recovered per unit amount of sludge by methane fermentation treatment is maximized. 10 and a methane fermentation tank 20 for collecting the methane gas G by retaining the low molecular weight sludge S for a predetermined time.

  Preferably, the hot water temperature T of the circulating hydrothermal reactor 10 is set to a temperature T at which the amount of methane gas recovered per unit amount of sludge by the methane fermentation treatment is maximized in a temperature range of 160 to 200 ° C. The hot water temperature T is preferably adjusted according to a predetermined circulation time of the concentrated sludge S (predetermined time for circulation between the hydrothermal reactor 11 and the gas-liquid separator 12). It is desirable to retain the methane fermentation tank 20 for 3 to 5 days to recover the methane gas G. More preferably, as in the illustrated example, a boiler or other energy conversion device 25 that inputs methane gas G recovered in the methane fermentation tank 20 and supplies heating energy to the hydrothermal reactor 11 is provided.

  The sludge hydrothermal reaction-based methane fermentation treatment method according to the present invention concentrates the sludge S to a predetermined moisture content and then circulates it in the circulation hydrothermal reactor 10 to recover the amount of methane gas per unit amount of sludge by the methane fermentation treatment. Is heated to the maximum hot water temperature T, the molecular weight is reduced, the low molecular weight sludge is retained in the methane fermentation tank 20 for a predetermined time, and the methane gas G is recovered.

(B) Since the hot water temperature T of the hydrothermal reactor 10 is adjusted so that the amount of methane gas recovered per unit amount of sludge is maximized, the energy of the entire system combining the hydrothermal reactor 10 and the methane fermentation tank 20 Efficiency can be optimized.
(B) Even when the type and concentration of the solid content in the sludge S are different, the predetermined time for circulation to the hydrothermal reactor 10 is adjusted, and the hot water temperature T is set to the sludge unit amount according to the circulation time. It can be expected that the amount of methane gas recovered from the sludge S can be maximized by adjusting the amount of methane gas recovered per unit to be maximum.
(C) By maximizing the amount of methane gas recovered from the methane fermentation tank 20, the heating energy of the hydrothermal reactor 10 will be covered by the energy of methane gas, and the system will be able to reduce and reduce the amount of sludge S voluntarily. Can do.

DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments and examples for carrying out the present invention will be described with reference to the accompanying drawings.
FIG. 3 is a block diagram of an embodiment of the present invention. These are an example of the graph which shows the relationship between the methane fermentation processing time (sludge residence time of a fermenter) with respect to low molecular weight sludge, and solid content decomposition rate. Is the low molecular weight treatment temperature (hydrothermal temperature) by hydrothermal reaction and the methane fermentation specific activity of the sludge after the low molecular weight treatment (the solid content of the treated sludge when the solid content decomposition rate of untreated sludge is 1) It is an example of the graph which shows the relationship with (fractionation rate). These are explanatory drawings of an example of the methane fermentation processing method using the conventional hydrothermal reaction. These are explanatory drawings of other examples of the methane fermentation processing method using the conventional hydrothermal reaction.

  FIG. 1 shows an embodiment in which the methane fermentation treatment system of the present invention is applied to volume reduction / reduction of sewage sludge S generated at a sewage treatment plant 1, for example. The system of the illustrated example includes a concentrating device 4 that concentrates sludge S, a circulating hydrothermal reaction device 10, and a methane fermentation tank 20. The raw sludge and excess sludge (for example, water content of about 97%) generated at the sewage treatment plant 1 are temporarily stored in the mixed sludge tank 2 and concentrated to a water content suitable for the hydrothermal reactor 10 by the concentrating device 4 and then concentrated. The sludge S (hereinafter sometimes referred to as concentrated sludge) is supplied to the reactor 10 via the supply tank 6. The filtrate separated from the concentrated sludge S in the concentration device 4 is directly introduced into the methane fermentation tank 20 via the raw material tank 19 without going through a hydrothermal reaction. Preferably, as shown in the illustrated example, the condensing device 4 includes the condensing agent adding device 5 to condense as much as possible the solid content of the small particle size in the sludge S and send it to the hydrothermal reactor 10, where the solid content together with the filtrate is methane. The direct introduction into the fermenter 20 is prevented.

  The concentrating device 4 can be, for example, a vacuum dehydrator, a centrifuge, a filter press (overpressure dehydrator), or the like, but may be a dry bed by sun drying or the like. The degree of concentration of the sludge S by the concentrator 4 can reduce the heating energy in the hydrothermal reactor 10 described later as the water content of the concentrated sludge S is lowered, and can improve the energy efficiency of the entire system. If the water content is too low, the viscosity (and frictional force) of the sludge S increases, which tends to cause a blockage of the reaction apparatus 10, and the sludge S may be fixed (burned) on the inner surface of the high temperature apparatus. Although it differs depending on the hot water temperature T of the hydrothermal reaction, it is possible to concentrate the sludge S in the range of 85 to 95% water content by the concentrator 4 and treat it in the hydrothermal reactor 10, preferably The moisture content of the concentrated sludge S is set to 90 to 95% to prevent sticking in the reactor 10, and when it is necessary to prevent the sticking, the moisture content of the concentrated sludge S is set to 93 to 95%.

  As shown by a dotted line in FIG. 1, the hydrothermal reaction device 10 is provided with a detection device 26 that detects the sticking of the sludge S inside, and the sludge S by the concentrating device 4 according to the sticking state detected by the detection device 26. The moisture content may be adjusted. In the illustrated example, a flow meter and a flow rate control valve are provided on the discharge side of the circulation pump 14 of the reaction device 10 to be described later to form a sticking detection device 26, and the flow rate is measured while circulating the concentrated sludge S at a predetermined flow rate that does not stick. The pressure loss of the circulation pipe of the reaction apparatus 10 is calculated from the opening degree of the control valve, and the state of sludge sticking inside the reaction apparatus 10 is detected from the increase amount of the pressure loss. For example, the sludge S concentrated to about 90% of the initial moisture content by the concentrator 4 is supplied to the reactor 10, and the concentrated sludge S contains water when the detection device 26 detects internal fixation (increase in pressure loss of the internal piping). The concentrator 4 is controlled so that the rate is 93 to 95%.

The concentrated sludge S from the concentrator 4 is temporarily stored in the supply tank 6 and then supplied to the hydrothermal reactor 10 described later by a supply unit 7 such as a mono pump. If necessary, the supply device 7 may be provided with a pulverizing device 8 or a stirring device 6a (see FIG. 5), and the solid content in the concentrated sludge S may be appropriately pulverized or stirred before being supplied to the reaction device 10. Further, as described above with reference to FIG. 5, the concentrated sludge S from the concentrating device 4 including the pulverization tank 8a and the pulverization pump 8b may be pulverized by the pulverization tank 8a and the pulverization pump 8b and then stored in the supply tank 6. Good. By reducing the particle diameter D _p of the solids by crushing the concentrated sludge S, to reduce the terminal settling velocity u of the solids described below, hardly occurs solids settling and fixed inside the reactor 10 can do.

  As in the case of FIG. 5, the circulating hydrothermal reactor 10 in the illustrated example includes a hydrothermal reactor 11, a gas-liquid separator 12, and a circulation pump 14 that heat the concentrated sludge S to a hot water temperature at a predetermined pressure. The concentrated sludge S supplied is circulated between the hydrothermal reactor 11 and the gas-liquid separator 12 for a predetermined time to reduce the molecular weight. For example, the concentrated sludge S can be efficiently reduced in molecular weight by setting the inside of the hydrothermal reactor 10 to a pressure of 0.72 to 8.7 MPa, a temperature of 160 to 300 ° C., and a circulation time of 15 to 120 minutes. Although possible (see Patent Document 3), increasing the temperature and pressure in the circulation path increases the energy consumption. In order to improve the energy efficiency of the entire system, the inventors suppress the hot water temperature in the circulation path to a relatively low range of 160 to 200 ° C. and suppress the pressure to about 1 MPa. It was experimentally found desirable. In order to make the concentrated sludge S a low molecular organic substance suitable for methane fermentation treatment at a temperature and pressure within this range, the circulation time needs to be about 30 to 90 minutes.

Further, as in the case of FIG. 5, the circulation type hydrothermal reaction device 10 makes the effective internal volume V inside the circulation line (wetted part) coincide with the product of the supply amount x of the concentrated sludge S and the circulation time y. (V = xy), in order to avoid the solid content in the concentrated sludge S from settling / adhering inside the pipe line, the concentrated sludge S inside the pipe line has a velocity v (> u higher than the terminal sedimentation speed u of the solid content. ). In general, the terminal sedimentation velocity u of the solid content is obtained by using the gravitational acceleration g, the density ρ _{f of} hot water and the viscosity μ _f , and the particle diameter D _p and the density ρ _p of the solid content suspended therein. It can be calculated by equation (1) based on the Stokes equation. In order to prevent sedimentation / fixation of the solid content inside the reaction apparatus 10, the present inventors are effective to set the circulation speed v of the reaction apparatus 10 at least 1.3 times the terminal sedimentation speed u of the solid content. It was found experimentally.
^{_{u = g · D p 2 ·}} (ρ p -ρ f) / 18μ f .................................... (1)

The formula (1) indicates that the terminal sedimentation velocity u increases as the solid content density ρ _p increases or the particle diameter D _p increases. Since it can be assumed that the solid content in the normal concentrated sludge S has a density ρ _p = 3500 to 1000 (kg / m ³ ) and a particle diameter D _p = 0.5 to 0.1 (mm), When calculated from the physical properties of hot water as density ρ _f = 886.9 (kg / m ³ ) and viscosity μ _f = 1.54 × 10 ⁻⁴ (kg / m · sec), the terminal settling velocity u is at least 0.004 The maximum range is 2.32 (m / sec). Therefore, if the circulation speed v is set to 2.32 × 1.3≈3 (m / sec) or more based on the above-described experimental findings of the present inventors, solid content settling and fixing inside the apparatus can be prevented. Can do. That is, if the circulation rate v of the circulation type hydrothermal reactor 10 is at least 3 (m / sec) or more, the normal concentrated sludge S can be reduced in molecular weight without causing solid matter to settle and stick.

  The hydrothermal reactor (heat exchanger) 11 of the circulating hydrothermal reactor 10 in the illustrated example heats methane gas G collected in a methane fermentation tank 20 described later to a heating medium (for example, steam, heat transfer oil, etc.) H. It is connected to an energy conversion device 25 (for example, a boiler or the like) that converts it into energy, and the concentrated sludge S is heated to the hot water temperature by heat exchange with the heating medium H. Further, the gas-liquid separator 12 of the reaction apparatus 10 is provided with a pressure gauge 17 (see FIG. 5) with a pressure valve 17a for maintaining the inside of the circulation path at a predetermined pressure, and the hydrothermal reactor 11 and the gas-liquid separator. The circulation time y in the circulation path consisting of 12 is maintained constant by the supply amount x of the concentrated sludge S by the supply device 7.

  In the circulating hydrothermal reactor 10, when the concentrated sludge S is circulated at a flow rate of 3 (m / sec) or more for about 30 to 90 minutes while heating to 160 to 200 ° C. under a pressure of 1 MPa, a gas-liquid separator The decomposition aqueous solution (low molecular weight sludge) S extracted from the liquid level control valve 15a (see FIG. 5) 12 to the outside of the apparatus 10 is made into a low molecular organic substance having high decomposability suitable for methane fermentation treatment. it can. The low molecular weight sludge S extracted from the hydrothermal reactor 10 is introduced into the methane fermentation tank 20 together with the filtrate of the concentrating device 4 through the raw material tank 19.

  The illustrated methane fermentation tank 20 is provided with a microbial fixed bed 21 that holds the methane fermentation microorganism group at a high concentration. By bringing the introduced low-molecular sludge S into contact with the methane fermentation microorganism group, the methane gas G is converted into methane gas G. Disassemble until For example, the fermenter 20 may be filled with a microbial carrier made of glass fiber or carbon fiber to form a fixed bed 21 (see Patent Documents 1 and 4). Further, the methane fermentation tank 20 can be provided with a heat retention device (not shown) that maintains the low molecular weight sludge S at the activation temperature of the methane fermentation microorganism group. For example, the sludge S in the fermentation tank 20 can be removed by the heat retention device. A fermentation temperature suitable for methane fermentation microorganisms, for example, a medium temperature (about 37 ° C.) or a high temperature (about 55 ° C.) is maintained (see Patent Document 4). However, the methane fermenter 20 used in the present invention is not limited to the illustrated example, and may be a floating bed type instead of a fixed bed.

  As described above, in order to subject the sludge S before molecular weight reduction to methane fermentation treatment, it is necessary to retain it in the fermenter 20 for 15 to 30 days or more, but the hydrothermal reactor 10 has improved degradability. The molecularized sludge S exhibits a high solid content decomposition rate even if the residence time (treatment time) in the fermenter 20 is shortened. The graph of FIG. 2 shows the solid content decomposition rate and the residence time in the methane fermentation tank 20 for the low molecular weight sludge S circulated by the hydrothermal reactor 10 in a state of pressure 1 MPa and temperature 160 to 200 ° C. for about 60 minutes. The experimental result which calculated | required this relationship is shown, Even if the residence time of the fermenter 20 is shortened to 3 to 5 days, the solid content decomposition rate of the low molecular weight sludge S is almost close to the maximum value (50%) 45. It represents that it can be maintained at% or more. Preferably, the residence time in the fermenter 20 is set to 5 days, which is the minimum residence time at which the maximum decomposition rate of solids is obtained, and 50% or more of the organic matter in the low molecular weight sludge S is decomposed to generate methane gas G. .

  Moreover, even if the residence time of the methane fermentation tank 20 is the same, the present inventors are different in the recovered amount of methane gas G generated during the methane fermentation of the low molecular weight sludge S depending on the hot water temperature T of the hydrothermal reactor 10. Experimentally found out. The graph of FIG. 3 shows the hot water temperature T in the circulation path of the hydrothermal reactor 10 and the specific activity of methane fermentation of the low molecular weight sludge S circulated to the temperature T for about 60 minutes (solid decomposition of untreated sludge). The experimental result which calculated | required the relationship with the solid content decomposition rate of a process sludge when a rate is set to 1 is shown. The graph shows that the low molecular weight sludge S having a hot water temperature T of 180 to 190 ° C has a higher methane fermentation specific activity than the low molecular weight sludge S having a hot water temperature T of 160 to 170 ° C. However, since the specific activity of methane fermentation is higher than that of the low molecular weight sludge S at 200 ° C., it indicates that the methane gas recovery amount per sludge unit amount is the optimum temperature during the methane fermentation treatment.

  In the graph of FIG. 3, the details of the cause of the maximum amount of methane gas recovered per unit amount of the low molecular weight sludge S when the hot water temperature T is 180 to 190 ° C. are unknown, but the hot water temperature T is 180. If the temperature is less than ℃, low molecular weight is insufficient due to hydrothermal reaction. Conversely, when the hot water temperature T is higher than 190 ° C, the low molecular weight is excessively advanced by hydrothermal reaction, and a part of the sludge S is decomposed into inorganic substances. It is thought that it has been. That is, in order to maximize the recovery amount of the methane gas G when the low molecular weight sludge S is subjected to methane fermentation treatment with the residence time of the methane fermentation tank 20 being 3 to 5 days, the pretreatment hydrothermal reactor 10 has 180. It is effective to reduce the sludge S at a hot water temperature T of ˜190 ° C. Preferably, the hot water temperature T in the hydrothermal reactor 10 is set to 180 ° C., and the heating energy (consumed energy) required for the hydrothermal reaction is kept small while maximizing the amount of methane gas recovered.

  In addition, the graph of FIG. 3 has shown the relationship between the hot water temperature T and the methane fermentation specific activity when the circulation time y of the sludge S in the hydrothermal reaction apparatus 10 is 60 minutes, and the present inventors further According to experiments, when the circulation time is shorter than 60 minutes, the optimum temperature T for maximizing the recovered amount of methane gas G is slightly higher than 180 to 190 ° C. Conversely, when the circulation time is longer than 60 minutes, the optimum temperature T Was found to be slightly lower than 180-190 ° C. That is, the hot water temperature T that maximizes the amount of methane gas recovered per unit amount of sludge is the circulation time of the concentrated sludge S in the reactor 10 (the time for circulation between the hydrothermal reactor 11 and the gas-liquid separator 12). It is effective to make adjustments according to

  The methane gas G generated in the methane fermentation tank 20 is taken out via the gas line, desulfurized with a desulfurizer as necessary, and then stored in the gas tank 23. As described above, the methane gas G stored in the gas tank 23 is converted into heating energy of a heating medium (e.g., steam, heat transfer oil, etc.) H by an energy conversion device 25 (e.g., boiler) to heat the hydrothermal reactor 10. Can be used. The digested sludge remaining in the methane fermentation tank 20 after the recovery of the methane gas G is sent to the dehydrator 28 via the return tank 27, the filtrate is returned to the sewage treatment plant 1 as the return water W, and the remaining dehydrated sludge D is returned to the conventional dewatering sludge D. Dispose in the same way as sludge S. For example, the sewage sludge S discharged from the sewage treatment plant 1 according to the embodiment of FIG. 1 is subjected to methane fermentation treatment by the system of the present invention, so that the dewatered sludge D after dehydration by the dehydrator 28 is 1/3 of the sewage sludge S. Volume can be reduced to 1/5.

  According to the present invention, since the hot water temperature T of the hydrothermal reactor 10 is adjusted so that the maximum amount of methane gas recovered per unit amount of sludge is obtained, a system in which the hydrothermal reactor 10 and the methane fermentation tank 20 are combined. The overall energy efficiency can be optimized. In addition, by maximizing the amount of methane gas recovered from the methane fermentation tank 20, the heating energy of the hydrothermal reactor 10 can be covered by the energy of methane gas, and the system can reduce and reduce the amount of sludge S by itself. it can.

  Thus, provision of “a methane fermentation treatment method and system capable of increasing the amount of methane gas recovered from sludge pretreated by hydrothermal reaction”, which is an object of the present invention, can be achieved.

DESCRIPTION OF SYMBOLS 1 ... Sewage treatment apparatus 2 ... Mixed sludge tank 4 ... Concentration apparatus 5 ... Condensing agent addition apparatus 6 ... Supply tank 6a ... Stirrer 6b ... Pressure gauge 6c ... Relief valve 7 ... Supply apparatus 8 ... Crushing apparatus 8a ... Crushing tank 8b ... Crushing pump 8c ... Three-way valve 10 ... Hydrothermal reactor 11 ... Hydrothermal reactor (heat exchanger)
DESCRIPTION OF SYMBOLS 11a ... Communication pipe 11b ... Pressure equalizing pipe 12 ... Gas-liquid separation apparatus 14 ... Circulation pump 15 ... Liquid level control apparatus 15a ... Liquid level control valve 16 ... Thermometer 17 ... Pressure gauge 17a ... Pressure relief valve 18 ... Safety valve 19 ... Raw material tank 19a ... metering pump 20 ... methane fermentation tank 21 ... microbial fixed bed 23 ... gas tank 25 ... energy conversion device 25a, 25b ... hot water circulation path 26 ... sticking detection device 27 ... return tank 28 ... dehydrator D ... dehydrated sludge G ... methane gas S ... Sludge W ... Return water

Claims (10)

After concentrating the sludge to a predetermined moisture content, the concentrated sludge is circulated for a predetermined time between the hydrothermal reactor and the gas-liquid separator, and hot water that maximizes the amount of methane gas recovered per unit amount of sludge by methane fermentation treatment. A sludge hydrothermal reaction type methane fermentation treatment method in which low molecular weight is reduced while heating to temperature, the low molecular weight sludge is retained in a methane fermentation tank for a predetermined time and methane gas is recovered. The treatment method according to claim 1, wherein the hydrothermal reaction utilizing methane fermentation treatment of sludge has a temperature at which the amount of methane gas recovered per unit amount of sludge by methane fermentation treatment is maximized in the temperature range of 160 to 200 ° C. Method. 3. The method according to claim 1 or 2, wherein the hot water temperature is adjusted according to a predetermined circulation time of the concentrated sludge, and the sludge hydrothermal reaction utilizing methane fermentation treatment method. 4. The methane fermentation treatment method utilizing sludge hydrothermal reaction, wherein the low molecular weight sludge is retained in a methane fermentation tank for 3 to 5 days to recover methane gas. 5. The methane fermentation treatment method using sludge hydrothermal reaction using the methane gas recovered in the methane fermentation tank and supplying heating energy by supplying the methane gas recovered in the methane fermentation tank according to claim 1. A concentrator for concentrating sludge to a predetermined moisture content, heat that circulates the concentrated sludge between the hydrothermal reactor and gas-liquid separator for a predetermined time and maximizes the amount of methane gas recovered per unit sludge by methane fermentation treatment Hydrothermal reaction type methane fermentation treatment of sludge equipped with a circulatory hydrothermal reactor that lowers the temperature while heating to water temperature, and a methane fermentation tank that collects methane gas by retaining the low molecular weight sludge for a predetermined time system. The treatment system of Claim 6 WHEREIN: The hydrothermal reaction of the sludge used as the temperature from which the amount of methane gas collection | recovery per sludge unit amount by a methane fermentation process becomes maximum in the hot water temperature of the said hydrothermal reaction apparatus in the temperature range of 160-200 degreeC. Utilization type methane fermentation treatment system. The treatment system of Claim 6 or 7 WHEREIN: The hydrothermal reaction utilization type | mold methane fermentation processing method of sludge which adjusts the hot-water temperature of the said hydrothermal reaction apparatus according to the predetermined circulation time of concentrated sludge. The treatment system according to any one of claims 6 to 8, wherein the sludge hydrothermal reaction type methane fermentation treatment system is obtained by retaining the low molecular weight sludge in the methane fermentation tank for 3 to 5 days and collecting methane gas. The treatment system according to any one of claims 6 to 9, wherein a sludge hydrothermal reaction utilization type comprising an energy conversion device that inputs methane gas recovered in the methane fermentation tank and supplies heating energy to the hydrothermal reactor. Methane fermentation treatment system.