Wastewater treatment method and wastewater treatment device

Abstract

To develop a method for facilitating fermentation without using granules in methane fermentation targeted on low concentration wastewater treatment such as sewage water.SOLUTION: A wastewater treatment method comprises: a fermentation step of carrying out fermentation with upward flow-type methane fermentation using low concentration wastewater (sewage water) containing organic matter as a raw material; a filtration step of taking in overflowing water from the fermentation step and carrying out membrane separation of sludge to discharge filtrated water; and a filtration sludge return step of returning the sludge remaining in filtration to the fermentation step. In the fermentation step, generated biogas is diffused from a lower part to promote the fermentation in an initial state in which granule sludge is not generated.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to an anaerobic biological treatment technology for treating wastewater containing organic matter generated in treatment facilities such as sewage treatment facilities. [Background technology]

[0002] Methane fermentation is used to treat wastewater containing organic matter, excess sewage sludge, food waste, and other organic waste. This is an anaerobic biological treatment method in which acetic acid and hydrogen obtained by anaerobic hydrolysis and acidification of polymeric organic matter are used to produce methane gas using methanogens. Anaerobic bacteria, including methanogens, grow slower than aerobic bacteria. Therefore, it takes time to start up methane fermentation, so it is important to speed up the start-up. It is also important to maintain a high concentration of anaerobic bacteria in the reactor after start-up. Methane fermentation treatment methods include completely mixed methane fermentation, in which wastewater and sludge (anaerobic bacteria) are mixed together and treated, and upflow methane fermentation, in which wastewater is supplied from the bottom of the reactor and treated by bringing it into contact with the sludge that has settled in the reactor. Completely mixed methane fermentation is a treatment process with a simple structure in which wastewater and sludge are mixed by mechanical agitation, but when the treated water is discharged, the sludge is also washed away, making it impossible to maintain a high concentration of anaerobic bacteria. Therefore, it is necessary to control the growth rate of anaerobic bacteria and the discharge rate. On the other hand, upflow methane fermentation is a treatment process that does not use mechanical stirring, but instead promotes contact between the wastewater and anaerobic bacteria through the linear velocity of the wastewater supplied from the bottom and the convection caused by the biogas generated. This process has the advantage of consuming less energy, since it does not require mechanical stirring and the treated water flows by overflow, so pumps are only required on the side supplying the reduced water. Since the sludge settles, it is possible to separate the treated water and sludge to some extent, but it is not possible to separate them completely, making it difficult to treat low-concentration wastewater such as sewage.

[0003] Regarding upflow methane fermentation, the following conventional techniques have been proposed. Patent Document 1 (JP 2019-42692 A) discloses a biological treatment device having a crushing section that crushes organic solid waste to 0.5 mm or more and 10 mm or less, a solid inlet, and a first treatment liquid outlet provided above the solid inlet. The invention relates to a miniaturized biological treatment device having a first reaction tank that contains an anaerobic microbial group including one or more of hydrolytic bacteria, acid fermentation bacteria, and methane fermentation bacteria, as well as a suspension containing the crushed solid waste, a solubilization reactor having a plurality of first inclined plates inclined with respect to the horizontal plane that are provided between the solid inlet and the first treatment liquid outlet in the first reaction tank and limit the movement of the solid waste to the first treatment liquid outlet, and a second reaction tank (methane fermentation reactor) equipped with granules. Patent Document 2 (JP 2005-224692 A) discloses an invention relating to wastewater treatment in which an ejector is placed on a transfer line that transports organic wastewater from an acid generation tank to an anaerobic reaction tank, and biogas discharged from the anaerobic reaction tank is guided to this ejector via a gas circulation line, mixed with the organic wastewater, and guided in a gas-liquid multiphase flow state containing the biogas as fine bubbles to a distributor installed at the bottom of the anaerobic reaction tank, where it is introduced into the anaerobic reaction tank, and the guided biogas is used for gas lift in the anaerobic reaction tank to swell granules.

[0004] Patent Document 3 (JP 2012-55837 A) discloses an invention relating to an anaerobic treatment device having a tank containing granules of anaerobic microorganisms, configured so that organic matter-containing wastewater is anaerobically treated by the anaerobic microorganisms in the tank, wherein the biochemical oxygen demand of the organic matter-containing wastewater is 1200 mg / L or less, and the tank is provided with an agitation means having an agitation blade located at the bottom, and the agitation means agitates the tank so that the flow velocity at a position 10 cm radially outward from the end of the agitation blade is more than 0.3 m / s and not more than 0.8 m / s. Patent Document 4 (JP 2012-110821 A) discloses an invention relating to a method for treating organic wastewater in which, when starting up a reaction tank, organic wastewater is started to flow into the reaction tank with non-biological carriers and methane bacteria granules present in the reaction tank at a volume ratio of carrier to methane bacteria granules in the range of 100:5 to 100:500, and then the flow of the organic wastewater is continued to break down and disperse at least a portion of the methane bacteria granules in the reaction tank, promoting the attachment of microorganisms to the carriers at the start of operation and causing a highly active biofilm to be formed quickly on the carrier surface, thereby significantly shortening the time required to start up the apparatus and providing efficient treatment even after the apparatus has been started up. As shown in these prior inventions, granules are generally used in upflow methane fermentation. To treat a large amount of wastewater such as sewage, it is necessary to prepare a large amount of granules, and it is also necessary to keep the granules in the fermentation tank from being destroyed or leaking out. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2019-42692 A [Patent Document 2] JP 2005-224692 A [Patent Document 3] JP 2012-55837 A [Patent Document 4] JP 2012-110821 A Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present invention is to develop a method for promoting methane fermentation in the treatment of low-concentration wastewater such as sewage, without using granules. [Means for solving the problem]

[0007] 1. A wastewater treatment method for fermenting wastewater containing organic matter using anaerobic bacteria, A fermentation process in which low-concentration wastewater containing organic matter is used as a raw material and fermented by upflow methane fermentation; A filtration process that takes in overflow water from the fermentation process, separates the sludge through a membrane, and releases the filtered water; A wastewater treatment method including a filtered sludge returning step in which sludge remaining after filtration is returned to a fermentation step, In this wastewater treatment method, the biogas generated in the fermentation process is diffused from the bottom to promote fermentation in the initial stage when no granular sludge is produced. 2. The method for wastewater treatment according to 1, characterized in that magnetite is added in the initial stage of the fermentation process. 3. Equipped with a reactor tank and a membrane separation tank for fermenting low-concentration wastewater containing organic matter by upflow methane fermentation; The reactor tank and membrane separation tank are equipped with an overflow pipe on the upper side and a sludge return pipe on the lower side. The reactor tank is equipped with a wastewater supply pipe at the bottom and a gas circulation pipe that circulates biogas from the top to the bottom. The membrane separation tank is equipped with a drainage pipe for the filtered water. A wastewater treatment device comprising: Effect of the Invention

[0008] 1. This invention has developed a method for promoting fermentation without using granules in methane fermentation for treating low-concentration wastewater such as sewage treatment water. By circulating the generated biogas to ensure agitation and returning sludge from the filtration process to the fermentation process, it is possible to maintain a high concentration of fermentation bacteria in the fermentation process, and to promote fermentation without using granules that generate a high-density bacterial flora. 2. By returning sludge from the filtration process to the fermentation process, the organic matter concentration in the filtration process can be reduced, which in turn suppresses clogging of the separation membrane. This prevents a decrease in the permeability of the wastewater due to clogging of the membrane, and reduces the load on the drainage pump. 3. By adding magnetite in the early stages of fermentation, methane production can be promoted and the start-up capacity of the fermenter can be improved. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 shows the steps of the present invention. [Diagram 2] FIG. 1 shows the configuration of the device of the present invention. [Diagram 3] FIG. 1 shows changes in methane gas concentration over time in Example 1. [Figure 4] FIG. 1 shows the cumulative methane gas production amount in Example 1. [Diagram 5] FIG. 1 shows the change in COD concentration of treated water after filtration in Example 1. [Figure 6] FIG. 13 is a graph showing changes in methane gas concentration over time in Example 2. [Figure 7] FIG. 1 shows the cumulative methane gas production amount in Example 2. [Figure 8] FIG. 1 shows the change in COD concentration of treated water after filtration in Example 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The present invention relates to an organic wastewater treatment method using upflow methane fermentation, which is a treatment process that promotes contact between wastewater and anaerobic bacteria by using the linear velocity of wastewater fed from the bottom and the convection of the generated biogas without mechanical stirring. The present invention is a wastewater treatment method comprising a fermentation process in which low-concentration wastewater (sewage) containing organic matter is used as a raw material and fermented by upflow methane fermentation, a filtration process in which overflow water from the fermentation process is taken in, the sludge is separated by membranes, and the filtrate is released, and a filtered sludge return process in which the sludge remaining after filtration is returned to the fermentation process. In the fermentation process, the generated biogas is diffused from the bottom to promote fermentation in an initial state in which granular sludge is not yet generated (see FIG. 1 for a process diagram).

[0011] The present invention is a wastewater treatment method for subjecting wastewater containing organic matter with a CODCr (hereinafter COD) concentration of 100 to (500) to 1500 mg / L to methane fermentation under anaerobic conditions without using granules. More preferably, the method is effective for wastewater containing organic matter with a CODCr (hereinafter COD) concentration of 300 to 700 mg / L. The generated biogas is returned to the fermentation process filled with low-concentration wastewater and aerated to generate an upward stirring flow, thereby promoting contact between bacteria and organic matter. In addition, by returning concentrated sludge in the filtration process to the fermentation process, the organic matter concentration in the filtration tank can be maintained low, clogging of the filtration membrane can be suppressed, and the maintenance frequency of the filtration membrane can be reduced. Since the returned sludge contains a large amount of bacteria, the amount of bacteria in the reaction tank can be increased, and methane production can be promoted.

[0012] This makes it possible to accelerate the start-up of the reaction process without using granules. In fermentation using granules, as disclosed in Patent Document 4 (JP Patent Publication 2012-110821 A) and the like, seed sludge in which methanogen granules are produced is put into a reaction tank to start up. The granules have a particle size of 0.5 to 2.0 mm. In particular, if the granules float and overflow from the reaction tank, they will adhere to the separation membrane and cause clogging. JP Patent Publication 2001-314839 A proposes a method of washing the separation membrane surface by diffusing biogas into the filtration membrane layer. In this method, organic matter in the filtration membrane tank floats, and precipitation and concentration do not proceed. In the present invention, since granules are not used, such granular organic matter does not adhere to the separation membrane, and the precipitated solids are collected, so that accumulation of organic matter in the membrane separation tank is prevented, the performance of the separation membrane can be maintained for a long period of time, and the maintenance period can be extended. Furthermore, the addition of magnetite at the beginning of the fermentation process can promote methane production, which can accelerate the start-up of the low-concentration wastewater treatment system and reduce the membrane load. By combining the circulation of biogas with the addition of magnetite, methane production can be further promoted. The appropriate concentration of magnetite is about 100 to 300 to 500 mg / L. For example, a ratio of magnetite to sludge concentration of about 20:1 is appropriate.

[0013] The configuration of the device of the present invention is shown in FIG. The reactor tank 1 and membrane separation tank 2 use low-concentration wastewater (sewage) containing organic matter as a raw material and ferment it by upward flow methane fermentation. The reactor tank 1 and membrane separation tank 2 are equipped with an overflow pipe 3 on the upper side and a sludge return pipe 4 on the lower side. The reactor tank 1 is equipped with a wastewater supply pipe 11 on the lower side and a gas circulation pipe 12 for circulating biogas from the top to the bottom. The membrane separation tank 2 is equipped with a filtration membrane 21 and a drainage pipe 22 for filtered water. The present invention is a closed system because it is carried out under anaerobic conditions. In addition, a connecting pipe 5 is provided between the reactor tank 1 and the membrane separation tank 2 to maintain the pressure in both tanks constant and allow free overflow.

[0014] In this wastewater treatment device, wastewater containing a low concentration of organic matter, which serves as raw water to be treated, is introduced into the reactor tank 1 through a wastewater supply pipe 11 provided at the bottom of the reactor tank 1 . As fermentation continues in the reactor tank 1, biogas containing methane is generated. The generated biogas is diffused from the bottom of the reactor tank 1 using a gas circulation pipe 12 that runs from the top to the bottom of the reactor tank 1. The diffused biogas rises through the wastewater in the reactor tank 1, stirring the wastewater and increasing contact between the bacteria and the wastewater. Because it is low-concentration wastewater, it is buoyant, which increases the stirring effect. However, since it is a relatively static stirrer rather than a strong stirrer like a screw, the concentration of solids such as organic matter is high at the bottom of the reactor tank, and low-concentration water with low solids is obtained at the top. Furthermore, when wastewater is supplied from the wastewater supply pipe 11 , treated water overflowing from the upper part of the reactor tank 1 is supplied to the membrane separation tank 2 from the overflow pipe 3 .

[0015] The membrane separation tank 2 is provided with a filtration membrane 21, and the membrane filtered water filtered by the filtration membrane 21 is sent from a drain pipe 22 to the outside of the apparatus. The treated water flowing into the membrane separation tank 2 contains a small amount of solid matter such as organic matter, but due to filtration, the solid matter concentration inside the membrane separation tank 2 becomes high and settles to the bottom. This settled sludge contains a large amount of bacteria. This settled sludge is fed from the bottom of the membrane separation tank 2 through the sludge return pipe 4 to the bottom of the reactor tank 1. The fermentation capacity of the reactor tank 1 is improved by adding the amount of bacteria contained in the returned sludge. This increases the amount of biogas generated, further increasing the stirring force. If the amount of biogas increases more than necessary, it is removed from the system (not shown) and used as fuel, etc. A communicating pipe 5 is provided between the upper part of the reactor tank 1 and the upper part of the membrane separation tank 2 to keep the pressure in both tanks in balance. This allows overflow to occur freely without the need for power. EXAMPLES

[0016] The experiment was carried out using an experimental setup similar to that shown in FIG. The equipment is divided into a reactor tank and a membrane separation tank. The effective volume of each tank is 10 L, and 2.5 L of concentrated sewage sludge is added to the reactor tank. The remaining 7.5 L is filled with tap water. 10 L of nitrogen-purged tap water was added to the membrane separation tank. The CODCr (hereinafter referred to as COD) concentration of the simulated wastewater was 500 mg / L. The hydraulic retention time (HRT) was 24 h (10 L / day). The supply of simulated wastewater and the discharge of treated water were controlled by intermittent operation. The water overflowing from the reactor tank was allowed to flow by free fall into the membrane separation tank. The reactor tank and the membrane separation tank were connected with a tube so that the pressure therebetween was constant. The initial sludge concentrations were MLSS 7600-8000 mg / L, MLVSS 4700-5600 mg / L, and the VSS / SS ratio was 0.62-0.68.

[0017] Biogas was circulated by pumping the gas from the top of the reactor tank to the bottom of the reactor tank. The pump had a flow rate of 2 L / min. The pump was operated for 5 seconds on and 60 minutes off to circulate the biogas. The experiment was carried out under two conditions: with and without biogas circulation. After the sludge was added, it was left to stand until methane was detected by gas chromatography, and then the supply of simulated wastewater was started. Biogas circulation was started immediately after the sludge was added. In addition, sludge return from the membrane separation tank to the reactor tank was only performed under the condition with biogas circulation (sludge return of 1 L / day was started on the 7th day after the start of the simulated wastewater supply). The simulated wastewater was supplied and the sludge was returned from the bottom of the reactor tank.

[0018] Figure 3 shows the change in methane gas concentration over time under each condition. Without biogas circulation, methane gas was detected after one week of standing (3%). On the other hand, with circulation, the methane gas concentration was close to 10% after four days of standing. Even after that, the methane gas concentration tended to be higher with circulation.

[0019] Figure 4 shows the cumulative amount of methane gas produced per cumulative amount of treated water. As shown in Figure 4, the amount of methane gas produced per unit of treated water volume increased by nearly 20% with biogas circulation.

[0020] Figure 5 shows the change over time in COD concentration of treated water under each condition. Immediately after the start, the concentration was higher with biogas circulation, but it gradually decreased, and on the final day the COD concentration with biogas circulation was about half of that without circulation.

[0021] Table 1 shows the amount of solids (sludge) in the treatment system. Comparing the cases with and without returning sludge from the membrane separation tank to the reactor tank, the total amounts are consistent in both cases, at approximately 4,500 MLSS and 2,600 MLVSS. By returning sludge, the MLSS is reduced to 40% in the membrane separation tank and increases to 21% in the reactor tank. The MLVSS is reduced to 43% in the membrane separation tank and increases to 23% in the reactor tank. Therefore, by returning the sludge, the organic matter and other solids in the membrane separation tank can be reduced by more than half, reducing the load on the separation membrane. On the other hand, the organic matter and other solids in the reactor tank have increased by more than 20%, which means that the amount of methane bacteria contained has also increased, and the increased sludge also becomes fermentation material, which increases the fermentation efficiency. The increase in fermentation efficiency in the reactor tank and the reduction in the load on the separation membrane have promoted the reduction in the COD concentration of the treated water, as shown in Figure 5.

[0022] Furthermore, for the reactor tank, the SVI30 (Sludge volume index, a numerical value that indicates the degree to which sludge settles; the lower the number, the higher the settleability) of the sludge was measured at the end of the experiment. The results showed that while the SVI of the sludge without biogas circulation was 65, the SVI of the sludge with biogas circulation was 60, indicating improved settleability. As a result of the improved settleability, the solids content in the overflow water is reduced, reducing the load on the membrane separation tank.

[0023] (Consideration) From the above results, the following can be stated: 1. Biogas circulation and sludge return improved the treatment capacity and sludge settling properties. 2. The generated biogas is diffused from the bottom of the reactor tank, which increases the processing capacity and enables early start-up. 3. Running costs are reduced because the load on the separation membrane is reduced. The sludge that flows out from the reactor tank to the membrane separation tank is concentrated by the separation membrane and returned, which makes it possible to reduce the amount of return and the membrane load. 4. Because it uses sludge discharged from sewage treatment plants, it can be applied to all sewage treatment plants (granular sludge must be procured from outside, which is costly, and it is difficult to procure large amounts of granules). 5. Since it is easy to switch to methane fermentation by modifying existing sewage treatment facilities (installing aeration pipes and creating an enclosed space), introduction costs can be reduced.

[0024] [Table 1] EXAMPLES

[0025] The experiment was carried out using an experimental setup similar to that shown in FIG. The equipment is divided into a reactor tank and a membrane separation tank. The effective volume of each tank is 10 L, and 2.5 L of concentrated sewage sludge is added to the reactor tank. The remaining 7.5 L is filled with tap water. The effective volume of each tank was 10 L, and 2.5 L of concentrated sewage sludge was added to the reactor tank. The remaining 7.5 L was filled with tap water. 10 L of nitrogen-purged tap water was added to the membrane separation tank. The CODCr (hereinafter referred to as COD) concentration of the simulated wastewater was 500 mg / L. The hydraulic retention time (HRT) was 24 h (10 L / day). The supply of simulated wastewater and the discharge of treated water were controlled by intermittent operation. The water overflowing from the reactor was allowed to flow by free fall into the membrane separation tank. The reactor and membrane separation tank were connected with a tube so that the pressure was constant. The initial sludge concentrations were MLSS 10,000 mg / L, MLVSS 6,000 mg / L, and the VSS / SS ratio was 0.60.

[0026] Biogas was circulated by pumping the gas from the top of the reactor to the bottom of the reactor. The pump had a flow rate of 2 L / min. The pump was operated for 5 seconds on and 60 minutes off to circulate the biogas. After the sludge was added, it was left to stand until methane was detected by gas chromatography, and then the supply of simulated wastewater was started. After the sludge was added, magnetite was added to the system at 300 mg / L (ratio to sludge concentration: 20:1), and operation was started. Biogas circulation was started immediately after the sludge was added.

[0027] The methane gas concentration, the cumulative methane gas production per cumulative treated water volume, and the COD concentration of the treated water over time under the test conditions (without circulation, with circulation) in Example 2 (with magnetite) in addition to the test conditions (without circulation, with circulation) in Example 1 are shown in Figures 6, 7, and 8. Each figure shows a combination of the graphs in Figures 3, 4, and 5. 6 and 7, it was confirmed that the addition of magnetite promotes methane production more than biogas circulation. In addition, the COD concentration of the treated water under the circulation + magnetite condition was higher than the other conditions immediately after the start of operation, but after 10 days of operation, the COD concentration was lower than the other conditions. This is thought to be due to the promotion of methane production, and the conversion of organic matter in the treated water to methane. [Explanation of symbols]

[0028] 1 Reactor tank 11 Drainage supply pipe 12 Gas circulation pipe 2 Membrane separation tank 21 Filtration Membrane 22 Drain pipe 3 Overflow pipe 4 Sludge return pipe 5 Communication pipe

Claims

1. A wastewater treatment method for fermenting wastewater containing organic matter using anaerobic bacteria, comprising: a fermentation step in which wastewater containing organic matter and having a COD concentration of 100 to 1,500 mg / L is used as a raw material and fermented by upflow methane fermentation in a reactor tank; A filtration process that takes in overflow water from the fermentation process, separates the sludge through a membrane, and releases the filtered water; A wastewater treatment method including a filtered sludge returning step in which sludge remaining after filtration is returned to a fermentation step, In this wastewater treatment method, the biogas generated in the fermentation process is diffused from the bottom of the reactor tank to promote fermentation in the initial state when no granular sludge is generated.

2. A wastewater treatment method as described in claim 1, characterized in that magnetite is added at an early stage of the fermentation process where granular sludge is not generated.

3. The system is equipped with a reactor tank and a membrane separation tank in which low-concentration wastewater containing organic matter is fermented by upflow methane fermentation, The reactor tank and membrane separation tank are equipped with an overflow pipe on the upper side and a sludge return pipe on the lower side. The reactor tank is equipped with a wastewater supply pipe at the bottom and a gas circulation pipe that circulates biogas from the top to the bottom. The membrane separation tank is equipped with a drainage pipe for the filtered water. A wastewater treatment device comprising:

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