Sewage treatment system

Abstract

To provide a sludge treatment system recovering a fibrous material from primary sludge extracted from a first sedimentation pond, subsequently digesting the fibrous material in a digestion tank to obtain digested sludge A, digesting the primary sludge after the fibrous material is recovered therefrom in a digestion tank different from the above-mentioned digestion tank to obtain digested sludge B, subsequently mixing the digested sludge A and the digested sludge B, and dehydrating the mixed sludge.SOLUTION: The sludge treatment system digesting the primary sludge separated by sedimentation of water to be treated containing a fibrous material in a first sedimentation pond and excess sludge separated by sedimentation in a final sedimentation pond, in a digestion tank, and subsequently dehydrating the digested sludge by a dehydrator, recovers the fibrous material contained in the water to be treated flowing into a sewage disposal plant prior to a front stage of a second digestion tank digesting sludge, digests the recovered fibrous material in a first digestion tank to produce digested sludge A, and digests the sludge after the fibrous material is recovered therefrom in the second digestion tank to produce digested sludge B, mixing the digested sludge A and the digested sludge B and subsequently dehydrating the mixture by a dehydrator. Thereby, the digestion efficiency and the dehydration efficiency can be improved.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a sludge treatment system that effectively dehydrates highly dewaterable sludge generated in a sewage treatment plant and improves digestion efficiency.

Background Art

[0002] Conventionally, generally, the water treatment system of a sewage treatment plant sediment-separates the water to be treated flowing into the treatment plant in a primary sedimentation tank, biologically treats the separated supernatant water in a reaction tank supplied with oxygen, then sediment-separates it in a final sedimentation tank, disinfects the supernatant water, and discharges it into a river or the like. On the other hand, the sludge treatment system gravity-concentrates the primary sludge drawn from the primary sedimentation tank to generate gravity-concentrated sludge, mechanically concentrates the excess sludge drawn from the final sedimentation tank to generate mechanically concentrated sludge, supplies these to a digestion tank for digestion treatment, and then performs dehydration treatment to generate a dehydrated cake.

[0003] Patent Document 1 discloses a technique of separating and recovering fibrous substances in raw sludge generated from a primary sedimentation tank, adding the fibrous substances to digested sludge and excess activated sludge respectively, and then mixing and dehydrating them in a mixer.

[0004] Patent Document 2 discloses a technique of solubilizing organic waste, solid-liquid separating the generated solubilized product into a solubilized liquid and a solubilized residue, and then anaerobically fermenting them in separate anaerobic fermentation tanks to increase the gas generation amount and the like.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] Conventionally, surplus sludge (hard-to-dewater sludge) and digested sludge (hard-to-dewater sludge) generated at sewage treatment plants had few fibrous substances that are the core of dehydration, resulting in poor dewaterability.

[0007] As shown in Patent Document 1, there was a technique of recovering fibrous substances from the raw sludge in the primary sedimentation tank and adding the recovered fibrous substances to digested sludge and surplus activated sludge respectively to enhance dewaterability. However, it was adding the recovered fibrous substances to the digested sludge after digestion treatment, not before the digestion treatment. Also, there was only one installed digester, and there was no description or suggestion of separately providing a digester for digesting only fibrous substances.

[0008] Patent Document 2 discloses a technique of fermenting the solubilized liquid and solubilized residue separated by solid-liquid separation in separate methane fermentation tanks to enhance digestion efficiency, but there is no description or suggestion of dehydrating the digested sludge after mixing the digested sludge generated from each methane fermentation tank.

[0009] The present invention provides a sludge treatment system capable of enhancing digestion efficiency and dewatering performance by digesting fibrous substances recovered from the primary sludge in the primary sedimentation tank in a first digester to generate digested sludge A, digesting a mixed sludge obtained by mixing the primary sludge (remaining sludge) after recovering the fibrous substances and surplus sludge drawn from the final sedimentation tank in a second digester to generate digested sludge B, and then mixing and dehydrating digested sludge A and digested sludge B.

Means for Solving the Problems

[0010] The present invention is a sludge treatment system that subjects the primary sludge obtained by sedimentation and separation of water to be treated containing fibrous substances in a primary sedimentation tank and surplus sludge obtained by sedimentation and separation in a final sedimentation tank to digestion treatment in a digester, and then subjects the digested sludge to dehydration treatment in a dehydration device. Before the front stage of a second digester for digesting sludge, from at least one of primary sludge or gravity thickened sludge obtained by gravity thickening primary sludge fibrous substances are recovered, only the fibrous material recovered digestion treatment is performed in a first digester to generate digested sludge A, and after recovering the fibrous substances, mixture of gravity thickened sludge and excess sludgeThe sludge is digested in a second digester to produce digested sludge B. After mixing digested sludge A and digested sludge B, dehydration treatment is performed, so that the fibrous substances recovered up to the front stage of the second digester for digesting the mixed sludge can be digested alone in a first digester provided separately from the second digester. Therefore, the digestion efficiency of the sludge is increased, and it is possible to reduce the digestion treatment days and increase the amount of digested gas generated.

[0012] Recover the fibrous substances floating in the water to be treated, together with the fibrous material recovered from the sludge Digest in a first digester to produce digested sludge A, and after recovering the fibrous substances mixture of gravity thickened sludge and excess sludge The sludge is digested in a second digester to produce digested sludge B. After mixing digested sludge A and digested sludge B, dehydration treatment is performed. Since there are few impurities other than fibrous substances in the water to be treated to be recovered, the fibrous substances can be easily separated and recovered.

[0013] After adding a part of the recovered fibrous substances in front of the dehydrator and then performing dehydration treatment, the fibrous substances become the core of dehydration, and the dewaterability of the difficult-to-dewater mixed digested sludge (digested sludge A and digested sludge B) sent to the dehydrator is improved, and a cake with a low water content can be obtained.

[0014] A part of the recovered fibrous substances is added to the mixed sludge and is installed in the front stage of the second digester (8) Mechanical concentrator (7) to thicken the mixed sludge By doing so, the mixed sludge (gravity thickened sludge and surplus sludge with difficult dewaterability) in a state where the fibrous substances are uniformly dispersed can be concentrated in the mechanical concentrator, so that it is possible to generate mechanically concentrated sludge with a high concentration of concentration, and it is also possible to reduce the amount of flocculant used.

Advantages of the Invention

[0015] The sludge treatment system according to the present invention recovers fibrous substances contained in the water to be treated flowing into a sewage treatment plant, digests the recovered fibrous substances in a first digestion tank to generate digested sludge A, and digests the sludge after the maximum amount of fibrous substances has been recovered in a second digestion tank to generate digested sludge B. After mixing digested sludge A and digested sludge B, the mixture is dehydrated. Since the fibrous substances recovered up to the stage before the second digestion tank that digests the mixed sludge are digested separately in a first digestion tank provided separately from the second digestion tank, the digestion efficiency can be increased compared to digesting the fibrous substances and the sludge in one digestion tank. As a result, the amount of generated digestion gas obtained in the entire sludge treatment system increases. When a part of the fibrous substances is added before a mechanical thickener or a dehydrator, the thickening property and dewatering property of the sludge are improved, so that the amount of flocculant used can also be reduced. Furthermore, since the fibrous substances (dewatering aids) are obtained by separating and recovering them from the sludge flowing into the sewage treatment plant, costs such as purchase, transportation, and storage are not required, and the costs associated with sludge treatment can also be reduced.

Brief Description of the Drawings

[0016]

Figure 1

Embodiments for Carrying Out the Invention

[0017] FIG. 1 is a flowchart of the system according to the present invention. In this embodiment, it includes a primary sedimentation tank 1 for sedimentation and separation of the water to be treated flowing into the sewage treatment plant, a recovery device 2 for selectively separating and recovering fibrous substances (dewatering aids) from the primary sludge separated in the primary sedimentation tank 1, a first digestion tank 3 for anaerobically treating the fibrous substances to generate digested sludge A, a gravity thickening tank 4 for gravity thickening of the primary sludge (remaining sludge) after the fibrous substances are recovered, a reaction tank 5 for blowing oxygen into the primary treated water supplied from the primary sedimentation tank 1 for biological treatment, a final sedimentation tank 6 for sedimentation and separation of the secondary treated water supplied from the reaction tank 5, disinfecting the supernatant water, and then discharging it into a river or the like, a mechanical thickener 7 for mechanically thickening the gravity thickened sludge supplied from the gravity thickening tank 4 and the excess sludge supplied from the final sedimentation tank 6, a second digestion tank 8 for anaerobically treating the mechanically thickened sludge, a sludge storage tank 9 for mixing the digested sludge B generated in the second digestion tank 8 with the digested sludge A generated in the first digestion tank 3 and temporarily storing them, a coagulation mixing tank 10 for adding a coagulant to the digested mixed sludge generated in the sludge storage tank 9 to generate coagulated sludge, and a dewatering device 11 for dewatering the coagulated sludge to generate dewatered cake. Incidentally, the recovery device 2 may be installed at the subsequent stage of the gravity thickening tank 4 to separate and recover the fibrous substances from the gravity thickened sludge obtained by gravity thickening the primary sludge.

[0018] Most of the excess sludge consists of high-moisture activated sludge microbial cells. Since it has a small particle size and contains moisture that cannot be removed by mechanical means inside the cells, it can be said to be sludge with poor dewaterability.

[0019] In the digested sludge A and the digested sludge B, a large number of fine colloidal particles that have an adverse effect on the filtration rate of the filter press remain due to the digestion treatment. Therefore, their dewaterability is extremely poor, and each can be said to be sludge with poor dewaterability. Incidentally, the mixed digested sludge obtained by mixing these can also be said to be sludge with poor dewaterability.

Example

[0020] <Primary sedimentation tank> The water to be treated flows into the primary sedimentation tank 1 first. The primary sludge contains 40% - 60% fibrous substances by solid content ratio. The supernatant water (suspended matter) separated by sedimentation in the primary sedimentation tank 1 is supplied to the reaction tank 5, and at the same time, the primary sludge is drawn out and supplied to the recovery device 2. Depending on the scale of the sewage treatment plant, the water to be treated (influent water) is retained in the primary sedimentation tank 1 for 1 to 2 hours.

[0021] <Recovery device> Next, in the recovery device 2, the fibrous substances contained in the water to be treated flowing into the sewage treatment plant are separated and recovered. Here, a large amount of fibrous substances are contained in the primary sludge. It has been found that most of the fibrous substances are derived from toilet paper with a fiber length of 0.1 mm - 0.5 mm and a fiber diameter of 1 μm - 50 μm. As the recovery device 2 for the fibrous substances, a known method can be used in which the primary sludge is supplied to a cylindrical screen provided with spiral screw blades, and while being conveyed inside, it is separated and recovered by the screen. In addition, a fiber storage tank (not shown) for temporarily storing the fibrous substances may be provided at the subsequent stage of the recovery device 2.

[0022] In this embodiment, the fibrous substances are recovered from the primary sludge sedimented and separated in the primary sedimentation tank 1, but it is not limited to this as long as they can be recovered before the previous stage of the second digestion tank 8. In the water to be treated flowing into the sewage treatment plant, it is sufficient if the fibrous substances can be recovered from at least one of the sludge sedimented and separated or gravity thickened before the previous stage of the second digestion tank 8. For example, it can be appropriately selected, such as recovering the fibrous substances from both the primary sludge sedimented and separated in the primary sedimentation tank 1 and the gravity thickened sludge gravity thickened in the gravity thickening tank 4, or recovering only from the gravity thickened sludge gravity thickened in the gravity thickening tank 4. Also, when recovering the fibrous substances from the sludge, the floating fibrous substances such as the supernatant water sedimented and separated in the primary sedimentation tank 1 may be recovered at the same time. By recovering not only from the sludge but also from the floating fibrous substances, the recovery amount of the fibrous substances that can be recovered before the second digestion tank front stage of 8 can be increased. In addition, the fibrous substances may be recovered from the water to be treated before it is sedimented and separated in the primary sedimentation tank 1.

[0023] The recovery of fibrous substances is generally carried out from the primary sedimentation sludge containing a large amount of fibrous substances. However, since fibrous substances are also mixed in the water to be treated before sedimentation separation in the primary sedimentation tank 1, and in the supernatant water after sedimentation separation from the sludge, the recovery position is not limited to the primary sedimentation sludge. When recovering, it is desirable to recover all the fibrous substances by the front stage of the second digestion tank 8 in order to remove the maximum amount of fibrous substances in the sludge sent to the second digestion tank 8. By separately digesting the recovered fibrous substances in the first digestion tank 3 different from the second digestion tank 8 to which the mixed sludge is supplied, the fibrous substances can be efficiently digested in the first digestion tank 3 so that the digestion efficiency of the whole system can be improved.

[0024] In addition, the fibrous substances contained in the water to be treated include, in addition to the fibrous substances derived from toilet paper, fibrous substances derived from plants or fibrous substances derived from food residues. As long as they are in a state of being finely divided into fibrous shapes, they function as a dehydration aid. The shape of the fibrous substances to be recovered may be made uniform by crushing the primary sedimentation sludge in the recovery device 2 or in the front stage of the recovery device 2. The recovered fibrous substances are transferred to the first digestion tank 3 located at the rear stage of the recovery device 2, but a part of them may be added to the front stage of the dehydration device 11 or the front stage of the mechanical thickener 7 and used as a dehydration aid. The sludge (remaining sludge) after recovering the fibrous substances is transferred to the gravity thickening tank 4.

[0025] When adding a part of the recovered fibrous material to the front stage of the dehydration device 11 or the front stage of the mechanical thickener 7, it may be added only to the front stage of the dehydration device 11 to improve the dewaterability of the sludge, or it may be added simultaneously to the front stage of the mechanical thickener 7 to increase the concentration of the sludge. When adding the fibrous material to the front stage of the mechanical thickener 7, it is added immediately before supplying the flocculant to the sludge fed into the mechanical thickener 7. Also, when adding it to the front stage of the dehydration device 11, it is desirable to add it before supplying the flocculant to the sludge in the coagulation mixing tank 10 at the front stage of the dehydration device 11. For example, it is supplied to the piping connected to the coagulation mixing tank 10 or the sludge storage tank 9. By adding the fibrous material before coagulating the sludge using the flocculant, the fibrous material can be uniformly dispersed in the sludge. Then, by adding the flocculant to the sludge in a state where the fibrous material is uniformly dispersed, a strong floc is formed, so that the chemical injection rate can be reduced. If there is a surplus in the recovery amount, it may be added to the generated mechanical thickened sludge or digested sludge. Also, it may be transferred to a neighboring sewage treatment plant through a communication pipe and used as a dewatering aid material.

[0026] <First digestion tank> In the first digestion tank 3, the fibrous material separated and recovered by the recovery device 2 is decomposed by anaerobic microorganisms into organic acids, carbon dioxide, etc. The organic acids are further decomposed by anaerobic microorganisms into methane and hydrogen sulfide. The digested sludge A that has completed decomposition and stabilized is significantly reduced in volume compared to before the digestion process. At this time, only the fibrous material recovered in the maximum amount by the recovery device 2 is supplied to the first digestion tank 3. Since a large amount of fibrous material exists in the digestion tank, the digestion rate is fast, and the sludge can be digested in a short time. Along with this, an increase in the amount of digestion gas generated in the entire system and a significant reduction in the digestion days can be realized.

[0027] <Gravity thickening tank> Next, in the gravity thickening tank 4, the sludge (remaining sludge) after recovering the fibrous material from the primary sedimentation sludge is thickened by gravity in the tank. The thickened gravity sludge is transferred for supply to the mechanical thickener 7. Since the fibrous material is recovered in the maximum amount by the recovery device 2 in the previous stage, the gravity sludge does not contain the fibrous material.

[0028] <Reaction tank> Next, in the reaction tank 5, activated sludge (sludge containing a large amount of aerobic microorganisms) is added to the primary treated water initially supplied from the sedimentation tank 1, and air is blown in and stirred. Due to the air (oxygen) and the organic matter in the primary treated water, the microorganisms in the activated sludge multiply, reducing the organic matter (dirt) in the primary treated water. At the same time, the activated sludge forms flocs and becomes in a state where it is easy to settle and flows out to the next stage. Depending on the scale of the sewage treatment plant, the primary treated water is retained in the reaction tank 5 for 6 to 10 hours.

[0029] <Final sedimentation tank> Next, in the final sedimentation tank 6, the secondary treated water mixed with the activated sludge supplied from the reaction tank 5 is retained for sedimentation separation. The supernatant water of the final sedimentation tank 6 is disinfected and then discharged into a river or the like. The sedimented sludge at the bottom of the final sedimentation tank 6 is returned from the final sedimentation tank 6 to the reaction tank 5. Also, the increased amount of microorganisms is discharged as excess sludge to the mechanical concentrator 7. This excess sludge is known to consist of polysaccharides, proteins, and nucleic acids secreted by microorganisms and to have extremely poor dewaterability. Depending on the scale of the sewage treatment plant, the secondary treated water is retained in the final sedimentation tank 6 for 3 to 4 hours.

[0030] <Mechanical concentrator> Next, in the mechanical concentrator 7, the mixed sludge (gravity thickened sludge and excess sludge) is concentrated by mechanical action. The concentrated mixed sludge is supplied to the second digestion tank 8. By mechanically concentrating with the mechanical concentrator 7 to increase the concentration of the sludge, the digestion efficiency in the subsequent second digestion tank 8 can be improved. Also, when a part of the fibrous material is added in front of the mechanical concentrator 7, the mixed sludge in a state where the fibrous material is uniformly dispersed can be mechanically concentrated in the mechanical concentrator 7, so that sludge with a high concentration can be obtained. Since the concentration is improved by the addition of the fibrous material, the amount of the flocculant used for supplying to the mechanical concentrator 7 can also be reduced. Incidentally, only the excess sludge may be mechanically concentrated by the mechanical concentrator 7 to generate mechanically concentrated sludge and supplied to the second digestion tank 8 separately from the gravity thickened sludge.

[0031] <Second digestion tank> In the second digestion tank 8, organic substances in the sludge are decomposed by anaerobic microorganisms into organic acids, carbon dioxide, etc. The organic acids are further decomposed by anaerobic microorganisms into methane, hydrogen sulfide, etc. The digested sludge B that has completed decomposition and stabilized is reduced by about 50 to 60% compared to before the digestion process. At this time, the second digestion tank 8 is supplied with the mixed sludge (gravity thickened sludge and excess sludge) after the maximum amount of fibrous substances has been recovered. Since the mixed sludge is concentrated to a high concentration by the mechanical thickener 7 and then transferred to the second digestion tank 8, efficient digestion treatment is performed in the second digestion tank 8, generating a large amount of digestion gas.

[0032] <Sludge storage tank> Next, in the sludge storage tank 9, the digested sludge A and the digested sludge B are mixed and temporarily stored. They are regularly stirred by known means to mix them uniformly. By providing the sludge storage tank 9, the digested sludge A and the digested sludge B can be mixed uniformly. Since the uniformly mixed digested sludge is dehydrated by the subsequent dehydration device 11, a dehydrated cake with a low moisture content and no variation can be obtained. Also, since the digested sludge A and the digested sludge B can be dehydrated after being mixed in one tank, it is possible to dehydrate them with one dehydration device 11, and it is not necessary to install one dehydration device 11 after each digestion tank. The digested mixed sludge generated by mixing the digested sludge A and the digested sludge B is transferred to the coagulation mixing tank 10.

[0033] <Coagulation mixing tank> Next, in the coagulation mixing tank 10, the digested mixed sludge supplied from the sludge storage tank 9 and the coagulant supplied from a coagulant supply device (not shown) are mixed while being uniformly stirred to form coagulated sludge that can be stably dehydrated by the subsequent dehydration device 11.

[0034] <Dehydration device> Next, in the dehydration device 11, the coagulated sludge supplied from the coagulation mixing tank 10 is dehydrated to generate a stable dehydrated cake with a low moisture content. Since the sludge immediately before dehydration has less fibrous substances (dehydration aids) due to anaerobic digestion, the dehydration property can be enhanced by adding fibrous substances (dehydration aids) immediately before dehydration, and a cake with a low moisture content can be obtained.

Industrial applicability

[0035] The sludge treatment system of the present invention digests the fibrous material recovered from the primary sludge withdrawn from the primary sedimentation tank in the first digestion tank to generate digested sludge A, and digests the mixed sludge obtained by mixing the primary sludge (residual sludge) after recovering the fibrous material and the excess sludge withdrawn from the final sedimentation tank in the second digestion tank to generate digested sludge B. By digesting the fibrous material and the sludge in separate digestion tanks, the amount of generated digestion gas can be increased. The large amount of generated digestion gas can be used in a power generation device, and energy can be generated without requiring a new energy source, so energy conservation and energy creation can be realized. Further, after mixing the fibrous material and the sludge generated in separate digestion tanks and then dehydrating, a cake with a low moisture content can be obtained, so the incineration treatment time in the post-treatment process is shortened, contributing to the reduction of carbon dioxide emissions.

Description of Symbols

[0036] 1 Primary sedimentation tank 3 First digestion tank 6 Final sedimentation tank 7 Mechanical thickener 8 Second digestion tank 11 Dehydration device

Claims

1. A sludge treatment system in which primary sludge obtained by sedimentation and separation of water to be treated containing fibrous substances in a primary sedimentation tank (1) and excess sludge obtained by sedimentation and separation in a final sedimentation tank (6) are digested in a digestion tank, and then the digested sludge is dehydrated by a dehydration device (11), wherein before the front stage of a second digestion tank (8) for digesting sludge, fibrous substances are recovered from at least one of the primary sludge or the gravity thickened sludge obtained by gravity thickening the primary sludge, and only the recovered fibrous substances are digested in a first digestion tank (3) to produce digested sludge A, and the mixed sludge of the gravity thickened sludge after recovering the fibrous substances and the excess sludge is digested in a second digestion tank (8) to produce digested sludge B, and after mixing digested sludge A and digested sludge B, dehydration treatment is performed A sludge treatment system characterized by the above.

2. Fibrous substances floating in the water to be treated are recovered, and digested in a first digestion tank (3) together with the fibrous substances recovered from the sludge to produce digested sludge A, and the mixed sludge of the gravity thickened sludge after recovering the fibrous substances and the excess sludge is digested in a second digestion tank (8) to produce digested sludge B, and after mixing digested sludge A and digested sludge B, dehydration treatment is performed The sludge treatment system according to claim 1, characterized by the above.

3. A part of the recovered fibrous substances is added before the front stage of the dehydration device (11), and then dehydration treatment is performed The sludge treatment system according to claim 1 or 2, characterized by the above.

4. A part of the recovered fibrous substances is added to the mixed sludge, and the mixed sludge is thickened by a mechanical thickener (7) installed at the front stage of the second digestion tank (8) The sludge treatment system according to any one of claims 1 to 3, characterized by the above.

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