TWI773110B - A method of removing solids from digestate obtained from a biogas plant - Google Patents

Abstract

The present invention provides a method of removing solids from digestate obtained from a biogas plant to provide a cleaned liquid phase. The method comprises the steps of a) separating a liquid phase and a solid phase from a digestate in a decanter centrifuge, wherein said digestate is the waste product from a biogas plant in which biogas has been produced from organic waste, and a step b) of separating a heavy phase and a liquid light phase from the liquid phase obtained in step a) in a disc-stack centrifugal separator, thereby providing a cleaned liquid phase.

Description

Method for removing solids from biogas slurry obtained from biogas production plants

The present invention relates to the field of biogas production, and more particularly to a method for removing solids from biogas slurry obtained from a biogas production plant.

Animal farms, such as pig farms, produce large amounts of manure. A technique to reduce nutrient loss from producing animals is to generate biogas from manure, usually through an anaerobic digestion process. The resulting biogas can then provide "green energy" for animal farms, ie heat that is used to generate electricity in generators.

However, even after the production of biogas, there is still a large amount of biogas slurry, ie, a slurry containing a large amount of liquid material. To remove solids from the liquid, a flocculant can be added to the biogas slurry prior to treatment in a decanter centrifuge. Flocculation is the action of the polymer to form bridges between the flocs and bind the solid particles into large agglomerates or agglomerates that are easier to separate. After treatment in a decanter centrifuge, a complex process is required in a membrane bioreactor system (MBR system) to remove the residual solid fraction from the liquid.

However, there is a need in the art for an improved method of removing solids from biogas slurry obtained after biogas production from organic waste, which is more environmentally friendly and which simplifies handling in MBR systems.

It is an object of the present invention to at least partially overcome one or more of the limitations of the prior art. Specifically, the purpose of the present invention is to provide a method for removing solids from biogas slurry obtained from a biogas production plant.

As a first aspect of the present invention, there is provided a method for removing solids from biogas slurry obtained from a biogas production plant to provide a cleaned liquid phase, wherein the method comprises the steps of: a) removing solids from the biogas slurry in a decanting centrifuge Separation of liquid and solid phases from a liquid, wherein the biogas slurry is waste from a biogas production plant, wherein biogas has been produced from organic waste; and b) in a disc stack centrifuge from step a) The heavy phase and the liquid light phase are separated from the liquid phase obtained to provide a cleaned liquid phase.

Thus, biogas slurry is the liquid slurry of organic waste remaining after anaerobic digestion in a biogas production plant, ie after biogas production in a biogas production facility. Therefore, biogas slurry contains a large amount of wastewater with suspended solids.

Decanter centrifuges (also known as solid bowl centrifuges) continuously separate solids from the liquid in the biogas slurry. In a decanter centrifuge, the biogas slurry is pumped into a rotating horizontal bowl. The rapid rotation of the bowl creates a centrifugal force that collects solid particles of higher density on the walls of the bowl. The solid phase is conveyed by a screw conveyor to one end of the bowl and then discharged therefrom. The separated liquid phase can be drained from the bowl, for example by means of an internal centripetal pump. Decanting centrifuges can be eg as disclosed in US 9962715.

Disc stack centrifuges are configured to separate solids and one or two liquid phases from each other in a single continuous process using extremely high centrifugal forces, such as in excess of 5000 G. When the denser solid experiences this force, it is forced outward against the rotating bowl wall, while the less dense liquid phase forms a concentric inner layer. The insertion of special plates ("disk stacks") provides additional surface settling area, which greatly speeds up the separation process. The concentrated solid phase formed by the particles can be removed continuously, intermittently or manually, depending on the type of separator and the amount of solids involved in the particular application.

The disc stack centrifuge separates the liquid phase discharged from the decanter centrifuge into at least phase and a liquid light phase, the heavy phase may therefore contain solids. The density of the liquid light phase is lower than the density of the liquid heavy phase.

The discs of the stack may be frustoconical discs, ie having separation surfaces forming frustoconical portions of the separation discs. However, axial discs are also contemplated. The stacked separation discs can be made of metal, for example.

The liquid light phase separated in the disc stack centrifuge is the cleaned liquid phase in the digestate which can be further processed downstream. The suspended solids content of the cleaned liquid phase may be less than 10%.

The first aspect of the present invention is based on the insight that a disc stack centrifuge is very effective for further removal of solids in the liquid phase obtained after processing the biogas slurry in a decanter centrifuge. As shown in the experimental examples below, the method is particularly effective for the treatment of biogas slurry obtained after the production of biogas from livestock manure, such as pig manure. This is useful for less complex liquid light phase processing, for example in membrane bioreactor (MBR) systems arranged downstream. For example, a decanter centrifuge may be able to remove 70% of suspended solids, while the remaining 30% of fine suspended solids can have a significant impact on downstream MBR systems. However, it has been found that with the further use of a disc stack centrifuge, over 90% of the suspended solids can be removed, thus providing better treatment in the MBR system.

Therefore, the method of the present invention involves a two-stage separation process of the liquid content remaining after the biogas production, ie a first treatment in a decanter centrifuge and a pan before the cleaned liquid can be further treated, for example in an MBR system The second treatment is carried out in a stack centrifuge.

Thus, in a specific example of the first aspect, the method further comprises the step of: c) processing the cleaned liquid phase obtained from step b) in a membrane bioreactor (MBR) system.

A membrane bioreactor (MBR) system is a system that combines a membrane process such as microfiltration or ultrafiltration with a biological wastewater treatment process to clean the liquid phase.

The type of organic waste can be very different, such as waste from kitchen waste, waste from paper mills or waste from animal farms.

In a specific example of the first aspect, the organic waste is livestock manure. For example, livestock manure The fertilizer can be pig manure. However, manure from eg cows can also be used. Thus, the biogas slurry may be manure waste from a biogas production plant, from which biogas has been produced from the manure.

In a specific example of the first aspect, the liquid phase treated in the disc stack centrifuge in step b) contains solid particles, most of which have a particle size of less than 20 μm.

Thus, the separated liquid phase discharged from the decanter centrifuge may contain a solid fraction, with the majority of the particles having a particle size of less than 20 μm.

In a specific example of the first aspect, the concentration of suspended solids in the cleaned liquid phase obtained in step b) is less than 850 mg/L. Therefore, the suspended solids content of the separated liquid light phase discharged from the disc stack centrifuge can be less than 850 mg/L. The suspended solids content may be "mixed liquor suspended solids (MLSS)", which refers to the concentration of suspended solids in the aeration tank during the activated sludge process (which occurs during wastewater treatment).

For example, the concentration of suspended solids in the cleaned liquid phase obtained in step b) is less than 500 mg/L.

For example, the suspended solids concentration of the cleaned liquid phase obtained in step b) may be between 150-850 mg/L, such as between 150-500 mg/L.

The light phase of the separated liquid discharged from the disc stack centrifuge, that is, the cleaned liquid phase, may contain most of the particles with a particle size of less than 2 μm.

The inventors have found that the method of the first aspect provides an environmentally friendly method of cleaning liquids in the biogas slurry obtained after biogas production without the need to add additional flocculants to the process prior to solids separation. Thus, in a specific example, no flocculant, such as a polymer, is added to the biogas slurry prior to separation in the decanting centrifuge of step a).

Therefore, the solids removed by decant centrifuges and disc stack centrifuges are suitable for composting and have high biodegradability.

In an embodiment of the first aspect, by decanting centrifuges and disc stack centrifuges Remove at least 90% of suspended solids in biogas slurry.

In a specific example of the first aspect, the biogas slurry of step a) is obtained by introducing the organic waste into an organic waste digester and converting at least some of the organic waste into biogas in an anaerobic process.

Accordingly, a biogas production plant may contain an organic waste digester, such as a manure digester, for producing biogas from organic waste under anaerobic conditions. The biogas can be methane.

In a specific example of the first aspect, the biogas slurry is conveyed to a storage tank before it is separated in the decanter in step a). Transport between the storage tank and the decanter centrifuge can be facilitated by, for example, a pump.

In an embodiment of the first aspect of the present invention, the disc stack centrifugal separator is a clarifier for solid-liquid separation. A clarifier disc stack centrifuge configured for solid-liquid separation removes solids (such as particles) and sediment from process liquids. Here, the liquid phase from the decanter becomes clearer after being processed in the disc stack centrifuge. A clarifier centrifuge may have an inlet for the liquid to be treated, a solids outlet arranged around the centrifuge bowl for discharging the separated heavy phase, and a single liquid outlet for the liquid light phase.

Furthermore, for example, a disc stack centrifuge can be hermetically sealed at the outlet for the light phase of the separated liquid. The airtight seal can be configured as a watertight seal or a mechanical airtight seal. This prevents the introduction of air into the separated liquid phase.

In an embodiment of the first aspect, the heavy phase separated in the disc stack centrifuge and the solid phase separated in the decanter are further conveyed to the same solids storage tank. This can reduce the footprint of the entire system used to provide cleaned liquid from the biogas slurry.

As a second aspect of the present invention, there is provided a biogas production system comprising - an anaerobic digester for producing biogas from organic wastes; - a storage tank for storing biogas produced by the anaerobic digester biogas slurry; - a decanting centrifuge, configured for separating liquid and solid phases from biogas slurry in the storage tank; - a disc stack centrifuge, configured for separating in the decanting centrifuge The liquid heavy phase and the liquid light phase are separated from the liquid phase of A second piping system for the decanting centrifuge, and a third piping system for conveying the separated liquid phase from the decanting centrifuge to the disc stack centrifuge.

This aspect may generally exhibit the same or corresponding advantages as the previous aspect. The efficacy and characteristics of the second aspect are largely similar to those described above in combination with the first and second aspects. The specific examples mentioned with respect to the first aspect are largely compatible with the second aspect.

In an embodiment of the second aspect, the biogas production system further comprises - a membrane bioreactor (MBR) system for removing solids from the cleaned liquid phase provided by the disc stack centrifuge, and - a fourth A piping system for conveying the clean liquid phase provided by the disc stack centrifuge to the membrane bioreactor (MBR) system.

Any of the first, second, third or fourth piping systems may comprise pipes and eg pumps for providing transport in the piping systems.

1: Biogas production system

2: Anaerobic digester

3: Biogas storage tank

4: Storage tank

5: Decanting centrifuge

6: Membrane Bioreactor (MBR) System

7: Solid storage tank

8: Pump

9a: Piping system

9b: Piping system

9c: Piping System

9d: Piping System

9e: Piping system

9f: Piping system

9g: Piping system

10: Disc stack centrifuge

11: Centrifugal rotor

12: Drive Spindle

13: drive motor

14: Frame

15: Bottom bearing

16: Top bearing

17: Inlet pipe

18: outlet pipe

19: Separation Space

20: Stacked

21: Entrance

22: Channel

23: Liquid light phase outlet

24: Channel

25: Operate the slider

26: Sludge outlet

100: Method

The foregoing and other objects, features and advantages of the inventive concept will be better understood from the following detailed description, illustrative and non-limiting, with reference to the accompanying drawings. In the drawings, similar reference numerals will be used for similar elements unless otherwise indicated.

[ FIG. 1 ] A schematic diagram showing a biogas production system according to an embodiment of the present invention.

[FIG. 2] A flow chart showing the method according to the present invention.

[FIG. 3] A schematic diagram showing a disc stack centrifuge. It can be used in the method of the present invention.

[FIG. 4] A schematic diagram showing a cross section of a centrifugal rotor of a disc-stack type centrifugal separator.

The centrifugation method and biogas production system according to the present invention will be further explained by the following description with reference to the accompanying drawings.

FIG. 1 shows a schematic diagram of a biogas production system 1 according to an embodiment of the present invention. The system 1 comprises an anaerobic digester 2 for producing biogas from animal manure and a first piping system 9a for transporting the biogas slurry from the anaerobic digester 2 to the storage tank 4 . Thus, biogas slurry is a liquid slurry containing solids. The produced biogas can be led to the biogas storage tank 3, for example via the piping system 9g. Therefore, the anaerobic digester 2, the biogas storage tank 3 and the storage tank 4 may be part of a biogas production plant.

The storage tank 4 is used to store the biogas slurry produced by the anaerobic digester 2 . Downstream of the storage tank 4 is equipment for removing solids from the biogas slurry in the form of a decanter centrifuge 5 and a disc stack centrifuge 10, which is configured for the removal of solids from the storage tank 4. The liquid and solid phases are separated in the biogas slurry, and the disc stack centrifuge 10 is configured to separate a liquid heavy phase and a liquid light phase from the liquid phase separated in the decanting centrifuge 5 . Figures 3 and 4 below describe the disc stack centrifuge 10 in more detail. The cleaned liquid phase is provided in the piping system 9d from the disc stack centrifuge 10.

The second piping system 9b is configured for conveying the biogas slurry from the storage tank 4 to the decanting centrifuge 5, and the third piping system 9c is for conveying the separated liquid phase from the decanting centrifuge 5 to the disc stack centrifuge The entrance of the separator 10. The solids removed in the decanter centrifuge 5 are directed to the solids storage tank 7 via the piping system 9f.

The biogas production system 1 further comprises a membrane bioreactor (MBR) system 6 for removing solids from the cleaned liquid phase provided by the free centrifuge 10 . Accordingly, the fourth piping system 9d is configured for conveying the separated liquid light phase discharged from the disc stack centrifugal separator 10 to the membrane bioreactor (MBR) system 6 .

At least 90% of the suspended solids of the biogas slurry present in the storage tank 4 can be removed by decanting centrifuges and disc stack centrifuges.

As shown in FIG. 1 , the separated heavy phase containing solids from centrifuge 10 is conveyed to the same solids storage tank 7 used to store separated solids from decanter centrifuge 5 .

In the specific example shown in Figure 1, only a single pump 8 for conveying the biogas slurry to the decanting centrifuge 5 in the second piping system 9b is shown. However, it should be understood that other pumps may be configured in other piping systems 9a, 9c, 9d, 9e, 9f, 9g to facilitate transportation. In particular, a pump may be provided in the piping system 9c for conveying the liquid phase from the decanter centrifuge 5 to the disc stack centrifuge 10 .

Figure 2 shows a schematic flow diagram of a method 100 for removing solids from biogas slurry obtained from a biogas production plant to provide a cleaned liquid phase according to the present invention. Method 100 comprises the steps of: a) separating liquid and solid phases from biogas slurry in a decanting centrifuge, wherein in this particular example, the biogas slurry is manure waste from a biogas production plant, which has been Biogas is produced from manure. The method further comprises step b): separating the heavy and light phases from the liquid phase obtained in step a) in a disc stack centrifuge to provide a cleaned liquid phase. Then, the method 100 may also comprise step c): processing the cleaned liquid phase obtained from step b) in a membrane bioreactor (MBR) system.

3 shows a cross-section of a specific example of a clarifier-type centrifuge 10 configured to separate a heavy phase and a liquid light phase from the liquid phase discharged from the decanter centrifuge in the methods and systems of the present invention .

The centrifugal separator 10 has a rotating part including a centrifugal rotor 11 and a drive spindle 12 . The centrifugal separator 1 is further provided with a drive motor 13 . This motor 13 1 transmits a drive torque to the spindle 12 and thus to the centrifugal rotor 11 during operation. The drive motor 13 may be an electric motor. Alternatively, the drive motor 3 may be connected to the spindle 12 by means of a transmission, eg in the form of a propeller shaft, drive belt or the like.

The centrifugal rotor 11 (shown in more detail in FIG. 4 ) is supported by a spindle 12 arranged in a frame 14 for rotation about a vertical axis of rotation (X) in a bottom bearing 15 and a top bearing 16 . fixed frame The rack 14 surrounds the centrifugal rotor 11 .

FIG. 3 also shows the inlet tube 17 extending axially from the top into the centrifuge rotor 11 . The liquid phase in the decanter centrifuge to be further cleaned in the disc stack centrifuge is thus introduced via the inlet pipe 17 . After the separation in the centrifuge rotor 11 , the light phase of the separated 1 liquid (ie the cleaned liquid phase) is discharged through the fixed outlet pipe 18 .

FIG. 4 shows a more detailed view of the centrifugal rotor 11 of the centrifuge 10 .

The centrifugal rotor 11 forms inside it a separation space 19 in which the stack 20 of separation discs is arranged coaxially around the axis of rotation (X) and axially below the top disc 11 and is thus arranged to rotate together with the centrifugal rotor 5 . The stack of separation discs provides efficient separation of the liquid phase conveyed from the decanter centrifuge.

The centrifuge 1 further comprises an inlet 21 in the form of a central inlet chamber into which a fixed inlet pipe 17 extends to supply the liquid feed to be separated. The inlet 21 communicates with the separation space 19 via a channel 22 formed in the distributor.

An outlet 23 for the separated liquid light phase is arranged in an outlet chamber which is in fluid communication with the fixed outlet pipe 18 to discharge the separated liquid light phase. The liquid light phase outlet 23 is in the form of a fixed pair of discs arranged in the outlet chamber.

The centrifugal rotor 11 is further provided with an outlet 26 at the radial periphery of the separation space 19 . The outlets 26 are evenly distributed around the rotor axis (X) and are configured to intermittently discharge the solid components of the liquid feed mixture. The solid content contains denser particles that form the sludge phase. The opening of the outlet 26 is controlled by an operating slider 25 actuated by the water in the operating channel 24, as known in the art. In the position shown in the figures, the operating slide 25 rests sealingly against the upper part of the centrifugal rotor 11 at its periphery, closing off the separation space 19 from being connected to the outlet 26 extending through the centrifugal rotor 11 .

During operation of the separator as shown in FIGS. 3 and 4 , the centrifugal rotor 11 is rotated by driving the motor 13 . Via the inlet pipe 17 , the liquid phase discharged in the process from upstream of the decanting centrifuge 5 is brought into the separation space 19 . Depending on the density, the different phases of the liquid phase are separated between the separation discs of the stack 20 . solid The particles move radially outward between the separation discs, while the liquid light phase moves radially inward between the separation discs and is forced through the outlet 23 . Here, the liquid light phase (ie the cleaned liquid phase) is discharged via the outlet pipe 18 .

By opening the sludge outlet 26 , the solids or sludge accumulated around the separation space 19 are intermittently evacuated from the centrifugal rotor 11 . However, the discharge of sludge can also be carried out continuously, in which case the sludge outlet 26 takes the form of an open nozzle and discharges a certain amount of sludge continuously by centrifugal force.

Experimental example

Biogas slurry obtained from pig manure biogas production was used to demonstrate the advantages of the present invention.

The biogas slurry is processed first in a decanting centrifuge and then the liquid phase in a centrifuge. Decanters and centrifuges were purchased from Alfa Laval.

"Mixed liquor suspended solids (MLSS)" was measured before treatment in a decanter centrifuge, after treatment in a decanter centrifuge and after further treatment in a disc stack centrifuge. The results are shown in Table 1 below.

These experimental examples clearly demonstrate that over 90% of MLSS can be removed in the process of the present invention.

The present invention is not limited to the specific examples disclosed, but may be varied and modified within the scope of the claims set forth below. The invention is not limited to the orientation of the axis of rotation (X) disclosed in the drawings. The term "centrifuge" also includes centrifuges having a substantially horizontally oriented axis of rotation. Above, the inventive concept has been mainly described with reference to a limited number of embodiments. However, as is readily understood by those skilled in the art, other embodiments than those disclosed above are equally possible within the scope of the inventive concept defined by the scope of the appended claims.

1: Biogas production system 2: Anaerobic digester 3: Biogas storage tank 4: Storage tank 5: Decanting centrifuge 6: Membrane Bioreactor (MBR) System 7: Solid storage tank 8: Pump 9a: Piping system 9b: Piping system 9c: Piping System 9d: Piping System 9e: Piping system 9f: Piping system 9g: Piping system 10: Disc stack centrifuge

Claims (14)

A method for removing solids from biogas slurry obtained in a biogas production plant to provide a cleaned liquid phase, wherein the method comprises the steps of: a) separating the liquid phase and the solid phase from the biogas slurry in a decanting centrifuge, wherein the biogas slurry is waste from a biogas production plant in which biogas has been produced from organic waste; b) the heavy phase and the The liquid light phase provides a cleaned liquid phase. The method of claim 1, wherein the organic waste is livestock manure. The method of claim 2, wherein the livestock manure is pig manure. The method of claim 1, wherein the method further comprises the step of: c) processing the cleaned liquid phase obtained from step b) in a membrane bioreactor (MBR) system. The method of claim 1 , wherein the liquid phase treated in the disc stack centrifuge in step b) contains solid particles, most of which have a particle size of less than 20 μm. The method of claim 1, wherein the concentration of suspended solids in the cleaned liquid phase obtained in step b) is less than 850 mg/L. The method of any one of claims 1 to 6, wherein no flocculant is added to the biogas slurry prior to the separation in the decanting centrifuge of step a). The method of claim 1 wherein the biogas slurry of step a) is obtained by introducing the organic waste into an organic waste digester and converting at least some of the organic waste into biogas in an anaerobic process. A method as claimed in claim 8, wherein the biogas slurry is conveyed to a storage tank before being separated in the decanter in step a). The method of claim 1, wherein the disc stack centrifuge is a solid-liquid Separate clarifier. The method of claim 10, wherein the disc stack centrifugal separator is hermetically sealed at the outlet for the light phase of the separated liquid. The method of claim 1, wherein the heavy phase separated in the disc stack centrifuge and the solid phase separated in the decanter are further conveyed to the same solids storage tank. A biogas production system (1) comprising - an anaerobic digester (2) for producing biogas from organic waste; - a storage tank (4) for storing the biogas produced by the anaerobic digester (2) the biogas slurry produced; - a decanting centrifuge (5) configured for separating liquid and solid phases from the biogas slurry in the storage tank (4); - a disc stack centrifuge (10) , configured to separate a liquid heavy phase and a liquid light phase from the liquid phase separated in the decanting centrifuge (5), thereby providing a cleaned liquid phase; and - for removing the biogas slurry from the The oxygen digester (2) is conveyed to the first piping system (92) of the storage tank (4), and the second piping system for conveying the biogas slurry from the storage tank (4) to the decanting centrifuge (5) (9b), and a third piping system (9c) for conveying the separated liquid phase from the decanter centrifuge (5) to the disc stack centrifuge (10). The biogas production system (1) of claim 13, further comprising - a membrane bioreactor (MBR) system (6) for the cleaned liquid phase provided from the disc stack centrifuge (10) Solids are removed in the , and - a fourth piping system (9d) for conveying the cleaned liquid phase provided by the disc stack centrifuge (10) to the membrane bioreactor (MBR) system (6).

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