GB2162195A - Improved anaerobic fermentation method and apparatus - Google Patents

Abstract

The present specification describes an apparatus (2) for and a method of anaerobic fermentation of organic waste material capable of being biodegraded into useful gaseous products. In one form the apparatus (2) consists of a double tank (8, 18) arrangement having a flexible membrane (10) or a cover securely attached to the top of the tank to collect the gaseous products. The flexible membrane (10) permits the volume of the gas being collected to vary whereas the double tank arrangement (8, 18) defines a tortuous path for the organic waste material to allow sufficient time for the fermentation to take place. <IMAGE>

Description

SPECIFICATION Improved anaerobic fermentation method and apparatus The present invention relates to methods of and apparatus suitable for anaerobic fermentation of organic waste materials which may be biodegraded to produce useful gas products such as for example methane, and render the waste material less polluting to the environment into which it is to be discharged.

The present invention finds particular application to the treatment of organic waste materials, such as for example, agricultural wastes, sewage, food processing waste, or the like but is not so limited and extends to cover any manner of materials or mixtures of material that may be anaerobically degraded.

Some of the problems of more traditional methods of anaerobic digestion which occur in substantially closed digesters include the following: (a) the formation of thick layers of scum material on the surface of liquid contained in the digester which ultimately leads to the failure of the digester due to clogging, (b) the accumulation of waste material solids on the surfaces on operative portions or components of the digester, such as for example the heat exchanger, which impairs the efficiency of the component and the digester generally, (c) the removal of useful bacteria from the digester by washing caused by the relatively high flow rate of material through the digester with the result that large populations of bacteria are not maintained in the system; under conditions of abnormally high flow through the digester, the removal of bacteria by washing may have a deleterious effect and may result in the failure of the system; (d) the complete conversion and biodegradation of the waste material may not always be accomplished and some untreated waste materials may flow freely out of the digester to contaminate the environment.

(e) the large size of digesters of traditional design required to effect treatment in some instances makes their installation economically unattractive.

Therefore, it is an aim of the present invention to provide an improved method of and apparatus for the anaerobic fermentation of organic waste materials which at least in part overcomes some of the problems of more traditional methods and apparatus.

According to the present invention there is provided an apparatus suitable for use in anaerobic fermentation of organic waste material comprising a rigid body portion having inlet means for the introduction of raw organic waste and an outlet means for discharging treated material, a flexible member connected to the body portion so as to form a closure means for the body portion, said flexible member and body portion defining a variable gas collection space therebetween, for accumulating gas derived from treatment of the waste wherein the flow path for the waste from the inlet means to the outlet means is somewhat tortuous so that waste may not flow directly from the inlet means to the outlet means.

In one embodiment, the flexible member is provided with an inflatable tube or ring portion which is sealed allowing it to be inflated. The tube portion when in an uninflated condition is received in a restraining means such as the cavity of a channel member, located around the inside of the outer wall of the body portion. Once the tube portion is received in the restraining means, the tube is inflated to fill the cavity and is retained in place.

In another embodiment, the apparatus of the present invention comprises a substantially cylindrical body portion which is divided into compartments, one of the compartments being centrally located and being provided with a heating means, baffle means and an agitation means, the other compartment, an outer compartment, being subdivided into two components by a barrier or wall means which restricts the access of waste material into the third compartment.

According to another aspect there is provided a method of anaerobically fermenting organic waste material to produce a gaseous phase comprising: (a) introducing waste material in an even distribution into a first compartment provided with an agitation means (b) allowing the material to ferment anaerobically in the first compartment to produce methane and carbon dioxide, a sludge and a treated liquid, whilst being agitated by the agitation means (c) introducing the waste material into a second compartment to continue anaerobic fermentation (d) allowing the waste material to enter a third compartment being a filter means to further permit further separation of solid and gaseous phases from the liquid phase (e) accumulating the gaseous phase produced in the overall process.

The present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a side view of one form of the anaerobic digester of the present invention, Figure 2 is a cross-sectional view of the anaerobic digester of figure 1, Figure 3 is a plan view of the digester shown in figure 1, Figure 4 is an enlarged view of one form of the means for connecting the flexible membrane to the body of the tank and Figure 5 is a cross-sectional view of one form of the bubbler unit suitable for use in the present invention.

In figure 1 there is shown a cylindrical tank, generally denoted by 2, comprising legs 4 for supporting the tank on a concrete pad 6 or similar, a rigid body portion 8 of concrete, metal or similar structurally rigid material, a flexible membrane 10 of rubber or similar flexible material, and a roof portion 12. The flexible membrane, which is in effect an expandable cap or cover for the tank which is closed in use may expand and contract in accord ance with the production and discharge of gases within and from the tank. Service pipe work, generally denoted as 14, is connected to the tank at appropriate and convenient locations and will be described in more detail later.

Turning now to figures 2 and 3 wherein the internal details of the tank 2 are illustrated,there is shown three separate compartments of the tank wherein different treatments occur. The first compartment 16 is a sludge filter reactor and is in the form of an inner cylindrical tank, centrally located within overall tank 2, and defined by walls 18 made from concrete or similar material. A waste inlet pipe 17 is located in wall 18 for admitting raw waste material into the interior of inner tank 16.

A gas bubbler agitation unit 20 is centrally located within tank 16 at its base for agitating andlor mixing the contents of this tank during operation of the digestor by periodically releasing bubbles of gas from its gas discharge pipe 22. In one form, as shown in figure 4, the bubbler comprises a bell arrangement of a cover 70 located over a cup 72 such that the open ends of the two are opposed to each other and a pipe, being the gas discharge pipe 22, is located in the upper surface of the cover to extend down into the cup. In operation the space immediately below the top of the cover 70 fills with gas to displace the liquid originally there until the level of the liquid falls below the lower end of pipe 22 whereupon the gas in the bubbler can escape through pipe 22 to rise to the surface of the liquid in tank 16.The bubbles of gas, emitted from pipe 22 rise to the surface of inner tank 16 and agitate the liquid in tank 16 as they rise to aid the treatment of the waste liquor and prevent a crust of scum forming on the surface of the liquid contained within inner tank 16. Bubbler 22 has a gas inlet pipe 21 for admitting gas into the bubbler to facilitate the periodic discharge of gas bubbles.

The bubbler is provided with a large diameter sludge removal pipe 23 for periodically discharging sludge from the inner tank as required or in accordance with the treatment of the waste material as and when a valve (not shown) in the pipe is opened. The withdrawal of sludge through the bubbler unit 22 is practiced periodically and has the effect of maintaining the bubbler unit in an unclogged condition. The periodic operation of the bubbler unit in releasing gas not only prevents the accumulation of deposits at the terminal end of the sludge withdrawal pipe 23, which overcomes one of the deficiencies of conventional digester designs by preventing blockages, but also prevents the buildup of materials on the heat exchanger surfaces.

A heat exchanger 24 in the form of a tubular arrangement is centrally located within inner tank 16 for maintaining the contents of the tank at the required temperature in accordance with the requirements of the process. Inlet 27 and outlet 25 are connected to heat exchanger 24 to supply hot fluid and to remove cool fluid from the heat exchanger.

In the embodiment illustrated and described the heat exchanger is located immediately above the bubbler 22.

A generally conical shaped baffle 26 having a centrally located opening 28 is located at the top of inner tank 16 and is connected to this tank at its outer rim around the circumference of the tank.

Baffle 26 is a barrier preventing the liquid in tank 16 from overflowing around the edges of the tank so that liquid must pass through opening 28 to exit from the tank. A baffle cap 30 of generally conical shape is located immediately above opening 28.

Baffle cap 30 is connected to baffle 28 by a plurality of brackets 32 at spaced apart intervals around the edge of baffle cap 30 and arranged so that the sloping sides of the baffle and baffle cap are oppositely arranged. Thus, liquid is free to flow from opening 28 through the spaces defined between brackets 32 for discharge from tank 16. The inner edge of baffle 26 underlies the outer edge of baffle cap 30 so that the liquid must flow through a somewhat convoluted or tortuous path to discharge from inner tank 16. Such a tortuous pathway contributes to maintaining agitation and movement of the liquid in inner tank 16 to prevent a crust of scum forming at the surface of the liquid in this tank.In addition, the gas bubbles discharged from bubbler 22 impinge against the under surface of baffle 26, thereby causing additional disturbance of the liquid in this region which further prevents the crust of scum forming on the liquid surface.

While any number of baffles may be employed to produce the desired result, at least one pair of conical baffles at the top of tank 16 are required.

Typically, the lowermost baffle 26 is an inverted, truncated cone with its walls sloping downwards at angles varying from 15 -30 , preferably at an angle of 20 to the horizontal. The diameter of the opening 28 of the cone is typically equal to or about greater than half the diameter of tank 16. The uppermost baffle or baffle cap 30 is also conical and is suspended above the lowermost cone by means of metal brackets. While the slope of the walls of this baffle may vary, a preferred slope is of the order of 45 to the horizontal. The diameter of the base of this baffle is equal to or about slightly greater than that of the opening 28 of the lowermost baffle 26.

The upper rim of tank 16 which forms a weir wall in effect, gently slopes from a high side 34 to a low side 36 so that preferentially liquid flows over low portion 36 into outer tank 40 which annularly surrounds inner tank 16 and is the second compartment of the apparatus of the present invention. Thus, liquids escaping the sludge filter reactor 16 pass between the two abovementioned baffles 26,30 and overflow the weir at low side portion 36 into the second compartment 40.

Outer tank 40 which is a high rate digester is defined between wall 18 and outer wall 42. The outer wall 42 is the outside wall of tank 2. Inner tank 16 is typically one third of the volume of outer tank 40. The walls of the two tanks are of different heights so that the liquid in outer tank 40 is typically 450 mm below the level of the inner tank and of lower edge portion 36. A number of gas bubbler/agitation units 44, similar to bubbler unit 22, are located at spaced apart intervals around the base of tank 40 and are typically of a smaller size than bubbler 22. Bubblers 44 function in a similar manner to bubbler 22 and are each provided with a gas inlet pipe 46, a gas discharge pipe 47 and a sludge discharge pipe 48.The gas emitted from discharge pipe 47 rises in the form of bubbles to the top of the tank as a whole and are collected in the gas collection space 50 defined between the top of the solid walls of the tank and accordingly, the level of the liquid in tanks 16 and 40 and the undersurface of flexible membrane 10. Liquid, after overflowing the weir formed by lower edge portion 36, gradually flows through outer tank 40. The liquid in tank 40 is agitated by the gas bubbles released by the gas bubblers 44 and further prevents the accumulation of scum at the surface of the liquid in outer tank 40. Sludge is removed by means of sludge removal pipe 48. Further degradation of the liquor wastes occurs in outer tank 40 and because of the tortuous pathway of liquid flow a longer time is permitted for treatment of the waste liquors to produce useful gas and purified liquid.

Outer tank 40 is provided with a further compartment 52, the third compartment of the tank, which is an anaerobic filter. The anaerobic filter 52 is located between two radially arranged walls 53 extending between the outer surface of inner wall 18 to the inner surface of outer wall 42. Top surface 54 of the anaerobic filter 52 is substantially solid so that no liquid may pass into the filter from above, whereas the underneath surface 55 is open and provided with a screen or mesh so that liquid may pass upwardly into and through the filter. In es sence, the function of the anaerobic filter is to re move any residual solid or organic material from the liquor to provide purified liquid and to produce gas. One form of the anaerobic filter 52 is the subject of copending Australian Application No. 45278/ 79.A gas discharge pipe 56 is located in the solid top 54 of the filter for discharging any gas pro duced in filter 52 into gas collection space 50, and a liquid discharge pipe 57 is located in side wall 42 for discharging liquid from anaerobic filter 52. Any sludge produced in anaerobic filter 52 falls through screen 55 located at the underneath surface to be collected at the base of tank 40 for removal through sludge discharge pipe 48 of the bubbler unit 44. As liquid flows through tank 40, it will also flow into anaerobic filter 52, after reaching a se lected predetermined level, so that the waste mate rial may undergo further treatment in both tank 40 and filter 52 to further degrade the waste material.

A scum hopper (not shown) is located along the top of each of the two radially arranged walls 53 to collect any scum or crust material which accumu lates on the surface of the liquid in tank 40. In one form, the scum hoppers comprise an open trough which is substantially cylindrical in cross-section having an opening of about a quarter of the cir cumference and extending almost the entire length of the trough. Each trough is aligned along the top of the respective walls 53 such that the opening faces upwardly and away from anaerobic filter 52 to prevent scum and liquid from flowing onto and accumulating on top of filter 52. A drain pipe interconnects the two scum hoppers to drain the hoppers and is connected to a suitable location for removing the scum material from the digester or delivering it for reprocessing such as recycling it to inner tank 16.

Flexible membrane 10 comprises a cap portion 60 and a wall portion 62. Wall portion 62 is securely connected to the inside of wall 42 of tank 2 so that when the flexible membrane expands as space 50 fills with gas, the membrane will not be forced away from the top of tank 2. One method of securely attaching the membrane comprises a generally C-shaped in cross-section ring 64 internally mounted to the inside of wall 42 around the entire circumference of tank 2 so as to form a partially enclosed channel section having a slit 65 (see figure 4). The ring 64 is located below the level of the liquid in the tank under normal operating conditions so that gas above the level of liquid cannot escape between the outer wall of the tank and the membrane. Typically the flexible membrane 10 is attached 2 metres below the selected liquid level of the outer tank 40.Rubber membrane 10 is provided at its edge with a tube portion 66 extending the entire circumference of the membrane. The tube portion is fitted with a valve arrangement (not shown) so that the tube portion may be inflated.

Tube portion when in an evacuated or uninflated condition is inserted through slit 65 of the Cshaped ring 64 around the circumference and inflated by means of the valve arrangement. As the tube portion expands to fill the space defined within the C-shaped channel, the membrane is securely held in place and is prevented from pulling free from the tank as the gas pressure in space 50 increases. The tube portion 66 is provided with reinforcement where it abuts against the edges of the C-shaped channel ring 64. A gas discharge port 68 is provided in cap 60 of the membrane for the removal of gas from space 50. The port 68 may be of any convenient structure and may comprise one or a number of discharge pipes.

In operation, waste liquors, including solid, liquid and gaseous phases are admitted into tank 16 via inlet pipe 17 to gradually fill the tank for the first stage of the treatment. The discharge end of pipe 17 is provided with a distribution means which provides an even distribution of waste liquor throughout tank 16, say around the lower quarter of the tank.

By means of both a careful start up procedure and the presence of the specially designed baffle arrangements at the top of the sludge filter reactor as described above, vast populations of appropriate anaerobic bacteria are developed and maintained in the sludge filter reactor 16.

Organic solids which form a part of the waste liquors which are introduced into the sludge filter reactor via pipe 17 are caused to flocculate into clumps and granules by virtue of the action of the bacteria contained in the reactor.

These clumps and granules are of a higher specific gravity than the liquid inside the reactor so that they precipitate toward the base of the reactor and form a dense layer known as sludge in the form of a blanket over about the lower one third of the reactor.

As the anaerobic bacteria act on, and break down, the organic material contained in each clump and granule, gases are formed which adhere to the outer surface of each clump and granule, causing them to rise towards the top of the reactor.

Due to both the multitude of particles rising in such a fashion at any given time and the effect of the aforementioned baffles located near the top of the reactor, significant turbulence is caused to occur in the upper regions of the reactor. This turbulence results in the release of the gas bubbles adhering to the outer surface of each particle, with the result that the particles gravitate towards the base of the reactor.

This rise and fall of each particle occurs repeatedly until substantially all of the organic material contained in the particle and accessible to the anaerobic bacteria has been broken down. The remaining inert material, which is more dense than other materials inside the reactor, forms a layer of sludge on the floor of the reactor and is withdrawn periodically through the sludge discharge pipe 23 of bubbler agitator unit 22 as previously mentioned.

The baffle arrangement contributes to the required degree of turbulence necessary to separate gases from the particles.

The design parameters used to establish the required capacity of the outer tank of the digester are the same as those used in the design of a conventional high rate digester as its performance and operation are similar to such conventional digesters.

This compartment is unique, however, from other high rate digesters in that it contains one or more bubbler gas agitator systems as means of agitation and has a unique flexible membrane roofing arrangement to contain gasses produced by the process.

Because much of the organic constituents of the original waste liquor are destroyed in the sludge filter reactor (the inner tank), the capacity of the second compartment or outer digester and, therefore, the digester as a whole, is very much reduced by comparison to other digesters.

After undergoing treatment in the outer digester, the waste is discharged from the digester via the third and final compartment which comprises a comprehensive anaerobic filter.

Some of the advantages of some embodiments of the present invention include the following: The inner reactor can handle an organic loading at least to five times the organic loading normally applied to a conventional digester.

If a conventional digester receives an organic loading equal to or greater than about 1/5th of that which can be coped with by the sludge filter, it will fail to operate.

Because the sludge filter can handle such a substantially higher loading, a smaller capacity is required for the sludge filter reactor. Further, because the sludge filter substantially breaks down the organic material introduced into it, the amount of organic material which overflows into the outer digester is very much lower than the amount originally introduced into the sludge filter. Therefore, the total loading on the outer digester is low so the required volume of the outer digester is substantially less than what would have been required had the waste material not first been treated in the sludge filter.

The net result is that, although the outer digester is designed on the same organic loading per unit volume basis as conventional digesters, the overall physical size of the entire digester -being the volume of the sludge filter plus the volume of the outer digester plus the volume of the anaerobic filter -is at least a quarter the volume of conventional digester required to treat the same waste.

The system achieves such higher levels of gas production and pollution control. Three complete and individually effective treatments.

Heating requirements are lower and are not as critical as they are in conventional digesters.

Because of the arrangement of the three treatments in one structure, it is not possible for fresh wastes entering the system to pass straight through without undergoing treatment.

There are no moving parts in the sludge filter/anaerobic filter digester.

The system is substantially more tolerant to shock loadings, and rapid temperature changes which are bottlenecks for conventional systems.

The anaerobic filter also acts as a bacteria drop which prevents the washout of bacteria thus maintaining high populations inside the digester.

Anaerobic filters have in the past been used to treat dilute wastes because they cannot handle strong wastes. Strong wastes are always directed to anaerobic digesters. Because of the arrangement where strong wastes undergo two treatments prior to entering the anaerobic filter, they are, at the time of entering the anaerobic filter, sufficiently dilute to ensure the effective operation of the anaerobic filter.

By its speedier treatment of wastes, this process reduces post treatment costs by its big lowering of B.O.D. in shorter retention times. Because there are no moving parts within the digester to wear out, the process also lowers digester maintenance costs. The bubblers which recycle the gas into a big bubble by differential pressure also have no moving parts.

The described arrangement has been advanced merely by way of explanation and many modifications may be made thereto without departing from the spirit and scope of the invention which includes every novel feature and combination of novel features herein disclosed.

Claims (24)

1. An apparatus suitable for use in anaerobic fermentation of organic waste material comprising a rigid body portion having inlet means for the introduction of raw organic waste and an outlet means for discharging treated material, a flexible member connected to the body portion so as to form a closure means for the body portion, said flexible member and body portion defining a variable gas collection space therebetween for accumulating gas derived from treatment of the waste wherein the flow path for the waste from the inlet means to the outlet means is somewhat tortuous so that waste may not flow directly from the inlet means to the outlet means.

2. An apparatus according to claim 1 in which the body portion is a tank divided into an inner tank and an outer tank separated by a common wall which defines a portion of the tortuous pathway.

3. An apparatus according to claim 2 in which the flexible member is securely attached to the inner wall of the outer tank.

4. An apparatus according to any one of claims 1 to 3 in which the flexible member is provided with an inflatable tube portion around its periphery, said tube portion when in an uninflated condition being adapted to be received in a retaining means located on the inner surface of the outer wall of the tank, and said tube portion when inflated being restrained in place by the retaining means.

5. An apparatus according to claim 5 wherein the retaining means is a substantially C-shaped channel member having a slit arrangement, the channel member being arranged within the tank to extend around the interior wall such that the slit arrangement faces into the interior of the tank.

6. An apparatus according to any one of the preceding claims wherein agitation means is located at or towards the base of the inner tank so as to agitate the contents of the inner tank.

7. The apparatus of claim 6 in which the agitation unit comprises means for periodically discharging of gas from a gas discharge mean for agitating the contents of the tank in use.

8. The apparatus of claims 6 or 7 in which the agitation unit has a gas inlet means for gas is recycling gas from the gas collection space to the agitation unit to facilitate periodic discharge of gas from the agitation means.

9. An apparatus according to any one of claims 6 to 8 wherein a heat exchanger is located within the body portion.

10. An apparatus according to claim 9 in which the heat exchanger is located immediately about the agitation unit in use.

11. An apparatus according to any one of claims 2 to 10 wherein the inner tank is provided with baffle means or barrier means to at least partially define a portion of the tortuous pathway for liquid flowing from the inner tank to the outer tank.

12. An apparatus according to claim 10 wherein baffle or barrier means comprises two oppositely inclined substantially conical or frusto-conical members spaced apart from each other, wherein one of the members is provided with an aperture and the other of the members has a peripheral portion of larger size than the aperture so as to overlie the aperture in use to define a portion of the tortuous pathway.

13. An apparatus according to any one of claims 6 to 12 in which additional agitation units are provided at spaced apart intervals in the outer tank, said agitation units all having gas discharge means and outlet means for removing treated substantially solid material.

14. An apparatus according to any one of the preceding claims having three compartments, wherein the outer tank is divided into two portions separated by wall or barrier means so that access to one of the portions is restricted to a preselected pathway, and wherein an anaerobic filter is located in said one portion of the outer tank.

15. An apparatus according to any one of the preceding claims wherein material removing means are located at the top of the outer tank for removing accumulated material at the surface of the liquid in the tank.

16. An apparatus according to claim 15 in which the material removing means are aligned with the wall of barrier means to prevent direct ingress of waste material into said one portion of the outer tank.

17. An apparatus according to any one of the preceding claims in which the tanks are cylindrical and the inner and outer are coaxial, the outer tank annularly surrounding the inner tank.

18. A method of anaerobically fermenting organic waste material to produce a gaseous phase comprising: (a) introducing waste material in an even distribution into a first compartment provided with an agitation means (b) allowing the material to ferment anaerobically in the first compartment to produce methane and carbon dioxide, a sludge and a treated liquid, whilst being agitated by the agitation means (c) introducing the waste material into a second compartment to continue anaerobic fermentation (d) allowing the waste material to enter a third compartment being a filter means to further permit further separation of solid and gaseous phases from the liquid phase (e) accumulating the gaseous phase produced in the overall process.

19. The method claim 18 further comprising recycling a portion of the collected gas to the agitation means.

20. The method of claim 18 or 19 further comprising periodically removing solid phase or sludge.

21. A method according to any one of claims 18 to 20 comprising removing accumulated solid phase from the surface of the liquid phase.

22. A method according to any one of claims 18 to 21 in which the waste material flows from the first to third compartment via the second compartment in a somewhat tortuous pathway.

23. An apparatus for anaerobic fermentation substantially as hereinbefore described with reference to the accompanying drawings.

24. A method of anaerobic fermentation substantially as hereinbefore described with reference to the accompanying drawings.