METHOD AND DEVICE FOR ANAEROBIC BREAKDOWN OF ORGANIC WASTE

Abstract: A method and a device for continuous anaerobic degradation of organic waste are disclosed. The waste is emptied into a hopper (1), which preferably contains rotary rollers (2) equipped with knives for disintegrating the waste. At the bottom of the hopper (1), there is provided an infeed means (5) feeding the waste through a conduit (6) to the downwardly-directed narrow end (7) of a funnel-shaped container (8) exhibiting an acute angle alpha of about 60-160 DEG . During the hydrolysis, the waste is fed upwards through the container (8) while being moistened by liquid injection through nozzles (13) arranged above the container (8) or nozzles (14) arranged inside the container (8). After the treatment, the waste is discharged from the container (8) by outfeed means (9), which preferably comprise two screw conveyors arranged at the upper, opposing edges of the funnel-shaped container. The liquid not absorbed by the waste is drawn off, e.g. through perforations (15), and collected in a tank (16), to subsequently be conducted to a filter (19) where biogas is separated (at 20). After an optional addition of fresh water (at 26), the liquid is recycled to the nozzles (13, 14) and supplied to the waste. When having undergone the inventive treatment and been discharged from the inventive device, the waste may undergo further treatment, such as a maturing stage.

Description: METHOD AND DEVICE FOR ANAEROBIC BREAKDOWN OF ORGANIC WASTE This invention concerns a method and a device for anaerobic breakdown or degradation of organic waste. By organic waste is generally meant organic house¬ hold waste, preferably pre-separated at source; organic industrial waste, such as slaughterhouse waste; hazardous organic waste, such as hazardous biological waste; and agricultural waste. According to the prior art, organic waste is treated by degradation, which comprises anaerobic degradation, including hydrolysis and digestion, as well as aerobic degradation or composting. The waste to be subjected to treatment is introduced into a container or chamber, where the anaerobic degradation proceeds while the waste is moistened by water or an aqueous liquid. A problem encountered in the prior art is that the liquid tends to find special ways when permeating down through the waste, thus forming channels. As a result of this formation of channels, the waste will be unevenly moistened, such that the degradation is incomplete and unsatisfactory. As an instance of the prior art, mention may be made of DE 37 19 564, which discloses a biogas plant for batchwise degradation of waste. Thus, the waste is fed from below into a chamber having a horizontal, perforated bottom and being covered by a hood. When the treatment in the chamber is completed, the hood is removed and the waste is transported from the premises by means of a grab bucket or a wheeled loader. This invention aims at counteracting or eliminating channel formation in anaerobic treatment of organic waste and to provide uniform, rapid and efficient degradation of the waste. According to the invention, this aim is achieved by feeding the waste being treated upwards through a funnel- shaped container while moistening it with water or an aqueous liquid. Since waste is being supplied from below and the funnel-shaped container widens in the upward direction, the waste is continuously in motion and agi¬ tated, the formation of channels being thus counteracted. These and other advantages of the invention will appear more clearly from the following description, as well as the appended claims. To be more specific, the invention provides a con¬ tinuous method for anaerobic degradation of organic waste, wherein the waste is emptied into a hopper and fed from the hopper to a container, the waste is moistened by the supply of water or an aqueous liquid, and the waste is removed from the container, said method being charac¬ terised in that the container is funnel-shaped, the down- wardly-directed narrow end of the container exhibiting an angle α of about 60-160°, that the waste is fed from below through the downwardly-directed narrow end of the funnel-shaped container, that the waste is fed upwards through the funnel-shaped container while being moisten- ed, and that the waste is fed from the funnel-shaped con¬ tainer via the upper side edges to outfeed means. The invention further provides a device for conti¬ nuous anaerobic degradation of organic waste, which com¬ prises a hopper into which the waste is emptied, an infeed means connected to the hopper, a container, a conduit interconnecting the hopper and the container, liquid-supplying means for supplying water or an aqueous liquid to the waste in the container, and dewatering means for separating liquid from the waste, said device being characterised in that the container is funnel-shap¬ ed, the downwardly-directed narrow end of the container exhibiting an angle α of about 60-160°, that the conduit is connected to the downwardly-directed narrow end of the funnel-shaped container for feeding of the waste into the container, and that the funnel-shaped container is adapt¬ ed, at its upper end, for feeding of the waste via the upper side edges to outfeed means. Further distinctive features and characteristics of the invention appear from the dependent claims. The invention will be described in more detail below with reference to the accompanying drawings, in which Fig. 1 is a schematic side view of a device accord¬ ing to the invention; Fig. 2 is a schematic longitudinal section of the funnel-shaped container in Fig. 1; and Fig. 3 is a schematic plan view of the funnel-shaped container in Fig. 2, the hood having been removed. The design of the device shown in Figs 1-3 will now be described. As appears from Fig. 1, there is provided a hopper 1 for receiving organic waste. The illustrated hopper 1 is in the form of a hollow, whose upper edge is on a level with the ground. However, it is understood that the hop¬ per 1 may also consist of e.g. a separate container placed above ground. The hopper 1 may comprise means for disintegrating the waste, for instance rollers 2 equipped with knives. If, however, the waste is sufficiently dis¬ integrated already at the outset, no such means are required. Preferably, the hopper 1 is also provided with a lid 3 and heating means 4, suitably in the form of nozzles for the injection of hot water vapour. At the bottom of the hopper 1, there is provided an infeed means 5, which preferably consists of a reciprocating hydraulic plunger, as shown in Fig. 1. Also other infeed means are conceivable, such as rotary screw feeders, but an infeed means 5 in the form of a hydraulic plunger (shown) is preferred. The infeed means 5 is connected to a conduit 6, which at its other end is connected to the downwardly- directed narrow end 7 of a funnel-shaped container 8. At the upper end, the container 8 is connected to outfeed means 9, which preferably consist of two screw conveyors arranged on opposing sides of the container 8, as shown in Figs 1-3. Conveniently, the screw conveyors 9 are com¬ bined with an additional screw conveyor 10 (Figs 2 and 3) . In order to prevent evil smell from spreading to the surroundings and to maintain the anaerobic conditions in the container 8, the latter is provided with a sealing hood 11. Preferably, the hood 11 has a chimney pipe 12 through which the generated gases can be drawn off, e.g. for combustion or some other type of destruction. As can be seen most clearly in Fig. 3, the container 8 is, for reasons of manufacture, preferably square. Above the upper end of the funnel-shaped container 8, there are provided liquid-supplying means 13 in the form of at least one nozzle for spraying water or an aqueous liquid (in the following referred to merely as "liquid") onto the waste in the container 8. Alternatively, the liquid- supplying means may consist of nozzles 14 introduced into the container 8, as hinted at in Fig. 1, but this alter¬ native is currently less preferred. The liquid not absorbed and retained by the waste is removed by dewater- ing means 15, preferably perforations formed in the lower part of the funnel-shaped container 8, as shown in Figs 1 and 3. A tank 16 surrounding the perforated area of the container 8 is arranged to collect the drawn-off liquid. Alternatively, or as a complement to the perforations 15, liquid may be drawn off via the outfeed screw conveyor means 9, which then compress the waste in the process of being discharged from the container, so as to extract liquid therefrom. The collecting tank 16 is provided with a line 17 having a pump 18 for feeding the drawn-off liquid to a filter 19. This filter, which is of conven¬ tional design, comprises a container holding a carrier, such as folded sheets of plastic, for methane bacteria generating methane (biogas) from the liquid supplied to the filter. The generated methane gas may, as indicated by the arrow 20, leave the filter 19, and the energy con¬ tent of the gas is recovered by combustion. A line 21 for collecting and supplying liquid, if any, from the outfeed means 9 may further be connected to the line 17. The fil¬ ter 19 is equipped with a return line 22 for recycling liquid to the nozzle 13 via a line 23, or alternatively to the nozzles 14 via a line 24, as shown in Fig. 1. The two opposing sides of the funnel-shaped con¬ tainer 8 not provided with outfeed means 9 are delimited by vertical walls, which for instance are formed integral with either the container 8 or the hood 11. These verti¬ cal walls are meant to prevent waste from being discharg¬ ed at these edges of the container 8 and to ensure that all the waste is discharged from the funnel-shaped con- tainer 8 via the upper side edges adjacent to the outfeed means 9. As mentioned in the foregoing, the outfeed means 9 normally compress the waste to a certain extent, in which case excess liquid may be extracted therefrom. The out- feed means 9 may be provided with means (not shown) for collecting such excess liquid, for instance in the form of perforations formed on the underside of the screw conveyors 9. The collected liquid is then drawn off through the line 21. The screw conveyors 9 are preferably connected to an additional screw conveyor 10, which is provided beneath the screw conveyors 9 and which receives the waste from the screw conveyors 9 and conveys it for further treat¬ ment, such as composting. When in the screw conveyor 10, the waste is suitably aired, for instance by air injec¬ tion (not shown) into the screw conveyor 10. The inventive device described above operates as follows. Organic waste is transported to the device, e.g. by lorry 25, and tipped into the hopper 1. In the hopper 1, the waste is disintegrated, preferably by means of rotary rollers 2 equipped with knives. When including e.g. hazardous waste or septic waste, the waste is preferably heat-treated as well in the hopper 1. During this treatment, the hopper 1 is sealed by means of the lid 3, and the waste is heated, for instance by the injection of vapour through the nozzles 4. In the heat treatment, the waste is brought to a temperature of about 60-125°C, preferably about 70-80°C, with the aid of water vapour under pressure, for instance at least about 1.5 atm gauge. In the heat treatment with water vapour, the latter condenses on the waste, which thus is moistened. From the bottom of the hopper 1, the waste is fed by means of the infeed device 5, preferably a hydraulic plunger, to the conduit 6, which ends in the lower, nar¬ row end 7 of the funnel-shaped container 8. Owing to the supply of more waste, the waste will move upwards through the funnel-shaped container 8. When pressed up through the container, the waste is agitated and pushes the mate¬ rial above upwards and to the sides of the funnel-shaped container 8, thereby counteracting undesirable channel formation in the waste. For optimum upward feeding of the waste through the funnel-shaped container 8, the narrow end of the container exhibits an angle α (Fig. 1) of 60- 160°, preferably about 60-90°, and most preferred about 60-80°, for instance about 65°. While fed upwards through the container 8, the waste is moistened by the supply of liquid, preferably by spraying from above via the spray nozzles 13, as described in the foregoing. The moistening reduces the dry-solids content (DS) of the waste to about 15-25% DS from, say, about 50% DS in the case of house- hold waste. Liquid not absorbed by the waste is drawn off through the perforations 15 and collected in the tank 16, as indicated above. While fed upwards through the con¬ tainer 8, the moist waste, which normally has a pH of about 5-6 and a temperature of about 20-55°C, preferably about 25-40°C, and most preferred about 25-35°C, is anaerobically broken down. The treatment time, which is the time it takes for the waste to be fed through the funnel-shaped container 8, depends on whether one wishes to achieve partial anaerobic degradation in the form of hydrolysis or complete anaerobic degradation, i.e. both hydrolysis and digestion, of the waste in the funnel- shaped container 8. If one merely wishes to achieve par- tial anaerobic degradation of the waste in the container 8, as is currently preferred, the treatment time suit¬ ably is at least about 2 days, preferably at least about 5 days, for instance about 5-7 days, and most preferred about 5 days. If, on the contrary, complete anaerobic degradation of the waste in the container 8 is to be obtained, a much longer treatment time of up to about 20-30 days is required. It should here be pointed out that, if complete anaerobic degradation is carried out in the container 8, also the digestion with ensuing generation of methane gas takes place in the container 8. The methane gas then leaves with the other gases, such as carbon dioxide, through the chimney pipe 12 and is disposed of, for instance by combustion. It will be appreciated that the filter 19 then is not needed for the generation of methane gas, but it may nevertheless be retained as an equalisation and buffer vessel in the liquid-circulation circuit described above. When the waste reaches the upper end of the con¬ tainer 8 and the treatment is completed, the waste is discharged from the container 8 in some suitable fashion, preferably with the aid of outfeed means 9 arranged on both sides of the container, as described above and shown in the drawings. When thus discharged, the waste is compressed by the outfeed means 9 so that excess liquid is extracted. When compressed, the waste forms a sealing stopper between the anaerobic environment in the con¬ tainer 8 and the surroundings. From the outfeed means 9, the waste is conveyed in a screw conveyor 10 where it is aired, before being collected for storage or continued biological degradation under aerobic conditions (maturing stage) . In order to create an optimally closed system, the liquid extracted from the waste during the treatment in the container 8 is conducted through lines 17, 21 to the filter 19, where the generated biogas is separated (see the arrow 20) and its energy content is recovered, for instance by combustion. Then, the liquid is recycled from the filter 19 to the container 8 through the line 22, optionally after an addition of fresh water, as indicated by the arrow 26. Owing to its compact design, the device according to the invention does not require much space. The hopper 1 is conveniently dimensioned for the volume of waste received in one day, and the funnel-shaped container 8 suitably has a square horizontal section, the upper side edges having a length of about 5 m, and the height of the container being about 4 m. The container 8 is suitably made of sheet metal or steel, as is the associated equip¬ ment. The inside of the container 8 is preferably coated with a wear- and friction-reducing material, such as rub¬ ber or plastic. It is evident from the above description that the method according to the invention is continuous, which constitutes an advantage over conventional, batchwise methods. Although the invention has been described with refe¬ rence to especially preferred embodiments, it will be appreciated that it is not restricted thereto, but may be modified and varied within the scope of the appended claims. The container 8 thus need not have a square horizontal section but may, for instance, have a circular section.

Claims: CLAIMS 1. A continuous method for anaerobic degradation of organic waste, wherein the waste is emptied into a hop¬ per ( 1 ) and fed from the hopper (1 ) to a container (8) , the waste is moistened by the supply of water or an aqueous liquid (13, 14), and the waste is removed from the container (8), c h a r a c t e r i s e d in that the container is funnel-shaped, the downwardly-directed nar¬ row end (7) of the container exhibiting an angle α of about 60-160°, that the waste is fed from below through the downwardly-directed narrow end (7) of the funnel- shaped container, that the waste is fed upwards through the funnel-shaped container (8) while being moistened, and that the waste is fed from the funnel-shaped con¬ tainer (8) via the upper side edges to outfeed means (9 ) . 2. A method as claimed in claim 1, c h a r a c ¬ t e r i s e d in that the treatment in the funnel-shaped container (8) is carried out at a temperature of about 20-55°C. 3. A method as claimed in claim 1 or 2, c h a r ¬ a c t e r i s e d in that the treatment in the funnel- shaped container (8 ) is carried out for at least 2 days. 4. A method as claimed in any one of claims 1-3, c h a r a c t e r i s e d in that the dry-solids content of the waste is reduced to about 15-25% as a result of the moistening in the funnel-shaped container (8). 5. A method as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the waste is heat-treated in the hopper ( 1) . 6. A device for continuous anaerobic degradation of organic waste, which comprises a hopper (1) into which the waste is emptied, an infeed means (5) connected to the hopper ( 1) , a container ( 8) , a conduit ( 6) inter¬ connecting the hopper ( 1 ) and the container (8), liquid- supplying means (13, 14) for supplying water or an aqueous liquid to the waste in the container ( 8) , and dewatering means (15) for separating liquid from the waste, c h a r a c t e r i s e d in that the container is funnel-shaped, the downwardly-directed narrow end (7) of the container exhibiting an angle α of about 60-160°, that the conduit ( 6) is connected to the downwardly- directed narrow end (7) of the funnel-shaped container for feeding of the waste into the container, and that the funnel-shaped container (8) is adapted, at its upper end, for feeding of the waste via the upper side edges to outfeed means (9) . 7. A device as claimed in claim 6, c h a r a c ¬ t e r i s e d in that the dewatering means (15) comprise perforations formed in the lower part of the funnel-shap- ed container (8). 8. A device as claimed in claim 6 or 7, c h a r ¬ a c t e r i s e d in that the outfeed means (9) comprise two screw conveyors arranged at the upper, opposing edges of the funnel-shaped container (8). 9. A device as claimed in any one of claims 6-8, c h a r a c t e r i s e d in that the liquid-supplying means (13, 14) comprise at least one nozzle (13) arranged above the funnel-shaped container (8). 10. A device as claimed in any one of claims 6-9, c h a r a c t e r i s e d in that the hopper ( 1 ) is equipped with heating means (4) .

Inventors: SIVERSSON BERTIL, SIVERSSON, BERTIL
Applicants: SIVERSSON BERTIL, SYDKRAFT AB, SIVERSSON, BERTIL
Application year: 1995
Main classification: C02F11
IPC codes: C02F11, C12M1