GB2282337A - Treating organic waste in an anaerobic digester by passage through perforate channel - Google Patents

Abstract

Apparatus for treating organic waste comprises a reception hopper 3 for receiving the waste, a vessel 1 within which anaerobic digestion can be carried out, a pipe 2 which has a perforate section 5, and an outlet tube 4. Waste from the hopper 3 is driven through the pipe 2 by means of a screw auger 15 and anaerobic digestion of the waste occurs though contact with the liquor 8 in which the pipe 2 is immersed. A back pressure caused by a spring loaded flap 13 or other means causes a net loss of fluid from the waste to the liquor 8 as the waste passes through the pipe 2 and exits via the outlet tube 4. Thus waste of relatively high solids content, eg straw bedding, can be treated. The fixed path for the movement of waste reduces the risk of material passing through the digester with little or no treatment. <IMAGE>

Description

APPARATUS AND METHOD FOR TREATING ORGANIC WASTE The present invention relates to an anaerobic digester apparatus for treating organic waste, and to a method of treating organic waste. The invention relates particularly, but not exclusively, to the treatment of farm waste including faecal material, urine, washing water and bedding material and to a combined de-watering and anaerobic digestion apparatus.

Organic waste is conventionally treated by allowing naturally occurring anaerobic bacteria to break down or digest the organic matter in an aqueous environment. This process helps to reduce the toxicity of the waste and produces potentially useful byproducts such as methane (biogas) and slurry for use as a fertiliser.

Different types of bacteria predominate at different temperatures in anaerobic digestion of organic waste.

Mesophilic bacteria operate in a relatively low temperature range, usually less than 400C, and they have a relatively slow rate of digestion. Thermophilic bacteria work effectively in a higher temperature range, around 40C to 650C. Digestion in the higher temperature thermophilic range occurs more rapidly than in the lower temperature mesophilic range, and it gives better control or elimination of pathogens and parasitic organisms.

Current methods or apparatus for treating farm waste include: (a) A weeping wall separator, consisting of a walled compound in which manure/slurry is fed daily, and periodically the contents are removed and spread on the farmland as farmyard manure. The separator walls are perforated to allow liquor to drain from the daily fed material, usually over several months. Such a weeping wall separator uses a large ground area, and needs careful siting to ensure that the run-off of liquor can be adequately contained or treated. The separator produces no useful energy.

(b) A mechanical separator which generally uses a belt, squeezer or screw type mechanism to remove liquid from the waste. The waste manure/slurry is de-watered to provide a liquor which is stored in tanks and fed to the land through conventional irrigation methods. The solids are stacked and periodically spread on the land as farmyard manure.

Mechanical separators (eg belt type) usually require additional pumps and have a complex mechanism with a limitation to a small particle input material.

Additionally, the liquor may require further treatment after separation.

(c) Anaerobic digestion which is performed in a closed tank into which the slurry is pumped so that it may be degraded by micro-organisms acting on the organic matter fed in with the slurry.

The majority of farm and other conventional anaerobic digesters have the following disadvantages: (1) The waste needs to be of low total solids content, typically 2% to 7% total solids. Because the solids content is low, a relatively large amount of water must be heated to treat a given weight of solids. Because of the cost of heating this water most conventional anaerobic digesters are operated in a mesophilic temperature range rather than in the more efficient thermophilic range.

(2) Completely or partially stirred anaerobic digesters have a potential risk of some of the input passing directly to the output, with little or no treatment, carrying pathogens and other items detrimental to health and hygiene; (3) Most farm and other conventional digesters will only accept liquids or near liquids (less than about 7% total solids) and a maximum particle size of, for example 50 mm, especially those digesters relying on a pumped input and a displacement output. This means that a lot of waste, for example straw bedding, is not suitable for treatment by conventional digesters.

According to the present invention there is provided anaerobic digester apparatus for the treatment of organic waste material, which apparatus comprises: a) a reception hopper into which waste material to be treated can be fed; b) a vessel within which anaerobic digestion can be carried out; c) a perforate member located within the vessel; d) a conduit connecting the reception hopper and the perforate member; e) means for advancing the waste material from the reception hopper along the perforate member; f) means by which a back pressure to the advance of material along the perforate member can be generated whereby fluid is expressed from material within the perforate member and can escape from the perforate member into the vessel; and g) an exit pipe which is connected to the perforate member, through which the treated waste material may be expelled.

The invention is of especial application in the treatment of animal wastes. However, it can also be used for the treatment of domestic sewage and other waste products, for example those from abattoirs, sugar cane or beet processing, food processing, brewing and the petrochemical industry.

For convenience, the invention will be described hereinafter in terms of the treatment of animal faecal waste material.

The perforate member is preferably tubular. It may have perforations of any shape. Preferably the perforations are axial slots to maximise the amount of fluid that can escape from waste inside the perforate member. A preferred length for the slots is in the range 2 to 800 mm, and preferably 200 to 600 mm. A suitable member may be made by welding, rivetting, or otherwise affixing bars of a suitable length together in parallel around a hoop.

The width of the perforations may be of any dimension which allows the escape of fluid, but which retains most or all of the solid component of the waste within the member. The width is preferably in the range 0.1 to 5 mm, and it is particularly preferred to be in the range 1 to 4 mm. The optimum perforation width will depend on the type of waste which is being digested; a typical perforation width for pig waste would be about 1 mm, while for cow waste with straw bedding 3-4 mm would be more appropriate.

The length of the perforate member is selected to enable sufficient digestion of the solid waste to take place before the waste exits the member. The optimum length will depend on the nature of the waste being digested, but a preferred length is in the range 2 to 10 metres, and a particularly preferred length is in the range 4 to 8 metres.

The perforate member may have any suitable internal diameter which allows the passage of the waste solids. A preferred diameter is in the range 100 to 1000 mm, and a particularly preferred diameter in the range 200 to 600 mm.

To maximise the exposure of the waste to digestive bacteria, the perforate member is preferably immersed along its whole length in active liquor contained in the vessel. It is preferred that the terminal end of the exit pipe is positioned above the level of liquor in the vessel, in order to eliminate any need for seals between the exit pipe and the vessel, and to allow fluid to drain from the solid waste under gravity before the waste exits from the exit pipe.

This may be achieved by using a horizontal perforate member and connecting it to an exit pipe which extends upwards at an angle to the horizontal. In a preferred embodiment the perforate member is disposed at an angle to the horizontal so that the end which is connected to the exit pipe is higher than its other end. By using this arrangement the waste is pushed upwards against gravity, which may help to provide the back pressure that causes fluid to escape from the waste.

The waste material may be advanced to and/or through the perforate member under gravity, or by some positive drive means. In order to allow for sufficient digestion of the waste, the retention time for a portion of waste material to pass from the hopper to the exit pipe is preferably in the range 1 to 10 days, and particularly preferably from 2 to 5 days. To achieve the preferred retention time, the drive speed and the dimensions of the perforate member are selected accordingly. The drive means may be any suitable means for advancing the waste material at the desired rate; for example electromagnetic, vibratory, suction or pressure drive means, or a piston, archimedean screw, screw auger, peristaltic pump or the like. In a preferred embodiment the drive means comprises a screw auger.

A typical screw auger will comprise a shaft on one end of which is mounted a helical flight. The shaft may be made hollow to provide a conduit for the passage of liquor from the vessel or any other suitable source into the waste within the perforate member. This may be useful if it is desired to increase the rate of digestion of the solid waste, or to modify the flow characteristics of the waste.

Preferably the back pressure which is generated is greater than one bar. It is preferred that the back pressure is in the range 1.5 to 6 bar, and particularly in the range 2 to 4 bar. Any suitable means may be used to provide the back pressure, for example a spring loaded flap at the terminal end of the exit pipe, or a narrowing of the bore of the pipe towards the terminal end.

If required, a plurality of hoppers and pipes may be used with a single vessel, or a multiple stage digestion system.

The vessel is preferably a closed vessel to promote more efficient anaerobic digestion. In a preferred embodiment the closed vessel has an outlet pipe for biogas and an outlet for venting liquor discharge. Other outlets may be provided for feeding liquor to the reception hopper, for heating (and cooling), for mixing and for grit removal from the contents, and for other processes associated with anaerobic digestion.

The conduit that connects the hopper to the perforate member may be a separate pipe member or it may comprise simply an end portion of the perforate member. Preferably the conduit is not perforated.

The exit pipe may be a separate pipe member or it may comprise simply an end portion of the perforate member.

Preferably the exit pipe is not perforated.

The vessel may be provided with one or more heaters or cooling devices to maintain the temperature of the liquor at a preferred level to optimise the digestion process.

The invention may be used in conjunction with a conventional anaerobic digester, for example an existing farm digester.

This may be done by connecting the vessel to the conventional digester by means of pipes, and using one or more pumps to circulate the fluid phase through the conventional digester. The conventional digester is preferably provided with an outlet pipe for biogas, and this may be connected to a biogas outlet pipe from the vessel to equalise the pressure.

The invention may be used to treat waste of a relatively high solids content, for example straw bedding. The use of high solids content waste means that it is more economical to heat the waste to the optimum thermophilic operating temperature. Because the invention provides a fixed path for the movement of the waste through the vessel, the risk of input material passing through the digester with little or no treatment is reduced.

The bacteria that digest the waste are generally present in the environment or in the waste material, so it is not usually necessary to add active liquor from the vessel to waste in the hopper to achieve digestion. Surprisingly, however we have found that by wholly or partially sterilising the waste before it is put in the hopper, or while it is in the hopper, and then adding liquor from the vessel to that waste, the process of digestion occurs more rapidly. The sterilisation may be effected by treatment of the waste with steam, or by treatment with alkali or any other suitable treatment.

The end products of the treatment are biogas, solids, and liquid. All of these may be put to use: biogas for producing heat energy; solids for use with muckspreaders on farmland, and for horticultural and forestry applications; liquids for use as liquid organic fertiliser.

Alternatively, if the liquid is not required as a fertiliser, it may be discharged immediately or after further treatment.

According to a second aspect of the invention, there is provided a method of treating organic waste material, which method comprises: a) feeding a slurry of the organic waste to at least one perforate member immersed in liquor undergoing anaerobic digestion; b) advancing the slurry along the perforate member while subjecting it to a back pressure whereby the solids content of the slurry within the member is exposed to anaerobic digestion and at least part of the fluid phase of the slurry escapes from the perforate member; and c) discharging the solids portion of the material in the perforate member from an outlet to the perforate member under a controlled pressure.

Preferably the method is carried out with liquor in the thermophilic temperature range.

The invention will now be further described, by way of example, with reference to the accompanying diagrammatic drawings in which: Figure 1 shows an anaerobic digester in accordance with the invention; Figure 2 shows an alternative embodiment of an anaerobic digester in accordance with the invention; Figure 3 shows a further alternative embodiment of an anaerobic digester in accordance with the invention; and Figure 4 shows a fourth embodiment of an anaerobic digester in accordance with the invention; Figure 5 shows the digester of Figure 2 when used with a conventional anaerobic digester.

Referring now to Figure 1, the anaerobic digester comprises a closed vessel 1 through which a separator tube or pipe line 2 passes linking a reception hopper 3 with a solids outlet tube 4. Within the vessel 1 the pipe 2 contains a perforate section 5 for separating liquid from solids. The pipe 2 contains organic material fed from the hopper 3 by means of an externally driven piston 6. A spring loaded flap 13 provides a back pressure on the waste in the pipe 2 so that fluid is made to escape through the perforations in the perforate section 5.

A bleed pipe 7 supplies the hopper 3 with liquor 8 from within the vessel 1 so that the raw material fed to the reception hopper 3 is sufficiently wetted to form a slurry which may be driven by the piston 6 through the pipe 2. If the digester is operated intermittently or if it is preferable to leave dry organic waste material in the hopper 3 a valve may be incorporated in the pipe 7 to hold back the liquor until required. Alternatively, the liquor may be fed into the pipe 2 and mixed in the pipe 2 before the separator section 5. By withholding the liquor from the hopper 3 noxious smell are reduced. The hopper 3 may also be fitted with a lid to reduce smells.

Any excess of liquid is drawn off from the vessel 1 through a liquids outlet pipe 9 by means of an overflow trap 10.

The biogas created by the digester is vented under the control of a valve 12 through a pipe 11 connected to the top of the vessel 1. The biogas may be used as an energy source.

In operation waste organic material such as faecal material, urine, washing water and bedding material is loaded into the hopper 3 which is wetted and seeded by digesting liquor from the vessel 1 through the bleed pipe 7. The material in the hopper is driven into the pipe 2 by the operation of the piston 6. As the waste material is compacted within the perforate section 5 of the pipe 2, liquid escapes through the section 5 into the liquor 8 within the vessel 1. The passage of the compacted waste material through the pipe 2 is controlled by the rate of advance of the piston 6. The piston 6 is advanced through about one fifth of the length of the pipe 2, compressing the waste material within the pipe 2, and it is then pulled back to allow a fresh charge of waste to be delivered to the pipe 2 from the hopper 3.

A non-return valve 14 prevents the compacted waste from moving back down the pipe 2. Pressure is maintained on the waste material in the pipe 2 between a spring loaded flap 13, at the solids exit pipe 4, and the non-return valve 14.

This compression promotes the de-watering process in the perforate section 5. The piston 6 may be operated continuously, very slowly, or it may be operated intermittently, for example once a day, to compact the organic material and to allow the liquid from the organic waste to escape slowly into the liquor.

The liquor 8 in the tank 1 is broken down with the aid of digester bacteria to produce biogas and liquid fertiliser.

The biogas may be drawn off from the top of the vessel by way of the pipe 11, and the liquid fertiliser may be drawn from the bottom of the vessel by way of the liquids outlet pipe 9. As the biogas is combustible and produces a significant calorific value it may be used for heating or combined heat and power generation purposes. The compacted rods of solids material from the exit pipe 4 may be stacked and stored for future use as a soil conditioning agent or as a fertiliser.

The embodiment shown in Figure 2 is substantially the same as Figure 1 and therefore similar components have been given the same reference numerals as in Figure 1. However, in Figure 2 a screw auger 15 replaces the reciprocating piston 6 which propels the slurry material from the hopper 3 along the tube 2. During its passage through the tube 2 the material is compressed and the liquid escapes through the perforate section 5. As can be seen from Figure 2 the tube 2 is straight which facilitates the action of the screw 15 and the passage of the solids material through the pipe 2.

The tube 2 is also angled upwards from the hopper 3 to the exit pipe 4; this means that the waste is pushed upwards against gravity which helps to increase the pressure on the waste.

The screw 15 operates against the end pressure of the flap 13 to compress the waste material. Fine material which passes through the perforations in the section 5 settles on the bottom of the vessel 1 and may be drawn off under the control of a valve 16 through a pipe 17, or, as shown in the embodiment of Figure 1, the fine material is drawn off with the discharge of liquor through the pipe 9.

As shown in Figure 2 the liquor 8 in the vessel 1 may be drawn off through a separate outlet pipe 18 under the control of a valve 19. If necessary this drawn off liquor or additional liquid may be added to the hopper 3 to wet down the solid waste materials which fills the reception hopper 3 waiting to be driven through the pipe 2.

The screw 15 may be turned continuously, very slowly, at a controlled variable speed, or intermittently, for example once a day. The screw 15 is driven by a motor 20 as shown in Figure 2 mounted at the high end of the pipe 2. If the lower end of the pipe 2 is accessible it may be more convenient to mount the motor 20 at the other end of the screw drive shaft. This shaft could be designed to conduct heat and so distribute it throughout the vessel 1. The shaft could also carry liquids and/or agitation gas into the vessel 1 to assist the digestion process.

The anaerobic digester as shown in Figures 1 and 2 is heated, or cooled, to maintain the temperature of the contents of the vessel 1 at its optimum temperature to assist the digestion process. If required external mixing or stirring may be applied to aid the digestion process.

The preferred method is to use an externally mounted grit remover, heat exchanger and mixing device.

The digester may be constructed so that the hopper 3 is relatively remote from the vessel 1 as shown schematically in Figure 3. The organic waste material in the hopper 3 is well wetted with recycled liquor which is returned to the hopper 3 via a pipeline 21. The pipeline 21 contains a pump to assist the flow of liquor to the hopper 3.

An additional modification is to provide a number of hoppers and/or pipes feeding into the vessel. As shown in Figure 4 separate hoppers 22, 23 and 24 may be situated at different locations such as a farm, an abattoir, and a sewage works.

The output pipes 25, 26 and 27 connecting the hoppers 22, 23 and 24 respectively to the vessel 1 may be arranged to discharge at a common point or, as shown in Figure 4, to remain separate in their passage through the vessel 1.

Figure 5 shows a modification of the digester of Figure 2.

A conventional farm anaerobic digester 32 is connected to the vessel 1 by means of pipes 28, 30. The liquor 8 is circulated through the vessel 1 and the farm digester 32 by means of pumps 29, 31. A pipe 33 is provided to connect the top of the farm digester 32 to the biogas outlet pipe 11 so that biogas produced in the farm digester 32 is controlled by the same valve 12 as controls the biogas output from the vessel 1. This arrangement helps to equalise the pressure in the system.

The modification shown in Figure 5 may be used if it is desired to modify an existing conventional anaerobic digester to handle higher solids content waste. Although, for the purpose of illustration, Figure 5 shows the conventional anaerobic digester being used with the archimedean screw modification of the invention, any other modification of the invention may also be used in conjunction with a conventional anaerobic digester.

The treatment method of the invention is particularly suitable for organic waste produced by farm animals, for example pigs, cows or poultry. With known methods of waste treatment problems occur where there is an imbalance of animal materials (faecal, urine and washing water) and bedding materials or other vegetable materials. If the volume of bedding or vegetable materials is too low the resultant manure/slurry is over-rich in nutrients and organic matter which produces scorched vegetation, deoxygenated water courses and increases the risk of disease through flies, vermin etc. With the present invention the anaerobic activity of the bacteria assists the breaking down of the waste products and contains them in a tank ready for use. The compacted solids obtained from the process are easily stored until required. An additional beneficial byproduct of the present invention is the biogas which may be used for heating or other purposes.

Claims (21)

1. Anaerobic digester apparatus for the treatment of organic waste material, which apparatus comprises: a) a reception hopper into which waste material to be treated can be fed; b) a vessel within which anaerobic digestion can be carried out; c) a perforate member located within the vessel; d) a conduit connecting the reception hopper and the perforate member; e) means for advancing the waste material. from the reception hopper along the perforate member; f) means by which a back pressure to the advance of material along the perforate member can be generated whereby fluid is expressed from material within the perforate member and can escape from the perforate member into the vessel; and g) an exit pipe which is connected to the perforate member, through which the treated waste material may be expelled.

2. Apparatus as claimed in claim 1, wherein the perforations are axial slots.

3. Apparatus as claimed in claim 2, wherein the length of the slots is in the range 2 to 800 mm.

4. Apparatus as claimed in claim 2, wherein the length of the slots is in the range 200 to 600 mm.

5. Apparatus as claimed in any one of the preceding claims, wherein the width of the perforations is in the range 0.1 to 5 mm.

6. Apparatus as claimed in any one of the preceding claims, wherein the width of the perforations is in the range 1 to 4 mm.

7. Apparatus as claimed in any one of the preceding claims, wherein the length of the perforate member is in the range 2 to 10 metres.

8. Apparatus as claimed in any one of the preceding claims, wherein the length of the perforate member is in the range 4 to 8 metres.

9. Apparatus as claimed in any one of the preceding claims, wherein the internal diameter of the perforate member is in the range 100 to 1000 mm.

10. Apparatus as claimed in any one of the preceding claims, wherein the internal diameter of the perforate member is in the range 200 to 600 mm.

11. Apparatus as claimed in any one of the preceding claims, wherein the terminal end of the exit pipe is positioned above the level of liquor in the vessel.

12. Apparatus as claimed in any one of the preceding claims, wherein positive drive means are provided to advance the waste material through the perforate member.

13. Apparatus as claimed in claim 12, wherein the drive means comprises a screw auger.

14. Apparatus as claimed in any one of the preceding claims, which in operation generates a back pressure in the range 1.5 to 6 bar.

15. Apparatus as claimed in any one of the preceding claims, which in operation generates a back pressure in the range 2 to 4 bar.

16. Apparatus as claimed in any one of the preceding claims, further including means for maintaining the liquor at a preferred temperature.

17. A method of treating organic waste material, which method comprises: a) feeding a slurry of the organic waste to at least one perforate member immersed in liquor .undergoing anaerobic digestion; b) advancing the slurry along the perforate member while subjecting it to a back pressure whereby the solids content of the slurry within the member is exposed to anaerobic digestion and at least part of the fluid phase of the slurry escapes from the perforate member; and c) discharging the solids portion of the material in the perforate member from an outlet to the perforate member under a controlled pressure.

18. A method as claimed in claim 17, which is carried out in the thermophilic temperature range as hereinbefore defined.

19. A method as claimed in claim 17 or claim 18, wherein the waste material is at least partially sterilised and then stored in a hopper, and then treated with some of the liquor in which the perforate member is immersed, before being fed from the hopper to the perforate member.

20. Apparatus substantially as hereinbefore described with reference to and as shown in any one of the drawings.

21. A method substantially as hereinbefore described.