GB1600979A

GB1600979A - Apparatuses for the anaerobic digestion of natural organic waste

Info

Publication number: GB1600979A
Application number: GB7562/77A
Authority: GB
Original assignee: Hamworthy Engineering Ltd
Current assignee: Hamworthy Engineering Ltd
Priority date: 1978-02-23
Filing date: 1978-02-23
Publication date: 1981-10-21

Description

(54) IMPROVEMENTS IN OR RELATING TO APPARATUSES FOR THE ANAEROBIC DIGESTION OF NATURAL ORGANIC WASTE (71) We, HAMWORTHY EN GINEERING LIMITED, of Fleets Corner, Poole, Dorset BH17 7LA, A British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described, in and by the following statement: The present invention relates to apparatuses for the anaerobic digestion of natural organic waste.

According to the invention there is provided an apparatus for the anaerobic digestion of natural organic waste comprising a digester, a feed tube which extends downwardly into the digester on one side thereof for feeding natural organic waste into the digester, an inclined outlet tube which extends upwardly from the digester on the opposite side thereof and has an in inclination such that, as waste is introduced into the container through the feed tube and the liquid overflows through the outlet tube, solids which had not settled to the bottom of the digester settle and remain in the digester, a gas outlet in the top of the digester, a diffuser located at the bottom of the digester, and a gas pump for pumping gas from the gas outlet through the diffuser, the diffuser comprising a flexible membrane perforated with holes which are sufficiently fine to open only when pressurized by gas from the gas pump.

The invention will be further described, by way of example, with reference to the drawings accompanying the provisional specification, in which: Figure I is a side elevation of a preferred apparatus for the anaerobic digestion of natural organic waste; Figure 2 is a plan view of the apparatus of Figure 1; and Figure 3 is a cross-sectioned side view of a gas pump of the apparatus of Figure 1.

The preferred apparatus comprises a feed tank 1, into which waste for treatment can be loaded, including a bucket elevator 2.

The bucket elevator 2 comprises an electric motor 3 arranged to drive an endless belt 4 on which are provided a plurality of buckets. The buckets are arranged to scoop waste from within the feed tank 1 and to empty the waste into a loading tube 5.

The loading tube 5 extends downwardly from the bucket elevator 2 into a digester 6 which is mounted on three pillars 22. The digester has convex conical upper and lower portions joined by a cylindrical portion. The digester 6 has a gas outlet pile 7 for removal of gases such as methane which collect over the fluid in the digester as a result of bacterial activity therein. A diffuser 8 is provided at the bottom of the digester 6 and is connected via a pipe 9 to a gas pump 10.

The diffuser 8 end incorporates a flexible membrane perforated with fine holes. These holes are such that they only open when pressurized by gas recirculated from the gas pump 10 so that they do not become clogged. Alternatively, the diffuser 8 may consist of a solid diffuser.

An outlet tube 11 extends upwardly from the digester 6 and is open to the atmosphere at its upper end. Adjacent its upper end, the outlet tube 11 is connected to an outlet tank 12 via a flexible hose 13.

The gas outlet pipe 7 is connected via a flame arrester 14 to a digital gas meter 15, whose outlet is connected to a digester pressure regulator 16. Gas from the pressure regulaor 16 is then connected via a pipe 17, for instance, to a gas stack or gasometer.

The outlet pipe 7 is also connected via a pipe 18 to an inlet of the gas pump 10.

Thermocouples and/or thermistors 19 are provided in the digester 6 for monitoring the temperature of the fluid therein. Gas alarms 20 and 21 are provided on the feed tank and the outlet tank respectively. A discharge pipe 23 and valve 24 are provided on the side of the digester 6 for sampling and a discharge tube 25 and valve 26 are provided on the bottom of the digester for removal of digested sludge.

The gas pump 10 is shown to a larger scale in cross-section in Figure 3 and comprises a vessel 27 containing a double acting piston 28 and cylinder 29 mounted on a frame 30.

The piston 28 is connected via a piston rod 31 and a coupling rod 32 to a first end of a rocking lever 33 which is pivotable about an axis 34. The other end of the rocking lever 33 is pivotably connected to one end of a driving link 35 whose other end is connected to a crank 36 (shown rotationally out of position for the sake of clarity). The crank 36 is mounted on a shaft 37 of a prime mover, such as an electric motor with a reduction drive 42.

The prime mover is separated from the piston 28 and cylinder 29 by a dividing wall 38 and liquid 39. The level of the liquid 39 on either side of the dividing wall 38 is such that the bottom of the dividing wall is below the liquid surface, so that the space 40 above the liquid and containing the cylinder 29 is air-tightly sealed from the space 41, which communicates with the atmosphere via a duct in the vessel 27.

The cylinder 29 is connected via valves (not shown) and pipes 43 to the pipes 9 and 18 so as to draw gas from the pipe 18 into the cylinder 29 on alternate sides of the piston 28 during consecutive strokes thereof and to force gas into the pipe 9 with alternate strokes of the double acting piston.

In use, waste is fed into the feed tank 1, from which it is continuously loaded into the digester 6 at a suitable predetermined rate by means of the bucket elevator 2 and the loading tube 5.

The temperature of the contents of the digester is substantially maintained at a predetermined value, usually between 20 and 60"C, depending on the type of waste to be treated. This can be achieved automatically by providing a control system arranged to control heating means provided in the digester according to the outputs from the thermo-couples and/or thermistors 19.

Gases such as methane and carbon dioxides are given off during the digestion of waste by the bacteria and are fed via the gas outlet pipe 7 to the gas pump 10 and to the gas stack or gasometer. The gas which is fed to the gas pump 10 is returned under pressure to the digester 6 via the pipe 9 and the diffuser 8. The inflow of gas via the diffuser produces efficient mixing of the contents of the digester and allows intimate contact between the gas and the bacteria, thus improving the efficiency of the digester.

The arrangement of the outlet tube 11, which acts as a settling vessel, encourages the maintaining of a suitable bacterial population in the digester, solid particles therein settling back into the digester, solid particles therein settling back into the digester under gravity. This permits a shorter hydraulic retention time for the liquids to be used, thus allowing a digester of smaller volume to be used. The solids are held in the digester for a relatively longer period allowing for more complete digestion and increasing the bacterial population.

Treated waste is fed from the digester 6 via the outlet tube 11 to the outlet tank 12, from which it may be discharged in any required way. Accumulated sludge can be removed from the bottom of the digester via the discharge tube 25 and the valve 26 and can be run off via the discharge pipe 23 and the valve 24.

The bucket elevator 2 allows water comprising organic material of almost any moisture content be treated. The feeding rate is variable so as to cater for different types of waste and for the required retention time.

Gravity feed of the digester 6 via the loading tube 5 has the advantage that no seals or valves are required and is thus relatively simple and reliable and makes any malfunction visibly obvious.

Excessive pressure build up within the digester 7 is prevented because the loading tube 5 and the outlet tube 11 are open to the atmosphere. A workable pressure can be maintained in the apparatus by means of a predetermined head of water in a digester pressure regulator (not shown).

The gas pump 10 is relatively simple and easy to maintain. The use of the liquid 39 acting as a hydraulic seal to prevent escape to the atmosphere of gas leaking through piston seals provided between the piston 28 or the piston rod 31 and the cylinder 29 makes the gas pump intrinsically safe.

The present apparatus can be of modular construction and can be made in batch production quantities to cater for a range of required sizes. However, various mechanisms, such as those for loading, mixing or temperature control, can be standard items.

Various modifications may be made within the scope of the invention. For instance, the digester 6 may be provided with a floating top, instead of a fixed top as shown, to provide a substantially constant gas pressure therein. Also, the bucket elevator 2 may be replaced by a pump to feed organic waste material into the digester 6.

WHAT WE CLAIM IS: 1. An apparatus for the anaerobic digestion of natural organic waste comprising a digester, a feed tube which extends downwardly into the digester on one side thereof for feeding natural organic waste into the digester, an inclined outlet tube which

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. side of the digester 6 for sampling and a discharge tube 25 and valve 26 are provided on the bottom of the digester for removal of digested sludge. The gas pump 10 is shown to a larger scale in cross-section in Figure 3 and comprises a vessel 27 containing a double acting piston 28 and cylinder 29 mounted on a frame 30. The piston 28 is connected via a piston rod 31 and a coupling rod 32 to a first end of a rocking lever 33 which is pivotable about an axis 34. The other end of the rocking lever 33 is pivotably connected to one end of a driving link 35 whose other end is connected to a crank 36 (shown rotationally out of position for the sake of clarity). The crank 36 is mounted on a shaft 37 of a prime mover, such as an electric motor with a reduction drive 42. The prime mover is separated from the piston 28 and cylinder 29 by a dividing wall 38 and liquid 39. The level of the liquid 39 on either side of the dividing wall 38 is such that the bottom of the dividing wall is below the liquid surface, so that the space 40 above the liquid and containing the cylinder 29 is air-tightly sealed from the space 41, which communicates with the atmosphere via a duct in the vessel 27. The cylinder 29 is connected via valves (not shown) and pipes 43 to the pipes 9 and 18 so as to draw gas from the pipe 18 into the cylinder 29 on alternate sides of the piston 28 during consecutive strokes thereof and to force gas into the pipe 9 with alternate strokes of the double acting piston. In use, waste is fed into the feed tank 1, from which it is continuously loaded into the digester 6 at a suitable predetermined rate by means of the bucket elevator 2 and the loading tube 5. The temperature of the contents of the digester is substantially maintained at a predetermined value, usually between 20 and 60"C, depending on the type of waste to be treated. This can be achieved automatically by providing a control system arranged to control heating means provided in the digester according to the outputs from the thermo-couples and/or thermistors 19. Gases such as methane and carbon dioxides are given off during the digestion of waste by the bacteria and are fed via the gas outlet pipe 7 to the gas pump 10 and to the gas stack or gasometer. The gas which is fed to the gas pump 10 is returned under pressure to the digester 6 via the pipe 9 and the diffuser 8. The inflow of gas via the diffuser produces efficient mixing of the contents of the digester and allows intimate contact between the gas and the bacteria, thus improving the efficiency of the digester. The arrangement of the outlet tube 11, which acts as a settling vessel, encourages the maintaining of a suitable bacterial population in the digester, solid particles therein settling back into the digester, solid particles therein settling back into the digester under gravity. This permits a shorter hydraulic retention time for the liquids to be used, thus allowing a digester of smaller volume to be used. The solids are held in the digester for a relatively longer period allowing for more complete digestion and increasing the bacterial population. Treated waste is fed from the digester 6 via the outlet tube 11 to the outlet tank 12, from which it may be discharged in any required way. Accumulated sludge can be removed from the bottom of the digester via the discharge tube 25 and the valve 26 and can be run off via the discharge pipe 23 and the valve 24. The bucket elevator 2 allows water comprising organic material of almost any moisture content be treated. The feeding rate is variable so as to cater for different types of waste and for the required retention time. Gravity feed of the digester 6 via the loading tube 5 has the advantage that no seals or valves are required and is thus relatively simple and reliable and makes any malfunction visibly obvious. Excessive pressure build up within the digester 7 is prevented because the loading tube 5 and the outlet tube 11 are open to the atmosphere. A workable pressure can be maintained in the apparatus by means of a predetermined head of water in a digester pressure regulator (not shown). The gas pump 10 is relatively simple and easy to maintain. The use of the liquid 39 acting as a hydraulic seal to prevent escape to the atmosphere of gas leaking through piston seals provided between the piston 28 or the piston rod 31 and the cylinder 29 makes the gas pump intrinsically safe. The present apparatus can be of modular construction and can be made in batch production quantities to cater for a range of required sizes. However, various mechanisms, such as those for loading, mixing or temperature control, can be standard items. Various modifications may be made within the scope of the invention. For instance, the digester 6 may be provided with a floating top, instead of a fixed top as shown, to provide a substantially constant gas pressure therein. Also, the bucket elevator 2 may be replaced by a pump to feed organic waste material into the digester 6. WHAT WE CLAIM IS:

1. An apparatus for the anaerobic digestion of natural organic waste comprising a digester, a feed tube which extends downwardly into the digester on one side thereof for feeding natural organic waste into the digester, an inclined outlet tube which

extends upwardly from the digester on the opposite side thereof and has an inclination such that, as waste is introduced into the container through the feed tube and the liquid overflows through the outlet tube, solids which had not settled to the bottom of the digester settle and remain in the digester, a gas outlet in the top of the digester, a diffuser located at the bottom of the digester, and a gas pump for pumping gases from the gas outlet through the diffuser, the diffuser comprising a flexible membrane perforated with holes which are sufficiently fine to open only when pressurized by gas from the gas pump.

2. An apparatus as claimed in claim 1, in which there is provided a feed tank and feed means arranged to feed waste to be digested from the feed tank to the digester at an adjustable continuous rate, the feed means comprising a bucket elevator, arranged to remove waste from the feed tank and to supply it to the feed tube for directing it under gravity into the digester.

3. An apparatus as claimed in claim 2, in which the bucket elevator comprises an electric motor arranged to drive an endless belt on which are provided a plurality of buckets.

4. An apparatus as claimed in any one of the preceding claims, in which the outlet tube is open at its upper end and communicates adjacent its upper end with an outlet tank.

5. An apparatus as claimed in any one of the preceding claims, in which the digester is provided at its side with a discharge pipe including a valve for allowing a sample of the digester contents to be removed.

6. An apparatus as claimed in any one of the preceding claims, in which the digester is provided at its bottom with a discharge tube including a valve for permitting removal of sludge from the digester.

7. An apparatus as claimed in any one of the preceding claims, in which the digester includes a movable upper part free to move under the influence of gas generated in the digester.

8. An apparatus as claimed in any one of the preceding claims, in which the digester comprises a container having upper and lower convex conical portions joined together by a cylindrical portion.

9. An apparatus as claimed in any one of the preceding claims, in which the gas pump comprises a container partially filled with liquid so as to provide a space above the liquid surface sealed by the liquid from the exterior of the container, pumping means being provided in the said space.

10. An apparatus as claimed in claim 9, in which the pumping means comprises a double acting piston and cylinder connected via valves to the gas outlet and the diffuser.

11. An apparatus as claimed in claim 10, in which the container includes a first portion which is open at its top and which is divided from a second portion containing the pumping means by a wall which extends below the normal liquid working level in the container.

12. An apparatus as claimed in claim 11, in which a lever is pivotably mounted below the dividing wall and includes a first part extending into the first portion of the container and connected to drive means for rocking the lever and a second part extending into the second portion of the container and connected to the piston by a connecting rod.

13. An apparatus as claimed in claim 12, in which the drive means comprises an electric motor and reduction gear having an output shaft on which is mounted a crank, the crank being connected to the first part of the lever by a further connecting rod.

14. An apparatus for the anaerobic digestion of natural organic waste, substantially as hereinbefore described with reference to and as illustrated in the drawings accompanying the provisional specification.