Technical Field

[1 ] The present invention relates to a prefabricated anaerobic biodigester, and in particular, to such a biodigester that can be used for the generation of biogas in areas in which there is little or no access to supplies of gas for domestic use (for cooking or heating for example).

Background of Invention

[2] In rural areas in many parts of the world, and in particular in developing countries, access to affordable and efficient energy sources remains inadequate. Many communities remain reliant on bioenergy in the form of firewood or other combustible collected materials, or on liquid fuels such as diesel to power generators for electricity. These sources of energy have drawbacks particularly in that their usage can have negative social, health and environmental impacts.

[3] One option for improving energy supply in rural areas is biogas, which is a clean-burning fuel gas consisting mainly of methane. Biogas may be produced through anaerobic biodigestion of locally available sources of biodigestible material, such as animal manure, green waste, kitchen waste and human waste. As an added advantage, the bioslurry (also known as digestate) co-produced in the process may be used as a fertiliser for crops. The generation of biogas in this form uses readily available material, has less impact on health and environment, allows biodigestion of waste product that would otherwise have to be disposed of and can form fertiliser as a by-product. These are all recognised and highly beneficial outcomes.

[4] Small-scale anaerobic biodigester devices or installations suitable for meeting the needs of small rural communities or single households have previously been developed. In installations known to Applicant, which are constructed on site and installed below ground level, biodigestible material and water are fed to a brick constructed anaerobic biodigestion chamber having a dome roof, where biodigestion commences, thereby producing biogas and bioslurry. The packed earth on top of the biodigestion chamber buttresses the brick dome roof structure, thereby providing the pressure resistance required to retain compressed biogas. In some of these installations, the biogas pressure build-up at the top of the biodigestion chamber forces bioslurry out of the chamber, such as through an outlet near the bottom of the chamber. The bioslurry can be directed or can flow up to ground level for collection or further distribution, for example as a fertilizer. A channel or outlet pipe can facilitate this travel of the bioslurry. In another development, the bioslurry is displaced upwardly through an outlet pipe which extends to ground level from within the biodigestion chamber, passing through the top cover or roof of the biodigestion chamber. In that development, the outlet pipe has a valve that can be opened to discharge gas from within the biodigestion chamber when needed, for example, for domestic cooking or heating purposes.

[5] The below ground installation of biodigesters makes them vulnerable to heavy rainfall and elevated ground water levels. Flood water or underground water can flow into the bioslurry outlet channel or pipe, potentially disrupting biodigestion in the biodigestion chamber or contaminating the water with bioslurry. Below ground installed biodigesters are therefore typically unsuited to flood-prone regions.

[6] Furthermore, the biogas stored within the biodigestion chamber is recoverable at pressure only while the liquid level of bioslurry in the bioslurry outlet channel remains above the liquid level in the biodigester. When gas is withdrawn from the biodigestion chamber, the liquid level in the outlet channel drops to meet the level in the biodigestion chamber. The relatively small amount of bioslurry in the outlet channel limits the fraction of the stored biogas that may be withdrawn in a single use, as is typically required for household cooking requirements

[7] In addition, prior art biodigesters suffer from loss of biogas as biogas has been able to escape from the biodigestion chamber, leading to poor gas recovery efficiencies. This has occurred due to the use of large openings leading from the biodigestion chamber to the outlet (or inlet) channels or pipes, or via leak-prone top or roof regions of the biodigestion chamber, such as the brick dome roofs of on-site constructed biodigester installations. [8] There is therefore an ongoing need to provide an anaerobic biodigester that provides improved performance, especially in flood-prone regions, that provides improved biogas retention and that permits improved utilisation of the biogas accumulated within the biodigestion chamber in a single withdrawal. Such a biodigester design should preferably be amenable to easy and low cost fabrication and installation. [9] A reference herein to a matter which is given as prior art is not to be taken as an admission that the document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

Summary of Invention [10] In accordance with one aspect the invention provides a prefabricated anaerobic biodigester, comprising: a biodigestion chamber for containing bioslurry and biogas, the biodigestion chamber including a feed port for ingress of biodigestible material into the chamber and a gas port for withdrawal of biogas out of the chamber; a displacement tank for containing bioslurry, the displacement tank being disposed above the biodigestion chamber; and a transfer conduit having an inlet opening in the biodigestion chamber and an outlet opening in the displacement tank for flow of bioslurry between the biodigestion chamber and the displacement tank, wherein the biodigester is operable such that upon biogas pressure build-up in the biodigestion chamber, bioslurry is displaced upwardly through the transfer conduit into the displacement tank, and upon withdrawal of biogas from the biodigestion chamber, bioslurry in the displacement tank flows downwardly through the transfer conduit into the biodigestion chamber.

[1 1 ] A prefabricated anaerobic biodigester according to the invention can provide improved flood resistance and capacity utilisation of produced biogas compared to prior art biodigesters. The biodigester comprises a displacement tank for receiving and containing bioslurry displaced upwardly through a transfer conduit from the biodigestion chamber, as a result of biogas build-up in the biodigestion chamber. The displacement tank is disposed above the biodigestion chamber. When installed, the displacement tank is substantially or entirely above ground level, thus providing protection against flooding, while the biodigestion chamber may optionally be installed below ground. When gas is withdrawn from the biodigestion chamber, bioslurry contained in the displacement tank can flow downwardly through the transfer conduit and back into the biodigestion chamber, maintaining pressure within the chamber by taking up space caused by the withdrawal of biogas. The resulting increase in the level of bioslurry in the biodigestion chamber allows an increased fraction, or substantially all, of the stored biogas to be withdrawn at a single time. The displacement tank is preferably capable of holding a volume of displaced bioslurry sufficient to substantially refill the biodigestion chamber when the biogas pressure is at a maximum and the bioslurry level therein is therefore at its minimum.

[12] In some embodiments, the inlet opening is in a side wall of the biodigestion chamber. This positioning advantageously reduces the amount of biogas that can escape the biodigestion chamber via the transfer conduit, since biogas formed in the bioslurry will bubble upwards past the inlet opening towards the top of the biodigestion chamber.

[13] In some embodiments, the transfer conduit is disposed externally of the biodigestion chamber. Since the transfer conduit therefore does not pass through a gas retaining roof of the biodigestion chamber, the leak-resistance of the biodigestion chamber may be improved, because there is no need to seal an opening in the roof. This can make the biodigester easier to fabricate.

[14] In some embodiments, the feed port is in a side wall of the biodigestion chamber. This positioning advantageously again reduces the amount of biogas that can escape the biodigestion chamber via the feed port.

[15] In some embodiments, the displacement tank is disposed directly above the biodigestion chamber. This is a preferred arrangement for a prefabricated biodigester, as it can improve the simplicity of fabrication and the transportability, and reduce the footprint of the biodigester.

[16] In some embodiments, the biodigester further comprises a bioslurry outlet port in the displacement tank, wherein bioslurry is discharged through the bioslurry outlet port when the level of bioslurry in the displacement tank exceeds a predetermined maximum level. The biodigester is thus able to automatically discharge bioslurry when the level of bioslurry reaches the predetermined maximum level in the displacement tank as biogas is generated during biodigestion, without requiring user intervention. The discharged bioslurry can be collected for use as fertiliser.

[17] In some embodiments, the biodigester further comprises a feed conduit in communication with the feed port, the feed conduit having a feed mouth for feeding biodigestible material into the biodigestion chamber via the feed conduit. In some such embodiments, the feed mouth is positioned above the predetermined maximum level in the displacement tank so that any discharge of bioslurry occurs through the bioslurry outlet port of the displacement tank rather than through the feed conduit. The feed conduit is thus able to maintain a liquid level during biodigestion, thereby providing an air lock to allow anaerobic biodigestion to proceed in the biodigestion chamber without the need to manually seal the feed conduit. In some embodiments, the feed conduit is disposed externally of the biodigestion chamber. Since the feed conduit therefore does not pass through a gas retaining roof of the biodigestion chamber, the leak-resistance of the biodigestion chamber may be improved, and the biodigester may be simpler to fabricate.

[18] In some embodiments, the inlet opening of the transfer conduit defines a minimum bioslurry level in the biodigestion chamber at which bioslurry can be displaced upwardly through the transfer conduit from the biodigestion chamber into the displacement tank. In some such embodiments, the feed port is positioned beneath the minimum bioslurry level. Once the bioslurry level in the biodigestion chamber drops to the minimum bioslurry level in operation, further biogas that is generated will escape through the transfer conduit and into the displacement tank where it will be lost to atmosphere. The minimum bioslurry level therefore sets the minimum working volume of bioslurry that is required to be maintained in the chamber for biogas generation, and also provides a pressure relief mechanism for the biodigester where pressure builds beyond an upper limit.

[19] In alternative embodiments, the feed port defines a minimum bioslurry level in the biodigestion chamber at which bioslurry can be displaced upwardly through the transfer conduit from the biodigestion chamber into the displacement tank. In some such embodiments, the inlet opening is positioned beneath the minimum bioslurry level. [20] In some embodiments, an internal volume of the biodigestion chamber located beneath the minimum bioslurry level is between 40% and 60% of the total internal volume of the biodigestion chamber. This ensures that a sufficient working volume of bioslurry containing a population of methogenic bacteria is maintained at all times in the chamber during ongoing operation.

[21 ] In some embodiments, the displacement tank is capable of containing a volume of bioslurry displaced from the biodigestion chamber that is at least 70%, preferably at least 90%, such as about 95%, of the internal volume of the biodigestion chamber located above the minimum bioslurry level. During a gas withdrawal from the biodigestion chamber, the biodigestion chamber can thus be substantially refilled with bioslurry, ensuring maintenance of pressure so that a high fraction of the stored biogas can be extracted.

[22] In some embodiments, the minimum bioslurry level in the biodigestion chamber is between 0.5 and 3.0m, preferably between 0.75 and 1 .5m, such as approximately 1 .1 m, below a predetermined maximum level at which bioslurry is contained in the displacement tank. The hydraulic head of bioslurry displaced into the displacement tank thus provides a suitable working pressure of biogas stored in the biodigester.

[23] In some embodiments, the cross-sectional area of the inlet opening in the biodigestion chamber is less than 10%, preferably less than 5%, more preferably less than 2%, such as less than 1 %, of the cross-sectional area of the biodigestion chamber taken horizontally through the biodigestion chamber at the minimum bioslurry level. In some embodiments, the cross-sectional area of the feed port of the biodigestion chamber is less than 10%, preferably less than 5%, more preferably less than 2%, such as less than 1 %, of the cross-sectional area of the biodigestion chamber taken horizontally through the biodigestion chamber at the minimum bioslurry level. In one working example, the cross-sectional area of the inlet opening and the feed port of the biodigestion chamber are both approximately 0.7% of the cross-sectional area of the biodigestion chamber taken horizontally through the biodigestion chamber at the minimum bioslurry level. The small inlet opening and feed port limits the loss of biogas through the transfer and feed conduits, thus improving gas efficiency. [24] In some embodiments, the displacement tank is capable of containing a volume of bioslurry displaced from the biodigestion chamber that is at least 25%, or at least 40%, of the total internal volume of the biodigestion chamber. This relative sizing may also ensure that a high fraction of the stored biogas can be utilised. [25] In some embodiments, the displacement tank is disposed on top of the biodigestion chamber. This positioning is particularly preferred for convenient fabrication and compactness of the biodigester.

[26] In some embodiments, the biodigestion chamber has a side wall and a roof that is integrally formed with upper portions of the side wall of the biodigestion chamber. The formation of the upper section of the biodigestion chamber as an integral structure may improve the gas-retention capability of the biodigestion chamber. In some embodiments, the roof is dome-shaped and is convex when viewed from outside of the biodigester. Such a configuration is preferred for retaining pressurised biogas. In some embodiments, the roof of the biodigestion chamber defines at least a part, and optionally the entire, floor of the displacement tank.

[27] In some embodiments, the outlet opening of the transfer conduit is located in a side wall of the displacement tank. The outlet opening of the transfer conduit may be located directly above a floor of the displacement tank, so that substantially all of the bioslurry contained within the displacement tank can flow downwardly through the transfer conduit.

[28] In embodiments in which the biodigestion chamber has a roof that defines at least a part of a floor of the displacement tank; the transfer conduit can be disposed internally of the biodigestion chamber; and the outlet opening of the transfer conduit can be located in the at least a part of the floor. A prefabricated anaerobic biodigester with such a configuration may be particularly compact, as the transfer conduit is located within the biodigestion chamber.

[29] In some embodiments, the gas port is fitted with a gas outlet pipeline equipped with a gas withdrawal valve for controllably withdrawing biogas from the biodigestion chamber. Alternatively, the gas port may be fitted directly with a valve, to which a gas pipeline may be connected. The gas pipeline and/or valve may also be fitted during an on-site installation of the prefabricated biodigester. [30] In some embodiments, the biodigestion chamber and the displacement tank are separately formed by rotational moulding of a thermoplastic. The biodigestion chamber and the displacement tank may then be welded together. The biodigester may be conveniently and affordably manufactured using this technique. [31 ] The biodigestion chamber and the displacement tank can be formed from

HDPE or LLDPE. LLDPE is a particularly preferred material of construction, as its inherent flexibility and toughness may provide enhanced resistance to mechanical stresses, for example resistance to earthquakes. In some embodiments, the transfer conduit, and optionally also the feed conduit, comprise PVC or LLDPE piping. [32] In some embodiments, the biodigestion chamber and the displacement tank are both cylindrical. In some such embodiments, the biodigestion chamber and the displacement tank have the same diameter and are coaxially aligned. In alternative embodiments, the displacement tank has a smaller diameter than the displacement tank, and the biodigestion chamber and the displacement tank are coaxially aligned. In some embodiments, the prefabricated anaerobic biodigester is substantially cylindrical. Cylindrical vessels may be convenient to fabricate, for example in a rotational moulding process, and also provide effective containment of pressurised fluids.

[33] In an alternative to embodiments where the biodigestion chamber and the displacement tank are coaxially aligned, a horizontal midpoint of the displacement tank may be offset from a horizontal midpoint of the biodigestion chamber. One advantage of this configuration is that a gas port at the apex of the biodigestion chamber roof does not open into the displacement tank.

[34] In some embodiments, the transfer conduit is substantially vertical, and can thus run parallel to and in close proximity to vertical side walls of the biodigestion chamber. In some embodiments, the biodigestion chamber has a substantially flat base, so that the biodigester may be conveniently installed on a flat surface, resting on the base. In some embodiments, the displacement tank has a roof for reducing bioslurry evaporation and rainwater ingress into the displacement tank. In some embodiments, the total internal volume of the biodigestion chamber is between 2 and 4 m ³ . Such a size may be well suited to supply the needs of a rural household. [35] Where the terms "comprise", "comprises" and "comprising" are used in this specification they are to be interpreted as specifying the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof. [36] Further aspects of the invention appear below in the detailed description of the invention.

Brief Description of Drawings

[37] Embodiments of the invention will herein be illustrated by way of example only with reference to the accompanying drawings in which: [38] Figure 1 is a perspective view of an anaerobic biodigester in accordance with an embodiment of the present invention.

[39] Figure 2 is a side and transparent view of an anaerobic biodigester in accordance with another embodiment of the present invention.

[40] Figure 3 is a side and transparent view of an anaerobic biodigester in accordance with still another embodiment of the present invention.

Detailed Description

[41 ] Figure 1 depicts prefabricated anaerobic biodigester 10 which comprises biodigestion chamber 1 1 for containing bioslurry and biogas. The biodigestion chamber 1 1 has a cylindrical side wall 12, a flat base 13 and dome-shaped top or roof (hereinafter "roof") 17 which is formed integrally with upper portions of side wall 12. Biodigester 10 further comprises displacement tank 14 for containing bioslurry. Displacement tank 14 is disposed on top of (and thus above) biodigestion chamber 1 1 , with roof 17 forming the floor of displacement tank 14. Displacement tank 14 has a cylindrical side wall 15 and roof 16 and while the roof 16 is not actually required, it advantageously reduces or minimises evaporation and rainwater ingress into displacement tank 14. Side wall 15 is joined to biodigestion chamber 1 1 along watertight seam 18, and provides containment for bioslurry in displacement tank 14. Biodigestion chamber 1 1 and displacement tank 14 are both cylindrical and have substantially the same diameter, so that biodigester 10 is also substantially cylindrical.

[42] Biodigester 10 further comprises transfer conduit 23 disposed externally of biodigestion chamber 1 1 . Transfer conduit 23 has inlet opening 25 in side wall 12 of biodigestion chamber 1 1 , and outlet opening 26 in side wall 15 of displacement tank 14. Transfer conduit 23 thus permits flow of bioslurry between biodigestion chamber 1 1 and displacement tank 14, both upwards and downwards. The positioning of inlet opening 25 in vertical side wall 12, and the consequent external configuration of transfer conduit 23, provides for efficient gas recoveries, since gas bubbles formed in biodigestion chamber 1 1 are prevented from escaping biodigester 10 via inlet opening 25.

[43] Inlet opening 25 defines a minimum bioslurry level in biodigestion chamber 1 1 at which bioslurry can be displaced upwardly through transfer conduit 23 into displacement tank 14. Outlet opening 26 in side wall 15 is located directly above seam 18, so that substantially all of the bioslurry contained within displacement tank 14 can flow into transfer conduit 23 and then downwardly towards biodigestion chamber 1 1 when the pressure of biogas within biodigestion chamber 1 1 reduces to allow return of bioslurry from the displacement tank 14 to the biodigestion chamber 1 1 . Transfer conduit 23 is formed from pipe 24, and is substantially vertical, extending parallel to side walls 12 and 15 between inlet opening 25 and outlet opening 26.

[44] Displacement tank 14 includes bioslurry outlet port 28 in side wall 15, which defines a predetermined maximum level for bioslurry in displacement tank 14, such that bioslurry is discharged through bioslurry outlet port 28 when the level of bioslurry rises to and exceeds the predetermined maximum level. Bioslurry outlet port 28 is immediately adjacent to roof 16, so that the predetermined maximum level is just beneath roof 16. Pipe 27 extends outwardly from bioslurry outlet port 28 to direct the flow of discharged bioslurry away from biodigester 10. Bioslurry discharged through pipe 27 may then be collected in a suitable vessel (not shown) or flowed under influence of gravity via a hose (not shown) to a preferred location. [45] Biodigestion chamber 1 1 includes feed port 22 for ingress of biodigestible matter into biodigestion chamber 1 1. Feed port 22 is located in side wall 12, positioned beneath the minimum bioslurry level in biodigestion chamber 1 1 at which bioslurry can be displaced upwardly through transfer conduit 23 into displacement tank 14.

[46] Biodigester 10 further comprises feed conduit 19 in communication with feed port 22, feed conduit 19 being disposed externally of biodigestion chamber 1 1 . Feed conduit 19 is formed from pipe 20, and is substantially vertical, extending parallel to side walls 12 and 15 from feed port 22. At the uppermost portion, feed conduit 19 widens to form feed mouth 21 for feeding biodigestible matter into biodigestion chamber 1 1 via feed conduit 19 and feed port 22. Feed mouth 21 is positioned at an elevation above the predetermined maximum level in displacement tank 14. As depicted in Figure 1 , feed mouth 21 is a funnel which is sealingly joined to pipe 20. Feed mouth 21 may optionally be provided with a removable cap (not shown) to prevent rainwater ingress into the biodigester.

[47] Biodigestion chamber 1 1 includes gas port 29 located at the apex of dome- shaped roof 17. When installed, a gas outlet pipeline (not shown) is secured to gas port 29. The gas outlet pipeline extends through the interior of displacement tank 14, passing through opening 30 in roof 16, and extending further to a suitable location where biogas is to be consumed (such as a kitchen with a gas-burning cooking appliance). Either gas port 29 or the gas outlet pipeline is provided with a valve, thereby allowing biogas to be withdrawn in a controlled manner through gas port 29.

[48] In use, water and biodigestible material, generally comprising one or more of animal manure, green waste, kitchen waste and human waste, is introduced into biodigester 10 through feed mouth 21 . The water and biodigestible material may be introduced manually, for example with a bucket, or by placing a feed pipe through feed mouth 21 . The water and biodigestible material then passes under influence of gravity into biodigestion chamber 1 1 via feed conduit 19 and feed port 22.

[49] Biodigestion chamber 1 1 is initially filled with feed slurry to a level higher than the minimum bioslurry level defined by inlet opening 25, allowing air present inside biodigestion chamber 1 1 to be displaced through gas port 29. As biodigestion chamber 1 1 is initially filled to a level above the minimum bioslurry level, corresponding liquid levels are maintained in feed conduit 19 and transfer conduit 23. If biodigestion chamber 1 1 is completely filled to the apex of roof 17, slurry will flow through outlet opening 26, creating a feed level in displacement tank 14. Once filled, gas flow through gas port 29 is blocked, for example by closing the valve provided on either gas port 29 or the gas outlet pipeline secured thereto.

[50] Air ingress into biodigestion chamber 1 1 is substantially blocked by the closure of gas port 29 and the narrow liquid columns in feed conduit 19 and transfer conduit 23, thereby providing conditions suitable for anaerobic biodigestion inside biodigestion chamber 1 1 . Methogenic bacteria naturally present in and/or added to the feed digest the organic matter in the feed, producing biogas (mainly methane, and carbon dioxide, with minor components such as sulphur dioxide) and convert the feed into bioslurry. [51 ] Biogas produced by the biodigestion accumulates at the top of biodigestion chamber 1 1 , trapped beneath dome-shaped roof 17. As the biogas pressure builds up, the bioslurry level in biodigestion chamber 1 1 is forced downwards, so that bioslurry is displaced through inlet opening 25 and upwardly through transfer conduit 23 into displacement tank 14. Continued bioslurry displacement through transfer conduit 23 increases the bioslurry level in displacement tank 14 until it reaches the predetermined maximum level, whereafter bioslurry is discharged from biodigester 10 through bioslurry outlet port 28. The discharged bioslurry is available for use, for example as a fertilizer for crops.

[52] During operation of biodigester 10, the level in feed conduit 19 varies to maintain the same elevation as the liquid level in transfer conduit 23 or displacement tank 14. However, since feed mouth 21 is positioned above the predetermined maximum level of displacement tank 14, bioslurry will not be discharged out of biodigester 10 via feed conduit 19.

[53] The pressure of the biogas stored in biodigestion chamber 1 1 increases as the bioslurry level in biodigestion chamber 1 1 and the bioslurry level in transfer conduit 23 or displacement tank 14 diverge, with the pressure of the biogas being balanced by the hydraulic head resulting from vertical displacement of bioslurry. Dome-shaped roof 17, formed integrally with side wall 15, provides gas-tight and pressure resistant retention of the pressurised biogas in biodigestion chamber 1 1. Since transfer conduit 23 and feed conduit 19 are disposed externally of biodigestion chamber 1 1 , they do not pass through roof 17. This also assists in providing leak- resistant containment of pressurised biogas beneath roof 17.

[54] Unless biogas is withdrawn or biodigestible organic material in the bioslurry is depleted, biogas pressure build-up due to ongoing biodigestion continues to displace bioslurry upwardly through transfer conduit 23 until the bioslurry level in biodigestion chamber 1 1 falls to the minimum bioslurry level defined by inlet opening 25. Further biogas generated in biodigestion chamber 1 1 is then able to escape by bubbling through the inlet opening 25, into the fluid-filled transfer conduit 23, and into displacement tank 24. The escaping gas then bubbles out of the bioslurry via the large surface area of bioslurry in displacement tank 14 (thereby avoiding excessive surface disturbance) and out into the atmosphere through opening 30 or bioslurry outlet port 28. Thus the biodigester 10 has an inherent pressure relief mechanism preventing over-pressuring of the biodigestion chamber 1 1 and preventing the level in biodigestion chamber 1 1 from dropping below the minimum bioslurry level. The maximum pressure of biogas stored in biodigestion chamber 1 1 is thus equal to the hydraulic head of bioslurry when the bioslurry level in biodigestion chamber 1 1 is at the minimum bioslurry level and the bioslurry level in displacement tank 14 is at the predetermined maximum level.

[55] During ongoing operation of biodigester 10, additional water and biodigestible material are added to biodigestion chamber 1 1 via feed mouth 21 as required to rebuild the level of bioslurry in biodigestion chamber 1 1 and to maintain an acceptable production rate of biogas and discharged bioslurry. As an example, approximately 20-40 kg per day of manure may be added to biodigester 10, when sized to meet the biogas supply needs of a typical household (5 people). If additional feed is added into the biodigestion chamber 1 1 when the level of bioslurry in displacement tank 14 is at the predetermined maximum level, a corresponding volume of bioslurry will be discharged from displacement tank 14 through bioslurry outlet port 28 when the additional feed is introduced. It may therefore be preferred to add feed to biodigester 10 when the level of bioslurry in displacement tank 14 is below the predetermined maximum level, for example, after a biogas withdrawal. The addition could thus be timed so that it occurs after a normal or regular biogas withdrawal event, such as after cooking has taken place, or the biogas could simply be discharged.

[56] When biogas is required for consumption (for example for domestic cooking purposes), it is controllably withdrawn through gas port 29. This may be done by opening a valve in the gas outlet pipeline secured to gas port 29. Withdrawal of biogas reduces the biogas pressure and volume in biodigestion chamber 1 1 , causing bioslurry to flow downwardly from displacement tank 14 through transfer conduit 23 into biodigestion chamber 1 1 . The level of bioslurry in biodigestion chamber 1 1 thus rises, while the level of bioslurry in displacement tank 14 drops. Simultaneously, the level of bioslurry in feed conduit 19 also drops. The reverse flow of bioslurry from displacement tank 14 into biodigestion chamber 1 1 continues during biogas withdrawal to maintain the equilibrium between the biogas pressure and the hydraulic head. If biogas withdrawal is not first voluntarily terminated (for example, by closing the valve), the level of bioslurry in biodigestion chamber 1 1 will eventually reach the same level as the level of bioslurry in transfer conduit 23 (or displacement tank 14). The biogas remaining in biodigestion chamber 1 1 is then at atmospheric pressure, and cannot be withdrawn under influence of pressure via gas port 29. However, biogas withdrawal is typically voluntarily terminated before the supply of withdrawable biogas is completely exhausted in this manner.

[57] After biogas withdrawal is terminated and gas port 29 is sealed, ongoing biodigestion in biodigestion chamber 1 1 generates further biogas, causing renewed bioslurry displacement through transfer conduit 23 into displacement tank 14 and - once the level again reaches the predetermined maximum level - further discharge of bioslurry from biodigester 10 through bioslurry outlet port 28.

[58] Biodigester 10 is generally installed partially or completely above ground level, which provides flood resistance since flood waters cannot enter biodigester 10 unless the flood waters rise to at least the level of bioslurry outlet port 28. In some embodiments, biodigester 10 is installed with base 13 at ground level. In other embodiments, biodigester 10 is installed with base 13 partially sunk below ground level.

[59] In a preferred embodiment, biodigester 10 is installed such that biodigestion chamber 1 1 is below ground level, while displacement tank 14 is substantially, or entirely, above ground level. The depth at which biodigester 10 is installed may depend on the flood line in the area where it is installed. Bioslurry outlet port 28 should be installed above the expected maximum flood line. However, installation of biodigestion chamber 1 1 below ground assists to regulate the temperature of the biodigesting bioslurry, thereby preventing temperature excursions due to unusually hot or cold ambient conditions, which might affect the rate of biodigestion. Furthermore, this mode of installation facilitates manual loading of feed by reducing the above-ground height of feed mouth 21 (preferably to approximately waist height), while nevertheless providing adequate flood protection. The configuration of biodigester 10 as an integrated, prefabricated unit comprising displacement tank 14 disposed on top of biodigestion chamber 1 1 , with roof 17 of biodigestion chamber 1 1 forming the floor of displacement tank 14, facilitates the installation of the biodigester 1 1 in this manner.

[60] Both the positioning of inlet opening 25 and feed port 22 in vertical side wall 12 and the relatively small diameter of these openings (relative to the diameter of biodigestion chamber 1 1 ) minimise the escape of biogas produced in biodigestion chamber 1 1 through transfer conduit 23 and feed conduit 19. In some embodiments, the cross-sectional area of inlet opening 25 is less than 5%, preferably less than 2%, such as less than 1 % of the cross-sectional area of biodigestion chamber 1 1 taken horizontally through biodigestion chamber 1 1 at the minimum bioslurry level. In some embodiments, the cross-sectional area of feed port 22 is less than 5%, preferably less than 2%, such as less than 1 % of the cross-sectional area of biodigestion chamber 1 1 taken horizontally through biodigestion chamber 1 1 at the minimum bioslurry level.

[61 ] The minimum bioslurry level defined by inlet opening 25 is at a suitable elevation on side wall 12 such that an adequate reserve of mature bioslurry is always maintained within biodigestion chamber 1 1 . In some embodiments, the internal volume of biodigestion chamber 1 1 located beneath the minimum bioslurry level is between 30% and 70%, preferably between 40% and 60%, such as approximately 50%, of the total internal volume of biodigestion chamber 1 1 .

[62] The downward flow of bioslurry from displacement tank 14 during biogas withdrawal builds the bioslurry level in biodigestion chamber 1 1 , thereby increasing the amount of the stored biogas that may be withdrawn through gas port 29. When filled with bioslurry to the predetermined maximum level, displacement tank 14 contains a sufficient volume of displaced bioslurry such that biodigestion chamber 1 1 can be at least partially, and preferably almost completely, refilled with bioslurry during gas withdrawal. In some embodiments, displacement tank 14 is thus capable of containing a volume of bioslurry displaced from biodigestion chamber 1 1 that is at least 50%, preferably at least 70%, most preferably at least 90%, such as approximately 95%, of the internal volume of biodigestion chamber 1 1 located above the minimum bioslurry level. Generally, however, displacement tank 14 should not be capable of containing a volume of bioslurry of more than 95%, or more than 100% of the internal volume of biodigestion chamber 1 1 located above the minimum bioslurry level, since this could result in components floating on the bioslurry, or the bioslurry itself, being displaced into gas port 29 if a gas withdrawal from biodigestion chamber 1 1 is not terminated timeously. In some embodiments, displacement tank 14 is capable of containing a volume of bioslurry displaced from the biodigestion chamber 1 1 that is at least 25%, preferably at least 40%, of the total internal volume of biodigestion chamber 1 1 .

[63] The percentage figures given above have been given in relation to the biodigester 10, but also apply to other forms of biodigesters according to the invention and which are not illustrated herein.

[64] The maximum pressure of biogas stored in biodigestion chamber 1 1 is directly correlated to the vertical elevation of the predetermined maximum level of bioslurry in displacement tank 14 (as defined by bioslurry outlet port 28) above the minimum bioslurry level of biodigestion chamber 1 1 (as defined by inlet opening 25). In some embodiments, the predetermined maximum level in displacement tank 14 is approximately 1 .1 m above the minimum bioslurry level in biodigestion chamber 1 1 . [65] Fabrication of prefabricated anaerobic biodigester 10 includes the step of producing biodigestion chamber 1 1 as a single integrated vessel by rotational moulding of a thermoplastic. Inlet opening 25, feed port 22 and gas port 29 are formed in biodigestion chamber 1 1 during the rotational moulding process. Side wall 15 and roof 16 of displacement tank 14 are separately produced by rotational moulding of a thermoplastic. Outlet opening 26, bioslurry outlet port 28 and port 30 are formed in side wall 15 or roof 16 during the rotational moulding process. Side wall 15 is then joined to biodigestion chamber 1 1 along seam 18 by plastic welding, thereby forming displacement tank 14 disposed on top of biodigestion chamber 1 1 . The thermoplastic used to fabricate biodigestion chamber 1 1 and displacement tank 14 is generally a polyolefin such as high density polyethylene (HDPE) or linear low density polyethylene (LLDPE). LLDPE is a particularly preferred material of construction, as its inherent flexibility and toughness may provide enhanced resistance to mechanical stresses to biodigester 10, for example resistance to earthquakes.

[66] Transfer conduit 23 is then formed by joining plastic pipe 24 to inlet opening 25 and outlet opening 26 formed in the side walls of biodigestion chamber 1 1 and displacement tank 14 respectively. Similarly, feed conduit 19 is formed by joining plastic pipe 20 to feed port 22. The pipes may be joined to the ports with pipe elbow joints. Plastic pipes 24 and 25 are generally polyvinyl chloride (PVC) or LLDPE pipes.

[67] As a result of its design, prefabricated anaerobic biodigester 10 is simple to construct, easily transported to rural locations as a single unit and readily installed on site without requiring a technically demanding installation procedure.

[68] It will be appreciated that biodigester 10 may be provided in different sizes, where the most appropriate size may be determined by the amount of biodigestible feed available and the needs of the user. In some embodiments, the total internal volume of biodigestion chamber 1 1 is between 2 and 4 m ³ , such as between 2.5 and 3 m ³ .

[69] Figure 2 depicts biodigester 10a in accordance with another embodiment of the invention. The biodigester 10a is shown in transparent side view so that the internal structure of the biodigester 10a can be seen. Biodigester 10a comprises biodigestion chamber 1 1 a having side wall 12a, base 13a and dome-shaped roof 17a, and displacement tank 14a disposed on top of (and thus above) biodigestion chamber 1 1 a, with roof 17a forming the floor of displacement tank 14a. Displacement tank 14a has side wall 15a and roof 16a. Displacement tank 14a includes bioslurry outlet port 28a in side wall 15a, bioslurry outlet port 28a defining a predetermined maximum level for bioslurry in displacement tank 14a. Pipe 27a extends outwardly from bioslurry outlet port 28a. Biodigestion chamber 1 1 a includes gas port 29a located at the apex of dome-shaped roof 17a. [70] Biodigester 10a further comprises transfer conduit 23a disposed internally of biodigestion chamber 1 1 a. Transfer conduit 23a has inlet opening 25a positioned inside biodigestion chamber 1 1 a, and outlet opening 26a formed in roof 17a, thus opening into the bottom of displacement tank 14a. Transfer conduit 23a is formed from pipe 24a which is connected to roof 17a by a gas tight seal around the periphery of outlet opening 26a, and is substantially vertical.

[71 ] Biodigester 10a further comprises feed conduit 19a disposed internally of biodigestion chamber 1 1 a and displacement tank 14a. Feed conduit 19a has inlet opening 22a positioned inside biodigestion chamber 1 1 a, and feed mouth 21 a positioned at an elevation above the predetermined maximum level in displacement tank 14a. Feed conduit 19a passes through roof 17a, through the interior of displacement tank 14a, and through roof 16a. Feed conduit 19a is formed from pipe 20a, and is substantially vertical. Roof 17a forms a gas-tight seal around the outside wall of pipe 20a.

[72] In the embodiment depicted in Figure 2, feed port 22a of feed conduit 19a defines the minimum bioslurry level in biodigestion chamber 1 1 a at which bioslurry can be displaced upwardly through transfer conduit 23a into displacement tank 14a. In this embodiment, inlet opening 25a of transfer conduit 23a is positioned beneath the minimum bioslurry level in biodigestion chamber 1 1 a.

[73] In use, water and biodigestible material is introduced into biodigester 10a through feed mouth 21 a. Biodigestion chamber 1 1 a is initially filled with feed slurry to a level higher than the minimum bioslurry level defined by feed port 22a, and is optionally completely filled. Corresponding liquid levels are initially formed in feed conduit 19a and transfer conduit 23a. Additional water and biodigestible material is added to biodigestion chamber 1 1 a during ongoing operation of biodigester 10a as required to maintain biogas and bioslurry production.

[74] Due to biodigestion of the organic matter in the feed, biogas accumulates at the top of biodigestion chamber 1 1 a, trapped beneath dome-shaped roof 17a. The liquid level in biodigestion chamber 1 1 a is thus forced downwards by the biogas pressure build-up, so that bioslurry is displaced upwardly through feed conduit 19a and upwardly through transfer conduit 23a into displacement tank 14a. Once the predetermined maximum level in displacement tank 14a is exceeded, bioslurry is discharged through bioslurry outlet port 28a.

[75] Biogas build-up will cause bioslurry to be displaced upwardly through both feed conduit 19a and transfer conduit 23a until the bioslurry level in biodigestion chamber 1 1 a falls to the minimum bioslurry level defined by feed port 22a. Further biogas generated in biodigestion chamber 1 1 a is then able to escape by bubbling through fluid-filled feed conduit 19a, and out into the atmosphere through feed mouth 21 a.

[76] Biogas is controllably withdrawn through gas port 29a when required for consumption. The withdrawal of biogas causes bioslurry to flow downwardly from displacement tank 14a through transfer conduit 23a into biodigestion chamber 1 1 a. The level of bioslurry in biodigestion chamber 1 1 a thus rises, while the level of bioslurry in displacement tank 14a and feed conduit 19a drops. The hydraulic head of the bioslurry in displacement tank 14a, and the building of liquid level in biodigestion chamber 1 1 a upon gas withdrawal, permits a higher fraction of the biogas stored within biodigester 10a to be utilised than would be the case without the assistance from the downward flow of a substantial volume of bioslurry from displacement tank 14a. Biogas may be withdrawn under positive pressure until the levels in biodigestion chamber 1 1 a and the levels in transfer conduit 23a have equalised. Preferably, the volume of bioslurry contained in displacement tank 14a (when filled to the predetermined maximum level) is sufficient to substantially fill biodigestion tank 1 1 a when the level of bioslurry is initially at the minimum bioslurry level, leaving only enough clearance beneath roof 17a to avoid contaminating or blocking gas port 29a.

[77] Figure 3 depicts biodigester 10b in accordance with yet another embodiment of the invention in transparent side view. Biodigester 10b comprises biodigestion chamber 1 1 b having cylindrical side wall 12b, base 13b and dome- shaped roof 17b, and displacement tank 14b having floor 31 b, cylindrical side wall 15b and roof 16b. Displacement tank 14b is above biodigester 10b, with floor 31 b being at substantially the same elevation as roof 17b. However, displacement tank 14b is radially offset from biodigestion chamber 1 1 b, so that only a portion of roof 17b and floor 31 b abut and are joined together at joint 18b. Displacement tank 14b includes bioslurry outlet port 28b in side wall 15b, bioslurry outlet port 28b defining a predetermined maximum level for bioslurry in displacement tank 14b. Pipe 27b extends outwardly from bioslurry outlet port 28b. Biodigestion chamber 1 1 b includes gas port 29b located at the apex of dome-shaped roof 17b. As a result of the off-set position of displacement tank 14b, gas port 29b does not open into displacement tank 14b, and a valve (not shown) fitted directly to gas port 29b is thus not immersed in bioslurry.

[78] Biodigester 10b further comprises transfer conduit 23b disposed externally of biodigestion chamber 1 1 b. Transfer conduit 23b has inlet opening 25b in side wall 12b of biodigestion chamber 1 1 b, and outlet opening 26b in floor 31 b of displacement tank 14b. Transfer conduit 23b is formed from pipe 24b, and is oriented at an angle to the vertical.

[79] Biodigestion chamber 1 1 b includes feed port 22b located in side wall 12b. Biodigester 10b further comprises feed conduit 19b in communication with feed port 22b, feed conduit 19b being disposed externally of biodigestion chamber 1 1 b. Feed conduit 19b is formed from pipe 20b, and is substantially vertical. At the uppermost portion, feed conduit 19b includes feed mouth 21 b, positioned at an elevation above the predetermined maximum level in displacement tank 14b.

[80] In the embodiment depicted in Figure 3, inlet opening 25b of transfer conduit 23b defines the minimum bioslurry level in biodigestion chamber 1 1 b at which bioslurry can be displaced upwardly through transfer conduit 23b into displacement tank 14b. In this embodiment, feed port 22b, in communication with feed conduit 19b, is positioned beneath the minimum bioslurry level in biodigestion chamber 1 1 b.

[81 ] In use, water and biodigestible material is introduced into biodigester 10b through feed mouth 21 b. Biodigestion chamber 1 1 b is initially filled with feed slurry to a level higher than the minimum bioslurry level defined by inlet opening 25b, and is optionally completely filled. After the initial feed addition, the liquid levels in biodigestion chamber 1 1 b, feed conduit 19b and transfer conduit 23b will be the same. Additional water and biodigestible material is added to biodigestion chamber 1 1 b during ongoing operation of biodigester 10b as required to maintain biogas and bioslurry production.

[82] Due to biodigestion of the organic matter in the feed, biogas accumulates at the top of biodigestion chamber 1 1 b, trapped beneath dome-shaped roof 17b. The liquid level in biodigestion chamber 1 1 b is thus forced downwards by the biogas pressure build-up, so that bioslurry is displaced upwardly through feed conduit 19b and upwardly through transfer conduit 23b into displacement tank 14b. Once the predetermined maximum level in displacement tank 14b is exceeded, bioslurry is discharged through bioslurry outlet port 28b.

[83] Biogas build-up will cause bioslurry to be displaced upwardly through both feed conduit 19b and transfer conduit 23b until the bioslurry level in biodigestion chamber 1 1 b falls to the minimum bioslurry level defined by inlet opening 25b. Further biogas generated in biodigestion chamber 1 1 b is then able to escape by bubbling through fluid-filled transfer conduit 23b, into displacement tank 24b and out into the atmosphere through bioslurry outlet port 28b. [84] Biogas is controllably withdrawn through gas port 29b when required for consumption. The withdrawal of biogas causes bioslurry to flow downwardly from displacement tank 14b through transfer conduit 23b into biodigestion chamber 1 1 b. The level of bioslurry in biodigestion chamber 1 1 b thus rises, while the level of bioslurry in displacement tank 14b and feed conduit 19b drops. The hydraulic head of the bioslurry in displacement tank 14b, and the building of liquid level in biodigestion chamber 1 1 b upon gas withdrawal, permits a higher fraction of the biogas stored within biodigester 10b to be utilised than would be the case without the assistance from the downward flow of a substantial volume of bioslurry from displacement tank 14b. Biogas may be withdrawn under positive pressure until the levels in biodigestion chamber 1 1 b and the levels in transfer conduit 23b have equalised. Preferably, the volume of bioslurry contained in displacement tank 14b (when filled to the predetermined maximum level) is sufficient to substantially fill biodigestion tank 1 1 b when the level of bioslurry is initially at the minimum bioslurry level, leaving only enough clearance beneath roof 17b to avoid contaminating or blocking gas port 29b. [85] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.