WO2017024337A9

WO2017024337A9 - A gas collection system

Info

Publication number: WO2017024337A9
Application number: PCT/AU2016/000277
Authority: WO
Inventors: Rajendra Gangadharan Pillai KURUP
Original assignee: Environmental Engineers International Pty Ltd
Priority date: 2015-08-12
Filing date: 2016-08-12
Publication date: 2017-07-13
Also published as: WO2017024337A1

Abstract

A cover and gas collection system (11) for capturing and collecting biogas produced in an anaerobic digester (13). The system (11) comprises a plurality of modules (30) each defining a separate gas holder 50. The gas holder modules (30) cooperate in an assembly (33) to overlie the contents within the anaerobic digester (13). The gas holder modules (30) are individually removable from the assembly (33). Each gas holder module (30) has a gas collection space (35) and a hydraulic seal (37) to trap biogas within the space (35). Biogas is captured within the gas collection space (35) from where is can be retrieved. The lower end of each gas holder module (30) is adapted to be immersed in liquid to provide the hydraulic seal (37). In one arrangement, the liquid may comprise a liquid constituted by the contents of the anaerobic digester (13). In another arrangement, the liquid may comprise a different liquid contained separately of the contents of the anaerobic digester (13).

Description

A Gas Collection System

TECHNICAL FIELD

[0001] This invention relates to a gas collection system for collecting liberated gas.

[0002] The liberated gas may emanate from a source of any appropriate form. By way of example, the source may comprise (i) a liquid, (ii) a liquid mixture, (iii) a mixture of solid(s) and liquid(s), such as a slurry or sludge, or (iv) solid material(s) which may comprise a mass of granular material (such as sand) or a porous solid (such as porous geological formation or porous rock) within a water bound or other liquid environment.

[0003] The gas collection system may be configured as a cover for the source. [0004] The invention has been devised particularly, although not necessarily solely, as a cover and gas collection system for a digester, including in particular an anaerobic digester, in which case the source would comprise fermenting contents of the digester. However, the invention may have application to any field as appropriate where there is a need for collection of liberated gas emanating from a source. BACKGROUND ART

[0005] The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application. [0006] As discussed above, the invention has been devised particularly, although not necessarily solely, in relation a cover and gas collection system for a digester, including in particular an anaerobic digester. Accordingly, the following discussion in relation to background art is provided in the context of collection of biogas generated by anaerobic digestion in a digester or reactor. [0007] It is known to treat organic waste, including for example wastewater, liquefied waste material, and sewage sludge, by way of a fermentation process comprising anaerobic digestion. The waste material undergoing anaerobic digestion produces biogas which can be captured and collected for subsequent use, such as for example in heat and power production. [0008] The fermentation process is typically performed in an anaerobic reactor and the waste material may be delivered as feedstock into the anaerobic reactor.

[0009] The anaerobic reactor may comprise a reservoir in which a volume of the feedstock is contained. The reservoir is covered to restrict oxygen from the anaerobic environment within the reactor and to restrict biogas escaping to atmosphere.

[0010] The reservoir can be in various forms; for example, the reservoir may comprise a tank, an earthen basin formed in the ground, a pond or a lagoon.

[001 1] The cover can be configured to provide a collector for capturing and collecting the biogas. [0012] Typical biogas collectors for anaerobic reactors are either fixed dome type or floating drum type. These are installed for small domestic or community type biogas plants, typically using animal waste. The floating drum type used as a gasholder in a small-scale biogas plant is supported by a central shaft that is fixed to the base of the reactor. The floating gasholder moves up or down depending up on the gas volume, but along the central shaft. Typically, these gasholders are made in mild or coated steel, fiberglass, plastic or ferrocement. This type of biogas plants is normally below 2 metres in height and is not detrimentally affected by wind force or rainfall. The fixed dome plants are made using concrete or brick, and typically the whole system is below ground and as such not affected by wind force.

[0013] The more recent double membrane and geodesic dome type gasholders in large- scale vertical type anaerobic digesters cover the entire reactor with a single gasholder. These gasholders are made up of polymers such as high-density polyethylene (HDPE), low density or linear low-density poly ethylene (LDPE or LLDPE), polypropylene (PP) or similar materials. The biogas pressure in the digester is regulated by means of air pressure between the gas liners/covers. The gas covers, either double membrane or geodesic dome type with metallic or plastic type covers, are fixed to the digester walls, and are subjected to extreme forces due to wind and rainfall. In addition, any malfunction in the digester (such as crust formation and mechanical breakdown of mixers), is practically impossible to manage without removing the cover material. Furthermore, any localized damage to the cover materials requires shutting down of the digester and downstream power generation system. [0014] Covered anaerobic lagoons (CAL) are commonly used for meat and livestock industry effluent treatment. The gasholders of CAL are made up of polymers such as HDPE, LDPE or LLDPE, PP or similar materials. CAL also uses a single cover spanning the entire lagoon, and the gas retention and prevention of gas leakage is ensured by encapsulating all the sides (entire perimeter) of the membrane cover through a concrete berm around the lagoon. Given the large plan area of the cover, the impact of rainfall on the cover is severe. The drainage cells do not often work properly. The impact of wind and rainfall reduces the life of the covers. Any tear or local damage on the covering material can cause leakage of gas and essentially obstruction of downstream energy production. In addition, CAL systems have problems in managing the process inside the system as fat, oil and grease can increase the thickness of the floating crust, which in fact will reduce the biogas storage volume. Removal of crust and sludge is difficult, and current systems do not have an option to remove excess crust and sludge. Any repair to the system requires cessation of its operation and downstream energy production.

[0015] The gas holders employed in upflow anaerobic sludge blanket reactor (UASB) systems are fixed type multiple channel type gas holders made up of steel, fiberglass or other plastic type materials. The gasholders of UASB are fixed to the floor of the reactor. The gasholder structure enables access to the reactor contents to remove any excess crust. However, the UASB reactors are normally considered to be unsuitable for wastewater with high suspended solids, fat, oil and grease as well as with low degrading organic matter. The UASB system requires periodic shut down maintenance. The rectification of any localised damages (rupture, holes, cracks etc) to the gasholders requires complete shut down of the UASB reactor, which in turn affects the energy production downstream. Besides, the biogas capturing technology of UASB reactors is not suitable for retrofitting to existing anaerobic reactors.

[0016] In summary, there are various types of covers currently used to provide collectors for capturing and collecting the biogas, including fixed covers, floating covers, membrane covers and gas-holder covers. Gas-holder covers have a facility for collection and temporary storage of biogas.

[0017] The current biogas covers are generally not satisfactory to resist extreme wind/cyclone and rainfall events. They are generally also unable to provide easily manageable operations of anaerobic digesters in terms of dealing with extensive crust formation on top of the liquid column and replacing any mixing mechanism without shutting down of the plant.

[0018] It is against this background, and the problems and difficulties associated therewith, that the present invention was developed. [0019] While the present invention was developed against this background, it need not necessarily overcome any or all of the problems and difficulties referred to above. Rather, the invention may merely offer an alternative arrangement for facilitating the capture and collection of biogas generated in an anaerobic system. Further, the invention may offer an arrangement for facilitating the capture and collection of liberated gas; that is, the invention is not necessarily limited to the capture and collection of liberated gas in the form of biogas generated in an anaerobic system.

[0020] SUMMARY OF INVENTION

[0021] According to a first aspect of the invention there is provided a gas collection system comprising a plurality of modules each defining a separate gas holder, the plurality of gas holder modules cooperating to overlie a source of upwardly flowing liberated gas.

[0022] The plurality of gas holder modules may overlie the entire surface of the source, or at least a substantial part thereof.

[0023] Each gas holder module may be configured to define a gas collection space above an underlying portion of the source and a hydraulic seal to trap gas within the space.

[0024] Each gas holder module may have a lower end adapted to be immersed in a liquid which acts as a seal, thereby to provide said hydraulic seal.

[0025] In one arrangement, the liquid may comprise a liquid constituted by the source; that is, the source may comprise the liquid. By way of example, the source may comprise a liquid mixture (including a slurry or sludge) having a liquid component, with the latter providing said liquid.

[0026] In another arrangement, the liquid may comprise a separate liquid; that is, the separate liquid may be contained separately of the source. [0027] Each gas holder module may comprise a top portion, a side portion and a lower end, the lower end being open to receive gas uprising from a location below gas holder module for entry into the gas collection space.

[0028] The gas collection space may be defined within the confines of the top and side portions.

[0029] The side portion may comprise a skirt depending from the top portion.

[0030] The lower end of the skirt may define an opening constituting the open lower end.

[0031] Each gas holder module may float on a volume of gas confined within the gas collection space thereof.

[0032] Each gas holder module may also be of buoyant construction to float upon the source when not floating above the source on gas confined within the gas collection space.

[0033] Each gas holder module may be adapted to rise and fall while maintaining said hydraulic seal.

[0034] Each gas holder module may be adapted for guided movement while rising and falling.

[0035] In one embodiment, the guided movement may be provided by a guide, such as a guide rod, along which the gas holder module is movable. [0036] In another embodiment, the guided movement may be provided by cooperation with adjacent gas holder modules.

[0037] Each gas holder module may be configured to have a centre of gravity and a centre of buoyancy, with the centre of gravity being lower than the centre of buoyancy.

[0038] Each gas holder module may have ballast to establish an arrangement in which the centre of gravity is lower than the centre of buoyancy.

[0039] Where the lower ends of the gas holder modules are adapted to be immersed in liquid constituted by the source to provide said hydraulic seals, adjacent gas holder modules may be configured to inhibit gas escaping upwardly from the source between the gas holders. In one embodiment, the gas holder modules may be provided in first and second configurations, with the skirt of the module of the first configuration having a deflector portion arranged to extend into a position below the skirt of the module of the second configuration, whereby the deflector portion is operable to deflect rising gas within the source into the gas collection space of the module of the first configuration. With this arrangement, each two adjacent gas holders are selected such that one is of the first configurations and the other is of the second configurations.

[0040] Where the lower ends of the gas holder modules are adapted to be immersed in liquid separate from the source, the liquid may be contained within a trough system having an open top into which the lower ends of the gas holder modules extend for immersion in the contained liquid. The liquid may be of any appropriate form, such as water or oil.

[0041] The trough system may comprise a trough structure. The trough structure may be buoyant and adapted to float upon the source. The trough structure may, for example, comprise a floating platform adapted to float upon a liquid or slurry which comprises the source. A covering may be provided to prevent or inhibit evaporation.

[0042] The gas collection system may further comprise retrieval means for retrieving collected gas from the gas collection space of each gas holder module.

[0043] The retrieval means may comprise gas withdrawal pipe having a flexible section for accommodating rising and falling motion of the gas holder module.

[0044] The source may be confined within a reservoir. The reservoir may be of any appropriate form as would it would be understood by a person skilled in the art, including for example a tank, an earthen basin formed in the ground, a pond or a lagoon. [0045] The source may comprise feedstock material for fermentation process. In particular, the source may comprise feedstock material for an anaerobic digestion process.

[0046] According to a second aspect of the invention there is provided a cover and gas collection system for an anaerobic digester, the system comprising a plurality of modules each defining a separate gas holder, the plurality of gas holder modules cooperating to overlie a surface of material within the digester in which biogas is to be generated.

[0047] According to a third aspect of the invention there is provided a digester comprising a reservoir to receive feedstock material for anaerobic digestion, and a cover and gas collection system comprising a plurality of modules each defining a separate gas holder, the plurality of gas holder modules cooperating to overlie a surface of feedstock material within the digester in which biogas is to be generated.

[0048] According to fourth aspect of the invention there is provided a gas holder for a gas collection system according to the first aspect of the invention. [0049] According to fifth aspect of the invention there is provided a gas holder for a cover and gas collection system according to the second aspect of the invention.

[0050] According to a sixth aspect of the invention there is provided a gas holder comprising a top portion, a side portion and a lower end, and a gas collection space, the lower end being open to receive gas uprising from a location below the below gas holder for entry into the gas collection space.

[0051] The gas collection space may be defined within the confines of the top and side portions.

[0052] The side portion may define with the lower end.

[0053] The side portion may comprise a skirt depending from the top portion. [0054] The lower end may comprise an open bottom, whereby uprising gas enter the gas collection space through the open bottom.

[0055] The lower end of the skirt may define an opening constituting the open bottom.

[0056] The gas holder may be adapted float on a volume of gas confined within the gas collection space thereof. [0057] The gas holder may be of buoyant construction for floating upon a body of liquid when not floating on gas confined within the gas collection space.

[0058] The gas holder may be adapted to establish a hydraulic seal in association with liquid to trap gas in the gas collection space. [0059] Each gas holder may be adapted to rise and fall while maintaining said hydraulic seal.

[0060] Each gas holder may be adapted for guided movement while rising and falling.

[0061] According to a seventh aspect of the invention there is provided a plurality of gas holders according to the sixth aspect of the invention in an assembly defining a gas collection system.

[0062] According to an eighth aspect of the invention there is provided a plurality of gas holders according to the sixth aspect of the invention in an assembly defining a cover and gas collection system. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Further features of the present invention are more fully described in the following description of several non-limiting embodiments thereof. This description is included solely for the purposes of exemplifying the present invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which:

Figure 1 is a schematic sectional side view of a first embodiment of a cover and gas collection system according to the invention in position on a digester;

Figure 2 is a schematic perspective view of the cover and gas collection system shown in figure 1 :

Figure 3 is a side view of the arrangement shown in figure 2:

Figure 4 is a fragmentary side view of part of the arrangement shown in figure 1 :

Figure 5 is a plan view of figure 5;

Figure 6 is a schematic sectional side view of two gas holders and a common trough establishing hydraulic seals with respect to the two gas holders; Figure 7 is a schematic plan view of a gas holder;

Figure 8 is a sectional side view of the gas holder of figure 7; Figure 9 is a perspective view of a floating platform providing a trough structure forming part of the arrangement shown in figure 1 ;

Figure 10 is a plan view of the floating platform of figure 9;

Figure 1 1 is a side view of the floating platform figure 9; Figure 12 is a perspective view of a guide system forming part of the arrangement shown in figure 1 :

Figure 13 is a plan view of the guide system;

Figure 14 is a side of the guide system;

Figure 15 is a schematic sectional side view of a second embodiment of a cover and gas collection system according to the invention, illustrating in particular two gas holders and a common trough establishing hydraulic seals with respect to the two gas holders;

Figure 16 is a schematic plan view of figure 15;

Figure 17 is a plan view of a third embodiment of a cover and gas collection system according to the invention in position on a digester;

Figure 18 is a sectional side view of the arrangement shown in figure 17;

Figure 19 is a schematic sectional side view of several gas holders of the arrangement shown in figure 17;

Figure 20 is a schematic plan view of figure 19; Figure 21 is a schematic sectional side view of a fourth embodiment of a cover and gas collection system according to the invention, illustrating in particular several gas holders in position;

Figure 22 is a schematic plan view of figure 21 ; Figure 23 is a perspective view of a fifth embodiment of a cover and gas collection system according to the invention in position on a digester;

Figure 24 is a sectional side view on line 24-24 of figure 23; Figure 25 is a plan view of the arrangement shown in figure 23;

Figure 26 is a side view of the arrangement shown in figure 23;

Figure 27 is a further side view of the arrangement shown in figure 23;

Figure 28 is a perspective view of a first configuration of gas holder for use in the fifth embodiment;

Figure 29 is an end view of the gas holder shown in figure 28;

Figure 30 is a cross-sectional view of the gas holder shown in figure 28;

Figure 31 is a perspective view of a second configuration of gas holder for use in the fifth embodiment; Figure 32 is an end view of the gas holder shown in figure 31 ;

Figure 33 is a cross-sectional view of the gas holder shown in figure 31

Figure 34 is a view similar to figure 30, but depicting liquid in a well forming part of the gas holder;

Figure 35 is a view similar to figure 33, but depicting liquid in a well forming part of the gas holder;

Figure 36 is a plan view of a sixth embodiment of a cover and gas collection system according to the invention in position on a digester;

Figure 37 is a sectional side view of the arrangement shown in figure 36;

Figure 38 is a perspective view of a first configuration of gas holder for use in the sixth embodiment; and

Figure 39 is a perspective view of a second configuration of gas holder for use in the sixth embodiment.

[0064] The drawings shown are not necessarily to scale, with emphasis instead generally being placed upon illustrating the principles of the present invention. [0065] The figures depict several embodiments of the invention. The embodiments illustrate certain configurations; however, it is to be appreciated that the invention can take the form of many configurations, as would be obvious to a person skilled in the art, whilst still embodying the present invention. These configurations are to be considered within the scope of this invention.

DESCRIPTION OF EMBODIMENTS

[0066] Referring to the drawings, there are various embodiments of a gas collection system 10 according to the invention for capturing and collecting liberated gas. In the arrangements described and illustrated, the gas collection systems 10 are each configured as a cover and gas collection system 1 1 for capturing and collecting biogas produced in an anaerobic treatment process.

[0067] The anaerobic treatment process is performed in an anaerobic digester 13 comprising a reservoir 15 adapted to contain a volume of feedstock material 17 for anaerobic digestions. In the arrangement shown, the volume of feedstock material 17 represents the contents 19 of the reservoir 15. The contents 19 present an upper surface 21 . Contents 19 are typically in the form of a liquid mixture, slurry or sludge, as would be understood by a person skilled in the art. In the arrangement shown, the reservoir 15 is in the form of a tank 23 having a base 25 and a side 27 defining the interior 29 of the reservoir in which the volume of feedstock material is contained. The reservoir 15 may, of course, take any other appropriate form as would be understood by a person skilled in the art, such as an earthen basin formed in the ground, a pond or a lagoon.

[0068] The anaerobic digester 13 further comprises the cover and gas collection system 1 1 which is mounted on the reservoir 15. The cover and gas collection system 1 1 is adapted to cover the contents 19 to restrict oxygen from the anaerobic environment within the reservoir 15 and to restrict biogas escaping to the atmosphere. Further, the cover and collection system 1 1 is adapted to capture and collect biogas, as will be explained in more detail later.

[0069] In the various embodiments, the cover and gas collection system 11 is of modular construction. More particularly, the cover and gas collection system 1 1 comprises a plurality of modules 30 each defining a separate gas holder. The plurality of gas holder modules 30 cooperate to overlie the contents 19 contained within the interior 29 of the reservoir 15 and thereby cover the contents 19. In other words, the plurality of gas holder modules 30 cooperate to define an assembly 33 which provides the cover and gas collection system 1 1 . The gas holder modules 30 are individually removable from the assembly 33. This is advantageous, as it facilitates access to the interior 29 of the reservoir 15, as well as the contents 19 therein, without necessarily requiring that operation of the anaerobic digester 13 be interrupted. Additionally, it also facilitates removal of one or several of the gas holder modules 30 for servicing, repair all replacement without again requiring operation of the anaerobic digester 13 to be interrupted. [0070] Each gas holder module 30 has a gas collection space 35 and a hydraulic seal 37 to trap biogas within the space 35. In this way, biogas is captured within the gas collection space 35. The lower end of the gas holder module 30 is adapted to be immersed in liquid to provide the hydraulic seal 37.

[0071] The captured biogas can then be retrieved from the gas collection space 35. For this purpose, there is provided a gas retrieval system 39 incorporating at least one gas exhaust port 41 opening onto the gas collection space 35.

[0072] Each gas holder module 30 is adapted to float on a volume of gas confined within the gas collection space 35. Additionally, each gas holder module 30 is adapted to float upon the surface 21 of the contents 19 of the reservoir 15 when not floating above the contents on gas confined within the gas collection space 35.

[0073] Various embodiments of the cover and gas collection system 11 according to the embodiment will now be described in more detail with reference to the accompanying drawings.

[0074] Referring now to Figures 1 to 14, there is shown a first embodiment of the cover and gas collection system 1 1. In the arrangement shown, the cover and gas collection system 1 1 comprises the plurality of gas collection modules 30, each defining a respective gas holder 50 of generally rectangular construction in plan.

[0075] The gas retrieval system 39 is not depicted in Figure 1 , but it is shown in Figures 4 and 5. [0076] In the arrangement shown, the assembly 33 of modules 30 is generally rectangular, so as to conform with a reservoir 15 of generally rectangular configuration, having side 27 defined by four side walls, as shown in Figures 1 . The reservoir 15 can, of course be of any other appropriate configuration, as would be understood by a person skilled in the art; for example, circular as depicted in later embodiments.

[0077] Each gas holder 50 comprises a top portion 51 , a side portion 53 and a lower end 55. The lower end 55 is open to receive biogas uprising from the fermenting contents 19 within the reservoir 15.

[0078] The side portion 53 is configured as a skirt 57 depending from the top portion 51. The skirt 57 is of rectangular configuration, comprising four sides 58. Each side 58 is configured as a wall. Accordingly, there are four walls connected together to provide the rectangular skirt 57. The lower end of the skirt 57 defines an opening 59 at the lower end 55 of the gas holder 50.

[0079] The gas collection space 35 defined within the confines of the top portion 51 and the side portion 53. More particularly, the gas collection space 35 is defined by an attic space 61 immediately below the top portion 51 . [0080] Each gas holder 50 further comprises an axial guide passage 63 extending downwardly from the top portion 51 , the purpose of which will be explained later. The guide passage 63 is defined by an internal portion 65 depending from the top portion 51 . The internal portion 65 is of hollow construction to define the guide passage 63, which opens onto the upper side of the top portion 51 . [0081] Each gas holder 50 is configured to have a centre of gravity and a centre of buoyancy, with the centre of gravity being lower than the centre of buoyancy. This is to ensure stability of the gas holder as it floats upon a volume of gas confined within the gas collection space 35 and also when floating upon the surface of the contents 19 of the reservoir 15. The gas holder 50 is so configured in this embodiment by having the internal portion 65 extending downwardly from the top portion 51 a sufficient extent to provide the necessary relationship between the centre of gravity and the centre of buoyancy. In the arrangement shown, the internal portion 65 extends downwardly through the opening 59.

[0082] The top portion 51 is of buoyant construction to provide the necessary buoyancy for the gas holder 50 to float upon the surface 21 of the contents 19 of the reservoir 15 when not floating above the contents on gas confined within the gas collection space 35. For this purpose, the top portion 51 is provided with flotation elements or materal 67. [0083] The lower end of the gas holder 50 is adapted to be immersed in liquid 70 to provide the hydraulic seal 37. In this embodiment, the liquid 70 is contained within a trough structure 71 spanning the interior 29 of the reservoir 15 above the space to be occupied by the contents 19. With this arrangement, the liquid 70 is contained separately of the contents 19 of the reservoir 15. The liquid 70 may be of any appropriate type, with water being considered to be particularly suitable. The liquid presents a liquid surface 70a.

[0084] The trough structure 71 is adapted to float upon the surface of the contents 19 within the reservoir 15. For this purpose, the trough structure 71 is of buoyant construction. In other words, the trough structure 71 comprises platform 72 adapted to float on the contents 19. In the arrangement shown, the floating platform 72 is rectangular in plan, as best seen in Figures 9 and 10.

[0085] With this arrangement, the cover and gas holder system 1 1 comprises a buoyant structure 74 adapted to float upon the surface 21 of the contents 19 within the reservoir 15, as shown in Figure 1 .

[0086] The trough structure 71 is in a grid formation, comprising a plurality of longitudinal channels 73 arranged in a crisscross formation. Buoyant supports 75 are provided on the underside of the longitudinal channels 73 to provide flotation for the trough structure. [0087] The criss-crossing channels 73 intersect one with another to provide a common trough assembly 77 comprising a plurality of interconnect trough sections 79. With this arrangement, the various trough sections 79 provide a common trough assembly 81 throughout which the liquid 70 can spread.

[0088] With this arrangement, the criss-crossing channels 73 define a plurality of rectangular zones 83, each bounded by four trough sections 79. The rectangular zones 83 correspond in number to the gas holders 50, with each gas holder being associated with a respective one of the rectangular zones. With this arrangement, the four sides 58 of the skirt 57 of each gas holder 50 aligns with the four trough section 67 bounding the respective rectangular zones 83 associated with the gas holder. The four sides 58 of the skirt 57 extend into the four trough sections 79, with the lower end portions 58a of the skirt sides 58 being immersed in the liquid 70 contained within the trough sections to establish the hydraulic seal 37. [0089] Adjacent gas holders 50 share a common trough section 79 therebetween. This is best seen in Figure 6, where skirts 57a, 57b of two adjacent gas holders 50 are shown sharing a common trough section 79a.

[0090] Each gas holder 50 is adapted to rise and fall while maintaining the hydraulic seal 37. A guide system 91 is provided for guiding the gas holders 50 while they rise and fall. In the arrangement shown, the guide system 91 comprises a guide structure 93 mounted upon the reservoir 15 to span the interior 29 thereof. The guide structure 93 comprises a support 95 and guide tracks 97 depending from the support. The support 95 comprises a plurality of longitudinal elements 99 arranged in criss-cross formation intersecting at nodes 101 , with the guide tracks 97 comprising guide rods 103 extending downwardly from the nodes. The guide rods 103 are adapted to be slidingly received in the axial guide passages 63 within the gas holders 50. With this arrangement, cooperation between each gas holder 50 and the respective guide rod 103 slidingly received in the axial passage of the gas holder, serves to guide movement of the gas holder vertically as the gas holder rises and falls. In providing for guided movement of the gas holder 50, the guide structure 93 serves to prevent the gas holder from tipping when exposed to strong wind conditions.

[0091] The gas holder 50 further comprises gas exhaust port 41 opening onto the gas collection space 35. The exhaust port 41 communicates with a gas withdrawal pipe 1 10 forming part of the gas retrieval system 39.

[0092] When installed in position, the various gas holders 50 cooperate to define the assembly 33 which provides the cover and gas collection system 1 1 of the anaerobic digester 13. The gas holders 50 are individually removable from the assembly 33 on a selective basis, as previously explained. [0093] As mentioned above, the cover and gas holder system 11 comprises a buoyant structure 74 adapted to float upon the surface 21 of the contents 19 within the reservoir 15, as shown in Figure 1 . In the arrangement shown, the buoyant structure 74 is rectangular in plan and designed to conform generally to the rectangular interior 29 of the reservoir 15, but with sufficient clearance to allow rising and falling movement of the buoyant structure 74 within the interior 29. In other arrangements, the buoyant structure 74 and the interior 29 of the reservoir 15 may be of different configurations; for example, the buoyant structure 74 may be rectangular in plan and the interior 29 of the reservoir 15 may be circular. [0094] A sealing arrangement (not shown), such as a flexible cover, may be provided between the buoyant structure 74 and the perimeter of the reservoir 15 for closing any gap therebetween to restrict oxygen from the anaerobic environment within the reactor, and to restrict biogas and odours escaping to atmosphere. This may be particularly so in cases where the buoyant structure 74 and the interior 29 of the reservoir 15 are of different configurations, thereby leading to larger gaps therebetween.

[0095] In operation, biogas generated in the anaerobic process within the contents 19 of the reservoir 15 rises within the contents and enters the various gas holders 50 through the bottom openings 59 therein. The biogas is captured and collected within the gas collection spaces 35 of the various gas holders 50. The collected biogas is retrieved from the gas holders 50 by way of the gas retrieval system 39. During operation, each gas holder 50 floats on the volume of gas confined within its respective gas collection space 35. In circumstances where there is not sufficient gas pressure within the gas collection to support the gas holder 50 in the floating condition, the gas holder can float upon the surface of the contents 19 within the reservoir 15. Such circumstances may arise when sufficient biogas is not being generated (such as at the commencement of the anaerobic digestion process) or in the event of some disruption leading to a loss of captured biogas or gas pressure.

[0096] The various gas holders 50 can rise and fall with changes in the volume of biogas being generated and also variations in the level of the contents 19 within the reservoirs. As each gas holder 50 undergoes rising and falling movement, the lower end of the gas holder remains immersed in the liquid 70, thereby maintaining the integrity of the hydraulic seal 37.

[0097] Referring now to Figures 15 and 16, there is shown a second embodiment of the cover and gas collection system 1 1 . The second embodiment is similar in many respects to the first embodiment and so corresponding reference numerals are used to identify corresponding parts as appropriate.

[0098] In the arrangement shown, the cover and gas collection system 1 1 comprises the plurality of gas collection modules 30, each defining a respective gas holder 50 of generally rectangular construction in plan, as was the case with the first embodiment. There is also guide system 91 for guiding the gas holders 50 while they rise and fall. In this second embodiment, the guide system 91 is different from that in the first embodiment. More particularly, the guide system 91 is supported in the floating platform 72 which provides the trough structure 71 . The guide system 91 comprises a plurality of guides rod 121 supported internally within the rectangular zones 83 bounded by the trough sections 79. Each guide rod 121 on a support frame 123 mounted within the rectangular zone 83. The guide rod 121 is slidingly received within a guide passage 125 provided on the top portion 21 of the gas holder. The guide passage 125 is defined by a tubular element 127 mounted on the top portion 21 of the gas holder 50. The guide passage 125 and the guide rod 121 are so is size as to define an annular space 129 therebetween which provides a flow path communicating with the gas retrieval system 39. The inlet end of the flow path defines the exhaust port 41 opening onto the gas collection space 35. A flexible section 131 is provided between the tubular element 127 and the section 133 of the gas retrieval system 39 immediately downstream thereof to accommodate rising and falling motion of the gas holder 50.

[0099] In the two previous embodiments, the hydraulic seal 37 was established in association with liquid 70 contained separately of the contents 19 of the reservoir 15. In other embodiments, the liquid component of the contents 19 within the reservoir 15 can be used to establish the hydraulic seal 37. Where, for example, the contents 19 comprise a slurry, the hydraulic seal 37 may be established in association with the slurry. Subsequent embodiments are directed to such arrangements.

[00100] Referring now to Figures 17 to 20, there is shown a third embodiment of the cover and gas collection system 1 1. The third embodiment is similar in some respects to the previous embodiments and so corresponding reference numerals are used to identify corresponding parts as appropriate.

[00101] In the arrangement shown, the third embodiment of the cover and gas collection system 1 1 comprises the plurality of gas collection modules 30, each defining a respective gas holder 50 of generally rectangular construction in plan, as was the case with the first embodiment. The gas collection modules 30 may be provided in various sizes, as shown in Figure 17, in order for the assembly 33 to better cover the surface 21 of the contents 19 of the reservoir 15, which in the arrangement shown is generally cylindrical. [00102] Each gas holder 50 has a buffer 141 provided on each of the four sides thereof. With this arrangement, adjacent gas holders 50 in the assembly 33 can interact one with another through contact with their respective buffers 141 . The buffers 141 are resiliently flexible to provide some cushioning, thereby reducing the impact of collisions between adjacent gas holders 50.

[00103] As a consequence of this arrangement, there exists a gap 143 between adjacent gas holders 53 through which biogas generated by way of the anaerobic reaction may potentially escape to atmosphere. The gas holders 50 are therefore configured to inhibit escape of biogas through the gaps 143. For this purpose, the gas holders 50 are provided in two complimentary configurations; that is, there are gas holders 50a in a first configuration and gas holders 50b in a second configuration.

[00104] The gas holders 50a, 50b are similar, apart from the configurations of the skirts 57 thereof. The skirts 57 are configured differently so as to interact in a way such that upwardly flowing biogas is directed away from the respective gaps 143.

[00105] More particularly, each of the gas holders 50a in the first configuration has the skirt 57 thereof configured to provide a deflector portion 145 operable to deflect rising biogas into the gas collection space 35 of gas holder 50a. In the arrangement shown, the deflector portion 145 comprises an outwardly flared portion 146 at the lower end of the skirt 57. The outwardly flared portion 146 provides a laterally extending portion 147 which presents a downwardly facing deflector surface 148. The laterally extending portion 147 extends downwardly and outwardly, thus having vertical and horizontal components. With this arrangement, the deflector surface 148 is configured to extend upwardly and inwardly towards the interior of the side portion 53. In this way, the deflector surface 148 is operable to direct oncoming upwardly flowing biogas inwardly towards the interior of the side portion 53 through the bottom opening 59 for capture and collection within the gas collection space 35 of the gas holder 50a.

[00106] Each of the gas holders 50b in the second configuration has the skirt 57 thereof configured to extend directly downwardly; that is, without any lateral extension. With this arrangement, each two adjacent gas holders are selected such that one is of the first configurations and the other is of the second configurations. With gas holders 50a and 50b are disposed in side-by-side relation, the deflector portion 145 of gas holder 50a is arranged to extend into a position below the skirt 57 of gas holder 50b, as best seen in Figure 19. In this way, the deflector portion 145 is operable to deflect rising biogas into the gas collection space 35 of gas holder 50a. [00107] In this embodiment, each gas holder 50 has a ballast member 151 mounted on the top portion 51 . The ballast member 151 extends downwardly from the top portion 51 into the interior space within the confines of side portion 53. The ballast member 151 is configured as an elongate element and is arranged to ensure that the centre of gravity of the gas holder 50 is lower than the centre of buoyancy of the gas holder.

[00108] The gas holders 50 are sized to minimise the gaps 143 between adjacent gas holders and to limit mobility of the gas holders within the assembly 33 to acceptable limits. [00109] In this embodiment, there is no guide system as such for guiding movement of the gas holders 50 as they rise and fall. Rather, the gas holders 50 are configured for self-regulation. Specifically, self-regulation is achieved by virtue of the centre of gravity being below the centre of buoyancy.

[00110] Referring now to Figures 21 and 22, there is shown a fourth embodiment of the cover and gas collection system 1 1 . The fourth embodiment is similar in many respects to the third embodiment and so corresponding reference numerals are used to identify corresponding parts as appropriate.

[0011 1] In this fourth embodiment, the ballast member 151 of each gas holder 50 is constructed somewhat differently to that of the third embodiment. More particularly, the ballast member 151 of this embodiment is configured such that it is accessible from above the gas holder 50 and of an arrangement such that the mass of the ballast member can be selectively varied.

[00112] In the previous embodiments, the cover and gas collection system 11 comprises the plurality of gas collection modules 30, each defining a respective gas holder 50 of generally rectangular, and more particularly substantially square, construction in plan. Other configurations of the gas holder 50 are possible. By way of example, the gas holders 50 may be of elongate construction. The next two embodiments are each directed to a cover and gas collection system 11 featuring gas holders 50 of elongate configuration. [00113] Referring now to Figures 23 and 35, here is shown a fifth embodiment of the cover and gas collection system 1 1. The fifth embodiment is similar in some respects to the previous embodiments and so corresponding reference numerals are used to identify corresponding parts as appropriate.

[001 14] In this fifth embodiment, the assembly 33 comprising as plurality of elongate gas holders 50 disposed one alongside another. [001 15] As with previous embodiments, each gas holder 50 comprises top portion 51 , side portion 53 and lower end 55. The lower end 55 is open to receive biogas uprising from fermenting contents 19 within the reservoir 15. However, in this embodiment, the top portion 51 , side portion 53 and lower end 55 are all elongate.

[001 16] The side portion 53 is configured as a skirt 57 depending from the top portion 51. The skirt 57 is of rectangular configuration, comprising four sides 58. The lower end of the skirt 57 defines an opening 59 at the open lower end 55 of the gas holder 50.

[00117] Gas collection space 35 is defined within the confines of the top portion 51 and the side portion 53 of each elongate gas holder 50. In this embodiment, the gas collection space 35 is divided into two gas collection compartments 35a, 35b, as will be described in more detail later. The two collection compartments 35a, 35b define respective attic spaces 61 a, 61 b immediately below the top portion 51. The two gas collection compartments 35a, 35b have respective exhaust ports 41 a, 41 b each communicating with the gas retrieval system 39. [00118] The plurality of elongate gas holders 50 in the assembly 33 are configured to span the interior of the reservoir 15, as best seen in Figures 23 and 25. Although the reservoir 15 can be of any other suitable configuration, including rectangular, it is cylindrical in the arrangement shown By way of example only, in an anaerobic digester 13 in which the reservoir 15 is about 32 metres in diameter, there may be 10 elongate gas holders 50, each of a width of about 3 metres and of varying lengths, as will be explained later.

[001 19] In the arrangement shown, interior 29 of the reservoir 15 is cylindrical. More particularly, the interior 29 is defined by a cylindrical side wall 155 which comprises the side 27 of the reservoir 15. Accordingly, the assembly 33 of elongate gas holders 50 is correspondingly generally circular in plan. With this arrangement, each elongate gas holder 50 can notionally be considered to align with a chord of a circle bounding the circular assembly 33. In this way, each intermediate gas holder 50 is of a length which corresponds generally to the counterpart notional chord of the circle bounding the circular assembly 33. By way of example only, in an anaerobic digester 13 in which the reservoir 15 is about 32 metres in diameter and in which there are 10 elongate gas holders 50, the gas holders may range in length from about 18.6 metres to 30 metres.

[00120] In the assembly 30 of elongate gas holders 50, there are longitudinal gaps 156 between adjacent gas holders 50, as will be explained further later.

[00121] Each elongate gas holder 50 is of a length selected such that there is an end gap 157 between its respective ends and the adjacent portion of cylindrical side wall 155 which defines the side 27 of the reservoir 15. The various end gaps 157 together form a space 158 between the circular assembly 33 and the cylindrical side wall 155 of the reservoir 15. The space 158 is provided for the purpose of affording clearance for fittings (not shown) on the gas holders 50 and the cylindrical side wall 155 of the reservoir 15. By way of example only, in an anaerobic digester 13 in which the reservoir 15 is about 32 m in diameter, each end gap 157 would typically be about 500mm.

[00122] The assembly 30 of elongate gas holders 50 may be considered as an assembly comprising two outermost gas holders 50c and a plurality of intermediate gas holders 50 d between the two outermost gas holders. Each outermost gas holder 50c comprises two longitudinal sides 161 , 162. The two longitudinal sides 161 , 162 may be parallel (as shown in the Figures 23 and 25) or longitudinal side 161 may be curved to conform to the profile of the circle bounding the circular assembly 33, and longitudinal side 162 is straight.

[00123] Each intermediate gas holder 50d comprises two longitudinal sides 163, 164 extending between opposed ends 165. The two longitudinal sides 163, 164 are straight. While the two opposed ends 165 may be straight (as shown in the drawings), they optionally may be curved to conform to the profile of the circle bounding the circular assembly 33.

[00124] Each gas holder 50 has at least one buffer 141 on each longitudinal side thereof. The buffer 141 is preferably configured as an elongate buffer 166 extending substantially the full length of the respective longitudinal side. The buffers 166 are provided to function as shock absorbers for cushioning impacts between adjacent gas holders 50, thereby preventing impact damage. The presence of the buffers 166 lead to the creation of the longitudinal gaps 156 between adjacent gas holders 50. By way of example only, in an anaerobic digester 13 in which the reservoir 15 is about 32m in diameter, each longitudinal gap 156 would typically be about 90 mm. [00125] With the elongate configuration of the gas holders 50, the skirts 57 thereof are also correspondingly elongate. There are four walls 58 connected together to provide the rectangular skirt 57. In the arrangement shown, the four walls 58 comprises two opposed longitudinal side walls 167 and two opposed end walls 168. .

[00126] As mentioned above, the elongate gas holders 50 are assembled one alongside another, with respective longitudinal gaps 156 between adjacent gas holders through which biogas generated by way of the anaerobic reaction may potentially escape to atmosphere. The gas holders 50 are therefore configured to inhibit escape of biogas through the gaps 156. For this purpose, the gas holders 50 are provided in two complimentary configurations; as was the case with several earlier embodiments. One such configuration is shown schematically in Figures 28 to 30 and identified as gas holder 50a, and the other configuration is shown schematically in Figure 31 to 33 and identified as gas holder 50b.

[00127] The gas holders 50a, 50b are similar, apart from the configurations of the skirts 57 thereof. The skirts 57 are configured differently so as to interact in a way such that upwardly flowing biogas is directed away from the longitudinal gaps 156 between adjacent gas holders 50.

[00128] More particularly, each of the gas holders 50a in the first configuration has the skirt 57 thereof configured to provide a deflector portion 145 operable to deflect rising biogas into the gas collection space 35 of gas holder 50a.. As was the case with previous embodiments, the deflector portion 145 comprises an outwardly flared portion 146 at the lower end thereof. The outwardly flared portion 146 provides a laterally extending portion 147 which presents a downwardly facing deflector surface 148. The laterally extending portion 147 extends downwardly and outwardly, thus having vertical and horizontal components. With this arrangement, the deflector surface 148 is configured to extend upwardly and inwardly towards the interior of the side portion 53. In this way, the deflector surface 148 is operable to direct oncoming upwardly flowing biogas inwardly towards the interior of the side portion 53 through the bottom opening 59 for capture and collection within the gas collection space 35 of the gas holder 50a . [00129] Each of the gas holders 50b in the second configuration has the skirt 57 thereof configured to extend directly downwardly; that is, without any lateral extension. With this arrangement, each two adjacent gas holders 50 are selected such that one is of the first configurations and the other is of the second configurations. With gas holders 50a and 50b are disposed in side-by-side relation, the laterally extending portion 147 of gas holder 50a is arranged to extend into a position below the skirt of gas holder 50b. In this way, the I the deflector portion 145 is operable to deflect rising biogas into the gas collection space 35 of gas holder 50a.

[00130] By way of example only, in an anaerobic digester 13 in which the reservoir 15 is about 32 metres in diameter and in which each longitudinal gap 156 is about 90 mm, the laterally extending portion 147 (which defines the deflector portion 145) would extend downward about 140mm and sideward about 90 mm; that is, the laterally extending portion 147 would be dimensioned to have a vertical component of about 140mm and a horizontal component about 90 mm. [00131] The top portion 51 of each gas holder 50 is of buoyant construction to provide the necessary buoyancy for the gas holder 50 to float upon the surface 21 of the contents 19 of the reservoir 15 when not floating above the contents on gas confined within the gas collection space 35. For this purpose, the top portion 51 is provided with flotation elements or material 67, as is the case with earlier embodiments. [00132] The gas holder 50 further comprises a well 181 extending downwardly from the top portion 51 . The well 181 is configured to receive and contain mass to provide ballast for the gas holder 50 and establish the required relationship between the centre of buoyancy and centre of matter (where the centre of mass is below the centre of buoyancy as described earlier). In this embodiment, the mass is constituted by a liquid such as, for example, water. Water is depicted schematically within the wells 181 of the gas holders 50a and 50b in Figures 34 and 35 respectively, and is identified by reference numeral 182

[00133] The well 181 is defined by an internal portion 183 depending from the top portion 51 . The internal portion 183 is of hollow construction, having a top end 185 opening onto the upper side of the top portion 51 and a closed bottom end 187. [00134] The internal portion 183 is elongate and extends along the full length of the gas collection space 35, dividing the latter into the two gas collection compartments 35a, 35b.

[00135] The well 181 is defined by a channel 191 extending longitudinally along the elongate gas holder 50. The internal portion 183 comprises two opposed longitudinal side walls 193, two opposed end walls 195 and a bottom wall 197 which cooperate to define the channel 191.

[00136] The bottom end 187 of the internal portion 183 extends to the open lower end 55 of the gas holder 50 and the bottom wall 197 of the internal portion 183 is configured to provide two opposed downwardly facing deflector surfaces 199, each operable to deflect rising biogas into a respective one of the two collection compartments 35a, 35b .

[00137] There may be provision (not shown) for discharging excess liquid from within the well 181 . This may comprise an overflow system operable to allow discharge of excess liquid from the well upon the liquid attaining a prescribed level.

[00138] There may be provision (not shown) for adjusting the level of liquid contained within the well 181 . This may comprise a system operable to deliver liquid into, and extract liquid from, the well 181 . The system may comprise a pump system.

[00139] The system operable to deliver liquid into, and extract liquid from, the well 181 may facilitate selective adjustment of the overall mass of the cover and gas collection system 1 1 . This may facilitate control of the level of the cover and gas collection system 1 1 above the contents within the reservoir. Such control may allow the internal gas pressure within the gas holders 50 to be selectively varied. Further, it may allow the assembly 30 of gas holders 50 to be lowered in the case of adverse wind or storm conditions, by increasing the liquid level in the wells 181 .

[00140] The wells 181 within the gas holders 50 may form part of a rainwater catchment system (not shown), whereby rain water falling on the assembly 30 can be directed to the wells for collection. The collected rainwater can then discharge from, or be extracted from, the respective wells and carried away from the cover and gas collection system 11 . [00141] It is advantageous for the mass within the wells 181 to comprise water, as this is conducive to implementation of the rainwater catchment system.

[00142] Referring now to Figures 36 to 39, there is shown a sixth embodiment of the cover and gas collection system 11 . This sixth embodiment is similar in many respects to the fifth embodiment, and so corresponding reference numerals are used to identify corresponding parts as appropriate.

[00143] sixth embodiment, the gas holders 50 are elongate but configured slightly differently. Specifically, the top portion 51 of each gas holder 50 is configured to direct rainwater falling upon the top portion into the well 181 . For this purpose, the upper surface 201 of the top portion 51 surrounding the well 181 may be configured to slope downwardly and inwardly towards the well. Additionally, each outermost gas holder 50c comprises two longitudinal sides 161 , 162 which converge at opposed ends of the gas holder. Specifically, longitudinal side 161 is curved to conform to the profile of the circle bounding the circular assembly 33, and longitudinal side 162 is straight. [00144] From the foregoing, it is evident that the various embodiments provide a cover and gas collection system for a digester, with the system being constructed in modular fashion. Each module can float independently. The system can provide variable gas pressure or storage volume. Additionally, problems such as excessive crust formation, and servicing of mixers of anaerobic digesters, can be easily managed. The system can also manage extreme wind, cyclone and rainfall events with no or minimal damage which can be rectified without shutting down the digester in contrast to the conventional systems. The various embodiments of the cover and gas collection system are self-regulating and designed against overturning or tilting. Any individual gas holder can be isolated and repaired without affecting the performance of the whole system, thus providing much needed flexibility during maintenance. Moreover, the system can be easily adapted to reactors/lagoons both as a retrofit to existing plants or new projects. The system comes with a high degree of customizability and a wide variety different materials such as but not limited to, concrete, ferro-cement, polymers including but not limited to HDPE, other type polyethylene, PP, PVC, or other type plastic membrane, latex or similar type rubber type materials, natural or processed fibre include wood, fibreglass, different types of steel, such as glass or epoxy coated steel, stainless steel, galvanized iron, aluminium, alloys and other materials on its own or in combination. The selection of materials for covers and size /dimensions are based upon the scale and applicability.

[00145] The foregoing disclosure is intended to explain how to fashion and use various embodiments in accordance with the invention, rather than to limit the true, intended, and fair scope and spirit of the invention. The foregoing description is not intended to be exhaustive, nor to be limited to the precise forms disclosed. Features described in relation to one embodiment may, as appropriate, be implemented in any one of the other embodiments, as would be understood by a person skilled in the art.

[00146] Further, it should be appreciated that various modifications can be made without departing from the principles of the invention. Therefore, the invention should be understood to include all such modifications within its scope. For example, in each of the embodiments discussed hereinbefore, the gas being captured and collected comprises biogas generated in an anaerobic digester or reactor. The invention may, however, have application to any field as appropriate where there is a need for collection of liberated gas or odour emanating from a source. By way of example, the invention may have application for collecting gas percolating upwardly out of a body of water, gas percolating upwardly out of the ground in a water bound environment, or gas percolating upwardly from the floor of a body of water (such as a lake, river, estuary or the ocean). [00147] Reference to any positional descriptions, such as "inner", "outer", "top", "bottom" and "side", are to be taken in context of the embodiments depicted in the drawings, and are not to be taken as limiting the invention to the literal interpretation of the term but rather as would be understood by the skilled addressee.

[00148] Additionally, where the terms "system", "device", and "apparatus" are used in the context of the invention, they are to be understood as including reference to any group of functionally related or interacting, interrelated, interdependent or associated components or elements that may be located in proximity to, separate from, integrated with, or discrete from, each other.

[00149] Throughout this specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims

1 . A gas collection system comprising a plurality of modules each defining a separate gas holder, the plurality of gas holder modules cooperating to overlie a source of upwardly flowing liberated gas.

2. The gas collection system according to claim 1 wherein the gas holder module is configured to define a gas collection space above an underlying portion of the source and a hydraulic seal to trap gas within the space.

3. The gas collection system according to claim 2 wherein the gas holder module has a lower end adapted to be immersed in a liquid which acts as a seal, thereby to provide said hydraulic seal.

4. The gas collection system according to any one of the preceding claims wherein each gas holder module comprises a top portion, a side portion and a lower end, the lower end being open to receive gas uprising from a location below gas holder module for entry into the gas collection space.

5. The gas collection system according to claim 4 wherein the gas collection space is defined within the confines of the top and side portions.

6. The gas collection system according to claim 4 or 5 wherein the side portion comprises a skirt depending from the top portion.

7. The gas collection system according to any one of the preceding claims wherein each gas holder module is configured to have a centre of gravity and a centre of buoyancy, with the centre of gravity being lower than the centre of buoyancy.

8. The gas collection system according to any one of claims 3 to 7 wherein said liquid comprises liquid contained separately of the source.

9. The gas collection system according to any one of claims 3 to 7 wherein said liquid comprises liquid constituted by the source.

10. The gas collection system according to claim 9 wherein the gas holder modules are configured to inhibit gas escaping upwardly from the source between adjacent gas holders.

1 1 . The gas collection system according to claim 10 wherein the gas holder modules are provided in first and second configurations, with the skirt of the module of the first configuration having a deflector portion arranged to extend into a position below the skirt of the module of the second configuration, whereby the deflector portion is operable to deflect rising gas within the source into the gas collection space of the module of the first configuration.

12. The gas collection system according to claim 8 wherein said liquid is contained within a trough system having an open top into which the lower ends of the gas holder modules extend for immersion in the contained liquid.

13. The gas collection system according to any one of the preceding claims further comprising retrieval means for retrieving collected gas from the gas collection space of each gas holder module.

14. A gas holder for a gas collection system according to any one of the preceding claims.

15. A cover and gas collection system for an anaerobic digester, the system comprising a plurality of modules each defining a separate gas holder, the plurality of gas holder modules cooperating to overlie a surface of material within the digester in which biogas is to be generated.

16. A gas holder for a cover and gas collection system according to claim 15.

17. A gas holder comprising a top portion, a side portion and a lower end, and a gas collection space, the lower end being open to receive gas uprising from a location below the below gas holder for entry into the gas collection space.

18. The gas holder according to claim 17 wherein the gas collection space is defined within the confines of the top and side portions.

19. The gas holder according to claim 17 or 18 wherein the side portion defines with the lower end.

20. The gas holder according to claim 17, 18 or 19 wherein side portion comprises a skirt depending from the top portion.

21 . The gas holder according to any one of claims 17 to 20 wherein lower end comprise an open bottom, whereby uprising gas enter the gas collection space through the open bottom.