PRIORITY DOCUMENTS

[0001] The present application claims priority from Australian Provisional Patent Application No. 2020903532 titled "ORGANIC WASTE DIGESTER" and filed on 30 September 2020, the contents of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

[0002] The present disclosure relates to organic waste digestion and apparatus therefor. In a particular form, the present disclosure relates to methods and apparatus for digesting high- solids waste containing at least a portion of organic waste for the production of biogas.

BACKGROUND

[0003] The decay of organic matter, which results in the production of methane (CH4) and carbon dioxide (CO2) biogas, is a naturally occurring process. This process involves a relatively dependant series of biological and chemical reactions. The generation of biogas in such a way is commonly referred to as anaerobic digestion. Decay of complex organic matter (such as present in the organic portion of solid waste) to produce methane-rich biogas is not however, in itself, a wholly anaerobic process. It actually occurs through two separate phases, distinguished by the relative availability of oxygen. The first phase (phase 1) is where the decay occurs aerobically (by oxygen-requiring microorganisms) and is also sometime described as the hydrolytic phase. Although this phase does not result in the production of methane, it is an important precursor step as it breaks down complex organic structures into simpler compounds that can be subsequently degraded by anaerobic bacteria (phase 2) when oxygen becomes depleted. In this respect, the effectiveness of phase 1 will be rate limiting on the production of methane in phase 2.

[0004] The generation of biogas from municipal solid organic waste (MSOW) deposited into landfill (landfill gas) is an example of this process in a relatively uncontrolled manner. Whilst the generation of landfill gas is proven as an affordable approach to capturing energy from untreated and contaminated solid waste, it is also a form of disposal and is thereby considered the least desirable form of waste management. A more sustainable means of capturing energy from organics is via a more controlled process or in-vessel anaerobic digestion. This process has been most well developed for the treatment of low solids homogenous wastes, such as from agriculture or sewage. Where this process has been applied to the treatment of MSOW however, it has been found to be significantly more expensive than landfilling and more technically challenging than digestion of low solids homogenous wastes.

[0005] The technical challenges of in-vessel anaerobic digestion for processing MSOW are greater than traditional homogenous waste feedstocks given the inherent heterogeneity and physical contamination of MSOW. Digesters, such as those adapted for low solids wastes, or otherwise reliant on highly fluidised waste for mobility within the digester, also introduce further complications when managing MSOW, such as the generation of excessive volumes of contaminated leachates. High solids-batch dry digesters have been developed that are better suited to MSOW predominately for this reason. Although more robust and suited to MSOW, these dry batch digesters provide lower levels of process control. For example, there is limited control over the two separate stages of digestion as the process occurs entirely within a single chamber. These high solids digesters are also typically large structures, often loaded and unloaded by heavy machinery. As with other forms of digesters, these dry batch digesters require significant capital investment for the basic infrastructure and long lead times for development that also contributes to their limited scalability.

[0006] Current approaches to commercial digesters offer limited or no flexibility in size or scale and are also potentially not well suited to the complexity and variability of MSOW feedstocks. Where uncertainty in feed stock quantity and quality exist (as is common with MSOW) the ability to first pilot trial and then scale up utilising a replicable technology would be of significant benefit. However, this approach is not possible with currently available commercial digesters that lack fundamental scalability. [0007] Current dry batch digesters that might be better suited to MSOW also might not have the right process controls to best optimise biogas production.

[0008] Thus, there is a need to provide improved methods and apparatus for digesting waste having high solids contents, or to at least provide a useful alternative to existing systems.

SUMMARY

[0009] According to a first aspect, there is provided a solid organic waste digester module including: an upper container housing: a hopper for receiving solid organic waste and an aerator system for aeration of the organic waste; a lower container for housing a plurality of moveable crates, the lower container in communication with the hopper through a dump valve having a discharge opening, the lower container including: a mouth positioned underneath the discharge opening; a first moveable crate housed within the lower container; the first moveable crate moveable from a loading position below the mouth to a non-loading position, the nonloading position laterally displaced from the loading position such that a second moveable crate is positionable in the loading position.

[0010] In one form the lower container includes: an entrance opening at a first end; and an exit opening at a second end, the second end opposite the first end, the first and second openings opening laterally to allow lateral entry and exit of the moveable creates respectively.

[0011] In one form the aerator system includes a blower and an outlet, the blower located within the upper container and the outlet opening into the hopper.

[0012] In one form the entrance opening includes an entrance door, and the exit opening includes an exit door, the doors both having an open condition allowing lateral movement of the moveable containers there-through and a closed condition substantially reducing free flow of gases into and out of the lower container.

[0013] In one form the upper container and the lower container are separable for transport.

[0014] In one form the digester module further includes: an offtake gas reticulation system, wherein the upper container is adapted for primarily aerobic digestion and the lower container is adapted for primarily anaerobic digestion, and wherein, in use, the gas reticulation system feeds CO2-rich gas produced within the upper container to displace oxygen containing air within the lower container thereby facilitating anaerobic digestion.

[0015] In one form the offtake gas reticulation system includes: an offtake gas extraction port in fluid communication with the hopper; an offtake gas feed port in fluid communication with the lower container; and a piping, valving and associated control system for selectively directing gas from the extraction port: to the feed port, or to by-pass the feed port, depending on control parameters including composition of offtake gas.

[0016] In one form the first moveable crate includes: a surround structure; and a base, the base removably connected to the surround, wherein the crate has an assembled condition in which the base is connected to the surround and a disassembled condition in which the base is spaced apart from the surround.

[0017] In one form the surround includes: an upper portion, the upper portion defining a plurality of apertures through which biogas can be vented; and a lower portion, the lower portion arranged and constructed to direct a percolate down towards the base. [0018] In one form the base includes a percolate outlet.

[0019] In one form the lower container includes a longitudinally disposed gutter, the gutter for receiving the percolate draining from the percolate outlet(s).

[0020] In one form the digester module further includes: a percolate system, the percolate system including: a percolate header tank within the upper container; a sprinkler system and a collection system within the lower container, the sprinkler system feed by the percolate header tank within the upper container.

[0021] In one form the collection system includes a longitudinal disposed gutter and the moveable crates, whereby the moveable crates are constrained to move within the lower container such that the outlets of the crates remain positioned above the gutter allowing continuous collection of percolate.

[0022] According to a second aspect, there is provided a digester system including a plurality of digester modules as described above.

[0023] According to a third aspect, there is provided a method of digesting organic waste including the steps of: depositing a first charge of waste in a hopper; retaining the first charge of waste in the hopper for a first period of time Tl; releasing the first charge of waste from the hopper such that it falls under gravity into a first moveable crate positioned underneath the hopper and within a lower container; displacing oxygen from within the lower container to promote anaerobic digestion; and laterally displacing the first moveable crate from underneath the hopper to a first storage position within the lower container. BRIEF DESCRIPTION OF DRAWINGS

[0024] Embodiments of the present disclosure will be discussed with reference to the accompanying drawings wherein:

[0025] Figure 1 is an isometric view of a solid organic waste digester module according to the disclosure;

[0026] Figure 2 is a diagrammatic partially exploded cross-sectional view of the digester module of Figure 1;

[0027] Figure 3 is an cutaway isometric view of a lower container portion of the digester module of Figure 1;

[0028] Figure 4 is an end view of the lower container portion of the digester module of Figure 1;

[0029] Figure 5 is a diagrammatic cross-sectional side view of the digester module of Figure 1 showing an aerator system;

[0030] Figures 6, 7 and 8 are isometric views of a crate forming part of the digester module of Figure 1;

[0031] Figure 9A is a similar isometric view to that of Figures 6, 7 and 8, but shows a base of the crate separated from a surround of the crate;

[0032] Figure 9B shows the base of the crate separated from a surround of the crate with a treated waste pile ready for disposal or post-digestion use;

[0033] Figure 10A is a similar diagrammatic cross-sectional side view to that of Figure 5, but shows an offtake gas reticulation system sending offtake gas to a biofilter;

[0034] Figure 10B is a similar diagrammatic cross-sectional side view to that of Figure 10A, but shows offtake gas with high methane concentration being fed to a generator or gas offtake system;

[0035] Figure 10C is a similar diagrammatic cross-sectional side view to that of Figure 10A, but shows a high flow gas purging system;

[0036] Figure 11 is a similar diagrammatic cross-sectional side view to that of Figure 10, but shows a water feed and percolate system;

[0037] Figure 12 is a again a similar diagrammatic cross-sectional side view to that of Figure 5, but shows the aeration system, the offtake gas reticulation system, the purge system and the water feed and percolate system in a single view; [0038] Figure 13 shows the digester module of Figure 1 with a truck 5000 in combination with a hopper loading arrangement;

[0039] Figure 14 is a process diagram illustrating organic waste digestion facilitated by the embodiments of the present disclosure; and

[0040] Figures 15 and 16 are isometric views of a solid organic waste digester module according to the disclosure similar to that of Figure 1, but also showing optional loading and unloading systems.

DESCRIPTION OF EMBODIMENTS

[0041] Referring to Figures 1, 2, 3, 4 and 5, an organic waste digester module 1 according to a first embodiment of the disclosure is shown. The digester module includes an upper container 100 housing a hopper 10 for receiving organic waste. An aerator system 300 for aeration of the organic waste is also included in the upper container as can be seen in Figure 5. The aerator system includes a blower 350 and an outlet 310, the outlet 310 opening into the hopper 10. In the embodiment illustrated, the aerator system includes a blower 350 that is also housed within the upper container 100. In other embodiments, the aerator system of the upper container 100 may simply include the outlet 310 and an air reticulation system with a blower or other means of inducing air flow external to the digester module 1.

[0042] The blower 350 receives air via an air inlet 305 connected to air inlet pipe 304 as is illustrated in Figure 10A.

[0043] Referring to Figure 1, a mesh or similar form of open screen panel (or mesh) 145 is provided to enclose portions of the upper container, while still allowing ventilation.

[0044] A lower container 200 for housing a plurality of moveable crates is also included in the digester module. Again referring to Figure 2, it can be seen that the lower container 200 is in fluid communication with the hopper 10 through a dump valve 12 having a discharge opening 16. As shown in Figure 3, a mouth 208 within the lower container 200 is positioned underneath the discharge opening 16, in use the mouth 208 receiving solid organic waste that has been processed within the upper container 100 through the discharge opening 16. [0045] A first moveable crate 210 housed within the lower container is also provided, as shown in Figures 1 and 5 for instance. The first moveable crate 210 is moveable from a loading position 290, shown in Figure 2, below the dump discharge to a non-loading position 291, the non-loading position laterally displaced from the loading position such that a second moveable crate 220 is positionable in the loading position as is shown in a transition from Figure 2 to Figure 3.

[0046] Referring again to Figure 3, it can be seen that lower container 200 has an entrance opening 205 at a first end 201; and an exit opening 295 at a second end 299, the second end opposite the first end. The first and second openings open laterally to allow lateral entry and exit of the moveable crates respectively. The entrance opening 205 includes an entrance door 202, and the exit opening includes an exit door 292, the doors both having an open condition allowing lateral movement of the moveable containers there-through and a closed condition substantially reducing free flow of gases into and out of the lower container. As is illustrated in Figures 1 and 4, the doors may be standard shipping container doors such as the doors 202 and 292 illustrated in Figure 1. Any doors that are sufficiently gas tight for the digester module to function may be used.

[0047] In other embodiments, vertical or horizontal sliding doors may be provided as an alternative to the hinged shipping container doors 202 and 292 shown in Figure 1.

[0048] As described above, the digester module 1 is made up of two distinct digester components being the upper container 100 and the lower container 200. The top mounted upper container 100 is the receiving point for input of MSOW. The upper container includes the hopper 10 that also provides the chamber 13 in which the first aerobic stage of digestion (stage 1 digestion) is undertaken. The lower container 200 is where the second anaerobic digestion stage occurs (stage 2: digestion). Functionally the two stages are joined in a gastight manner with an opening between them that can be closed with a gas-tight dump valve 12.

[0049] It should be understood that 'gas-tight', when used in this specification, refers to a condition which is functionally gas-tight, in the sense that at the low pressures of normal operation, gas leakage will be small as a proportion of gas passing to the capture and purging systems.

[0050] A second gas-tight valve 14, housed within a housing 14' as shown in Figure 1, is located at a top of the upper container 100. When open, this valve 14 provides the external loading point for the MSOW.

[0051] The modular design of the digester module is of great practical benefit, allowing the upper container and the lower container to be separated for transport. Once in the field, the two modules provide a self-contained system with minimal external services required. For example, in the embodiment illustrated, the aerator system, including the blower 350 and an outlet 310, the blower 350 (fan) is located within the upper container. Also, the offtake gas reticulation system, including piping, valving and an associated control system is all located within the upper container 100. Further, the percolate system includes a percolate header tank 160 within the upper container and a sprinkler system and a collection system within the lower container. Therefore the digester module 1 can easily and quickly be deployed at a waste site with minimal services connections required.

[0052] Referring to Figure 6, which is a similar view to that of Figure 4, it can be seen that with this embodiment the upper container and the lower container of the digester module are both in the form of shipping containers stacked on top of each other. Together the two shipping containers form a single digester module.

[0053] The digester module includes an offtake gas reticulation system as can be seen in Figures 10A, 10B and 12. The upper container is adapted for primarily aerobic digestion and the lower container is adapted for primarily anaerobic digestion. In use, the gas reticulation system feeds CO2-rich gas produced within the upper container to displace oxygen containing air within the lower container thereby facilitating anaerobic digestion.

[0054] Again referring to Figures 10A, 10B and 12, it can be seen that the offtake gas reticulation system includes: an offtake gas extraction port 604 in fluid communication with the hopper; an offtake gas feed port 620 in fluid communication with the lower container; and a piping, valving and associated control system for selectively directing gas from the extraction port: to the feed port, or to by-pass the feed port, depending on control parameters including composition of offtake gas. The valving includes control valves 650, 660, 670 and 675 shown in Figures 10A, 10B and 12.

[0055] An offtake gas extraction port 622 opening into the lower container 200 is also provided, as is shown in Figure 10B. Finally, a control valve 715 connected to a continuous gas analyser is located near the outlet of the extraction port 714, as shown in Figure 10C

[0056] The time required to achieve initial aerobic digestion in the hopper 10 (hours to days) is short relative to the time required to achieve good anaerobic digestion within the lower container 200 (weeks to months). Therefore the lower container 200, where anaerobic digestion occurs, is designed to hold a larger volume of (partially processed) organic waste than the hopper 10 in the upper container 100. In order to hold and manage the partially processed organic waste in the lower container 200, five moveable crates 210, 220, 230, 240, 250 are provided, as is shown in the cutaway drawing of Figure 3. The first moveable crate 210 includes: a surround structure 260; and a base 270, the base 270 removably connected to the surround structure 260. The crate 210 has an assembled condition in which the base 270 is connected to the surround structure 260 and a disassembled condition in which the base 270 is spaced apart from the surround structure 260. In other embodiments, the sizes of the crates maybe varied so that a smaller or larger number are able to fit within the lower container 200 (e.g 3 or 6 crates).

[0057] The surround includes: an upper portion 262, the upper portion 262 defining a plurality of apertures 140 through which biogas can be vented; and a lower portion 267, the lower portion 267 arranged and constructed to direct a percolate down towards the base 270. 35mm thick FRP grating is used to provide the apertures 140. Other types of mesh or perforated materials may be used in other embodiments.

[0058] Referring to Figure 8, it can be seen that the base 270 includes a percolate outlet 275 and the lower container 200 includes a longitudinally disposed gutter 282. The gutter 282, shown in Figure 4, receives the percolate draining from the percolate outlet(s) 275.

[0059] As can be seen in Figure 9A, pins 269 are provided to releaseably join the surround structure 260 to the base 270. This allows contained and safe unloading. A forklift truck can lift the surround off the base, utilising channels 268 within the upper portion 262 as shown in Figure 8, leaving spadeable processed waste on the base 270 that is easily disposed of after complete digestion process, as is illustrated in Figure 9B. Again referring to Figure 8, it can be seen that channels 278 are provided within the base to facilitate easy forklift handling.

[0060] Each of the other crates 220, 230, 240 and 250 are identical to crate 210.

[0061] Referring to Figure 11, a percolate system that also forms part of the digester is shown. The percolate system includes; a percolate header tank 160 within the upper container 100; a sprinkler system and a collection system within the lower container 200, the sprinkler system feed by the percolate header tank(s) 160 within the upper container 100.

[0062] The collection system includes a longitudinal disposed gutter 282 shown in Figure 4 and the moveable crates, whereby the moveable crates are constrained to move within the lower container 200 such that the outlets 275 of the crates remain positioned above the gutter 282 allowing continuous collection of percolate. The gutter 282 has a gutter end 283 joined to a sump 284 as can be seen in Figure 3. The sump forms a collection point for pumping the percolate up to the percolate header tank(s) 160 within the upper container 100. A sump pump may be used and connected to a flexible hose leading up to the percolate tank(s) 160. Alternatively, other suitable plumbing and pumping arrangements may also be used.

[0063] Figure 14 shows the digester module of Figure 1 with a truck 5000 in combination with a hopper loading arrangement. The truck 500 is shown loading a load of waste for discharge into the hopper intake mouth 112 of the hopper 10. [0064] Referring to Figures 15 and 16, a loading system that can be used with the digester module 1 is shown. The loading system includes a track 910 upon which a bin loader assembly 820 travels. The bin loader assembly 820 receives a bin 810 and lifts the bin 810 upwards, as is shown in Figure 17. The bin loader assembly 120 tips the bin 810 into the hopper intake mouth 112 thereby loading organic waste into the digester 1.

[0065] The track 910 also has an associated carriage 920 conveying and loading crates, such as the crate 250 shown in Figures 15 and 16.

[0066] A similar track 960 is provided at the opposite end of the digester 1 as is shown in Figures 15, 16 and 17. Again, the track 960 has an associated carriage 970 to facilitate easy unloading of the crates, such as crate 210 illustrated in Figure 16.

[0067] In other embodiments, alternate loading and unloading systems may be used.

[0068] The description of the present disclosure above, with reference to Figures 1 to Figure 13, describes the operation of a single digester module. In this condition it may be used as either a small capacity digester, or as part of a dedicated anaerobic digestion pilot system. The system's capacity can be increased or scaled up with the addition of further modules with identical operating systems. For example, a further digester module 1 can be provided with rows of modules forming larger digester system. Such a digester system comprising many modules may include conveyor systems for loading and unloading and may include a roof structure.

[0069] The digester module 1 is loaded incrementally with sub-batches, with the volume of each sub-batch limited by the usable volume of the hopper 10. An illustrative example of loading is shown in Figure 15 where a truck 5000 delivers waste. In other embodiments, loading occurs using bins, or other vessels, lifted up and tipped into the hopper 10 as shown in Figure 1.

[0070] Following digestion within the hopper 10, each sub-batch is released under gravity from the upper container 100 to the lower container 200 via the opening of the gas-tight dump valve 12. The material released from the hopper 10 of the upper container 100 is captured within moveable crate 210, or basket 210. As such, the holding capacity of the crate 210 is no less than the volume of the hopper 10. The lower container 200, which may be in the form of a standard shipping container for example, is able to hold multiple subbatches in respective multiple crates 210, 220, 230, 240, 250 which is reflective of the difference of time required to achieve initial digestion in the hopper 10 (hours to days) versus primarily anaerobic digestion (weeks to months) within the lower container 200.

[0071] An advantage of embodiments of the disclosure as described herein and illustrated in the Figures is a process that can treat multiple small sub-batches simultaneously. This provides for a high level of process control over "stage 1 digestion" that takes place in the upper container for each sub-batch. Such batch management also provides a high degree of segregation of MSOW from differing sources and/or contamination levels while simultaneously digesting within a single module, thus maximising the value of the residual components for further use (such as for compost). It also allows for contaminated subbatches to be separated if required allowing a very high level of quality control to be achieved.

[0072] The crates 210, 220, 230, 240, 250 are constructed to allow both containment of the solid waste and also the ability for biogas generated from the waste contained within them to be freely vented to the lower container 200 (digester). The crates 210, 220, 230, 240, 250 are on rollers 508 that may either be fixed to the base of the lower container 200 or the crates themselves. This facilitates ease of loading and unloading.

[0073] The crates may be connected to each other for ease of transport within the lower container 200. Various means may be used to move the crates in and out along the rollers 508 of the digester (as shown in Figure 2). For example, a winch or other conveyance system may be provided.

[0074] Operationally, the material is loaded into the digester via the upper hopper valve 14 with the dump valve 12 at the bottom of the hopper 10 closed. Once the chamber 13 of the hopper 10 is loaded, the upper hopper valve 14 is closed and the organic material is aerated with the air pump (blower) 350 directing air into the organic waste via at least one aerator outlet 310 and through perforations built into the hopper 10.

[0075] By aerating this waste, natural processes of aerobic microbial decomposition are activated that amongst other things results in the production of thermophilic temperatures within the organic waste mass. By sustaining these conditions for an appropriate period of time the objective of stage 1 treatment (hydrolysis and partial pasteurisation of the MSOW feedstock) can be obtained. Quality assurance of this step will be provided via continuous temperature and moisture monitoring of the waste mass.

[0076] The intent of the embodiments described is to improve the optimisation of this phase over existing similar technologies in order to improve biogas generation rates through the partial hydrolysis of the feedstock. By pasteurisation of the waste mass it will also improve final organic product quality, in respect of the efforts and cost required for further processing that would render it suitable for agricultural use. Advantageously, this first phase within the upper container 100 can be applied to sub-batches providing a relatively high degree of monitoring and control at a very discrete level.

[0077] At or prior to the completion of stage 1 digestion, a crate 210 (or 220, 230, 240, 250) is placed in the lower container 200 centred below the discharge opening 16. The crate 210 (or 220, 230, 240, 250) will enter the stage 2 digestion area within the lower container 200 via the door 202. With the first crate 210 located in a receiving position under the discharge opening 16, the now aerated waste is released from the hopper 10 by opening the dump valve 12. With the crate 210 now fully in the lower container 200, it can be moved forward, allowing room for the next crate 220 to be added. This process of crate filling is repeated until the digester is loaded to capacity (as shown in Figure 3). A feature of this design is that the handling process minimises or eliminates the need for human or machine contact (physical material handling) with the waste once placed into the digester module.

[0078] During the stage 1 operation of the system within the upper container 100, gas generated from the aerobic digestion of waste will be collected through a gas port 604 at the top of the hopper, as shown in Figure 10A. This gas will be directed under vacuum to a first control valve 610. A sensor on the control valve will measure gas concentrations (nominally CO2 and/or 02). Where gas concentrations detected at or near the control valve 610 is outside of a permitted range, the gas will be directed using the control valves 610, 650, 660 670 and 675 to the off-gas disposal piping 611 to a biofilter connected to offtake port 630 (or other treatment system) as is shown in Figure 10A. Where the gas concentrations are within the permitted range at or near the control valve 610, the gas will be directed to the lower container 200 via piping 612 to offtake gas feed port 620, as is illustrated in Figure 10B. By directing the CO2 rich "off-gas" into the lower container it can be used to assist in conditioning the container 200 to anaerobic conditions (oxygen starvation), whilst also providing a heat source to warm the lower container 200 to assist in achieving or maintaining thermophilic conditions within it (conditions known to best favour biogas production).

[0079] From stage 1 within the upper container 100 to stage 2 within the lower container 200, anaerobic digestion will be initiated through both the MSOW entering the oxygen- starved conditions in the stage 2 digestion within the lower container 200.

[0080] Digestion within the lower container 200 may be further enhanced by the application of alkaline solution or other soluble substrates within irrigation water supplied through percolate sprinkler system 180, illustrated in Figure 11. This percolate irrigation solution may be added at the time of or in conjunction with material being released from the upper container 100, which also provides for a hopper washing functionality using an upper sprinkler system 183 as shown in Figure 3. It also provides the opportunity for moisture to be added to the organic mass, along with supplementary nutrients or microbial inoculum to further enhance the production of biogas. For this process all the aforementioned amendments to the irrigation water (and any others taken) are based on readily available (published) scientific and/or engineering knowledge of anaerobic digestion.

[0081] Once in the lower container 200, MSOW is held in the perforated crates 210, 220, 230, 240, 250 within the sealed volume of the lower container 200 until biogas production has substantially passed peak production. This will be assessed by continuous monitoring of gas composition and biogas flow rate through the extraction port 714 of the lower container 200. In order to monitor the environment within the digester and control the quality of gas being fed to any utilisation device where minimum calorific values may be required, an automated control valve 715 connected to a continuous gas analyser is located near the outlet of the extraction port 714, as shown in Figure 10C. This control valve monitors and reacts to gas concentrations, with sub-optimum gases directed to the off-gas disposal piping 716 (to a biofilter or other treatment) and optimum gases directed to gas recovery piping. Off-gas disposal piping 716, connected to an air pump (blower) 750 as shown in Figure 10, also provides a method of venting the lower container 200 when the door 202 or 292 is open to reduce the expression of odour or noxious gases to those in the vicinity of the crate, or under any other emergency situation. This venting phase will be actuated through interlocks that sense opening or unlocking of the door 202 or 292.

[0082] Any water released during the digestion process (or other percolate gathered) will be collected by a drainage or collections system 190 installed into the base of the lower container 200 as is shown in Figure 11. This percolate will be channelled into collection points and directed through common plumbing (including the longitudinal gutter 282 draining to the sump 286 as described above). Harvested percolate recovered through percolate intake line 168 will then be pumped back to a central storage tank or header tank, such as the percolate tank(s) 160 shown in various Figures including Figure 1 and Figure 11. From the header tank 160, the percolate can be directed back into the MSOW batches via an interior roof mounted irrigation system within the lower container 200 allowing a semi- continuous or controlled irrigation cycles to be undertaken. Supplementary water, chemicals, nutrients or microbial inoculum to further enhance the production of biogas may also be added at this stage to further enhance biogas production via the irrigation percolate.

[0083] At the completion of the anaerobic stage of digestion (after several weeks of containment) the digester can be emptied for re-use. Emptying begins by opening door 202 at the load end (or from door 292 if fitted at the end opposite the load end) which also actives a purge cycle which controls the release of fugitive gas from the door opening and rapidly shuts down the anaerobic process by introducing air (oxygen). This is shown in Figure 10C. During this purge cycle, the hopper valve 14 is open, as is the dump valve 14, allowing air to be drawn down into the lower container 200 and then out through the purge outlet 730 under vacuum, as illustrated by the large arrows on Figure IOC.

[0084] Now referring to Figure 15, a process diagram illustrating organic waste digestion facilitated by the embodiments of the present disclosure described above is provided.

[0085] A method of digesting organic waste, according to further aspect of the disclosure, includes the steps of:

- depositing a first charge of waste in a hopper. This first charge could be municipal solid organic waste for instance. The municipal solid organic waste may be contaminated with a proportion of non-organic waste;

- retaining the first charge of waste in the hopper for a first period of time Tl. This time will typically be relatively short, in the order of hours or days;

- releasing the first charge of waste from the hopper such that it falls under gravity into a first moveable crate positioned underneath the hopper and within a lower container;

- displacing oxygen from within the lower container to promote anaerobic digestion; and

- laterally displacing the first moveable crate from underneath the hopper to a first storage position within the lower container.

[0086] The above method may include the step of aeration of the charge of waste within the hopper so as to facilitate anaerobic digestion.

[0087] Embodiments of the disclosure are modular self-contained, two-stage dry batch digesters. Being modular and self-contained device, it can be scaled from pilot capacities to any required scale through the addition of any number of further modules. Embodiments are preferably made from shipping containers, however other methods of construction that afford an equivalent flexibility can also be used.

[0088] An advantage of the construction methodology of embodiments of the disclosure are that they may be substantially constructed off-site and require no, or limited, site preparation work to be installed in the intended location. Also the method of construction provides a relatively light weight and sturdy structure that can be located on ground of low stability such as closed landfills and thus be able to provide beneficial use of otherwise wasted land.

[0089] Throughout the specification and the claims that follow, unless the context requires otherwise, the words "comprise" and "include" and variations such as "comprising" and "including" 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.

[0090] In some cases, a single embodiment may, for succinctness and/or to assist in understanding the scope of the disclosure, combine multiple features. It is to be understood that in such a case, these multiple features may be provided separately (in separate embodiments), or in any other suitable combination. Alternatively, where separate features are described in separate embodiments, these separate features may be combined into a single embodiment unless otherwise stated or implied. This also applies to the claims which can be recombined in any combination. That is a claim may be amended to include a feature defined in any other claim. Further a phrase referring to "at least one of" a list of items refers to any combination of those items, including single members. As an example, "at least one of: a, b, or c" is intended to cover: a, b, c, a-b, a-c, b-c, and a-b-c.

[0091] The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement of any form of suggestion that such prior art forms part of the common general knowledge.

[0092] It will be appreciated by those skilled in the art that the disclosure is not restricted in its use to the particular application or applications described. Neither is the present disclosure restricted in its preferred embodiment with regard to the particular elements and/or features described or depicted herein. It will be appreciated that the disclosure is not limited to the embodiment or embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope as set forth and defined by the following claims.