FIELD OF THE INVENTION

[0001 ] The invention relates to a digestion tank. In particular, the invention relates, but is not limited, to a modular, demountable anaerobic digestion tank configured to process liquid waste or by-products such as effluent, or the like.

BACKGROUND TO THE INVENTION

[0002] Reference to background art herein is not to be construed as an admission that such art constitutes common general knowledge.

[0003] Historically, waste liquids, such as sewerage or industrial by products, were discharged into waterways such as rivers, lakes, or the sea. Such discharges can have serious environmental consequences and, over time, the waste liquids were treated prior to discharge. In many cases the liquid waste products were delivered to treatment ponds. The waste water can then be treated in the pond and waste by-products disposed of responsibly which is resource intensive and costly.

[0004] Anaerobic digestion is more recently utilised process to treat suitable liquid waste into one or more desirable products by providing an environment in which microorganisms can break down biodegradable materials in the liquid. For example, lagoons can be covered and made anaerobic to simultaneously improve the waste or by-product liquid and to generate useful gasses. For example, methane can be generated and utilised to provide energy, such as for a to provide a heat source and/or electricity generation.

[0005] A downside of anaerobic digesters is that they are costly and relatively permanent. This limits their uptake to long term larger scale operations. They are therefore more well suited to on-going waste water processing plants, such as for sewerage treatment, but in certain operations the need to treat waste water might be temporary. In such circumstances, it is impractical to construct a permanent anaerobic digestion plant. OBJECT OF THE INVENTION

[0006] It is an aim of this invention to provide a digestion tank which overcomes or ameliorates one or more of the disadvantages or problems described above, or which at least provides a useful alternative.

[0007] Other preferred objects of the present invention will become apparent from the following description.

SUMMARY OF INVENTION

[0008] In one form, although it need not be the only or indeed the broadest form, there is provided a digestion tank comprising: a demountable tank comprising a plurality of precast wall panels retained by a tensioning system to form a continuous wall; and a roof mounted on the continuous wall to fluidly seal a cavity formed between the roof, the wall panels, and the floor.

[0009] The roof may be flexible. The roof may comprise an inner roof and an outer roof. At least one of the inner roof and outer roof may be flexible. The roof may comprise one or more membranes. The roof may comprise an inner membrane. The roof may comprise an outer membrane. In a preferred form the roof is a double membrane roof comprising an inner membrane spaced apart from an outer membrane. The roof may comprise a flexible plastic or polymer material such as, for example, polypropylene.

[0010] The digestion tank may comprise a roof support member to support at least a portion of the roof. The support member may be in the form of a support column. The support column may comprise a central support column. The support column may extend from the floor to the roof. In a double membrane roof, the support column may extend from the floor to the inner membrane.

[0011 ] The wall panels may be made of a cementitious material. The wall panels may comprise precast concrete panels. The wall panels may be substantially planar. The wall panels may be rectangular. The wall panels may have side walls at a transverse angle to the plane of the panel. The side walls may be angled from 0° to 5° relative to the orthogonal axis of the plane of the panel. Adjacent wall panels of the continuous wall may directly abut each other. The wall panels may have at least one channel that receives the tensioning system. The continuous wall is preferably circular or, at least, substantially circular. The demountable tank may comprise a liner configured to fluidly seal the wall panels and floor of the demountable tank. In a preferred embodiment there are between 20-500 wall panels, more preferably between 20 and 400 wall panels, and even more preferably between approximately 25 and 300 wall panels.

[0012] The tensioning system may comprise a cable tensioning system. The cable tensioning system may comprise at least one cable extending around a plurality of the wall panels. The cable may be external to the wall panels, may be internal to the wall panels, or partially external and partially internal to the wall panels. The cable tensioning system may comprise a cable anchor at least one end of the cable. The cable tensioning system may comprise a cable anchor at each end of the cable. The tensioning system may comprise a single cable tensioning system extending the perimeter of the continuous wall. The cable tensioning system may comprise a plurality of cables each extending around a portion of the perimeter of the continuous wall. The continuous wall preferably has no support structures, such as buttresses, extending radially or perpendicularly to the wall panels.

[0013] The digestion tank is preferably an anaerobic digestion tank. The digestion tank is preferably configured to maintain a constant pressure between the inner roof and outer roof. The pressure is preferably constant regardless of the volume of liquid contained in the digestion tank. The constant pressure is preferably above atmospheric pressure.

[0014] A method of constructing a digestion tank comprising: erecting a plurality of precast wall panels at a site to form a continuous wall; tightening a tensioning system to hold the plurality of wall panels in an interference fit; constructing a roof over the continuous wall to fluidly seal a cavity formed between the roof, wall panels, and the floor.

[0015] The method may comprise levelling a ground surface to form the floor. The floor may be earthen. The method may comprise creating a pad for form the floor. The method may comprise creating a concrete pad. The method may comprise supporting the plurality of precast wall panels with temporary supports until the tensioning system is tightened to hold the wall panels in the interference fit. The method may comprise applying a liner to fluidly seal the wall panels and floor.

[0016] The method of constructing a digestion tank preferably comprises constructing the digestion tank as hereinbefore described.

[0017] Further features and advantages of the present invention will become apparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] By way of example only, preferred embodiments of the invention will be described more fully hereinafter with reference to the accompanying figures, wherein:

Figure 1 illustrates a cross sectional view of a digestion tank;

Figure 2 illustrates a perspective view of a demountable tank; and

Figure 3 a cross sectional view of adjacent wall panels of the demountable tank.

DETAILED DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 illustrates a digestion tank 10 having a wall 100 extending upwards from a floor 140, a roof mounted on the wall 100, comprising an outer membrane 200 and inner membrane 220, and a roof support member in the form of a central support column 300 that supports the inner membrane 220. The illustrated digestion tank 10 also comprises optional agitators 400 located in a lower region of the tank 10, to stimulate movement of liquids container therein, and an inspection platform and/or blower support 500 located outside the tank 10.

[0020] The digestion tank 10 has an inlet 160, a gas outlet 180, an effluent outlet 182, and a sludge discharge outlet 184. The inlet 160 is located in a lower region of the tank 10 to allow fluid communication of liquid to be processed 20, such as effluent or other biological or industrial waste water, to the inside of the tank 10. The gas outlet 180 is located in an upper region of the tank to allow fluid communication of gasses 40 that accumulate between the liquid 20 and inner membrane 220. The effluent outlet 182 and sludge discharge outlet 184 are located in a low region of the tank 10. It should be appreciated that the sludge discharge outlet could be via a sump (not shown).

[0021 ] Although the floor 140 is illustrated as being integral with the wall 100 in figure 1 , in preferred forms the wall 100 is independent, resting upon the floor 140. The floor 140 may be earthen or may comprise a pad depending on the size and nature of the tank 10 and surrounding environment (e.g. whether an earthen floor is suitable or otherwise).

[0022] Figure 2 illustrates the wall 100 during construction with optional temporary supports 102. The wall 100 is substantially circular comprising a plurality of precast wall panels 110 arranged adjacent one another. The panels 110 are planar such that each forms a circle segment. In the illustrated wall 100 there are approximately 200 wall panels 110. Once the wall 100 has been erected and retained by a tensioning system the temporary supports 102 can be removed and the wall 100 should be self standing.

[0023] As shown in figure 3, the wall panels 110 have side walls 1 12 that are at a transverse angle, preferably between 0° and 5° to an orthogonal axis of the plane of the panels 110. The wall panels 110 also have a channel 114 configured to receive a cable from a cable tensioning system. The tensioning cable (not shown) is passed through the channels 114 of adjacent wall panels 110 and tensioned at cable anchors 120 in selected wall panels 110. A plurality of tensioning cables, anchored by a plurality of cable anchors 120, may be provided to extend around the continuous wall. The angled side walls 112 allow the wall panels 110 to abut each other and provide support in a ‘keystone’ manner such that they cannot fall inward. The cable tensioning system retains the wall panels 110 in such an arrangement and provide support such that the wall panels cannot fall outward. In use, radial forces from liquid 20 contained therein provides a radial force against the wall panels 110. The tensioning system must therefore be of sufficient strength to not just hold the wall panels 110 in place in a freestanding manner, but also to be able to withstand the radial forces caused by the liquid 20 contained therein.

[0024] To construct the digestion tank 10, a suitable site is preferably first determined. The site may be prepared first, such as by levelling a ground surface and/or creating a pad to form a floor of the tank 10. One a suitable ground surface has been determined, a plurality of precast wall panels 110 can be erected to form a continuous wall 100. Temporary supports 102 may be utilised to support the wall panels 110 during construction. A tensioning system is then applied to the wall panels 110 and tightened to old the wall panels 110 in an interference fit.

[0025] A roof 200, 220 may then be constructed over the continuous wall 100 to fluidly seal a cavity formed between the roof 200, 220, wall 100, and floor 140. The outer roof 200 and inner roof 220 are preferably flexible. In preferred forms they may comprise a flexible plastics material such as polypropylene.

[0026] In this specification, adjectives such as first and second, left and right, top and bottom, and the like may be used solely to distinguish one element or action from another element or action without necessarily requiring or implying any actual such relationship or order. Where the context permits, reference to an integer or a component or step (or the like) is not to be interpreted as being limited to only one of that integer, component, or step, but rather could be one or more of that integer, component, or step etc. [0027] Advantageously, the digestion tank 10 can be constructed quickly and relatively cost effectively at a site in a modular and adaptable manner. It can be a temporary or permanent installation with casting performed off-site. Waste liquids, such as effluent or industrial wastes, can then be processed in the digestion tank 10, preferably through anaerobic digestion. Gasses, such as methane, can be extracted and used to provide energy such as, for example, to heat boilers or to generate electricity.

[0028] The above description of various embodiments of the present invention is provided for purposes of description to one of ordinary skill in the related art. It is not intended to be exhaustive or to limit the invention to a single disclosed embodiment. As mentioned above, numerous alternatives and variations to the present invention will be apparent to those skilled in the art of the above teaching. Accordingly, while some alternative embodiments have been discussed specifically, other embodiments will be apparent or relatively easily developed by those of ordinary skill in the art. The invention is intended to embrace all alternatives, modifications, and variations of the present invention that have been discussed herein, and other embodiments that fall within the spirit and scope of the above described invention.

[0029] As used herein, an element or operation recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural elements or operations, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.

[0030] In this specification, the terms ‘comprises’, ‘comprising’, ‘includes’, ‘including’, or similar terms are intended to mean a non-exclusive inclusion, such that a method, system or apparatus that comprises a list of elements does not include those elements solely, but may well include other elements not listed.