Waste digestion

Abstract

Removing inorganic particles from a digester (10 figure 1) for digesting organic waste, the digester comprising at least an aerobic digestion vessel (14 figure 1) and an anaerobic digestion vessel 16.

Description

FIELD OF THE INVENTION The present invention relates to methods and apparatus for digesting organic waste materials. In particular, the present invention relates to methods and apparatus for removing inorganic particles from a digester. BACKGROUND TO THE INVENTION Organic waste materials are regularly produced by abattoirs, farms, food-processing plants and households. Such waste may be treated by a digestion process to render it suitable for further use or for discharge into the environment, and to produce useful by-products. Organic waste materials may be digested under anaerobic conditions in a single-stage digestion vessel, to produce useful methane-containing biogas, and solids and / or liquids waste materials which are environmentally more acceptable than the organic waste feedstock materials. Other suitable digestion processes may involve two or more stages. In a first, fermentation stage, waste is digested under aerobic conditions in a solids digestion vessel to produce a treated solids waste material and a liquid digestate fraction. The liquid digestate is then digested further in a second, anaerobic stage, to produce methane gas. Some of the liquid digestate may be drawn off and added to a subsequent load of organic waste material feedstock prior to the fermentation stage. It has been found that using a separate anaerobic stage in this way considerably improves the yield of useful biogas. A significant problem with the digestion of organic waste is the accumulation in digestion vessels of inorganic particulate material (which may be known as ‘grit ’in the waste digestion industry). Over time, the accumulation of grit reduces the useful volume of digestion vessels, resulting in a reduction in output and efficiency of a digester. Periodically therefore, it is often necessary for digestion vessels to be opened, the digestate emptied, and the grit to be removed, which means that the digestion process is interrupted. In some digesters, mechanical apparatus such as an auger may be used to remove grit from a digestion vessel, but this approach may only be suitable for aerobic digestion vessels which need not be sealed, and can add expense and complexity to the manufacture and operation of digestion apparatus. Emptying of a single-stage digester, or the aerobic digestion vessel of a two-stage digester, means that biogas production is halted for a period of time, and further time may be lost in re-establishing the anaerobic digestion process following grit removal. It is against this background that the present invention is devised. It is an object of the present invention to overcome at least one problem associated with the prior art, whether referred to herein or otherwise. SUMMARY OF THE INVENTION According to a first aspect of the invention there is provided a method of removing inorganic particles from a digester for digesting organic waste, the digester comprising at least an aerobic digestion vessel and an anaerobic digestion vessel, the anaerobic digestion vessel being arranged to receive at least some digestion liquid from the aerobic digestion vessel, and the method comprising: allowing inorganic particles of a first type to accumulate in the aerobic digestion vessel and removing the inorganic particles of the first type from the aerobic digestion vessel; allowing inorganic particles of a second type to accumulate in the anaerobic digestion vessel and providing a plurality of pulses of digestion liquid into the anaerobic digestion vessel, thereby to suspend the inorganic particles of the second type in the digestion liquid in the anaerobic digestion vessel; and removing the suspended inorganic particles of the second type from the digester by removing a portion of the digestion liquid containing inorganic particles of the second type from the anaerobic digestion vessel; wherein the inorganic particles of the second type have a smaller average particle size than an average particle size of the inorganic particles of the first type. Advantageously, by removing a portion of inorganic particles (grit) from an aerobic stage of the digester prior to an anaerobic digestion stage, inorganic particles which accumulate subsequently at the anaerobic stage have properties which allow them to be removed from the anaerobic stage without the need to interrupt the anaerobic digestion process. In this way, efficiency of methane-rich biogas production may be maintained by preventing build-up of inorganic particles in the anaerobic stage and therefore biogas can be produced continuously without interruption. Preferably, the inorganic particles of the second type have a lower density than the inorganic particles of the first type. The inorganic particles of the second type may have a lower bulk density than a bulk density of the inorganic particles of the first type. Preferably, the inorganic particles of the second type are removed from the digester directly with liquid from the anaerobic digestion vessel and the inorganic particles of the second type are not returned to any vessel of the digester. Preferably, the inorganic particles of the second type are removed from the digester directly with liquid from the anaerobic digestion vessel (for example for disposal or end use) and the inorganic particles of the second type are not returned to the aerobic digestion vessel or a reception vessel (where provided) of the digester. Preferably, during intervals between the pulses of liquid, substantially no digestion liquid enters the anaerobic digestion vessel. Preferably, during intervals between the pulses of liquid, substantially no digestion liquid enters the anaerobic digestion vessel from the aerobic digestion vessel. At least a portion of the inorganic particles of the second type may be formed by precipitation of inorganic substances dissolved or suspended in liquid in the anaerobic digestion vessel. The inorganic particles of the second type may form a bed on a base of the anaerobic digestion vessel. Preferably the step of removing the inorganic particles of the second type comprises using each pulse of liquid to suspend at least part of the bed, thereby to remove the inorganic particles with a portion of liquid digestate from the anaerobic digestion vessel. The inorganic particles of the second type may form a silt-like layer or fluidised bed at the bottom of the anaerobic digestion vessel. Removing the inorganic particles of the second type may comprise feeding liquid into the anaerobic digestion vessel to disturb the fluidised bed, thereby causing a portion of the inorganic particles of the second type to be displaced from the anaerobic digestion vessel. Preferably the step of removing inorganic particles of the first type comprises removing mechanically the inorganic particles of the first type from the aerobic digestion vessel. For example, the inorganic particles may be removed from the aerobic digestion vessel by digging, or by tipping the aerobic digestion vessel, or by using an auger or pump. Removing inorganic particles of the first type may comprise opening the aerobic digestion vessel. The digester may comprise a reception vessel for receiving organic waste material prior to transferring the organic waste into the aerobic digestion vessel and the method may comprise removing inorganic particles of the first type from the reception vessel. The organic waste may be treated in the reception vessel and subsequently transferred from the reception vessel to the aerobic digestion vessel. In some embodiments, organic waste material may be loaded directly into the aerobic digestion vessel. In other words, in some embodiments, the reception vessel and aerobic digestion vessel may be provided by the same single vessel. That is, the aerobic digestion vessel may also be the reception vessel such that aerobic digestion is completed in the reception vessel. The inorganic particles of the first type may comprise inorganic particles of a first sub-type and inorganic particles of a second sub-type. Preferably the step of removing inorganic particles of the first type from the reception vessel comprises removing inorganic particles of the first sub-type from the reception vessel. Preferably the step of removing inorganic particles of the first type from the aerobic digestion vessel comprises removing inorganic particles of the second sub-type from the aerobic digestion vessel. At least a portion of the inorganic particles of the second sub-type may be formed by precipitation of inorganic substances dissolved and / or suspended in liquid in the anaerobic digestion vessel. The inorganic particles of the second sub-type have a greater density than the inorganic particles of the first sub-type and a greater density than the inorganic particles of the second type. The inorganic particles of the second sub-type may have an average particle size of between about 0.1 mm and about 5 mm. The inorganic particles of the second type may have an average particle size of between about 1 mm and about 5 mm. The digester may comprise an intermediate digestion vessel (preferably arranged between the aerobic digestion vessel and the anaerobic digestion vessel), wherein digestion liquid from the aerobic digestion vessel is directed to the intermediate digestion vessel for digestion and subsequently directed to the anaerobic digestion vessel. In this case the method may comprise removing inorganic particles of the second sub-type from the intermediate digestion vessel. The inorganic particles of the second type may have an average particle size of less than 1 mm. The inorganic particles of the second type may have an average particle size of less than 0.1 mm. An interval between each pulse of liquid may be between about 30 minutes and about 36 hours. For example, a pulse of liquid may be fed into the anaerobic digestion vessel once per day (i.e. once per twenty four hours). Between about 1 and 100 pulses of liquid may be fed into the anaerobic digestion vessel per day, for example 5 to 25 pulses per day. Preferably a volume of each pulse of liquid is between about 0.1% and about 20% of the volume of the anaerobic digestion vessel. More preferably a volume of each pulse of liquid is about 0.5% of the volume of the anaerobic digestion vessel. In a preferred embodiment, the volume of each pulse is about 0.2% of the volume of the anaerobic digestion vessel and a plurality of pulses per day are fed into the anaerobic digestion vessel, such that a volume of liquid approximately equal to the total volume of liquid in the anaerobic digestion vessel passes through the anaerobic digestion vessel about every 15 to 25 days. In this way, inorganic particles may be removed from the anaerobic digestion vessel in batches. Preferably, removal of inorganic particles of the second type is a batch process. The method may comprise providing, proximate a floor of the anaerobic digestion vessel, a heating arrangement configured to generate convection currents in the liquid in the anaerobic digestion vessel, thereby to cause movement of inorganic particles of the second type across the floor of the anaerobic digestion vessel. The inorganic particles of the first type may comprise stones, sand, metal items, bone fragments and other entrained materials contained in the organic waste. The inorganic particles of the second type may comprise chemically or biochemically formed deposits formed in situ in the aerobic digestion vessel and carried into the anaerobic digestion vessel in digestion fluid, and / or formed in situ in the anaerobic digestion vessel. Preferably the aerobic digestion vessel is a liquids digestion vessel in which the organic waste comprises solids materials. Preferably the aerobic digestion vessel is arranged to receive solids waste materials or a mixture of solids and liquids waste materials. Preferably the anaerobic digestion vessel is a liquids digestion vessel in which the organic waste is in a liquid phase. Preferably the method comprises feeding at least a part of the digestion liquid from the anaerobic digestion vessel to the reception vessel and / or to the aerobic digestion vessel to provide a source of bacterially active waste from the anaerobic digestion vessel to the reception vessel and / or to the aerobic digestion vessel. The aerobic digestion vessel may comprise a mobile or portable vessel such as a skip or trailer. The step of removing inorganic particles of the first type may comprise tipping the mobile vessel to empty the inorganic particles of the first type from the mobile vessel. According to a second aspect of the invention there is provided apparatus for digesting organic waste, the apparatus comprising: an aerobic digestion vessel; an anaerobic digestion vessel arranged to receive digestion liquid from the aerobic digestion vessel and comprising an outlet for removing liquid digestate and inorganic particles from the anaerobic digestion vessel; a pulsing arrangement configured to provide a plurality of pulses of digestion liquid into the anaerobic digestion vessel, such that inorganic particles are removed from the anaerobic digestion vessel via the outlet together with a portion of the liquid digestate. Preferably the anaerobic digestion vessel comprises an inlet through which the pulses of digestion liquid enter the anaerobic digestion vessel, and preferably the inlet is disposed proximate a base of the anaerobic digestion vessel such that each pulse of liquid is directed across the base of the anaerobic digestion vessel. Preferably the apparatus comprises a reception vessel arranged to receive organic waste prior to transferring the organic waste to the aerobic digestion vessel. Preferably the reception vessel comprises a mixing arrangement for mixing the inorganic waste in the reception vessel. The apparatus may comprise an intermediate digestion vessel (preferably arranged between the aerobic digestion vessel and the anaerobic digestion vessel), wherein digestion liquid from the aerobic digestion vessel is directed to the intermediate digestion vessel for digestion and subsequently directed to the anaerobic digestion vessel. Preferably the anaerobic digestion vessel comprises a heating arrangement configured to generate convection currents in the liquid in the anaerobic digestion vessel, thereby to cause movement of inorganic particles of the second type within the anaerobic digestion vessel. The convection currents may cause movement of the inorganic particles of the second type across the floor of the anaerobic digestion vessel. Preferably the heating arrangement comprises a heat exchanger. The heat exchanger may be supplied with fluid (e.g. water) heated using heat recovered from a generator fuelled by biogas from the apparatus for digesting organic waste. The anaerobic digestion vessel may comprise a rigid receptacle and a flexible liner arranged in the rigid receptacle for containing the digestion liquid. The heating arrangement may comprise a length of pipe for carrying a heating fluid, the length of pipe being disposed between the rigid receptacle and the flexible liner. With the arrangements described herein, a pulse of liquid entering the anaerobic digestion vessel causes inorganic particles of the second type to form a suspension in the digestion liquid so that they are removed from the anaerobic digestion vessel with the liquid digestate displaced from the anaerobic digestion vessel. In this way, no stirring or mixing arrangement may be necessary in the anaerobic digestion vessel as a pulse of incoming liquid serves to displace a portion of liquid from the vessel which contains inorganic particles of the second type. Preferably the or each pulse of liquid enters the anaerobic digestion vessel via an inlet of the anaerobic digestion vessel and inorganic particles of the second type are displaced from the anaerobic digestion vessel via an outlet of the anaerobic digestion vessel. Preferably the outlet is disposed on an opposite side of the anaerobic digestion vessel to the inlet. A further advantage of the multi-stage digestion process described herein is that compounds containing ammonia and sulphur are found in the inorganic particles, which helps to reduce or eliminate ammoniacal and sulphurous emissions (e.g. gaseous emissions) from the digester. In particular, the inorganic particles of the second sub-type, and / or the inorganic particles of the second type, may comprise compounds containing sulphur and / or ammonia (e.g. sulphate and / or ammonium-containing compounds). BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which like reference numerals indicate like features and in which: Figure 1 is a schematic diagram illustrating a digester according to an embodiment of the invention; Figure 2 is a schematic diagram illustrating a variant of the digester of Figure 1; and Figures 3A, 3B, 3C and 3D illustrate stages of a process for removing inorganic particles from a digestion vessel according to an embodiment of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention provides a process for removing inorganic solids particles from a multi-stage digester for treating organic waste. Inorganic particles are removed from different stages of the digester in different ways, which enables inorganic particles to be removed from an anaerobic stage of the digester without opening or otherwise interrupting an anaerobic digestion vessel in which the anaerobic stage occurs. In this way, anaerobic digestion, which may be used to provide useful biogas such as methane, may be allowed to continue uninterrupted whilst preventing accumulation of unwanted inorganic particles. In particular, inorganic particles of at least a first type may be removed mechanically from one or more aerobic stages of the digester. Inorganic particles of a second type, having different properties from the inorganic particles of the first type, are removed from one or more anaerobic stages of the digester together with a flow of liquid digestate. With this arrangement, inorganic particles of the second type may be removed from the anaerobic stage of the digester without interrupting the anaerobic stage by exposure to air or by needing to empty an anaerobic digestion vessel. Inorganic particles which collect at different stages of a multi-stage digester have different properties. By removing at least a first type of inorganic particles from early stages of the digester which are relatively insensitive to exposure to air, inorganic particles of a second type which form or collect in the later anaerobic stage of the digester may be removed without interrupting the anaerobic stage. The inorganic particles of the second type have different bulk properties from the inorganic particles of the first type. In particular, properties of the inorganic particles which collect in the later anaerobic stage (or stages) of the digester mean that these particles may be removed with fluid digestate from the anaerobic stage. Advantageously, these inorganic particles of the second type may be removed from an anaerobic digestion vessel by being agitated into suspension by liquid introduced into the anaerobic digestion vessel. The liquid digestate, together with inorganic particles of the second type, may then be disposed of or directed to another end use, such as use as a fertiliser. Figure 1 is a schematic diagram illustrating apparatus according to an embodiment of the invention, in the form of a multi-stage digester 10. The digester 10 comprises a reception vessel 12, an aerobic digestion vessel 14 and an anaerobic digestion vessel 16. In use, organic waste material to be digested is loaded into the reception vessel 12, and subsequently fed from the reception vessel 12 into the aerobic digestion vessel 14. The reception vessel 12 is arranged to receive a feedstock of organic waste materials to be digested. The reception vessel 12 may comprise a mixing arrangement for mixing and / or macerating the waste material prior to the waste material being fed into the aerobic digestion vessel 14. For example the reception vessel 12 may comprise a shredder, chopper pump or macerating pump, comminuter or similar. The reception vessel 12 may be provided by any suitable vessel for holding the waste material, such as a tank or container. The reception vessel may be portable. For example, the reception vessel may comprise a trailer or shipping container. In this embodiment, the aerobic digestion vessel 14 is a solids digestion vessel suitable for aerobic digestion of organic waste material that includes at least some solids waste materials. The aerobic digestion vessel 14 is arranged to receive waste material from the reception vessel 12. For example, the aerobic digestion vessel 14 may be connected to the reception vessel 12 by a pipe. The aerobic digestion vessel 14 may be provided by any suitable vessel for holding the waste material, such as a tank or container. The aerobic digestion vessel may be portable. For example, the aerobic digestion vessel may comprise a trailer, skip or shipping container. The reception vessel 12 is optional, and in some embodiments, a separate reception vessel is not provided. In such embodiments, waste material feedstock is loaded directly into the aerobic digestion vessel and may be mixed and / or macerated, and subsequently digested in the aerobic digestion vessel. In this case, the reception vessel and aerobic digestion vessel are provided by the same vessel, so that the reception and aerobic digestion stages are combined. Optional digestion vessels are shown in dashed lines in Figures 1 and 2. In some embodiments, a screening arrangement (not shown) may be provided to screen solids waste items from the contents of the aerobic digestion vessel so that only a liquid fraction of the digesting organic waste is transferred to the anaerobic digestion vessel. In this preferred embodiment, the anaerobic digestion vessel 16 is a liquids digestion vessel suitable for anaerobic digestion of liquids waste materials. The anaerobic digestion vessel 16 is arranged to receive waste materials from the aerobic digestion vessel 14. The anaerobic digestion vessel 16 is arranged to receive a liquid fraction of waste materials from the aerobic digestion vessel 14. For example the anaerobic digestion vessel 16 may be connected to the aerobic digestion vessel 14 by a pipe. In some embodiments, a weir is provided between the aerobic digestion vessel 14 and the anaerobic digestion vessel 16, such that digestion liquid flows from the aerobic digestion vessel 14 to the anaerobic digestion vessel 16 via the weir. The weir may be provided by the position of a connecting pipe (not shown) between the aerobic digestion vessel 14 and the anaerobic digestion vessel 16. In this case the pipe is arranged such that an inlet end of the pipe is at a height above a base of the aerobic digestion vessel 14, such that only an upper portion of digestion liquid in the aerobic digestion vessel 14 is able to flow through the pipe to the anaerobic digestion vessel 16. In this way, heavy particulate matter (e.g. inorganic particles) which has sunk to the base of the aerobic digestion vessel 14 is not transferred to the anaerobic digestion vessel 16. In use, waste material is loaded into the reception vessel 12 and optionally mixed and / or macerated. Make-up water and / or other liquid may be added to the waste material. Preferably, at least some liquid digestate from the anaerobic digestion vessel 16 is added to waste material feedstock in the reception vessel 12 to provide a source of suitable microorganisms for digestion. The organic waste material may be mixed in the reception vessel 12 for a period of time, for example about 24 hours. The mixed organic waste is then fed into the aerobic digestion vessel 14 for digestion. Waste material in the aerobic digestion vessel 14 is heated to a suitable temperature (e.g. in the thermophilic range) for digestion. A preferred temperature range for this stage is from about 55°C to about 70°C. In particular, waste material in the aerobic digestion vessel 14 undergoes acid phase hydrolysis fermentation by aerobic bacteria. The contents of the aerobic digestion vessel 14 may be mixed during digestion, for example using a mechanical stirrer, or by blowing air into the vessel 14. Digested solids waste material may be removed from the aerobic digestion vessel, for example by screening or sieving the solids waste material in order to remove solids waste materials above a predetermined size. The separated fermented solids waste material may be subsequently further dewatered and / or dried and / or pasteurised, to be used as a soil improver, fertiliser or peat substitute, or in some cases to provide a fibrous material for manufacturing biodegradable items such as plant pots and bird nesting boxes. Once the aerobic digestion stage is substantially completed, at least a portion of a liquid fraction from the aerobic digestion vessel 14 is transferred to the anaerobic digestion vessel 16. This digestion liquid (e.g., the liquid fraction of the waste material) undergoes methanogenic anaerobic digestion in the anaerobic digestion vessel 16 to produce useful methane-rich biogas. Liquid in the anaerobic digestion vessel 16 is maintained in the mesophilic temperature range, preferably from about 30 to about 45°C. In some cases higher thermophilic digestion temperatures of 50° to 65° can be used. During digestion, the anaerobic digestion vessel 16 may be substantially sealed to prevent ingress of air or other sources of oxygen which could disrupt the anaerobic conditions. As mentioned above, at least a portion of liquid digestate from the anaerobic digestion vessel 16 may be mixed with waste material feedstock in the reception vessel 12, to provide active bacteria to the reception vessel 12. Advantageously, liquid is transferred from the aerobic digestion vessel 14 to the anaerobic digestion 16 vessel in batches rather than continuously, since this approach helps to remove inorganic particles from the anaerobic digestion vessel 16 (and can also help to reduce disturbance to the conditions needed for the anaerobic digestion process) as described further below. Optionally, a variant of the digester 10 may comprise an intermediate digestion vessel 18 arranged between the aerobic digestion vessel and the anaerobic digestion vessel, as shown in Figure 2. Preferably the intermediate digestion vessel 18 is an anaerobic digestion vessel. In this case, digestion liquid from the aerobic digestion vessel 14 is directed to the intermediate digestion vessel 18 for digestion and subsequently directed to the anaerobic digestion vessel 16 for further digestion. The intermediate digestion vessel 18 therefore provides an additional anaerobic digestion stage. This optional step helps to increase the yield of biogas from the digester 10. It will be appreciated that in some embodiments, the reception vessel and the intermediate digestion vessel are not present, so that the digester has only two stages: the aerobic digestion vessel and the anaerobic digestion vessel. In the present invention, inorganic particles are removed from each stage of the digester. The present applicant has found that in a multi-stage digester, for example of the kind described above, different types of inorganic particles accumulate at different stages of the digestion process. Accordingly, different methods of removal may be used for each type of inorganic particles, which has advantages for the digestion process. Inorganic particles of a first type 20 accumulate in the reception vessel 12 and in the aerobic digestion vessel 14. Inorganic particles of a second type 22, having different bulk properties to the inorganic particles of the first type 20, accumulate in the anaerobic digestion vessel 16. The present applicant has also found that the inorganic particles of the first type 20 comprise inorganic particles of a first sub-type 20a and inorganic particle of a second sub-type 20b. In embodiments in which a separate reception vessel 12 is provided, inorganic solids or particles of the first sub-type 20a, such as larger stones and pebbles, sand, fragments of glass, bone, and metal and other debris may sink to the bottom of the reception vessel 12. Such inorganic particles 20a may be released from the waste material during mixing and / or maceration in the reception vessel 12. Some or substantially all of these inorganic particles may be entrained objects carried into the reception vessel 12 with the waste materials. For example, stones, grit and the like may be entrained in plant waste material such as grass or weeds cuttings, or in manure or slurry in the case where animal waste materials are used. These inorganic particles of the first sub-type 20a have a relatively larger average particle size (e.g. particle diameter) compared with inorganic particles of the second subtype 20b and may have a relatively higher density compared with inorganic particles of the second sub-type 20b. The inorganic particles of the first sub-type 20a may have a larger average particle size than particles of the second type 22 present at later stages of the digestion process. The processing of waste in the reception vessel 12 is relatively insensitive to conditions such as temperature, pH, or the presence of oxygen, compared to the later stages of the digester. Also, waste may be processed in batches in the reception vessel 12, with a batch load of waste materials being loaded into the reception vessel 12, mixed for a period of time (for example about 24 hours) and subsequently transferred to the aerobic digestion vessel 14. Accordingly, the reception vessel 12 may be open (or easily openable) to the air, and the reception vessel 12 may be at least partially emptied, periodically, for example when transferring waste material to the aerobic digestion vessel 14. The reception vessel 12 may be opened safely, because little or no methane (which can form explosive mixtures) is formed in the reception vessel 12. Inorganic particles of the first subtype 20a may therefore be easily removed mechanically from the reception vessel, for example by digging (e.g. using an excavator or tractor bucket), using an auger, suction means such as a pump, or by tipping the reception vessel 12. Additionally or alternatively, the reception vessel 12 and / or the aerobic digestion vessel 14 may be provided with a particle trap arranged to trap and collect inorganic particles. It will be appreciated that in embodiments without a reception vessel, in which waste feedstock is first loaded into the aerobic digestion vessel 14 (i.e. in which the reception vessel and aerobic digestion vessel are combined), inorganic particles of the first type 20, comprising inorganic particles of both the first and second subtypes 20a, 20b, may be removed together from the aerobic digestion vessel 14. In the present embodiment, in the aerobic digestion vessel 14, inorganic particles of the second sub-type 20b accumulate. While the waste material is undergoing digestion in the aerobic digestion vessel 14, inorganic particles of the second subtype 20b sink to the bottom of the aerobic digestion vessel 14. These inorganic particles 20b generally comprise both entrained debris such as smaller stones and sand and also inorganic precipitates which form during the aerobic digestion process. Various inorganic compounds may be present (in solid particulate form, or in solution) in solid and / or liquid waste materials in the aerobic digestion vessel 14. During digestion in the aerobic digestion vessel 14, inorganic compounds may be released from solid waste materials into the digestion liquid. Some of these inorganic compounds may react and precipitate to form inorganic particles. For example, the inorganic particles may comprise calcium phosphate, calcium sulphate, ammonium sulphate, calcium carbonate, Struvite and other compounds. The digestion process in the aerobic digestion vessel 14 is relatively robust and is less sensitive to fluctuations in temperature and pH, compared with digestion in the anaerobic digestion vessel 16. In some embodiments, the aerobic digestion vessel 14 may be open to the atmosphere. Waste material may be processed in batches in the aerobic digestion vessel 14. For example, a batch of mixed waste material may be transferred from the reception vessel 12 to the aerobic digestion and digested in the aerobic digestion vessel 14 for a period of time, for example from about one day to about four days. Following this period of digestion the aerobic digestion vessel 14 may be partially or completely emptied. Accordingly, the aerobic digestion vessel 14 may be open (or easily openable) to the air, and the aerobic digestion vessel is at least partially emptied, periodically, allowing safe removal of the inorganic particles. The inorganic particles of the second sub-type 20b have a relatively greater bulk density (compared with inorganic particles of the first sub-type 20a and particles of the second type 22 formed at later stages of the digester 10) and therefore remain settled at the bottom of the aerobic digestion vessel 14, even during mixing of contents in the vessel 14. The inorganic particles of the second sub-type 20b may be too dense to be removed easily with any fluid flow from the aerobic digestion vessel 14. The inorganic particles of the second sub-type 20b may be easily removed mechanically from the aerobic digestion vessel 14, for example by digging (e.g. using an excavator bucket), using an auger, suction means such as a pump, or by tipping the aerobic digestion vessel 14. In some embodiments, each of the reception vessel and / or the aerobic digestion vessel is provided by a trailer (such as an agricultural trailer), such that the digestion vessel can be tipped periodically to remove inorganic particles. Compared with the processing of waste in the reception vessel 12 and aerobic digestion vessel 14, processing of waste in the anaerobic digestion vessel 16 is relatively sensitive to changes in temperature, pH and the presence of oxygen. The anaerobic digestion vessel 16 may be substantially sealed against ingress of air, to allow anaerobic digestion to occur. Accordingly, opening of the anaerobic digestion vessel 16 to remove inorganic particles would disturb the anaerobic digestion process. Inorganic particles of the second type 22 accumulate in the anaerobic digestion vessel 16 and sink towards the bottom of the anaerobic digestion vessel 16. Again, various inorganic compounds may be present (in solid particulate form in suspension, or in solution) in the liquid fraction of waste materials from the aerobic digestion vessel 14 which is subsequently transferred to the anaerobic digestion vessel 16. In the anaerobic digestion vessel 16, at least in part due to changes in temperature and / or pH, some inorganic compounds may precipitate to form inorganic particles. The inorganic particles of the second type 22 may also comprise some particles residually entrained or suspended in the liquid from the aerobic digestion vessel, such as sand and silt-like particles. The inorganic particles of the second type 22 have a smaller average particle size (such as particle diameter) than the inorganic particles of the first type 20. In some examples the inorganic particles of the second type 22 have an average particle size of less than 1 mm. The inorganic particles of the second type 22 may also have a lower bulk density than the inorganic particles of the first type 20. In particular, in the anaerobic digestion vessel 16, the inorganic particles of the second type 22 are substantially silt-like, settling towards the bottom of the anaerobic digestion vessel 16. At least some of the inorganic particles of the second type 22 do not aggregate to form a closely packed sediment but remain loosely accumulated or partially suspended in liquid at the bottom of the anaerobic digestion vessel 16. The bulk properties of the inorganic particles of the second type 22 are such that these particles settle to form a loose bed or semi-suspended bed (similar to a fluidised bed) at the bottom of the anaerobic digestion vessel 16. The introduction of digestion liquid into the anaerobic digestion 16 vessel disturbs the bed of inorganic particles of the second type 22 and causes a portion of the particles to be held in suspension. A portion of the inorganic particles of the second type 22 may therefore be carried from the anaerobic digestion vessel 16 with a portion of fluid which is removed from the anaerobic digestion vessel 16. Figures 3A, 3B, 3C, and 3D illustrate the introduction of a pulse of liquid into the anaerobic digestion vessel 16. In this preferred embodiment, the anaerobic digestion vessel 16 comprises an inlet 24 for introducing digestion liquid into the anaerobic digestion vessel 16. The inlet 24 is disposed on a first side of the vessel, proximate a floor or base 26 of the vessel 16. The inlet 24 is spaced above the base 26. The vessel 16 further comprises an outlet 28 for removal of liquid digestate from the anaerobic digestion vessel 16. The outlet 28 is disposed on a second side of the vessel 16 generally opposite to the first side. The outlet 28 is disposed proximate the floor 26 of the vessel 16, but spaced at a distance above the floor 26. Figure 3A shows the anaerobic digestion vessel 16 when liquid has not been introduced into the vessel 16 for a period of time (for example for several hours, or about one day). Inorganic particles of the second type 22 have settled in the digestion liquid to form a bed on the base 26 of the anaerobic digestion vessel 16. Biogas 30 formed by anaerobic digestion collects above the surface of the digestion liquid 32 in the vessel 16, from where it may be removed. In the present invention, to remove the inorganic particles of the second type 22 from the anaerobic digestion vessel 16, batches of liquid from the aerobic digestion vessel 14 are surged or pulsed into the anaerobic digestion vessel 16 at intervals. As shown in Figure 3B, a pulse of liquid from the aerobic digestion vessel 14 is introduced into the anaerobic digestion vessel 16 via the inlet 24 (as indicated by the arrow in Figure 3B). As a result, the inorganic particles of the second type 22 are disturbed and become suspended in the digestion liquid 32. Whilst in suspension, some of the inorganic particles 22 may travel towards the outlet 28 carried by the momentum of the pulse of liquid. The pulses of liquid are provided using a pump and / or gravity to move liquid from the aerobic digestion vessel 14 (or optionally from the intermediate digestion vessel where provided) to the anaerobic digestion vessel 16. It will be appreciated that any suitable pumping arrangement may be used. As shown in Figure 3C, digestion liquid is removed from the anaerobic digestion vessel 16 via the outlet 28, and a portion of the suspended inorganic particles of the second type 22 are removed with the liquid 32. The liquid 32 and suspended particles 22 may be removed after the incoming pulse has finished, or removal may occur simultaneously, or at least partly overlap with the incoming pulse of liquid. Removal of liquid 32 is indicated by an arrow in Figure 3C. Liquid 32 may be actively removed via the outlet by pumping, or passively by displacement caused by introduction of liquid into the vessel 16 via the inlet 24. Digestion liquid 32 removed in this way, which has at least partially undergone anaerobic digestion, may be referred to as liquid digestate. Following each pulse, flow of liquid into or out of the anaerobic digestion vessel 16 is paused for a period of time before a subsequent pulse of liquid is introduced. In this way, an interval is provided between pulses during which no liquid flows into the anaerobic digestion vessel 16. With this arrangement, because no liquid from the aerobic digestion vessel 14 is introduced for a period of time, disruption to the anaerobic digestion conditions is minimised, which helps to maintain the efficiency of biogas production. During this period, the inorganic particles of the second type 22 may begin to sink to the bottom of the vessel, as illustrated in Figure 3D. The introduction of a pulse of liquid into the anaerobic digestion vessel 16 provides a source of further inorganic particles into the anaerobic digestion vessel 16. Some inorganic particles may be in suspension in the introduced liquid and subsequently sink to the bottom of the anaerobic digestion vessel 16. Additionally, the digestion liquid introduced with each pulse contains dissolved inorganic compounds. During the anaerobic digestion process, these compounds subsequently precipitate to form inorganic particles of the second type 22. For example, changes in temperature, pH, and the action of bacteria in the anaerobic vessel 16 may cause precipitation and formation of the inorganic particles. The majority of the inorganic particles of the second type 22 may be formed in situ in this way. The present applicant has also found that, by removing larger and / or denser inorganic particles of the first type 20 at earlier stages of the digestion process, the bed of inorganic particles of the second type 22 is at least partially self-levelling and that this property can be utilised to help remove the inorganic particles of the second type 22 from the anaerobic digestion vessel 16. Gas bubbles forming in the bed of inorganic particles 22 during anaerobic digestion cause movement and mixing of the inorganic particles 22 as the bubbles escape into the digestion liquid 32 above. This effect causes the inorganic particles 22 to become evenly distributed across the base 26 of the anaerobic digestion vessel 16. In this way, localised build-up of inorganic particles in only some areas of the vessel 16 is prevented, as particles become spread across the base 26 of the vessel 16 by this self-levelling effect. This effect also facilitates the removal of inorganic particles 22 using pulses of liquid, because the inorganic particles 22 do not accumulate in only some regions of the anaerobic digestion vessel 16 (for example near the inlet 24) but instead become evenly distributed across the base 26, with overall movement towards other regions of the vessel (e.g. generally towards the outlet 28). Accordingly, the inorganic particles of the second type 22 can be removed from the anaerobic digestion vessel 16 without needing to open the anaerobic digestion vessel 16. With these arrangements, digestion in the anaerobic vessel 16 may continue uninterrupted for many years without significant accumulation of inorganic particles or any reduction in biogas evolution. In contrast, continuous flow of liquid through the anaerobic digestion vessel may simply form a channel through any particulate matter on the base of the digestion vessel, such that inorganic particles continue to accumulate and are not removed in sufficient quantities. In a preferred embodiment, the volume of each pulse is about 0.5% of the volume of the anaerobic digestion vessel 16 and a plurality of pulses (e.g. 10 pulses) per day are fed into the anaerobic digestion vessel 16, such that a volume of liquid approximately equal to the total volume of liquid in the anaerobic digestion vessel 16 passes through the anaerobic digestion vessel about every 20 days. The duration of each pulse may range from about 10 seconds to about 1 hour, depending on a flow rate of fluid pumped into the anaerobic digestion vessel 16 and the volume of the anaerobic digestion vessel 16. The rate of flow of each pulse is sufficient to stir up and suspend the inorganic particles of the second type 22. Where an intermediate digestion vessel such as the intermediate digestion vessel 18 is provided, inorganic particles of the second sub-type also accumulate in the intermediate digestion vessel and are removed from the intermediate digestion vessel, with the outflow of liquid. In this case, inorganic particles may be removed from the intermediate digestion vessel uses pulses of liquid, substantially as described above in relation to the anaerobic digestion vessel. In the present invention, as described above, because larger inorganic particles and organic solids can be removed safely at earlier stages in the digestion process, they do not accumulate in the later anaerobic stage, and the finer inorganic particles of the second type which accumulate in the later anaerobic stage may be removed from the anaerobic digestion vessel in the manner described above. A related advantage is that there is no need for a mechanical mixing arrangement in order to mix the contents of the anaerobic digestion vessel. In some examples the contents of the anaerobic digestion vessel need not be actively mixed at all, as bubbling of gas during the digestion process may provide sufficient mixing. In a preferred embodiment however, the anaerobic digestion vessel is provided with a heating arrangement which acts to heat the contents of the anaerobic digestion vessel and also acts to mix the contents of the vessel by producing convection currents in the digestion liquid. In one example a floor mounted heat exchanger is connected to a hot water supply (for example using heat recovered from a generator powered by biogas from the digester, and / or from a boiler) and acts to transfer heat to the digestion liquid. With this arrangement, the heating arrangement also generates convection currents in the digestion liquid, thereby mixing the contents of the anaerobic vessel. The inorganic particles of the second type may also be gently mixed and suspended in the liquid by this arrangement. Advantageously, in preferred embodiments therefore, the anaerobic digestion vessel is not stirred by any mechanical mixing apparatus, but mixing is provided solely by convection currents. In some prior art anaerobic digesters, such as those using a continuous stirred tank reactor (CSTR) with mechanical stirring or agitating means, the vessel needs substantial reinforcement to support the mixing apparatus, which transmits vibrations and stresses to the material from which the vessel is constructed. In the present invention therefore, a substantially simpler and lower cost vessel may be used. With the arrangements described above, the anaerobic digestion vessel need not be opened in order to remove inorganic particles, thereby allowing the anaerobic digestion vessel to be operated continuously without interrupting the digestion process by opening the vessel. Another advantage of using pulses of fluid is that disturbance to the anaerobic digestion process conditions is minimised. Maintenance of some particulate matter in the anaerobic digestion vessel can help to improve digestion and to improve the output of biogas. In the present invention, an equilibrium may be reached in which a limited quantity of inorganic particles is maintained in the anaerobic digestion vessel, whilst removal of such particles prevents any unwanted build-up of such inorganic particles to a level detrimental to the digestion process.

Claims

1. A method of removing inorganic particles from a digester for digesting organic waste, the digester comprising at least an aerobic digestion vessel and an anaerobic digestion vessel, the anaerobic digestion vessel being arranged to receive at least some digestion liquid from the aerobic digestion vessel, and the method comprising:allowing inorganic particles of a first type to accumulate in the aerobic digestion vessel and removing the inorganic particles of the first type from the aerobic digestion vessel;allowing inorganic particles of a second type to accumulate in the anaerobic digestion vessel and providing a plurality of pulses of digestion liquid into the anaerobic digestion vessel, thereby to suspend the inorganic particles of the second type in the digestion liquid in the anaerobic digestion vessel; andremoving the suspended inorganic particles of the second type from the digester by removing a portion of the digestion liquid containing inorganic particles of the second type from the anaerobic digestion vessel;wherein the inorganic particles of the second type have a smaller average particle size than an average particle size of the inorganic particles of the first type.

2. The method of Claim 1, in which the inorganic particles of the second type have a lower density than the inorganic particles of the first type.

3. The method of Claim 1 or Claim 2, in which, during intervals between the pulses of liquid, substantially no digestion liquid enters the anaerobic digestion vessel.

4. The method of any preceding claim, in which at least a portion of the inorganic particles of the second type are formed by precipitation of inorganicsubstances dissolved or suspended in liquid in the anaerobic digestion vessel.

5. The method of any preceding claim, in which the inorganic particles of the second type form a bed on a base of the anaerobic digestion vessel and the step of removing the inorganic particles of the second type comprises using each pulse of liquid to suspend at least part of the bed, thereby to remove the inorganic particles with a portion of liquid digestate from the anaerobic digestion vessel.

6. The method of any preceding claim, in which the step of removing inorganic particles of the first type comprises removing mechanically the inorganic particles of the first type from the aerobic digestion vessel.

7. The method of any preceding claim, in which the digester comprises a reception vessel for receiving organic waste prior to transferring the organic waste into the aerobic digestion vessel and the method comprises removing inorganic particles of the first type from the reception vessel.

8. The method of Claim 7, in which the inorganic particles of the first type comprise inorganic particles of a first sub-type and inorganic particles of a second sub-type and the step of removing inorganic particles of the first type from the reception vessel comprises removing inorganic particles of the first sub-type from the reception vessel.

9. The method of Claim 8, in which the step of removing inorganic particles of the first type from the aerobic digestion vessel comprises removing inorganic particles of the second sub-type from the aerobic digestion vessel.

10. The method of Claim 9, in which at least a portion of the inorganic particles of the second sub-type are formed by precipitation of inorganic substances dissolved and / or suspended in liquid in the anaerobic digestion vessel.

11. The method of Claim 9 or Claim 10, in which the inorganic particles of the second sub-type have a greater density than the inorganic particles of the first sub-type and a greater density than the inorganic particles of the second type.

12. The method of any of Claims 8 to 11, in which the inorganic particles of the second sub-type have an average particle size of between about 0.1 mm and about 5 mm.

13. The method of any of Claims 8 to 12, in which the digester comprises an intermediate digestion vessel, wherein digestion liquid from the aerobic digestion vessel is directed to the intermediate digestion vessel for digestion and subsequently directed to the anaerobic digestion vessel, and wherein the method comprises removing inorganic particles of the second sub-type from the intermediate digestion vessel.

14. The method of any preceding claim, in which the inorganic particles of the second type have an average particle size of less than 1 mm.

15. The method of any preceding claim, in which an interval between each pulse of liquid is between about 30 minutes and about 36 hours.

16. The method of any preceding claim, in which a volume of each pulse of liquid is between about 0.1% and about 20% of the volume of the anaerobic digestion vessel.

17. The method of Claim 16, in which a volume of each pulse of liquid is about 0.5% of the volume of the anaerobic digestion vessel.

18. The method of any preceding claim, comprising providing, proximate a floor of the anaerobic digestion vessel, a heating arrangement configured togenerate convection currents in the liquid in the anaerobic digestion vessel, thereby to cause movement of inorganic particles of the second type across the floor of the anaerobic digestion vessel.

19. The method of any preceding claim, in which the aerobic digestion vessel comprises a trailer and the step of removing inorganic particles of the first type comprises tipping the trailer to empty the inorganic particles of the first type from the trailer.

20. Apparatus for digesting organic waste, the apparatus comprising:an aerobic digestion vessel;an anaerobic digestion vessel arranged to receive digestion liquid from the aerobic digestion vessel and comprising an outlet for removing liquid digestate and inorganic particles from the anaerobic digestion vessel;a pulsing arrangement configured to provide a plurality of pulses of digestion liquid into the anaerobic digestion vessel, such that inorganic particles are removed from the anaerobic digestion vessel via the outlet together with a portion of the liquid digestate.

21. Apparatus according to Claim 20, in which the anaerobic digestion vessel comprises an inlet through which the pulses of digestion liquid enter the anaerobic digestion vessel, and in which the inlet is disposed proximate a base of the anaerobic digestion vessel such that each pulse of liquid is directed across the base of the anaerobic digestion vessel.

22. Apparatus according to Claim 20 or Claim 21, comprising a reception vessel arranged to receive organic waste prior to transferring the organic waste to the aerobic digestion vessel, the reception vessel comprising a mixing arrangement for mixing the inorganic waste in the reception vessel.

23. Apparatus according to any of Claims 20 to 22, comprising an intermediatedigestion vessel, wherein digestion liquid from the aerobic digestion vessel is directed to the intermediate digestion vessel for digestion and subsequently directed to the anaerobic digestion vessel.

24. Apparatus according to any of Claims 20 to 23, in which the anaerobic digestion vessel comprises a heating arrangement configured to generate convection currents in the liquid in the anaerobic digestion vessel, thereby to cause movement of inorganic particles of the second type within the anaerobic digestion vessel.

25. Apparatus according to Claim 24, in which the anaerobic digestion vessel comprises a rigid receptacle and a flexible liner arranged in the rigid receptacle for containing the digestion liquid, and in which the heating arrangement comprises a heat exchanger.

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