WO2012168341A1 - Waste digestion - Google Patents

Waste digestion Download PDF

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Publication number
WO2012168341A1
WO2012168341A1 PCT/EP2012/060771 EP2012060771W WO2012168341A1 WO 2012168341 A1 WO2012168341 A1 WO 2012168341A1 EP 2012060771 W EP2012060771 W EP 2012060771W WO 2012168341 A1 WO2012168341 A1 WO 2012168341A1
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WO
WIPO (PCT)
Prior art keywords
vessel
solids
digestion
digestion vessel
liquids
Prior art date
Application number
PCT/EP2012/060771
Other languages
French (fr)
Inventor
Christopher Paul Reynell
Original Assignee
Christopher Paul Reynell
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Christopher Paul Reynell filed Critical Christopher Paul Reynell
Priority to EP12728051.9A priority Critical patent/EP2718247A1/en
Publication of WO2012168341A1 publication Critical patent/WO2012168341A1/en

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Classifications

    • CCHEMISTRY; METALLURGY
    • C12BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
    • C12MAPPARATUS FOR ENZYMOLOGY OR MICROBIOLOGY; APPARATUS FOR CULTURING MICROORGANISMS FOR PRODUCING BIOMASS, FOR GROWING CELLS OR FOR OBTAINING FERMENTATION OR METABOLIC PRODUCTS, i.e. BIOREACTORS OR FERMENTERS
    • C12M21/00Bioreactors or fermenters specially adapted for specific uses
    • C12M21/04Bioreactors or fermenters specially adapted for specific uses for producing gas, e.g. biogas
    • CCHEMISTRY; METALLURGY
    • C05FERTILISERS; MANUFACTURE THEREOF
    • C05FORGANIC FERTILISERS NOT COVERED BY SUBCLASSES C05B, C05C, e.g. FERTILISERS FROM WASTE OR REFUSE
    • C05F17/00Preparation of fertilisers characterised by biological or biochemical treatment steps, e.g. composting or fermentation
    • C05F17/40Treatment of liquids or slurries
    • CCHEMISTRY; METALLURGY
    • C05FERTILISERS; MANUFACTURE THEREOF
    • C05FORGANIC FERTILISERS NOT COVERED BY SUBCLASSES C05B, C05C, e.g. FERTILISERS FROM WASTE OR REFUSE
    • C05F17/00Preparation of fertilisers characterised by biological or biochemical treatment steps, e.g. composting or fermentation
    • C05F17/50Treatments combining two or more different biological or biochemical treatments, e.g. anaerobic and aerobic treatment or vermicomposting and aerobic treatment
    • CCHEMISTRY; METALLURGY
    • C05FERTILISERS; MANUFACTURE THEREOF
    • C05FORGANIC FERTILISERS NOT COVERED BY SUBCLASSES C05B, C05C, e.g. FERTILISERS FROM WASTE OR REFUSE
    • C05F17/00Preparation of fertilisers characterised by biological or biochemical treatment steps, e.g. composting or fermentation
    • C05F17/90Apparatus therefor
    • C05F17/964Constructional parts, e.g. floors, covers or doors
    • C05F17/971Constructional parts, e.g. floors, covers or doors for feeding or discharging materials to be treated; for feeding or discharging other material
    • C05F17/986Constructional parts, e.g. floors, covers or doors for feeding or discharging materials to be treated; for feeding or discharging other material the other material being liquid
    • CCHEMISTRY; METALLURGY
    • C12BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
    • C12MAPPARATUS FOR ENZYMOLOGY OR MICROBIOLOGY; APPARATUS FOR CULTURING MICROORGANISMS FOR PRODUCING BIOMASS, FOR GROWING CELLS OR FOR OBTAINING FERMENTATION OR METABOLIC PRODUCTS, i.e. BIOREACTORS OR FERMENTERS
    • C12M21/00Bioreactors or fermenters specially adapted for specific uses
    • C12M21/16Solid state fermenters, e.g. for koji production
    • CCHEMISTRY; METALLURGY
    • C12BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
    • C12MAPPARATUS FOR ENZYMOLOGY OR MICROBIOLOGY; APPARATUS FOR CULTURING MICROORGANISMS FOR PRODUCING BIOMASS, FOR GROWING CELLS OR FOR OBTAINING FERMENTATION OR METABOLIC PRODUCTS, i.e. BIOREACTORS OR FERMENTERS
    • C12M23/00Constructional details, e.g. recesses, hinges
    • C12M23/58Reaction vessels connected in series or in parallel
    • CCHEMISTRY; METALLURGY
    • C12BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
    • C12MAPPARATUS FOR ENZYMOLOGY OR MICROBIOLOGY; APPARATUS FOR CULTURING MICROORGANISMS FOR PRODUCING BIOMASS, FOR GROWING CELLS OR FOR OBTAINING FERMENTATION OR METABOLIC PRODUCTS, i.e. BIOREACTORS OR FERMENTERS
    • C12M45/00Means for pre-treatment of biological substances
    • C12M45/04Phase separators; Separation of non fermentable material; Fractionation
    • CCHEMISTRY; METALLURGY
    • C02TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02FTREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F11/00Treatment of sludge; Devices therefor
    • C02F11/02Biological treatment
    • C02F11/04Anaerobic treatment; Production of methane by such processes
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02EREDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E50/00Technologies for the production of fuel of non-fossil origin
    • Y02E50/30Fuel from waste, e.g. synthetic alcohol or diesel
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02PCLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P20/00Technologies relating to chemical industry
    • Y02P20/141Feedstock
    • Y02P20/145Feedstock the feedstock being materials of biological origin
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02WCLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO WASTEWATER TREATMENT OR WASTE MANAGEMENT
    • Y02W10/00Technologies for wastewater treatment
    • Y02W10/20Sludge processing
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02WCLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO WASTEWATER TREATMENT OR WASTE MANAGEMENT
    • Y02W30/00Technologies for solid waste management
    • Y02W30/40Bio-organic fraction processing; Production of fertilisers from the organic fraction of waste or refuse

Definitions

  • the present invention relates to methods and apparatus for digesting organic waste materials.
  • Organic waste is regularly produced by abattoirs, farms and food- processing plants as well as households. It requires treatment to render it suitable for discharge into the environment. b. Related Art
  • GB 2230004 describes a two stage digestion process which uses an installation comprising a fluids digestion vessel and a solids digestion vessel which are connected together.
  • the solids digestion vessel is in the form of a tower or other fixture which is located in or on the ground, and the fluids digestion vessel is an adjacent tank.
  • Bacterially active waste is fed from the fluids digestion vessel into the solids digestion vessel where it percolates through the solid content.
  • the solid waste is subjected to anaerobic bacterial digestion for seven to 14 days to produce a methane-rich gas fraction and a solid fraction which is environmentally more acceptable than the feed solids.
  • the digested solids material may be used as a soil conditioner.
  • GB 2459881 describes a waste treatment process in which solid wastes are first subjected to a fermentation process in a solids digestion vessel to produce a compostable material and carbon dioxide gas. Liquid from the fermentation stage is fed to a liquids digestion vessel and subjected to anaerobic digestion to produce methane gas. Because methane gas is not produced in significant quantities in the solids digestion vessel, there is no danger of an explosive mixture being formed in the event that air is drawn into the solids digestion vessel.
  • One aspect of the invention allows a user to select whether to produce a relatively high-nutrient plant growth medium, or a fibrous, relatively low-nutrient, material suitable for use as a plant growth medium, a solid fuel, or a building material. Liquid obtained during preparation of the low-nutrient material is used to produce useful methane.
  • Another aspect of the invention provides improved process control and reduced operating costs by removing grit from at least the solids digestion vessel and the liquids digestion vessel, and preferably from all of the vessels used in the process.
  • the invention provides the ability to use a higher rate or faster methane- generating digester than would otherwise be possible with current methods. I have found that retention times may be substantially reduced (halved in a number of cases) for all types of anaerobic digester.
  • An aspect of the invention provides an improved 'front end' stage that can be used with nearly any type of conventional anaerobic (biogas) digester by removing oversize, mainly non- or slow-digestible materials and optionally grit/heavy material.
  • the new front end enables the use of a higher rate but more sensitive and easily blocked digester such as UASB (upflow anaerobic sludge blanket) or anaerobic filter.
  • Figures 1 and 2 are schematic diagrams illustrating an embodiment of the invention
  • FIG. 3 illustrates apparatus for use in an embodiment of the invention.
  • FIG. 4 is a schematic diagram showing another embodiment of the invention. DETAILED DESCRIPTION
  • Apparatus 1 for digesting organic waste material comprises a solids digestion vessel 3, a liquids digestion vessel 5, and means 6 for feeding fluid between the two vessels 3,5, and a reception vessel 2, for receiving organic waste material suitable for anaerobic digestion and composting .
  • the reception vessel 2 is optionally provided with means for mixing and macerating the waste material prior to the waste being fed into the solids digestion vessel.
  • the mixing and macerating means may comprise a slurry wall tank mixer or a specialist comminuter. For finely-chopped waste material, a chopper pump or macerating pump may be used. Make-up water is added, and an optional feed from the liquids digestion vessel may provide a fluid phase with bacteria at a temperature suitable to initiate digestion of the solids waste.
  • an incoming batch of solids waste material is mixed and macerated with make-up water and fluid from the liquids digestion vessel 5 for about one day prior to its being pumped to the solids digestion vessel 3.
  • the pumpable fluid typically has a solids content in the range 2-20%.
  • Waste material in the solids digestion vessel 3 is mixed with microorganism-active fluid from the liquids digestion vessel 5 via pipework 6, and heated to a temperature in the thermophilic range for one to four days depending on the level of nutrients present and the pH.
  • a temperature in the thermophilic range for one to four days depending on the level of nutrients present and the pH may be reduced by blowing air into the vessel, which aids the acid phase hydrolysis process by aerobic bacteria.
  • a preferred temperature range for this stage is from about 55°C to about 70°C, which accelerates the digestion process and helps reduce or eliminate pathogens.
  • the retention time in the solids digestion vessel may be reduced to about one day.
  • the waste material undergoes acid phase hydrolysis fermentation and produces a biogas which consists principally of carbon dioxide.
  • Other byproducts of the process are ethanol and acetic acid, which lowers the pH.
  • the pH for this stage is typically about 6.5.
  • solids material over a predetermined size is separated, as will be described in more detail later.
  • the liquid fraction is fed to the liquids digestion vessel 5, in this embodiment via an intermediate optional anaerobic digestion vessel 4.
  • the liquid fraction may be digested for several days in the anaerobic digestion vessel 4, typically 1 -3 days, at a lower temperature than the preceding stage, typically about 45°C.
  • biogases are evolved, including carbon dioxide and hydrogen.
  • the liquid fraction that is fed to the liquids digestion vessel 5 is maintained in the mesophilic temperature range, preferably about 33-38°C.
  • the vessel 5 may be heated to maintain this temperature, but typically we have found that the inflow of hotter liquid, together with heat generated by the digestion process, is sufficient to maintain the desired temperature range.
  • the liquids digestion vessel 5 may be a conventional anaerobic digester, for example a completely stirred tank reactor (CSTR), plug flow, an anaerobic filter, or that using a sludge blan ket or a combination of these methods.
  • each vessel is provided with a gas mixer or blade or paddle for stirring the contents.
  • the process is substantially methanogenic because most of the carbon dioxide evolution has already taken place.
  • the methane content of the biogas at this stage is about 50-80% with the remainder carbon dioxide.
  • the evolved methane gas may be used for heat or power generation.
  • the retention time for the liquid digestion phase will vary depending on the type of digestion vessel 5 which is used: from 10-20 days for a CSTR to as little as 12 hours to five days for an anaerobic filter, upflow anaerobic sludge blanket or other high rate methods.
  • Liquid may be retained in the solids digestion vessel 3 or the optional anaerobic digestion vessel 4 until required by the liquids digestion vessel 5.
  • the volume and/or rate of gas evolution from the liquids digestion vessel 5 is measured, and the measurement used to control the rate of liquid feed into the liquids digestion vessel 5.
  • the liquids digestion vessel 5 receives only liquids, it may be selected from a range of digester types, including a filter digester such as is typically used in beverage residue treatment.
  • a filter digester requires a liquid-only feed and provides a more efficient digestion process.
  • Incoming organic waste is often contaminated with inorganic solids such as stones, grit, or other undesirable heavy materials.
  • large stones have been manually removed from the reception vessel 2 from time to time, and grit or other fine particles have been caught in a trap in the solids digestion vessel 3 and removed . Trapping and removing grit and other heavy particles allows improved process control and reduces operating costs.
  • the reception vessel 2, the solids digestion vessel 3 and the liquids digestion vessel 5 are each provided with means for trapping grit and heavy particles.
  • the optional vessel 4 is also provided with its own means for trapping grit and heavy particles.
  • Suitable types of particle trap which are well known in the art per se may be used; for example, a U- bend, or a cyclone trap which funnels particles into a pot with a tap or bung that can be opened to remove the accumulated particles; or a rotating screen or centreless auger may be employed.
  • Heavy particle removal is particularly beneficial for anaerobic liquid digesters 5 such as an up-flow anaerobic sludge blanket (UASB).
  • UASB up-flow anaerobic sludge blanket
  • debris forms a blanket on the liquid layer, and liquid percolates up through the floating blanket.
  • Bacteria are principally active in the blanket, and by percolating liquid through the blanket at a controlled flow rate, the liquid gets a lot of bacterial interaction. For process efficiency it is important not to break the blanket, so the controlled flow rate is also important. If grit or lignin fibre builds up in the tank 5, an increased flow of liquid is needed to get the heavy particles into suspension. Too great a flow-rate increase causes the blanket to break and process efficiency to be substantially reduced.
  • Figure 1 illustrates the removal of heavy particles separately from the passage of liquid from the solids digestion vessel 3, the optional anaerobic digestion vessel 4 and the liquids digestion vessel 5.
  • inorganic particles may advantageously be removed by carrying them with a stream of some or all of the liquid digestate so that the particles are returned to the reception vessel 2.
  • liquids digestion vessel 5 This may be done for the liquids digestion vessel 5 by providing a flow pattern within the liquid digestion vessel 5 which impels inorganic particles to flow out with the liquid digestate via pipework 6.
  • inorganic particles trapped in a U-bend of other particle trap may also be fed back to the reception vessel 2 along with a portion of liquid fraction via the pipeline 6.
  • Inorganic particles returned to the reception vessel in this way may readily be removed using mechanical apparatus or a sludge pump. Typically such removal is carried out about once a month depending on the level of usage of the apparatus 1 .
  • the apparatus 1 includes a screen 8 for separating fermented solids material 7 above a predetermined size after the acid phase hydrolysis stage has been sufficiently completed.
  • the screen size may be from 1 -50 mm, preferably 1 -25 mm, notably 5-10 mm.
  • the screen 8 is a downward-sloping metal mesh, and treated solids waste 7 passes down the screen under gravity. Some liquid is lost from the solids waste 7 during this process through the action of gravity. The resulting material has a solids content typically in the range 15-25%.
  • Other screening means 8 may be used; for example a run-down screen or a brush screen.
  • the operator can choose whether or not to apply the roller 9, which can be driven by a motor (not shown) via axial drive spigots 10.
  • dewatering means may be employed, singly or in multiple stages; for example a compression auger, filter press, doctor blade or squeegee, or a centrifuge.
  • the screening of solids may take place internally or externally to the solids digestion vessel 3.
  • the roller pressurises the wet solids waste 7 and drives out much of the excess of liquid 1 1 , leaving low-nutrient fibrous material 12 suitable for use as a plant growth medium or as a solid fuel or building material.
  • Liquid 1 1 which is pressed from the solids material 7 is returned to the apparatus, preferably into the liquids digestion vessel 5 to enable the organic content to be digested and produce additional useful methane.
  • the further dewatered solids waste 12 has a solids content substantially above 25%, notably at least 40%. Typically the solids content is in the range 50-60% after the further dewatering..
  • the wet solids waste 7 provides a high- nutrient plant growth medium suitable for putting on fields or soil.
  • the separated fermented solids materials may optionally be further treated, for example by composting.
  • the early stage of the process ie up to the point where the material enters the methanogenic stage
  • Being able to add-on the early stage to an existing digester provides the benefit of being able to treat higher solids content materials an increase in throughput of the existing plant and/or the abil ity to treat a greater range of materials.
  • the process removes substances which are detrimental to catalytic processes; accordingly in another aspect of the invention, the early stage of the process may be used to provide feedstock for fuel cells or other apparatus which uses catalytic processes
  • another embodiment of the invention includes apparatus for measuring gas evolution from the liquid digestion vessel 5.
  • the gas measuring apparatus is a floating roof gasholder which is connected to the liquid and gas outlets of the liquids digestion vessel 5.
  • the passage of warm liquid can help keep the gasholder from freezing in winter weather.
  • Electronic feedback from the gasholder may be used to control the rate of passage of liquid digestate into the liquids digestion vessel.
  • an oil-fired boiler receives oil from a fuel tank and may be used to heat the contents of the solids digestion vessel 3, or other vessels, via a heat exchanger.
  • a combined heat and power generator set (CHP Genset) or a gas-fired boiler is illustrated, either of which may receive and burn methane-rich gas from the liquids digestion vessel 5 to provide useful heat which may be optionally be distributed via the heat exchanger.
  • a control room houses electronic controls for the process.
  • the digester system is conveniently housed in shipping containers.
  • liquid will be aqueous but will include a variety of dissolved solutes and may contain a suspension of small particles.
  • 'high nutrient' and 'low nutrient' are relative.
  • dissolved nutrients in the fibrous material render it particularly suitable for use as a plant-growth material.
  • much of the dissolved nutrients are removed along with the water, rendering the fibrous material particularly suitable for applications such as fuels or use as a building material when suitably processed.

Abstract

A method for digesting organic waste comprises the steps of: providing a source of organic material undergoing anaerobic bacterial digestion in a liquid phase within a liquids digestion vessel (5); providing a reception vessel (2) for receiving organic waste materials; feeding organic waste material containing biodegradable solids from the reception vessel (2) to a solids digestion vessel(3); feeding at least a part of the liquid phase from the liquids digestion vessel (5) to the reception vessel (2); after a predetermined time, separating solids materials above a specified size from other materials in the solids digestion vessel to produce a high-nutrient plant-growth material (7); optionally further dewatering at least some of the separated solids materials (7) to produce a fibrous low-nutrient material (12) and feeding at least some of the liquid from the dewatering process to at least one of the digestion vessels, preferably the liquids digestion vessel (5); feeding at least some liquid from the solids digestion vessel (3) to the liquids digestion vessel (5); and removing inorganic particles from each of the digestion vessels (3,5).

Description

WASTE DIGESTION
BACKGROUND a. Field of the Invention
The present invention relates to methods and apparatus for digesting organic waste materials. Organic waste is regularly produced by abattoirs, farms and food- processing plants as well as households. It requires treatment to render it suitable for discharge into the environment. b. Related Art
GB 2230004 describes a two stage digestion process which uses an installation comprising a fluids digestion vessel and a solids digestion vessel which are connected together. The solids digestion vessel is in the form of a tower or other fixture which is located in or on the ground, and the fluids digestion vessel is an adjacent tank. Bacterially active waste is fed from the fluids digestion vessel into the solids digestion vessel where it percolates through the solid content. The solid waste is subjected to anaerobic bacterial digestion for seven to 14 days to produce a methane-rich gas fraction and a solid fraction which is environmentally more acceptable than the feed solids. The digested solids material may be used as a soil conditioner.
GB 2459881 describes a waste treatment process in which solid wastes are first subjected to a fermentation process in a solids digestion vessel to produce a compostable material and carbon dioxide gas. Liquid from the fermentation stage is fed to a liquids digestion vessel and subjected to anaerobic digestion to produce methane gas. Because methane gas is not produced in significant quantities in the solids digestion vessel, there is no danger of an explosive mixture being formed in the event that air is drawn into the solids digestion vessel. SUMMARY OF THE INVENTION
Aspects of the invention are specified in the independent claims. Preferred features are specified in the dependent claims.
One aspect of the invention allows a user to select whether to produce a relatively high-nutrient plant growth medium, or a fibrous, relatively low-nutrient, material suitable for use as a plant growth medium, a solid fuel, or a building material. Liquid obtained during preparation of the low-nutrient material is used to produce useful methane.
Another aspect of the invention provides improved process control and reduced operating costs by removing grit from at least the solids digestion vessel and the liquids digestion vessel, and preferably from all of the vessels used in the process.
The invention provides the ability to use a higher rate or faster methane- generating digester than would otherwise be possible with current methods. I have found that retention times may be substantially reduced (halved in a number of cases) for all types of anaerobic digester.
An aspect of the invention provides an improved 'front end' stage that can be used with nearly any type of conventional anaerobic (biogas) digester by removing oversize, mainly non- or slow-digestible materials and optionally grit/heavy material. The new front end enables the use of a higher rate but more sensitive and easily blocked digester such as UASB (upflow anaerobic sludge blanket) or anaerobic filter. BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be further described, by way of example only, with reference to the following drawings in which:
Figures 1 and 2 are schematic diagrams illustrating an embodiment of the invention;
Figure 3 illustrates apparatus for use in an embodiment of the invention; and
Figure 4 is a schematic diagram showing another embodiment of the invention. DETAILED DESCRIPTION
Apparatus 1 for digesting organic waste material comprises a solids digestion vessel 3, a liquids digestion vessel 5, and means 6 for feeding fluid between the two vessels 3,5, and a reception vessel 2, for receiving organic waste material suitable for anaerobic digestion and composting . The reception vessel 2 is optionally provided with means for mixing and macerating the waste material prior to the waste being fed into the solids digestion vessel. The mixing and macerating means may comprise a slurry wall tank mixer or a specialist comminuter. For finely-chopped waste material, a chopper pump or macerating pump may be used. Make-up water is added, and an optional feed from the liquids digestion vessel may provide a fluid phase with bacteria at a temperature suitable to initiate digestion of the solids waste. In a preferred embodiment, an incoming batch of solids waste material is mixed and macerated with make-up water and fluid from the liquids digestion vessel 5 for about one day prior to its being pumped to the solids digestion vessel 3. The pumpable fluid typically has a solids content in the range 2-20%.
Waste material in the solids digestion vessel 3 is mixed with microorganism-active fluid from the liquids digestion vessel 5 via pipework 6, and heated to a temperature in the thermophilic range for one to four days depending on the level of nutrients present and the pH. I have found that the retention time in the solids digestion vessel 3 may be reduced by blowing air into the vessel, which aids the acid phase hydrolysis process by aerobic bacteria. A preferred temperature range for this stage is from about 55°C to about 70°C, which accelerates the digestion process and helps reduce or eliminate pathogens. By optimising the temperature and blowing air into the solids digestion vessel, the retention time in the solids digestion vessel may be reduced to about one day. During this stage, the waste material undergoes acid phase hydrolysis fermentation and produces a biogas which consists principally of carbon dioxide. Other byproducts of the process are ethanol and acetic acid, which lowers the pH. The pH for this stage is typically about 6.5. Once the acid phase hydrolysis has been substantially completed, solids material over a predetermined size is separated, as will be described in more detail later. The liquid fraction is fed to the liquids digestion vessel 5, in this embodiment via an intermediate optional anaerobic digestion vessel 4. The liquid fraction may be digested for several days in the anaerobic digestion vessel 4, typically 1 -3 days, at a lower temperature than the preceding stage, typically about 45°C. During mixing and digesting, biogases are evolved, including carbon dioxide and hydrogen.
The liquid fraction that is fed to the liquids digestion vessel 5 (either directly from the solids digestion vessel 3 or via the anaerobic digestion vessel 4) is maintained in the mesophilic temperature range, preferably about 33-38°C. The vessel 5 may be heated to maintain this temperature, but typically we have found that the inflow of hotter liquid, together with heat generated by the digestion process, is sufficient to maintain the desired temperature range. The liquids digestion vessel 5 may be a conventional anaerobic digester, for example a completely stirred tank reactor (CSTR), plug flow, an anaerobic filter, or that using a sludge blan ket or a combination of these methods. Preferably, each vessel is provided with a gas mixer or blade or paddle for stirring the contents. During this stage of digestion, the process is substantially methanogenic because most of the carbon dioxide evolution has already taken place. Typically the methane content of the biogas at this stage is about 50-80% with the remainder carbon dioxide. The evolved methane gas may be used for heat or power generation. The retention time for the liquid digestion phase will vary depending on the type of digestion vessel 5 which is used: from 10-20 days for a CSTR to as little as 12 hours to five days for an anaerobic filter, upflow anaerobic sludge blanket or other high rate methods. Liquid may be retained in the solids digestion vessel 3 or the optional anaerobic digestion vessel 4 until required by the liquids digestion vessel 5. In a preferred embodiment, the volume and/or rate of gas evolution from the liquids digestion vessel 5 is measured, and the measurement used to control the rate of liquid feed into the liquids digestion vessel 5.
Because the liquids digestion vessel 5 receives only liquids, it may be selected from a range of digester types, including a filter digester such as is typically used in beverage residue treatment. A filter digester requires a liquid-only feed and provides a more efficient digestion process.
Incoming organic waste is often contaminated with inorganic solids such as stones, grit, or other undesirable heavy materials. In prior art systems, large stones have been manually removed from the reception vessel 2 from time to time, and grit or other fine particles have been caught in a trap in the solids digestion vessel 3 and removed . Trapping and removing grit and other heavy particles allows improved process control and reduces operating costs.
I have now found that, surprisingly, grit and heavy particles are not necessarily present, or entirely present, in the incoming waste feedstock. Rather, I have found th at heavy particles are present at each stage of th e process and are advantageously removed from each of the digestion vessels. Without wishing to be bound by theory, I believe that heavy particles (ie, particles more dense than water) are produced by the interaction of ions such as silicates, carbonates and phosphates which are initially dissolved in the water content of the waste, or which result from the degradation of biological material, and which subsequently react and precipitate from solution. Such precipitations of inorganic particles may be promoted by changes in pH and/or the hardness of the water.
Accordingly, the reception vessel 2, the solids digestion vessel 3 and the liquids digestion vessel 5 are each provided with means for trapping grit and heavy particles. In a particularly preferred embodiment, the optional vessel 4 is also provided with its own means for trapping grit and heavy particles. Suitable types of particle trap which are well known in the art per se may be used; for example, a U- bend, or a cyclone trap which funnels particles into a pot with a tap or bung that can be opened to remove the accumulated particles; or a rotating screen or centreless auger may be employed. By removing inorganic material at each stage of the process, digestion speed and efficiency may be improved for all types of anaerobic digester 5 because the active volume within the digester is maintained. Heavy particle removal is particularly beneficial for anaerobic liquid digesters 5 such as an up-flow anaerobic sludge blanket (UASB). In a UASB digester, debris forms a blanket on the liquid layer, and liquid percolates up through the floating blanket. Bacteria are principally active in the blanket, and by percolating liquid through the blanket at a controlled flow rate, the liquid gets a lot of bacterial interaction. For process efficiency it is important not to break the blanket, so the controlled flow rate is also important. If grit or lignin fibre builds up in the tank 5, an increased flow of liquid is needed to get the heavy particles into suspension. Too great a flow-rate increase causes the blanket to break and process efficiency to be substantially reduced.
While a traditional CSTR typically has a retention time of 15-50 days, depending on temperature and materials, operation of the present process with heavy particle removal from all vessels typically reduces the retention time to 10-20 days. When a UASB or other high-rate system is used, the retention time can be as little as 12 hours. For clarity, Figure 1 illustrates the removal of heavy particles separately from the passage of liquid from the solids digestion vessel 3, the optional anaerobic digestion vessel 4 and the liquids digestion vessel 5. However, I have found that inorganic particles may advantageously be removed by carrying them with a stream of some or all of the liquid digestate so that the particles are returned to the reception vessel 2. This may be done for the liquids digestion vessel 5 by providing a flow pattern within the liquid digestion vessel 5 which impels inorganic particles to flow out with the liquid digestate via pipework 6. For the solids digestion vessel 3 and/or the anaerobic digestion vessel 4, inorganic particles trapped in a U-bend of other particle trap may also be fed back to the reception vessel 2 along with a portion of liquid fraction via the pipeline 6.
Inorganic particles returned to the reception vessel in this way may readily be removed using mechanical apparatus or a sludge pump. Typically such removal is carried out about once a month depending on the level of usage of the apparatus 1 .
Turning now to Figures 2 and 3, in this embodiment the apparatus 1 includes a screen 8 for separating fermented solids material 7 above a predetermined size after the acid phase hydrolysis stage has been sufficiently completed. The screen size may be from 1 -50 mm, preferably 1 -25 mm, notably 5-10 mm. In this example, the screen 8 is a downward-sloping metal mesh, and treated solids waste 7 passes down the screen under gravity. Some liquid is lost from the solids waste 7 during this process through the action of gravity. The resulting material has a solids content typically in the range 15-25%. Other screening means 8 may be used; for example a run-down screen or a brush screen.
A dewatering means 9, in this example a roller, is arranged and adapted to allow pressure to be applied to the wet solids waste 7 as it passes down the screen 8. The operator can choose whether or not to apply the roller 9, which can be driven by a motor (not shown) via axial drive spigots 10. It will be understood that other dewatering means may be employed, singly or in multiple stages; for example a compression auger, filter press, doctor blade or squeegee, or a centrifuge. The screening of solids may take place internally or externally to the solids digestion vessel 3.
If the dewatering means 9 is operated, the roller pressurises the wet solids waste 7 and drives out much of the excess of liquid 1 1 , leaving low-nutrient fibrous material 12 suitable for use as a plant growth medium or as a solid fuel or building material. Liquid 1 1 which is pressed from the solids material 7 is returned to the apparatus, preferably into the liquids digestion vessel 5 to enable the organic content to be digested and produce additional useful methane. The further dewatered solids waste 12 has a solids content substantially above 25%, notably at least 40%. Typically the solids content is in the range 50-60% after the further dewatering..
If the dewatering means 9 is not operated, the wet solids waste 7 provides a high- nutrient plant growth medium suitable for putting on fields or soil.
The separated fermented solids materials, whether further dewatered or not, may optionally be further treated, for example by composting. The early stage of the process (ie up to the point where the material enters the methanogenic stage) may be used with other anaerobic or aerobic systems as an extra processing stage, and may also be used as a pre-treatment for hydrogen production. Being able to add-on the early stage to an existing digester provides the benefit of being able to treat higher solids content materials an increase in throughput of the existing plant and/or the abil ity to treat a greater range of materials.
The process removes substances which are detrimental to catalytic processes; accordingly in another aspect of the invention, the early stage of the process may be used to provide feedstock for fuel cells or other apparatus which uses catalytic processes
Referring now to Figure 4, another embodiment of the invention includes apparatus for measuring gas evolution from the liquid digestion vessel 5. In this embodiment, the gas measuring apparatus is a floating roof gasholder which is connected to the liquid and gas outlets of the liquids digestion vessel 5. We have found that the passage of warm liquid can help keep the gasholder from freezing in winter weather. Electronic feedback from the gasholder may be used to control the rate of passage of liquid digestate into the liquids digestion vessel.
After solid waste has been sufficiently digested, it passes to a separator from which screened solids are removed as previously described, and liquid from the screened solids is fed to the liquids digestion vessel 5, in this embodiment, via an optional anaerobic digestion vessel 4.
In this example an oil-fired boiler receives oil from a fuel tank and may be used to heat the contents of the solids digestion vessel 3, or other vessels, via a heat exchanger. Also in this example, a combined heat and power generator set (CHP Genset) or a gas-fired boiler is illustrated, either of which may receive and burn methane-rich gas from the liquids digestion vessel 5 to provide useful heat which may be optionally be distributed via the heat exchanger. A control room houses electronic controls for the process.
In this embodiment, the digester system is conveniently housed in shipping containers.
It will be understood that the terms 'dewatering' and similar terms, are used herein to refer to a process of separating liquid from solids. The liquid will be aqueous but will include a variety of dissolved solutes and may contain a suspension of small particles.
The terms 'high nutrient' and 'low nutrient' are relative. Prior to final dewatering, dissolved nutrients in the fibrous material render it particularly suitable for use as a plant-growth material. After dewatering, much of the dissolved nutrients are removed along with the water, rendering the fibrous material particularly suitable for applications such as fuels or use as a building material when suitably processed.
The articles, 'a' and 'an' are used herein to denote 'at least one' unless the context otherwise requires.
It is to be recognized that various alterations, modifications, and/or additions may be introduced into the constructions and arrangements of parts described above without departing from the ambit of the present invention as specified in the claims.

Claims

1 . A method for digesting organic waste comprising the steps of:
providing a source of organic material undergoing anaerobic bacterial digestion in a liquid phase within a liquids digestion vessel;
providing a reception vessel for receiving organic waste materials;
feeding organic waste material containing biodegradable solids from the reception vessel to a solids digestion vessel;
feeding at least a part of the liquid phase from the liquids digestion vessel to the reception vessel;
after a predetermined time, separating solids materials above a specified size from other materials in the solids digestion vessel to produce a high-nutrient plant-growth material;
optionally further dewatering at least some of the separated solids materials to produce a fibrous low-nutrient material and feeding at least some of the liquid from the dewatering process to at least one of the digestion vessels, preferably the liquids digestion vessel;
feeding at least some liquid from the solids digestion vessel to the liquids digestion vessel; and
removing inorganic particles from each of the digestion vessels.
2. A method according to claim 1 , further comprising dewatering at least some of the separated solids material and feeding at least some of the liquid from the dewatering process to at least one of the digestion vessels, preferably the liquids digestion vessel.
3. A method according to claim 1 or claim 2, wherein the apparatus further comprises an anaerobic digestion vessel; the method further comprising feeding fluid from the solids digestion vessel to the anaerobic digestion vessel and feeding fluid from the anaerobic digestion vessel to the liquids digestion vessel, and removing inorganic particles from the anaerobic digestion vessel.
4. A method according to any preceding claim, further comprising the step of feeding removed inorganic material from each of the digestion vessels to the reception vessel.
5. A method according to claim 4, wherein the removed inorganic materials are fed to the reception vessel via the same pipework that liquid is fed from the liquids digestion vessel to the reception vessel.
6. A method according to any preceding claim, wherein separation of the solids material is carried out by means of a screen, preferably having a screen size in the range 1 -50 mm, notably 1 -25 mm, particularly preferably 5-10 mm.
7. A method according to claim 6, wherein the optional further dewatering is carried out by applying pressure, preferably by means of a powered roller, to solids materials when on the screen.
8. A method according to claim 1 , wherein the optional further dewatering is carried out by means of vacuum vibration, a dewatering auger, a screw centrifuge or a belt press.
9. A method according to any preceding claim, further comprising macerating solids waste in the reception vessel and feeding the macerated waste to the solids digestion vessel.
10. A method according to any preceding claim, further comprising blowing air into the solids digestion vessel to promote acid phase hydrolysis.
1 1 . A method according to claim 2, further comprising using the resulting dewatered solids material as a fuel, a soil conditioning agent, or a building material.
12. Use of fluid from the solids digestion vessel produced according to the method of any preceding claim as a feed material for the production of hydrogen gas by anaerobic digestion.
13. Use of fluid from the solids digestion vessel produced according to the method of any of claims 1 -1 1 as a feedstock for a catalytic process.
14. Use of fluid from the solids digestion vessel produced according to the method of any of claims 1 -1 1 as a feedstock for a fuel cell.
15. Use of fluid from the solids digestion vessel produced according to the method of any of claims 1 -1 1 as a feedstock for an anaerobic or aerobic digester.
16. Apparatus for digesting organic waste, the apparatus comprising:
a reception vessel for receiving organic waste to be digested;
a solids digestion vessel;
a liquids digestion vessel;
means for feeding organic waste from the reception vessel to the solids digestion vessel;
means for feeding liquid from the liquids digestion vessel to the reception vessel;
means for receiving screened solids materials above a specified size from the solids digestion vessel;
further dewatering means suitable for applying pressure to the screened solids materials;
means for feeding liquid removed from the screened solids materials to the liquids digestion vessel, optionally via an anaerobic digestion vessel; and
means for removing inorganic particles from each of said digestion vessels.
PCT/EP2012/060771 2011-06-08 2012-06-06 Waste digestion WO2012168341A1 (en)

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