GB2491818A

GB2491818A - Waste disposal

Info

Publication number: GB2491818A
Application number: GB1109564.3A
Authority: GB
Inventors: Christopher Paul Reynell
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-06-08
Filing date: 2011-06-08
Publication date: 2012-12-19
Also published as: EP2718247A1; GB201109564D0; WO2012168341A1

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 organic waste material containing biodegradable solids to a solids digestion vessel 3, feeding at least a part of the liquid phase from the liquids digestion vessel 3 to the solids digestion vessel 5, 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, feeding at least some liquid from the solids digestion vessel 3 to the liquids digestion vessel 5 and optionally further dewatering at least some of the separated solids materials 7 to produce a fibrous low-nutrient material 12 suitable for use as a solid fuel and/or a building material, and feeding at least some of the liquid 11 from the dewatering process to at least one of the digestion vessels, preferably the liquids digestion vessel 5. In a preferred embodiment, heavy material such as grit is removed from each of the process stages.

Description

WASTE DIGESTION

BACKGROUND

a. Field of the Invention

The present invention relates to methods 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 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; and Figure 3 illustrates apparatus for use in an embodiment of the invention.

DETAILED DESCRIPTION

Apparatus I 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. In the embodiment of Figure 1, an optional reception vessel 2 is provided, 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 2-5%.

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 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. During this stage, the waste material undergoes acid phase hydrolysis 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 blanket 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, uplfow anaerobic sludge blanket or other high rate methods.

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 particies have been caught in a trap in the soiids 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 that heavy particles are present at each stage of the 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 may be promoted by changes in pH and/or the hardness of the water.

Accordingly, in the preferred embodiment, 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, each optional vessel 2,4 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 so may be used; for example, 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 grit particles 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. Jf 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.

Turning now to Figures 2 and 3, in this embodiment the apparatus I 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 11, leaving low-nutrient fibrous material 12 suitable for use as a plant growth medium or as a solid fuel or building material. Liquid 11 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 dewateri ng..

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 ability 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 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

CLAIMS1. 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 organic waste material containing biodegradable solids to a solids digestion vessel; feeding at least a part of the liquid phase from the liquids digestion vessel to the solids digestion 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; and feeding at least some liquid from the solids digestion vessel to the liquids digestion vessel.
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.
4. A method according to any preceding claim, further comprising the step of removing heavy particles from each of the digestion vessels.
5. A method according to any preceding claim, wherein separation of the solids material is carried out by means of a screen.
6. A method according to claim 5, wherein the screen has a screen size in the range 1-50 mm, preferably 1-25 mm, particularly preferably 5-10 mm.
7. A method according to claim 5 or 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 a reception vessel and feeding the macerated waste to the solids digestion vessel.
10. A method according to any preceding claim, further comprising the step of removing heavy particles from each of the solids digestion vessel and the liquids digestion vessel.
11. A method according to any preceding claim, wherein the step of feeding at least a part of the liquid phase from the liquids digestion vessel to the solids digestion vessel further comprises the steps of feeding fluid from the solids digestion vessel to an anaerobic digestion vessel and feeding fluid from the anaerobic digestion vessel to the liquids digestion vessel.
12. A method according to claim 11, further comprising the step of removing heavy particles from the anaerobic digestion vessel.
13. A method according to claim 2, further comprising using the resulting -11 -dewatered solids material as a fuel, a soil conditioning agent, or a building material.
14. 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.
15. Use of fluid from the solids digestion vessel produced according to the method of any of claims 1-12 as a feedstock for a catalytic process.
16. Use of fluid from the solids digestion vessel produced according to the method of any of claims 1-12 as a feedstock for a fuel cell.
17. Use of fluid from the solids digestion vessel produced according to the method of any of claims 1-12 as a feedstock for an anaerobic or aerobic digester.
18. Apparatus for digesting organic waste, the apparatus comprising: a solids digestion vessel, a liquids digestion vessel, and means for feeding fluid between the two vessels; means for receiving screened solids materials above a specified size from the solids digestion vessel; and further dewatering means suitable for applying pressure to the screened solids materials.
19. Apparatus for digesting organic waste, the apparatus comprising: a solids digestion vessel, a liquids digestion vessel, and means for feeding fluid between the two vessels; and an optional anaerobic digestion vessel arranged and adapted to receive fluid from the solids digestion vessel and to feed fluid to the liquids digestion vessel; wherein each of said vessels is provided with a trap for removing heavy particles.