WO2008144830A1

WO2008144830A1 - Methods and apparatus for denitrifying a biomass product stream

Info

Publication number: WO2008144830A1
Application number: PCT/AU2008/000767
Authority: WO
Inventors: Rodney Gerald France; Sandeep Chugh
Original assignee: Grl Investments Pty Ltd
Priority date: 2007-05-28
Filing date: 2008-05-28
Publication date: 2008-12-04
Also published as: AU2008255644A1

Abstract

The invention generally relates to methods and apparatus for the denitrification of a product stream from a biomass digestion system. The product stream can include an aqueous process stream derived from an anaerobic biomass digestion circuit. More particularly, but not exclusively, the invention also relates to methods and apparatus for the removal of excess ammonia via a denitrification process from an anaerobic biomass digestion circuit that maintains the integrity of the biomass component of the digestion circuit in that the denitrification process does not destroy the biological activity of the reaction system or result in a detrimental loss of biomass.

Description

METHODS AND APPARATUS FOR DENITRIFYING A BIOMASS PRODUCT STREAM

Field of the Invention

The present invention generally relates to methods and apparatus for the denitrification of a product stream from a biomass digestion system.

The present invention relates more particularly but not exclusively to methods and apparatus for the denitrification of a product stream from an aqueous process stream derived from an anaerobic biomass digestion circuit.

The present invention relates more particularly but not exclusively to methods and apparatus for the removal of excess ammonia contained in a product stream derived from an anaerobic biomass digestion circuit that is used to process municipal solid wastes via a denitrification process.

The present invention relates more particularly but not exclusively to methods and apparatus for the removal of excess ammonia via a denitrification process from an anaerobic biomass digestion circuit that maintains the integrity of the biomass component of the digestion circuit in that the denitrification process does not destroy the biological activity of the reaction system or result in a detrimental loss of biomass.

The present invention relates more particularly but not exclusively to the removal of excess ammonia via a denitrification process from an anaerobic biomass digestion circuit that maintains the integrity of the biomass component of the digestion circuit in such a manner that there is minimal use of chemical reagents and/or physical process steps thereby bringing about a reduction in capital and operating costs.

Background to the Invention

In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was, at the priority date: (i) part of the common general knowledge, or;

(ii) known to be relevant to attempt to solve any problem with which this specification is concerned.

Disposal of municipal solid wastes into landfill is becoming increasing less attractive in both environmental and economic terms and there is an increasing trend to treat such wastes by various combinations of chemical, physical and biological means to recover recyclable by-products as well as generate energy by decomposition/degradation of the organic components of the waste material.

Decomposition of the organic waste material leads to the generation of ammonia and because of the recycle nature of the decomposition/degradation system, the ammonia concentration in the aqueous process stream tends to increase quite rapidly. Biological activity can be significantly curtailed as the ammonia concentration increases, so that it is necessary to eliminate excess ammonia from the recycled process stream.

Various means for removing ammonia from aqueous process streams have been proposed but all suffer from a range of process and cost constraints. Some processes remove the ammonia as such, while others involve oxidation through to nitrate, which in some cases will be reduced back to nitrogen. Included in such processes are combined biological nitrification/denitrification, air stripping, steam stripping, ion exchange, membrane denitrification and solvent extraction.

The biological processes can be characterised as requiring a high level of process control and generally have long residence times, leading to complex, large processing facilities. Moreover, these processes are prone to operating difficulties because of the presence of toxic components present in the waste stream, while it is critical that a constant feed rate be maintained in order to achieve sustainable biological activity.

Physical/chemical processes have potential advantages over biological systems in that they have greater flexibility in terms of their ability to be adjusted in concert with the content and flow of the incoming waste stream. However, many suffer from significant operational and maintenance problems such as carbonate scaling. This is a particular problem with the conventional air and steam stripping options. Some of the proposed chemical processes involve high reagent consumptions and thus high operating costs. For example, the conventional air stripping approach requires the pH of the waste stream to be increased to at least pH 11 by means of addition of a suitable chemical base such as caustic soda. In cases where the denitrified process stream is returned to an anaerobic biomass digestion circuit it is necessary to lower the pH to a value of about pH 9 so that the biological activity in the biomass digestion circuit can be maintained. This lowering of the pH typically involves the addition of a suitable acid reagent such as citric acid. This adds to the overall operating costs.

Summary of the Invention

According to one aspect of the present invention there is provided a method for the denitrification of a product stream from a biomass digestion system, the method including the steps of: a) ■ subjecting the product stream to a solid/liquid separation stage to provide a recovered biomass component and a liquid component, whereby the recovered biomass component is returned to the biomass digestion system and the separated liquid component is forwarded to a denitrification system; b) heating the separated liquid component to facilitate the removal of ammonia; c) aerating the heated liquid component from step b) with a flow of air to remove at least a portion of the ammonia from the liquid component by transferring ammonia into the flow of air to form a denitrified liquid component; and d) returning the denitrified liquid component to the biomass digestion system.

In a preferred embodiment, the method may include the further step of: e) removing ammonia from the flow of air from step c) containing ammonia.

) Preferably, the biomass digestion system is used to process municipal solid wastes.

In another preferred embodiment, the biomass digestion system is an anaerobic biomass digestion system.

Preferably, the method according to the above aspect is configured and operated to enable the sustainable biological degradation of the biomass in the biomass digestion system by controlling any one of: the concentration of ammonia in the system or level of denitrification; the amount of biomass lost from the system; and/or the flow rates or volumes of the product stream, the liquid component or biomass component.

In a preferred embodiment, the sustainability of the biological degradation in the biomass digestion system is achieved without the need for adding into the system an externally sourced biologically active component.

In another preferred embodiment, in step a) the solid/liquid separation stage is configured and operated such that the separation of the biomass component from the liquid component is sufficient to allow the recovery and return of biomass to the biomass digestion system at a level where the overall activity of the biomass in the biomass digestion system is not detrimentally affected. The solid/liquid separation stage may also include the use of a centrifuge system to separate the biomass component from the liquid component. Although a large range of solid/liquid separation devices or systems would be suitable, it is a preferred embodiment of the invention that any such device or system is capable of separating the biomass component, particularly entrained biomass component, from the liquid component so that, in use, loss of biomass from the system (via containment in the separated liquid component) is substantially limited or eliminated such that the system can operate sustainably without the need for introducing externally sourced bioactive component.

Preferably, step c) includes aerating the heated liquid component with a flow of air in a mixing device to remove excess ammonia from the liquid component by transferring ammonia into the flow of air to form a denitrified liquid component. In step c) the pH value of the liquid component can be substantially less than pH 11. Preferably, prior to or during step c), a chemical or base component is not added to the liquid component to facilitate removal of ammonia by increasing the pH value of the liquid component. In a preferred embodiment of the invention, the pH value of the biomass digestion system, the product stream, the liquid component or the biomass component, is not controlled by the addition of a chemical or base component.

In another preferred embodiment, step e) can include scrubbing the ammonia from the air flow with sulphuric acid to form an ammonium sulphate component suitable for disposal. Preferably, the ammonia is present in the separated liquid component when it is returned to the biomass digestion system at a level sufficient to maintain an acceptable level of overall system water and ammonia balance such that the overall activity of the biomass is not detrimentally affected. In another preferred embodiment, in step b) the separated liquid component is heated to a predetermined temperature in the range of about 6O⁰C to about 7O⁰C. More preferably, the predetermined temperature is about 65⁰C.

Preferably, in step c) the method of aerating the liquid component includes using an aeration vessel capable of operably controlling the retention time of the liquid component and the volume of airflow to ensure that the ammonia concentration of the denitrified liquid component exiting the aeration vessel and re-entering the biomass digestion system is sufficiently low to maintain the efficacy of the biomass digestion system. The aerating can be controlled to reduce the ammonia concentration in the liquid component to a range of about 1500 to 2500 mg/L, more preferably to about 2000 mg/L.

In another aspect, the invention provides an apparatus for the denitrification of a product stream from a biomass digestion system, the apparatus including: a solid/liquid separation means for separating the product stream into a liquid component and a biomass component; a biomass component return means for returning the biomass component to the biomass digestion system after being separated from the liquid component; a liquid component transfer means for transferring the separated liquid component between the solid/liquid separation means, a heating means, an aeration means and the biomass digestion system; the heating means for heating the separated liquid component to facilitate the removal of ammonia; the aeration means for aerating the heated liquid component with a flow of air to remove at least a portion of the ammonia from the liquid component by transferring ammonia into the flow of air before the liquid component is returned as a denitrified liquid component to the biomass digestion system.

In a preferred embodiment, the apparatus includes an ammonia removal means for removing the ammonia from the flow of air containing ammonia. Preferably, the solid/liquid separation means includes a centrifuge system for separating the biomass component from the liquid component.

Preferably, the aeration means includes an aeration vessel configured to operably control the retention time of the liquid component and the volume of airflow to ensure, when in use, that the ammonia concentration of the denitrified liquid component exiting the aeration vessel and re-entering the biomass digestion system is sufficiently low to maintain the efficacy of the biomass digestion system. Preferably, the aeration vessel is configured and designed for being operated to reduce the ammonia concentration in the liquid component, when in use, to a range of about 1500 to 2500 mg/L, preferably about 2000 mg/L.

In another aspect, the invention provides a biomass digestion system, or anaerobic biomass digestion system, that includes the apparatus according to above aspect of the invention.

According to another aspect of the present invention there is provided a method for the denitrification of the product stream exiting an anaerobic biomass digestion circuit, the method comprising the steps of: a) subjecting the product stream to a solid/liquid separation stage by means of a suitable centrifuge system whereby the recovered biomass is returned to the biomass digestion circuit and the clarified solution is forwarded to the denitrification circuit: b) heating the clarified solution to facilitate removal of excess ammonia; c) aerating the hot clarified solution with a flow of air in a mixing device to remove excess ammonia which dissolves in the air flow and is transported out of the clarified solution; d) returning the denitrified solution to the anaerobic biomass digestion circuit; and e) scrubbing the ammonia from the air flow exiting the mixing device with sulphuric acid to yield an ammonium sulphate solution for disposal by suitable means.

According to a further aspect of the present invention there is provided an apparatus for the denitrification of a product stream from a biomass digestion system, the apparatus including solid/liquid separation means for the product stream whereby biomass is recovered and returned to the biomass digestion system and the separated liquid component is forwarded to a denitrification stage, heating means for heating the separated liquid component to facilitate the removal of ammonia, aeration means for aerating the heated liquid component with a flow of air to remove at least a portion of the ammonia from the liquid component by transferring ammonia into the flow of air, recycle means for returning the denitrified liquid component to the biomass digestion system, and ammonia removal means for removing the ammonia from the flow of air containing ammonia. The inventors have identified a means by which the ammonia content of the waste process stream can be reduced to a level that permits the sustainable biological degradation of the organic component of the incoming municipal solid waste stream in a relatively simple manner that requires relatively simple process control procedures, is easily maintained in terms of operating criteria, and does not require the addition of large amounts of expensive and potentially toxic reagents. Moreover, it can be operated in a manner that can be readily adjusted to comply with changes to the volume and composition of the process stream to be treated.

It has long been recognized that high levels of ammonia are toxic to the biomass in anaerobic digestion systems. However, provided that the ammonia concentration of the process water is kept below acceptable limits then the overall activity of the biomass is not detrimentally affected.

The inventors have identified that it is not necessary to completely remove all of the dissolved ammonia in the solution exiting the anaerobic biomass digestion circuit before that solution is returned to the anaerobic biomass digestion circuit in order to maintain the required level of overall circuit water and ammonia balance.

In turn, this led the inventors to identify that it is possible to reduce the ammonia concentration of the process water returned to the anaerobic digestion circuit to an acceptably low level by a combination of relatively simple steps that can be characterised as having at least one of the following advantages:

(i) Low capital cost;

(ii) Low operating cost;

(iii) Operational simplicity; and

(iv) No requirement for pH adjusting chemical both into and out of the system.

The inventors have also identified that it is necessary to substantially preserve anaerobic biomass by means of separating and recycling the entrained biomass in the solution exiting the anaerobic biomass digestion system by means of a solid/liquid separation device, such as a centrifuge. If this entrained biomass is not recovered and recycled, then the anaerobic digestion system becomes inhibited and potentially compromised to the point of collapse of the biological population. Hence according to a further aspect of the present invention there is provided a method for substantially maintaining the biological decomposition of an anaerobic biomass digestion system by denitrifying the system according to the first aspect of the invention, wherein the biomass component is returned to the biomass digestion system after being separated from the liquid component, the denitrified liquid component is returned to the biomass digestion system, and wherein the level of denitrifying -is sufficient to permit the sustainable biological decomposition of the biomass in the biomass digestion system.

According to another aspect of the present invention there is provided a method for denitrifying a product stream from a biomass digestion system, wherein the method includes the steps of separating the product stream into a liquid component and a biomass component, heating the separated liquid component, and denitrifying the liquid component by aeration.

According to a further aspect of the present invention there is provided an apparatus for denitrifying a product stream from a biomass digestion system, the apparatus including mechanical means for separating the product stream into a liquid component and a biomass component, heating means for heating the separated liquid component, and aeration means for denitrifying the liquid component.

Preferably, the denitrification methods and apparatus according to the invention are performed or operated without adding, or adding any significant quantity of, a chemical component to control the pH.

Typically, the product stream is obtained from an anaerobic biomass digestion circuit, for example a municipal effluent or waste digestion circuit containing organic matter. Denitrifying the liquid component by aeration means allows the transfer of nitrogen, substantially in the form of ammonia, from the liquid component into the flow of air provided by the aeration means.

Separation of the product stream into a liquid component and a biomass component will typically be achieved by mechanical means. A separation device, such as a centrifuge system, can be used for this purpose. Typically, the separated biomass component is returned to the biomass digestion system. Preferably, the mechanical means is capable of separating entrained biomass from the product stream. In a preferred embodiment, the method is adapted to substantially preserve anaerobic biomass by means of sufficiently separating and recycling the biomass in the product stream back into the anaerobic biomass digestion system, for example by using a centrifugal solid/liquid separation device.

The separated liquid component is preferably heated to a temperature in the range of about 60⁰C to about 70⁰C₁ and more preferably to a temperature of about 65°C.

Preferably, the aeration means will include an aeration vessel designed to operably control the retention time of the liquid component and the volume of airflow to ensure that the ammonia concentration of the denitrified liquid component exiting the aeration vessel and re- entering the biomass digestion system is sufficiently low to maintain the efficacy of the anaerobic biomass digestion system. Preferably, the aeration means is controlled to reduce the ammonia concentration in the liquid component to a range of about 1500 to 2500 mg/L, and more preferably to about 2000 mg/L. Preferably, the ammonia transferred into the flow of air is removed or scrubbed by reaction with sulphuric acid to yield an ammonium sulphate component suitable for disposal.

Preferably, the level of denitrification will be sufficient to permit the sustainable biological decomposition of the biomass in the biomass digestion circuit. The flow rate and volume of the product stream can also be controlled such that the level of denitrification is sufficient to permit the sustainable biological decomposition of the biomass in the biomass digestion circuit.

Detailed Description of the Invention

The following detailed description of a presently preferred but non-limiting embodiment of the present invention refers to the overall process flowsheet outlined in Figure 1.

In the embodiment shown in Figure 1, the organic component of municipal solid waste [1] is treated in an anaerobic biomass digestion circuit [2] where the organic component of the waste undergoes a series of decomposition reactions, leading amongst other things, to the generation of ammonia which becomes dissolved in the aqueous reaction system. Following the required retention time, the aqueous reaction product [3] is discharged from the anaerobic biomass digestion circuit. This aqueous reaction product [3] typically contains 2-3% entrained biomass. This finely dispersed material is removed from the aqueous reaction product [3] by means of a suitable biomass retention system, for example a mechanical means in the form of a centrifuge [4]. The recovered biomass [5] is returned to the anaerobic biomass digestion circuit [2].

The clarified aqueous reaction product containing a significant ammonia concentration produced by the biological decomposition of the organic component of the municipal solid waste [6] is directed to a suitable heat exchanger [7] where the temperature is increased to the optimum temperature, which typically is in the range of about 60⁰C to about 7O⁰C, preferably about 65⁰C.

The heated, clarified ammonia-containing reaction solution [8] is pumped into a suitable aeration vessel [9] through which fresh air [10] is sparged whereby the free ammonia in the solution dissolves in the stream of air and is transported out of the water. The design of the aeration vessel itself, as well as retention time and the volume of air are collectively designed to ensure that the ammonia concentration of the treated (denitrified) water [11] exiting the aeration vessel [9] is at or below a preset level. This preset level is such that the contained ammonia concentration does not have any deleterious effect on the efficacy of the anaerobic biomass digestion circuit [2]. Typically this upper ammonia concentration will be of the order of about 2000 mg/L.

Various designs of the aeration vessel [9] may be used. The preferred aeration vessel consists of a fully enclosed, insulated, rectangular vessel divided into six equally sized cells arranged in a "U" configuration such that the treated solution exits at the same end as the incoming solution enters. Each cell is provided with a baffle plate that directs the incoming water from the previous cell to the bottom of the receiving cell to minimise short- circuiting. Each cell contains a sparger array fitted to the bottom through which air is introduced. Those skilled in the art will appreciate that alternative designs for the aeration vessel may be applicable.

The ammonia-containing air collects in the gas space above the aqueous phase in the aeration vessel [9] and is ducted [12] out of the aeration vessel and forwarded to an ammonia scrubbing circuit [13] which consists of a packed bed column with a re-circulating sulphuric acid solution. The ammonia contained in the air stream is converted to ammonium sulphate [14] and is periodically bled from the scrubber. The scrubbed air [15] exists the top of the scrubber and is discharged to the atmosphere via a suitable biofiltration system [16].

In the preceding description of the invention and in the claims which follow, except where the context requires otherwise due to express language or necessary implications, the words "comprise" or variations such as "comprises" or "comprising" are used in an inclusive sense, ie., specify the presence of the stated features, but not to preclude the presence or addition of further features in various embodiments of the invention.

It is to be understood that this invention and the preferred embodiments are not limited to the particular materials described, as they may vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present invention in any way.

It is also to be noted that, as used herein, the singular forms of "a", "an" and "the" include the plural unless the context clearly requires otherwise. Unless defined otherwise, all technical and scientific terms herein have the same meanings as commonly understood by one of ordinary skill in the art to which the invention belongs.

Claims

What is claimed is:

1. A method for the denitrification of a product stream from a biomass digestion system, the method including the steps of: a) subjecting the product stream to a solid/liquid separation stage to provide a recovered biomass component and a liquid component, whereby the recovered biomass component is returned to the biomass digestion system and the separated liquid component is forwarded to a denitrification system; b) heating the separated liquid component to facilitate the removal of ammonia; c) aerating the heated liquid component from step b) with a flow of air to remove at least a portion of the ammonia from the liquid component by transferring ammonia into the flow of air to form a denitrified liquid component; and d) returning the denitrified liquid component to the biomass digestion system.

2. The method of claim 1 , wherein the method includes the further step of: e) removing ammonia from the flow of air from step c) containing ammonia.

3. The method of claim 1 or claim 2, wherein the biomass digestion system is used to process municipal solid wastes.

4. The method of any one of claims 1 to 3, wherein the biomass digestion system is an anaerobic biomass digestion system.

5. The method of any one of claims 1 to 4, the method being configured and operated to enable the sustainable biological degradation of the biomass in the biomass digestion system by controlling any one of: the concentration of ammonia in the system or level of denitrification; the amount of biomass lost from the system; and/or the flow rates or volumes of the product stream, the liquid component or biomass component.

6. The method of claim 5, wherein the sustainability of the biological degradation in the biomass digestion system is achieved without the need for adding into the system an externally sou reed biologically active component.

7. The method of any one of claims 1 to 6, wherein in step a) the solid/liquid separation stage is configured and operated such that the separation of the biomass component from the liquid component is sufficient to allow the recovery and return of biomass to the biomass digestion system at a level where the overall activity of the biomass in the biomass digestion system is not detrimentally affected.

8. The method of any one of claims 1 to 7, wherein in step a) the solid/liquid separation stage includes the use of a centrifuge system to separate the biomass component from the liquid component.

9. The method of any one of claims 1 to 8, wherein step c) includes aerating the heated liquid component with a flow of air in a mixing device to remove excess ammonia from the liquid component by transferring ammonia into the flow of air to form a denitrified liquid component.

10. The method of any one of claims 2 to 9, wherein step e) includes scrubbing the ammonia from the air flow with sulphuric acid to form an ammonium sulphate component suitable for disposal.

11. The method of any one of claims 1 to 10, wherein in step c) the pH value of the liquid component is substantially less than pH 11.

12. The method of any one of claims 1 to 11, wherein prior to or during step c), a chemical or base component is not added to the liquid component to facilitate removal of ammonia by increasing the pH value of the liquid component.

13. The method of any one of claims 1 to 11, wherein the pH value of the biomass digestion system, the product stream, the liquid component or the biomass component, is not controlled by the addition of a chemical or base component.

14. The method of any one of claims 1 to 13, wherein ammonia is present in the separated liquid component when it is returned to the biomass digestion system at a level sufficient to maintain an acceptable level of overall system water and ammonia balance such that the overall activity of the biomass is not detrimentally affected.

15. The method of any one of claims 1 to 14, wherein in step b) the separated liquid component is heated to a predetermined temperature in the range of about 6O⁰C to about 7O⁰C.

16. The method of claim 15, wherein the predetermined temperature is about 65⁰C.

17. The method of any one of claims 1 to 16, wherein in step c) the method of aerating the liquid component includes using an aeration vessel capable of operably controlling the retention time of the liquid component and the volume of airflow to ensure that the ammonia concentration of the denitrified liquid component exiting the aeration vessel and re-entering the biomass digestion system is sufficiently low to maintain the efficacy of the biomass digestion system.

18. The method of any one of claims 1 to 17, wherein in step c) the aerating is controlled to reduce the ammonia concentration in the liquid component to a range of about 1500 to 2500 mg/L.

19. The method of any one of claims 1 to 18, wherein in step c) the aerating is controlled to reduce the ammonia concentration in the liquid component to about 2000 mg/L.

20. An apparatus for the denitrification of a product stream from a biomass digestion system, the apparatus including: a solid/liquid separation means for separating the product stream into a liquid component and a biomass component; a biomass component return means for returning the biomass component to the biomass digestion system after being separated from the liquid component; a liquid component transfer means for transferring the separated liquid component between the solid/liquid separation means, a heating means, an aeration means and the biomass digestion system; the heating means for heating the separated liquid component to facilitate the removal of ammonia; the aeration means for aerating the heated liquid component with a flow of air to remove at least a portion of the ammonia from the liquid component by transferring ammonia into the flow of air before the liquid component is returned as a denitrified liquid component to the biomass digestion system.

21. The apparatus according to claim 20, including an ammonia removal means for removing the ammonia from the flow of air containing ammonia.

22. The apparatus according to claim 20 or claim 21 , wherein the solid/liquid separation means includes a centrifuge system for separating the biomass component from the liquid component.

23. The apparatus according to any one of claims 20 to 22, wherein the heating means is capable of providing a predetermined temperature in the range of about 60⁰C to about 70⁰C.

24. The apparatus according to any one of claims 20 to 23. wherein the aeration means includes an aeration vessel configured to operably control the retention time of the liquid component and the volume of airflow to ensure, when in use, that the ammonia concentration of the denitrified liquid component exiting the aeration vessel and re-entering the biomass digestion system is sufficiently low to maintain the efficacy of the biomass digestion system.

25. The apparatus according to claim 24, wherein the aeration vessel is configured and designed for being operated to reduce the ammonia concentration in the liquid component, when in use, to a range of about 1500 to 2500 mg/L, preferably about 2000 mg/L.

26. A biomass digestion system including the apparatus according to any one of claims 20 to 25.

27. A anaerobic biomass digestion system including the apparatus according to any one of claims 20 to 25.