WO2008037429A1

WO2008037429A1 - A method and apparatus for the treatment of organic slurry

Info

Publication number: WO2008037429A1
Application number: PCT/EP2007/008328
Authority: WO
Inventors: Lorna Lawrence; Gordon Forbes
Original assignee: FINRONE Ltd
Current assignee: FINRONE Ltd
Priority date: 2006-09-25
Filing date: 2007-09-25
Publication date: 2008-04-03
Anticipated expiration: 2009-03-25
Also published as: GB0618760D0

Abstract

A method and an apparatus for the treatment of organic slurry is disclosed. The method comprises the steps of separating the slurry into a solid and a liquid fraction and treating the liquid fraction using anoxic and aerobic microbial digestion process. The liquid fraction is filtered during the aerobic process. The apparatus comprises a reactor having two vessels one of which is for the anoxic process and the other is for the aerobic process. The vessels are in liquid communication and, preferably, wherein the vessels are concentrically disposed.

Description

A Method and Apparatus for treating Organic Slurry

The present invention is concerned with a method and apparatus for the treatment of organic slurry, which may be used to recover organic matter and nutrients from animal manure slurry, for instance pig slurry, as well as any industrial or agricultural waste with high organic matter and nutrient concentrations. The solids extracted from such slurry can then be used as a high energy biomass renewable energy source.

Over the last twenty years, and especially in the last five years due to increased regulatory and public pressure placed upon intensive farming production and other high strength waste generators, extensive research has been done in the US, Europe and Japan to develop various processes to treat high-organic waste streams, such as pig slurry and other animal manures. See for example United States Patent Specification No. US-A-5 885 461, United States Patent Specification No. US-A-6 054 044; United States Patent Specification No. ϋS-A-6 083 386; and United States Patent Specification No. US-A-6 692 642.

The present invention has bean developed to provide an improved method for treating such organic slurry, and in particular to provide a method capable of significantly increased volumetric loading rates, reduced reactor volume, reduced retention time, long solids retention time and reduced solids production. According to an aspect of the present invention, there is provided a method as specified in claim 1. According to another aspect of the present invention, there is provided an apparatus as specified in claim 23.

According to a first aspect of the present invention there is provided a method for the treatment of organic slurry, the method comprising the steps of separating the slurry into a solid and a liquid fraction; treating the liquid fraction using anoxic and aerobic microbial digestion processes; and filtering the liquid fraction during the aerobic process thereof .

Preferably, the method of separating the solid fraction comprises adding to the slurry a coagulant or flocculant for the agglomeration of particles present in the slurry.

Preferably, the flocculant comprises a polymer in the form of a powder, and/or beads, and/or emulsions, and/or liquids or dispersions.

Preferably, the flocculant is cationic and binds with anionic components in the slurry to form an insoluble flocculant .

Preferably, the step of adding the flocculant comprises using a metered polymer device that effects polymer inversion where the polymer passes from a continuous oil phase to a continuous water phase and finally a dissolution phase.

Preferably, the polymer emulsion is added to water and not the reverse .

Preferably, the step of adding the flocculant to the slurry comprises mixing and applying shear energy when the flocculant comes into contact with the slurry.

Preferably, the polymer emulsion comprises fresh water and polymer .

Preferably, the method comprises separating the solid fraction using a centrifugal decanter, rotary press or other separation technology which is capable of bringing about the separation of solids from an organic slurry.

Preferably, the treatment of the liquid fraction comprises removing suspended solids by including induced air flotation.

Preferably, the treatment of the liquid fraction comprises removing nitrogen and/or phosphorous compounds by dosing the liquid fraction with magnesium chloride or other salts.

Preferably, the method further comprises precipitating out the nitrogen and/or phosphorous compounds of the liquid fraction and feeding them back to said untreated organic slurry.

Preferably, the method further comprises agitating the liquid fraction during the aerobic process.

Preferably, the anoxic process comprises metabolising facultative anaerobic denitrifying bacteria.

Preferably, the aerobic process comprises metabolising nitrifying bacteria.

Preferably, the aerobic process comprises aerating the liquid fraction.

Preferably, the treatment of the liquid fraction comprises recycling the liquid fraction between the aerobic process and anoxic process.

Preferably, the level of recycling is controlled by monitoring the liquid fraction.

Preferably, the filtration of the liquid fraction comprises passing the fraction through a membrane.

Preferably, the step of aerating the liquid fraction comprises directing a stream of air against the membrane .

Preferably, the method further comprises using the solid fraction as a biomass fuel. Preferably, the method further comprises using the extracted biomass nutrients as a feed for a fluidised bed reactor; combusting the biomass material and producing energy for heating or generation of electricity.

Preferably, the method further comprises drying, compressing and pelletizing the dried solid fraction.

The invention is also directed to apparatus for carrying out the disclosed method and including apparatus parts for performing each described method step, be it by way of hardware components, computer programme by appropriate software, by any combination of the two or in any other manner.

According to a second aspect of the invention there is provided an apparatus for the treatment of organic slurry, the apparatus comprising a reactor comprising a first vessel in which, in use, anoxic processing of a liquid fraction of the slurry occurs, and a second vessel in which, in use, aerobic processing of the liquid fraction occurs and wherein the vessels are in liquid communication.

Preferably, a membrane is located within the second vessel, through which the liquid fraction can be filtered. Preferably, the reactor is adapted to effect recycling of the liquid fraction between the first and second vessels .

Preferably, the first vessel is an outer vessel and the second vessel is located substantially concentrically within the outer vessel .

The invention will be understood in greater detail from the following description of a preferred embodiment thereof given by way of example only and with reference to the accompanying drawings in which: -

Figure 1 illustrates a side elevation of a preferred embodiment of an apparatus for treating organic slurry according to the present invention;

Figure 2 illustrates a front elevation of the apparatus illustrated in Figure 1; and

Figure 3 illustrates an end elevation of the apparatus of Figures 1 and 2.

Referring now to the drawings, there is illustrated an apparatus, generally indicated as 10, which is adapted to effect a method of treating organic slurry as described in detail hereinafter. An embodiment of the invention provides a process for treating organic slurry waste comprising a mixture of solids suspended in a liquid, colloidal solids and dissolved pollutants such as nitrogen, phosphorus and organic matter, in a continuous process involving a number of steps as described hereinafter in detail .

Initially the slurry waste is received, stored and mixed in conventional fashion in a reservoir 12 of the apparatus 10. The first step in the method of treatment is the addition of a polymer to the feed stream. This is added through a metered system (not shown) that provides a mixing chamber blade and orifice that minimises polymer requirements. The slurry is then separated into a solid fraction and a liquid fraction. This separation is preferably performed using a centrifugal decanter or rotary press 14 or any other suitable alternative. Solids capture efficiency is enhanced by cationic polymer addition, described above and below. The solids fraction, or "cake", retains a significant portion of the total amount of solids, organic matter, nitrates and phosphorus from the slurry. The solids fraction is transported from the centrifugal decanter or rotary press 14 by means of an auger 16 or any other suitable alternative. The liquid fraction or filtrate or centrate still retains some particulate and colloidal matter and all of the dissolved pollutants.

The next step in the method is the downstream treatment of the liquid and solid fractions. The liquid fraction or filtrate or centrate now contains, for example, between 1000 - 2000 mg.l^"1 suspended solids (SS). Induced air flotation or the like may be used to remove most of the suspended solids and increase the efficiency of the downstream biological processes. Other forms of separation may of course be employed to achieve this removal of suspended solids. This step may be performed in a holding tank 18.

The liquid fraction in the tank 18 is now ready to be passed to a reactor 20 for the biological processing thereof. However, prior to entry into the reactor 20, the liquid fraction is, preferably, dosed with magnesium chloride or other salts that will bring about the complexing and precipitation of nitrogen and phosphorous compounds. This precipitate is preferably fed back to the raw slurry storage tank where it will be removed by the initial solid/liquid separation as the raw slurry begins the treatment process.

The steps of inducing air flotation and dosing with magnesium chloride are dependant on the efficiency of phosphate removal by the primary solid/liquid separation stage and may be omitted if the phosphorous removal is high.

The reactor 20 is preferably in the form of a membrane bioreactor (MBR) . The MBR 20 comprises two zones - an anoxic zone defined by an outer vessel 22 where the liquid is gently mixed by a mixer (not shown) located near the bottom of the vessel 22. The role of the mixer is to keep solids in suspension and encourage the growth and metabolism of facultative anaerobic denitrifying bacteria that dominate this environment. These bacteria use oxides of nitrogen rather than oxygen as an electron acceptor and thereby reduce nitrates through a process of denitrification to form nitrogen gas.

The second zone of the MBR 20 is an aerobic zone defined by an inner vessel 24 located concentrically within the outer vessel 22. In the inner vessel 24, air is forced through the suspension to encourage the growth and metabolic activity of nitrifying bacteria which oxidise ammonia or nitrite as their sole energy source. Heterotrophic bacteria will also convert carbon to water and carbon dioxide within this zone. This zone will contain typically 1-3. mgl^"1 dissolved oxygen.

The liquid suspension is cycled between the aerobic and anoxic zones 22, 24 via a transfer pipe 26 to ensure that denitrification is optimised, and, preferably, so that the effluent will meet the legislative requirements of the European Union Nitrates Council Directive 91/676/EEC. The level of recycling is, preferably, controlled by monitoring the effluent from this process and the allowable levels may depend on various factors, for example the spread land available for disposal or the downstream applications required for recycling of water, depending on the system requirements .

The inner vessel 24 incorporated a membrane module 28 comprising a number of membranes (not shown) . In use, the membrane module 28 will be submerged in the liquid fraction therein. These membranes, in the preferred embodiment described herein, have a pore size not exceeding 0.2μm. The bacterial suspension within the aerobic zone 24 is in direct contact with the surface of the membrane module 28 and some of the water is separated from the suspension and passes through the membrane. The water that passes through the membrane is called "permeate", and may be withdrawn via an outlet 30 in the apparatus 10. The part of the biomass including wastewater, suspended solids, and dissolved organic solids that do not pass through the membrane as permeate is called "reject". The amount of permeate a membrane can produce per unit of membrane area is called "flux rate" and is a function of the differential pressure across the membrane and the effectiveness of cross flow at keeping the membrane from fouling. The membranes can be tubular or in sheet format . Sheet membranes are made by modifying the surface of a thicker sheet to form a dense, microporous film on top and serves as the working membrane that actually rejects the solids and large molecular weight solutions as water flows through. The function of the remaining and thicker part of the sheet is to provide support .

The membranes are submerged in the bioreactor 20. The head pressure of the wastewater on the outside of the submerged membranes provides lower but sufficient differential pressure to drive the wastewater through the membranes and concentrate the biomass in the bioreactor 20. The head pressure may be supplemented by a suction pump (not shown) connected to the permeate outlet side to create a higher differential pressure across the membranes.

The solids in the biomass (sometimes called "sludge") can build up and foul the membranes. For this reason air or other suitable gas is preferably continuously bubbled through the aerobic zone 24 and through the use of diffusers air or other suitable gas is continuously- forced from the bottom of the bioreactor 20 across the surface of the membranes to dislodge any build up of biomass on the membrane surface.

Since the apparatus 10 removes solids by filtration rather than settling, the bioreactor 20 can operate at a much higher mixed liquor suspended solids (MLSS) concentration making the bioreactor volume much smaller than that of conventional technology. The bioreactor 20 can be operated at an MLSS of up to 35 000 mg.l^"1 compared to 3 000 to 5 000 mg.l^'1 for a conventional system. The higher MLSS concentration in the MBR 20, coupled with a longer solids retention time, means that sludge production can be reduced significantly when compared to conventional systems. Also the time taken for denitrification, nitrification and reduction of harmful pollutants to harmless products is significantly reduced from conventional systems.

This process also reduces the Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) of the effluent when compared to conventional systems with reductions of up to 98% being achieved. The approach of the method of treatment according to the present invention to organic slurry and manure treatment differs from that of conventional processes, which usually rely on anaerobic digestion. The present organic slurry treatment process is based on effectively separating solids from liquids in the raw slurry stream. Solids can be combusted without any further processing using a fluidised bed reactor or they may be further treated and may be mixed with other organic material such as wood, peat, straw or other plant material to produce energy rich biomass for renewable energy production. The liquid fraction is subjected to physical, chemical, and/or biological treatment. The treatment of the liquid fraction using the above process removes suspended solids, colloidal solids, nitrogen, phosphorus and dissolved organic matter thereby reducing the biological oxygen demand (BOD) and chemical oxygen demand (COD) to levels whereby the liquid is below the regulated requirement.

Advantageously, the process according to the present invention can eliminate the need for waste lagoons and/or land application when treating animal manure slurries.

Alternatively, for treating organic slurries having a low solids content, for example wastewater, the slurry need not be separated initially to remove solids but can be treated directly according to the above process steps .

The synergies brought about by the biological nutrient removal/membrane bioreactor 20 combination enable the system to remove efficiently very high dissolved organic matter, nitrogen and phosphorus loads. Additionally, this combination provides high solids retention time, which introduces process stability, reduces biomass production and enables slow-growing organisms, such as nitrifiers, to establish a healthy population.

Effective separation of liquid and biomass achieved with microfiltration membranes submerged in the aerobic zone 24 allows the apparatus 10 to operate with extremely high biomass concentrations and long solids retention times.

The liquid, or permeate, passing through the membrane has no suspended solids, extremely low phosphorus, nitrogen and dissolved organic matter. Microfiltration removes a significant portion of pathogen indicator organisms and produces an effluent with low turbidity. If necessary, the permeate is polished by using reverse osmosis and can be disinfected prior to reuse or may be discharged depending on spread land available.

The combination of the suspended growth in the anoxic 22 and aerobic zones 24, the biomass recycle pattern, and the microfiltration membranes producing permeate from the aerobic zone 24 creates a synergistic effect that enables biological removal of high loads of nitrogen, phosphorus and dissolved organic matter contained in any liquid waste with high concentrations thereof .

As mentioned above, polymer blending storage units and metering units (not shown) may add cationic polymer to the separator feed. Polymer addition rate is automatically controlled based on flow. In-line high- energy mixing enables adequate polymer dispersion in the decanter feed line. The cationic polymer addition enhances solids and phosphorus recovery. Over 85% of phosphorus in manure slurry is contained in organic particulate matter and the majority of this can be removed along with the solids.

Approximately ten to twenty percent of the feed slurry volume is separator output which contains solids "cake" with twenty five to thirty five percent dry solids content. The rest exits the decanter as liquid fraction, filtrate or centrate.

Operation as a membrane bioreactor has the advantage of allowing MLSS concentrations as high as 35 000 mg.l^"1, compared with 3 000 mg.l^"1 of conventional activated sludge bioreactors. This allows increased volumetric loading rates, reduced reactor 20 volume, reduced retention time, long solids retention time, and reduced solids production. Permeate (e. g., membrane effluent) contains less than 1 mg.l^"1 of TSS and extremely low turbidity. Membranes achieve coliform bacteria reductions of over 6 log orders. Permeate is well suited for further disinfection if required. Coarse air bubbles scour the membranes to reduce biological build-up .

Treated effluent flow through the membranes is controlled by available gravity head above membrane cases. In-situ chemical cleaning with a backwash of 0.5 percent sodium hypochlorite solution is recommended periodically depending on membrane performance. This requires approximately 6 hours off-line.

Nutrient and organic matter removal in the system takes place as follows. Most nitrogen is removed by denitrification in the anoxic zone 22. For de- nitrification to occur, ammonium-nitrogen contained in the waste must be first oxidized to nitrate (nitrified) . Nitrification occurs in the aerobic zone 24, downstream of the anoxic zone 22. Nitrified liquor is recycled from the aerobic zone 24 to the anoxic zone 22 at a rate that is dependent on the required level of denitrification and according to the spread land available, or any other factors to be considered.

The present invention does not depend on microbial decomposition for generation of heat energy or utilisation of gases produced from the combustion of the organic biomass as described in the background art . The invention provides a method of treatment whereby the organic biomass material can be directly combusted using for example a fluidised bed reactor or similar alternative, or the cake can be further treated on its own or mixed with other biomass materials such as wood, peat, straw or other crop material, and compressed into pellets or bricks as a source of biofuel for combustion and production of heat that can either be used for heat exchange or for generation of electricity.

The synergies created by the specific sequencing of treatment systems in this process maximize treatment capacity and recovery of organic matter and nutrients, reduce treatment time and significantly reduce the footprint required for a bioreactor system. A further feature of this invention is that the organic slurry treatment process can be customised to fit with the land requirements as specified in the EU Nitrates

Directive. This Directive stipulates that the maximum nitrogen loading for land should not exceed 70kg/ha/year . The organic treatment process described can produce effluent in line with each individual farmer's (or other user) land area. The process can be controlled to produce a liquid effluent that when spread over the relevant land area it will meet the EU Nitrates Directive requirements and other water and nutrient based legislative requirements.

The present invention is not limited to the embodiment described herein, which may be amended or modified without departing from the scope of the present invention .

Claims

Claims :

1. A method for the treatment of organic slurry, the method comprising the steps of separating the slurry into a solid fraction and a liquid fraction; treating the liquid fraction using anoxic and aerobic microbial digestion processes; and filtering the liquid fraction during the aerobic process thereof.

2. A method as claimed in claim 1 wherein the method of separating the solid fraction comprises adding to the slurry a coagulant or flocculant for the agglomeration of particles present in the slurry.

3. A method as claimed in claim 2 wherein the flocculant comprises a polymer in the form of a powder, and/or beads, and/or emulsions, and/or liquids or dispersions .

4. A method as claimed in claim 2 wherein the flocculant is cationic and binds with anionic components in the slurry to form an insoluble flocculant .

5. A method as claimed in any of claims 2-4 wherein the step of adding the flocculant comprises using a metered polymer device that effects polymer inversion where the polymer passes from a continuous oil phase to a continuous water phase and finally a dissolution phase.

6. A method as claimed in any of claims 2-5, the step of adding the flocculant to the slurry comprises mixing and applying shear energy when the flocculant comes into contact with the slurry.

7. A method as claimed in claim 3 wherein the polymer emulsion comprises fresh water and polymer.

8. A method as claimed in any of claims 1-7 further comprising separating the solid fraction using a centrifugal decanter, rotary press or other separation technology which is capable of bringing about the separation of solids from an organic slurry.

9. A method as claimed in any of claims 1-8 wherein, the treatment of the liquid fraction comprises removing suspended solids by including induced air flotation.

10. A method as claimed in any of claims 1-8 wherein, the treatment of the liquid fraction comprises removing nitrogen and/or phosphorous compounds by dosing the liquid fraction with magnesium chloride or other salts.

11. A method as claimed in any of claims 1-10 which further comprises precipitating out the nitrogen and/or phosphorous compounds of the liquid fraction and feeding them back to said untreated organic slurry.

12. A method as claimed in any of claims 1-11 which further comprises agitating the liquid fraction during the aerobic process .

13. A method as claimed in any of claims 1-12 wherein, the anoxic process comprises metabolising facultative anaerobic denitrifying bacteria.

14. A method as claimed in any of claims 1-13 wherein, the aerobic process comprises metabolising nitrifying bacteria .

15. A method as claimed in any of claims 1-14 wherein, the aerobic process comprises aerating the liquid fraction.

16. A method as claimed in any of claims 1-15 wherein, the treatment of the liquid fraction comprises recycling the liquid fraction between the aerobic process and anoxic process .

17. A method as claimed in any of claims 1-16 wherein, the level of recycling is controlled by monitoring the liquid fraction.

18. A method as claimed in any of claims 1-17 wherein, the filtration of the liquid fraction comprises passing the fraction through a membrane.

19. A method as claimed in any of claims 1-18 wherein, the step of aerating the liquid fraction comprises directing a stream of air against the membrane.

20. A method as claimed in any of claims 1-19 wherein, the method further comprises using the solid fraction as a biomass fuel .

21. A method as claimed in any of claims 1-201 wherein, the method further comprises using the extracted biomass nutrients as a feed for a fluidised bed reactor; combusting the biomass material and producing energy for heating or generation of electricity.

22. A method as claimed in any of claims 1-21 wherein, the method further comprises drying, compressing and pelletizing the dried solid fraction.

23. An apparatus for the treatment of organic slurry, the apparatus comprising a reactor comprising a first vessel in which, in use, anoxic processing of a liquid fraction of the slurry occurs, and a second vessel in which, in use, aerobic processing of the liquid fraction occurs and wherein the vessels are in liquid communication.

24. An apparatus as claimed in claim 23 wherein, a membrane is located within the second vessel, through which the liquid fraction can be filtered.

25. An apparatus as claimed in claim 23 or 24 wherein means is provided to effect recycling of the liquid fraction between the first vessel and the second vessel .

26. An apparatus as claimed in any of claims 23-25 wherein the first vessel is an outer vessel and the second vessel is located substantially concentrically within the outer vessel .