WO2012096574A1

WO2012096574A1 - Process for treating manure

Info

Publication number: WO2012096574A1
Application number: PCT/NL2012/050012
Authority: WO
Inventors: Hendrik Arie Boele
Original assignee: Agri Separation Technologies B.V.
Priority date: 2011-01-10
Filing date: 2012-01-10
Publication date: 2012-07-19
Also published as: NL2005980C2

Abstract

The present invention is directed to a process and apparatus for processing manure. The process comprises feeding a manure feed (a,a') stream to a dewatering step (4) producing a filtrate (a') having a lower solids content than (a) and a retentate (m), followed by feeding said filtrate (a') to system comprising a centrifugal type separator (1), wherebyin two effluents (b) and (c) are obtained, wherein effluent (b) contains the majority of the solids present in said feed stream (a) and wherein at least part of effluent (c) is fed to a flotation device (2), wherein flocculation agent (d) is added, from which flotation device a stream of flocculated solids (f) and a permeate stream (e) are obtained.

Description

Title: Process for treating manure

The invention is directed to a process for processing manure, in particular for processing liquid or semi-liquid manure, which is in the form of a slurry.

In agriculture effective control of manure streams is of great importance, both from an economic and environmental point of view. If possible, manure is used as fertilizer by spreading it out on the fields.

However, many farms have limited possibility to spread the manure on fields nearby, so the manure needs to be stored and shipped to other places. Storage, transportation and handling of the liquid manure often add up considerably, e.g. to more than 10% of the total expense of growing cattle. To cut these costs the manure is often split e.g. in a dry cake for use as fertilizer, a stream of mineral free water that may be discharged e.g. in surface water, and a stream of concentrated brine which can be used as artificial fertilizer.

In most farms, in particular pig farms and cattle farms, manure is produced as a mixture of urine and feces. As a result, it has a relatively low dry solids content, typically between 5-10 wt.%, e.g. around 8 wt.%. This is commonly referred to as liquid manure (Dutch: "gief) or semi-liquid manure or floating manure (in Dutch: "drijfmest"), both types are covered by the term "manure" as used herein.

In the art, manure is separated into a solid and a liquid fraction by flotation techniques and decanter centrifuges, belt presses and the like. These techniques require feeding air through a manure stock to which is added FeCl3 or other salts such as calcium hydroxide. The addition of these salts results in flocculation of phosphate salts, such as FeP0₄ or Ca3(P0₄)2. As a result, the phosphate salts end up in the solid fraction. However, an undesired effect of the use of FeCh and similar salts is that a relatively high amount of it is required and high amounts of it end up in the solid fraction, which limits the usability of this solid fraction as fertilizer, since high amounts of FeCl3 are not tolerable in most situations. Also as a result of the addition of these salts is that it becomes difficult for plants to effectively take up phosphates, because the salts have a flocculating effect and the phosphates need to be in dissolved form to be effectively taken up by the plant. Because phosphates are

increasingly scarce, the resulting decreased take-up in plants is another disadvantage of the prior art.

It is an object of the present invention to provide a process that results in a solid fraction, which is relatively low in flocculation salts such as FeCl₃.

A further object of the invention is to provide a process which requires less addition of chemicals such as FeCl3 and CaOH, so that products are obtained which are relatively low in these added salts.

US-A-2006/0138053 describes a wastewater processing method comprising a rotating filter thickener, which discharges into a screw thickener. The rotating filter thickener uses a mesh size from about 250 μιη to about 500 μηι. This means that particles having a smaller size will be contained in the filtrate. As a result, in a subsequent step more flocculent needs to be added to remove phosphates and other compounds. Also rotating filter thickeners are not capable of operating more concentrated slurries, such as an effluent from a filter press. Also this type of thickener requires the use of a plurality of sprays nozzles and tubing for rinsing and cleaning of the filtration disks.

WO-A-03/086977 describes a method for treating manure for use at medium size farms. This known method comprises a biological passive flotation followed by a coarse solid particles separation step, such as

decantation. This known method does not solve the problems indicated above. In particular it requires the addition of large amounts of flocculating agents.

WO-A-2005/03039 also describes methods for clarifying and deodorizing slurries. This method also requires the addition of large amounts of flocculating agents. WO-A-2008/037429 discloses a membrane bioreactor (MBR) like system. It comprises a flocculation step followed by a separation step, which employs a centrifuge. Again the above-mentioned disadvantages apply.

The publication "Wastewater Management Fact Sheet Membrane Bioreactors" (Unit States Environmental Protection Agency, September 2007, XP00265188, pages 1-9) reviews the use of MBRs in wastewater management. No specific attention is given to separators.

US-A-4 765 900 discloses methods for waste treatment. It does not mention or suggest the use of centrifugal separators. Similar disclosures can be found in GB-A-2 252 310, US-A-3 645 893 and US-B1-6 245 121. In these known methods also no centrifuges are employed. Like in the other prior art processes mentioned above, large amounts of flocculants are necessary.

The manure solid-liquid mixture typically contains suspended solids, dissolved components such as salts and gases, and solids which are

electrochemically bound to the liquid. After removing the suspended solids from the manure, a clear fluid remains, typically yellow in color. This fluid is then ready for further processing e.g. by membrane technology. Membrane filtration is typically used to produce mineral free water. This type of further processing operations put strong demands on the quality of their feed, in particular with regards to their solids content.

One of the further objects of the present invention is to provide a process for treatment of a manure stream to yield a product stream that is suitable to be further processed, e.g. by microfiltration or other membrane technologies. At the same time it is an object of the present invention to produce from the manure stream a dry product stream that is dry enough to facilitate carefree handling and storing of the material.

To effectively process commercial streams of manure, it is important to separate the solids from the liquids. The separation should result in a solid product stream and a liquid product stream. The process should have a high separation efficiency, viz. the solid content in the liquid product stream should be as low as possible and the liquid content of the solid product stream should also be as low as possible.

The liquid product stream can for instance be discharged into surface water, provided the amount of organic matter and solids is sufficiently low. The solid product stream can be further processed for instance into pellets or bricks, so that it becomes easily manageable and takes up minimal storage volume.

Figure 1 shows a schematic representation of a process in accordance with the present invention.

Figure 2 is a schematic representation of a MBR.

Figure 3 shows by way of example a flow diagram of a manure purification process in which the process of the present invention is used.

Figure 4 is a schematic perspective view on an arrangement of a centrifugal separator for use in the present invention.

Figure 5 is a schematic perspective cut-open view of a centrifugal separator for use in the present invention.

Figure 6 is a schematic view on a centrifugal separation device used during operation in accordance with the present invention.

It was found that the above-mentioned objects can be met by using a specific setup, which inter alia employs a centrifugal separation device, in particular a device which is commercially available under the trade name Evodos™ (obtainable from Evodos BV, Breda (NL)).

Thus in a first aspect, the invention is directed to a process for processing manure, comprising feeding manure feed (a, a') stream to a system comprising a centrifugal type separator (1), wherein two effluents (b) and (c) are obtained, wherein effluent (b) contains the majority (viz. more than 50 wt.%) of the solids in said feed stream and wherein at least part of effluent (c) is fed to a flotation device (2), wherein flocculation agent (d) is added, from which flotation device a stream of flocculated solids (f) and a permeate stream (e) are obtained. The action of the centrifugal separation device results in a concentration of the solids (or fines) into effluent (b), which typically has a paste-like appearance. The remainder of the solids (viz. 50 wt.% or less of the solids originally present in said feed stream) ends up in effluent (c) and is fed to the flocculation unit (2). Preferably effluent (b) contains more than 90 wt.% of the solids of feed stream (stream (a) or stream (a')), more preferably more than 92 wt.%, even more preferably more than 93 wt.%, typically from 97-99 wt.%.

The present inventors realized that the amount of flocculating agent that is normally required to obtain flocculation of phosphates and the like is approximately linearly proportional to the total mass of these compounds to be flocculated from the liquid stream. Therefore it is highly desirable to device a process that result in a liquid stream that has a concentration of these compounds that is as low as possible. Surprisingly, by employing the setup outlined above, a very high proportion of these compounds can be

preconcentrated in stream (b). The volume of liquid to be subjected to the flocculating step is thus minimized. As a result the amount of flocculating agents to be added in stream (d) is relatively low as compared to prior art methods, since stream (c) is relatively low in compounds to be flocculated (such as phosphates). Because of this, the present invention enables not only the use of less common flocculating agents (such as FeCl3 and Ca(OH)2) but it enables also the use of flocculating agents that can pose a limited burden on the environment because they are degradable. Normally the use of such

ecologically "friendly" compounds would be economically prohibited, because they would have to be employed in high amounts and their costs are considerably higher than the normal flocculating agents, such as FeCl3. Thus, in a preferred embodiment, the present invention uses environmental friendly flocculating agents, such as biopolymers and bioflocculants.

Also, by result, the amount of flocculating agents in stream (e) is correspondingly low, so even if less ecologically friendly flocculating agents (such as FeC-3) are employed, still a considerable ecological advantage is obtained with the present invention.

In the present description and claims the term "manure" refers to any manure slurry that has free flowing properties. It is a dispersion of solids in liquid, typically having a dry solid content of up to 15 wt.%, preferably from 1-10 wt.%, more preferably from 5-9 wt.%, e.g. around 8 wt.%.

A screw press (sometimes also referred to as press auger or press screw) may be used to pre-concentrate the manure stream prior to feeding it to centrifugal separator (1). A screw press is a machine in which a screw rotates within a cylindrical trough with perforated holes of typically 0.15 to 1.0 mm. The liquid fraction is separated from the rest of the manure, in particular from the more solid fraction which mainly comprises coarse fibers. The screw results in a (gradually) increasing pressure. The screw shaft presses the more solid fraction in an effluent pipe. A screw press may result in a more solid fraction typically ranging from 25 to 45%.

In a preferred embodiment, the centrifugal separator is an Evodos™ centrifugal separator. The Evodos™ centrifugal separator comprises a cylindrical outer cover, which cover during solid collection supports vanes or plates that touch on its surface on the inside and which plates are at least partly flexible and/or rotatably connected to a central second carrier, so that said plates are able to spread out during solid discharge, after said cover is removed, and wherein said second carrier is positioned essentially

concentrically in said cover and during solids collection may rotate at essentially the same speed as said cover.

Preferably the centrifugal separator for the present invention, is characterized by its separation efficiency, by which particles of 10 μηι or larger are separated off with high efficiency, typically 90 wt.% or more (based on the original amount of particles). Thus in a preferred embodiment the centrifugal separator has a separation efficiency of 90 wt.% or more (preferably 95 wt.% or more, more preferably 98 wt.% or more, even more preferably 99 wt.% or more) for particles of 10 μηι or larger, more preferably for particles of 5 μιη or larger. Particle sizes as used herein corresponds to the largest dimension of a particle (e.g. to the diameter for spherical particles and the length for particles having an elongated shape).

The present inventors realized that such a centrifugal separator enables very cost effective operation when subsequently a biological wastewater treatment step is carried out, in particular when a MBR is used. Biological wastewater treatment typically requires the addition of oxygen in order to make aerobic reactions possible. This oxygen is most efficiently provided in the form of air. An important parameter is the chemical oxygen demand (COD), which is a degree for the level at which the water is

contaminated with organic pollutants. It is the amount of oxygen that is require to fully oxidize the pollutants. A practical limitation in operating aerobic waste water plants results from the amount of air that can be fed to the reactor. This amount can not be unlimited, since pumping air into the reactor vessel consumes considerable amounts of energy. As a result, streams having a COD that is too high, for instance more than 100 g/dm³, can not be treated cost-effectively. However, by using the centrifugal separator, in particular the type mentioned above, a stream can be produced that has a COD that is half of the original value or less, for instance 40-50 g/dm³, without requiring the addition of chemicals to the streams. When this stream is subsequently fed to a flocculator, the resulting stream has a COD that can be as low as 10 to 20 g/dm³. Such a stream can be processed very efficiently and with great economic advantage in accordance with the invention. For this reason, stream (c) preferably has a COD that is half the value of stream (a), more preferably from 40 to 50 g/dm³. Stream (e) preferably has a COD of 5 to 40 g/dm³, more preferably 10 to 20 g/dm³.

Thus a very suitable centrifugal separator for the present invention comprises an upwardly extending first element with vanes connected thereto which is rotatably arranged with respect to an upwardly extending second element, wherein the first element and the second element are approximately concentrically arranged with respect to each other resulting in an inner element and an outer element and wherein the vanes extend from the first element towards the second element, further comprising a supply opening arranged at an outer end of the separator for supplying a dispersion to be separated and a discharge opening arranged at an opposite outer end for discharging at least one centrate, the retentate may be cost and energy efficiently separated in a particles stream and at least one centrate. Such a type of separator is commercially available under the trademark Evodos™. This type of centrifugal separator is described in WO-A-2009/05355, which is incorporated herein by reference in its entirety. After stopping of the centrifuging, the outer element is removed with respect to the inner element, the inner element with the vanes connected thereto is rotated again and due to the rotation under influence of the centrifugal forces the vanes stretch and the accumulated particles are swung loose from the vanes and can be collected. By providing a centrifugal separator of which the supply opening is in radial distance adjacent the outer element, the retentate can be supplied near the more efficient region of the centrifugal separator. The more efficient region of the centrifugal separator is the region where the centrifugal forces during centrifuging may be the highest, i.e. the outer region of the separator. By supplying the retentate more near the more efficient region of the centrifugal separator, separation may be more efficient, less time consuming and more energy saving.

In particular the Evodos™ separator comprises a plate type rotational centrifugal separator for separating one or more components of a feed stream having rotatable carriers (41,42) comprising a rotatable first outer carrier (41) and a rotatable second inner carrier (42), wherein the second carrier (42) is coaxially arranged in the first carrier (41) around an axis wherein both carriers (41,42) are rotatable around the axis and comprising one or more curved plates (46), wherein the one or more plates (46) are supported by the first carrier (41) and wherein said plates (46) are at least partly flexible and/or rotatably connected to said second carrier (42), so that said plates (46) are able to spread out during solid discharge, further comprising feeding means at one end (416) of the separator for supplying a feed stream to be separated and discharging means at an opposite end (417) of the separator for discharging separated streams, wherein between adjacent curved plates (46) and the first and second carrier (41,42) confined spaces are defined for separation of the feed stream under influence of centrifugal forces,

characterized in that the first carrier (41) is axially removable from the second carrier (42) for removing components collected on the plates (46).

Preferably at least part of the plates (46) are made of a flexible material. Preferably at least part of the plates (46) are rotatably connected to the second carrier (42). Preferably at least part of the plates (46) are made of a stiff material wherein these plates (46) are able to pivot relative to the second carrier (42). Preferably the first carrier (41) is in the form of a closed drum, which is rotatably driven on a rotation shaft. Preferably said one or more plates (46) are mounted on said second carrier (42), which plates (46) at least partly extend from said second carrier (42) to said first carrier (1). Preferably the angle a between the radial of the second carrier (42) and the tangent of the plates (46) at the joint of the plate with the second carrier (42) is more than 0°. Preferably said one or more plates (46) are curved or shaped in a

pre-determined arc. Preferably said one or more plates (46) are mounted on said second (2) carrier by means of a flexible joint. Preferably said plates (46) comprise at least two different rows of plates (46) and at least one baffle plate (47) is provided in a space between two rows of plates (46), preferably a circular baffle plate. Preferably said flexible plates (46) are elastic, i.e. return to their initial form after deformation. Preferably the angle a between the radial of the second carrier (42) and the tangent of the plates at the joint of the plate (46) with the second carrier (42) is 5-85°, more preferably 10-45°. Figure 4 depicts a schematic front view of the inner part of an Evodos™ separator. It shows plurality of curved plates 46 that can be used in combination with the second carrier 42. In the embodiment shown in Figure 2, the plurality of curved plates 46 are constructed as a plate pack which can be easily mounted on the central body 45 of the second carrier 42 as shown in Figure 4. The plurality of curved plates 46 are preferably homogeneously distributed on the second carrier 42, i.e. substantially equidistant. The plates 46 are mounted between the flanges 43 and 44. Figure 5 shows the device of the invention comprising the first carrier 1 and the second carrier 42 on which the plurality of curved plates 46 is mounted.

Figure 5 shows a schematic view of an Evodos™ separator comprising the first carrier 41 and the second carrier 42 on which the plurality of curved plates 46 is mounted.

Figure 6 depicts a schematic view on the Evodos™ separation device used during operation. In operation, the feed stream (a') which is fed into the rotating device via the feeding means on one end 416 of the device flows parallel, or substantially parallel, to the axis of rotation 410 to discharging means 414, 415 positioned on the other end 417 of the device, from which stream (c) can be discharged. Preferably, the device has at least two

discharging means 414, 415, one discharging means 414 can be placed near wall 418 of the first carrier and is thus suitable for discharging a heavy fraction, another discharging means 415 can be placed near the central body 45 of the second carrier 42 and is thus suitable for discharging a light fraction.

In a preferred embodiment, permeate stream (e) is subsequently treated in a biological wastewater treatment plant, which are known per se. The wastewater treatment may comprise aerobic and/or anaerobic digestion steps, which may be followed by filtration, e.g. using a filtration device, such as an ultrafiltration device and/or other purification devices, such as a reverse- osmosis (RO) setup. In this way a product water stream can be obtained that is sufficiently pure to be discharged on surface water. Preferably the biological treatment is carried out using a membrane bioreactor (MBR), a technology that has become increasingly used in the past ten years. MBRs are described for instance in the publication "Wastewater Management Fact Sheet

Membrane Bioreactors", United States Environmental Protection Agency EPA, Washington, DC (2007) (http://www.epa.gov/owm/mtb/etfs_membrane- bioreactors.pdf). A schematic representation of an MBR is given in figure 2. This figure shows how permeate stream (e) is fed to a bioreactor system (6, 7), which may comprise multiple reactors. Air may be fed through line (g) and is distributed by sieve plate mounted in the bottom of the reactor resulting in a stream of air bubbles. Sludge is continuously taken from the bioreactor and fed to membrane unit (8). The retentate contains sludge, which is recycled to the bioreactor, as shown in figure 2, or to a point upstream thereof. The filtrate of the membrane unit is subjected to further processing. In this way slow growing biomass, in particular bacteria, can grow in the bioreactor. The biocultures thus formed are very stable and can maintain active for a long period of time.

By way of example, the present invention is illustrated with reference to figure 3, which shows schematically a manure processing plant, in which manure is converted into different product streams:

- a concentrated ("thick") fraction (m) and a paste-like fraction (b), which both may be used as fertilizer, optionally after further drying;

- a permeate stream (h), which is sufficiently low in concentration of salts and other components that it may be discharged on surface water; and

- a concentrate stream (i), which contains all dissolved salts, such as alkali salts, in particular alkali chloride and other chlorides. This concentrate stream may be used as an additive to the produced fertilizer streams.

The setup of figure 3 comprises a pretreatment section 21, which comprises the process of the present invention. It furthermore comprises membrane bioreactor (MBR) section 22, in which the organic matter present in permeate stream (e) is biologically treated to remove organic matter therefrom. In particular this section contains an anaerobic and an aerobic section, which are carried out in denitrification reactor 6 and nitrification reactor 7, respectively, in which stream (e) is treated. Air (g) is added to reactor 7. Sludge that is formed in these two steps may be recycled in part to denitrification reactor 6 via line (j) and/or to the upstream pretreatment section 21, for instance prior to centrifugal separator 2, via line (k). The effluent of the bioreactors is treated in a subsequent filtration step, e.g. using ultrafiltration module 8. The retentate of this step may be recycled to lines (j) and/or (k) as indicated in figure 3. The filtrate of this step is passed to the final section.

The final section 23 may for instance comprise a reverse osmosis step to produce permeate (h), which meets the desired requirements, e.g. with respect to purity for discharging to surface water. Concentrate stream (i) can be used for as an additive to the produced fertilizer streams.

Example The setup depicted in figure 3 was used to process a stream of fresh manure. The dewatering step was carried out using a screw press. This resulted in a thick fraction (m) having a dry solids (ds) content of 32 wt.% and a thin fraction (a'), which was fed to an Evodos™ centrifuge. This resulted in a thick fraction (b) having a ds of 23 wt.% and a thin fraction (c). The thin fraction was temporarily stored and then fed to a flotation device, after mixing with a biodegradable poly-electrolyte (200 g per 1000 kg fresh manure). The solids (f) were recycled to the screw press. The liquid fraction was fed to an MBR, where it is subjected to nitrification/denitrifaction, resulting in all N- containing components being converted into nitrogen gas. In the MBR all organic matter was converted into CO2. Water leaving the MBR (about 800 liter per 1000 kg fresh manure) contains low concentration of potassium and other salts. This stream could be treated with nanofiltration or reverse osmosis to produce a stream of water that can be discharged onto surface water.

The following table shows a mass balance of the process.

Table 1. Mass balance for different streams

Starting Product Mass % total ds Nitrogen P2O5 Potassium material /kg weight /(£/kg) /(g/dm¾ /(g/dm3) /(g/dm¾

(a) (a') 160.5 91.9 50 5.15 2.84 4.2

(a) (m) 14.1 8.1 298 7.02 4.95 3.9

(a') (b) 9.7 13.5 196 10.70 18.20 4.1

(a') (0 62.2 86.5 27 4.22 0.23 4.2

(0 (f) 38.4 18.0 62 6.29 0.87 3.7

(c) (e) 174.4 82.0 16 3.28 0.09 3.9

Claims

1. Process for processing manure, comprising feeding an optionally concentrated manure feed (a') stream to a system comprising a centrifugal type separator (1), whereby two effluents (b) and (c) are obtained, wherein effluent (b) contains the majority of the solids present in said feed stream (a') and wherein at least part of effluent (c) is fed to a flotation device (2), wherein flocculation agent (d) is added, from which flotation device a stream of flocculated solids (f) and a permeate stream (e) are obtained.

2. Process according to claim 1, wherein a manure feed (a), prior to being fed to said centrifugal type separator is dewatered, preferably using a screw press, whereby concentrated manure effluent (a') is obtained having a higher solid content than said manure feed (a) and in which a retentate stream (m) is produced.

3. Process according to any of the previous claims, wherein the centrifugal separator has a separation efficiency of 90 wt.% or more for particles of 10 μηι or larger.

4. Process according to any of the previous claims, wherein stream (b) contains more than 90 wt.%, preferably more than 93 wt.%, more preferably more than 95 wt.%, most preferably between 97 and 99 wt.% of the solids present in said feed stream.

5. Process according to any of the previous claims, wherein said centrifugal type separator is an Evodos type separator, which comprises a cylindrical outer cover, which cover during solid collection supports vanes or plates that touch on its surface on the inside and which plates are at least partly flexible and/or rotatably connected to a central second carrier, so that said plates are able to spread out during solid discharge, after said cover is removed, and wherein said second carrier is positioned essentially concentrically in said cover and during solids collection may rotate at essentially the same speed as said cover.

6. Process according to any of the previous claims, wherein said flocculation agent comprises biodegradable bio-flocculants or bio-coagulants.

7. Process according to any of the previous claims, wherein said flocculating agent is essentially free of Ca(OH)2, FeCl3 or both.

8. Process according to any of the previous claims, wherein stream (b) comprises more than 50 wt.% of the phosphate content of stream (a), preferably more than 60 wt.%, even more preferably more than 70 wt.%.

9. Process according to any of the previous claims, wherein stream (b) comprises more than 15 wt.% of the content of nitrogen containing compounds of stream (a), preferably more than 18 wt.%, even more preferably more than 20 wt.%.

10. Process according to any of the previous claims 2-8, wherein stream (b) and (m) combined comprise more than 80 wt.% of the phosphate compounds present in stream (a), and preferably more than 30 wt.% of the nitrates present in stream (a).

11. Process according to any of the previous claims, wherein permeate stream (e) is fed to a biological wastewater treatment step.

12. Process according to the previous claim, wherein said biological wastewater treatment step comprises a membrane bioreactor, which comprises a nitrification reactor, a denitrification reactor and an ultrafiltration step.

13. Process according to the previous claim, wherein the permeate effluent of said ultrafiltration step is fed to a reverse osmosis device, which produces a permeate that can be discharged into surface water.

14. Apparatus for processing manure, comprising feeding a dewatering device (4), to which is connected manure feed (a), filtrate line (a') and retentate line (m), wherein filtrate line (a') is connected to centrifugal separator (1) to which is connected retentate line (b) and liquid line (c), wherein liquid line (c) discharges into flotation device (2), to which a discharge for flocculated solids (f) is connected and a discharge for permeate stream (e).

15. Apparatus according to the previous claim, wherein permeate stream (e) is connected to a biological wastewater treatment plant, which produces an effluent that is connected to a filtration device, such as an ultrafiltration device and/or other purification devices, such as a reverse- osmosis (RO) setup.