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Title:
FLOATING BIOREACTOR
Document Type and Number:
WIPO Patent Application WO/2017/188875
Kind Code:
A1
Abstract:
The present invention relates to a mobile, floating biofilm bioreactor for wastewater treatment, comprising a frame having an upper portion and a lower portion, at least one buoyant member attached to the upper portion of the frame such that the bioreactor remains afloat when submerged into wastewater, an aerator comprising, an air dispensing device arranged at the lower end of the rigid frame, a biofilm support media attached to the frame below the at least one buoyant member, wherein the biofilm support media comprises pipes having perforated walls arranged in an upright position alongside each other, said pipes forming a planar biofilm platform having an upper side and a lower side,, said air dispensing device comprises a collar for provision of a desired distribution pattern of air bubbles, positioned in connection to the lower side of the biofilm platform.

Inventors:
HOLBY, Ola (Fadderortsgatan 27, KARLSTAD, 654 66, SE)
PETERSEN, Svend (Rastavägen 9, CHARLOTTENBERG, 673 92, SE)
Application Number:
SE2017/050304
Publication Date:
November 02, 2017
Filing Date:
March 29, 2017
Export Citation:
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Assignee:
HOLBY, Ola (Fadderortsgatan 27, KARLSTAD, 654 66, SE)
PETERSEN, Svend (Rastavägen 9, CHARLOTTENBERG, 673 92, SE)
International Classes:
C02F3/06; B01F3/04
Domestic Patent References:
WO2007092701A22007-08-16
Foreign References:
US6344144B12002-02-05
US20110132822A12011-06-09
US20050269262A12005-12-08
JP2005152757A2005-06-16
Other References:
See also references of EP 3448816A4
Attorney, Agent or Firm:
BERGENSTRÅHLE & PARTNERS GÖTEBORG AB (Lindholmspiren 5A, Göteborg, SE-417 56, SE)
Download PDF:
Claims:
CLAIMS

A mobile, floating biofilm bioreactor (1) for wastewater treatment, comprising a frame having an upper portion (8) and a lower portion (6), at least one buoyant member (7) attached to the upper portion (8) of the frame such that the bioreactor

(1) remains afloat when submerged into wastewater, an aerator (3) comprising an air dispensing device (31) arranged at the lower end of the frame, a biofilm support media (2, 20) attached to the frame below the at least one buoyant member (7), characterized in that the biofilm support media (2, 20) comprises pipes (20) having perforated walls (21) arranged in an upright position alongside each other, said pipes forming a planar biofilm platform (2) having an upper side and a lower side, said air dispensing device (31) comprises a collar (4) for provision of a desired distribution pattern of air bubbles, positioned in connection to the lower side of the biofilm platform (2).

The mobile, floating biofilm bioreactor (1) according to claim 1, characterized in that said biofilm platform (2) has a width (X) in the range of 1 - 15 m, preferably in the range of 2 - 8 m, and a height (H) in the range of 0,4 - 4.0 m, preferably 0,4 - 2.0 m and more preferred 0,5-1,0 m.

The mobile, floating biofilm bioreactor (1) according to claim 2, characterized in that said biofilm platform (2) has a square, oval or circular form.

The mobile, floating biofilm bioreactor (1) according to claim 1, characterized in that the impeller (31) is positioned substantially at the center of the biofilm platform

(2) .

The mobile, floating biofilm bioreactor (1) according to claim 4, characterized in that the impeller (31) is positioned in the same plane as the lower side of the biofilm platform (2), preferably max 0.1 m above the lower side of the biofilm platform to max 0.3 m below the lower side of the biofilm platform, or extending a short distance below the lower side, preferably max 1.0 m below the lower side of the biofilm platform (2), and further characterized in a column frame (60) which extends through the biofilm platform (2) and forms a central shaft for the aerator (3).

6. The mobile, floating biofilm bioreactor (1) according to claim 1, characterized in that said collar (4) comprises an upper circular disc (40) and a wall (41) which extends downwardly, outwardly relative the upper disc (40), in that said wall (41) comprises grooves (43) arranged in the radial direction on its inside (42), the height of the wall (41) being adapted to the height of radial openings between impeller blades (5) at the circumferential surface of the impeller (31) such that the wall (41) extends in the region radially outside said openings.

7. The mobile, floating biofilm bioreactor (1) according to claim 6, characterized in that the wall (41) has an angle (v) in the range of 30 - 60°, preferably v = 40 - 50° and more preferred v = 45°, in relation to radial extension of the impeller (31), in that said grooves (43) have a depth in the range of 0.5 - 1.5 mm, preferably 0.8 -1.2 mm and most preferred 0.9-1.1 mm and are arranged along substantially the whole height of the wall (41) with a mutual angle distance of 5-10° in the circumferential direction.

8. The mobile, floating biofilm bioreactor (1) according to claim 1, characterized in that the perforated pipes (20) have an inner diameter in the range of 20 - 120 mm, preferably 40 - 80 mm and more preferred 50 - 70 mm, and in that the pipe walls (21) being a mesh having a void percentage in the range of 85-95%, preferably 88-

90%, said void having a diameter in the range of 5 - 20 mm.

9. The mobile, floating biofilm bioreactor (1) according to claim 1, characterized in that the at least one buoyant member (7) is arranged at least 0.1 m, preferably at least 0.2 m above the upper side of the biofilm platform (2) .

10. The mobile, floating biofilm bioreactor (1) according to claim 9, characterized in that it the at least one buoyant member (7) comprises several floats (7) which are fixed to bars (80) of the upper portion (8) of the rigid frame, said floats (7) having the shape of flattened globes or tubes and positioned along said bars (80) such that there is obtained a maximum distance between individual floats (7) or group of floats (7).

11. The mobile, floating biofilm reactor (1) according to any of claims 1 to 10, wherein the aerator comprises a motor (33) arranged at the upper portion (8) of the frame and an air pipe (32) extending between the motor (33) and the air dispensing device (31).

12. The mobile, floating biofilm reactor (1) according to claim 11, wherein the collar (4) is arranged circumferentially outside the motor (33). 13. The mobile, floating biofilm reactor (1) according to any of claims 1 to 12, wherein the frame is rigid.

Description:
FLOATING BIOREACTOR

TECHNICAL FIELD

The present invention relates to a mobile, floating biofilm bioreactor for wastewater treatment, comprising a frame having an upper portion and a lower portion, at least one buoyant member attached to the upper portion of the frame such that the bioreactor remains afloat when submerged into wastewater, an aeration system comprising an air dispensing device arranged at the lower end of the frame, a biofilm support media attached to the ridged frame below the at least one buoyant member.

STATE OF THE ART

Eutrophication is a problem today and in the handling of sewage/leachate one of the major challenges is the reduction of Biochemical Oxygen Demand (BOD) and nitrogen. Both are usually taken away by biological methods to relatively high costs. If BOD and nitrogen is not removed waters will be nutrient rich and lead to eutrophication and the sediments might become anaerobic. Today, more and more stakeholders have restrictions in their nitrogen emissions which often is the limiting nutrient in the ocean environment.

Subsurface aeration and bioreactors are used today for treating sewage and wastewater. The document US 2011/0132822 describes an aeration and microbial reactor system for use in decontaminating water including a housing adapted to float within the medium such that a top portion thereof remains adjacent a top surface of the contaminated water while the bioreactor containing inoculated carrier media is attached below. Beneficial microbial populations thrive and spread throughout the liquid medium, and consume or fix the contaminant such that the contaminant is removed from the water.

The document US 2005/0269262 describes a biological film module for wastewater treatment system, a frame for supporting porous biological-growth support media is provided with at least one pontoon float. Also disposed on the frame is an aerator in the form of a removable diffused aeration grid including a plurality of individual diffusers disposed in a rectangular planar array. BRIEF DESCRIPTION OF THE INVENTION

It is an object of the present invention to eliminate or at least minimize the above mentioned problems which can be achieved as the biofilm support media comprises pipes having perforated walls arranged in an upright position alongside each other, said pipes forming a planar biofilm module having an upper side and a lower side, said air dispensing device comprising an impeller positioned in connection to the lower side of the biofilm module and further comprises a collar arranged circumferentially outside the impeller.

Thanks to the invention a bioreactor increases the decomposition of organic matter, favors nitrification and denitrification. The content of organic material and nutrients are reduced in wastewater and other waters with high concentrations of these substances.

According to one aspect of the invention said biofilm module has a width in the range of 1 - 15 m, preferably in the range of 2 - 8 m, and a height in the range of 0.4 - 4.0 m, preferably 0.5 - 2.0 m and even more preferred 0.5 - 1.0 m.

According to another aspect of the invention said biofilm module has a square, oval or circular form and the impeller is positioned substantially at the center of the biofilm module.

According to still another aspect of the invention the impeller is positioned essentially in the same plane as the lower side of the biofilm module, typically in a position max 0.1 m above the lower side of the biofilm module to max 0.3 m below the lower side of the biofilm module.

According to yet another aspect of the invention the impeller is positioned at a distance of up to 1 m below the lower side of the biofilm module.

According to one aspect of the invention the bioreactor comprises a column frame which extends through the biofilm platform and forms a central shaft for the aerator.

According to yet another aspect of the invention said collar comprises an upper circular disc and a wall which extends downwardly, outwardly relative the upper disc, said wall having grooves arranged in the radial direction on its inside, the height of the wall being adapted to the height of radial openings between impeller blades at the circumferential surface of the impeller such that the wall extends in the region radially outside said openings.

According to another aspect of the invention the funnel wall has an angle v in the range of 30 - 60°, preferably 40 - 50°, and more preferred 45°, in relation to radial extension of the impeller, in that said grooves have a depth in the range of 0.5 - 1.5 mm, preferably 0.8 - 1.2 mm and most preferred 0.9 - 1.1 mm and are arranged along substantially the whole height of the wall with a mutual angle distance of 5 - 10° in the circumferential direction.

According to another aspect of the invention the perforated pipes have an inner diameter in the interval of 20 - 120 mm, preferably in the interval 40 - 80 mm and more preferred in the interval 50 - 70 mm, and in that the walls being a mesh having a void percentage in the range of 85 - 95 %, preferably 88 - 90 %, said void having a diameter in the range of 5 - 20 mm.

According to another aspect of the invention the at least one buoyant member is arranged at least 0.1 m, preferably at least 0.2 m above the upper side of the planar biofilm module.

According to another aspect of the invention the at least one buoyant member comprises several floats which are fixed to radially extending arms of the frame, said arms preferably being at least three, more preferred at least four, in number and

symmetrically arranged in a circular direction, said floats having the shape of flattened globes or tubes and positioned along said arms in a pattern such that there is obtained a maximum distance between individual floats.

BRIEF DESCRIPTION OF FIGURES

In the following, the invention will be described in greater detail with reference to the attached figures of the drawings, in which:

Fig. 1 illustrates a side view of a bioreactor according to the invention,

Fig. 2 illustrates a side view of an aerator according to the invention,

Fig. 3 illustrates a cross sectional view of a lower part of an aerator according to the invention,

Fig 4 illustrates a perspective view of a collar according to the invention, and

Fig 5 illustrates a cross sectional view of a collar according to the invention. Fig 6 illustrates a module arranged in a bioreactor according to the invention, Fig. 7 illustrates a perspective view of a bioreactor, active in water, according to the invention,

DETAILED DESCRIPTION OF FIGURES

The following detailed description, and the examples contained therein, are provided for the purpose of describing and illustrating certain embodiments of the invention only and are not intended to limit the scope of the invention in any way.

Figure 1 illustrates a side view of a mobile, floating bioreactor 1 according to the invention. The bioreactor 1 comprises a biofilm platform 2, an upper framework 8, a lower framework 6 and floats 7. The frame of the bioreactor may be rigid. The floats are arranged above the platform 2 and are attached by vertical struts (not shown). Each strut extends from the upper framework 6, through the center of a float 7 further through the platform 2 to the lower framework 8. The vertical struts also acts as fixing and distance devices whereby the floats 7 and the platform 2 are kept in accurate positions. The bioreactor 1 is arranged with an aerator 3 for controlled oxygenation which extends vertically through the biofilm platform and has a motor 33 and an air intake above the biofilm platform 2 and an air dispensing device in the form of an impeller 31 for distribution of air below the biofilm platform 2. For example, the aerator 3 may be a product named Airturbo™ marketed by the company Eden Aquatech.

In the shown embodiment, the platform 2 has a square form with a side length X and a width Y in the interval 1 - 15 m, preferably in the interval 2 - 8 m and more preferred in the interval 3 - 6 m. The platform 2 described in the figures has a length X and width Y that is 2.8 m. The height H of the platform 2 is the same as a length L of the pipes 20 and in the described example the height H is 55 cm. For example, the biofilm platform 2 may comprise several BIO-BLOK ® modules marketed by the company C.A.P.

Technology assembled side by side. The BIO-BLOK ® modules comprises perforated pipes and will be more described in accordance with figure 4.

Figure 2 illustrates a side view of an aerator 3 according to the invention. The aerator 3 comprises a motor 33 arranged at the top of a tubular drive shaft 32 for an impeller 31 arranged at its lower end. The tubular drive shaft 32 also acts as air duct for provision of air to the impeller 31. An upper mounting sleeve 35 for the tubular drive shaft 32 is arranged with the motor 30. The upper mounting sleeve 35 comprises an air intake opening 34 (or openings). The impeller 31, which shall be described in detail in figure 3, is provided with a collar 4 for disintegrating of the air bubbles and provision of a desired distribution pattern of the air bubbles below the biofilm platform 2.

Figure 3 illustrates a cross section of the impeller 31. A lower mounting sleeve 36 is arranged with an inner axial bore 37 for fixing of the lower end of the tubular drive shaft 32. The axial bore has an upper section with a diameter adapted to the outer diameter of the tubular drive shaft 32 such that the tubular drive shaft 32 is snugly fitted inside the axial bore.

A lower section of the axial bore extends axially outside the end of the tubular drive shaft 32 and has a diameter that is smaller than the diameter of the upper section of the axial bore, preferably about the same as the inner diameter of the tubular drive shaft 32. Between the upper and lower section is a beveled step 45 in the wall of the axial bore. The lower end of the tubular drive shaft 32 is provided with a circumferential sealing 46 and rests against this beveled step 45 for provision of an airtight sealing of the lower end against the inside of the axial bore 37.

The lower section of the axial bore is provided with radially extending holes 38 through the wall of the mounting sleeve 36. At the center of a short side of the lower end of the mounting sleeve 36, the tubular drive shaft 32, i.e. the mounting sleeve 36, is carried by a bearing 62 at a cross brace 63 in a manner known per se. The arms of the cross brace 63 is attached at lower cross-ties 61 of a column frame 60 which extends through the biofilm platform 2 and forms a central shaft for the aerator 3. The column frame 60 also attaches to the upper frame section 8 and the lower frame section 6 which are fixed in parallel planes at a suitable distance relative to each other by the column frame 60 (see figure 1). The distance between the upper frame section 8 and lower frame section 6 is adapted to the height of the biofilm platform 2 and the buoyant members 81 such that these are either clamped between the frame sections 6, 8 or such that a distance of at least 0.1 m, preferably at least 0.2 m is formed between the underside of the buoyant members 81, which are mounted to the underside of the upper frame section 8, and the upper side of the biofilm platform 2. Hereby, the buoyant members will affect the flow less and a larger water depth between the biofilm platform 2 and the water surface will improve flow conditions.

The impeller 31 and the collar 4 is fixed on the outside of the mounting sleeve 36 via screws (not shown) to a mounting flange 39. Mating mounting holes 44 for the screws are arranged in the flange 39, collar 4 and impeller 31 in a conventional manner. The impeller 31 is mounted outside the holes 38 and air flowing down the tubular drive shaft (indicated with arrow at A) will flow through the holes 38 and into a central compartment of the impeller 31 and be accelerated outwards from the center of rotation, out through the openings between the impeller blades 5, as is conventional for impellers.

In figure 4 the collar 4 is seen in a perspective view from below and figure 5 shows a cross sectional view of said collar 4. The collar 4 comprises a substantially flat, circular upper disc 40 having through going mounting holes 44 for fixing the collar 4 to the impeller 31, for example by screws (not shown). From the circumferential edge of said upper disc 40 a funnel-like side wall 41 extends downwardly, outwardly at an angle v of about 45° relative the upper disc 40. The inner side 42 of the side wall 41 comprises evenly distributed grooves 43 which extend radially from the upper edge of the side wall to the lower free end of the side wall and over the edge which is beneficial from a flow perspective. As the collar 4 rotates together with the impeller 31, the grooves 43 shatter and shred the air bubbles that are pumped out from the impeller. In the described example the grooves 43 have a depth of about 1 mm and there are fifty grooves 43 evenly arranged at the inner side 42 but within the concept of the invention said grooves may have a depth in the range of 0.5 - 1.5 mm, preferably 0.8 -1.2 mm and most preferred 0.9-1.1 mm and may be arranged along substantially the whole height of the wall with a mutual angle distance of 5-10° in the circumferential direction.

In figure 6 is illustrated a perspective view of a module 200 which forms the biofilm platform. The module 200 comprises a structured filter media developed by the company CAP Technology in England,

http://www.captechnology.co.uk/pdf/cap_design.pdf. The filter media has proved extremely efficient in biological treatment of domestic sewage, industrial wastewater and process water within the aquaculture field. The media is made from the

environmentally friendly material polyethylene and consists of net tubes 20 which are fixed lengthwise to each other and welded together, preferably at their upper and lower ends, to form a square block. The unique surface structure of the many net tubes provides a large accessible surface area for enhanced biological growth on the filter media. The filter media is called BIO-BLOK ® and the surface structure of the many net tubes acts as a substrate for specialized bacterial strains, which are able to treat and degrade a wide range of wastewater qualities. The treatment capacity of a biological filter depends on the quantity of bacteria that the filter can sustain. Naturally, other pipes 20 which are assembled side by side and has mesh pipe walls 21 in a material and with a structure that would provide a suitable surface for biological growth could be used for the purpose without departing from the concept of this invention. The perforated pipe 20 has a length L in the interval 40 - 400 cm, preferably in the interval 40 - 200 cm. In a preferred embodiment, the platform 2 is designed to be shallow, typically in the range of 50 - 100 cm. The shallow depth is preferred as a suitable flow rate of the air bubbles through the pipes 20 is thereby obtained. The perforated pipe 20 has an outer diameter D in the interval 20 - 120 mm, preferably in the interval 40 - 80 mm and more preferred in the interval 50 - 70 mm. The inner diameter shall not be too small as there is a risk that the pipes will become clogged and not too large as this will impair the biodegradation efficiency of the bioreactor 1. In this described example, BIO-BLOK ® 100 modules were used. In the modules 200, the perforated pipes 20 have a length L of 55 cm, an outer diameter D of 67.5 mm and an inner diameter of 62.5 mm. A module 200 has the dimension 54 x 54 x 55 cm (width x depth x height). The BIO-BLOK ® 100 module have a specific surface of 100 m 2 /m 3 (biofilm excluded), an area of flow of 70 % and a void percentage of 90 %. In a variant, the perforated pipes 20 have an outer diameter D of 55 mm and an inner diameter of 50 mm. This module have a specific surface of 125 m 2 /m 3 , an area of flow of 67 % and a void percentage of 89 %. The larger the specific surface area is, the larger the bacterial population. As a rule of thumb, the voids in the mesh shall have a diameter in the range of 5 - 20 mm.

In figure 7 is illustrated a bioreactor 1 in operation. The platform 2 is situated below the water surface and the upper framework 8 and the floats 7 are situated above the water surface as well as the motor 33 and the upper mounting sleeve 35 with the air intake opening 34 of the aerator 3. The floats 7 are arranged above the platform 2 and are fixed to bars 80 of the upper framework 8. The upper framework 8 also comprises fasteners 81 for example wires 82 that may be fixed to a pier or to the water bottom to keep the bioreactor 1 in place. The framework 8 also fixes the aerator 3 in the center of the platform 2. A grating 83 is arranged between the vertical bars that form the column frame around the aerator 3. The grating 83 is positioned in the area between the motor 33 and the upper surface of the platform 2 to prevent debris from entering the shaft around the aerator 3 and get entangled in the rotating tubular drive shaft 32 or the impeller 31.

In this described example the bioreactor 1 is arranged with eight floats 7 which have the shape of flattened globes, that is, ellipsoids. The floats 7 are symmetrically positioned in order to balance the bioreactor 1 in the water and distribute the load on the upper framework 8 evenly. A first group of four floats 7 are positioned in the corners of an inner square frame approximately halfway from the center of the platform 2 and its outer corners. A second group of four floats are each positioned at the edge of the platform 2, in the middle of the side between two corners. The bars 80 that fix the second group of floats 7 extend from the inner square frame. The skilled person realize that the floats 7 may vary in number, shape and placement depending on the size and form of the platform 2 and that it is beneficial to arrange the floats so that the distance between individual floats 7 are as large as possible from a flow restriction perspective. In a variant with eight floats, the floats 7 are instead grouped two and two and arranged along diagonally extending bars which may be even better from a flow restriction perspective than the arrangement described above.

The bioreactor 1 according to the invention operates by that the impeller 31 rotates and air is sucked through the tubular drive shaft 32. The air that is hurled from the impeller hits the collar 4 which shatters and spreads the bubbles of air laterally under the platform 2. Thanks to the fact that the platform 2 comprises pipes 20 a pumping effect is achieved, according to the air-lift pump principle, which will cause a mixture of water and air bubbles to pass through the perforated pipes 20 of the platform 2. On the perforated pipe walls 21 a biofilm of specialized bacterial strains evolves over time and when the water and the air bubbles passes, the contaminants diffuses into the biofilm and the bacterial strains treat and degrade the contaminants during consumption of the oxygen in the air bubbles and the denitrification process (anaerobic) is enhanced.

The collar 4 shatters the air bubbles coming from the aerator 3 and according to the inventive concept it is possible to control the size and the spreading pattern of the air bubbles by designing the collar 4, e.g. the depth and orientation of the grooves 43 and the inclination of the side wall 41, in an appropriate manner. The actual position of the impeller relative the lower side of the platform 2 also is of some importance as well as the dimension, e.g. diameter, of the impeller. It is a great advantage to be able to control the size of the air bubbles since the velocity of which the air bubbles rise through the pipes 20 and the oxygen transfer rate will be effected. To be able to distribute the air bubbles evenly underneath the platform 2, or at least underneath the majority of the platform 2, is naturally also advantageous.

The inclination of the side wall 41 affects the spreading pattern of the air bubbles.

Depending on the length X and the width Y of the platform 2 it may be desired that the air bubbles have a wide or narrow distribution. The choice of wide or narrow spreading pattern is also dependent on the conditions at the installation site. A wider spreading pattern may be desired in shallow sites and vice versa. A greater angle v between the upper part 40 and the side wall 41 (see Fig. 5) generates a narrower spreading pattern than a smaller angle v which generates a wider spreading pattern. With a wide spreading pattern there is a risk that the bubbles are spread outside the central area which is undesired. With a narrow spreading pattern air bubbles go deeper below the platform 2 and creates a high pumping effect which is beneficial. An optimal spreading pattern of the air bubbles is achieved at an angle in the range of 40 - 50°, typically of around 45°. Further, the distribution of the bubbles is affected, at least to some extent, by the actual position of the impeller relative the lower side of the platform 2. The impeller 31 may be positioned a short distance inside the platform 2, typically max 10 cm above the lower side, provided that the side wall 41 of the collar 4 have a sufficient inclination to direct the air bubbles past the lower end of the innermost pipes 20. However, the impeller 31 is preferably positioned so that it essentially aligns with the lower side of the platform 2 or a short distance, typically max 30 cm, below the lower side of the platform 2 which can be seen in Fig. 1. In a variant it is however conceivable to let the impeller extend up to 1 m below the lower side of the platform 2. Hereby, the pumping effect, i.e. the amount of water that is lifted through the pipes, is increased. The spreading of the air bubbles sideways is also improved due to the increased time before the bubbles start to rise and enter into the pipes 20.

By the design of the grooves 43 it is possible to achieve desired air bubble size which will result in a distribution of air bubbles along the entire under side of the platform 2, or at least the main part of it, and an even pumping effect over the entire platform area. The size of the bubbles will be smaller than if the side wall 41 are without grooves 43 and significantly smaller than if no collar 4 is used at all. The size of all air bubbles will not be exactly the same but significantly more even in size. Empirical tests have shown that a suitable depth of the grooves 43 is in the range of 0.5 - 1.5 mm, preferably 0.8 - 1.2 mm and most preferred 0.9 - 1.1 mm and they are arranged along substantially the whole height of the wall with a mutual angle distance of 5 - 10° in the circumferential direction. Tests have shown that a depth of the grooves of 1.5 mm or more will not be as effective in shuttering the air bubbles. Yet a disadvantage is that the side wall 41 of the collar 4 needs to be thicker which is undesired for a number of reasons, e.g.

manufacturing cost and energy consumption.

From the discussion above it is evident that several parameters has to be modulated in order to obtain a desired size and distribution pattern of the air bubbles. In combination with the design of the platform 2 it will contribute to a bioreactor 1 with improved performance compared to prior art reactors.

The column frame 60 also serves to protect the impeller 31 from getting damaged in case there is a risk that the bioreactor 1 will touch the bottom if the water level drops. Since it is possible to spread air bubbles far to the sides the bioreactor 1 may be designed to be shallow but widespread sideways instead. This is a great advantage in e.g. leachate ponds since they often are 1-3 m deep and prior art bioreactors are too deep to be used therein. In order to avoid bottom sediments from being whirled up and dragged along by the mixture of air bubbles and water, it is suitable to keep a margin of at least 0.2 m but preferably at least 1 m between the impeller 31 and the bottom. An even greater distance is preferred in order to allow good flow conditions between the ejected air and water mixture from the impeller 31 and the circulating water which is sucked in from the water surrounding the bioreactor 1. By the design of the bioreactor 1 a circulation of water will evolve that covers a large area around the bioreactor. It will, however, take some time, up to two to three weeks, before a stable circulation has fully established.

The skilled person realizes, however, that the bioreactor 1 may be designed to be used in water ponds with greater depth. In a variant, the platform 2 may be designed with a large depth, e.g. the thickness of the platform 2 may be up to 3 - 4 m. A large thickness is less advantageous as the velocity of the air bubbles tend to be too high through the perforated pipes 20. As an alternative, or as a complement, the impeller may be extended up to 2 m below the platform 2. If the tubular drive shaft 32 of the impeller 31 is made very long there is a risk that the shaft 32 starts to wobble, especially in combination with a high rotational speed of the impeller, which increases the risk of failure. In that case the tubular drive shaft 32 may need to be supported by rotational bearings in intermediate positions between the upper and lower ends. Overall, a bioreactor 1 with large depth is less advantageous than one with shallow depth. It is also more complicated to handle. It may, for example, be problematic to transport, unless demounted. Therefore, a bioreactor according to the preferred embodiment comprises a shallow bioreactor having a circular biofilm platform 2 having a central impeller 31 positioned essentially in the same plane as the underside of the pipes 20.

An advantage with the inventive bioreactor 1 is that the pipes 20 will not be clogged with time which is a problem with today's bioreactors. Thanks to the construction the bioreactor 1 will not become clogged and cease to function even if it is not cleaned as the biofilm will fall off when its thickness has become too thick and the innermost bacterial strains will suffer from lack of nutrients or oxygen and die. Anyhow, it is preferable to flush the platform 2 in connection with service intervals of the motor 33, maybe once a year, in order to check the status of the modules 200 and the perforated pipes 20. By that operation the biofilm will be essentially flushed off, but it will regrow relatively quickly and the bioreactor 1 will soon be operating at normal level after such service.

The term mobile as used herein refers to that the bioreactor can be either untethered such that it floats freely, or tetherable to a fixed point but easily demountable and repositionable. A bioreactor being rigidly fixed to the shore is not mobile, whereas a bioreactor being tethered to, for example, a pier, but which can be untethered and repositioned is mobile.

PERFORMED TESTS

An impeller with a collar according to the invention was compared with an impeller without a collar and the air bubble size and the distribution pattern of air bubbles were studied visually in a 6 m 3 tank with glass walls. It could be seen that the air bubbles were smaller and that the air bubbles were distributed more evenly inside the tank when the impeller was equipped with the collar. PILOT SCALE TEST

A pilot plant bioreactor 1 was installed for about four months (June to September) in an aerobic sewage treatment plant having a volume of 5400 m 3 (60 m long, 45 m wide, 2 m deep). The bioreactor 1 measured 2.77 x 2.77 m, consisted of one layer of BIO-BLOK ® 100 matrices and had an area of 8 m 2 . The aerator 3 comprised an Airturbo™ impeller 0200 mm provided with an inventive collar 0 296 mm (lower end of side wall). The air flow was 14.5 1/s and the impeller rotated with 1450 rpm. The flow of sewage water through the treatment plant was approximately 20 m 3 /d.

At the time of evaluation the bioreactor was removed (e.g. lifted up and inspected). At inspection of the platform it could be concluded that the perforated pipes had an even fouling of a biofilm which indicates good flow and function. The temperature in the sewage water was 16 °C at that time.

-BLOK ® 100 matrix was used for the platform:

The result for September shows a reduction of the ammonium content from 59 mgN/1 to 37 mgN/1. The test was performed by a certified laboratory and has an accuracy of +10%. Part of the ammonium may have been stripped out but as the pH-value is quite low that should not be a significant problem. A total of 440 gN/d (d = 24 h) was removed from the sewage water which corresponds to an active biosurface (surface area of biofilm at the bioreactor 1) of at least 630 m 2 at optimal conditions. However, the conditions in the sewage treatment plant is probably not ideal for biological life. Hence, at optimal conditions the active biosurface could be even larger and the biodegradation capacity of the bioreactor 1 even better.

The theoretical biodegradation capacity of a bioreactor 1 according to the invention depends on a number of factors. The bioreactor 1 used in the pilot scale test can be estimated to oxidize 0,9 gNH 4 /m 2 d och 20 gBOD/m 2 d at a sewage water temperature of 20°C and 0,7 gNH 4 /m 2 d och 17 gBOD/m 2 d at a sewage water temperature of 15 °C. The evolved biosurface is estimated to have an area of 300 m 2 /m 3 . The area of the biosurface depends on the kind of pipes that are used and the thickness of the biofilm but normally it is in the range of 250-500 m 2 /m 3 when fully evolved (around 3 mm).

ALTERNATIVE EMBODIMENTS

In the foregoing, the invention has been described with reference to a conceivable embodiment. It is appreciated, however, that also other embodiments and variants are possible within the scope of the following claims.

In the described example, the platform 2 has a square form but a circular form is probably more effective from an air bubble spreading pattern perspective. If used in streaming water, an oval form may be preferable. It is also conceivable to have hexagonal or octagonal platform from a manufacturing point of view. Thanks to the perforated pipes 20 being manufactured in modules, any desired form may be made which can be adapted to the actual form at the site, eg. at golf courses. It may even be designed from aesthetical reasons and if made long and narrow it may be provided with several small aerators instead of one large. It is possible to manufacture small bioreactors arranged with solar panels for remote locations having a platform with a diameter in the range of 0.5 - 3 m.

The grooves 43 on the inner side 42 of the side wall 41 of the impeller may be arranged differently than described in this example. For example, the grooves may extend in an inclined fashion. The upper end of the grooves are positioned in front of the lower end of the grooves, leaning forward in the direction of rotation of the impeller 31, which may increase the spreading of the air bubbles and result in somewhat less energy consumption. It is however more complicated and expensive to manufacture a collar 4 with inclined grooves which is why radially extending grooves has been chosen. Instead of grooves it is also possible to arrange small cavities. The typical pattern of cavities arranged on a golf ball may for example be conceivable.

The pipes 20 may be arranged in a more compact manner where parallel pipe rows may be positioned offset each other, a distance corresponding to half the diameter of the pipes.

The aerator 3 may comprise a system for the provision of air and need not be restricted to the motor 33, impellor 31 arrangement described herein. For example, the aerator 3 may comprise a compressed air system for the provision of air to the air dispensing device 31.

The skilled person understands that the floats 7 preferably are arranged within the area which the platform 2 occupies although it is conceivable to let the floats project outside the occupied area in order to extend the distance between the floats to further improve the flow conditions.