SE1650586A1 - Floating bioreactor - Google Patents

Abstract

16 ABSTRACT The present invention relates to a mobile, floating bio film bioreactor (l) for Wastewatertreatment, comprising a rigid 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 rigid frame suchthat the bioreactor (l) remains aflo at when submerged into Wastewater, an aerator (3)comprising an motor (33) arranged at the upper portion (8) of the rigid frame, an airdispensing device (3 l) arranged at the lower end of the rigid frame, an air pipe (32)extending between the motor (33) and the air dispensing device (3 l), a biof1lm supportmedia (2, 20) attached to the rigid frame below the at least one buoyant member (7),wherein the biof1lm support media (2, 20) comprises pipes (20) having perforated pipewalls (2l) arranged in an upright position alongside each other, said pipes forrning aplanar bio film platform (2) having an upper side and a lower side, said bio film platform(2) having a horizontal extension (X, Y) which extends its vertical extension (H), saidair dispensing device (3 l) comprising an impeller (3 l) positioned in connection to thelower side of the bio film platform (2) and further comprises a collar (4) arranged circumferentially outside the impeller (3 l).

Description

FLOATING BIOREACTOR TECHNICAL FIELD The present invention relates to a mobile, floating bio film bioreactor for Wastewatertreatment, comprising a rigid frame having an upper portion and a lower portion, at leastone buoyant member attached to the upper portion of the rigid frame such that thebioreactor remains afloat when submerged into Wastewater, an aeration systemcomprising an motor arranged at the upper portion of the rigid frame, an air dispensingdevice arranged at the lower end of the ridged frame an air pipe extending between themotor and the air dispensing device, a bio film 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 themajor 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 andnitrogen is not removed waters will be nutrient rich and lead to eutrophication and thesediments might become anaerobic. Today, more and more stakeholders haverestrictions 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 foruse in decontaminating water including a housing adapted to float within the mediumsuch that a top portion thereof remains adj acent a top surface of the contaminated waterwhile the bioreactor containing inoculated carrier media is attached below. Beneficialmicrobial 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 wastewatertreatment system, a frame for supporting porous biological-growth support media isprovided with at least one pontoon float. Also disposed on the frame is an aerator in theform 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 abovementioned problems which can be achieved as the bio film support media comprisespipes having perforated pipe walls arranged in an upright position alongside each other,said pipes forrning a planar biofilm module having an upper side and a lower side, saidbio film module having a horizontal extension which extends its vertical extension, saidair dispensing device comprising an impeller positioned in connection to the lower sideof the bio film module and further comprises a collar arranged circumferentially outsidethe 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 bio film module has a width in the rangeof 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 bio film module has a square, oval orcircular 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 essentiallyin the same plane as the lower side of the biofilm module, typically in a position max0.1 m above the lower side of the biofilm module to max 0.3 m below the lower side ofthe bio film 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 bio film platform and forms a central shaft for the aerator.

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

According to another aspect of the invention the fiannel wall has an angle V in the rangeof 30 - 60°, preferably 40 - 50°, and more preferred 45°, in relation to radial extensionof 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 alongsubstantially 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 diameterin the interval of 20 - 120 mm, preferably in the interval 40 - 80 mm and morepreferred in the interval 50 - 70 mm, and in that the walls being a mesh having a voidpercentage 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 isarranged 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 comprisesseveral floats which are fixed to radially extending arms of the frame, said armspreferably being at least three, more preferred at least four, in number andsymmetrically arranged in a circular direction, said floats having the shape of flattenedglobes or tubes and positioned along said arms in a pattem such that there is obtained a maximum distance between individual flo ats.

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 tothe 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 forthe 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 l illustrates a side view of a mobile, floating bioreactor l according to theinvention. The bioreactor l comprises a biofilm platforrn 2, an upper framework 8, alower framework 6 and floats 7. The floats are arranged above the platforrn 2 and areattached by vertical struts (not shown). Each strut extends from the upper framework 6,through the center of a float 7 further through the platforrn 2 to the lower framework 8.The vertical struts also acts as fixing and distance devices whereby the floats 7 and theplatforrn 2 are kept in accurate positions. The bioreactor l is arranged with an aerator 3for controlled oxygenation which extends vertically through the biofilm platforrn andhas a motor 33 and an air intake above the bio film platforrn 2 and an air dispensingdevice in the form of an impeller 3l for distribution of air below the biofilm platforrn 2.For example, the aerator 3 may be a product named AirturboTM marketed by the company Eden Aquatec.

In the shown embodiment, the platforrn 2 has a square form with a side length X and awidth Y in the interval l - l5 m, preferably in the interval 2 - 8 m and more preferredin the interval 3 - 6 m. The platform 2 described in the figures has a length X and widthY that is 2.8 m. The height H of the platforrn 2 is the same as a length L of the pipes 20and in the described example the height H is 55 cm. For example, the biofilm platform 2may 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 3comprises a motor 33 arranged at the top of a tubular drive shaft 32 for an impeller 3larranged at its lower end. The tubular drive shaft 32 also acts as air duct for provision ofair to the impeller 31. An upper mounting sleeve 35 for the tubular drive shaft 32 isarranged with the motor 30. The upper mounting sleeve 35 comprises an air intake opening 34 (or openings). The impeller 3l, which shall be described in detail in figure 3, is provided with a collar 4 for disintegrating of the air bubbles and provision of adesired distribution pattern of the air bubbles below the bio film platforrn 2.

Figure 3 illustrates a cross section of the impeller 3 l. A lower mounting sleeve 36 isarranged with an inner axial bore 37 for fixing of the lower end of the tubular driveshaft 32. The axial bore has an upper section with a diameter adapted to the outerdiameter 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 driveshaft 32 and has a diameter that is smaller than the diameter of the upper section of theaxial 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 46and 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 ho les 38 throughthe wall of the mounting sleeve 36. At the center of a short side of the lower end of themounting sleeve 36, the tubular drive shaft 32, i.e. the mounting sleeve 36, is carried bya bearing 62 at a cross brace 63 in a manner known per se. The arms of the cross brace63 is attached at lower cross-ties 6l of a column frame 60 which extends through thebiofilm platform 2 and forms a central shaft for the aerator 3. The column frame 60 alsoattaches to the upper frame section 8 and the lower frame section 6 which are fixed inparallel planes at a suitable distance relative to each other by the column frame 60 (seefigure l). The distance between the upper frame section 8 and lower frame section 6 isadapted to the height of the bio film platform 2 and the buoyant members 8l such thatthese are either clamped between the frame sections 6, 8 or such that a distance of atleast 0.l m, preferably at least 0.2 m is formed between the underside of the buoyantmembers 8 l, which are mounted to the underside of the upper frame section 8, and theupper side of the bio film platform 2. Hereby, the buoyant members will affect the flowless and a larger water depth between the bio film platform 2 and the water surface will improve flow conditions.

The impeller 3l and the collar 4 is fixed on the outside of the mounting sleeve 36 viascrews (not shown) to a mounting flange 39. Mating mounting holes 44 for the screwsare arranged in the flange 39, collar 4 and impeller 3l in a conventional manner. Theimpeller 3l is mounted outside the ho les 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 3l 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 across sectional view of said collar 4. The collar 4 comprises a substantially flat, circularupper disc 40 having through going mounting ho les 44 for fixing the collar 4 to theimpeller 31, for example by screws (not shown). From the circumferential edge of saidupper disc 40 a funnel-like side wall 4l extends downwardly, outwardly at an angle v ofabout 45° relative the upper disc 40. The inner side 42 of the side wall 4l comprisesevenly distributed grooves 43 which extend radially from the upper edge of the sidewall to the lower free end of the side wall and over the edge which is beneficial from aflow perspective. As the collar 4 rotates together with the impeller 3 l, the grooves 43shatter and shred the air bubbles that are pumped out from the impeller. In the describedexample the grooves 43 have a depth of about l mm and there are fifty grooves 43evenly arranged at the inner side 42 but within the concept of the invention said groovesmay have a depth in the range of 0.5 - l.5 mm, preferably 0.8 -l .2 mm and mostpreferred 0.9-l .l mm and may be arranged along substantially the whole height of the wall with a mutual angle distance of 5-l0° in the circumferential direction.

In figure 6 is illustrated a perspective view of a module 200 which forms the bio filmplatform. The module 200 comprises a structured filter media developed by thecompany CAP Technology in England, http://www.captechno lo gy. co.uk/pdf/ cap_design.pdf. The filter media has provedextremely efficient in biological treatment of domestic sewage, industrial wastewaterand process water within the aquaculture field. The media is made from theenvironmentally friendly material polyethylene and consists of net tubes 20 which arefixed lengthwise to each other and welded together, preferably at their upper and lowerends, to form a square block. The unique surface structure of the many net tubesprovides a large accessible surface area for enhanced biological growth on the filtermedia. The filter media is called BIO-BLOK® and the surface structure of the many nettubes acts as a substrate for specialized bacterial strains, which are able to treat anddegrade a wide range of wastewater qualities. The treatment capacity of a biologicalfilter depends on the quantity of bacteria that the filter can sustain. Naturally, otherpipes 20 which are assembled side by side and has mesh pipe walls 2l in a material andwith 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 theinterval 40 - 200 cm. In a preferred embodiment, the platforrn 2 is designed to beshallow, typically in the range of 50 - 100 cm. The shallow depth is preferred as asuitable flow rate of the air bubbles through the pipes 20 is thereby obtained. Theperforated pipe 20 has an outer diameter D in the interval 20 - 120 mm, preferably inthe interval 40 - 80 mm and more preferred in the interval 50 - 70 mm. The innerdiameter shall not be too small as there is a risk that the pipes will become clogged andnot too large as this will impair the biodegradation efficiency of the bioreactor l. In thisdescribed example, BIO-BLOK® 100 modules were used. In the modules 200, theperforated pipes 20 have a length L of 55 cm, an outer diameter D of 67.5 mm and aninner diameter of 62.5 mm. A module 200 has the dimension 54 x 54 x 55 cm (width xdepth x height). The BIO-BLOK® 100 module have a specific surface of 100 m2/m3(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 50mm. This module have a specific surface of l25 m2/m3, an area of flow of 67 % and avoid percentage of 89 %. The larger the specific surface area is, the larger the bacterialpopulation. As a rule of thumb, the voids in the mesh shall have a diameter in the rangeof 5 - 20 mm.

In figure 7 is illustrated a bioreactor l in operation. The platform 2 is situated below thewater surface and the upper framework 8 and the floats 7 are situated above the watersurface as well as the motor 33 and the upper mounting sleeve 35 with the air intakeopening 34 of the aerator 3. The floats 7 are arranged above the platform 2 and are fixedto bars 80 of the upper framework 8. The upper framework 8 also comprises fasteners8l for example wires 82 that may be fixed to a pier or to the water bottom to keep thebioreactor l in place. The framework 8 also fixes the aerator 3 in the center of theplatform 2. A grating 83 is arranged between the vertical bars that form the columnframe around the aerator 3. The grating 83 is positioned in the area between the motor33 and the upper surface of the platform 2 to prevent debris from entering the shaftaround the aerator 3 and get entangled in the rotating tubular drive shaft 32 or the impeller 3 l.

In this described example the bioreactor l is arranged with eight floats 7 which have theshape of flattened globes. The floats 7 are symmetrically positioned in order to balancethe bioreactor l in the water and distribute the load on the upper framework 8 evenly. Afirst group of four floats 7 are positioned in the comers of an inner square frameapproximately halfway from the center of the platform 2 and its outer comers. A secondgroup 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 extendfrom the inner square frame. The skilled person realize that the floats 7 may vary innumber, shape and placement depending on the size and form of the platform 2 and thatit is benef1cial to arrange the floats so that the distance between individual floats 7 are aslarge as possible from a flow restriction perspective. In a variant with eight floats, thefloats 7 are instead grouped two and two and arranged along diagonally extending barswhich may be even better from a flow restriction perspective than the arrangement described above.

The bioreactor l according to the invention operates by that the impeller 3l rotates andair is sucked through the tubular drive shaft 32. The air that is hurled from the impellerhits the collar 4 which shatters and spreads the bubbles of air laterally under theplatform 2. Thanks to the fact that the platform 2 comprises pipes 20 a pumping effect isachieved, according to the air-lift pump principle, which will cause a mixture of waterand air bubbles to pass through the perforated pipes 20 of the platform 2. On theperforated pipe walls 2l a bio film of specialized bacterial strains evo lves over time andwhen the water and the air bubbles passes, the contaminants diffuses into the bio filmand 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 theinventive concept it is possible to control the size and the spreading pattem of the airbubbles by designing the collar 4, e.g. the depth and orientation of the grooves 43 andthe inclination of the side wall 4l, in an appropriate manner. The actual position of theimpeller relative the lower side of the platform 2 also is of some importance as well asthe dimension, e.g. diameter, of the impeller. It is a great advantage to be able to controlthe size of the air bubbles since the velocity of which the air bubbles rise through thepipes 20 and the oxygen transfer rate will be effected. To be able to distribute the airbubbles evenly undemeath the platform 2, or at least undemeath the majority of the platform 2, is naturally also advantageous.

The inclination of the side wall 4l affects the spreading pattem of the air bubbles.Depending on the length X and the width Y of the platform 2 it may be desired that theair bubbles have a wide or narrow distribution. The choice of wide or narrow spreadingpattem is also dependent on the conditions at the installation site. A wider spreadingpattem may be desired in shallow sites and vice versa. A greater angle v between theupper part 40 and the side wall 4l (see Fig. 5) generates a narrower spreading pattem than a smaller angle v which generates a wider spreading pattem. 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 platforrn 2and creates a high pumping effect which is benef1cial. An optimal spreading pattern ofthe 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 actualposition of the impeller relative the lower side of the platforrn 2. The impeller 3l maybe positioned a short distance inside the platform 2, typically max l0 cm above thelower side, provided that the side wall 4l of the collar 4 have a sufficient inclination todirect the air bubbles past the lower end of the innerrnost pipes 20. However, theimpeller 3l is preferably positioned so that it essentially aligns with the lower side ofthe platforrn 2 or a short distance, typically max 30 cm, below the lower side of theplatforrn 2 which can be seen in Fig. l. In a variant it is however conceivable to let theimpeller extend up to l m below the lower side of the platforrn 2. Hereby, the pumpingeffect, i.e. the amount of water that is lifted through the pipes, is increased. Thespreading of the air bubbles sideways is also improved due to the increased time beforethe 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 whichwill 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 platforrn area.The size of the bubbles will be smaller than if the side wall 4l are without grooves 43and signif1cantly smaller than if no collar 4 is used at all. The size of all air bubbles willnot be exactly the same but significantly more even in size. Empirical tests have shownthat a suitable depth of the grooves 43 is in the range of 0.5 - 1.5 mm, preferably 0.8 -l.2 mm and most preferred 0.9 - l.l mm and they are arranged along substantially thewhole height of the wall with a mutual angle distance of 5 - l0° in the circumferentialdirection. Tests have shown that a depth of the grooves of l .5 mm or more will not beas effective in shuttering the air bubbles. Yet a disadvantage is that the side wall 4l ofthe 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 inorder to obtain a desired size and distribution pattem of the air bubbles. In combinationwith the design of the platform 2 it will contribute to a bioreactor l with improved performance compared to prior art reactors.

The column frame 60 also serves to protect the impeller 3l from getting damaged in case there is a risk that the bioreactor l will touch the bottom if the water level drops.

Since it is possible to spread air bubbles far to the sides the bioreactor l may bedesigned to be shallow but widespread sideways instead. This is a great advantage ineg. leachate ponds since they often are l-3 m deep and prior art bioreactors are too deepto be used therein. In order to avoid bottom sediments from being whirled up anddragged along by the mixture of air bubbles and water, it is suitable to keep a margin ofat least 0.2 m but preferably at least l m between the impeller 3l and the bottom. Aneven greater distance is preferred in order to allow good flow conditions between theejected air and water mixture from the impeller 3l and the circulating water which issucked in from the water surrounding the bioreactor l. By the design of the bioreactor la circulation of water will evo lve 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 fullyestablished.

The skilled person realizes, however, that the bioreactor l may be designed to be usedin water ponds with greater depth. In a variant, the platform 2 may be designed with alarge depth, e.g. the thickness of the platform 2 may be up to 3 - 4 m. A large thicknessis less advantageous as the velocity of the air bubbles tend to be too high through theperforated pipes 20. As an altemative, or as a complement, the impeller may beextended up to 2 m below the platform 2. If the tubular drive shaft 32 of the impeller 3lis made very long there is a risk that the shaft 32 starts to wobble, especially incombination with a high rotational speed of the impeller, which increases the risk offailure. In that case the tubular drive shaft 32 may need to be supported by rotationalbearings in interrnediate positions between the upper and lower ends. Overall, abioreactor l with large depth is less advantageous than one with shallow depth. It is alsomore complicated to handle. It may, for example, be problematic to transport, unlessdemounted. Therefore, a bioreactor according to the preferred embodiment comprises ashallow bioreactor having a circular bio film platform 2 having a central impeller 3l positioned essentially in the same plane as the underside of the pipes 20.

A huge advantage with the inventive bioreactor l is that the pipes 20 will not beclogged with time which is a problem with today's bioreactors. Thanks to theconstruction the bioreactor l will not become clogged and cease to function even if it isnot cleaned as the bio film will fall off when its thickness has become too thick and theinnerrnost 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 ofthe motor 33, maybe once a year, in order to check the status of the modules 200 andthe perforated pipes 20. By that operation the biof1lm will be essentially flushed off, but 11 it will regrow relatively quickly and the bioreactor 1 will soon be Operating at normal level after such service.

PERFORMED TESTS An impeller with a collar according to the invention was compared with an impellerwithout a collar and the air bubble size and the distribution pattern of air bubbles werestudied visually in a 6 m3 tank with glass walls. It could be seen that the air bubbleswere smaller and that the air bubbles were distributed more evenly inside the tank whenthe impeller was equipped with the collar.

PILOT SCALE TEST A pilot plant bioreactor 1 was installed for about four months (June to September) in anaerobic sewage treatment plant having a volume of 5400 m3 (60 m long, 45 m wide, 2 mdeep). 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 m2. The aerator 3 comprised an AirturboTM impellerø 200 mm provided with an inventive collar ø 296 mm (lower end of side wall). Theair flow was 14.5 l/s and the impeller rotated with 1450 rpm. The flow of sewage water through the treatment plant was approximately 20 m3/ d.

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

A BIO-BLOK® 100 matrix was used for the platform: Specific Surface Area of Void Outer tube Standard surface structure flow Percentage diameter module form (m2/m3) (%) (%) (mm) LxWxH(Om) 100 Rough 70 90 67,5 54x54x55 The result for September shows a reduction of the ammonium content from 59 mgN/l to37 mgN/l. The test was performed by a certified laboratory and has an accuracy ofi10%. Part of the ammonium may have been stripped out but as the pH-value is quitelow that should not be a significant problem. A total of 440 gN/ d (d = 24 h) was 12 removed from the sewage Water which corresponds to an active biosurface (surface areaof biofilm at the bioreactor 1) of at least 630 m2 at optimal conditions. However, theconditions 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 inventiondepends on a number of factors. The bioreactor 1 used in the pilot scale test can beestimated to oxidize 0,9 gNH4/m2 d och 20 gBOD/mz d at a sewage water temperatureof 20°C and 0,7 gNH4/m2 d och 17 gBOD/mz d at a sewage water temperature of 15 °C.The evolved biosurface is estimated to have an area of 300 m2/m3. The area of thebiosurface depends on the kind of pipes that are used and the thickness of the bio film but norrnally it is in the range of 250-500 m2/m3 when fully evolved (around 3 mm).

ALTERNATIVE EMBODIMENTS In the foregoing, the invention has been described with reference to a conceivableembodiment. 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 isprobably more effective from an air bubble spreading pattem perspective. If used instreaming water, an oval form may be preferable. It is also conceivable to havehexagonal or octagonal platform from a manufacturing point of view. Thanks to theperforated pipes 20 being manufactured in modules, any desired form may be madewhich can be adapted to the actual form at the site, eg. at golf courses. It may even bedesigned from aesthetical reasons and if made long and narrow it may be provided withseveral small aerators instead of one large. It is possible to manufacture smallbioreactors 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 arrangeddifferently than described in this example. For example, the grooves may extend in aninclined fashion. The upper end of the grooves are positioned in front of the lower endof the grooves, leaning forward in the direction of rotation of the impeller 31, whichmay increase the spreading of the air bubbles and result in somewhat less energyconsumption. 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 13 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 thepipes.

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

Claims (10)

1. A mobile, floating bio film bioreactor (1) for Wastewater treatment, comprising arigid frame having an upper portion (8) and a lower portion (6), at least one buoyantmember (7) attached to the upper portion (8) of the rigid frame such that thebioreactor (1) remains aflo at when submerged into Wastewater, an aerator (3)comprising an motor (33) arranged at the upper portion (8) of the rigid frame, an airdispensing device (31) arranged at the lower end of the rigid frame, an air pipe (32)extending between the motor (33) and the air dispensing device (31), a biofilmsupport media (2, 20) attached to the rigid frame below the at least one buoyantmember (7), characterized in that the biofilm support media (2, 20) comprisespipes (20) having perforated pipe walls (21) arranged in an upright positionalongside each other, said pipes forrning a planar bio film platform (2) having anupper side and a lower side, said biofilm platform (2) having a horizontal extension(X, Y) which extends its vertical extension (H), said air dispensing device (31)comprising an impeller (31) positioned in connection to the lower side of the bio filmplatform (2) and further comprises a collar (4) arranged circumferentially outsidethe impeller (31). A mobile, floating biofilm bioreactor (1) according to claim 1, characterized inthat said biofilm platform (2) has a width (X) in the range of 1 - 15 m, preferably inthe 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. A mobile, floating biofilm bioreactor (1) according to claim 2, characterized in that said bio film platform (2) has a square, oval or circular form. A 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 (z). A mobile, floating biofilm bioreactor (1) according to claim 4, characterized inthat the impeller (31) is positioned in the same plane as the lower side of the bio film platform (2), preferably max 0.1 m above the lower side of the bio filmplatform to max 0.3 m below the lower side of the bio film platform, or extending a short distance below the lower side, preferably max 1.0 m below the lower side of 10. the bio film platform (2), and further characterized in a column frame (60) Which extends through the bio film platform (2) and forrns a central shaft for the aerator (3). A mobile, floating biof1lm bioreactor (1) according to claim 1, characterized inthat said collar (4) comprises an upper circular disc (40) and a Wall (41) Whichextends 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 heightof the Wall (41) being adapted to the height of radial openings between impellerblades (5) at the circumferential surface of the impeller (31) such that the Wall (41) extends in the region radially outside said openings. A mobile, floating biof1lm bioreactor (1) according to claim 6, characterized inthat 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), inthat said grooVes (43) haVe a depth in the range of 0.5 - 1.5 mm, preferably 0.8 -1.2mm and most preferred 0.9-1.1 mm and are arranged along substantially the Wholeheight of the Wall (41) With a mutual angle distance of 5-10° in the circumferential direction. A mobile, floating biof1lm bioreactor (1) according to claim 1, characterized inthat 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. A mobile, floating biof1lm bioreactor (1) according to claim 1, characterized inthat 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) . A mobile, floating biof1lm bioreactor (1) according to claim 9, characterized inthat it the at least one buoyant member (7) comprises seVeral floats (7) Which arefixed to bars (80) of the upper portion (8) of the rigid frame, said floats (7) haVingthe shape of flattened globes or tubes and positioned along said bars (80) such thatthere is obtained a maximum distance between indiVidual floats (7) or group offloats (7).