Free-flowing carrier elements

Abstract

A carrier element (1; 10; 20) for growth of biofilm thereon is designed for free-flowing in liquid to be purified and has surfaces (3; 13) for biofilm growth which are protected from the abrasion from other carrier elements or surfaces in a container containing the liquid to be purified by ridges (4; 12) having a height corresponding to a desired thickness of a biofilm intended to grow on the protected surfaces (3; 13). The ratio between the surfaces (3; 13) for biofilm growth and the area of the ridges ranges from 1:1 to 1:20.

Claims

1. Carrier element for growth of biofilm thereon, comprising protected surfaces for biofilm growth between ridges which are protected by the ridges from abrasion from other carrier elements or surfaces while free flowing in a container containing a liquid to be purified, wherein said carrier element is in a form of a plate with a pattern on each side, said pattern being a grid of the ridges, wherein the ratio between area of the protected surfaces for biofilm growth between the ridges and side area of the ridges ranges from 1:1 to 20:1, and a height of the ridges, which corresponds to a desired thickness of a biofilm intended to grow on the protected surfaces, is within the interval of 0.05 to 1.0 mm.

2. Carrier element according to claim 1, wherein the height of the ridges, which corresponds to the desired thickness of a biofilm intended to grow on the protected surfaces, is within the interval of 0.1 to 0.5 mm.

3. Carrier element according to claim 1, wherein the height of the ridges, which corresponds to the desired thickness of a biofilm intended to grow on the protected surfaces, is within the interval of 0.15 to 0.45 mm.

4. Carrier element according to claim 1, wherein said plate is of round or oval shape.

5. Carrier element according to claim 1, wherein said plate is saddle shaped.

6. Carrier element according to claim 1, wherein the protected surfaces for biofilm growth between the ridges are square or rectangular.

7. Carrier element according to claim 1, wherein said grid is in a shape of a honeycomb with hexagonal protected surfaces for biofilm growth between the ridges.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other aspects, features and advantages of which the invention is capable of will be apparent and elucidated from the following description of embodiments of the present invention, reference being made to the accompanying drawings, in which

(2) FIG. 1 is a schematic perspective view showing a hexagonal carrier element according to a first embodiment of the invention;

(3) FIG. 2 is a schematic perspective view showing a hexagonal carrier element according to a second embodiment of the invention;

(4) FIG. 3 is a schematic perspective view showing a hexagonal carrier element according to a third embodiment of the invention;

(5) FIG. 4 is a schematic perspective view showing a hexagonal carrier element according to a fourth embodiment of the invention;

(6) FIG. 5 is a schematic perspective view showing a hexagonal carrier element according to a fifth embodiment of the invention;

(7) FIG. 6. is a schematic perspective view showing a flat round carrier element according to a sixth embodiment of the invention;

(8) FIG. 7. is a schematic perspective view showing a saddle shaped carrier element according to a seventh embodiment of the invention; and

(9) FIG. 8. is a schematic cross section view showing a carrier element with an arched biofilm surface layer.

DESCRIPTION OF EMBODIMENTS

(10) A first embodiment the present invention is shown in FIG. 1 and relates to a carrier element 1 for allowing growth thereon of a biologically active layer of microbes. The carrier element 1 comprises a multitude of through holes 2, each through hole 2 being limited by walls 3 and ridges 4 extending along a length of the through hole 2. The through holes 2 and their ridges 4 are arranged to be cleaned by a rod-like structure (not shown) having a diameter fitting within the area defined by the ridges 4. In the shown embodiment, the through holes 2 are hexagonally shaped and arranged, but other arrangements are also possible within the scope of the invention.

(11) The arrangement with a rod-like cleaning element and the ridges of the holes has the effect that a microbial film growing in the holes will be limited with regards to its thickness.

(12) FIG. 2 shows an embodiment that is similar to the embodiment of FIG. 1, however with a dividing wall extending between two identical elements provided with holes having a shape corresponding to the through holes of FIG. 1.

(13) FIG. 3 shows an embodiment that is similar to the embodiment of FIGS. 1 and 2, however with a rod-like cleaning element 5 extending from an outer surface of the carrier 1.

(14) FIG. 4 shows an embodiment that is similar to the embodiment of FIG. 2, however with a rod-like cleaning element 5 extending from an outer surface of the carrier 1.

(15) FIG. 5 shows still another embodiment having six rod-like cleaning elements 5.

(16) During operation the elements will float around, contacting other elements and surfaces in a reactor (not shown). The inner surfaces of the through holes will be protected from wear, however the rod-like cleaning elements 5 described in embodiments 3 to 5, having a diameter fitting within the area defined by the ridges 4, will be able to scrape out the biofilm in the center of the through holes, leaving a protected biofilm layer with a thickness defined by the height of the ridges 4 extending along a length of the through hole 2.

(17) The elements 1 can be manufactured from e.g. polyethylene plastic or polypropylene plastic by extrusion or injection molding, they may have a thickness of 2 to 20 mm, a diameter of 10 to 50 mm, a through hole diameter of 2 to 5 mm, wherein the ridges may extend 0.1 to 0.5 mm from an internal surface of the through hole. The rod like cleaning element 5 has preferably a diameter such that it may enter the holes, i.e. its diameter is smaller than the hole diameter subtracted by the height of the ridges 4. Its length is preferably somewhat larger than half of the thickness of the carrier.

(18) The elements 1 described in embodiments 2, 3 and 6 can have a small protrusion (not shown) on the dividing wall in the bottom of the holes. This small protrusion may extend 0.1 to 0.5 mm from the dividing wall of the hole and have a diameter smaller than the hole diameter subtracted by the height of the ridges 4. The dividing wall surface of the hole will be protected from wear from the rod-like cleaning element 5, leaving a protected biofilm layer with a thickness defined by the height of the small protrusion.

(19) As a result of this the biofilm growing within the holes will have a limited thickness, thereby avoiding anaerobic portions of the biofilm.

(20) In FIG. 6, a carrier 10 according to another embodiment of the present invention is shown. The carrier 10 comprises a disc-like body 11 comprising a grid 12 of ridges 4 extending over the entire area of the disc-like body 11. The ridges 4 of the grid 12 have a height over the disc-like body 11 corresponding to a desired thickness of a biofilm growing thereon. It should be noted that the disc-like body should have a shape avoiding sharp corners, i.e. a circular or oval shape, since sharp corners will scrape off biofilm from the disc-like body, even if protected by the grid 12 of ridges 4.

(21) During operation, wherein a large number of carriers according to this embodiment are free-floating in the reactor, biofilm growing on the surface thereof will be scraped off. However, due to the provision of the grid 12 and the fact that there are no sharp corners, areas 13 between the ridges 4 of the grid 12 will be protected from wear. However, if the biofilm thickness becomes too large the excess will be scraped off. Accordingly, the biofilm thickness may be controlled in an efficient manner.

(22) In FIG. 7, a carrier 20 is shown, the carrier 20 being substantially identical to the carrier 10 of FIG. 6, however, formed into a saddle-like configuration. This configuration has been shown to perform excellently in terms of aeration properties.

(23) FIG. 8 illustrates when a biofilm surface between the ridges 4 of the grid 12 on a disc-like body 11 carrier has not been scraped off at a straight angle. When two carrier elements 10 have scraped against each other, the circular or oval shape of the disc-like bodies 11 may leave a correspondingly arched biofilm surface 14 shape between the ridges 4 of the grid 12. The shape of such an arched surface 14 will depend on the diameter of the disc-like body and the grid 12 density.

(24) The carriers 10 and 20 may be manufactured by injection molding. They may have a diameter of 10 to 50 mm, a grid 12 density of 5 to 10 mm and a ridge 4 height of 0.05 to 1.0 mm. The thickness of the body is preferably as thin as possible with regards to the strength thereof. Another manufacturing method may be to roll a plate of plastic such that ridges 4 thereon are formed and thereafter stamp carriers of the desired shape and size from the rolled plate.

(25) The surface area for biofilm growth is primarily the wall 3 or area 13 of the carrier elements. The ridge 4 and grid 12 constitute similar or smaller biofilm growth area and primarily protects and define the thickness of the biofilm.

(26) The ratio of biofilm growth area between the protected surfaces walls 3 and areas 13 between the ridges 4 in comparison to the ridges 4 and the grid 12 of ridges 4 of the carrier elements 1 and 10 and 20 is in the range of 1:1 to 20:1.

(27) Because they are the engine that drives biological wastewater treatment, it is critical to closely monitor the quantity and quality of microorganisms in bioreactors.

(28) Earlier developments of the MBBR-process and carrier elements has been solely focused on the protective surface provided by the carrier material and to some extent to how the carrier elements should be formed to increase the mass transport to the biofilm, when the biofilm has an extensive thickness.

(29) A major limitation of all prior art carriers and MBBR processes is that the thickness of the biofilm is not controlled to a range where the mass transfer is effective throughout the biofilm, thus only the outer part of the biofilm is utilizing for the desired reactions.

(30) When the biofilm grows thick on the carrier elements, the effective area of the biofilm exposed to the environment is reduced, making a large portion of biomass inactive with respect to the desired reactions and the MBBR process efficiency decreases. The process can thus exhibit a varying effectiveness depending on how the biofilm develops and a complete in-growth of biofilm in the support elements results in a drastic reduction in efficiency. For a thick biofilm, reactions averse to the MBBR-process may take place further into the biofilm, for example the reduction of sulfate to hydrogen sulfide. These problems are well known, but no solution has existed for controlling them.

(31) When the over-growth of the inert carrier elements becomes acute, different types of washing devices has been used to mechanically knock away the biofilm from the carriers. For the individual carrier elements, however, the result of the cleaning is random, and the biofilm is not controlled to an optimum thickness. The present invention relates to a carrier element in which thickness of the biofilm in a MBBR process can be predetermined and maintained in an optimum range for the desired reactions.

(32) The wear which occurs when the carrier elements moves against other carrier elements or other surfaces in the process has previously been considered as a disadvantage of the MBBR process, since it limits the size of the area of the support material which can be colonized with biofilm compared with other biofilm processes where the carriers are stationary and all surfaces are colonized. The general solution according to the invention is to instead use this wear to provide a biofilm with controlled thickness within an optimum range for the reactions to be achieved, which provides significant advantages over prior MBBR processes as well as other biofilm processes.

(33) In comparisons between the invention and the prior embodiments, it has been found that it is equally important to keep the protected biofilm thickness of a carrier element within the optimal range as achieving a maximum protected surface of the material.

(34) It has been shown that the efficiency of the process is in practice determined by the amount of biomass that actively participates in the desired reactions and this is found in the part of the biofilm which is reached by the reactants which are to be renewed. Because of the nature of the biofilm process, a carrier material surface which stops the liquid flow, the transport of reactants into the biofilm will be limited by the diffusion rates for the substances in question. This type of mass transport limitation in a biofilm applies to all kinds of substrates, such as oxygen in aerobic processes, volatile fatty acids in anaerobic processes, the nitrification of ammonium and phosphate in the biological phosphorus removal.

(35) More particularly, MBBR trials according to the present invention has shown that in order to achieve a maximum active biofilm, the thickness thereof has to be kept in the range of 0.05-1.0 mm, preferably in the range of 0.1-0.5 mm, in particular in the range of 0.15 to 0.45 mm.

(36) For biofilm thinner than 0.05 mm, the amount of active biomass is limited by the biofilm thickness, so that full capacity is not reached. For biofilm thicker than 1.0 mm, there is a significant amount of inactive biomass in the interior of the biofilm and unwanted reactions begin to assert themselves.

(37) The protected biofilm thickness is the thickness to which biofilm can grow on the support element without being subjected to scraping against other surfaces in the process, the surfaces of other carriers, reactor walls or other parts of the reactor. The protected biofilm thickness will thus depend on both the carrier material design as well as the design of other parts of the reactor.

(38) The present invention has the advantage over the prior art that it provides a carrier element which uses the wear and tear from when the carrier elements moves against other carrier elements or other surfaces in the process to control the thickness of the biofilm within an optimum range for the reactions to be achieved, which provides significant advantages over prior MBBR processes as well as other biofilm processes.

(39) Although the present invention has been described above with reference to specific embodiments, it is not intended to be limited to the specific forms set forth herein. Rather, the invention is limited only by the accompanying claims and, other embodiments than the specific above are equally possible within the scope of these appended claims.

(40) In the claims, the term comprises/comprising does not exclude the presence of other elements or steps. Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly advantageously be combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. In addition, singular references do not exclude a plurality. The terms a, an, first, second etc do not preclude a plurality. Reference signs in the claims are provided merely as a clarifying example and shall not be construed as limiting the scope of the claims in any way.