Title of Invention	A HOLLOW-FIBER MEMBRANE FILTRATION APPARATUS IN THE CLEANING OF WASTE WATER, AND A MEMBRANE BIOREACTOR
Abstract	The invention relates to a filtration apparatus for separating particles from a liquid by means of hollow-fiber membranes which are joined into a fiber bundle. The hollow-fiber membranes are flowed through from the outside to the inside by the liquid and liquid purified from the particles is drawn off at least at one of the ends of the hollow-fiber membranes. The hollow-fiber bundle is wound up on a support whose outer circumferential surface can be flowed through by gas at least partly from the inside to the outside. The winding of the hollow-fiber bundle on the support allows on the one hand a compact configuration and on the other hand a reliable cleaning of the hollow-fiber membranes from any deposited materials. The filtration apparatus can be used alone or with several combined in a filtration module for cleaning waste water. The invention relates further to a method for purifying waste water by using the filtration module and a membrane bioreactor.

Title of Invention

A HOLLOW-FIBER MEMBRANE FILTRATION APPARATUS IN THE CLEANING OF WASTE WATER, AND A MEMBRANE BIOREACTOR

Abstract

The invention relates to a filtration apparatus for separating particles from a liquid by means of hollow-fiber membranes which are joined into a fiber bundle. The hollow-fiber membranes are flowed through from the outside to the inside by the liquid and liquid purified from the particles is drawn off at least at one of the ends of the hollow-fiber membranes. The hollow-fiber bundle is wound up on a support whose outer circumferential surface can be flowed through by gas at least partly from the inside to the outside. The winding of the hollow-fiber bundle on the support allows on the one hand a compact configuration and on the other hand a reliable cleaning of the hollow-fiber membranes from any deposited materials. The filtration apparatus can be used alone or with several combined in a filtration module for cleaning waste water. The invention relates further to a method for purifying waste water by using the filtration module and a membrane bioreactor.

Full Text	A HOLLOW-FIBER MEMBRANE FILTRATION APPARATUS AND ITS USE IN THE CLEANING OF WASTE WATER, AND A MEMBRANE BIOREACTOR TECHNICAL FIELD The invention relates to a filtration apparatus for separating particles from a liquid by means of hollow-fiber membranes which are joined into a fiber bundle. This filtration apparatus and a filtration module in which several filtration apparatuses are combined are especially suitable for separating biomass from water or waste water. The invention further relates to a membrane bioreactor and a method for (waste) water treatment. BACKGROUND AMD PRIOR ART REFERECENS The use of membrane filters for purifying water or waste water is principally known. The porous materials used for filtration consist for example of ceramics or polymer membranes, for example, made of polyethylene, polypropylene, polyethersulfone or the like. Depending on the field of application, the pore sizes of the membrane can be in the region of between 0.001 and 1 pm. Three different modes of operation are principally distinguished in membrane filtration, namely "dead end", "crossflow" and "submerged". "Dead end" are usually designated as such applications in which the liquid to be cleaned is pressed without any further circulation in batch operation through the membrane. The particles held back by the membrane will accumulate on the membrane and lead to blockages or growths on the membrane in the course of operation. In the course of the operating period it is necessary to increase the pressure in order to ensure even throughput. Conversely, the throughput will decrease with unchanged pressure. This filtration method is therefore not suitable for large-scale use in the field of continuous systems for the treatment of drinking water or waste water. In the "crossflow" method, the liquid to be cleaned is guided in a circulation along the membrane surface and is pressed through the membrane as a result of the differential pressure between said membrane side and the opposite side of the membrane, with the particles to be separated being held back. This method requires differential pressures of clearly above 500 mbar. It is further necessary to recirculate a large portion of the as yet non-cleaned liquid for detaching the deposits forming on the membrane. This causes considerable operating costs. In the "submerged" method, the membrane is immersed into the liquid to be cleaned and in the case of hollow-fiber membranes permeate is pulled from the outside into the interior of the hollow fiber according to the vacuum principle and is carried off in the interior of the hollow fiber. The differential pressure between the outside of the membrane and the interior of the membrane is clearly lower than in the case of the "crossflow" filtration. When using membrane filtration for treating water or waste water, the "submerged" method is usually applied, in which the membrane is immersed into the liquid to be cleaned. One problem in the use of membrane filters in the field of waste water purification is that as a result of so-called "fouling" or "bio-fouling", coatings will form on the membrane or the precipitation of chemical substances will lead to deposits on the membranes ("scaling"). Different methods and arrangements have been proposed in order to relieve the membranes from such deposits. US Pat. No. 6,214,231 B1 for example describes a filtration system which uses hollow-fiber membranes. Several hollow-fiber membranes are combined into a substantially cylindrical fiber bundle. The upper and lower ends of the hollow-fiber membranes are each embedded in a holding apparatus. A hollow-fiber module is thus obtained. Several filtration modules situated adjacent to each other form a filtration cassette which is placed into the liquid to be cleaned. A permeate suction line is connected with each of the cassettes, which line is in connection with the upper fixing devices of the membrane filter bundles and removes the liquid purified from the particles from the interior of the individual hollow-fiber membranes. In order to relieve the hollow-fiber membranes from deposits, the direction of flow can be reversed. Liquid is thus supplied under high pressure through the suction line into the interior of the hollow-fiber membranes and passes through the latter from the inside to the outside, with deposits being detached from the membrane surface. Such a method means an interruption of the regular filtration operation and thus a reduced throughput. Moreover, this method cannot prevent the depositing of impurities on the membrane, but at best it can remove them after they occur. US Pat. No. 6,156,200 A also describes filtration modules with hollow membrane fiber bundles which in their configuration principally correspond to those of the US Pat. No. 6,214,231 B1. The fiber bundle is also placed in this case vertically into the liquid to be cleaned. The fixing device in which the lower ends of the hollow-fiber membranes are embedded comprises gas outlet openings through which gas bubbles rise along the outside of the hollow- fiber membranes and thus reduce the formation of deposits on the outside surfaces of the membranes on the one hand and remove deposits already formed on the other hand. A similar arrangement is also described in WO 97/06880 A2. It is also mentioned in this case that an especially good cleaning can be achieved in such a way that the fibers are longer by between five and ten percent than the distance between the fixing devices in which the fiber ends are embedded. The described apparatuses for cleaning the membrane fibers with compressed air come with the disadvantage that very large quantities of compressed air are required in order to ensure an effective cleaning of the membrane fibers. The increased supply of compressed air in the liquid to be cleaned can have a negative influence on other process parameters, e.g. it may make observing the set oxygen values considerably more difficult. A further disadvantage is that the applicable pressures are limited at most to the hydrostatic pressure level in the region of the introduced fiber. Moreover, the rising speed of the bubbles is predetermined not by the introduced air quantity but by the size of the produced bubbles. A purposeful control of the cleaning effect of the apparatuses is thus principally not possible. An even distribution of the action of the air along the length of the hollow-fiber membranes is also impossible. An even cleaning over the entire length of the fibers of mostly one to two meters is therefore not possible with the described systems. The publications as mentioned above further merely generally describe the application of hollow-fiber membranes for separating biomass in membrane bioreactors. They focus alone on the separation of biomass which is present and has been obtained as biocynosis from the existing nutrients under the respective environmental conditions. The mechanisms which promote the depositing on the membrane surfaces are not considered thereby. Within the scope of the examinations underlying this invention it was found however that certain environmental conditions have a considerable influence on the separating power of membranes. It was noticed in particular that certain environmental conditions strongly promote the biological growth on the membrane surface (bio-fouling) and the adherence of the suspended material. One aspect of the invention is therefore also ensuring such conditions in the filtration medium which reduce bio-fouling and the depositing on the membrane surface. It is therefore the object of the present invention to provide a filtration apparatus which is arranged in a simple and compact way and maintains a high throughput over a long period of time. In particular, it should be possible to evenly clean the hollow-fiber membranes which are present in the filtration apparatus with the lowest possible quantity of gas, and conditions should be created in a waste water treatment apparatus which reduce the formation of deposits on the membrane right from the beginning. This object is achieved by the filtration apparatus according to claim 1 which is a component of the filtration module according to claim 9. The invention further relates to the use of the filtration apparatus or filtration module according to claim 10 and a method for treating water or waste water according to claim 13 as well as a membrane bioreactor according to claim 24. Preferred embodiments and method variants are shown in the respective subclaims. SUMMARY OF THE INVENTION Accordingly, the present invention provides a filtration apparatus for separating particles from a liquid by means of hollow-fiber membranes which are joined into a fiber bundle. The hollow-fiber membranes are flowed through from the outside to the inside by the liquid and liquid purified from the particles is drawn off at least at one of the ends of the hollow-fiber membranes. The hollow-fiber bundle is wound up on a support whose outer circumferential surface can be flowed through by gas at least partly from the inside to the outside. The winding of the hollow-fiber bundle on the support allows on the one hand a compact configuration and on the other hand a reliable cleaning of the hollow-fiber membranes from any deposited materials. The filtration apparatus can be used alone or with several combined in a filtration module for cleaning waste water. The invention further provides a method for purifying waste water by using the filtration module and a membrane bioreactor. DETAILED DESCRIPTION OF THE INVENTION In a first aspect the invention relates to a filtration apparatus for separating particles from a liquid by means of hollow-fiber membranes joined into a fiber bundle. The hollow-fiber membranes can be flowed through from the outside to the inside by the liquid and the filtered liquid is drawn off from at least one of their ends. The filtration apparatus further comprises a gas supply apparatus in order to flush the outside of the hollow-fiber membranes with a gas. A part of the gas supply apparatus is according to the invention a support whose outer circumferential surface is permeable for the gas at least partly from the inside to the outside. The fiber bundle is wound around said outer circumferential surface of the support. This construction of the filtration apparatus in accordance with the invention allows on the one hand a compact configuration and on the other hand an even gas supply along the length of the hollow-fiber membranes. By using the gas-permeable support and the winding of the fiber bundles on the outer circumferential surface of the same, the outlet locations for the gas are always situated in the direct vicinity of the outer surfaces of the hollow-fiber membranes. This ensures an even and direct action of the gas upon the membrane surfaces. The quantity of supplied gas is so low that virtually no negative influences due to increased oxygen content or other disturbances caused by the gas are observed. The size of the emerging gas bubbles can be set in a simple way through the configuration of the outer circumferential surface of the support. For example, the outer circumferential surface can comprise pass-through openings in a suitable number and size. Holes or slots in the outer circumferential surface are also suitable like a grating-like or skeleton-like configuration of the outer circumferential surface. Depending on the desired distribution of the gas outlet, the pass-through openings can be present in the region of the entire outer circumferential surface or only in partial areas. The outer circumferential surface can also consist fully or partly of porous material for an especially fine distribution of the gas bubbles. The choice of the material for the support and its outer circumferential surface is not especially limited. All materials are principally suitable which are stable in the medium to be filtered and under the filtration conditions, e.g. all types of suitable plastic. Ceramics or metallic materials can principally also be used. The form of the support and the shaping of the outer circumferential surface are not limited to any special forms. Outer circumferential surfaces in form of a cylinder jacket are especially suitable for winding the hollow-fiber membranes around. The size of the support and its outer circumferential surface is adjusted to the conditions of the application. Outer circumferential surfaces of the support with a length of 1 to 100 cm, preferably 5 to 70 cm, and a diameter of 1 to 40 cm, preferably 5 to 20 cm, have proven to be suitable for the use of the filtration apparatus in accordance with the invention in waste water purification. For introducing the gas, the support appropriately comprises a connection for the gas at one of its face-side ends. Compressed air is preferably used as gas. Other gases such as nitrogen or the like can be used for special applications, e.g. the filtration under anaerobic conditions. In order to combine the hollow-fiber membranes into a fiber bundle as used in the filtration apparatus in accordance with the invention, the ends of the hollow-fiber membranes are appropriately fastened in at least one connection head. It appropriately comprises a suction connection which can be connected with the pump in order to withdraw cleaned liquid from the interior of the hollow-fiber membranes. The configuration of the connection head and the embedding of the ends of the hollow-fiber membranes can occur as principally known from the state of the art. Examples are disclosed in the publications as mentioned above. It is possible in accordance with the invention to fasten both ends of a hollow-fiber membrane in the same connection head. It is also possible alternatively to embed the two ends of a hollow-fiber membrane in separate connection heads. In both cases of fastening, the winding of the fiber bundle on the outer circumferential surface of the support appropriately occurs in such a way that all ends of the hollow-fiber membranes are oriented towards the suction connection. The suction connection is appropriately situated in the region of the end on the face side which is opposite the one where the gas connection is situated. The length, number and diameter of the hollow-fiber membranes are based on the desired application. Such filtration apparatuses have proven their worth for waste water purification in membrane bioreactors in which the overall surface of the hollow-fiber membranes comprises a filtration surface of 0.1 to 10 m2, especially 0.5 to 5 m2. All such hollow-fiber membranes can principally be used which have already been used in the state of the art for filtration purposes. Suitable for waste water purification are such materials which are mentioned in WO 97/06880 A2 and the specifications mentioned therein. Preferable in accordance with the invention are hollow-fiber membranes made of ceramics, and in particular aluminum oxide ceramic, and polymer membranes made of polyethylene, polypropylene, polyethersulfone or mixtures thereof. Suitable pore sizes are in the region of 0.001 to 1 pm, for example. The pressure conditions can also correspond to those as described in WO '880. Several filtration apparatuses in accordance with the invention can be combined into a filtration module which is also a subject matter of the invention. The combination of the filtration apparatuses into a module can principally occur according to the manner as described in the state of the art for similar apparatuses. For example, the filtration module can comprise suitable fixing devices in which the filtration apparatuses can be fastened in a specific orientation in relation to each other. Preferably, the filtration apparatuses are placed vertically adjacent to each other, with the side at which the gas is supplied appropriately being located at the bottom. Preferably, as many filtration apparatuses are arranged in the filtration module so that a filtration surface of 50 to 700 m2/m3 of covered space is obtained, in particular 100 to 400 m2/m3. A common feed line for the gas is appropriately provided for the parallel operation of the filtration apparatuses contained in the filtration module. A common discharge line for the purified liquid, which is the permeate, is appropriately also provided. Distribution lines for gas supply line and permeate discharge line to the individual filtration apparatuses can advantageously be integrated in the holding apparatuses for the filtration apparatuses. As already mentioned, the filtration apparatus and the filtration module in accordance with the invention are especially suitable for the treatment of water or waste water, especially for separating activated sludge in the so-called membrane bioreactors. The object of the invention is further a method for treating water or waste water in which water contaminated with biologically active material is introduced into a filtration basin in which at least one filtration module is arranged. The water purified in the filtration module is thereafter withdrawn. Any deposits formed during the filtration process on the outside surfaces of the hollow-fiber membranes are removed in such a way that gas is blown through the outer circumferential surfaces of the support. As a result of the small distance between the gas outlet openings and the hollow-fiber membranes as well as the even supply of gas over the length of the hollow-fiber membranes, an excellent cleaning effect at a very low gas volume is obtained. The ejection of the gas preferably occurs discontinuously or in pulses. This pulsating gas supply ensures that in the case of a vertically standing support the liquid column situated in the interior of the support will oppose the applied gas stream with its weight and flow resistance. This leads to the consequence that a high flow speed and high pressure difference build up in the direct boundary region to the membrane. Any forming deposits and coatings are shorn off in an especially favorable way as a result of this. As already mentioned, the invention is based on the finding that the production of deposits on the membrane surface is influenced strongly by conditions under which the filtration is performed. Within the scope of the examinations which preceded the present invention it was determined that in membrane bioreactors it is not only the depositing of biomass that leads to a reduction of the separating performance of the hollow-fiber membranes. The separating effect of the membranes can deteriorate substantially especially under disadvantageous ambient conditions. Extracellular polymers (ECP) are produced during the degradation of the biomass in the activated sludge, the quantity of which increases considerably under certain physical stress conditions. The extracellular polymers promote growths on the membrane surface (bio-fouling). If the content of ECPs exceeds or falls below the desired amount, the filterability of the biomass decreases substantially. Moreover, particles suspended in the activated sludge will clearly adhere more easily to the membrane surface under these conditions. It was noticed on the other hand that certain filamentous organisms (e.g. bacteria of type microthrix parvicella, noccardia, type 021N, etc.) have similarly negative effects on the throughput of the hollow-fiber membranes as the extracellular polymers. A further aspect of the invention therefore relates to measures with which unfavorable ambient conditions can be avoided which reduce the throughput of the hollow-fiber membranes in membrane bioreactors. A respective method and special membrane bioreactor are also a subject matter of the invention. The aforementioned problem is solved in such a way that the filtration basin which contains the filtration apparatuses in accordance with the invention is provided upstream with at least one basin to which highly contaminated waste water (hereinafter referred to as "raw waste water") is supplied. This upstream basin is referred to below as contactor. At least a part of the biologically active material from the filtration basin is supplied back to said contactor. The microorganisms which are formed in the activated sludge are thus subjected to a change in the ambient conditions. This change in stress leads to the dying off of the filamentous organisms. The effects are especially favorable when the change in stress is repeated several times. For this reason the waste water to be purified is circulated several times between contactor and filtration basin. It is preferable when the recirculation quantity from the filtration basin is 5 to 300 % by volume, especially 10 to 100 % by volume, of the supplied daily quantity of raw waste water. The dwell time of the return flow from the filtration basin in the contactor is appropriately between 2 and 120 minutes, especially between 20 and 90 minutes. It has further proven to be useful to ensure a certain ratio of biomass to organic load in the basin upstream of the filtration basin. In accordance with the invention, a certain ratio of bio- chemical oxygen demand (BOD) of the raw waste water to the activated sludge (AS) recirculated from the filtration basin is ensured for this purpose, namely in such a way that in the region of the contactor in which biologically active material is supplied from the filtration basin the ratio is set to 1 to 100 kg BOD/kg AS per day and preferably 5 to 70 kg BOD/kg AS per day in the raw waste water. The setting of the ratio appropriately occurs by respective dimensioning of the contactor and/or the capacity of its supply and discharge lines. Appropriately, the ratio between biochemical oxygen demand (BOD) and activated sludge (AS) in the contactor is allowed to drop to such a value that after the recirculation of the waste water from the contactor to the filtration basin a ratio is obtained in the filtration basin between biochemical oxygen demand (BOD) and activated sludge (AS) of 0.01 to 1 kg BOD/kg AS per day, preferably 0.02 to 0.6 kg BOD/kg AS per day. If the ratio of BOD/AS is within the stated range, optimal conditions are obtained in the filtration basin for membrane filtration. The formation of deposits on the membrane surface is clearly reduced. The contactor can consists merely of one contactor basin. The contactor is however preferably subdivided into at least two successively switched contactor basins, with the direction of flow extending from a first contactor basin to a last contactor basin which is directly upstream of the filtration basin. The introduction of raw waste water and the recirculation of biologically active material from the filtration basin occur to the first contactor basin and the waste water from the last contactor basin is conducted back to the filtration basin. The above highest ratio of BOD/AS is thus present in the first contactor basin and decreases towards the last basin, from where the waste water is guided back to the filtration basin with especially low concentrations of extracellular polymers and a clearly reduced share of microorganisms. The amount of the production of deposits on the membranes is thus clearly reduced. Preferably, the number of contactor basins is between 2 and 20, preferably 3 to 12. The decrease of the ratio of BOD/AS in these basins preferably occurs in substantially even steps. The decrease of the ratio BOD/AS is achieved by biosorption and incorporation of organic material. A large part of the free extracellular polymers is thus also removed, because they are bound on an increased level in form of flocculation when using a contactor. Large quantities of organic macromolecules which are otherwise difficult to filter are already bound directly to activated sludge flocks, with the extracellular polymers strongly promoting this process. Suspensions are obtained which are considerably better to filtrate in this way. It is preferable for the flocculation when there is no thorough mixing in the contactor. Mechanical stirring and mixing are preferably omitted. Instead, invert walls, which are especially horizontal or vertical baffles, are used for setting a certain flow speed. It is appropriately in the region of 1 to 60 m per hour, preferably 10 to 40 m per hour. The contactor or certain parts of the contactor basin are configured especially preferably as a plug-flow or tubular-flow reactor. Moreover, the membrane bioreactor in accordance with the invention may further comprise further components as are conventionally used in the state of the art. For example, a ventilation device for the contactor can be provided in order to set suitable aerobic, anaerobic or anoxic conditions depending on the substrate to be treated. The membrane bioreactor in accordance with the invention preferably uses the filtration apparatuses or filtration modules in accordance with the invention. It is not limited to the same however. It is also possible to use other membrane filter units, e.g. the flat membrane systems or the ones mentioned above in connection with the state of the art. BRIEF DESCRIPTION OF ACCOMPANYNG DRAWINGS The invention is now explained in closer detail by reference to the enclosed schematic drawings, wherein: Fig. 1 shows a filtration apparatus in accordance with the invention in a top view; Fig. 2 shows a hollow-fiber membrane bundle for use in a the filtration apparatus in accordance with the invention as shown in fig. 1; Fig. 3 shows a filtration module in accordance with the invention, and Fig. 4 shows a membrane bioreactor in accordance with the invention in a cross-sectional view. Fig. 1 shows a top view of a filtration apparatus 1 in accordance with the invention. The filtration apparatus 1 comprises a gas supply apparatus 5 with a substantially cylindrical support 6. The outer circumferential surface 7 of the support 6 comprises pass-through openings which are evenly distributed over its entire surface and which are not shown here for reasons of clarity of the illustration. A compressed air connection 8 is present in the lower face-side region of the carrier 6 through which compressed air is supplied to the interior of the support 6. Said compressed air passes to the outside through the openings on the outer circumferential surface 7 of the support 6. A fiber bundle 2 is wound on the outer circumferential surface 7. This fiber bundle is shown in this case only schematically and limited to a partial area of the outer circumferential surface 7. In actual fact, however, it is evenly wound over the entire outer circumferential surface 7. The fiber bundle 2 is shown more clearly in fig. 2. It consists of a plurality of thin hollow-fiber membranes 3 which have a length of up to 3 m for example. So many hollow-fiber membranes 3 are combined in the fiber bundle 2 that a filtration surface of 4 m2 is obtained. The respective ends 4 and 4' of the hollow-fiber membranes 3 are fastened to the connection heads 9 and 9'. Pass-through openings for the fiber ends 4 and 4' are provided in the connection heads 9 and 9', in which the latter are embedded. The hollow-fiber membranes 3 remain open at the two ends 4 and 4f through the embedding in the connection heads 9 and 9'. The embedding can occur in the known manner, as described for example in the above publications. For the purpose of winding on the support 6, one of the two connection heads 9 or 9' is inserted onto one end of the T-like suction connection 10 which is located at one face-side end of the support 6. Thereafter the fiber bundle 2 is wound around the outer circumferential surface 7 of the support 6 in the direction towards the pressure connection 8 and thereafter back to the suction connection 10. The second connection head is thereafter placed on the second end of the T-like connection. Several filtration apparatuses 1 in accordance with the invention can be combined into a filtration module 11 which is shown schematically in fig. 3. The filtration apparatuses 1 are set with the compressed air connection 8 downwardly into a suitable fixing device which is not shown in closer detail. In the filtration module 11, all compressed air connections 8 of the filtration apparatuses 1 are connected to a common compressed air connection 12. Similarly, all suction connections 10 in the upper region of the filtration apparatus 1 are connected to a common discharge line 13 for the permeate drawn off from the hollow-fiber membranes 3. The discharge line 13 can be connected with a suitable pump. In the method in accordance with the invention for cleaning water or waste water, the filtration module 11 as shown in fig. 3 is placed in a filtration basin in which the liquid to be filtered is located. The method in accordance with the invention will be explained by reference to the example of fig. 4 together with the membrane bioreactor in accordance with the invention. The membrane bioreactor 16 is used for the purification of communal waste water by the activated sludge method. It comprises a filtration basin 14 which is filled with pre-purified waste water 22. The filtration basin 14 comprises several filtration modules 11 which are switched in parallel and which comprise several filtration apparatuses 1. The filtration modules 11 are each connected with a discharge line 13 for the filtrated permeate and a compressed air supply line 12 for injecting compressed air. Compressed air is supplied through this compressed air line 12 to the individual filtration modules 11 and the filtration apparatuses 1 which are combined in the same. This preferably occurs in the described discontinuous pulsed manner. The compressed air reaches the compressed air connections 8 of the individual filtration apparatuses 1 via the supply line 12 and goes from there to the interior of support 6. The compressed air passes through the pass-through openings in the outer circumferential surfaces 7 of the filtration apparatuses in an even manner over the entire surface. It produces an even and very effective shearing of deposits formed on the membrane surfaces of the hollow-fiber membranes 3. Permeate which has entered the interior of the hollow-fiber membranes 3 is drawn off from the membrane bioreactor 16 via the suction connections 10 of the individual filtration apparatuses 1 and the discharge line 13 for the permeate. The discharge line 13 can also be operated in reverse operation. This occurs when a liquid cleaning of the hollow-fiber membranes is to be performed, especially by the addition of cleaning chemicals. This backflushing process is principally known and is described in the aforementioned US Pat No. 6,214,231 B1. Liquid is pumped under increased pressure through the line 13 and the individual suction connections 10 into the interior of the hollow-fiber membranes 3 and passes to the outside through the membrane surfaces. This leads to the detachment of deposits adhering to the membrane surface. Such backflushing processes are not frequently necessary due to the highly effective compressed air cleaning in the apparatus in accordance with the invention. In a further aspect of the invention, the waste water purification can also be performed in such a manner that deposits are reduced right from the start as a result of the meaningfully controlled conditions in the membrane bioreactor 16 in accordance with the invention. For this purpose, the filtration basin 14 of the bioreactor 16 in accordance with the invention is provided upstream with a contactor 15. The contactor 15 is divided in this case by a perpendicular separating wall 17 on the floor of the reactor into two basin sections 18 and 19. The contactor can also comprise merely one basin. In practice, usually more than two basins will be present. For the sake of better clarity, only two basins are shown in this case here. First basin 18 and second basin 19 are joined in this case by a transfer line (not shown). Similarly, there is a transfer line between the second basin 19 and the filtration basin 14. Two supply lines lead to the first basin 18 of the contactor 15, namely a supply line 20 for the raw waste water and a supply line 21 with which material can be recirculated from the filtration basin 14 to the contactor, or more precisely to the first basin 18. The feed quantity from the two supply lines 20 and 21 relative to the filling quantity of the first contactor basin 18 is set through a respective dimensioning of the contactor basin and the pump output of the pumps (not shown) allocated to the supply lines 20 and 21. The regulation occurs in accordance with the invention in such a way that in the first basin 18 a ratio is set of 1 to 100 kg of biochemical oxygen demand of the raw waste water from the supply line 20/kg of recirculated activated sludge form the supply line 21. Preferably, the regulation of the supply lines 20 and 21 occurs in such a way that a ratio of 5 to 70 kg BOD/kg of recirculated sludge per day is obtained. The recirculated quantity of biomass from the filtration basin 14 through line 21 is appropriately set in such a way that it is 5 to 300 % by volume, especially 10 to 100 % by volume, of the supplied daily quantity of raw waste water. The speed with which the supply from the filtration basin 14 is recirculated from the contactor 15 to the filtration basin 14 is appropriately between 2 and 120 minutes, preferably between 20 and 90 minutes. The flow speed within the contactor 15 is appropriately set to 1 to 60 m per hour, especially 10 to 40 m per hour. Horizontal or vertical baffles can be installed for example for setting the flow speed and for the better thorough mixing within the contactor 15. They are not shown here for reasons of clarity, but they are generally known. Preferably, the contactor with its first basin 18 and second basin 19 is arranged as plug-flow or tubular-flow reactor. In the course of the dwell time of the biomass in the contactor 15, the proportion of extracellular polymers and organic macromolecules which are difficult to filter decreases through flocculation. The presence of extracellular polymers promotes the bonding of the organic macromolecules to activated sludge flocks. At the same time, the concentration of microorganisms decreases because they die in the highly contaminated basin parts into which they were recirculated from the filtration basin 14. Accordingly, the ratio BOD/AS also decreases with longer dwell time in the contactor 15. It is therefore lower in the second reactor 19 than in the first reactor 18. The waste water is preferably left in the second reactor 19 for such a time until during the recirculation of the waste water 22 from the last contactor basin 19 to the filtration basin 14 a ratio of BOD/AS is obtained in the latter in a region of between 0.01 and 1 kg BOD/AS per day and preferably 0.02 to 0.6 kg BOD/AS per day. If these values are observed in the filtration basin 14, conditions are obtained which render the formation of deposits on the hollow-fiber membranes 3 more difficult. A clearly improved filterability of the waste water 22 in comparison with conventional operation of membrane bioreactors has been observed. The throughput as compared with conventional reactors can therefore be increased considerably. Maintenance and cleaning work is required less often. The membrane bioreactor 16 in accordance with the invention comprises not only the illustrated embodiment with the filtration modules 11 in accordance with the invention, but also such membrane bioreactors with other embodiments of membrane filter apparatuses. For example, such hollow-fiber membrane filtration apparatuses can be used as described above in the state of the art. Flat membrane filtration apparatuses can also be used. Moreover, the membrane bioreactor 16 in accordance with the invention can comprise further components or apparatuses which are conventionally used in the state of the art. It is possible for example to provide an aeration device for the contactor 15 in order to provide aerobic, anoxic or anaerobic ambient conditions depending on the sewage sludge to be cleaned. Stirring apparatuses for circulating the waste water in the individual basins can be used, even though it is currently preferable not to provide a stirring apparatus in the contactor 15. I /WE Claims: A filtration apparatus (1) for separating particles from a liquid by means of hollow-fiber membranes (3) which are joined into fiber bundles (2) and which can be flowed through from the outside to the inside by the liquid and from which filtered liquid (4) is withdrawn from at least one of its respective ends (4), and in which a gas supply apparatus (5) is present in order to flush the outside of the hollow-fiber membranes (3) with a gas, characterized in that the gas supply apparatus (5) comprises a support (6) whose outer circumferential surface (7) is at least partly permeable from the inside to the outside for the gas, and that the fiber bundle (2) is wound up on the outer circumferential surface (7) of the support (6). A filtration apparatus according to claim 1, characterized in that the outer circumferential surface (7) comprises pass-through openings in form of holes or slots or is made of porous material and preferably has the shape of a cylinder jacket. A filtration apparatus according to claim 1 or 2, characterized in that the outer circumferential surface (7) of the support (6) has a length of 1 to 100 cm, preferably 5 to 70 cm, and a diameter of 1 to 40 cm, preferably 5 to 20 cm. A filtration apparatus according to one of the claims 1 to 3, characterized in that the support (6) comprises at one face-side end a connection (8) for gas, especially compressed air. A filtration apparatus according to one of the claims 1 to 4, characterized in that the ends (4) of the hollow-fiber membranes (3) are fastened to at least one connection head (9), which connection head is joined to a suction connection (10) for withdrawing permeate from the hollow-fiber membranes (3). A filtration apparatus according to claim 5, characterized in that the first ends (4) of each hollow-fiber membrane (3) are connected with a first connection head (9) and the respective other ends (41) of the hollow-fiber membranes (3) are connected with a second connection head (9'). A filtration apparatus according to claim 5 or 6, characterized in that the connection head (9, 9') and the gas connection (8) are arranged on opposite face-side ends of the suoDort (6). A filtration apparatus according to one of the claims 1 to 7, characterized in that several hollow-fiber membranes (3) are joined in such a way into a fiber bundle (2) that the latter has a filtration surface of 0.1 to 10 m2, especially 0.5 to 5 m2. A filtration module (11), characterized in that it comprises several filtration apparatuses (1) according to one of the claims 1 to 8, especially several filtration apparatuses which are arranged vertically adjacent to one another and whose gas supply side faces downwardly. ). A filtration module according to claim 9, characterized in that the filtration apparatuses (1) are arranged in such a way that a filtration surface of 50 to 700 m2/m3, especially 100 to 400 m2/m3, is obtained. . A filtration module according to claim 9 or 10, characterized in that the filtration apparatuses (1) have a common feed line (12) for gas, preferably compressed air, and/or a common discharge line (13) for permeate. . The use of the filtration apparatus (1) according to one of the claims 1 to 8 or the filtration module (11) according to one of the claims 9 to 11 for treating water or waste water. .A method for treating water or waste water, comprising the following steps: - the introduction of (waste) water (22) which is contaminated with biologically active material into a filtration basin (14) in which at least one filtration module (11) according to one of the claims 9 to 11 is arranged, and - withdrawal of the water purified from the biologically active material, with deposits formed on the outside surfaces of the hollow-fiber membranes (3) being removed by the ejection of gas through the outer circumferential surfaces (7) of the supports (6) of the hollow-fiber membranes (3). . A method according to claim 13, characterized in that the gas is supplied in a discontinuous way. . A method according to one of the claims 13 or 14, characterized in that a contactor (15) is provided upstream of the filtration basin (14) and at least a portion of the biologically active material which is separated in the filtration basin (14) is recirculated to the contactor (15). . A method according to claim 15, characterized in that the recirculation quantity from the filtration basin (14) is 5 to 300 % by volume, especially 10 to 100 % by volume, of the daily supplied quantity of raw waste water. A method according to claim 15 or 16, characterized in that the dwell time of the return flow from the filtration basin (14) in the contactor (15) is 2 to 120 minutes, especially 20 to 90 minutes. A method according to one of the claims 15 to 17, characterized in that in the region of the contactor (15) in which biologically active material is recirculated from the filtration basin (14) the ratio between biochemical oxygen demand of the raw waste water (BOD) and activated sludge (AS) recirculated from the filtration basin (14) is set to 1 to 100 kg BOD/kg AS per day, preferably 5 to 70 kg BOD/kg AS per day. A method according to claim 18, characterized in that the ratio between biochemical oxygen demand (BOD) and activated sludge (AS) is allowed to decrease to such a value in the contactor (15) that after the recirculation of the waste water from the contactor (15) to the filtration basin (14) a ratio is obtained in the filtration basin (14) between biochemical oxygen demand (BOD) and activated sludge (AS) of 0.01 to 1 kg BOD/kg AS per day, preferably 0.02 to 0.6 kg BOD/kg AS per day. A method according to one of the claims 15 to 19, characterized in that the contactor (15) is divided into at least two successively switched contactor basins (18, 19), having a direction of flow from a first contactor basin (18) to a last contactor basin (19) being directly upstream of the filtration basin (14), that the introduction of raw waste water and the recirculation of biologically active material from the filtration basin (14) are made to the respectively first contactor basin (18), and that the waste water from the last contactor basin (19) is recirculated to the filtration basin (14). A method according to one of the claims 15 to 20, characterized in that the flow speed within the contactor (15) is set to 1 to 60 m per second, especially 10 to 40 m per second. A method according to one of the claims 15 to 21, characterized in that invert walls for controlling the liquid flow are present in the contactor (15). A method according to claim 22, characterized in that the contactor (15) is arranged as a plug-type or tubular-type reactor. . A membrane bioreactor (16) with a filtration basin (14) in which a membrane filtration unit (11) is situated, characterized in that the filtration basin (14) is provided upstream with a contactor (15) with at least one contactor basin, into which open a supply line (20) for raw waste water and a supply line (21) for recirculating biologically active material from the filtration basin (14). . A membrane bioreactor according to claim 24, characterized in that the contactor (15) is divided into at least two successively arranged contactor basins (18, 19), having a direction of flow from a first contactor basin (18) to a last contactor basin (19) being directly upstream of the filtration basin (14), and the supply line (20) for introducing raw waste water and the supply line (21) for recirculating biologically active material from the filtration basin (14) each open into the first contactor basin (18). A membrane bioreactor according to claim 24 or 25, characterized in that the contactor (15) is dimensioned in such a way that in the region in which biologically active material is recirculated from the filtration basin (14) a ratio is obtained between biochemical oxygen demand of the raw waste water (BOD) and activated sludge (AS) recirculated from the filtration basin (14) of 1 to 100 kg BOD/kg AS per day, preferably 5 to 70 kg BOD/kg AS per day. A membrane bioreactor according to claim 25 or 26, characterized in that the ratio between biochemical oxygen demand and activated sludge decreases from the first contactor basin (18) to the last contactor basin (19). A membrane bioreactor according to one of the claims 24 to 27, characterized in that the contactor (15) is divided into 2 to 20 contactor basins, especially 3 to 12 contactor basins. A membrane bioreactor according to one of the claims 24 to 28, characterized in that the ratio between biochemical oxygen demand (BOD) and activated sludge (AS) in the recirculation from the contactor (15) to the filtration basin (14) has such a value that in the filtration basin (14) a ratio is obtained between biochemical oxygen demand (BOD) and activated sludge (AS) of 0.01 to 1 kg BOD/kg AS per day, preferably 0.02 to 0.6 kg BOD/kg AS per day. A membrane bioreactor according to one of the claims 24 to 29, characterized in that invert walls for controlling the liquid flow are present in the contactor (15). A membrane bioreactor according to one of the claims 24 to 30, characterized in that an aeration apparatus is provided for the contactor (15). A membrane bioreactor according to one of the claims 24 to 31, characterized in that the membrane filtration unit consist of at least one filtration module (11) according to one of the claims 9 to 11. A filtration apparatus for separating particles from a liquid by means of hollow-fiber membranes which are joined into a fiber bundle herein described with reference to examples and accompanying drawings. A membrane bioreactor herein described with reference to examples and accompanying drawings A method for (waste) water treatment as herein described with reference to examples.

Full Text

A HOLLOW-FIBER MEMBRANE FILTRATION APPARATUS AND ITS USE IN THE CLEANING OF WASTE WATER, AND A MEMBRANE BIOREACTOR
TECHNICAL FIELD
The invention relates to a filtration apparatus for separating particles from a liquid by means of hollow-fiber membranes which are joined into a fiber bundle. This filtration apparatus and a filtration module in which several filtration apparatuses are combined are especially suitable for separating biomass from water or waste water. The invention further relates to a membrane bioreactor and a method for (waste) water treatment.
BACKGROUND AMD PRIOR ART REFERECENS
The use of membrane filters for purifying water or waste water is principally known. The porous materials used for filtration consist for example of ceramics or polymer membranes, for example, made of polyethylene, polypropylene, polyethersulfone or the like. Depending on the field of application, the pore sizes of the membrane can be in the region of between 0.001 and 1 pm.
Three different modes of operation are principally distinguished in membrane filtration, namely "dead end", "crossflow" and "submerged".
"Dead end" are usually designated as such applications in which the liquid to be cleaned is pressed without any further circulation in batch operation through the membrane. The particles held back by the membrane will accumulate on the membrane and lead to blockages or growths on the membrane in the course of operation. In the course of the operating period it is necessary to increase the pressure in order to ensure even throughput. Conversely, the throughput will decrease with unchanged pressure. This filtration method is therefore not suitable for large-scale use in the field of continuous systems for the treatment of drinking water or waste water.
In the "crossflow" method, the liquid to be cleaned is guided in a circulation along the membrane surface and is pressed through the membrane as a result of the differential pressure between said membrane side and the opposite side of the membrane, with the particles to be separated being held back. This method requires differential pressures of clearly above 500 mbar. It is further necessary to recirculate a large portion of the as yet non-cleaned liquid for detaching the deposits forming on the membrane. This causes considerable operating costs.

In the "submerged" method, the membrane is immersed into the liquid to be cleaned and in the case of hollow-fiber membranes permeate is pulled from the outside into the interior of the hollow fiber according to the vacuum principle and is carried off in the interior of the hollow fiber. The differential pressure between the outside of the membrane and the interior of the membrane is clearly lower than in the case of the "crossflow" filtration.
When using membrane filtration for treating water or waste water, the "submerged" method is usually applied, in which the membrane is immersed into the liquid to be cleaned. One problem in the use of membrane filters in the field of waste water purification is that as a result of so-called "fouling" or "bio-fouling", coatings will form on the membrane or the precipitation of chemical substances will lead to deposits on the membranes ("scaling"). Different methods and arrangements have been proposed in order to relieve the membranes from such deposits.
US Pat. No. 6,214,231 B1 for example describes a filtration system which uses hollow-fiber membranes. Several hollow-fiber membranes are combined into a substantially cylindrical fiber bundle. The upper and lower ends of the hollow-fiber membranes are each embedded in a holding apparatus. A hollow-fiber module is thus obtained. Several filtration modules situated adjacent to each other form a filtration cassette which is placed into the liquid to be cleaned. A permeate suction line is connected with each of the cassettes, which line is in connection with the upper fixing devices of the membrane filter bundles and removes the liquid purified from the particles from the interior of the individual hollow-fiber membranes. In order to relieve the hollow-fiber membranes from deposits, the direction of flow can be reversed. Liquid is thus supplied under high pressure through the suction line into the interior of the hollow-fiber membranes and passes through the latter from the inside to the outside, with deposits being detached from the membrane surface. Such a method means an interruption of the regular filtration operation and thus a reduced throughput. Moreover, this method cannot prevent the depositing of impurities on the membrane, but at best it can remove them after they occur.
US Pat. No. 6,156,200 A also describes filtration modules with hollow membrane fiber bundles which in their configuration principally correspond to those of the US Pat. No. 6,214,231 B1. The fiber bundle is also placed in this case vertically into the liquid to be cleaned. The fixing device in which the lower ends of the hollow-fiber membranes are embedded comprises gas outlet openings through which gas bubbles rise along the outside of the hollow-

fiber membranes and thus reduce the formation of deposits on the outside surfaces of the membranes on the one hand and remove deposits already formed on the other hand.
A similar arrangement is also described in WO 97/06880 A2. It is also mentioned in this case that an especially good cleaning can be achieved in such a way that the fibers are longer by between five and ten percent than the distance between the fixing devices in which the fiber ends are embedded.
The described apparatuses for cleaning the membrane fibers with compressed air come with the disadvantage that very large quantities of compressed air are required in order to ensure an effective cleaning of the membrane fibers. The increased supply of compressed air in the liquid to be cleaned can have a negative influence on other process parameters, e.g. it may make observing the set oxygen values considerably more difficult.
A further disadvantage is that the applicable pressures are limited at most to the hydrostatic pressure level in the region of the introduced fiber. Moreover, the rising speed of the bubbles is predetermined not by the introduced air quantity but by the size of the produced bubbles. A purposeful control of the cleaning effect of the apparatuses is thus principally not possible.
An even distribution of the action of the air along the length of the hollow-fiber membranes is also impossible. An even cleaning over the entire length of the fibers of mostly one to two meters is therefore not possible with the described systems.
The publications as mentioned above further merely generally describe the application of hollow-fiber membranes for separating biomass in membrane bioreactors. They focus alone on the separation of biomass which is present and has been obtained as biocynosis from the existing nutrients under the respective environmental conditions. The mechanisms which promote the depositing on the membrane surfaces are not considered thereby.
Within the scope of the examinations underlying this invention it was found however that certain environmental conditions have a considerable influence on the separating power of membranes. It was noticed in particular that certain environmental conditions strongly promote the biological growth on the membrane surface (bio-fouling) and the adherence of the suspended material. One aspect of the invention is therefore also ensuring such conditions in the filtration medium which reduce bio-fouling and the depositing on the membrane surface.

It is therefore the object of the present invention to provide a filtration apparatus which is arranged in a simple and compact way and maintains a high throughput over a long period of time. In particular, it should be possible to evenly clean the hollow-fiber membranes which are present in the filtration apparatus with the lowest possible quantity of gas, and conditions should be created in a waste water treatment apparatus which reduce the formation of deposits on the membrane right from the beginning.
This object is achieved by the filtration apparatus according to claim 1 which is a component of the filtration module according to claim 9. The invention further relates to the use of the filtration apparatus or filtration module according to claim 10 and a method for treating water or waste water according to claim 13 as well as a membrane bioreactor according to claim 24. Preferred embodiments and method variants are shown in the respective subclaims.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a filtration apparatus for separating particles from a liquid by means of hollow-fiber membranes which are joined into a fiber bundle. The hollow-fiber membranes are flowed through from the outside to the inside by the liquid and liquid purified from the particles is drawn off at least at one of the ends of the hollow-fiber membranes. The hollow-fiber bundle is wound up on a support whose outer circumferential surface can be flowed through by gas at least partly from the inside to the outside. The winding of the hollow-fiber bundle on the support allows on the one hand a compact configuration and on the other hand a reliable cleaning of the hollow-fiber membranes from any deposited materials. The filtration apparatus can be used alone or with several combined in a filtration module for cleaning waste water. The invention further provides a method for purifying waste water by using the filtration module and a membrane bioreactor.
DETAILED DESCRIPTION OF THE INVENTION
In a first aspect the invention relates to a filtration apparatus for separating particles from a liquid by means of hollow-fiber membranes joined into a fiber bundle. The hollow-fiber membranes can be flowed through from the outside to the inside by the liquid and the filtered liquid is drawn off from at least one of their ends.
The filtration apparatus further comprises a gas supply apparatus in order to flush the outside of the hollow-fiber membranes with a gas. A part of the gas supply apparatus is according to the invention a support whose outer circumferential surface is permeable for the gas at least

partly from the inside to the outside. The fiber bundle is wound around said outer circumferential surface of the support.
This construction of the filtration apparatus in accordance with the invention allows on the one hand a compact configuration and on the other hand an even gas supply along the length of the hollow-fiber membranes. By using the gas-permeable support and the winding of the fiber bundles on the outer circumferential surface of the same, the outlet locations for the gas are always situated in the direct vicinity of the outer surfaces of the hollow-fiber membranes. This ensures an even and direct action of the gas upon the membrane surfaces. The quantity of supplied gas is so low that virtually no negative influences due to increased oxygen content or other disturbances caused by the gas are observed.
The size of the emerging gas bubbles can be set in a simple way through the configuration of the outer circumferential surface of the support. For example, the outer circumferential surface can comprise pass-through openings in a suitable number and size. Holes or slots in the outer circumferential surface are also suitable like a grating-like or skeleton-like configuration of the outer circumferential surface. Depending on the desired distribution of the gas outlet, the pass-through openings can be present in the region of the entire outer circumferential surface or only in partial areas. The outer circumferential surface can also consist fully or partly of porous material for an especially fine distribution of the gas bubbles. The choice of the material for the support and its outer circumferential surface is not especially limited. All materials are principally suitable which are stable in the medium to be filtered and under the filtration conditions, e.g. all types of suitable plastic. Ceramics or metallic materials can principally also be used.
The form of the support and the shaping of the outer circumferential surface are not limited to any special forms. Outer circumferential surfaces in form of a cylinder jacket are especially suitable for winding the hollow-fiber membranes around.
The size of the support and its outer circumferential surface is adjusted to the conditions of the application. Outer circumferential surfaces of the support with a length of 1 to 100 cm, preferably 5 to 70 cm, and a diameter of 1 to 40 cm, preferably 5 to 20 cm, have proven to be suitable for the use of the filtration apparatus in accordance with the invention in waste water purification.

For introducing the gas, the support appropriately comprises a connection for the gas at one of its face-side ends. Compressed air is preferably used as gas. Other gases such as nitrogen or the like can be used for special applications, e.g. the filtration under anaerobic conditions.
In order to combine the hollow-fiber membranes into a fiber bundle as used in the filtration apparatus in accordance with the invention, the ends of the hollow-fiber membranes are appropriately fastened in at least one connection head. It appropriately comprises a suction connection which can be connected with the pump in order to withdraw cleaned liquid from the interior of the hollow-fiber membranes. The configuration of the connection head and the embedding of the ends of the hollow-fiber membranes can occur as principally known from the state of the art. Examples are disclosed in the publications as mentioned above.
It is possible in accordance with the invention to fasten both ends of a hollow-fiber membrane in the same connection head. It is also possible alternatively to embed the two ends of a hollow-fiber membrane in separate connection heads. In both cases of fastening, the winding of the fiber bundle on the outer circumferential surface of the support appropriately occurs in such a way that all ends of the hollow-fiber membranes are oriented towards the suction connection. The suction connection is appropriately situated in the region of the end on the face side which is opposite the one where the gas connection is situated.
The length, number and diameter of the hollow-fiber membranes are based on the desired application. Such filtration apparatuses have proven their worth for waste water purification in membrane bioreactors in which the overall surface of the hollow-fiber membranes comprises a filtration surface of 0.1 to 10 m2, especially 0.5 to 5 m2. All such hollow-fiber membranes can principally be used which have already been used in the state of the art for filtration purposes. Suitable for waste water purification are such materials which are mentioned in WO 97/06880 A2 and the specifications mentioned therein. Preferable in accordance with the invention are hollow-fiber membranes made of ceramics, and in particular aluminum oxide ceramic, and polymer membranes made of polyethylene, polypropylene, polyethersulfone or mixtures thereof. Suitable pore sizes are in the region of 0.001 to 1 pm, for example. The pressure conditions can also correspond to those as described in WO '880.
Several filtration apparatuses in accordance with the invention can be combined into a filtration module which is also a subject matter of the invention. The combination of the filtration apparatuses into a module can principally occur according to the manner as described in the

state of the art for similar apparatuses. For example, the filtration module can comprise suitable fixing devices in which the filtration apparatuses can be fastened in a specific orientation in relation to each other. Preferably, the filtration apparatuses are placed vertically adjacent to each other, with the side at which the gas is supplied appropriately being located at the bottom. Preferably, as many filtration apparatuses are arranged in the filtration module so that a filtration surface of 50 to 700 m2/m3 of covered space is obtained, in particular 100 to 400 m2/m3.
A common feed line for the gas is appropriately provided for the parallel operation of the filtration apparatuses contained in the filtration module. A common discharge line for the purified liquid, which is the permeate, is appropriately also provided. Distribution lines for gas supply line and permeate discharge line to the individual filtration apparatuses can advantageously be integrated in the holding apparatuses for the filtration apparatuses.
As already mentioned, the filtration apparatus and the filtration module in accordance with the invention are especially suitable for the treatment of water or waste water, especially for separating activated sludge in the so-called membrane bioreactors.
The object of the invention is further a method for treating water or waste water in which water contaminated with biologically active material is introduced into a filtration basin in which at least one filtration module is arranged. The water purified in the filtration module is thereafter withdrawn. Any deposits formed during the filtration process on the outside surfaces of the hollow-fiber membranes are removed in such a way that gas is blown through the outer circumferential surfaces of the support. As a result of the small distance between the gas outlet openings and the hollow-fiber membranes as well as the even supply of gas over the length of the hollow-fiber membranes, an excellent cleaning effect at a very low gas volume is obtained.
The ejection of the gas preferably occurs discontinuously or in pulses. This pulsating gas supply ensures that in the case of a vertically standing support the liquid column situated in the interior of the support will oppose the applied gas stream with its weight and flow resistance. This leads to the consequence that a high flow speed and high pressure difference build up in the direct boundary region to the membrane. Any forming deposits and coatings are shorn off in an especially favorable way as a result of this.

As already mentioned, the invention is based on the finding that the production of deposits on the membrane surface is influenced strongly by conditions under which the filtration is performed. Within the scope of the examinations which preceded the present invention it was determined that in membrane bioreactors it is not only the depositing of biomass that leads to a reduction of the separating performance of the hollow-fiber membranes. The separating effect of the membranes can deteriorate substantially especially under disadvantageous ambient conditions. Extracellular polymers (ECP) are produced during the degradation of the biomass in the activated sludge, the quantity of which increases considerably under certain physical stress conditions. The extracellular polymers promote growths on the membrane surface (bio-fouling). If the content of ECPs exceeds or falls below the desired amount, the filterability of the biomass decreases substantially. Moreover, particles suspended in the activated sludge will clearly adhere more easily to the membrane surface under these conditions. It was noticed on the other hand that certain filamentous organisms (e.g. bacteria of type microthrix parvicella, noccardia, type 021N, etc.) have similarly negative effects on the throughput of the hollow-fiber membranes as the extracellular polymers. A further aspect of the invention therefore relates to measures with which unfavorable ambient conditions can be avoided which reduce the throughput of the hollow-fiber membranes in membrane bioreactors. A respective method and special membrane bioreactor are also a subject matter of the invention.
The aforementioned problem is solved in such a way that the filtration basin which contains the filtration apparatuses in accordance with the invention is provided upstream with at least one basin to which highly contaminated waste water (hereinafter referred to as "raw waste water") is supplied. This upstream basin is referred to below as contactor. At least a part of the biologically active material from the filtration basin is supplied back to said contactor. The microorganisms which are formed in the activated sludge are thus subjected to a change in the ambient conditions. This change in stress leads to the dying off of the filamentous organisms. The effects are especially favorable when the change in stress is repeated several times. For this reason the waste water to be purified is circulated several times between contactor and filtration basin. It is preferable when the recirculation quantity from the filtration basin is 5 to 300 % by volume, especially 10 to 100 % by volume, of the supplied daily quantity of raw waste water. The dwell time of the return flow from the filtration basin in the contactor is appropriately between 2 and 120 minutes, especially between 20 and 90 minutes.
It has further proven to be useful to ensure a certain ratio of biomass to organic load in the basin upstream of the filtration basin. In accordance with the invention, a certain ratio of bio-

chemical oxygen demand (BOD) of the raw waste water to the activated sludge (AS) recirculated from the filtration basin is ensured for this purpose, namely in such a way that in the region of the contactor in which biologically active material is supplied from the filtration basin the ratio is set to 1 to 100 kg BOD/kg AS per day and preferably 5 to 70 kg BOD/kg AS per day in the raw waste water. The setting of the ratio appropriately occurs by respective dimensioning of the contactor and/or the capacity of its supply and discharge lines.
Appropriately, the ratio between biochemical oxygen demand (BOD) and activated sludge (AS) in the contactor is allowed to drop to such a value that after the recirculation of the waste water from the contactor to the filtration basin a ratio is obtained in the filtration basin between biochemical oxygen demand (BOD) and activated sludge (AS) of 0.01 to 1 kg BOD/kg AS per day, preferably 0.02 to 0.6 kg BOD/kg AS per day. If the ratio of BOD/AS is within the stated range, optimal conditions are obtained in the filtration basin for membrane filtration. The formation of deposits on the membrane surface is clearly reduced. The contactor can consists merely of one contactor basin. The contactor is however preferably subdivided into at least two successively switched contactor basins, with the direction of flow extending from a first contactor basin to a last contactor basin which is directly upstream of the filtration basin. The introduction of raw waste water and the recirculation of biologically active material from the filtration basin occur to the first contactor basin and the waste water from the last contactor basin is conducted back to the filtration basin.
The above highest ratio of BOD/AS is thus present in the first contactor basin and decreases towards the last basin, from where the waste water is guided back to the filtration basin with especially low concentrations of extracellular polymers and a clearly reduced share of microorganisms. The amount of the production of deposits on the membranes is thus clearly reduced.
Preferably, the number of contactor basins is between 2 and 20, preferably 3 to 12. The decrease of the ratio of BOD/AS in these basins preferably occurs in substantially even steps.
The decrease of the ratio BOD/AS is achieved by biosorption and incorporation of organic material. A large part of the free extracellular polymers is thus also removed, because they are bound on an increased level in form of flocculation when using a contactor. Large quantities of organic macromolecules which are otherwise difficult to filter are already bound directly to activated sludge flocks, with the extracellular polymers strongly promoting this process. Suspensions are obtained which are considerably better to filtrate in this way.

It is preferable for the flocculation when there is no thorough mixing in the contactor. Mechanical stirring and mixing are preferably omitted. Instead, invert walls, which are especially horizontal or vertical baffles, are used for setting a certain flow speed. It is appropriately in the region of 1 to 60 m per hour, preferably 10 to 40 m per hour. The contactor or certain parts of the contactor basin are configured especially preferably as a plug-flow or tubular-flow reactor.
Moreover, the membrane bioreactor in accordance with the invention may further comprise further components as are conventionally used in the state of the art. For example, a ventilation device for the contactor can be provided in order to set suitable aerobic, anaerobic or anoxic conditions depending on the substrate to be treated.
The membrane bioreactor in accordance with the invention preferably uses the filtration apparatuses or filtration modules in accordance with the invention. It is not limited to the same however. It is also possible to use other membrane filter units, e.g. the flat membrane systems or the ones mentioned above in connection with the state of the art.
BRIEF DESCRIPTION OF ACCOMPANYNG DRAWINGS
The invention is now explained in closer detail by reference to the enclosed schematic drawings, wherein:
Fig. 1 shows a filtration apparatus in accordance with the invention in a top view;
Fig. 2 shows a hollow-fiber membrane bundle for use in a the filtration apparatus in accordance with the invention as shown in fig. 1;
Fig. 3 shows a filtration module in accordance with the invention, and
Fig. 4 shows a membrane bioreactor in accordance with the invention in a cross-sectional view.
Fig. 1 shows a top view of a filtration apparatus 1 in accordance with the invention. The filtration apparatus 1 comprises a gas supply apparatus 5 with a substantially cylindrical support 6. The outer circumferential surface 7 of the support 6 comprises pass-through openings which are evenly distributed over its entire surface and which are not shown here for reasons of

clarity of the illustration. A compressed air connection 8 is present in the lower face-side region of the carrier 6 through which compressed air is supplied to the interior of the support 6. Said compressed air passes to the outside through the openings on the outer circumferential surface 7 of the support 6. A fiber bundle 2 is wound on the outer circumferential surface 7. This fiber bundle is shown in this case only schematically and limited to a partial area of the outer circumferential surface 7. In actual fact, however, it is evenly wound over the entire outer circumferential surface 7.
The fiber bundle 2 is shown more clearly in fig. 2. It consists of a plurality of thin hollow-fiber membranes 3 which have a length of up to 3 m for example. So many hollow-fiber membranes 3 are combined in the fiber bundle 2 that a filtration surface of 4 m2 is obtained. The respective ends 4 and 4' of the hollow-fiber membranes 3 are fastened to the connection heads 9 and 9'. Pass-through openings for the fiber ends 4 and 4' are provided in the connection heads 9 and 9', in which the latter are embedded. The hollow-fiber membranes 3 remain open at the two ends 4 and 4f through the embedding in the connection heads 9 and 9'. The embedding can occur in the known manner, as described for example in the above publications.
For the purpose of winding on the support 6, one of the two connection heads 9 or 9' is inserted onto one end of the T-like suction connection 10 which is located at one face-side end of the support 6. Thereafter the fiber bundle 2 is wound around the outer circumferential surface 7 of the support 6 in the direction towards the pressure connection 8 and thereafter back to the suction connection 10. The second connection head is thereafter placed on the second end of the T-like connection.
Several filtration apparatuses 1 in accordance with the invention can be combined into a filtration module 11 which is shown schematically in fig. 3. The filtration apparatuses 1 are set with the compressed air connection 8 downwardly into a suitable fixing device which is not shown in closer detail. In the filtration module 11, all compressed air connections 8 of the filtration apparatuses 1 are connected to a common compressed air connection 12. Similarly, all suction connections 10 in the upper region of the filtration apparatus 1 are connected to a common discharge line 13 for the permeate drawn off from the hollow-fiber membranes 3. The discharge line 13 can be connected with a suitable pump.
In the method in accordance with the invention for cleaning water or waste water, the filtration module 11 as shown in fig. 3 is placed in a filtration basin in which the liquid to be filtered

is located. The method in accordance with the invention will be explained by reference to the example of fig. 4 together with the membrane bioreactor in accordance with the invention.
The membrane bioreactor 16 is used for the purification of communal waste water by the activated sludge method. It comprises a filtration basin 14 which is filled with pre-purified waste water 22. The filtration basin 14 comprises several filtration modules 11 which are switched in parallel and which comprise several filtration apparatuses 1. The filtration modules 11 are each connected with a discharge line 13 for the filtrated permeate and a compressed air supply line 12 for injecting compressed air. Compressed air is supplied through this compressed air line 12 to the individual filtration modules 11 and the filtration apparatuses 1 which are combined in the same. This preferably occurs in the described discontinuous pulsed manner. The compressed air reaches the compressed air connections 8 of the individual filtration apparatuses 1 via the supply line 12 and goes from there to the interior of support 6. The compressed air passes through the pass-through openings in the outer circumferential surfaces 7 of the filtration apparatuses in an even manner over the entire surface. It produces an even and very effective shearing of deposits formed on the membrane surfaces of the hollow-fiber membranes 3.
Permeate which has entered the interior of the hollow-fiber membranes 3 is drawn off from the membrane bioreactor 16 via the suction connections 10 of the individual filtration apparatuses 1 and the discharge line 13 for the permeate. The discharge line 13 can also be operated in reverse operation. This occurs when a liquid cleaning of the hollow-fiber membranes is to be performed, especially by the addition of cleaning chemicals. This backflushing process is principally known and is described in the aforementioned US Pat No. 6,214,231 B1. Liquid is pumped under increased pressure through the line 13 and the individual suction connections 10 into the interior of the hollow-fiber membranes 3 and passes to the outside through the membrane surfaces. This leads to the detachment of deposits adhering to the membrane surface. Such backflushing processes are not frequently necessary due to the highly effective compressed air cleaning in the apparatus in accordance with the invention.
In a further aspect of the invention, the waste water purification can also be performed in such a manner that deposits are reduced right from the start as a result of the meaningfully controlled conditions in the membrane bioreactor 16 in accordance with the invention. For this purpose, the filtration basin 14 of the bioreactor 16 in accordance with the invention is provided upstream with a contactor 15. The contactor 15 is divided in this case by a perpendicular separating wall 17 on the floor of the reactor into two basin sections 18 and 19. The

contactor can also comprise merely one basin. In practice, usually more than two basins will be present. For the sake of better clarity, only two basins are shown in this case here. First basin 18 and second basin 19 are joined in this case by a transfer line (not shown). Similarly, there is a transfer line between the second basin 19 and the filtration basin 14. Two supply lines lead to the first basin 18 of the contactor 15, namely a supply line 20 for the raw waste water and a supply line 21 with which material can be recirculated from the filtration basin 14 to the contactor, or more precisely to the first basin 18. The feed quantity from the two supply lines 20 and 21 relative to the filling quantity of the first contactor basin 18 is set through a respective dimensioning of the contactor basin and the pump output of the pumps (not shown) allocated to the supply lines 20 and 21. The regulation occurs in accordance with the invention in such a way that in the first basin 18 a ratio is set of 1 to 100 kg of biochemical oxygen demand of the raw waste water from the supply line 20/kg of recirculated activated sludge form the supply line 21. Preferably, the regulation of the supply lines 20 and 21 occurs in such a way that a ratio of 5 to 70 kg BOD/kg of recirculated sludge per day is obtained.
The recirculated quantity of biomass from the filtration basin 14 through line 21 is appropriately set in such a way that it is 5 to 300 % by volume, especially 10 to 100 % by volume, of the supplied daily quantity of raw waste water. The speed with which the supply from the filtration basin 14 is recirculated from the contactor 15 to the filtration basin 14 is appropriately between 2 and 120 minutes, preferably between 20 and 90 minutes. The flow speed within the contactor 15 is appropriately set to 1 to 60 m per hour, especially 10 to 40 m per hour. Horizontal or vertical baffles can be installed for example for setting the flow speed and for the better thorough mixing within the contactor 15. They are not shown here for reasons of clarity, but they are generally known. Preferably, the contactor with its first basin 18 and second basin 19 is arranged as plug-flow or tubular-flow reactor.
In the course of the dwell time of the biomass in the contactor 15, the proportion of extracellular polymers and organic macromolecules which are difficult to filter decreases through flocculation. The presence of extracellular polymers promotes the bonding of the organic macromolecules to activated sludge flocks. At the same time, the concentration of microorganisms decreases because they die in the highly contaminated basin parts into which they were recirculated from the filtration basin 14. Accordingly, the ratio BOD/AS also decreases with longer dwell time in the contactor 15. It is therefore lower in the second reactor 19 than in the first reactor 18. The waste water is preferably left in the second reactor 19 for such a time until during the recirculation of the waste water 22 from the last contactor basin 19 to the filtration basin 14 a ratio of BOD/AS is obtained in the latter in a region of between 0.01 and 1

kg BOD/AS per day and preferably 0.02 to 0.6 kg BOD/AS per day. If these values are observed in the filtration basin 14, conditions are obtained which render the formation of deposits on the hollow-fiber membranes 3 more difficult. A clearly improved filterability of the waste water 22 in comparison with conventional operation of membrane bioreactors has been observed. The throughput as compared with conventional reactors can therefore be increased considerably. Maintenance and cleaning work is required less often.
The membrane bioreactor 16 in accordance with the invention comprises not only the illustrated embodiment with the filtration modules 11 in accordance with the invention, but also such membrane bioreactors with other embodiments of membrane filter apparatuses. For example, such hollow-fiber membrane filtration apparatuses can be used as described above in the state of the art. Flat membrane filtration apparatuses can also be used. Moreover, the membrane bioreactor 16 in accordance with the invention can comprise further components or apparatuses which are conventionally used in the state of the art. It is possible for example to provide an aeration device for the contactor 15 in order to provide aerobic, anoxic or anaerobic ambient conditions depending on the sewage sludge to be cleaned. Stirring apparatuses for circulating the waste water in the individual basins can be used, even though it is currently preferable not to provide a stirring apparatus in the contactor 15.

I /WE Claims:
A filtration apparatus (1) for separating particles from a liquid by means of hollow-fiber membranes (3) which are joined into fiber bundles (2) and which can be flowed through from the outside to the inside by the liquid and from which filtered liquid (4) is withdrawn from at least one of its respective ends (4), and in which a gas supply apparatus (5) is present in order to flush the outside of the hollow-fiber membranes (3) with a gas,
characterized in that the gas supply apparatus (5) comprises a support (6) whose outer circumferential surface (7) is at least partly permeable from the inside to the outside for the gas, and that the fiber bundle (2) is wound up on the outer circumferential surface (7) of the support (6).
A filtration apparatus according to claim 1, characterized in that the outer circumferential surface (7) comprises pass-through openings in form of holes or slots or is made of porous material and preferably has the shape of a cylinder jacket.
A filtration apparatus according to claim 1 or 2, characterized in that the outer circumferential surface (7) of the support (6) has a length of 1 to 100 cm, preferably 5 to 70 cm, and a diameter of 1 to 40 cm, preferably 5 to 20 cm.
A filtration apparatus according to one of the claims 1 to 3, characterized in that the support (6) comprises at one face-side end a connection (8) for gas, especially compressed air.
A filtration apparatus according to one of the claims 1 to 4, characterized in that the ends (4) of the hollow-fiber membranes (3) are fastened to at least one connection head (9), which connection head is joined to a suction connection (10) for withdrawing permeate from the hollow-fiber membranes (3).
A filtration apparatus according to claim 5, characterized in that the first ends (4) of each hollow-fiber membrane (3) are connected with a first connection head (9) and the respective other ends (41) of the hollow-fiber membranes (3) are connected with a second connection head (9').
A filtration apparatus according to claim 5 or 6, characterized in that the connection head (9, 9') and the gas connection (8) are arranged on opposite face-side ends of the suoDort (6).

A filtration apparatus according to one of the claims 1 to 7, characterized in that several hollow-fiber membranes (3) are joined in such a way into a fiber bundle (2) that the latter has a filtration surface of 0.1 to 10 m2, especially 0.5 to 5 m2.
A filtration module (11), characterized in that it comprises several filtration apparatuses (1) according to one of the claims 1 to 8, especially several filtration apparatuses which are arranged vertically adjacent to one another and whose gas supply side faces downwardly.
). A filtration module according to claim 9, characterized in that the filtration apparatuses (1) are arranged in such a way that a filtration surface of 50 to 700 m2/m3, especially 100 to 400 m2/m3, is obtained.
. A filtration module according to claim 9 or 10, characterized in that the filtration apparatuses (1) have a common feed line (12) for gas, preferably compressed air, and/or a common discharge line (13) for permeate.
. The use of the filtration apparatus (1) according to one of the claims 1 to 8 or the filtration module (11) according to one of the claims 9 to 11 for treating water or waste water.
.A method for treating water or waste water, comprising the following steps:
- the introduction of (waste) water (22) which is contaminated with biologically active material into a filtration basin (14) in which at least one filtration module (11) according to one of the claims 9 to 11 is arranged, and
- withdrawal of the water purified from the biologically active material, with deposits formed on the outside surfaces of the hollow-fiber membranes (3) being removed by the ejection of gas through the outer circumferential surfaces (7) of the supports (6) of the hollow-fiber membranes (3).
. A method according to claim 13, characterized in that the gas is supplied in a discontinuous way.
. A method according to one of the claims 13 or 14, characterized in that a contactor (15) is provided upstream of the filtration basin (14) and at least a portion of the biologically active material which is separated in the filtration basin (14) is recirculated to the contactor (15).

. A method according to claim 15, characterized in that the recirculation quantity from the filtration basin (14) is 5 to 300 % by volume, especially 10 to 100 % by volume, of the daily supplied quantity of raw waste water.
A method according to claim 15 or 16, characterized in that the dwell time of the return flow from the filtration basin (14) in the contactor (15) is 2 to 120 minutes, especially 20 to 90 minutes.
A method according to one of the claims 15 to 17, characterized in that in the region of the contactor (15) in which biologically active material is recirculated from the filtration basin (14) the ratio between biochemical oxygen demand of the raw waste water (BOD) and activated sludge (AS) recirculated from the filtration basin (14) is set to 1 to 100 kg BOD/kg AS per day, preferably 5 to 70 kg BOD/kg AS per day.
A method according to claim 18, characterized in that the ratio between biochemical oxygen demand (BOD) and activated sludge (AS) is allowed to decrease to such a value in the contactor (15) that after the recirculation of the waste water from the contactor (15) to the filtration basin (14) a ratio is obtained in the filtration basin (14) between biochemical oxygen demand (BOD) and activated sludge (AS) of 0.01 to 1 kg BOD/kg AS per day, preferably 0.02 to 0.6 kg BOD/kg AS per day.
A method according to one of the claims 15 to 19, characterized in that the contactor (15) is divided into at least two successively switched contactor basins (18, 19), having a direction of flow from a first contactor basin (18) to a last contactor basin (19) being directly upstream of the filtration basin (14), that the introduction of raw waste water and the recirculation of biologically active material from the filtration basin (14) are made to the respectively first contactor basin (18), and that the waste water from the last contactor basin (19) is recirculated to the filtration basin (14).
A method according to one of the claims 15 to 20, characterized in that the flow speed within the contactor (15) is set to 1 to 60 m per second, especially 10 to 40 m per second.
A method according to one of the claims 15 to 21, characterized in that invert walls for controlling the liquid flow are present in the contactor (15).
A method according to claim 22, characterized in that the contactor (15) is arranged as a plug-type or tubular-type reactor.

. A membrane bioreactor (16) with a filtration basin (14) in which a membrane filtration unit (11) is situated, characterized in that the filtration basin (14) is provided upstream with a contactor (15) with at least one contactor basin, into which open a supply line (20) for raw waste water and a supply line (21) for recirculating biologically active material from the filtration basin (14).
. A membrane bioreactor according to claim 24, characterized in that the contactor (15) is divided into at least two successively arranged contactor basins (18, 19), having a direction of flow from a first contactor basin (18) to a last contactor basin (19) being directly upstream of the filtration basin (14), and the supply line (20) for introducing raw waste water and the supply line (21) for recirculating biologically active material from the filtration basin (14) each open into the first contactor basin (18).
A membrane bioreactor according to claim 24 or 25, characterized in that the contactor (15) is dimensioned in such a way that in the region in which biologically active material is recirculated from the filtration basin (14) a ratio is obtained between biochemical oxygen demand of the raw waste water (BOD) and activated sludge (AS) recirculated from the filtration basin (14) of 1 to 100 kg BOD/kg AS per day, preferably 5 to 70 kg BOD/kg AS per day.
A membrane bioreactor according to claim 25 or 26, characterized in that the ratio between biochemical oxygen demand and activated sludge decreases from the first contactor basin (18) to the last contactor basin (19).
A membrane bioreactor according to one of the claims 24 to 27, characterized in that the contactor (15) is divided into 2 to 20 contactor basins, especially 3 to 12 contactor basins.
A membrane bioreactor according to one of the claims 24 to 28, characterized in that the ratio between biochemical oxygen demand (BOD) and activated sludge (AS) in the recirculation from the contactor (15) to the filtration basin (14) has such a value that in the filtration basin (14) a ratio is obtained between biochemical oxygen demand (BOD) and activated sludge (AS) of 0.01 to 1 kg BOD/kg AS per day, preferably 0.02 to 0.6 kg BOD/kg AS per day.
A membrane bioreactor according to one of the claims 24 to 29, characterized in that invert walls for controlling the liquid flow are present in the contactor (15).

A membrane bioreactor according to one of the claims 24 to 30, characterized in that an aeration apparatus is provided for the contactor (15).
A membrane bioreactor according to one of the claims 24 to 31, characterized in that the membrane filtration unit consist of at least one filtration module (11) according to one of the claims 9 to 11.
A filtration apparatus for separating particles from a liquid by means of hollow-fiber membranes which are joined into a fiber bundle herein described with reference to examples and accompanying drawings.
A membrane bioreactor herein described with reference to examples and accompanying drawings
A method for (waste) water treatment as herein described with reference to examples.

Documents:

« Previous Patent

Next Patent »

Patent Number

225903

Indian Patent Application Number

2491/CHENP/2004

PG Journal Number

02/2009

Publication Date

09-Jan-2009

Grant Date

01-Dec-2008

Date of Filing

04-Nov-2004

Name of Patentee

SFC UMWELTTECHNIK GmbH

Applicant Address

Julius-Welser-Str. 15, A-5020 Salzburg,

Inventors:

#	Inventor's Name	Inventor's Address
1	DEMOULIN, Gunnar	Uberfuhrstr, 12, A-5026 Salzburg,

PCT International Classification Number

B01D65/02

PCT International Application Number

PCT/EP03/04224

PCT International Filing date

2003-04-23

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	102 20 916.2	2002-05-10	Germany