MEMBRANES WITH IMMOBILIZED MICROORGANISMS THEREON AND/OR THEREIN, PROCESS FOR OBTAINING SUCH MEMBRANES, REACTOR COMPRISING SAID MEMBRANES AND PROCESS INVOLVING THE USE OF SAID MEMBRANES, IN PARTICULAR FOR THE ELIMINATION OF METALS OR THE DEGRADATION OF XENOBIOTIC ORGANIC COMPOUNDS

Abstract: The invention relates to membranes with immobilized microorganisms thereon and/or therein wherein the microorganisms are alive or in a viable form, and the membranes are of porous material such that the microorganisms, particularly bacteria, can settle in the pores, and such that the release of the microorganisms is not significant. The invention also relates to new reactors, enabling to eliminate metals and/or to degrade xenobiotic organic compounds in an effluent containing them or the synthesize compounds.

Description: MEMBRANES WITH IMMOBILIZED MICROORGANISMS THEREON AND/OR THEREIN, PROCESS FOR OBTAINING SUCH MEMBRANES, REACTOR COMPRISING SAID MEMBRANES AND PROCESS INVOLVING THE USE OF SAID MEMBRANES, IN PARTICULAR FOR THE ELIMINATION OF METALS OR THE DEGRADATION OF XENOBIOTIC ORGANIC COMPOUNDS The invention relates to membranes with immobilized microorganisms thereon and/or therein, to a process for obtaining such membranes, and to reactors comprising said membranes and to a process involving the use of said membranes, in particular for the elimination of metals or of xenobiotic organic compounds. Soluble metal removal is a technical challenge which must be met before the recycling process water or avoiding toxic discharges in plant effluent. Heavy metals such as cadmium, lead, copper and zinc draw the attention of the hygienists because of their toxicity. The public health is directly concerned with the occurrence of heavy metals in water and soil even at low concentrations owing to their accumulation in vegetables through soil solution. In the same way, sewage sludges produced in biological waste treatment can be also loaded with heavy metals and are often used as fertilizers. It is a tendency towards water reuse and rivers as reservoirs for drinking water. It is the reason why national and international authorities have issued directives on limit value for heavy metals in industrial effluents as well as in aquatic systems. So, it appears desirable that these health hazards should be avoided. For this purpose, the best way should be the removal of heavy metals at the emission point i.e. in the industrial effluents. The removal of heavy metals from industrial wastewaters can be performed by several strategies. Among them, chemical precipitation by addition of hydroxides or calcium oxides, ion-exchange on resin or electrolyses are of common practices. These methods are used when rather large amounts of metals, i.e. more than 500 ppm, are involved. The use of ion exchange is more interesting at very low concentrations (less than about 5 ppm) . The disadvantages of ion exchange is high resin costs. The disadvantages of electrolyses is high energy costs. The disadvantages of hydroxides or calcium oxides is high sludge production. However, they are not appropriate to remove intermediate amounts of metals or to degrade xenobiotic organic compounds. It has also been investigated to use biomass which is immobilized in porous polysulfone beads for extracting toxic and heavy metals from dilute waste streams.. The beads were fabricated from high-density polysulfone dissolved in dimethylformamide (DMF) . Dried, thermally-killed biomass produced by algae, yeast, bacteria, and aquatic flora were blended into the polysulfone-DMF solution, and spherical beads were formed by injecting the mixture into water. Contaminants removed from the waters included arsenic, cadmium, copper, mercury, lead, manganese, and zinc. Laboratory tests indicated that the beads may be especially useful in treating dilute wastewaters containing metal concentrations up to about 15 mg/1 (cf. Jeffers T.H. et al., 1989, "Biosorption of metal contaminants using immobilized biomass", Biohydrometallurgy p. 317-327) . This is a method applicable for very low concentrations. In this case, it is necessary to regenerate the bacteria, because there is adsorption of the metals to be removed, on the sites of the bacteria, and hardly little precipitation and then the sites of the bacteria become saturated. When the bacteria have been regenerated, their efficiency to adsorb metal is lowered because all the sites cannot be regenerated or some sites are destroyed. No high upconcentration can be obtained. It is necessary to use a large amount of beads, because it is possible to adsorb only between 5 and 10% of the metal with respect to the biomass, whereas if precipitation could take place (which is not the case) , it would be possible to eliminate up to above 50%- of metal with respect to the biomass. Debus O. et al. ("Aerobic mineralization of benzene, toluene and xylenes by microorganisms attached to gas-permeable membranes", Technical University of Hamburg-Harburg, FRG, April 1990) have disclosed the use of pure or mixed cultures of microorganisms to biodegrade volatile organics like benzene, toluene, ethylbenzene and the iso eric xylenes (BTEX) , under aerobic conditions. In order to avoid the conventional aeration systems producing a large number of bubbles, leading to a BTEX loss by stripping, gas permeable membranes are used, such as silicon rubber. The loss of volatile organics can be minimized by allowing BTEX- mineralizing microorganisms to colonize the membrane surface and form a barrier to the escaping substances. In this case, the bacteria receive only oxygen from the membranes. Moreover, the bacteria form a biofil on the membranes, because the pores are too small for the bacteria to be immobilized on or in the membrane, and the nutrients are in the effluents, which involves a contamination of the effluents. It is known that some microorganisms can immobilize metals up to high concentrations in their cellular materials specially when they are attached on a support. Among the culture devices which promote the growth of microorganisms on a surface, the biological fluidized bed is attractive because it has excellent adhesion potentialities for bacteria. The biological fluidized bed is composed of a cylinder packed with inert particles such as sand, anthracite, glass beads, plastic, stone gravels which provide support for microbial growth. The particles are freely suspended in the nutritive solution by an upward flow (cf. Remacle J. et al., Heidelberg 1983, "Uptake of heavy metals from industrial effluents by microorganisms developed in a biological fluidised bed" p. 936-939) . One of the drawbacks of this method is the fact that the nutritive medium for the microorganisms is mixed with the effluent to be treated, which involves a contamination of the effluent. Moreover, important amounts of carbon are needed, because they are added into the effluent. Besides, there is a big release of the microorganisms; both these factors increase the cost of the treatment of the effluent. Reticulated polyurethane foams are interesting supports allowing a high retention and an easy recovery of biomass just by squeezing (Cooper P.F. et al., 1986, in Process Engineering Aspect of Immobilised Cell Systems p. 205-217 Webb C. et al. (eds) , I. Chem. Eng.). These are essential characteristics for the development of a process of metal recovery from industrial effluents. The immobilized cell cultures were conducted in fixed bed microfermenters (V = 0.41) continuously fed with a nutrient medium. The support particles consisted in 3.4 cm cubes with an internal porosity of 98% and a pore aperture of 30 ppi (Colombi Y. et al., 1987, "Cadmium uptake by Alcaliqenes eutrophus immobilized in reticulated polyurethane foam", Proc. 4th European Congress on Biotechnology, 1 : 120) . In this case, the biofilm is on the surface of the pores, which implies that there is a big release of cells. Besides, there is a need for important amounts of carbon. During the last years, bacteria resistant to a variety of heavy metals were isolated and identified (Silver S. et al., 1988, "Plasmid-mediated heavy metal resistances" Ann. Rev. Microbiol. 4_2, 717-743). The mechanisms for these resistances are often controlled by plasmid borne genes or by transposons. A remarkable example of those resistant bacteria is Alcaliqenes eutrophus var. metallotolerans. The representative strain CH34 was isolated in sediments from a decantation basin of a zinc factory (Mergeay M. et al., 1978, "Extrachromosomal inheritance controlling resistance to cadmium, cobalt and zinc ions: evidence from curing in a Pseudomonas" Arch. Int. Physiol. Biochim. 86_, 440-441) . Strain CH34 bears two large plasmids (Mergeay M. et al., 1985, "Alcaliqenes eutrophus CH34 is a facultative chemolithotroph with plasmid-bound resistance to heavy metals". J. Bacteriol. 162, 328-334) controlling resistance against Cd ++ , Co ++ , Zn ++ , Hg ++ , Tl + , Cu ++ , Pb ++ (pMOL30, 240 kb) and Co ++ , Zn ++ , Ni ++ , Hg**, Cr0 4 " , Tl + (pMOL28, 165 kb) . On pMOL28 nickel, cobalt and zinc genes (cnr) are probably on the same cluster very near to the chromate genes (chr) . One DNA fragment (in pMOL30) of about 9 kb (Nies D. et al., 1987, "Cloning of plasmid genes encoding resistance to cadmium, zinc and cobalt in Alcaliqenes eutrophus" Bacteriol. 169, 4865-4868) is responsible for the resistance against cadmium, zinc and cobalt (czc) . And another gene cluster seems to code for copper and lead resistance (cup) . Both plasmids contain a mercury transposon : Tn4378 and Tn4380 (Diels L. et al., 1985, "Mercury transposons from plasmids governing multiple resistance to heavy metals in Alcaliqenes eutrophus CH34", Arch. Int. Physiol. Biochim. 93_, B27- B28 ; Diels L. et al., 1989, "Large plasmids governing multiple resistances to heavy metals : a genetic approach" Toxic. Environm. Chem. 2ϊ., 79-89) . Different heavy metal resistances genes are cloned and sequenced, namely czc (Nies D.H. et al., 1989, "Expression and nucleotide sequence of a plasmid determined divalent cation efflux system from Alcaliqenes eutrophus", Proc. Natl. Acad. Sci. USA 86, 7351-7356), cnr (Siddiqui R.A. et al., 1989, "Cloning of pMOL28-encoded nickel resistance genes and expression of the qene in Alcaliqenes eutrophus and Pseudomonas spp", J. Bacteriol. 171, 5071-5078) and chr (Nies A. et al., 1990, "Nucleotide sequence of chr genes responsible for resistance to chromate in Alcaliqenes eutrophus", J. Biol. Chem. 265, 5648-5653) . From the copper and lead qenes, mutants are available. The czc and mercury genes were used as probes for hybridization with total DNA from strains isolated from different mining and industrial sites in Belgium and Zaire (Diels L. et al., 1988, "Isolation and characterization of resistant bacteria to heavy metals from mining areas of Zaire", Arch. Int. Physiol. Biochim. 9_6, B13 ; Diels L. et al., 1988, "Detection of heterotrophic bacteria with plasmid-bound resistances to heavy metals from belgian industrial sites", Arch. Int. Physiol. Biochim. 96, B14) . From these different sites, strains hybridizing with these probes could be isolated (Diels L. et al., 1990, "DNA probe-mediated detection of resistant bacteria from soils highly polluted by heavy metals" Appl. Environm. Microbiol. 56, 1485-1491) . As could be shown by Nies D. et al. (1989, "Plasmid determined inducible efflux is responsible for resistance to cadmium, cobalt and zinc in Alcaliqenes eutrophus" J. Bacteriol. 171, 896-900 and 1989, "Metal ion uptake by a plasmid-free metal sensitive Alcaliqenes eutrophus strain", J. Bacteriol. 171, 4073-4075) resistance to chromate is inducible and based on decreased net accumulation of the metal anion. Resistance to zinc, cadmium, cobalt and nickel are resulting from inducible, energy dependent cation efflux systems. In some physiological circumstances Alcaliqenes eutrophus can also accumulate and precipitate heavy metals (Diels L. , 1990, "Accumulation and precipitation of Cd and Zn ions by Alcaliqenes eutrophus strains", Biohydrometallurgy (1989) 369-377 ; Diels L. et al., 1989, "Isolation and identification of bacteria living in environments severely contaminated with heavy metals", 7th International Conference on Heavy Metals in the Environment, 12-15 September 1989, Geneva) . At increased concentrations of Cd or Zn ions, a removal of these metals from the solution is observed during the late log phase and the stationary phase. This accumulation and precipitation is correlated with the concentration and kind of carbon source (lactate or gluconate) , with the progressive alkalinization of the periplasmic space and the surrounding medium, due to the proton antiporter system of the resistance mechanism, with the concentration of phosphate and appears to be associated with the outer cell membrane. The precipitation of CdC0 3 and Cd(0H) 2 is proved by IR-spectroscopy. The interpretation of this feature is that the metal speciation will change at the cell surface due to the progressive pH increase, the steep pH gradient on this site, and the production of C0 2 by the cell metabolism. For electrochemical purposes, membranes have been developed (Leysen R. et al., 1987, "The use of heterogeneous membranes in electrochemical systems", in "Synthetic polymeric membranes"", Eds. B. Sedlacek and J. Kahovec, W. de Gruyter, Berlin) composed of a polymeric polysulfone material in which zirconium oxide grains are distributed in a homogeneous way in order to form a composite membrane ; these membranes are formed using the phase inversion technique (evaporation - crystallization) . This type of membrane has already been produced in three different configurations : flat membranes (Doyen W. et al., 1988, "The use of Zr0 2 -based composite membranes for the separation of oil-water emulsions", in Proceedings of the Symposium on "Particle Technology in relation to Filtration Separation", Antwerp, October 3-4) with or without a reinforcing support, hollow fibers (Matthys J. et al., 1989, "Development of hollow fibers for the production of secretory products by living cells, in Proceedings of the Symposium on "Down stream processing in Biotechnology", Bruges, April 10-11) and tubes (Doyen W. et al., 1989, "New composite tubular membranes for ultrafiltration" in Proceedings of the "6th International Symposium on Synthetic Membranes in Science and Industry", Tubingen, September 4-8) ; however, the hollow fibers (when their diameter is smaller than about 4 mm) are very sensitive to clogging especially in the presence of an effluent which contains suspended materials. An aspect of the invention is to provide with membranes with immobilized microorganisms thereon and/or therein, enabling to eliminate low metal concentrations, particularly of heavy metals. Another aspect of the invention is to provide with membranes with immobilized microorganisms thereon and/or therein, enabling to degrade xenobiotic organic compounds. Another aspect of the invention is to provide with membranes with immobilized microorganisms thereon and/or therein, enabling to synthesize compounds when the microorganisms contain the appropriate cellular machinery. Another aspect of the invention is to provide with membranes with immobilized microorganisms thereon and/or therein, enabling to separate the effluent to be treated from the nutrient medium necessary for the life of microorganisms, i.e. enabling to recover a non contaminated effluent. Another aspect of the invention is to provide with reactors comprising membranes with immobilized microorganisms thereon and/or therein enabling to treat effluents contaminated with metals, said effluents being separated from the nutrient medium necessary for the life of the microorganisms. Another aspect of the invention is to provide with reactors having both a good metal removal capacity and a reduced need for the concentration of nutrients. The membranes of the invention with immobilized microorganisms thereon and/or therein are characterized by the fact that the microorganisms are alive or in a viable form and are liable to precipitate one or several metals, when they are in the presence of said metal and/or to degrade xenobiotic organic compounds, when they are in the presence of said compounds, and the membranes are of porous material, said material being either an inorganic oxide or a composite material containing an inorganic oxide, the membrane being such that the microorganisms can settle in the pores, said pores communicating between themselves, so that it makes it possible an appropriate colonization of the membranes by the microorganisms, and such that the release of the microorganisms is not significant and advantageously less than about 10 4 microorganisms/ml/h, preferably than about 10 2 microorganisms/ml/h. "Not significant" means that there is advantageously a release of less than about 10 4 microorganisms/ml/h and preferably less than about 10 2 microorganisms/ml/h. By "microorganisms", one may cite, for instance, bacteria, algae, yeasts, fungi. Bacteria are advantageously used for the membranes of the invention. An important feature of the membranes of the invention is the fact that the size of the pores is appropriate to the size of the microorganism which is immobilized in the membrane, i.e. large enough for the microorganisms to settle and to grow and not too large to prevent the release of said microorganisms, especially in the effluent to be treated. The membranes with immobilized microorganisms, especially bacteria, thereon and/or therein according to the invention can be used to remove metals if the bacteria can induce the accumulation and/or the precipitation of said metals when the latter are in a solution (hereafter called "effluent") , when said effluent is in the presence of said membranes. Accumulation corresponds to the adsorption of the metals on the sites of the bacteria, said sites being on the bacteria walls or in the extracellular polymers of said bacteria walls. The precipitation of the metal takes place on the bacteria walls, as well as on the membranes and in the vessel in which the membranes can be located. By metal, one designates the transition metals, the rare earth, the elements having metallic properties in the families Ilia, IVa, Va and Via of the Mendelieff table. By metals, one may cite for example cadmium, zinc, cobalt, copper, lead, mercury, thallium, chromium and manganese under the form of salts, either in a soluble or non soluble state. The membranes with immobilized bacteria thereon and/or therein can also be used to degrade xenobiotic organic compounds, if the microorganisms have the genes responsible for the degradation or mineralization of these compounds and if they can express them. The xenobiotic organic compounds designate the compounds which may endanger health and which are man made chemicals (non naturally occurring compounds) . By way of example, one may cite fungicides, herbicides, pesticides, insecticides, chloroorganic compounds, particularly biphenyl compounds. Advantageously, the membranes of the invention are used to eliminate metals and/or to degrade xenobiotic organic compounds. The membranes contain pores at its surface and in its thickness in which the microorganisms can nest and grow, which implies that the pores must be large enough for the size of the microorganisms and must not be too large otherwise the microorganisms would be released from the pores. The expression immobilized microorganisms and/or in the membrane means that the microorganisms are trapped in the pores of the membrane, but can migrate from one pore to another one, especially if space is needed after cellular division. When the membrane has immobilized microorganisms thereon and/or therein, the microorganisms can migrate in the direction of the nutrient medium in the presence of which the membrane can be. The expression "appropriate colonization" means that, after the incorporation of the bacteria in the pores of the membrane and due to the growth of the bacteria, the pores of the membranes can be filled by bacteria in a sufficient amount for the process to be efficient. An efficient process is a process in which the bacteria can grow on the membrane and exert their function at the outside of the membrane (being the effluent side) . The microorganisms on and/or in the membranes must be non killed, i.e. alive or liable to be brought back to life, for instance lyophilized, in so far as the lyophilization process does not destroy most of the microorganisms. The membranes must be such that the release of bacteria is less than about lxlO 4 cells/ml/h, preferably less than about lxlO 2 cells/ml/h. For heavy metal removal a direct contact between the metals and the bacteria is necessary to induce metal precipitation and crystallisation. When there is no cell release from the membrane, the metal moves to the immobilized cells and precipitates around them. This results in crystal growth in and on the membrane. With this system heavy metal removal is also very slowly. And once the membrane is saturated an acid treatment is necessary for regeneration of the membrane, but with the risk of eliminating all viable immobilized cells. When there is indeed cell release, the metals come immediately in contact with the bacteria around which they cristallize. This is a faster process and gives no membrane regeneration problems. Besides, the bacteria with crystallized metals must be removed by a recuperation column (such as a glass bead column) . When no more metal is available, the free bacteria will bind with much difficulty to the recuperation column; this means that cell release must be steered in function of metal concentration. The cell release can be influenced by the diameter of the skin pores of the membrane or by the pump flow rate in the effluent at the membrane. If the release is above these values, there is a contamination of the effluent containing the metal or organic compound to be eliminated. The membranes of the invention can enable to eliminate up to 90% of metals contained in an effluent to be treated or up to 90% of xenobiotic organic compounds contained in an effluent to be treated. An advantageous group of membranes with immobilized -bacteria thereon and/or therein is constituted by the membranes in which the pores of the membranes have a maximum size of about 1 μ at the skin side (hereafter defined) and between about 1 μ and about 5 μ at the other side. An advantageous group of membranes with immobilized bacteria thereon and/or therein of the invention is constituted by membranes wherein the porosity is of about 50% to about 80%, preferably of about 65% to about 75%. The porosity is defined by the ratio between the volume of the pores and the volume of the total material which constitutes the membranes. The thickness of the membranes must advantageously be such that there is appropriate tightness between the two parts located respectively on each of the sides of the membrane, said parts being created by the division of the space, by the membranes. When the membranes are thicker (i.e. more than about 200 μ) , the surface of the membranes can be smaller, for the same results of elimination of metals or xenobiotic organic compounds. Moreover, in the case of thicker membranes, i.e. of more than about 200 μ , the release of the bacteria is smaller. To give an idea, a thickness of about 250 μ reduces the release of microorganisms of about 10 times with respect to a membrane of about 60 μ. Besides, when the membranes are thicker, i.e. more than about 200 μ, there is a good accumulation of the metal on the membrane. To give an idea, a thickness of about 250 μ enables to increase the accumulation of metal of about 5 times with respect to a membrane of about 60 μ. The thickness of the membranes is an important aspect, especially when the metal concentration is low. In fact, when the metal concentration is low, i.e. of less than about 1 ppm, the precipitation does not work very well, and the fact that the metal can accumulate on the bacterial wall plays an important part in the efficiency of the metal removal. To give an idea, 50% of a metal can be eliminated from a solution containing 0,5 ppm of said metal. Advantageously, the membranes with immobilized microorganisms, particularly bacteria, thereon and/or therein are such that the thickness of the membranes is of about 50 μ to about 700 μ, preferably of about 70 to 500 μ, and more advantageously of about 250 μ to about 500 μ. Preferably, the membranes with immobilized microorganisms, especially bacteria, thereon and/or therein according to the invention are substantially homogeneous in the whole of their thickness. Said homogeneity corresponds both to the fact that the pores are substantially equally distributed in the membrane and to the fact that the size of the pores is substantially the same. It is advantageous that the membrane should not contain large pores or finger like structures (i.e. pores of over 10 μ) , to prevent an important release of bacteria. It is also advantageous that the pores are not finger like shaped, because the trapping of microorganisms is not good, and consequently, release can be enhanced. According to an advantageous embodiment of the invention, one of the sides of the membrane form a skin having pores, the size of which prevents the bacteria from being released, the size of these pores being advantageously smaller than about 1 μ, and preferably smaller than about 0.5 μ. The side of the membrane which contains the smaller pores is hereafter named the "skin side". According to another advantageous embodiment of the invention, on one of the sides of the membranes, externally, the bacteria are in the form of a biofilm of a thickness of about 1 to about 50 μ, preferably of about 10 to about 20 μ and internally the bacteria form a colonizing front, around single dispersed bacteria in the membrane. The biofilm is always located externally on one of the sides of the membranes, but it is not always present. The biofilm can be defined as a mass of bacteria which are fixed to each other by their own polymers. The biofilm results from the growth of the bacteria, which have previously been introduced into some pores on the surface of said membrane. The colonizing front is always present in the membranes and it corresponds to the filling of the pores by the bacteria, said colonizing front extends from one side of the membrane and have a thickness of about 10 μ, preferably 50 μ, to about the totality of the thickness of the membrane. The growth of the microorganisms can be controlled by the nutrient medium and by the supply of oxygen. After being introduced into some of the pores of the membrane, in the membrane and on the surface of the membrane, the bacteria grow inside the membrane, and on its surface, provided the size of the pores enables it and provided the pores communicate between themselves. When the membrane comprises a skin side, the biofilm is opposite to the skin side. As to the colonizing front, it is inside the membrane and limited to the membrane thickness. To give an idea, when the growth of the bacteria on the membrane has been monitored such that the biofilm is opposite to the skin side, outside the membrane, the colonizing front can have a thickness of about 10 μ, preferably 50 μ, to about the totality of the thickness of the membrane, and preferably of about 100 μ to about 200 μ. The shape of the membrane must be such that it forms a surface liable to be a separation for two different parts located respectively on each side of the membrane. Advantageously, the surface of the membrane is flat or tubular, with or without a support. When the membrane is flat, its size can be of about 10 cm 2 to about 1 m 2 . When the membrane is flat, it is advantageously combined with a support which prevents the membrane from being tome in case the membrane is subjected to high pressures, or to pressure differences between the two parts located respectively on each side of the membrane. It is advantageous to combine a support on either side of the membrane, or on both sides of the membrane, preferably perpendicular to the direction of the pression. Said support has substantially the surface size of the membrane and has a thickness of about 100 μ to about 1000 μ, and is provided with holes of about 50 μ to about 500 μ, preferably of about 100 μ. Said support is advantageously adjacent to the membrane. Said support is advantageously constituted of the following material : polyester with a pore diameter between about 10 μ to about 200 μ. When the membrane surface is flat, there is advantageously a biofilm of microorganisms, particularly bacteria located on the external side of the membrane which has the larger pore size, i.e. opposite the skin side. When the membrane surface is tubular, the colonizing front of bacteria is necessarily on the internal side of the membrane, and can move up to the internal part of the skin side. Advantageously, when the membrane is tubular, a colonizing front is close to the internal side of the membrane which has the smaller pore size. The tubular membrane advantageously is supported by a tube, and is located either on the external surface of said tube or on the inner surface of said tube. The tubes used are porous tubes, preferably carbon tubes or polyester tubes. When the membranes are tubular and coat the inner surface of the tube and have a skin side, the larger size pores are preferably in contact with the inner surface of the tube and the colonizing front is preferably close to the skin side, inside the membrane. When the tubular membrane is not supported by a tube, it is called a hollow fiber membrane. For instance, the length is from about 30 to about 100 cm, the diameter is from about 5 to about 10 cm. Advantageously, the membrane is made of polysulfone comprising an inorganic material, such as Zr0 2 . The membranes used are for instance described in European Patent Application n' 241995. Pore formers such as Sb 2 0 5 , CaC0 3 or polyvinylpyrrolidone (PVP) can be added during the manufacture of the membrane for the production of pores, but are eliminated at the end of the production process of the membrane. Preferred membranes have the following composition: about 80% Zr0 2 , about 20% polysulfone. Other preferred membranes are made of inorganic materials, such as inorganic oxides, such as Zr0 2 or A1 2 0 3 . In these cases, the material of the membranes is designated by ceramics. Advantageously, the microorganisms are liable to precipitate one or several metals, when they are in the presence of said metal and/or to degrade xenobiotic organic compounds, when they are in the presence of said compounds, or to synthesize compounds when a substrate to be transformed is present. Advantageous microorganisms used are bacteria. An advantageous group of bacteria is Alcaliqenes eutrophus CH34 (ATCC N" 43123) , which can be used to precipitate one or several heavy metals such as zinc, cadmium, nickel, lead and copper. Another advantageous bacteria is A5 (deposited at the C.N.C.M., Institut Pasteur, 28 rue du Dr Roux, 75015 Paris, on February 28, 1991, under N° 1-1047), liable to degrade xenobiotic organic compounds. The membranes of the invention enable to remove up o about 225 ppm of cadmium, up to about 720 ppm of c 19 zinc, up to about 150 ppm of nickel up to about 150 ppm of lead, and up to about 60 ppm of copper. The lowest concentrations which can be removed are at least in the order of 1 ppm. The invention also relates to membranes with immobilized bacteria according to the invention, wherein the pressure difference between each side of the membrane is about 0. The pressure difference over the membrane will be defined as : ΔP = P2 — PI in which PI = pressure in effluent P2 = pressure in nutrient When the reactor will be loaded (immobilization) with bacteria, ΔP will be slightly larger than zero in order to suck the bacteria into the membrane. During the growth of the bacteria in the membrane (colonization process) ΔP will be zero. For heavy metal removal, ΔP can be zero or slightly positive and for xenobiotic degradation, ΔP, can be zero or slightly negative, in order to eliminate cell release in this case. The preparation of the membranes with the immobilized microorganisms thereon and/or therein, can be carried out as follows : - the filtration of a suspension of microorganisms, containing an amount of microorganisms such that the membrane is not clogged and containing appropriate amounts of microorganisms, said suspension containing advantageously 10 8 to 10 9 microorganisms/ml, preferably 10 8 microorganisms/ml, the filtration being carried out through the membrane, preferably in a tangential manner with respect to the surface of the membrane, in order to immobilize the microorganisms in and/or on the membrane. The preparation of the membranes with the immobilized bacteria thereon and/or therein, can be carried out as follows : - the filtration of a suspension of bacteria, containing an amount of bacteria such that the membrane is not clogged and containing an appropriate amount of bacteria, said suspension containing advantageously 10 7 to 10 8 bacteria/ml, preferably 10 8 bacteria/ml, the filtration being carried out through the membrane, preferably in a tangential manner with respect to the surface of the membrane, in order to immobilize the bacteria in and/or on the membrane. If the filtration is tangential with respect to the membrane, the bacteria can settle in the pores or if not are released in the suspension, but clogging of the membrane is avoided. If the filtration is perpendicular with respect to the membrane, bacteria settle upon each other and clogging appears. The bacteria used are preferably constituted by an overnight culture (10 9 bacteria/ml) , which is preferably about 10 times diluted, to avoid clogging. If the suspension contains at least 10 9 bacteria/ml, there is a risk of clogging of the membrane. It is possible to use less than 10 7 bacteria/ml, but under these conditions, it would be necessary to wait until enough bacteria have grown, otherwise the metal or xenobiotic elimination would last too long. In the process for immobilizing the microorganisms on the membrane, it is advantageous to colonize the immobilized microorganisms, until the pores of the membrane are filled by the microorganisms on a thickness of about 100 μ to about 200 μ, preferably in the whole thickness of the membrane, in the presence of an appropriate nutrient medium. The colonization corresponds to the growth of the microorganisms inside the membrane, and also possibly on the membrane until a biofilm is obtained. The colonization until the pores of the membrane are completely filled lasts advantageously 4 days. The advantage of colonization is that there is a high concentration of bacteria and the treatment process is more efficient. To prepare the membranes of the invention, the microorganisms are immobilized on the membranes after the preparation of the membranes, to be sure that the microorganisms are microorganisms trapped in the pores and not in the mass of the material of the membranes. To prepare tubular membranes, resort may be had to the process described in European Patent Application n°241995. To prepare tubular membranes coated inside tubes or outside tubes, it is possible to resort to process known by the man skilled in the art, such as bringing a suspension of the components of the membrane to the shape of a hollow tube by means of a casting bob. The membranes with bacteria immobilized thereon and/or therein are advantageously used in new reactors. Said reactors of the invention comprise a recipient (cell) containing .: - a membrane with immobilized microorganisms thereon and/or therein, said microorganisms being alive or in a viable form, said membrane being of a porous material such that the microorganisms can settle in the pores, said pores communicating between themselves so that it makes it possible an appropriate colonization of the membrane by the microorganisms, and such that the release of microorganisms is not significant, - possibly a support, - two parts with each an inlet and an outlet and separated by a membrane, one of the two parts being in contact with one side of the membranes, and containing a nutrient medium to enable the life and growth of the microorganisms, and the other one, being in contact with the other side of the membranes and containing an effluent to be treated. "Not significant" means that there is advantageously a release of less than about 10 4 microorganisms/ml/h and preferably less than about 10 2 microorganisms/ml/ . The reactors of the invention are new, not only because of the membranes used, but also because they correspond to a new configuration, in so far as these reactors comprise two parts corresponding respectively to the effluent to be treated and to the nutrient medium, said two parts being created in the cell of the reactor by the separation of said cell with an appropriate membrane. Said reactors of the invention comprise a recipient (cell) containing : - a membrane with immobilized bacteria thereon and/or therein, said bacteria being alive or in a viable form, said membrane being of a porous material such that the bacteria can settle in the pores, said pores communicating between themselves so that it makes it possible an appropriate colonization of the membrane by the bacteria, and such that the release of bacteria is less than about 1.10 4 bacteria/ml/h, preferably than about 1.10 2 bacteria/ml/h, - possibly a support, - two parts with each an inlet and an outlet and separated by a membrane, one of the two parts being in contact with one side of the membranes, and containing a nutrient medium to enable the life and growth of the bacteria, and the other one, being in contact with the other side of the membranes and containing an effluent to be treated. Said reactor can comprise anyone of the membranes of the invention above defined. The parts hereto above-mentioned will be designated by "chambers". The bacteria, immobilized on a membrane, receive the nutrient by diffusion of said nutrient medium through the membrane. It is necessary to bring oxygen to the membrane to promote the development of the abovesaid biofilm. Preferably, oxygen is brought to the membrane on the effluent side. Preferably, there is no oxygen in the nutrient side. The maximum amount of oxygen corresponds to saturation (i.e. about 10 mg of oxygen per liter) . The conditions in the reactor are such that there is growth of the microorganisms inside the membrane and not too much growth of the biofilm. When the thickness of the biofilm is above about 100 μ bacteria can be released from the biofilm and this is preferably avoided, because it is a loss of bacteria. If the biofilm is on the nutrient medium side, then there is a loss of nutrient medium, because of the released bacteria. If the biofilm is on the effluent side, there is a contamination of the effluent medium by the released bacteria. Moreover, when the thickness of the biofilm is above about 100 μ, the bacteria which are close to the membrane are under anaerobic conditions and inefficient. The effluent to be treated is advantageously a continuous flow. The conditions in the reactor above referred to are : - the composition of the nutrient medium, - the amount of the nutrient medium, - the amount of oxygen, - the ratio of nutrient volume to effluent volume, - the flow rate of the effluent volume and - the flow rate of the nutrient medium. The flow rate of the effluent medium must not be too high so that too many microorganisms are not released into the effluent medium. An advantageous flow rate of the effluent medium is of about 1 ml/mn/cm 2 to about 4 ml/mn/cm 2 , preferably of about 2 ml/mn/cm 2 . As . to the nutrient medium, it can be or not continuous and its flow rate can preferably vary from about 10 μl/mn/cm 2 to about 200 μl/mn/cm 2 , preferably of about 0 to about 100 μl/mn/cm 2 . An advantageous ratio between the volume of nutrient medium with respect to the volume of the effluent is about 1/10 to about 1/1000. By way of example, in a reactor of the invention, the effluent volume is about 5 1 and the nutrient medium volume is of about 500 ml, for a surface membrane of about 10 cm 2 . The effluent must have a pH such that it does not kill the microorganisms. The effluent must be previously treated so that the pH is about 6 to about 8, preferably about 7 to about 7.5. The effluent must contain no organic solvent in an amount liable either to kill the microorganisms or to dissolve the membrane. The organic solvent, if any, must be under about 5% (expressed in weight) . In the nutrient medium, it must be taken into account the fact that the immobilized microorganisms need less nutrient elements than when they are free. The amount of the nutrient medium needed for the immobilized bacteria is about 10 times less than the amount of nutrient medium needed for the free microorganisms. The nutrient medium comprises : - a carbon source, - a phosphate, - a nitrogen. As a carbon source, one may use lactate or acetate. Lactate is advantageously used, in an amount of about 1 g/1 for 10 9 bacteria/ml, i.e. about 0,1 g/1 for 10 8 bacteria/ml. This corresponds to about 0.3 g/1 of effluent. The phosphate used is advantageously under the form Na 2 HP0 4 . It is used in a concentration of about 1 mM for 10 9 bacteria/ml, i.e. 0.1 mM for 10 8 bacteria/ml. This corresponds to about 0.02 mM/1 of effluent. The nitrogen used is advantageously under the form NH 4 0H. It is used in a concentration of about 1 M for 10 9 bacteria/ml, i.e. 0.1 mM for 10 8 bacteria/ml. This corresponds to about 0.2 mM/1 of effluent. The pH of the nutrient medium is advantageously from about 7.5 to about 9, and is advantageously about 7,8. An advantageous reactor of the invention comprises a flat membrane with immobilized microorganisms, especially bacteria, thereon and/or therein according to the invention, said membrane separating two chambers with each an inlet and an outlet, one of the chambers containing a nutrient medium and the other chamber containing an effluent to be treated. The chamber containing the nutrient medium can be either in contact with the side of the membrane with the smaller pores or can be in contact with the side of the membrane with the larger pores. When there is a biofilm, it can be either in contact with the effluent or in contact with the nutrient medium. Advantageously, the chamber containing the effluent medium is in contact with the skin side of the membrane and the chamber containing the nutrient is in contact with the side of the membrane with the larger pores. Advantageously, the biofilm of bacteria is located externally on the side of the membrane with the smaller pores, and the biofilm is in contact with the effluent. In this case, the bacteria are immobilized, preferably by tangential filtration of a suspension of bacteria introduced into one of the chambers. The suspension of bacteria must be introduced into the chamber which is in contact with the side of the membrane with larger pores, i.e. opposite to the skin side. The suspension of bacteria is introduced either into the chamber which is destined to receive the effluent, after immobilization of the bacteria on the membrane or into the chamber which is destined to receive, after immobilization the nutrient medium, depending on the orientation of the skin side. During the immobilization, it is not necessary that the bacteria be in the presence of a nutrient medium. When the immobilization of the bacteria is over, colonization is carried out in the presence of a nutrient medium which diffuses to the bacteria immobilized. When the colonization is over, the effluent to be treated is introduced into one of the chambers to receive it. In one chamber nutrient solution is pumped during reactor use and at the other side an effluent solution is pumped. With this system a reduction of nutrients can be obtained. Bacteria immobilized on and/or in the membrane are fed by nutrients coming from the nutrient side. At the effluent side the immobilized microorganisms remove the heavy metals from the effluent. Siphons are used to keep the pressure difference over the membrane at zero. The leakage between the nutrient medium and the effluent is prevented in so far as the amount of nutrient medium is such that it is taken up by the microorganisms immobilized in and/or on the membrane. The leakage between the effluent and the nutrient medium can be avoided by keeping in a constant way, low concentrations of nutrients, in the nutrient solution. Another advantageous reactor of the invention comprises : - a tubular recipient, said recipient containing hollow tubes of a porous material, preferably carbon tubes, the inner surface or the outer surface of the carbon tubes being coated with a membrane according to the invention, said membrane being in contact with the effluent to be treated, the thickness of the carbon tubes being - such that the microorganisms can be immobilized in the membranes by filtration of a suspension of microorganisms substantially along the outer or the inner surface of the tubes, and - such that the nutrient medium which is either in the tube or in the intertubular space can diffuse from the outer or the inner surface of the tube respectively to the inner or to the outer surface of the tube coated with the abovesaid membrane. In this embodiment, the membrane with immobilized microorganisms, especially bacteria, thereon and/or therein can be on the outer surface of the tubes. In these conditions, the side of the membrane having the skin side is preferably opposite to the side of the membrane which is in contact with the outer surface of the membrane. In this embodiment, the effluent to be treated is preferably in the intertubular space and the nutrient medium is inside the tube. In this case, the microorganisms have been immobilized, preferably by tangential filtration, along the inner surface of the tube, of a suspension of bacteria. Advantageously, the membrane with immobilized bacteria thereon and/or therein coat the inner surface of the membrane. In these conditions, the side of the membrane having the skin side is preferably opposite to the side of the membrane which is in contact with the inner surface of the tube. In this embodiment, the effluent to be treated is preferably inside the tube and the nutrient medium is in the intertubular space. In this case, the bacteria have been immobilized preferably by filtration, along the outer surface of the tube, of a suspension of bacteria. Another advantageous reactor of the invention comprises : - a tubular recipient, said recipient containing hollow tubes (supporting tubes) of a porous material, said supporting tubes being carbon tubes or polyester tubes, the inner surface of the supporting tubes being coated with a membrane according to the invention, said membrane being in contact with the effluent to be treated, the thickness of the carbon tubes being - such that the microorganisms can be immobilized in the membranes by filtration of a suspension of microorganisms substantially along the outer surface of the tubes, and - such that the nutrient medium which is in the intertubular space can diffuse from the outer surface of the tube respectively to the inner surface of the tube coated with the abovesaid membrane. Preferably, the thickness of the carbon tube is about 1 to about 3 mm, and the thickness of the polyester tubes is about 100 to about 500 μ. In this embodiment, the bacteria are first immobilized by a filtration of a suspension of bacteria tangentially along the outer surface of the tube, said bacteria going through the supporting tube and settling in and/or on the membrane. Then, the introduction of a nutrient medium in • the intertubular space, which diffuses through the tubes to the membrane allow the bacteria to grow up to the skin side, forming thus a colonizing front close to the skin side. Then the effluent to be treated can be introduced into the inner surface of the membrane, through the tube. The nutrient medium can also be inside the inner surface of the membrane and the effluent medium can also be in the intertubular space. The invention also relates to a reactor according to the invention, comprising - a tubular recipient, said recipient containing hollow tubes (supporting tubes) of a porous material, said supporting tubes being carbon or polyester tubes, the inner surface of the supporting tubes being coated with a membrane according to the invention, said membrane having preferably its larger pores in contact with the inner side of the tubes and said membrane having preferably its smaller pores in contact with an effluent medium and comprising preferably an internal biofilm close to the side of the smaller pores, the intertubular space being preferably filled with a nutrient medium, the thickness of the carbon tubes being - such that the bacteria can be immobilized in the membranes by filtration of a suspension of bacteria substantially along the outer surface of the tubes, and - such that the nutrient medium can diffuse from the outer surface of the tubes to the inner surface of the tubes coated with the abovesaid membranes, the thickness of the carbon tubes being preferably of about 1 to about 3 mm and of the polyester tubes being preferably of about 100 to about 500 μ. When xenobiotics must be degraded by specialized bacteria immobilized in the membranes of flat sheet reactor, tubular membrane reactor or continuous tubular membrane reactor, cell release needs to be reduced by a very low minimum) . The reactors of the invention can be used very well for the introduction of degradation of recalcitrant molecules by the principle of co etabolism. The advantage of this invention is that low amounts of substrate can be used for induction of cometabolism, compared to the stirred tank reactors where sometimes until 7 times higher substrate concentrations than the pollutant concentration are necessary. The flat sheet reactor (FSR) , continuous flat sheet reactor (CFSR) , tubular membrane reactor (TMR) and continuous tubular membrane reactor (CTMR) can be used for the degradation of xenobiotic organic compounds, thanks to the immobilization of specialized xenobiotic organic compound degrading microorganisms. At the nutrient side, the bacteria can be provided with essential elements with or without some substrate in the use of very low pollutant concentrations, or in the case of cometabolism. This system is then able to degrade the xenobiotics, such as chlorobiphenyl compounds, completely into water, C0 and NaCl (mineralization) if the right bacteria are used and if an appropriate substrate is used. Examples of substrates are lactic acid, aromatic compounds, chloroaromatic compounds and polyaromatic compounds. The invention also relates to a reactor wherein a recuperation column is integrated downstream of reactor, said column being such that it enables to recover metal which has been precipitated together with bacteria involved in said precipitation, said column being preferably filled with a material liable to adsorb metal and bacteria, such as glass beads, glass wool, glass powder or other form of silicate like sand particles. In order to recover the biologically precipitated metals, a recuperation column can be integrated in the circuit downstream of the FSR (Flat Sheet Reactor) , CFSR (continuous flat sheet reaction) , or TMR (Tubular Membrane Reactor) , or CTMR. Bacteria are released slowly from the reactor into the effluent, where they interact with the metals around them and afterwards they must be removed from the suspension. For this a column, preferentially filled with glass beads, glass wool or glass powder, or other silicate like sand particles, is installed downstream of the reactor. The bacteria together with the crystallized metals can bind to the glass beads, the glass wool or glass powder, or other materials of the column. When the column is completely filled and saturated with metals, she can be replaced by another one without causing some problems at the level of the membrane in the reactor. The metals can be eluted from the column by acids (this eluate can be used in electrolyses) , and the column, regenerated in this way, can be reused. The invention also relates to a process for precipitating metals or degrading xenobiotic organic compounds wherein a nutrient is administered into one of the chambers of the reactor according to the invention, and effluent to be treated containing metals or xenobiotic organic compounds are circulated in the other chamber of said reactor and wherein the bacteria in and on the membrane induce - either precipitation and crystallization processes which results in the settling of the metals, or mineralization of xenobiotic organic compounds, for instance by co-metabolism of said xenobiotic compounds in the presence of a substrate, which results in degradation of said xenobiotic organic compounds into water, C0 2 and a mineral salt originating from the inorganic ions of said xenobiotic organic compounds and of said substrate, said process for precipitating metals or degrading xenobiotics being carried out continuously or in batch. The invention also relates to a process for precipitating metals, wherein the release of bacteria from the membrane into the effluent chamber is such that the bacteria come into contact with the metals in solution, and induce the precipitation and crystallization of the metals around them. The invention also relates to a process for precipitating metals, wherein the release of bacteria from the membrane into the effluent chamber is such that the bacteria come into contact with the metals in solution, induce the precipitation and crystallization of the metals around them, and wherein said bacteria are recovered together with crystallized metals by a column downstream of the reactor. The invention also relates to a process for degrading xenobiotic organic compounds, wherein a substrate for induction of co-metabolism is used for the mineralization of xenobiotics by the bacteria into water, C0 2 and either inorganic salts originating from inorganic ions of said xenobiotic organic compounds and of said substrate or gaz, or wherein no substrate is used and xenobiotic organic compounds in the solutions to be treated are mineralized into water, C0 2 and possibly CI " (in the case of chlorinated compounds) by the bacteria. FIGURES Figure 1A represents a reactor of the invention. This reactor consists of two chambers with each an inlet and outlet and separated by each other by a membrane and a supporting frame. In one chamber nutrient solution is pumped during reactor use and at the other side a synthetic or a real effluent solution is pumped. With, this system a reduction of nutrients can be obtained. The bacteria are fed at the membrane in the biofilm by nutrients (A) coming from the nutrient side and induce in . that way the precipitation of the metals. Moderate amounts of nutrients (P, C, N) must be added to the effluent stream. At the effluent side, the immobilized bacteria remove the heavy metals from the effluent (B) . The siphons C and D are used to keep the pressure difference over the membrane at zero. Figure.. IB represents a cross section of the reactor, represented on Figure 1A in which : - M represents the membrane with immobilized bacteria therein and/or thereon, - S represents the support, - N represents the nutrient medium, - E represents the effluent. - Figure 2A and Figure 2B correspond to Figures 1A and IB in which, downstream of the reactor, a recuperation column (F) is installed. - Figure 3A represents a Tubular Membrane Reactor (TMR) . This consists of a tubular membrane (E) connected at the outer shell side with the nutrient (A) and at the inner tubular membrane side to the effluent (B) . The siphons (C) and (D) regulate the pressure difference. Downstream of the reactor a recuperation column (F) is installed. - Figure 3B represents the tubular reactor (E) . It shows the composite membrane (H) at the inner side of a carbon or polyester support (I) . The effluent flows through the inner tubular space (J) . The nutrient through the outer tubular space (G) . - Figure 3C represents a general view of a multipipe tubular membrane reactor. - Figure 4 represents a continuous tubular membrane reactor. The system consists of an input (J) , with metal contaminated effluent, an output (K) , a nutrient vessel (A) and an effluent vessel (B) each foreseen with a filter (L) . Further pump (I) pumps the effluent to vessel (B) , from these it is pumped by pump (G) through the TMR (M) with immobilized bacteria in the membrane (E) into siphon (C) over a glass bead column (F) again into (B) . Nutrient is pumped by pump (H) from (A) to through the outer tubular space of (M) into siphon (D) back to (A) . EXAMPLE 1 : Removal of heavy metals : The Alcaliqenes eutrophus strain CH34 can be used for induction of precipitation and crystallisation of heavy metals like cadmium, lead, zinc, nickel and copper. At the nutrient side a carbon source (0.1% to 0.8% lactate), phosphate (from 0.1 to 1.0 mM) and ammonia (from 0.5 to 5 mM) are administered to keep the bacteria efficient for the precipitation process. At the effluent side, water solutions containing cadmium (from 5 to 250 ppm) , nickel (from 20 to 100 ppm) , zinc (from 20 to 600 ppm) , copper (from 20 to 60 ppm) and lead (from 5 to 250 ppm) can be circulated. The bacteria in the membrane will induce precipitation and crystallization processes at the membrane which results in a settling of these metal precipitates on the membranes or on the walls of the recirculation tank. The metals can be removed from the walls by treatment with a IN HCI solution. More precisely, a membrane of a surface of 10 cm 2 is used. Said membrane is a polysulfonic membrane (18% of polysulfone) and contains 82% of Zr0 2 . It has a porosity of 70%, the size of the pores is from about 1 to about 2 μ and it has a thickness of 130 μ and one of its sides is a "skin side". The immobilization of the bacteria is carried out with 100 ml of solution containing 10 8 bacteria/ml (it is a dilution by 10 of an overnight culture) submitted to a tangential filtration which lasts 4 hours. The colonization is carried out for 4 days in the presence of a nutrient medium containing 0.2% of lactate, 1 mM of Na 2 HP0 4 , 5 mM of NH 4 0H. The pH of the effluent to be treated is of 7.8. The flow rate of the nutrient medium is of 23 ml/mn and the flow rate of the effluent medium to be treated is of 23 ml/mn. The treatment of the effluent lasts 36 hours. With a solution containing 224 ppm of cadmium the yield of removal of cadmium is of about 98%. EXAMPLE 2 : Degradation of PCB : In the followinq example, the conditions used, except for the strain, are the same ones as the ones described in example 1. However, the pH of the effluent to be treated is between 6 and 8. The Alcaliqenes eutrophus strain A5 can be used for cometabolism of chlorinated biphenyl molecules. At the nutrient side a co etabolizing carbon source (0.1 - 0.2%) is administered to keep the bacteria catabolizing (for instance biphenyl for the degradation of PCB). Also some phosphate (from 0.1 to 1.0 mM) , and ammonia (from 0.5 to 5 mM) are fed to the immobilized bacteria. At the effluent side a water suspension containing some chlorinated biphenyls (from 0 to 2000 ppm) can be circulated. The biphenyls can bind easily to the hydrophobic membrane and there they will be catabolized by the immobilized bacteria. In the case of A5 only mono-, di- and trichlorobiphenyls can be degraded. EXAMPLE 3 : Synthesis of a melanine like polymer : In the following example, the conditions are the same as the ones described in example 1, except for the pH of the process solution which is from about 6 to about 8. Some spontaneous mutants of Alcaliqenes eutrophus CH34 (available to the man skilled in the art) can transform compounds like tyrosine and meta hydroxylated aromatics into black colored melanine like polymers. At the nutrient side a mineral medium with gluconate (0.2%) as carbon source and 0.1% of tyrosine (the compound to be transformed) are administered (tyrosine can also be fed at the process solution side) . At the effluent side the end product polymer will appear and can be recovered without contamination of the original compound (e.g. tyrosine). EXAMPLE 4 : Continuous tubular membrane reactor (CTMR) Figure 2 presents a Continuous Tubular Membrane Reactor (CTMR) . The system consists of an input (J) , with metal contaminated effluent, an output (K) , a nutrient vessel (A) and an effluent vessel (B) each foreseen with a filter (L) . Further pump (I) pumps the effluent to vessel (B) , from these it is pumped by pump (G) through the TMR (M) with immobilized bacteria in the membrane (E) into siphon (C) over a glass bead column (F) again into (B) . Nutrient is pumped by pump (H) from (A) through the outer tubular space of (M) into siphon (D) back to A. With this system 1 liter effluent with 125 ppm Cd can be treated in minor than 20 hours to below 1 ppm Cd with a membrane surface of 16 cm 2 . The same is true for the reduction of 70 ppm Zn to below 1 ppm Zn. The lactate consumption with this system is 0.3 g lactate/1 effluent. At the membrane a minimum flow rate of between 100 and 200 ml/mn is necessary. EXAMPLE 5 : The use of a TMR for degradation of organic compounds In a FSR, 500 ml of a solution containing 0.8% lactic acid was treated by immobilized A^ eutrophus CH34. The degradation of lactic acid was measured spectrophotometrically. A degradation rate of 1 mg lactic acid/liter.hour.cm 2 membrane surface or 1 ppm lactic acid/hour.cm 2 membrane surface was obtained in a real crude first assay. Higher rates can probably be obtained in a TMR and a CTMR. The example shows the utility of the reactor system for degradation of small amounts (ppm to ppb level) of organic compounds in liquid effluents. EXAMPLE 6 : Comparison between polysulfone and composite (polysulfone + ZrO 2 ) membrane Two FSR reactors were run with 1.0 mM Cd in the effluent stream. In one reactor, a composite Zirfon membrane (Zirfon = polysulfone + Zr0 2 ) was used and in the other a commerical polysulfone membrane. The removal of Cd was measured in function of time. Two results were obtained : 1. Metal diffusion through the polysulfone membrane was higher than through the Zirfon membrane (3 times higher) , which implies that a substantial amount of metal can be found in the nutrient, which is something to be avoided as it is a contamination of the nutrient. 2. Metal removal was faster with the Zirfon membrane than with the polysulfone membrane (depending on the Zirfon membrane that is used, it can go up to 4 times faster) . With Zirfon membranes with no cell release, only a slight difference could be seen with a polysulfone membrane (due to the high impact of the released cells in the metal removal process) .

Claims: CLAIMS 1. Membranes with immobilized microorganisms thereon and/or therein are characterized by the fact that the microorganisms are alive or in a viable form and are liable to precipitate one or several metals, when they are in the presence of said metal and/or to degrade xenobiotic organic compounds, when they are in the presence of said compounds, and the membranes are of porous material, said material being either an inorganic oxide or a composite material containing an inorganic oxide, the membrane being such that the microorganisms can settle in the pores, said pores communicating between themselves, so that it makes it possible an appropriate colonization of the membranes by the microorganisms, and such that the release of the microorganisms is not significant and advantageously less than about 10 4 microorganisms/ml/h, preferably than about 10 2 raicroorganisms/ml/h. 2 . Membranes according to claim 1, wherein the microorganisms are fungi, yeast, algae and preferably bacteria. 3. Membranes with immobilized microorganisms, particularly bacteria, according to claims 1 or 2, wherein the pores of the membranes have an average size of about 1 μ to about 10 μ , preferably about 1 μ to about 3 μ. 4. Membranes with immobilized microorganisms, particularly bacteria, according to claims l to 3, wherein the porosity is of about 50% to about 80%, preferably of about 65% to about 75%. 5. Membranes with immobilized microorganisms, particularly bacteria, according to anyone of claims 1 to 4, wherein the thickness of the membranes is of about 50 μ to about 700 μ, preferably of about 70 to 500 μ, and more advantageously of about 250 μ to about 500 μ. 6. Membranes with immobilized microorganisms, particularly bacteria, according to anyone of claims l to 5, wherein one of the sides of the membrane form a skin having pores, the size of which prevents the microorganisms from being released, the size of these pores being advantageously smaller than about 0.5 μ. 7. Membranes with immobilized microorganisms, particularly bacteria, according to anyone of claims 1 to 6, wherein on one of its sides, the microorganisms are in the form of a biofilm of a thickness of about l to about 50 μ, preferably of about 10 to about 20 μ, and internally the bacteria form a colonizing front, around single dispersed bacteria in the membrane. 8. Membranes with immobilized microorganisms, particularly bacteria, according to anyone of claims 1 to 7, wherein the microorganisms are settled in the pores of the membranes, to form a colonizing front on a thickness which varies from 10 μ, preferably 50 μ, to the totality of the thickness of the membranes. 9. Membranes with immobilized microorganisms, particularly bacteria, according to claim 1, wherein the membranes are flat or tubular, with or without a support, and when they are tubular, the membranes are such that they have an inner diameter, preferably higher than about 2 mm. 10. Membranes with immobilized microorganisms, particularly bacteria, according to anyone of claims l to 9, wherein the membranes are flat and a biofilm of microorganisms is on the external side of the membranes which has the larger pore size. 11. Membranes with immobilized microorganisms, particularly bacteria, according to anyone of claims l to 10, wherein the membranes are tubular and a colonizing front of bacteria is close to the internal side of the membranes which has the smaller pore size. 12. Membranes with immobilized microorganisms, particularly bacteria, according to anyone of claim 11, wherein the membranes coat the inner surface of tubes, preferably carbon tubes or polyester tubes, the larger size pores of the membranes being in contact with the inner surface of the tube and the colonizing front is preferably close to the skin side, inside the membrane. 13. Membranes with immobilized microorganisms, particularly bacteria, according to anyone of claims 1 to 12, wherein the membrane is made of polysulfone comprising an inorganic material, such as polysulfone comprising Zr0 2 or is made of Zr0 2 . 14. Membranes with immobilized bacteria according to anyone of claims 1 to 13, wherein the pressure difference between each side of the membrane is about 0. 15. Process for preparing a membrane according to anyone of claims 1 to 14, with the immobilized microorganisms comprising the following steps : - the filtration of a suspension of microorganisms, containing an amount of microorganisms such that the membrane is not clogged and containing appropriate amounts of microorganisms, said suspension containing advantageously 10 8 to 10 9 microorganisms/ml, preferably 10 8 microorganisms/ml, the filtration being carried out through the membrane, preferably in a tangential manner with respect to the surface of the membrane, in order to immobilize the microorganisms in and/or on the membrane. 16. Process for preparing a membrane according to anyone of claims 1 to 14, with the immobilized microorganisms comprising the following steps: the filtration of a suspension of bacteria, containing an amount of bacteria such that the membrane is not clogged and containing appropriate amounts of bacteria said suspension containing advantageously 10 7 to 10 8 bacteria/ml, preferably 10 8 bacteria/ml, the filtration being carried out through the membrane, preferably in a tangential manner with respect to the surface of the membrane, in order to immobilize the bacteria in and/or on the membrane. 17. Reactor comprising a cell containing : - the membrane according to anyone of claims 1 to 14, - possibly a support, - two .parts with each an inlet and an outlet and separated by a membrane, one of them in contact with one side of the membranes, containing a nutrient medium to enable the life and growth of the microorganisms, and the other one, in contact with the other side of the membranes, containing an effluent to be treated. 18. Reactor according to claim 17, comprising - a flat membrane according to anyone of claims 1 to 14, said membrane separating two chambers with each an inlet and an outlet, one of them containing a nutrient medium and the other one containing an effluent to be treated, the chamber containing the nutrient medium being preferably in contact with the side of the membrane with the smaller pores and the chamber containing the effluent medium being preferably in contact with the side of the membrane with the larger pores, and more preferably with a biofilm of microorganisms located externally on the side of the membrane with larger pores. 19. Reactor according to claim 17, comprising : - a tubular recipient, said recipient containing hollow tubes (supporting tubes) of a porous material, said supporting tubes being preferably carbon tubes or polyester tubes, the inner surface or the outer surface of the supporting tubes being coated with a membrane according to the invention, said membrane being in contact with the effluent to be treated, the thickness of the carbon tubes being - such that the microorganisms can be immobilized in the membranes by filtration of a suspension of microorganisms substantially along the outer or the inner surface of the tubes, and - such that the nutrient medium which is either in the tube or in the intertubular space can diffuse from the outer or the inner surface of the tube respectively to the inner or to the outer surface of the tube coated with the abovesaid membrane. 20. Reactor according to claim 17, comprising - a tubular recipient, said recipient containing hollow tubes (supporting tubes) of a porous material, said supporting tubes being carbon tubes or polyester tubes, the inner surface of the supporting tubes being coated with a membrane according to anyone of claims 1 to 14, said membrane having preferably its larger pores in contact with the inner side of the tubes and said membrane having preferably its smaller pores in contact with an effluent medium and comprising an internal colonizing front, preferably close to the side of the smaller pores, the intertubular space being preferably filled with a nutrient medium, the thickness of the carbon tubes being - such that the microorganisms part is immobilized in the membranes by filtration of a suspension of microorganisms part is substantially along the outer surface of the tubes, and - such that the nutrient medium can diffuse from the outer surface of the tubes to the inner surface of the tubes coated with the abovesaid membranes, the thickness of the carbon tubes being preferably of about 1 to about 3 mm and of the polyester tubes being preferably of about 100 to about 500 μ. 21. Reactor according to anyone of claims 17 to 20, wherein a recuperation column is integrated downstream of reactor, said column being such that it enables to recover metal which has been precipitated together with bacteria involved in said precipitation, said column being preferably filled with a material liable to adsorb metal and bacteria, such as glass beads, glass powder, glass wool or any form of silicate like sand particles. 22. Process for precipitating metals or degrading xenobiotic organic compounds wherein a nutrient is administered into one of the chambers of the reactor according to anyone of claims 17 to 21, and effluents to be treated containing metals or xenobiotic organic compounds are circulated in the other chamber of said reactor and wherein the bacteria in and on the membrane induce - either precipitation and crystallization processes which results in the settling of the metals, - or mineralization of xenobiotic organic compounds, for instance by co-metabolism of said xenobiotic compounds in the presence of a substrate, which results in degradation of said xenobiotic organic compounds into water, C0 2 . and a mineral salt originating from the inorganic ions of said xenobiotic organic compounds, and from said substrate, said process for precipitating metals or degrading xenobiotics being carried out continuously or in batch. 23. Process for precipitating metals according to claim 22, wherein the release of bacteria from the membrane into the effluent chamber is such that the bacteria come into contact with the metals in solution, and induce the precipitation and crystallization of the metals around them. 24. Process for precipitating metals according to claim 22, wherein the release of bacteria from the membrane into the effluent chamber is such that the bacteria come into contact with the metals in solution, induce the precipitation and crystallization of the metals around them, and wherein said bacteria are recovered together with crystallized metals by a column downstream of the reactor. 25. Process for degrading xenobiotic organic compounds according to claim 22, wherein a substrate for induction of co-metabolism is used for the mineralization of xenobiotics by the bacteria into water, C0 2 and either inorganic salts originating from inorganic ions of said xenobiotic organic compounds and of said substrate or gaz, or wherein no substrate is used and xenobiotic organic compounds in the solutions to be treated are mineralized into water, C0 2 and possibly CI " by the bacteria.

Inventors: DIELS LUDO, DOYEN WILLY, LEYSEN ROGER, MERGEAY MAXIMILIEN, VAN ROY SANDRA, DIELS, LUDO, DOYEN, WILLY, LEYSEN, ROGER, MERGEAY, MAXIMILIEN, VAN ROY, SANDRA
Applicants: VITO
Application year: 1992
Main classification: C02F3
IPC codes: B01D69, C02F3, C12M1