TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Culture: The microorganisms are selected from chemolithoautotrophic nitrifying organisms, such as ammonia and nitrite oxidants, e.g. bacteria of the nitrosomonas, nitrosococcus, nitrosospira, nitrosovibrio and nitrosospira types. Preferred species are Nitrosomonas halophila, Nitrosomonas eutropha, Nitrosomonas europaea, Nitrosomonas oligotropha, Nitrosomonas ureae, Nitrosomonas aestuarii Nitrosomonas marina, Nitrosomonas sp. 3 Nm 52, Nitrosomonas communis, Nitrosomonas nitrosa Nitrosomonas sp. 1 Nm 33, Nitrosomonas sp. 2 Nm 41 and Nitrosomonas cryotolerans. Also suitable are nitrobacter and nitrospira, especially Nitrobacter winogradskyi, heterotrophic nitrifying species, e.g. fungi of the aspergillus, penicillium and cephalosporium, types, algae, arthrobacter species, Alcaligenes faecalis, nocordia species, heterotrophic denitrifying species, e.g. paracocus and pseudomonas species. Any mixtures of these can be used, especially mixtures of (a) ammonia oxidizing and (b) nitrite oxidizing and optionally (c) nitrate- and nitrite-reducing bacteria. The bacteria are immobilized in the form of a cell suspension in a concentration of 5 x 108 - 6 x 109 cells/ml. If a mixture is used, the ratios of cell counts in the immobilizate are a:b 1:(10000-1), preferably 1:(1000-10) and b:c (1000-1):1, preferably (100-5):1.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Matrix: The bacteria are immobilized in a matrix of capsules, gel and/or gel beads, preferably with a particle diameter of 1-10000, especially 100-5000 microm. The matrix material is selected from natural and/or synthetic polymers and may have several layers. Preferred matrix materials include foliated and 3-dimensional framework silicate, preferably zeolites. The matrix may contain a buffer system, e.g. hydrogen carbonate/carbonate (HCO3-/CO32-), phosphoric acid/H2PO3-/HPO32-, hydrogen sulfate/sulfate HSO4-/SO42- or solid carbonate.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Matrix: The matrix may contain a buffer system, e.g. citric acid/citrate or lactic acid/lactate.    TECHNOLOGY FOCUS - POLYMERS - Preferred Matrix: Preferred matrix materials include polymeric polysaccharides, e.g. agarose or cellulose; proteins, e.g. gelatin, gum arabic, albumin or fibrinogen; ethyl-, methyl- and carboxymethylethyl-cellulose; cellulose acetate, acetate-butyrate and phthalate; alkali cellulose sulfate; polyaniline; polypyrrole; polyvinylpyrrolidone; polystyrene; polyvinyl chloride; polyvinyl alcohol; polyethylene; polypropylene; styrene/maleic anhydride copolymers; epoxide resins; polyethylene-imine; styrene/methyl methacrylate copolymers; polystyrenesulfonate; poly(meth)acrylates; polycarbonate; polyester; silicone; chitosan; polydialkyldimethylammonium chloride; and also mixtures of gelatin and water glass, polyphosphate or maleic anhydride/methyl vinyl ether copolymer; mixtures of polyacrylic acid and polydiallyldimethylammonium chloride; and any mixtures of the cited polymers. If the matrix is multi-layered, it preferably comprises gel-forming polymer, especially alginate and/or alginate derivative, and a film-forming polymer, especially alkali cellulose sulfate, polyethylene-imine and/or polydialkyldimethylammonium chloride. SPECIFIC MICROORGANISMS - Specific examples of the microorganisms are Nitrosomonas halophila, Nitrobacter winogradskyi and Alcaligenes faecalis.