Abstract: The invention relates to a process for the biological treatment of ammonium-rich aqueous media in the presence of ammonia oxidizing bacteria, by supplying nitrogen dioxide (NO2) into said aqueous media, thus effecting a nitrification and a denitrification, wherein further nitrogen oxide (NO) is supplied into said media such that the ratio of NO:NO2 is in the range from 1:2 to 1:500 (v/v). Further the use of off-gases in this process is disclosed as well as microorganisms cultured in the presence of NO and NO2.

Abstract: A method is described for the biological treatment of ammonium-rich wastewater in at least one reactor which has a temperature of at least 25° C., which involves the wastewater being passed through the said reactor(s) with a population, obtained by natural selection in the absence of sludge retention, in the suspended state of nitrifying and denitrifying bacteria to form, in a first stage with the infeed of oxygen, a nitrite-rich wastewater and by the nitrite-rich wastewater thus obtained being subjected, in a second stage without the infeed of oxygen, to denitrification in the presence of an electon donor of inorganic or organic nature, in such a way that the contact time between the ammonium-rich wastewater and the nitrifying bacteria is at most about two days, and the pH of the medium is controlled between 6.0 and 8.5 and the excess, formed by growth, of nitrifying and denitrifying bacteria and the effluent formed by the denitrification are extracted.

Abstract: A method is described for the biological treatment of ammonium-rich wastewater in at least one reactor which has a temperature of at least 25.degree. C., which involves the wastewater being passed through the said reactor(s) with a population, obtained by natural selection in the absence of sludge retention, in the suspended state of nitrifying and denitrifying bacteria to form, in a first stage with the infeed of oxygen, a nitrite-rich wastewater and by the nitrite-rich wastewater thus obtained being subjected, if required, in a second stage without the infeed of oxygen, to denitrification in the presence of a carbon source such as methanol, in such a way that the contact time between the ammonium-rich wastewater and the nitrifying bacteria is at most about two days, and the pH of the medium is controlled between 6.5 and 8.