NOVELTY - Operating biogas plant involves: (i) fractionating organic waste material with ammonium nitrogen into solid and liquid fractions; (ii) leading solid fraction to biogas reactor; (iii) removing ammonium nitrogen from liquid fraction by applying the fraction to organic, synthetic ion exchanger adsorbing (more than 1.2, preferably more than 2) equivalents (eq.)/l, and allowing ammonium nitrogen from liquid fraction to adsorb to ion exchanger; and (iv) collecting from exchanger, permeate from which ammonium nitrogen has been extensively removed and leading portion of permeate to the reactor.    USE - For operating a biogas plant (claimed) used as central fertilizer distribution and exchange facility, and for transferring nitrogen, phosphorus and potassium in degassed manure from pigs, cattle and poultry.    ADVANTAGE - The method is an efficient, simple and environment-friendly procedure for controlling the level of ammonium in biogas plants to enhance its productivity.    DETAILED DESCRIPTION - Operating a biogas plant, involves: (i) fractionating an organic waste material with a content of ammonium nitrogen into a solid fraction and a liquid fraction; (ii) leading the solid fraction to a biogas reactor; (iii) removing ammonium nitrogen from the liquid fraction by: a) applying the liquid fraction to an organic, synthetic ion exchanger adsorbing, in use, more than 1.2 equivalents (eq.)/l, preferably more than 2 eq./l; b) allowing ammonium nitrogen from the liquid fraction to adsorb to the ion exchanger, where the concentration of ammonium nitrogen in the liquid fraction exceeds 2 g/l at the time of application of the waste water to the ion exchanger, and the liquid fraction has a content of organic matter of not greater than 8 wt./wt.% at the time of application of the liquid fraction to the ion exchanger, where the organic matter is dissolved or is in particles of a maximum extension of 25 mu m; and (iv) collecting from the ion exchanger, a permeate from which ammonium nitrogen has been extensively removed and leading a portion of the permeate to the biogas reactor; and optionally (v) regenerating the ion exchanger with a solution of sodium nitrite (NaNO3) of a molality of 3 mol/kg to full saturation and at a temperature of 5-40 degrees C, and/or with a solution of sodium carbonate (Na2CO3) of a molality of 1 mol/kg to full saturation and at a temperature of 5-40 degrees C, and/or with a solution of sodium chloride (NaCl) of a molality of 3 mol/kg to full saturation and at a temperature of 5-40 degrees C, and/or with a solution of sodium sulfate (Na2SO4) of a molality of 1 mol/kg to full saturation and at a temperature of 30-40 degrees C, and/or with a solution of potassium carbonate (K2CO3) of a molality of 4 mol/kg to full saturation and at a temperature of 5-40 degrees C, and/or with a solution of potassium hypophosphate (K2HPO4) of a molality of 4 mol/kg to full saturation and at a temperature of 5-40 degrees C.