A process and an associated facility for treating ammonium-containing wastewater are specified. Ammonium present in the wastewater is first oxidized to nitrite by use of aerobically oxidizing bacteria in an activation unit. Then ammonium and nitrite are reduced to elemental nitrogen anaerobically by use of ANAMMOX bacteria. Excess sludge arising in this operation is removed from the activation unit. ANAMMOX bacteria removed with the excess sludge are separated and returned to the activation unit. To facilitate the returning of the ANAMMOX bacteria, magnetic or magnetizable expanded glass particles are added, as colonization bodies for the ANAMMOX bacteria, to the wastewater in the activation unit. Expanded glass particles removed from the activation unit with the excess sludge are separated from the excess sludge magnetically and returned to the activation unit.