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An autotrophic, synthetic medium for the enrichment of anaerobic ammonium-oxidizing (Anammox) micro-organisms was developed. This medium contained ammonium and nitrite, as the only electron donor and electron acceptor, respectively, while carbonate was the only carbon source provided. Preliminary studies showed that the presence of nitrite and the absence of organic electron donors were essential for Anammox activity. The conversion rate of the enrichment culture in a fluidized bed reactor was 3 kg NH₄ ⁺ m₋₃ d₋₁ when fed with 30 mM NH₄ ⁺. This is equivalent to a specific anaerobic ammonium oxidation rate of 1000–1100 nmol NH₄ ⁺h⁻¹ (mg volatile solids)⁻¹. The maximum specific oxidation rate obtained was 1500 nmol NH₄ ⁺h⁻¹ (mg volatile solids)⁻¹. Per mol NH₄ ⁺ oxidized, 0.041mol CO₂ were incorporated, resulting in a estimated growth rate of 0.001 h₋₁. The main product of the Anammox …