This paper reports the discovery of anaerobic ammonium oxidation (Anammox) in a denitrifying fluidized bed reactor treating effluent from a methanogenic reactor. The study found that NH4+ was disappearing from the reactor, with both nitrate and ammonium consumption increasing, leading to gas production. A maximum ammonium removal rate of 0.4 kg N·m⁻³·d⁻¹ was observed. The evidence for Anammox was based on nitrogen and redox balances in continuous-flow experiments, showing that 3 mol of nitrate were required to oxidize 5 mol of ammonium, producing 4 mol of dinitrogen gas. Batch experiments confirmed that NH4+ conversion was nitrate-dependent. The process, named 'Anammox,' involves the oxidation of ammonium with nitrate as the electron acceptor under anaerobic conditions, producing dinitrogen gas. This biological process has been patented. The study highlights the potential of Anammox for wastewater treatment, particularly in reducing nitrogen pollution.This paper reports the discovery of anaerobic ammonium oxidation (Anammox) in a denitrifying fluidized bed reactor treating effluent from a methanogenic reactor. The study found that NH4+ was disappearing from the reactor, with both nitrate and ammonium consumption increasing, leading to gas production. A maximum ammonium removal rate of 0.4 kg N·m⁻³·d⁻¹ was observed. The evidence for Anammox was based on nitrogen and redox balances in continuous-flow experiments, showing that 3 mol of nitrate were required to oxidize 5 mol of ammonium, producing 4 mol of dinitrogen gas. Batch experiments confirmed that NH4+ conversion was nitrate-dependent. The process, named 'Anammox,' involves the oxidation of ammonium with nitrate as the electron acceptor under anaerobic conditions, producing dinitrogen gas. This biological process has been patented. The study highlights the potential of Anammox for wastewater treatment, particularly in reducing nitrogen pollution.