A new anaerobic process for ammonium oxidation, called Anammox, was discovered in a denitrifying fluidized bed reactor. This process uses nitrate as an electron acceptor to oxidize ammonium under anaerobic conditions, producing dinitrogen gas. The discovery was based on nitrogen and redox balances in continuous-flow experiments, showing that 3 mol of nitrate are required to oxidize 5 mol of ammonium, resulting in 4 mol of dinitrogen gas. Batch experiments confirmed that ammonium conversion depends on nitrate availability. The process was named Anammox and has been patented. The study highlights the potential of this biological process for nitrogen removal in wastewater treatment. The Anammox process was observed in a reactor treating effluent from a methanogenic reactor, where ammonium and nitrate concentrations decreased, and gas production increased. The process was confirmed through fed-batch experiments, where ammonium and nitrate were simultaneously converted to dinitrogen. The results indicate that the observed ammonium loss can be explained by anaerobic ammonium oxidation. This discovery fills a gap in understanding of nitrogen cycling in nature and has implications for wastewater treatment technologies. The Anammox process has been patented and further research is needed to isolate the responsible microorganisms and apply this process for wastewater treatment.A new anaerobic process for ammonium oxidation, called Anammox, was discovered in a denitrifying fluidized bed reactor. This process uses nitrate as an electron acceptor to oxidize ammonium under anaerobic conditions, producing dinitrogen gas. The discovery was based on nitrogen and redox balances in continuous-flow experiments, showing that 3 mol of nitrate are required to oxidize 5 mol of ammonium, resulting in 4 mol of dinitrogen gas. Batch experiments confirmed that ammonium conversion depends on nitrate availability. The process was named Anammox and has been patented. The study highlights the potential of this biological process for nitrogen removal in wastewater treatment. The Anammox process was observed in a reactor treating effluent from a methanogenic reactor, where ammonium and nitrate concentrations decreased, and gas production increased. The process was confirmed through fed-batch experiments, where ammonium and nitrate were simultaneously converted to dinitrogen. The results indicate that the observed ammonium loss can be explained by anaerobic ammonium oxidation. This discovery fills a gap in understanding of nitrogen cycling in nature and has implications for wastewater treatment technologies. The Anammox process has been patented and further research is needed to isolate the responsible microorganisms and apply this process for wastewater treatment.