Nitrogen removal is the most complicating factor in urban wastewater treatment. Nitrification accounts for more than 50% of the oxygen need and requires long sludge ages. Denitrification is often hampered by a lack of COD. In fact it would be better to use this COD to generate methane. Recent research has shown that it is possible to remove ammonium by anaerobic ammonium oxidation leading to a much reduced need for aeration energy, no need for COD in the N-removal, and a considerable lower sludge production. The state of the art and the potential role in urban wastewater treatment are discussed.

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