Nitrogen conversion processes are essential for most wastewater treatment systems. Due to the many possible conversions, and the complexity of analysing the reactions under actual conditions, there is much room for confusion. This review provides an overview of the possible microbiological nitrogen conversions described in literature. The relevance of these conversions with respect to wastewater treatment processes is discussed. Rates described for aerobic denitrification or denitrification by autotrophic nitrifiers are so low that these conversions probably do not play a significant role under practical conditions. Heterotrophic nitrification may be of relevance only when the wastewater contains a high COD/N ratio (>10). Anaerobic ammonium oxidation can occur in fully autotrophic systems with very long sludge retention times or biofilm systems. This conversion offers great opportunities since it allows us to denitrify with ammonium as electron donor, i.e. no organic substrate is needed in the nitrogen removal process.

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