Until now, oxidation of ammonium has only been known to proceed under aerobic conditions. Recently, we observed that NH4+ was disappearing from a denitrifying fluidized bed reactor treating effluent from a methanogenic reactor. Both nitrate and ammonium consumption increased with concomitant gas production. A maximum ammonium removal rate of 0.4 kg N · m−3 · d−1 (1.2 mM/h) was observed. The evidence for this anaerobic ammonium oxidation was based on nitrogen and redox balances in continuous‐flow experiments. It was shown that for the oxidation of 5 mol ammonium, 3 mol nitrate were required, resulting in the formation of 4 mol dinitrogen gas. Subsequent batch experiments confirmed that the NH4+ conversion was nitrate dependent. It was concluded that anaerobic ammonium oxidation is a new process in which ammonium is oxidized with nitrate serving as the electron acceptor under anaerobic conditions, producing dinitrogen gas. This biological process has been given the name ‘Anammox” (anaerobic ammonium oxidation), and has been patented.

Until now, oxidation of ammonium has only been known to proceed under aerobic conditions. Recently, we observed that NH: was disappearing from a denitrifying fluidized bed reactor treating effluent from a methanogenic reactor. Both nitrate and ammonium consumption increased with concomitant gas production. A maximum ammonium removal rate of 0.4 kg N. me3 . d-' (1.2 mM/h) was observed. The evidence for this anaerobic ammonium oxidation was based on nitrogen and redox balances in continuous-flow experiments. It was shown that for the oxidation of 5 mol ammonium, 3 mol nitrate were required, resulting in the formation of 4 mol dinitrogen gas, Subsequent batch experiments confirmed that the NH: conversion was nitrate dependent. It was concluded that anaerobic ammonium oxidation is a new process in which ammonium is oxidized with nitrate serving as the electron acceptor under anaerobic conditions, producing dinitrogen gas.This biological process has been given the name 'Anammox' (anaerobic ammonium oxidation), and has been patented.

A newly discovered process by which ammonium is converted to dinitrogen gas under anaerobic conditions (the Anammox process) has now been examined in detail. In order to confirm the biological nature of this process, anaerobic batch culture experiments were used. All of the ammonium provided in the medium was oxidized within 9 days. In control experiments with autoclaved or raw wastewater, without added sludge or with added sterilized (either autoclaved or gamma irradiated) sludge, no changes in the ammonium and nitrate concentrations were observed. Chemical reactions could therefore not be responsible for the ammonium conversion. The addition of chloramphenicol, ampicillin, 2,4-dinitrophenol, carbonyl cyanide m-chlorophenylhydrazone (CCCP), and mercuric chloride (Hg II Cl 2) completely inhibited the activity of the ammoniumoxidizing sludge. Furthermore, the rate of ammonium oxidation was proportional to the initial amount of sludge used. It was therefore concluded that anaerobic ammonium oxidation was a microbiological process. As the experiments were carried out in an oxygen-free atmosphere, the conversion of ammonium to dinitrogen gas did not even require a trace of O 2. That the end product of the reaction was nitrogen gas has been confirmed by using 15 NH 4 ؉ and 14 NO 3 ؊. The dominant product was 14-15 N 2. Only 1.7% of the total labelled nitrogen gas produced was 15-15 N 2. It is therefore proposed that the N 2 produced by the Anammox process is formed from equimolar amounts of NH 4 ؉ and NO 3 ؊ .

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In this paper the sustainability of current available and future nitrogen removal systems has been investigated. For the assessment of the sustainability six indicators were used; sludge production; energy consumption, resource recovery; area requirement and N2O-emission. For the evaluation of the position of the individual nitrogen removal systems in the anthropogenic nitrogen cycle a broad outline for a life-cycle analysis has been presented.

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