To date, two anaerobic ammonia-oxidizing bacteria species have been isolated and preliminarily classified in the order Planctomycetales as candidatus Brocardia anammoxidans and candidatus Kuenenia stuttgartiensis. Under anaerobic conditions ammonia is oxidized to nitrite in membrane-bound intracytoplasmic anammoxo-soms (Schmidt et al., 2003).
In the Anammox process ammonia is oxidized to nitrogen, with nitrite serving as electron acceptor (Van de Graaf et al., 1995). Use of N15 isotopes indicated that nitrite is reduced to hydroxylamine (Eq. 31), which then reacts with ammonia to yield hydrazine (N2H4, Eq. 32). By oxidation of hydrazine to molecular nitrogen (Eq. 33), four reducing equivalents are generated, which are required for nitrite reduction to hydroxylamine (Van de Graaf et al., 1997).
Since the redox state is balanced in the above reactions, reducing equivalents for CO2 reduction by the autotrophic microorganisms must be generated by oxidation of nitrite to nitrate (Eq. 34):
Per mol of ammonia, 0.2 mol nitrate is generated and 20 mg biomass is produced (Van de Graaf et al., 1996). The Anammox process seems to be suitable for nitrogen removal in ammonia-rich effluents of anaerobic reactors that are fed with wastewater rich in TKN compounds. Nitrogenous compounds can be eliminated from this wastewater by a combination of nitrification for nitrite supply and anaerobic ammonia oxidation (Strous et al., 1997). The process is sensitive to high, toxic nitrite concentrations of 70-180 mg N L-1 (depending on the kind of biomass). Furthermore, due to the low growth rate of the anammox bacteria, a startup time of 100-150 days with activated sludge as inoculum seems to be necessary (Schmidt et al., 2003).
In oxygen-limited environments (e.g. oxic-anoxic interfaces of biofilms or sludge flocs) aerobic and anaerobic ammonia oxidizers are natural partners. The ammonia is oxidized to nitrite and concomitantly the oxygen level is decreased. The anammox bacteria disproportionate the nitrite and the remaining ammonia to N2. When am monia is limited, the affinities of aerobic and anaerobic ammonia oxidizers may lead the natural partners to change to competitors (Schmidt et al., 2002). Other examples of 'uncontrolled' anammox activities were observed in different wastewater treatment plants with uncharacterized high ammonium losses.
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