An autotrophic, synthetic medium for the enrichment of anaerobic ammonium-oxidizing (Anammox) micro-organisms was developed. This medium contained ammonium and nitrite, as the only electron donor and electron acceptor, respectively, while carbonate was the only carbon source provided. Preliminary studies showed that the presence of nitrite and the absence of organic electron donors were essential for Anammox activity. The conversion rate of the enrichment culture in a fluidized bed reactor was 3 kg NH m d when fed with 30 mM NH . This is equivalent to a specific anaerobic ammonium oxidation rate of 1000-1100 nmol NH h (mg volatile solids). The maximum specific oxidation rate obtained was 1500 nmol NH h (mg volatile solids). Per mol NH oxidized, 0.041mol CO were incorporated, resulting in a estimated growth rate of 0.001 h. The main product of the Anammox reaction is N, but about 10% of the N-feed is converted to NO . The overall nitrogen balance gave a ratio of NH -conversion to NO -conversion and NO -production of 1:1-31++0.06:0.22+0.02. During the conversion of NH with NO , no other intermediates or end-products such as hydroxylamine, NO and NO could be detected. Acetylene, phosphate and oxygen were shown to be strong inhibitors of the Anammox activity. The dominant type of micro-organism in the enrichment culture was an irregularly shaped cell with an unusual morphology. During the enrichment for Anammox micro-organisms on synthetic medium, an increase in ether lipids was observed. The colour of the biomass changed from brownish to red, which was accompanied by an increase in the cytochrome content. Cytochrome spectra showed a peak at 470 nm gradually increasing in intensity during enrichment.


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