AUTOTROPHIC GROWTH OF ANAEROBIC AMMONIUM-OXIDIZING MICROORGANISMS IN A FLUIDIZED-BED REACTOR

An autotrophic, synthetic medium for the enrichment of anaerobic ammon ium-oxidizing (Anammox) micro-organisms was developed. This medium con tained ammonium and nitrite, as the only electron donor and electron a cceptor, respectively, while carbonate was the only carbon source prov ided. 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 rea ctor was 3 kg NH4+ m(-3) d(-1) when fed with 30 mM NH4+. This is equiv alent to a specific anaerobic ammonium oxidation rate of 1000-1100 nmo l NH4+ h(-1) (mg volatile solids)(-1). The maximum specific oxidation rate obtained was 1500 nmol NH4+ h(-1) (mg volatile solids)(-1). Per m ol NH4+ oxidized, 0.041 mol CO2 were incorporated, resulting in a esti mated growth rate of 0.001 h(-1). The main product of the Anammox reac tion is N-2, but about 10% of the N-feed is converted to NO3-. The ove rall nitrogen balance gave a ratio of NH4+-conversion to NO2--conversi on and NO3--production of 1:1.31 +/- 0.06:0.22 +/- 0.02. During the co nversion of NH4+ with NO2-, no other intermediates or end-products suc h as hydroxylamine, NO and N2O 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 enrich ment for Anammox micro-organisms on synthetic medium, an increase in e ther lipids was observed. The colour of the biomass changed from brown ish to red, which was accompanied by an increase in the cytochrome con tent. Cytochrome spectra showed a peak at 470 nm gradually increasing in intensity during enrichment.