Autotrophic growth of anaerobic ammonium-oxidizing micro-organisms in a fluidized bed reactor

Abstract

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 NH4 + m−3 d−1 when fed with 30 mM NH4 +. This is equivalent to a specific anaerobic ammonium oxidation rate of 1000–1100 nmol NH4 +h−1 (mg volatile solids)−1. The maximum specific oxidation rate obtained was 1500 nmol NH4 +h−1 (mg volatile solids)−1. Per mol NH4 + oxidized, 0.041mol CO2 were incorporated, resulting in a estimated growth rate of 0.001 h−1. The main product of the Anammox reaction is N2, but about 10% of the N-feed is converted to NO3 −. The overall nitrogen balance gave a ratio of NH4 −-conversion to NO2 −-conversion and NO3 −-production of 1:1·31±0·06:2·02±0·02. During the conversion of NH4 + with NO2 −, no other intermediates or end-products such 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 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.