J. Gijs Kuenen

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.

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 NHf m-3 d-l when fed with 30 mM NHf. This is equivalent to a specific anaerobic ammonium oxidation rate of 1000-1100 nmol NH; h-l (mg volatile solids)-l. The maximum specific oxidation rate obtained was 1500 nmol NH; h-l (mg volatile solids)-'. Per mol NH; oxidized, 0041mol 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 NOS-production of 1 :la31 &0-06:022+002. During the conversion of NH; with NO; , no other intermediates or end-products such as hydroxylamine, NO and N, O 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.

\n Contains fulltext :\n 84019.pdf (Publisher’s version ) (Closed access)\n