Extensive loss of nitrogen (up to 70% of ammonium oxidized) is observed in a nitrifying rotating biological contactor (RBC) treating ammonium-rich leachate from a hazardous-waste landfill site. Due to pretreatment (precipitation, flocculation, sedimentation, BOD removal, activated carbon adsorption), the DOC is less than 20 mg/l. Heterotrophic denitrification can therefore be excluded. The nitrification rate reaches 3-4 g NH4-N m−2 d−1 at a pH of 7-7.3 in the first two compartments of the three-stage RBC. An increasing partial pressure of oxygen and ammonium concentration favor nitrogen removal over ammonium oxidation. The reduction of nitrite might therefore be strongly coupled with ammonium oxidation. The degradation mechanism is not yet known. Two possibilities are suggested: (i) nitrite produced in the aerobic biofilm layer close to the surface diffuses into deeper anoxic layers of the biofilm and is reduced in parallel with ammonium oxidation similarly to the Anammox process, or (ii) nitrite produced by Nitrosomonas is reduced by NADH2+ generated during hydroxylamine oxidation.

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