Extended loss (up to 70%) of nitrogen is observed in a nitrifying rotating biological contactor (RBC) treating ammonium - rich leachate of a hazardous waste landfill. Due to pretreatment (flocculation, BOD-removal, activated carbon) DOC was less than 20 mg/l so heterotrophic denitrification can 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 of three RBC compartments. An increasing partial pressure of oxygen and ammonium concentration favor nitrogen removal over ammonium oxidation. The reduction of nitrite produced in the aerobic biofilm layer close to the surface might therefore be coupled with ammonium oxidation and takes place in the deeper or temporarily anoxic layer of the biofilm.

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