A lab-scale Rotating Biological Contactor (RBC) was operated with the purpose of oxygen-limited (autotrophic) nitrification-denitrification of an ammonium-rich synthetic wastewater without Chemical Oxygen Demand (COD). Based on the field observations that RBCs receiving anaerobic effluents come to anoxic ammonium removal, the RBC was inoculated with methanogenic sludge. Some 100 days after the addition of the anaerobic sludge to the reactor as a possible means of a rapid initiation of the nitrogen (N) removal process, a maximum ammonium removal of 1,550 mg N m(-2) d(-1) was achieved. Batch tests with 15N labeled ammonium and nitrite indicated that a large part of that N was removed via oxygen-limited oxidation of ammonium with nitrite as the electron acceptor. The other part was removed via conventional denitrification, presumably with COD released from lysis of cells. Species identification of the most abundant microorganisms revealed that Nitrosomonas spp. were the dominant ammonium-oxidizers in the sludge. Thus far, the molecular characterization of the sludge could not show the presence of Planctomycetes among the most dominant species. Overall this experiment confirms the property of the RBC system to remove ammonium to nitrogen gas without the use of heterotrophic carbon source.

Rotating Biological Contactors (RBC) treating highly nitrogenous wastewaters are known to give rise to nitrogen losses, generally assumed to be due to concomitant nitrification and denitrification. In this study, a lab-scale nitrifying RBC reactor was shown to bring about losses of nitrogen of the order of 10 to 20% at ammonium loading rates of up to 2129 mg N m(-2) d(-1), when no extra carbon source was added. These higher removal values could be due to Oxygen-Limited Autotrophic Nitrification and Denitrification (OLAND), because the reactor was operated at oxygen limitation. When methanol was added as a soluble organic carbon source to further diminish oxygen transfer into the deeper parts of the biofilm, the RBC achieved 84% nitrogen removal (loading rate 2300 mg N m(-2) d(-1)) at a COD/N ratio of 3.1. The latter also suggests that the process was occurring parallel to conventional denitrification.

The hypothesis is formulated that in case of oxygen limitation in the sediment, nitrifiers switch from nitrification to oxygen-limited autotrophic nitrification-denitrification (OLAND) in order to survive and maintain activity. During OLAND, ammonium is oxidised using nitrite as e-acceptor to form dinitrogen gas. As an additional advantage they benefit from the gaseous N(2) formed as a means of transport. In this way, the nitrifiers can move out of the sediment and rise through the water column towards more favourable conditions. At the surface, the bacteria could take up oxygen, and recommence nitrification. In order to test this hypothesis, nitrifying sediment with an overlaying water column was simulated in lab-scale columns. Nitrogen transformations and material transport through the water column were followed after addition of different forms of nitrogen under oxygen-limited conditions. (15)N-labelling experiments showed a large contribution of OLAND to the observed nitrogen deficits. Nitrifier enumerations, fluorescent in situ hybridisation and 16S rRNA gene analysis revealed increased populations of ammonia oxidising nitrifiers in the upper water layers. The results presented support the proposed hypothesis of transport using OLAND. Nitrifying activity in the sediment immediately recovered almost completely from prolonged oxygen-limited incubation when oxygen concentrations were increased.