Nitrification of ammonium nitrogen and urea nitrogen was studied in a submerged biofilm reactor. The reactor was operated as a fed-batch reactor to which an excessive mass loading was applied. The increases in TKN, ammonium-N, NOx-N and NO2-N concentrations inside the reactor were investigated to assess inhibitory behaviour under non-steady state conditions. The maximum ammonium oxidation rate was about 60 mg NH4-N/h that corresponded to a surface removal rate of 8.5 g NH4-N/m2.d. The major factor for nitrite accumulation was the presence of free ammonia. In the oxidation of urea where the free ammonia concentrations were very small no nitrite accumulation was observed. Overall, the results showed that the fed-batch reactor technique could be applied to a nitrifying biofilm reactor to evaluate the effect of slug doses in a short time period.

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