Partial nitrification to nitrite has three practical advantages: lower oxygen consumption, lower need for organics and lower sludge production. In order to develop a stable partial nitrification a novel operational strategy was studied in a sequencing batch rotating disk reactor under oxygen concentrations lower than 1.0 mg/L throughout 270 days. The strategy was based on a supervisory pH control and an automatic interruption of aeration at the endpoint of ammonia oxidation. The supervisory control enabled the maintenance of a concentration of 3-4 mg NH3-N/L for optimal growing of ammonia oxidizing bacteria. For this reason on-line monitoring of sodium carbonate consumption was implemented during nitrification. The results showed that it is possible to reach a stable partial nitrification with high nitrite accumulations of 84-88% during long-term assays, and a relatively high ammonia conversion rate of 1.45-4.25 kg NH4+-N/(m2 day). A high impact was observed by decreasing the oxygen concentration from 1.0 to 0.8 mg O2/L; the ammonium removal rate declined significantly from 4.25 to 1.62 kg/(m2 day). The molecular analyses by dot-blot hybridizations with 16S rRNA revealed the presence of more than 95% of Nitrosomonas sp., and only 5% of Nitrobacter sp. during the initial phase of the biofilm formation.