The aim of this article is to define the possibilities of applying multivariate statistical analysis (PCA and control charts) in the monitoring of the effectiveness of biological nitrification in a wastewater treatment plant working for the municipality of Sanok. The difference in oxygen affinity between ammonium and nitrite oxidizers results in a bacteria competition between AOM and NOM. A more stable nitrification process was obtained in reactor I for mean oxygen concentration of 1.13-2.05 mgO₂·dm^-3. The lowest mean concentrations of ammonia nitrogen were obtained in the range 3.43-3.62 mgN-NH₄⁺·dm^-3. Reactor II worked at mean oxygen concentration 1.69-4.56 mgO₂·dm^-3, which caused lower stability in this study period. The mean concentration of ammonium nitrogen ranged from 4.06 to 9.08 mgN-NH₄⁺·dm^-3. April 2016 was considered the most stable period of work of nitrification reactors. In that month, in reactor I the upper specification limit USL was not exceeded. In reactor II the USL was exceeded only 10% of the time. The index of the process capacity C_pk was higher for reactor I, and achieved a value of 1.71. The process of nitrification in both reactors was qualified as stable when oxygen concentration was between 1 and 2 mgO₂·dm^-3.