The aim of this study was to assess the reliability of removal of nutrients nitrogen compounds using the Weibull reliability method. The studies, conducted during the 4 years period from January 2009 to December 2012 in the collective wastewater treatment plant. The designed size of the treatment plant, expressed in PE is 101200 residents. Seventy Three samples of raw and treated sewage samples were collected and subjected to physico-chemical analysis within the study period. Based on the results in raw wastewater it was found much higher than average concentration of total nitrogen. The high in equality concentrations of nitrogen compounds in wastewater influent had an impact on the volatility of nitrification and denitrification processes in a sequential biological reactor. As a result of the volatility process in biological reactor it was found, that 46% cases of concentration of total nitrogen in the waste waterflowing into the river was higher, than the limit specified in the law permit for this facility. In the paper proposes some solutions to the problems associated with the operation of the biological reactor, which in the future should result in increase defficiency of the elimination of nitrogen compounds in the treatment plant.