The aim of the research was to determine, using statistical methods, the interdependence between COD and such parameters as: BOD5, TN and TP in raw wastewater and in mechanically treated wastewater, which would allow to estimate the values of these parameters based on COD. Moreover, the aim of the study was to determine the susceptibility of wastewater before and after mechanical treatment to decomposition of organic and biogenic pollutants. The research was conducted since 2006 to 2015 in a collective wastewater treatment plant in Nowy Sącz. During the research period, 120 samples of raw and mechanically treated wastewater were collected and analyzed once a month. In the analytical part of the research the quality of sewage was characterized, the strength of correlation of the examined indicators was determined and the susceptibility of sewage to biological decomposition was assessed. On the basis of the analysis it was found that knowing the value of the COD parameter it is possible to determine the values of the other indicators, i.e. BOD5, TN and TP. Moreover, it was shown that wastewater is largely susceptible to biological decomposition of pollutants, and during the period when there occurs the inflow of sewage with reduced susceptibility, wastewater with high content of organic carbon should be dosed.