The subject of the study was to evaluate the stability and reliability of the sewage treatment plant in Nowy Sącz. The scope of the analysed indicators included the main indicators of wastewater contamination: BOD5 (biochemical oxygen demand), CODCr (chemical oxygen demand), total suspended solids (TSS), total nitrogen (Ntot), and total phosphorus (Ptot). The operation stability of the sewage treatment plant in Nowy Sącz was determined on the basis of control cards x for 24 observations made in the period 2018–2019 (2 years). Moreover, the technological reliability of the tested sewage treatment plant (WN) was determined based on the values of the analysed pollution indicators in treated sewage and their permissible values. On the basis of the conducted analyses, full stability of the removal process of most of the analysed contaminants was found. In no case was there any crossing of the control lines, only a single grouping of samples above the help line in the case of total nitrogen, which could indicate a periodical disturbance in the stability of the removal process of this compound. On the basis of the obtained values of the reliability coefficient, which were below WN = 1.00, reliable operation of the analysed facility was found, with a high degree of reduction (ƞ) of the analysed pollutants. The method of determining the technological reliability and stability of the treatment plant with the use of control cards is an effective and easy tool for detecting any disturbances and instabilities in the processes taking place in the tested facility. It enables the operator to take quick action to remove them, thus ensuring a safe wastewater treatment process for the environment and human health.