The aim of the research was to determine the efficiency and reliability of the removal of pollutants from wastewater by the mechanical–biological treatment plant A2/O. The system disposed of on average 4,000–5,000 m3 /d of wastewater, of which about 320 m3 /d was industrial wastewater. The evaluation was based on data from 5 y (2015–2019) of the work of this facility. The following pollution indicators were analyzed: biochemical demand for oxygen (BOD5 ), chemical oxygen demand (COD), total suspended solids (TSS), total nitrogen (TN), total phosphorus (TP). The efficiency of wastewater treatment during the period under study was good. The pollutant reduction coefficients averaged over the years 2015–2019 were: for TSS – 95%, for BOD5 – 98%, COD – 93% for TN – 82%, and TP – 88%. The results of the Weibull-based reliability analysis in the case of TSS and COD indicated that the treatment plant analyzed during the whole test period worked with high efficiency. The study showed that in the case of BOD5 there were 25 d in the year in which the limit may be exceeded. Studies have shown that 44% of collected samples of treated wastewater exceeded total nitrogen concentrations of 15 mg/L. Thus, effluent discharged to the receiver from the treatment plant analyzed, on approximately 160 d during a year total nitrogen concentrations are above limit values. On this basis, it can be concluded that the treatment plant under examination does not comply with the requirements of the Regulation for almost half of the year. The cause of this condition is the instability of nitrification and denitrification processes which are responsible for the removal of nitrogen compounds in a biological reactor. In the case of the treatment plant under consideration, the reliability of the removal of general phosphorus on 55 d/y is reduced, thus the values obtained during these days are at a level that does not meet the reliability requirements.