The study aimed to assess the operation of the mechanical and biological wastewater treatment plant in Michów (Poland) in terms of the effectiveness of removing contaminants from mixed wastewater (dairy and domestic) and technological reliability. The wastewater treatment plant is owned by the Dairy Cooperative "Michowianka." It is intended to treat mixed sewage, consisting of technological and sanitary sewage as well as part of rainwater from the plant, domestic wastewater supplied by a sewerage system from Michów, and wastewater delivered from septic tanks. In 2017 and in the first quarter of 2018, the permeate from the whey thickening process was also sent to the treatment plant. The period 2017-2021 was covered by the analysis. The analysis included the indicators from the fundamental group (TSS, BOD5, COD), biogenic indices (total phosphorus, total nitrogen, and ammonium nitrogen). The applied technology ensured the removal of total suspended solids, BOD5, and COD at the level of over 96%. Total phosphorus was removed with an average efficiency of 91%. Slightly lower effects were found in the elimination of total nitrogen — 78.5% on average, while the ammonium nitrogen removal rate was 88% on average. Due to the high efficiency of the facility, the values of the standard contamination indicators at the outflow, as a rule, met the requirements specified in the water permit. The technological reliability of the wastewater treatment plant in Michów, determined by the Weibull method, was at a high level, usually exceeding 90%. The reliability analysis shows that the facility in Michów has a high capacity to treat wastewater to the extent required for the wastewater receiver, and that the treated wastewater discharge does not cause negative changes in the environment. It indicates a high probability of obtaining the wastewater quality at the outflow from the treatment plant meeting the water permit requirements.