In this study, the current operation efficiency of the wastewater treatment plant (WWTP) in the city of Kłodzko in the Lower Silesia Voivodeship (Poland) was analyzed and the predicted amount of the wastewater flowing to the WWTP in a 20-year period (until the year 2036) was estimated. The data on the quality and the quantity of raw (entering to the WWTP) and treated sewage from the years 2012-2016 were analyzed. The five essential pollutants indicators: biochemical oxygen demand (BOD5), chemical oxygen demand (CODCr), total suspended solids (TSS), total nitrogen and total phosphorus, regulated in the water permit were used to evaluate the efficiency of contaminants removal. On the basis of the percentage reduction of the individual pollutants indicators and their acceptable concentrations in the treated sewage, the effectiveness of the operation of the object was determined. Furthermore, a so-called the treatment plant reliability factor (RF) was calculated and its value below 1.0 indicates the correct operation of the wastewater treatment plant. After determining the prospective amount of raw sewage in 2036 (based on the decreasing population in the city) it has been found that the WWTP will work with a significant reserve of capacity and the modernization of the treatment system, leading to a decrease of its capacity should be considered.