The aim of the study was to determine the technological reliability and to forecast the efficiency of wastewater treatment in a constructed wetland wastewater treatment plant (CW WWTP) in long-term operation. The research was conducted at a facility which in the mechanical part had a 3-chamber settling tank and the biological stage consisted of 4 constructed wetland beds with horizontal sewage flow, planted with common reed (Phragmites australis). During 12 years of the study, the WWTP was hydraulically loaded up to 97.7% of the designed mean daily inflow, equal to Qd,av. = 116 m3/d. The study showed that despite the high degree of reduction of such parameters as biochemical oxygen demand (BOD5) (90.0%), chemical oxygen demand (COD) (88.4%) and total suspended solids (TSS) (93.5%) the achieved reliability with respect to the limit values determined by the Weibull distribution model amounted to 48%, 62% and 77% for BOD5, COD and TSS, respectively. In the case of biogenic parameters, the technological reliability with respect to permissible values was 3% for total nitrogen and 13% for total phosphorus. Based on the performed analysis, it was concluded that the analyzed technological system did not guarantee the concentrations of pollutants in treated wastewater to reach the admissible levels defined in the related Polish legal act. In order to increase the efficiency of pollutants removal in the discussed wastewater treatment plant, the application of additional constructed wetland bed with vertical sewage flow (VF) preceding the horizontal beds (HF) was suggested.