The paper presents the results of a study of the efficiency and technological reliability of total nitrogen and total phosphorus removal during long term operation of a one-stage constructed wetland system with subsurface horizontal wastewater flow. The flow rate of the wastewater treatment plant was 1.2 m3/d during the research period. Physical and chemical analyses of raw wastewater and treated effluent were carried out in the years 1997–2010 (14 years). During this study period, 56 series of analyses were performed and 112 wastewater samples were collected. It was observed that the average efficiency of total nitrogen and total phosphorus removal amounted to 51% and 73%, respectively. The technological reliability, determined by means of the Weibull reliability method, was 45% for total nitrogen removal and 48% in the case of total phosphorus removal. Total nitrogen and total phosphorus concentrations in the treated wastewater did not exceed the permissible values in 39.3% and 37.5% of cases, respectively. A negative correlation was found between the sorption capacity of the soil-plant filter for the removal of total phosphorus and the time of plant operation. The efficiency of total nitrogen removal was stable over time.