The aim of this work was to determine the technological reliability of pollutions removal in different seasons in one-stage constructed wetland system with horizontal flow operating in the conditions of moderate climate. In the above-mentioned system, willow (Salix Viminalis L.) was planted and the average flow of domestic sewage were 1.2 m3/d. The analyzed system is located in south-eastern Poland, where the climate is moderate transitional. The tests were carried out during the 14-year exploitation of the wastewater treatment plant (1997–2010). During this period, sewage samples were collected in 4 seasons (winter – February, spring – may, summer – August and autumn – November) to be analyzed. The average long-term air temperatures in these months were respectively: −1.8; 13.8; 18.3 and 3.5 °C. Altogether 56 series of analyzes were carried out during the research and 112 samples of the sewage, both from the inflow and the outflow of the wastewater treatment plant, were collected. The measured parameters were: BOD5, COD, total nitrogen, total phosphorus. On the grounds of the reliability analysis performed on the basis of the Weibull probability model, it was found that the reliability of pollutions removal in the tested constructed wetland system are higher in summer and autumn (August, November) than in winter and in spring (February, May). It was shown that exceedance of the admissible values ​​of pollutant indicators in treated wastewater is dependent on the season (air temperature) and it occurred mainly in the winter period (February). It was proved that the tested CW does not provide effective elimination of biogenic indicators (nitrogen and phosphorus), as evidenced by small values ​​of reliability. To increase the technological reliability of the tested treatment plant, it is proposed to expand the existing system and create a hybrid system consisting of two beds with vertical and horizontal flow and a special P-filter.