A full-scale hybrid constructed wetland (CW) wastewater treatment plant (Qav.d.=74 m3∙d-1) was analyzed in terms of its operational efficiency and reliability in the start-up period (12 months, from April 2023 to March 2024). The focus was only on the biological part consisting of 2 parallel systems of CW beds - with vertical flow (VF) and horizontal flow (HF). Three sampling points were chosen, i.e. mechanically treated wastewater, effluent from the VF-type beds and effluent from the HF-type beds. Several pollution indicators were measured: BOD5, COD, TSS, TP and TN. The studied CW ensured high efficiency of wastewater treatment which reached: 96% for BOD5, 89% for COD, 82% for TSS, 41% for TP and 50% for TN. At higher air temperatures in the spring-summer season (the average from April to September was 16.8°C), higher removal rates of pollutants expressed by COD, TP, and TN in the VF bed and TSS reduction in the HF bed were achieved compared to the autumn-winter period, when the average temperature from October to March was 3.9°C. This indicates that large-scale constructed wetland wastewater treatment plants serving entire communities (as opposed to small household systems) are more vulnerable to seasonal fluctuations in air temperature, leading to reduced pollutant removal efficiency during cold periods. In this paper it was shown that the higher values of the BOD5/TN and BOD5/TP did not increase the susceptibility of wastewater to decomposition of nitrogen and phosphorus compounds. Further analyses should be carried out to assess how the functioning of the facility changes over time, after the start-up period.