Two biolog. wastewater treatment plants with hybrid hydrophyte systems located at Wieszyno and Wiklino, PL (vertical flow constructed wetlands) were analyzed for efficiency of removal of impurities. The efficiency depended on quality and hight of org. matter (COD values 8.0 and 31.0 g/m(2)d, resp.). The Wieszyno plant was less efficient (COD redn. by 41.2% to 47.4% only) than that of Wiklino (COD redn. by 80.1% to 89.2%) because of lower oxygenation intensity (O(2) diffusion coefs. 0.0132 and 0.0552 cm/s, resp.).