The performance of three constructed wetland systems treating landfill leachate, two located in northern Poland (Szadółki near Gdańsk and Gatka near Miastko) and one in southern Sweden (Örebro), is discussed. The CWs differ in size, hydraulic regime, type of hydrophytic plants, and type of leachate pre-treatment before discharging to the CW. The CW in Szadółki consists of two parallel beds with sub-surface, horizontal flow of leachate (HSSF) planted with reed. The leachate is discharged to the CW without pre-treatment. The facility in Gatka is a willow plantation that receives leachate after preliminary sedimentation in a retention pond. The system at Örebro consists of a series of ponds with a surface flow of leachate (the free water surface - FWS wetland), preceded by pre-treatment in an aerated tank with nitrogen stripping. A comparison of treatment conditions and results is presented. The best treatment efficiencies were observed at the CW Örebro (98% TSS, 91% BOD, 65% COD, 99.5% N-NH₄⁺), which resulted from:
(i) an effective pre-treatment of leachate before it was discharged to the CW, and
(ii) the CW type – with surface flow of leachate (FWS).
At the sub-surface flow wetlands (Szadółki, Gatka), clogging problems occurred due to lack of or unsatisfactory pre-treatment of leachate, resulting in lower treatment efficiencies. The ammonia nitrogen was still removed at the clogged facilities with the efficiencies varying from 52 to 89%, while the organics removal efficiencies were substantially lower (27-61% for BOD₅ and 2-35% for COD).