The possibility of enhancing the denitrification of a newly established LECA-based horizontal subsurface flow (HSSF) soil filter receiving pretreated wastewater from a vertical flow filter was studied. The pilot-scale experiment offers evidence regarding the survival and reproduction of introduced microbes taken from an LECA-based HSSF constructed wetland (CW) that has similar internal conditions, after bioaugmentation into newly established LECA-based HSSF CW mesocosms. Bioaugmentation resulted in a trend towards higher and more stable denitrification in the supplemented mesocosms during the nearly half-year study period. (C) 2009 Elsevier Ltd. All rights reserved.