“…The inadequate oxygen transfer of typical subsurface flow wetlands resulted in the progress of improved treatment systems, which are able to assure adequate oxygen levels for nitrification, degradation of organic matter, and prevention of P leaching. The systems include introduction of oxygen to the wetland by means of regular water level oscillations, passive air pumps or powered mechanical aeration of the reed bed (Nivala et al, 2007). Yet, the elimination of nitrogen in ET has been enhanced by the different flows, cascades, open areas, and with the application of recirculation of the treated outflow back to the inflow (Griessler Bulc & Šajn-Slak, 2009;Griessler Bulc et al, 2011).…”