Competition of Phragmites australis and Phalaris arundinacea in constructed wetlands with horizontal subsurface flow - does it affect BOD5, COD and TSS removal?

Phalaris arundinacea and Phragmites australis are frequently planted together in constructed wetlands for wastewater treatment in the Czech Republic. However, it has been observed that Phragmites usually outcompetes Phalaris over time but the speed of this process varies among systems. In this paper we evaluated the process of Phragmites encroachment into Phalaris stands in two constructed wetlands with horizontal subsurface flow. The results revealed that 50% of the area originally planted with Phalaris was overgrown by Phragmites in 5 and 9 years in the monitored systems while the complete elimination of Phalaris occurred after 9 and 13 years. Replacement of Phalaris did not have any effect on removal of BOD5 and COD while removal efficiency of TSS increased after Phalaris was replaced with Phragmites. However, it is difficult to determine out whether replacement was the only reason for better TSS removal.