In North America, FWS CWs started with the ecological engineering of natural wetlands for
wastewater treatment at the end of the 1960s and beginning of the 1970s [8-10]. This treatment
technology was adopted in North America not only for municipal wastewaters but all kinds of
wastewaters [11]. Subsurface flow technology spread more slowly in North America but, at present,
thousands of CWs of this type are in operation [11].
Various types of constructed wetlands may be combined in order to achieve higher treatment effect,
especially for nitrogen. Hybrid systems comprise most frequently VF and HF systems arranged in a
staged manner but, in general, all types of constructed wetlands could be combined in order to achieve
more complex treatment efficiency [12].