The first experiments using wetland macrophytes for wastewater treatment were carried by out by Kathe Seidel in Germany ¨
in early 1950s. The horizontal sub-surface flow constructed wetlands (HF CWs) were initiated by Seidel in the early 1960s
and improved by Reinhold Kickuth under the name Root Zone Method in late 1960s and early 1970s and spread throughout
Europe in 1980s and 1990s. However, cohesive soils proposed by Kickuth got clogged very quickly because of low hydraulic
permeability and were replaced by more porous media such as gravel in late 1980s in the United Kingdom and this design
feature is still used. In fact, the use of porous media with high hydraulic conductivity was originally proposed by Seidel. HF
CWs provide high removal of organics and suspended solids but removal of nutrients is low. Removal of nitrogen is limited by
anoxic/anaerobic conditions in filtration beds which do not allow for ammonia nitrification. Phosphorus removal is restricted
by the use of filter materials (pea gravel, crushed rock) with low sorption capacity. Various types of constructed wetlands may
be combined in order to achieve higher treatment effect, especially for nitrogen. However, hybrid systems are comprised most
frequently of vertical flow (VF) and HF systems arranged in a staged manner. HF systems cannot provide nitrification because
of their limited oxygen transfer capacity. VF systems, on the other hand, do provide a good conditions for nitrification but no
denitrification occurs in these systems. In hybrid systems (also sometimes called combined systems) the advantages of the HF
and VF systems can be combined to complement processes in each system to produce an effluent low in BOD, which is fully
nitrified and partly denitrified and hence has a much lower total-N outflow concentrations.
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