This survey of 60 hybrid constructed wetlands in 24 countries, finds that these systems are more efficient than single horizontal flow (HF) or vertical flow (VF) units for total nitrogen removal from wastewater.
Hybrid constructed wetlands (CWs) were developed in the 1960s, but their use has only really increased since the late 1990s, mostly because of more stringent discharge limits for nitrogen and also more complex wastewaters treated in constructed wetlands.
Early hybrid CWs consisted of several stages of vertical flow (VF) followed by several stages of horizontal flow (HF) beds. During the 1990s, HF–VF and VF–HF hybrid systems were introduced.
However, to achieve higher removal of total nitrogen or to treat more complex industrial and agricultural wastewaters, other types of hybrid constructed wetlands – including free water surface (FWS) CWs and multistage CWs – have also been used recently.
This review paper by Jan Vymazal at the Czech University of Life Sciences Prague surveys 60 hybrid constructed wetlands in 24 countries reported since 2003. Hybrid CWs are primarily used in Europe and Asia, with only limited use on other continents.
The most commonly used hybrid system is a VF–HF constructed wetland, which has been used for treatment of both sewage and industrial wastewaters. On the other hand, the use of a HF–VF system has been reported only for treatment of municipal sewage.
Out of 60 surveyed hybrid systems, 38 have been designed to treat municipal sewage, while 22 hybrid systems were designed to treat various industrial and agricultural wastewaters.
More detailed analysis reveals that VF–HF hybrid constructed wetlands are slightly more efficient in ammonia removal than hybrid systems with free water surface (FWS) CWs, HF–VF systems, or multistage VF and HF hybrid CWs.
All types of hybrid CW are comparable with single VF CWs in terms of NH4-N removal rates. On the other hand, CWs with FWS units remove substantially more total nitrogen than other types of hybrid constructed wetlands.
However, all types of hybrid constructed wetlands are more efficient in total nitrogen removal than single HF or VF constructed wetlands.
