The initial use of natural wetlands for treatment of wastewater has been practiced for several decades in the past (Vymazal, 2011). Since the 1950s, the wetland technology emerged and the use of controlled wetland environments for wastewater treatment was developed. Constructed wetlands (CWs) appear nowadays as a reliable, effective and feasible solution in the field of wastewater treatment. They can be classified by the type of plants (free floating, rooted emergent and submergent systems) or by the type of flow (horizontal or vertical) (Haberl et al., 1995; Cooper, 2007).
The use of Vertical Flow CW systems (VFCWs) in Europe became very popular in the 1990s. The adoption of strict disposal limits for nitrogen favoured the application of VFCWs compared to horizontal systems (Vymazal, 2011), due to their enhanced ability to oxidize ammonia nitrogen. In VFCW systems, the wastewater fills periodically the wetland matrix and then drains completely by gravity. This way of feeding provides high levels of oxygen transfer within the substrate, which allows for good treatment performance, especially for organic matter and nitrogen (Reed et al., 1995; Brix and Arias, 2005; Kadlec and Wallace, 2009), as also prevents substrate clogging. Phosphorus retention is comparatively low (Luederitz et al., 2002; Brix and Arias, 2005; Vohla et al., 2011). Across Europe, there have been proposed different design guidelines in many countries, varying from 1 to 5 m²/pe (Stefanakis and Tsihrintzis, 2009). The high ammonia oxidizing ability of VF systems also promoted their use in treatment of high ammonia wastewater, such as landfill leachates (Sun and Austin, 2007; Yalcuk and Ugurlu, 2009) and food processing (Healy et al., 2007; Wood et al., 2007), among others.
A typical layout of a VFCW system consists of a sedimentation tank for wastewater pre-treatment, the VFCW beds and an effluent ditch (Brix and Arias, 2005; Vymazal, et al, 2006). However, designs in France do not include a sedimentation tank. The French version of a VFCW system comprises a coarse bar screen and 2 stages of beds, without any wastewater pre-treatment (Boutin and Liénard, 2003; Molle et al., 2006). Since denitrification practically does not take place in VFCWs (Tietz et al., 2007; Vymazal, 2011), the design of hybrid systems (combination of vertical and horizontal flow systems) has also been proposed to exploit the anoxic areas within the horizontal bed for denitrification (Cooper et al., 1999; Molle et al., 2008; Kadlec and Wallace, 2009). Recirculation of treated effluent has also been evaluated as a possible denitrification enhancement configuration (Arias et al., 2005). A promising application of VFCWs also is activated sludge dewatering and mineralization (Nielsen 2003; Stefanakis et al., 2009a; Melidis et al., 2010).