The French systems

In France, several VSF systems were adapted to treat raw wastewater by using gravel as a substrate. The system was developed by CEMAGREF (Institut de recherche pour l'ingénierie de l'agriculture et de l'environnement) and promoted by SINT (La Société d’Ingénierie Nature et Techniques) company. The idea behind this system is that sludge management can be simpler comparing to the conventional imhoff or digesting tank (Molle et. al., 2005).

It is recommended to divide the system into 2 stages,

1^st stage:3 filters with >30 cm of fine gravel (2-8 mm) as a 1^st layer substrate

2^nd stage: 2 filters with >30 cm of (0.25 < d₁₀ < 0.4 mm) sand as a 1^st layer substrate

Both stages also constitute of transition layer (2^nd layer, 10-20 cm) and drainage layer (10-20 cm). The feeding phase generally lasts for 3 to 4 days, after that it is needed to rest for twice this time in order to maintain “unsaturated� (aerobic) condition within the wetland bodies as well as to mineralise the organic accumulated due to suspended solid (SS). The plant uses special-designed siphon to maintain the hydraulic condition without an external energy source, provided the appropriate topography. In term of its performance, significant removal of COD, TSS and almost complete nitrification can be expected (Boutin et. al., 1997). The sludge withdrawal should be performed approximately once every 10-15 years, and this has no subsequent effect to the regrowth of reeds from the rhizomes. Schematic of the first stage CW can be seen below;

Figure 6: Schematic of the first stage French system (Molle et al., 2005)