Constructed wetlands must be sealed at the bottom and sidewalls to avoid any groundwater pollution. As natural sealing there are different recommendations as shown in the following table:
SWAMP 2002
DWA 2004 Germany
Permeability coefficient kf
< 10-7 m/s
< 10-8 m/s
Thickness of sealing
> 30 cm
> 60 cm
Table 4: Recommendation for natural sealing of Constructed wetlands
If natural soil is not available, an artificial layer with impermeable layer is required. The material should be acid resistant and alkali proof, frost and UV resistant, root and rodent resistant, non toxic, easy to carry and move and preferably made of recyclable materials (preferred HDPE or LDPE).
Construction and sizing of the bed
The design criteria for HSF Constructed wetlands can be seen in the table below;
Criteria
Germany, DWA 2004
US EPA 2000
EC Guidelines CEMAGREF
United Kingdom, Cooper 1996
Surface area
5 m2/pe
minimum size 20 m2
5 m2/pe for BOD > 300 mg/l, otherwise 10 m2/pe
5 m2/pe**
0.5-1 m2/pe***
Hydraulic surface load
40 mm/d
< 50 mm/d**
< 200 mm/d***
Max organic load
BOD: 8 g/(m2*d)*
BOD: 6 g/(m2*d)
Depth
0.5 m
0.6 m
Table 5: Design criteria for horizontal flow submerged beds (HSF)
* calculated with 40 g BOD/(PE*d)
** for secondary treatment
*** for tertiary treatment
Apart from those specified in the presented guidelines, Rousseau et. al. (2004) performed a review concerning different design methods. In general, the rules of thumb suggested by several works can be served as a safe bed. However the investment costs tend to be higher due to conservative aspects of this approach.
For the vertical subsurface-flow type, the design criteria are shown below;
Germany, ATV 2004,
Lange & Otterpohl 1998
4 m2/pe
minimum size 16 m2
1-2 m2/pe for greywater
80 mm/d
COD: 20 g/(m2*d)
Table 6: Design criteria for vertical subsurface-flow Constructed wetlands (VSF)
Primary treatment
Primary treatment