Aquatic weeds and their utilization 373deep enough to support the growth of the emergent plant’s roots, i.e. about 1.0 –
1.5 m deep.
The cross-sectional area of a SF constructed wetland is calculated as follows
(U.S. EPA 1988):
Ac = Q/(Ks.S) (7.14)Where,
Ac = cross sectional area of wetland bed perpendicular to the direction of flow (or
depth × width), m^2 ,
Ks = hydraulic conductivity of the medium, m^3 /(m^2 -day), as given in Table 7.13
S = slope of the wetland bed (as a fraction or decimal)
To ensure the sufficient contact time for wastewater, the value of Ks.S or
Q/Ac should be less than 8.6 m/day.
Table 7.13 Typical media characteristics for SF wetlands (WEF 2001)
Media type D 10 Effective size a, mm Porosity (Ȗ) Ks,(m^3 /m2-day)Coarse sand 2 0.28 – 0.32 100 – 1000
Gravely sand 8 0.30 – 0.35 500 – 5000
Fine gravel 16 0.35 – 0.38 1000 – 10000
Medium
gravel32 0.36 – 0.40 10000 – 50 000Coarse rock 128 0.38 – 0.35 50000 – 250 000
aD 10 = effective size of the media at 10 % cumulative weight of total, or the size of the media such
that 10 % by weight are smaller.
Example 7.5
Compare the area requirements of FWS and SF constructed wetlands (planted
with cattails) to be used to treat a wastewater having the following
characteristics: average flow rate = 760 m^3 /day, influent BOD 5 = 200 g/m^3 , and
water temperature = 20oC. To meet the discharge standard for reuse, the effluent
BOD 5 concentration has to be 10 g/m^3 or less. For FWS constructed wetland: Ȗ
= 0.75 and bed depth is 0.7 m (cattail plants root is 0.5 and free water depth is
0.2 m)
For SF constructed wetland: Ȗ = 0.3 and bed depth is 0.5m
Equation 7.11 is applicable to describe the BOD 5 removal efficiency of
constructed wetlands.