WETLANDS - Water Quality Engineering in Natural Systems - page 301

EXAMPLE 8.3 P = 3), the area and volume of each tank are

given by

A wetland is to be designed to treat 3785 m 3 /d (1 mgd)
of wastewater containing 5 mg/L of total phosphorus
(TP). The average annual rainfall in the area is 1397 mm/
yr, the average annual ET is 914 mm/yr of which 50%
is transpiration, and infiltration to groundwater within
the wetland is only 25.4 mm/year. The average depth of
water within the wetland is expected to be 0.40 m, the
porosity of the wetland with this depth of water is 0.95,
and the average temperature of the water is 23°C. Esti-
mate the effluent concentration for a wetland area of
20 ha. What wetland area would be required to meet a
effluent standard of 0.50 mg/L?

5

A
P

2 10
3

×

A = = = 6 67 10 . ×

4

m

2

tank

V = nAy = ( . 0 95 6 67 10 )( . ×

4

)( . ) 0 4 = 2 52 10 . ×

4

m

3

tank

Using the given and derived data, the water budget in
each tank can be calculated applying Equation (8.25)
consecutively to each tank, which yields the results
shown in Table 8.6, where the outflow from Tank 3 is
the outflow from the wetland. Based on the above
results, it is apparent that the flow rate in the wetland
increases due to accumulated net rainfall. Applying
Equation (8.27) to each consecutive tank with the
given and derived parameters yields the results shown
in Table 8.7, and hence the TP concentration in the
wetland effluent is 0.73 mg/L. These results also show an
84.5% removal rate for the TP load, most of which is
stored within the wetland.
If the concentration of TP in the effluent is required
to be less than or equal to 0.50 mg/L, then the above
calculations must be repeated with different assumed
(total) wetland areas until the effluent concentration is
equal to 0.50 mg/L. This can be done by using an elec-
tronic spreadsheet to perform the above calculations.
Applying this approach to the present case, the required
wetland area for an effluent concentration of 0.50 mg/L
is found to be 25.9 ha.

Solution

From the given data: Q in = 3785 m 3 /d, c in =
5 mg/L, R = 1397 mm/y = 0.003827 m/d, ET = 914 mm/
yr = 0.002 5 05 m/d, α = 0.50, I = 25.4 mm/yr = 6.96 ×
10 −5 m/d, y = 0 4. m , n = 0.95, T = 23°C, and A = 20 ha =
2 × 10 5 m 2 . From the data in Tables 8.4 and 8.5, it can
be assumed that C * = 0.01 mg/L, k 20 = 18 m/yr =
0.0493 m/d, and θ = 1.006. Hence, the areal rate coeffi-
cient, k , at 23°C is given by Equation (8.23) as

k = k θ

T − 20

= ( . 0 0493 1 006 )( . )

23 20 −

= 0 0502 . m/d

20

Using the typical characterization of a wetland cell as a
series of three continuously stirred tank reactors (i.e.,

TABLE 8.6. Water Budget in Wetland

Inflow Tank 1 Tank 2 Tank 3 Total

Flow rate, Q (m 3 /d)

3785 3869 3952 4036

rainfall, R (m 3 /d) - 255 255 255 765

Evapotranspiration, ET (m 3 /d) - 167 167 167 501

Infiltration, I (m 3 /d) - 5 5 5 14

TABLE 8.7. Concentrations and Mass Fluxes of Total Phosphorus in Wetland
Inflow Tank 1 Tank 2 Tank 3 removed (kg) reduction (%)

Concentration, C (mg/L) 5.00 2.60 1.36 0.73 - -

Influent/effluent load (kg/yr) 6908 - - 1070 5839 84.5

Load infiltrated (kg/yr) - 4 2 1 8 0.1

Load stored (kg/yr) - - - - 5831 84.4