WETLANDS - Water Quality Engineering in Natural Systems - page 300

constituent, it is necessary to separate the ET into
its evaporation and transpiration components, since
the transpiration component will transport some of
the constituent mass into the soil. If it is assumed
that the transpiration is equal to α ET, where α is the
transpiration fraction , then the mass balance for the j th
segment is

TABLE 8.4. Typical Values of Background Concentration,
C *, in FWS Wetlands

Constituent Lightly Loaded Heavily Loaded

BOD 5 (mg/L) 2 10

TSS (mg/L) 2 15

Organic-N (mg/L) 1 3

Ammonia-N (mg/L)

<0.1 <0.1

Oxidized-N (mg/L) <0.1 <0.1

Q C Q C = − ( IAC ) − ( αET A C ) − kA C C ( −

(8.26)

*

)

Total-P (mg/L) <0.01 0.04

j j j − 1 j − 1 j j j j j j

Fecal coliforms
(CFU/dL)

10-50 100-500

where C j is the constituent concentration in the outflow
(ML 3 ), C j −1 is the constituent concentration in the inflow
(ML 3 ), and infiltration is assumed to occur at the outlet
concentration. Combining Equations (8.25) and (8.26)
gives the following expression for calculating the
outflow concentration in the j th segment,

Source of data : Kadlec and Wallace (2009).

TABLE 8.5. Areal Rate Coeficient, k 20 , and Temperature
Factor, θ, in FWS Wetlands

Constituent k 20 (m/year) θ (-)

BOD 5 33-41 ≈1

*

Q C + kAC

j − 1 j − 1 j

C = (8.27)

Organic-N 17.3 ≈1

j

Q + αET A IA kA + +

Ammonia-N 14.2 1.049

j j j j

TKN 21.0 1.036

Oxidized-N 30.6 1.102

Equation (8.27) is then applied sequentially to each cell
in the wetland system. The input parameters required
to apply Equation (8.27) to the wetland system are the
number of tanks ( P ), the input concentration ( C in ), the
background concentration ( C *), the rate coefficient ( k ),
and the transpiration factor ( α ). The number of tanks,
P , reflects the hydraulic efficiency of the wetland and
measures the number of well-mixed areas within a
wetland; P is under limited control of the designer, and,
usually the required size of the wetland is not very sensi-
tive to P . A typical value for any given wetland cell is
P = 3. In cases where the size of the wetland is sensitive
to P , which usually occurs when the target concentra-
tion is close to C *, the value of P becomes more impor-
tant. In these cases, it is desirable to use wetland cells in
series. The use of three cells in series typically gives P -
values in the range of 6-9, beyond which is a region of
diminishing returns (Kadlec and Wallace, 2009). The
input concentration, ( C in ) is specified based on the type
and quality of water to be handled by the wetland cell.
Typical values of C * are shown in Table 8.4; however,
for operational purposes, the value of C * should be
determined from field measurements once the wetland
is constructed. The rate coefficient, k , can vary substan-
tially between wetlands and can depend on such factors
as the quality of the water, types of vegetation, and
climate. Ideally, values of k in design applications should
be expressed by probability distributions. Typical
(median) values of the value of k at 20°C, k 20 , and the
temperature correction factor, θ , to be used in the
Arrhenius equation (Eq. 8.23) are given in Table 8.5.
The values shown in Table 8.5 indicate that the rate

Total-N 21.5 1.056

Total-P 18 1.006

Fecal coliforms 83 0.963

Source of data : Kadlec and Wallace (2009).

coefficients of BOD 5 and organic-N are typically not
sensitive to temperature, although some individual
studies have shown the contrary. A constituent that is
conspicuously absent from Table 8.5 is TSS; this is
because decay rates of TSS are usually sufficiently high
that this is not the limiting constituent in sizing FWS
wetlands. Values of the transpiration fraction, α , are
generally a function of the wetland vegetation and the
fraction of open water surface. As a general guideline
α = 0.50-0.67 for fully vegetated wetlands (Kadlec,
2006).
Equations (8.22) and (8.27) are the primary wetland
design equations used for performance-based designs of
FWS wetlands. Performance objectives can be varied;
however, the three most common objectives are: (1)
meet target constituent concentration(s) in the wetland
outflow, (2) meet target constituent loads in the wetland
outflow, and (3) meet target constituent removal rates
in the wetland. For analysis and design of wetlands to
meet these objectives, Equations (8.25) and (8.27) are
particularly suited for spreadsheet computations, where
the area of the wetland can be varied until the most
stringent design objective is met. A variety of flows
should be considered to investigate seasonal variations
in the performance of the wetland.