Design Models for Treatment Wetland Systems
Concentrations: DMSTA
at
Low Phosphorus
William W. Walker
Robert H. Kadlec
The goal of the Everglades Protection Project (EPP) is to remove phosphorus
from runoff waters before it enters the Everglades Protection Area (EPA).
Treatment wetlands are the surviving option, out of dozens that have been tested.
Those wetlands are characterized by varying proportions of three classes of
communities: emergent plants, submerged plants, and periphyton (algae). The
acronyms SAV (submergent aquatic vegetation) and PSTA (periphyton
stormwater treatment area) have been commonly used to represent the submersed
and periphyton communities, respectively. All three assemblages are capable of
phosphorus removal.
The STA design model (Walker, 1995) embodied a first order, areal net uptake of
phosphorus, and was constrained to be applied to long term average wetland
performance. A modified version of the first order, areal model, containing such a
lower limit, was proposed (Kadlec and Knight, 1996). That modification was
subsequently adopted for design of STA3/4. For long term average wetland
performance:
J net  k(C  C*)
(1)
The STA design model effectively used C* ≈ 2 ppb, a result driven by rainfall P.
The later k-C* version allowed C* as a calibration parameter, with resulting
values in the 4 - 20 ppb range.

The goal of this work was to develop and calibrate the simplest, highly aggregated
model that could mimic the major features of event driven behavior of treatment
wetlands in the runoff environment. A simple extension of the STA and k-C*
design models is the addition of P storage in the biota of the wetland ecosystem
(Figure 1). During periods of high phosphorus availability, that storage will
increase, and in periods of P-famine, the storage will decrease. At all times, the
ecosystem produces the residual sediments containing unavailable P, that
characterized the WCA2A calibration data of the STA design model (Walker,
1995). This extension has been named the Dynamic STA Design Model
(DMSTA). It is a unsteady state model that removes phosphorus to permanent
burial in proportion to the amount of labile P in storage, and is patterned after a
similar model calibrated for other marshes (Kadlec, 1997). The labile pool is
presumed to be drawn down by a return flux, or bleed-back. Temporal variations
in the water budget (flows, rainfall, evapotranspiration, seepage, storage) are also
simulated. The underlying philosophy of DMSTA is to maximize simplicity,
while retaining features absolutely necessary for explaining P removal.
La
Li = QiCi
QC
VC = variable = M
Water Column P Storage
Jup
Jret
S = variable
Labile P Storage
Jburial
Figure 1. Concepts of the DMSTA structure.
C
=
P concentration in water, mg P/m3
J
=
P flux, mg P/m2•yr
L =
P loading, mg P/m2•yr
M
=
water column P storage, mg P/ m2
Q =
water outflow, 106 m3/yr
M
=
water column P storage, mg P/ m2
S
=
biomass labile P storage, mg P/ m2
V =
water volume, 106 m3
The model presumes P removal from the water to labile biomass storage that is
proportional to the water concentration and the storage size.
J up  FzK1CS
(2)
This process transfers P from the water column to temporary (labile) storage.
A second principal premise of this model is a P return from labile storage to the
water that is proportional to the square of 
the amount of stored labile P. This
recycle rate is:
J ret  K2S2

(3)
A third premise is that labile stored P is partially converted to biologically
unavailable P in soils and sediments:
J burial  K 3S
(4)
Three rate constants K1, K2, and K3 are adjustable parameters. The combination:
C0  K 3 / Fz K1
(5)

is the lowest steady state (long-term average) concentration that can be attained
by a specific calibration wetland community. Further details are given in Walker
and Kadlec (2002).

Calibrations of DMSTA have been accomplished for three genera of treatment
wetlands receiving low phosphorus waters: emergent, submergent and periphyton
dominated systems. Calibration prototypes were selected based upon vegetation
characteristics, size, and dataset duration. Central tendency parameters
determined from single-platform calibrations have been determined for the three
wetland types, and for combinations.
DMSTA has been used to investigate alternatives for future improvements to
phosphorus removal from waters entering the EPA.
References
W. W. Walker and R.H. Kadlec, 2002. Dynamic Model for Stormwater Treatment
Areas, website: http://www.wwwalker.net/dmsta
Kadlec, R. H., 1997. “An Autobiotic Wetland Phosphorus Model,” Ecological
Engineering, Vol. 8, No. 2, pp. 145-172.
Walker, W. W., 1995. “Design basis for Everglades stormwater treatment areas,”
Water Resources Bulletin, Vol. 31, No. 4, pp. 671-685.
Kadlec, R.H. and R.L. Knight, 1996. Treatment Wetlands. Lewis Publishers,
Boca Raton, FL.
Separate Figure:
La
Li = QiCi
QC
VC = variable = M
Water Column P Storage
Jup
Jret
S = variable
Labile P Storage
Jburial