Linking Water Treatment with Wetland Restoration - Restorative Redevelopment of Devastated Ecocultural Landscapes - page 365

renewal, they are building really a layer of soil by laying out a new layer of detritus,
which becomes the substrate for the microbes that provide the basis for much of the
treatment. Physical settling processes also take place. The important point is that
though we see the plants and the plants provide a layer of this fuel for the system,
most of the processes are happening at the microscopic level. The big issue of con-
cern for designers is exactly what elements are adjustable in a system like this, and
how can one really manage and maintain and operate these systems? It turns out
that there are really many factors that are readily amenable to direct manipulation
such as water depth, the concentration of the pollutant input relative to the size of
the system, the hydraulic loading rate in terms of how much water is being applied,
the configuration and the type of wetlands, and the abundance and type of plants.
All these variables can have a big impact on the type of performance that can be
expected from any constructed wetland. As Bays (2004) stated, treatment wetlands
do have a few “knobs” that can be easily turned one direction or another in order to
obtain the desired outflow quality goal.
Past performance provides preliminary design loading criteria. The first question
with a treatment wetland design is how large does the system have to be to achieve
the goals, either in terms of receiving the flow that is available or for processing the
wastewater to the desired goal (Bays 2004)? If contaminant loading versus outflow
concentration from hundreds of treatment wetlands is plotted, spanning a range of
input loads on the order of magnitude, a proportional change in response is observed.
In other words, the more the system is loaded, the higher the outflow concentration.
The important point is that it is a predictable relationship that produces design infor-
mation (i.e., simply fitting a regression equation into this provides coefficients and
constants and a loading rate).
For biological oxygen demand (BOD), a number of processes occur within wet-
lands that are important (Bays 2004). Dissolved oxygen is supplied to the wetlands
through passive aeration and through the re-aeration of soils by the plants themselves.
But dissolved oxygen is taken up in proximity to the sediments through the microbial
activity that is consuming it at a great rate. Longitudinally within a wetland, the con-
sumption of the oxygen produces a marked gradient in concentration. Construction
of weirs between adjacent cells is a good strategy for re-aeration. Oxygen is impor-
tant not only in its own right but also through its influence on the nitrogen cycle. The
oxygen content of water is also dependent upon temperature and salinity, and this is
a factor worthy of consideration in Iraq (Bays 2004) (i.e., the higher the temperature,
the lower the oxygen saturation; the higher the salinity, the lower the oxygen con-
tent). And so, if water has been present on the landscape for a long time it is already
going to have a low oxygen content which will be a factor of potential significance
for fish populations.
A well-established wetland has a good cover of plants that shade the water col-
umn and as a result limit algal growth, which in effect controls the pH of the sys-
tem. Influent pH is often very variable, whereas outflow pH is much more stable.
Wetlands are, therefore, actually buffering systems.
Wetlands naturally assimilate nitrogen though sedimentation, uptake, and atmo-
spheric losses. Nitrogen is taken up by the plants, incorporated into their biomass,
and returned back to the wetlands and water column as an organic nitrogen source.