Constructed Wetlands
Description
A
constructed wetland uses natural geochemical and biological processes in a
wetland ecosystem to treat metals, explosives, and other contaminants in groundwater. Usually, the
constructed wetland has three primary components: an impermeable layer (generally clay), a gravel
layer that provides a substrate (i.e., an area that provides nutrients and
support) for the root zone, and an above-surface vegetation zone. The
impermeable layer prevents infiltration of wastes down into lower aquifers. The gravel layer and root zone is
where water flows and bioremediation and denitrification take place.
The above ground vegetative layer contains the plant material. Both aerobic and anaerobic systems (i.e., systems with and
without oxygen) exist within the wetland, and these can be divided into
separate cells. Groundwater is either pumped or allowed to naturally flow
through the wetland. The anaerobic cell uses plants in concert with natural
microbes to degrade the contaminant. The aerobic cell further improves water
quality through continued exposure to the plants and the movement of water
between cell compartments. Straw, manure or compost is used, with little or no
soil, in wetlands constructed primarily for the removal of metals. For wetlands
constructed to treat explosives-contaminated water, certain plant species are
used to support degradation. The process of using plants to break down
contaminants is also referred to as phytoremediation.
The
process filters some materials and degrades others. The technology incorporates
the principal components of wetland ecosystems that promote degradation and
control of contaminants by plants: degradation by microbial activity and increased sorption, filtering, and precipitation. The
technology can be adapted to treatment needs by selecting a design, such as
surface or subsurface-flow, single or multiple cells, and parallel or series
flow. Constructed wetlands are sometimes built as part of a treatment train
that may include processes in series such as settling ponds, oil/water
separators, and physical/chemical treatment methods.
Removal
mechanisms can act uniquely, sequentially, or simultaneously on each
contaminant group or species. Volatile organic compounds (VOCs)
in contaminated groundwater are primarily removed through the physical
mechanism of diffusion-volatilization. However, mechanisms such as adsorption to suspended matter, photochemical
oxidation, and biological degradation may also play a role. Major physical
removal mechanisms in wetlands include settling, sedimentation, and volatilization. Gravitational settling is
responsible for most of the removal of suspended solids.
Limitations
and Concerns
The
long-term effectiveness of constructed wetlands to contain or treat some
contaminants is not well known. Wetland aging may contribute to a decrease in contaminant
removal rates over time.
Constructed
wetlands, like other biological methods, are limited by the ability of the
biota to withstand exposure to their environment. Natural systems must
establish themselves in order for this method to be successful. Weather events,
wildlife, and contaminant concentrations may be problematic in establishing the
systems. For example at one demonstration, a hailstorm decimated one of the few
plants able to reestablish itself; a tadpole infestation severely defoliated the
plants within two months of planting; and there was difficulty encountered in
reestablishing plant growth because photo-degradation of explosives in the
contaminated groundwater colored the water a dark red, which in turn inhibited
photosynthesis.
High
contaminant concentrations with low permissible effluent concentrations require
long retention times, hence large wetland areas.
In
cases where metals are the key contaminant, constructed wetlands do not destroy
the metals; they restrict their mobility through sorption.
During
operation of the constructed wetland, wildlife may be adversely affected by the
presence of metals that have accumulated in plants.
After
the pumping of contaminated water ceases, the artificial wetland ecosystem
changes. This could severely affect the plant and animal life that comes to
depend on the wetland, and it may leave a waste byproduct contaminated with
metals and other contaminants. This residue or sludge may have to be disposed or capped.
The
outlet of the monitored wetland should be carefully monitored. Underlying
aquifers must also be monitored to assure that the impermeable base has not
leaked.
When
developing a constructed wetland, exotic and invasive species should not be
used, and a plan should be prepared to remove these species if they appear.
Applicability
Constructed
wetlands have most commonly been used in wastewater treatment to control organic matter, nitrogen, and phosphorus. The
wetland process is also used for controlling trace metals and other toxic
materials in groundwater. Most experimental work is geared
towards groundwater contaminated with explosives such as trinitrotoluene
(TNT), Royal Demolition Explosive (RDX), and High-Melt
Explosive (HMX), as well as perchlorate and nitrates.
Technology
Development Status
Constructed
wetlands have been commercially used to control and degrade municipal and
industrial wastewater. For more exotic wastes such as explosives, it is being
field tested.
Web
Links
http://www.frtr.gov/matrix2/section4/4-43.html
http://www.itrcweb.org/Documents/WTLND-1.pdf
Other
Resources and Demonstrations
See
description of Phytoremediation.
Also
see http://www.itrcweb.org/Documents/WTLND-2.pdf
for related document on mitigated wetlands.
See The Use of Constructed Wetlands to Phytoremediate Explosives-Contaminated Groundwater at the Milan Army Ammunition Plant, Milan, Tennessee, July 1999, http://www.estcp.org/documents/techdocs/199520.pdf for a description of a demonstration at the Milan Plant.