Constructed Wetlands for Water Treatment and Pollutant Removal

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Constructed Wetlands for Water Treatment
andPollutant Removal
T.P. Cathcart Biological Engineering Mississippi
State University
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Its a regenerative technology to manage human
waste products
What is a constructed wetland?
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A process that makes use of one or more
regenerative cycles.
What is a regenerative technology?
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A cycle in which the end product of one process
becomes the raw material of the next.
What is a regenerative cycle?
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Why are regenerative cycles and technologies good
things?

• They use solar energy (an unlimited and non-
• polluting form of energy) to operate.

• They function independently of humans
• (unless humans disrupt them).

• They are inherently sustainable.

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Why are regenerative cycles and technologies
important now?
Projected 9-12 Billion by 2050
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There are enough of us now to adversely effect
the natural systems upon which we depend.

• 7000 sq. mile dead (anoxic) zone in
• the northern Gulf of Mexico.

• World wide loss of biodiversity.

• Global warming

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Were a bit like a giraffe growing up in a garage.
There will come a time when we have to consider
our limits and our options.
Regenerative technologies will help us meet our
needs without degrading the earth.
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Constructed wetlands
What is a wetland?
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A wetland is a site that has saturated soil for a
long enough duration each year so that its
primary population consists of plants especially
adapted to survive in saturated soils
Emergent wetland plants have specialized internal
tubes, called aerenchyma, that allow oxygen to
diffuse to their root zones. Aerenchyma allow the
plant to persist in anaerobic sediments when
other plants will die.
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A constructed wetland is a wetland created where
one would not otherwise exist.
Its created To meet a human need (usually waste
treatment)
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Natural wetlands cannot be used for waste
Treatment or as part of a system for pollutant
removal (Clean Water Act, 1980).
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There are 3 types of constructed wetlands

• Surface flow (Free water surface)

• Sub-surface flow (rock-reed)

• Stormwater

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• The surface flow and subsurface flow constructed
  wetlands are used for waste treatment and are
  continually loaded.
• The Storm water wetland is loaded like a storm
  water detention pond (intermittently, following
  rain).
• Waste removal mechanisms are similar for all
  three, but they each have their differences as
  well.

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Surface Flow Constructed Wetlands

• Don Hammer was one of the pioneers of surface
  flow constructed wetlands.

• The technology emerged in the 1980s

• Excellent for the removal of sediment, BOD, and
  nitrogen

• Has been used for heavy metals, treating acid
  mine waters, and pesticides

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Construction is pretty simple
Commonly, levees are pushed up from the inside
out (volume is partly dug and partly raised
freeboard of the levees.
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If built on a site having relatively impermeable
soil, a lining may not be required. Alternately,
clay or a plastic liner may be used to prevent
groundwater contamination.
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An easy way to control hydraulic loading is using
orifice flow. For small systems, a simple and
cheap PVC orifice can be constructed.
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Here is a research wetland shortly after planting
at Mississippi State University.
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This is the same system approximately 1 year
later.
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Influent is distributed using a manifold to
promote uniform flow.
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At the effluent end of small systems, water is
collected using a PVC manifold. Plugging can be
prevented using river gravel. Water elevation is
determined by the height of the outlet.
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In larger systems, concrete drop box structures
are used at the effluent end.
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Emergent wetland plants fulfill a variety of
roles. They promote microbial digestion of the
wastes.
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They promote uniform flow, and shade the water
(which reduces phytoplankton growth a source of
BOD)
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Natural wetlands have pools. This led to the
Marsh-Pond-Marsh constructed wetland.
This system was designed specifically to remove
nitrogen.
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In the marsh-pond-marsh system, the middle pool
was made deep enough so that emergent plants
would not grow there.
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Surface flow constructed wetlands as a
regenerative technology

• Very low energy input (typically gravity
• flow may use lift pump in larger
• systems).
• Rich and diverse population of bacteria,
• insects, amphibians, and reptiles.
• Transforms organic wastes to carbon
• dioxide and water.
• Transforms organic nitrogen and ammonia
• to nitrate and then to nitrogen gas.

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The second type of constructed wetland is the
sub-surface flow or rock-reed system
Developed by Bill Wolverton (NASA) for space
travel.
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Here is a sub-surface flow system under
construction
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Subsurface flow
In a subsurface flow constructed wetland, the
wetland is filled with river gravel or similar
size stones. The treatment volume is the space
between the stones.
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Subsurface flow
The stones are a colonizing surface for
microorganisms. The root zones in SSF systems are
an important component for treatment, supplying
much of the oxygen required for aerobic treatment.
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Subsurface flow

• Sediment removal prior to the
• wetland is critical to prevent
• clogging.
• Sub-surface discharge is common.
• Many systems appear to be zero-
• discharge for much of the year,

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If you see water at the surface of a SSF wetland,
thats a good indication of plugging or some
other problem.
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Bill Wolverton developed the SSF system to be
used inside a space ship. His system has been
used inside buildings.
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There are currently plans to include such a
system in the new Landscape Architecture facility
next door.
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Storm water wetlands differ from surface flow and
subsurface flow in that loading is neither
uniform nor continuous.
These systems are designed as shallow detention
ponds or as off-line first flush systems for
treating runoff.
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Constructed wetlands are effective at removing a
variety of pollutants.
A properly designed system will remove 80 to 90
of many pollutants, including BOD, nitrogen, and
sediment. The systems must maintain adequate
oxygen to support aerobic microbes to be
effective.