Constructed Treatment Wetlands

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Constructed Treatment Wetlands
Dr. Yi-Ching Chen Dept. of Environmental Engineer
ing Dayeh University

• Special Topics on Environmental Engineering (I)

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What is a Constructed Wetland? (1/6)

• Wetlands are transitional areas between water and
  land.
• The 1977 Clean Water Act Amendments provide a
  broad definition of wetlands "The term
  'wetlands' means those areas that are inundated
  or saturated by surface or groundwater at a
  frequency and duration sufficient to support, and
  that under normal circumstances do support, a
  prevalence of vegetation typically adapted for
  life in saturated soil conditions.
• Water, Soil, Vegetation.

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What is a Constructed Wetland? (2/6)
Ecotonebetween dry terrestrial and permanently
flooded systems
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What is a Constructed Wetland? (3/6)
Isolated basinslittle outflow and no adjacent
deep- water system
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What is a Constructed Wetland? (5/6)

• In brief, a constructed wetland is a water
  treatment facility. Duplicating the processes
  occurring in natural wetlands, constructed
  wetlands are complex, integrated systems in which
  water, plants, animals, microorganisms and the
  environment-sun, soil, air-interact to improve
  water quality.
• Whereas geology, hydrology and biology create
  natural wetlands, constructed wetlands are the
  result of human skill and technology. Humans
  design, build and operate constructed wetlands to
  treat wastewater.

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What is a Constructed Wetland? (6/6)

• Advantage of constructed wetlands is that
  operation and maintenance costs are likely to be
  less than a conventional treatment plant. Less
  energy and supplies are needed, and constructed
  wetland facilities can get by with periodic
  on-site labor, rather than continuous, full-time
  attention.

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What is a Constructed Wetland? (4/6)

• Wetlands
• engineers applying natural purifying abilities
  of wetland systems emulate and enhance for human
  benefit (possibly for wildlife/nature benefit).

combination of modern industrial world and
natural world
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Wastewater treatment

• Traditional environmental engineeringlarge,
  centralized facilities are resource and energy
  intensive solutions to pollution problems
• Ecological engineeringminimize environmental
  stress (e.g. effluent discharge with nutrients to
  water bodies)

Cluster greywater recycling system
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How do treatment wetlands work? (1/3)

• Natural wetland systems have often been described
  as the earths kidneys because they filter
  pollutants from water that flows through on its
  way to receiving lakes, streams and oceans.
• Because these systems can improve water quality,
  engineers and scientists construct systems that
  replicate the functions of natural wetlands.
• Constructed wetlands are treatment systems that
  use natural processes involving wetland
  vegetation, soils, and their associated microbial
  assemblages to improve water quality.

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How do treatment wetlands work? (2/3)

• As water flows through a wetland, it slows down
  and many of the suspended solids become trapped
  by vegetation and settle out. Other pollutants
  are transformed to less soluble forms taken up by
  plants or become inactive.
• Wetland plants also foster the necessary
  conditions for microorganisms to live there.
  Through a series of complex processes, these
  microrganisms also transform and removepollutants
  from the water.

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How do treatment wetlands work? (3/3)

• Nutrients, such as nitrogen and phosphorous, are
  deposited into wetlands from stormwater runoff,
  from areas where fertilizers or manure have been
  applied and from leaking septic fields. These
  excess nutrients are often absorbed by wetland
  soils and taken up by plants and microorganisms.
• Wetland microbes can convert organic nitrogen
  into useable, inorganic forms (NO3 and NH4) that
  are necessary for plant growth and into gasses
  that escape to the atmosphere.

This hog operation in Taylor County, Iowa, uses a
wetland system constructed on a series of
hillside terraces to filter and purify wastewater.
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Design and Planning Considerations (1/2)
If planned and maintained properly, treatment
wetlands can provide wastewater treatment and
also promote water reuse, wildlife habitat, and
public use benefits (recreation,
education). Guidelines for proper planning,
design, construction and operating techniques
(a) Construct treatment wetlands, as a rule, on
uplands and outside floodplains in order to avoid
damage to natural wetlands and other aquatic
resources, unless pretreated effluent can be used
to restore degraded systems. (b) Consider the
role of treatment wetlands within the watershed
(e.g., potential water quality impacts,
surrounding land uses and relation to local
wildlife corridors).
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Design and Planning Considerations (2/2)
(c) Closely examine site-specific factors, such
as soil suitability, hydrology, vegetation, and
presence of endangered species or critical
habitat, when determining an appropriate location
for the project in order to avoid unintended
consequences, such as bioaccumulation or
destruction of critical habitat. (d) Use water
control measures that will allow easy response to
changes in water quantity, quality, depth and
flow. (e) Create and follow a long-term
management plan that includes regular
inspections, monitoring and maintenance.
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Processes taking place in a constructed wetland
may be abiotic (physical/chemical) or biotic
(microbial/phytological). The mechanisms used
for treatment/removal of a contaminant depend on
the specific contaminant, site conditions,
remedial objectives, and regulatory issues.
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• The primary physical and chemical processes
• settling, sedimentation
• sorption
• Chemical oxidation/ reduction precipitation
• photodegradation/ oxidation
• volatilization

Abiotic Mechanisms Treating Organic Compounds in
Wetland Treatment Systems
Abiotic Mechanisms Treating Inorganic Compounds
in Wetland Treatment Systems
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• biodegradation and plant uptake are major
  contributors for contaminant removal
• aerobic/anaerobic biodegradation
• phytoaccumulation/ phytostabilization
• phytodegradation/ rhizodegradation
• phytovolatilization/ evapotranspiration

Biotic Mechanisms Treating Organic Compounds in
Wetland Treatment Systems
Biotic Mechanisms Treating Inorganic Compounds in
Wetland Treatment Systems
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Primary Contaminant Removal Mechanisms
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Summary of contaminant removal efficiency in
treatment wetlands
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Percentage of Constructed Wetland Uses, North
American
Typical Contaminants Found in Various Wastewaters
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Types of constructed wetland systems

• The two main types surface flow (SF) and
  subsurface flow (SSF).
• SF wetlands require more land than SSF wetlands
  for the same pollution reduction but are easier
  and cheaper to design and build.
• SSF systems are often more efficient but can cost
  significantly more than equivalent SF wetlands.
• ( however, recent data may show efficiencies
  are more equitable)
• SF systems are preferred in the United States (
  SF SSF21 )in Europe the reverse is true.

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Surface Flow (SF) Systems

• The SF system is usually a basin or channel
  surrounded by a barrier of ponded wastewater and
  soil to support the growth of rooted emergent
  vegetation.
• SF wetlands are better suited for large community
  systems in milder climates for several reasons
  the system can be fenced to prevent public
  contact, mosquito habitat is not a major issue,
  freezing is unlikely, and the amount of gravel is
  minimal, therefore lowering cost.

Open water SF wetland
Hydroponic SF wetland
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Subsurface Flow (SSF) Systems
Subsurface flow constructed wetland

• In the SSF system, the water level is maintained
  below the surface of the gravel substrate by a
  stand-pipe structure at the discharge end of the
  cell which minimizes the risk of exposure to
  people and animals and greatly reduces mosquito
  breeding.
• The SSF is the most common constructed wetland
  system used for small flows and is often used for
  individual homes, small clusters of houses, or
  resorts.

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Riparian Buffer System (1/2)

• Technically not a constructed wetland system
• establishment or enhancements to riparian buffers
  can be used to treat nonpoint sources of
  pollution.
• Surface water bodies can be impacted from
  nonpoint sources of pollution derived from runoff
  from roadways, agricultural fields, and urban
  areas.
• These systems are typically situated parallel to
  the banks of the water body to be protected along
  the entire length.
• Similarly to wetland treatment systems, riparian
  buffers can use all classes of vegetation on the
  wetland indicator status.

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Riparian Buffer System (2/2)
Obligate wetland plants (OBL) occur almost
exclusively in wetlands (gt99 of the time).
Facultative wetland plants (FACW) usually occur
in wetlands (6799). Facultative plants (FAC)
are equally likely to occur in wetlands or
nonwetlands (3466). Facultative upland plants
(FACU) usually occur in nonwetlands (6799).
Upland plants (UPL) occur almost exclusively in
uplands (gt99).
Cross-Sectional Schematic of a Riparian Buffer
System
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Constructed Wetland at Gao-Ping River, Taiwan
Area30ha Pollutant sources river water Design
pollutant load 456.4 kg-BOD5 / ha-day Design
hydraulic load200 CMD/ha Detention time 7
days System FWS (Free Water Surface)
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Constructed Wetlands at Chu-Nan Coast
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Hong Kong Wetland Park (1/2)
The site of Hong Kong Wetland Park was originally
intended to be an ecological mitigation area
(EMA) to compensate for the wetlands lost due to
Tin Shui Wai New Town development.
The Wetland Reserve is a constructed wetlands of
re-created habitats specially designed for
waterfowls and other wildlife.
Area 61 ha Visitor centre 1 ha
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Hong Kong Wetland Park (2/2)