constructed wetlands - michigan technological university12/9/2011 8 reasons for constructing...

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Constructed Wetlands

Functions Design Criteria

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Functions of Wetlands

bull Flood mitigationbull Flood mitigationbull Water filtrationpurification (treatment)bull Wildlife habitat biodiversitybull Biogeochemical cycles

R tibull Recreation

Reasons for wetland construction

Miti ti Mi i i l f l d dMitigation Mitigation means replacement of wetlands and their functions It is required in the Corps process if alternative siting and efforts to reduce impacts still result in wetland loss

Banking Banking refers to the creation of artificial wetlands storage of ldquocreditsrdquo for these wetlands and subsequent permission to destroy other wetlands with acreages less than the credits banked

Treatment Creation of wetland specifically for purpose of treating a specific waste (eg agricultural runoff storm runoff domestic wastewater mine drainage)

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Advantages of Treatment Wetlands

1 Wetlands can be less expensive (both construction operation amp maintenance) than conventional treatment facilities

2 Wetlands can tolerate fluctuations in flow2 Wetlands can tolerate fluctuations in flow3 Wetlands facilitate water reuse and recycling4 Wetlands provide habitat5 Wetlands enhance landscape aesthetics and

functions

Limitationsdisadvantages of wetlands as treatment systems

bull Require large land areasbull Performance may be temporally variable

(temperature floods snowmelt)bull Biological components are susceptible toBiological components are susceptible to

shock loadings (toxics)bull Require minimum amount of water

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Important characteristics of wetlands

bull Position in landscapebull Position in landscapebull Water sourcebull Vegetationbull Depth of water tablebull Temporal heterogeneity - variability in p g y y

water level (hydroperiod)bull Spatial heterogeneity - vegetation flow

paths soil or substrate quality

Position in landscape 1

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Site SelectionUPLAND

Lowland

bull No permits required for inputsbull Preferred for treatment wetlandsbull Easier to control hydrology

bull Permit required for inputsbull Can restore degraded wetlandsbull Can result in net environmental benefitbullCWA Sec 319 NPS control

Other considerationsOther considerations

bull Public perceptionsbull Role in watershed (water quality wildlife corridors hydrologic)bull Hydrology geomorphology endangered specieshabitat zoning health

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Site Selection Criteriabull Historic wetland sitesbull Previously drained or degraded sites of certain wetland types (eg forested or riparian wetlands)bull Sites with reliable adequate and available water supply necessary to provide for proposed wetland functions and valuesbull Sites situated within the landscape such that self-sustaining hydrology can be ensuredbull Sites containing majority of drained or hydrologically modified hydric soilsbull Sites with predominately hydric or hydrologically modified soilsbull Sites in close proximity to designated priority areas such as state management areas public lands preservesbull Sites connected to existing wetland systems andor areas of wildlife significancebull Sites with sufficient distance from incompatible land uses andor urban or populated areas that may reduce long-term wetland functioningbull Sites that contain adequate associated upland areas to serve as a wetland bufferbull Sites in which construction will not adversely affect ecologically significant aquatic or upland areas cultural sites or habitat for threatened and endangered speciesorbull Sites that will enhance habitat and species diversity outdoor recreation and scientific and research valuesEnviron Law Inst (ELI) 2002 Banking and Fees

VegetationObjectivesbull Maximize species diversitybull Maximize genetic diversityMaximize genetic diversitybullOptimize wildlife habitat

StrategiesMi i i ldquo d rdquo i di i i ibull Minimize ldquoweedyrdquo nonindigenous or invasive species

bull Use local indigenous speciesbull Optimize physical heterogeneitybull Create temporal variations in flowbull Use multiple ldquocellsrdquobull Plan capacity for sediment accumulation

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Reasons for constructing wetlandswetlands

1 Mitigation (replacement of destroyed wetlands)2 Banking2 NPS pollution control2 NPS pollution control3 Point source pollution treatment (sewage storm water runoff

acid mine drainage ldquoindustrialrdquo waste)4 Wildlife habitat5 Wetland restoration

Flow regimes in constructed wetlands

S f flSurface flow ndasheasier but require more areatemporally amp spatially variable

Subsurface flow saturated ndashlargely anaerobic

Subsurface flow unsaturated ndashhorizontal trickling filter both aerobic andanaerobic

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Flow regimes cell layout

Surface feed unsaturated subsurface flow

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Subsurface flow

Surface feedsurface flow

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Performance Potential

Frequent pollutants1 Sediments - 1) settling basin design τmin gt settling

time 2) sediment accumulation ratedictates storage capacity (dredging willdictates storage capacity (dredging willrequire CWA sec 404 permit as well aseasy access) Short circuiting of flow isproblematic avoid scouring

2 Nitrogen ndash two removal mechanisms 1) burial of organic N2) d it ifi ti T fi t h i i i2) denitrification To use first mechanism maximizeplant growth and sediment accumulation (saturated soil)For denitrification anaerobic zones without shortcircuiting are important subsurface feed and flow aredesirable For feed lots initial nitrification may requireaerated flow

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Further pollutantsPhosphorus ndash primary removal mechanisms are burial of organic

matter and sorption on oxic soils Maximize organicmatter preservation (anaerobic soils) sedimentaccumulation rate and areas with oxic sediments avoidshort circuiting ndash multiple cells desirable

Vectors amp pathogens1 Mosquitoes ndash minimize stagnant water

- utilize natural predators2 Bacteria ndash retention time adequate to kill bacteria

base on low temperature conditions- solids filtration (subsurface flow) and settling

Processes in wetlands

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Empirical design criteria

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Quasi-mechanistic design criteria

1 Hydrology Q ndash Q + PmiddotA - ETA = dVdt1 Hydrology Qin ndash Qout + P A - ET A = dVdt1 Darcyrsquos Law Q = KA(dhdx)

2 BOD or bacteria removal

( )175

2

3

1exp refT Tv

o

v

C A k A L w dC Q

mA specific area for bacterial growth

θ φminus⎛ ⎞= sdot minus sdot sdot sdot sdot sdot sdot sdot⎜ ⎟

⎝ ⎠⎛ ⎞

= sdot sdot sdot sdot sdot⎜ ⎟⎝ ⎠

3

dim

v p f f gm

L w d physical ensionsporosityφ

⎜ ⎟⎝ ⎠

= sdot=

Constructed Wetland or Pond

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What functions are fulfilled

Wetland or lagoon or pond

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References

1 U S EPA 6261-88022 Sept 1988 Design Manual1 US EPA 6261-88022 Sept 1988 Design Manual Constructed wetlands and aquatic plant systems for municipal wastewater treatment

2 Moshiri GA (ed) 1993 Constructed wetlands for water quality improvement CRC Press Boca Raton FL 632 pp

3 Hammer DA (ed) 1989 Constructed wetlands for wastewater treatment Municipal Industrial and Agricultural Lewis p g Publishers Chelsea MI 831 pp

4 Hammer DA 1997 Creating Freshwater Wetlands 2nd ed CRC Press Boca Raton FL 406 pp

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R tibull Recreation





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functions





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Lowland





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Subsurface flow


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( )175

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3

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v

C A k A L w dC Q



⎝ ⎠⎛ ⎞


3

dim

v p f f gm


⎜ ⎟⎝ ⎠

= sdot=


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References





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R tibull Recreation





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functions





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Lowland





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Subsurface flow


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( )175

2

3

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v

C A k A L w dC Q



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dim

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References





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functions





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Lowland





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Subsurface flow


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( )175

2

3

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v

C A k A L w dC Q



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References





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Lowland





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Subsurface flow


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( )175

2

3

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C A k A L w dC Q



⎝ ⎠⎛ ⎞


3

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⎜ ⎟⎝ ⎠

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References





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Lowland





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Subsurface flow


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( )175

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v

C A k A L w dC Q



⎝ ⎠⎛ ⎞


3

dim

v p f f gm


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References





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Subsurface flow


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( )175

2

3

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v

C A k A L w dC Q



⎝ ⎠⎛ ⎞


3

dim

v p f f gm


⎜ ⎟⎝ ⎠

= sdot=


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References





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Subsurface flow


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( )175

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v

C A k A L w dC Q



⎝ ⎠⎛ ⎞


3

dim

v p f f gm


⎜ ⎟⎝ ⎠

= sdot=


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References





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Subsurface flow


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( )175

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C A k A L w dC Q



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References





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Subsurface flow


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( )175

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C A k A L w dC Q



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dim

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References





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( )175

2

3

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C A k A L w dC Q



⎝ ⎠⎛ ⎞


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References





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( )175

2

3

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v

C A k A L w dC Q



⎝ ⎠⎛ ⎞


3

dim

v p f f gm


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References





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( )175

2

3

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C A k A L w dC Q



⎝ ⎠⎛ ⎞


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dim

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References





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( )175

2

3

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o

v

C A k A L w dC Q



⎝ ⎠⎛ ⎞


3

dim

v p f f gm


⎜ ⎟⎝ ⎠

= sdot=


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References





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References





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References





constructed wetlands - michigan technological university12/9/2011 8 reasons for constructing...

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