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Constructed Wetlands for the Treatment of Municipal
Wastewater
Rebecca NewtonCivil and Environmental Engineering
November 28, 2006BZ 572
http://www.engr.colostate.edu/~rnewton/Intro.html
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Introduction – Wastewater
• People generate 50-100 gallons of wastewater every day.
• Comes from sinks, showers, toilets, dishwashers, laundry, factory waste, food service waste, and shopping centers
• Mostly water with organic solids and other things that are flushed
• Typically BOD = 500 mg BODL/liter, total nitrogen = 60 mg TKN/liter, extra phosphorous
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Why Clean Wastewater?
Contributes to eutrophication– High oxygen demand via organics– High nitrogen and phosphorous content– Low dissolved oxygen
• Carries pathogenic organisms
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Normal Wastewater Treatment• Activated Sludge Process
– Primary settling – removes solids & grit
– Aerate water to promote microbiological degradation of organics & nitrogen
– Settle again– Disinfect (with UV or chlorine)
• Wastewater plants are very expensive ($20-30 million)
• Require highly trained operators onsite all of the time
• Can be difficult to operate because of ecological changes in microbes
• Does not work well at small scale
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Natural Wetlands
• Natural wetlands have been used to treat waste for hundreds of years
• Typically occur in low lying areas that are inundated by surface and groundwater
• Known nutrient sinks and transformers
• Also good with removing metals and organic pollutants
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Constructed Wetlands
• More than 6,000 constructed wetlands in use for wastewater treatment worldwide
• Constructed wetlands built in upland areas and outside of floodplains to prevent wastewater from escaping the wetland
• They can replace the activated sludge part of the conventional wastewater treatment system
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General Constructed Wetland Considerations
• Planted after construction– may take some time, up to a
year, to become fully developed
• Generally natural wetland plants from area are used
• Usually little vegetation management required
• Work well in cold climates – if allowed to develop an ice
layer above an air layer for insulation
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Types of Constructed Wetlands
• Free Water Surface (FWS)– Has areas of open water
and emergent vegetation– Most resembles a natural
wetland
• Vegetated Submerged Bed (VSB)– Gravel bed that water flows through
– Can be planted or unplanted
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FWS vs. VSB
• Usually divided into three segments– Anoxic, oxic, anoxic
• Allows for flocculation and sedimentation, nitrification, denitrification, pathogen removal, organic oxidation
• Can be made into three zones with cyclic operation– Gravel with physical
processes dominating the system
• Allows for flocculation, sedimentation, and filtration
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FWS vs. VSB Removal Mechanisms
• Biological Oxygen Demand– Microbial decomposition
• Total Suspended Solids– Flocculation, sedimentation and
filtration, interception • Nitrogen
– Nitrification in oxic zones and denitrification in anoxic zones
• Phosphorous– Plant uptake, physical adsorption
• Fecal Coliforms & Pathogens– Removal with solids and competition
with wetland microbes• Metals
– cation exchange and chelation with wetland soils, binding with humic materials, and precipitation
• Biological Oxygen Demand– Flocculation, settling, and filtration of
suspended particles. – Microbial degradation of larger
particles• Total Suspended Solids
– same physical mechanisms as BOD and FWS
• Nitrogen– Not as easily removed in a vegetative
submerged – Usually requires a separate process
• Phosphorous– Physical adsorption
• Fecal Coliforms & Pathogens– Removal with solids, not as much
competition, requires disinfection• Metals
– Particulate separation
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Plants in FWS Wetlands
• The type of plant does not matter because primary role is providing structure for enhancing flocculation, sedimentation, and filtration of suspended solids
• Even though plant type does not matter, there are some common varieties
Sedges, Water Hyacinth, Common Cattail, Duckweed, Spatterdock, Waterweed
• In the past monocultures or a combination of two species were used• Currently more diverse representative of natural ecosystem plantings
occur
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VSB Plants
• Plants are not required in VSB wetlands
• Aesthetic and habitat value
• When plants are used, they are chosen for compatibility with the site and local ecosystems
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FWS Case Studies
• Fort Deposit, Alabama• Small town with sewage lagoon
that was outgrown• Replaced with a 15 acre wetland
for 0.24 mgd flow• Good removal of all
contaminants
• Sacramento Constructed Wetlands Demonstration Project
• Demonstration project to see if wetlands could remove metals to meet upcoming regulations
• 22 acre wetland for 1.2 mgd flow• Successful metal removal as well
as general operations
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VSB Case Studies
• Grailville, Ohio • Retreat center with broken septic
tank• Replaced with VSB with filtration
tanks before wetland• Planted with varied local flora• USEPA/Univ. Cincinnati study
• Mandeville, Louisiana • Fast growing town with outdated
lagoons• Aerated one lagoon, followed by
planted VSB for 1.5 mgd flows• Ammonia problems in colder
weather due to no nitrification in lagoons
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Constructed Wetland Costs
• Constructed wetlands are generally more affordable than conventional plants
• The main cost is land area, which varies greatly with location• Cost per acre is based on land cost and how many acres are
necessary to treat the water– VSB - $87,000/acre– FWS - $22,000/acre
• A more accurate measure of cost is $/gallon of treated water– VSB - $0.62/gallon of wastewater treated – FWS - $0.78/gallon of wastewater treated
• Capital costs are more for VSB systems– Cost to transport and install media
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Conclusions
• Constructed wetlands are good for smaller communities with smaller flows– Fort Collins would need between 133 and 833
acres of wetland to treat its 33 mgd wastewater flow
• Constructed wetlands can provide habitat and educational benefits to a community
• Which type and what plants depends on the community and their desires for the wetland
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Questions?