Constructed Wetlands for the Treatment of Municipal Wastewater Rebecca Newton Civil and...
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Transcript of Constructed Wetlands for the Treatment of Municipal Wastewater Rebecca Newton Civil and...
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
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
Why Clean Wastewater?
Contributes to eutrophication– High oxygen demand via organics– High nitrogen and phosphorous content– Low dissolved oxygen
• Carries pathogenic organisms
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
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
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
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
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
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
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
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
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
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
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
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
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
Questions?