Transcript Constructed Wetlands for the Treatment of Municipal Wastewater

Constructed Wetlands for the
Treatment of Municipal
Wastewater
Rebecca Newton
Civil and Environmental Engineering
November 28, 2006
BZ 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
• Usually divided into three
segments
– Anoxic, oxic, anoxic
• Allows for flocculation and
sedimentation, nitrification,
denitrification, pathogen
removal, organic oxidation
vs.
VSB
• 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
• Mandeville, Louisiana
• Retreat center with broken septic
tank
• Replaced with VSB with filtration
tanks before wetland
• Planted with varied local flora
• USEPA/Univ. Cincinnati study
• 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?