Constructed Wetlands for the Treatment of Municipal Wastewater Rebecca Newton Civil and...

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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

Transcript of Constructed Wetlands for the Treatment of Municipal Wastewater Rebecca Newton Civil and...

Page 1: Constructed Wetlands for the Treatment of Municipal Wastewater Rebecca Newton Civil and Environmental Engineering November 28, 2006 BZ 572 rnewton/Intro.html.

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

Page 2: Constructed Wetlands for the Treatment of Municipal Wastewater Rebecca Newton Civil and Environmental Engineering November 28, 2006 BZ 572 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

Page 3: Constructed Wetlands for the Treatment of Municipal Wastewater Rebecca Newton Civil and Environmental Engineering November 28, 2006 BZ 572 rnewton/Intro.html.

Why Clean Wastewater?

Contributes to eutrophication– High oxygen demand via organics– High nitrogen and phosphorous content– Low dissolved oxygen

• Carries pathogenic organisms

Page 4: Constructed Wetlands for the Treatment of Municipal Wastewater Rebecca Newton Civil and Environmental Engineering November 28, 2006 BZ 572 rnewton/Intro.html.

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

Page 5: Constructed Wetlands for the Treatment of Municipal Wastewater Rebecca Newton Civil and Environmental Engineering November 28, 2006 BZ 572 rnewton/Intro.html.

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

Page 6: Constructed Wetlands for the Treatment of Municipal Wastewater Rebecca Newton Civil and Environmental Engineering November 28, 2006 BZ 572 rnewton/Intro.html.

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

Page 7: Constructed Wetlands for the Treatment of Municipal Wastewater Rebecca Newton Civil and Environmental Engineering November 28, 2006 BZ 572 rnewton/Intro.html.

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

Page 8: Constructed Wetlands for the Treatment of Municipal Wastewater Rebecca Newton Civil and Environmental Engineering November 28, 2006 BZ 572 rnewton/Intro.html.

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

Page 9: Constructed Wetlands for the Treatment of Municipal Wastewater Rebecca Newton Civil and Environmental Engineering November 28, 2006 BZ 572 rnewton/Intro.html.

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

Page 10: Constructed Wetlands for the Treatment of Municipal Wastewater Rebecca Newton Civil and Environmental Engineering November 28, 2006 BZ 572 rnewton/Intro.html.

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

Page 11: Constructed Wetlands for the Treatment of Municipal Wastewater Rebecca Newton Civil and Environmental Engineering November 28, 2006 BZ 572 rnewton/Intro.html.

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

Page 12: Constructed Wetlands for the Treatment of Municipal Wastewater Rebecca Newton Civil and Environmental Engineering November 28, 2006 BZ 572 rnewton/Intro.html.

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

Page 13: Constructed Wetlands for the Treatment of Municipal Wastewater Rebecca Newton Civil and Environmental Engineering November 28, 2006 BZ 572 rnewton/Intro.html.

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

Page 14: Constructed Wetlands for the Treatment of Municipal Wastewater Rebecca Newton Civil and Environmental Engineering November 28, 2006 BZ 572 rnewton/Intro.html.

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

Page 15: Constructed Wetlands for the Treatment of Municipal Wastewater Rebecca Newton Civil and Environmental Engineering November 28, 2006 BZ 572 rnewton/Intro.html.

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

Page 16: Constructed Wetlands for the Treatment of Municipal Wastewater Rebecca Newton Civil and Environmental Engineering November 28, 2006 BZ 572 rnewton/Intro.html.

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

Page 17: Constructed Wetlands for the Treatment of Municipal Wastewater Rebecca Newton Civil and Environmental Engineering November 28, 2006 BZ 572 rnewton/Intro.html.

Questions?