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?