Treatment Wetlands – Constructed Wetlands

Chapter 20

History

• German scientists used constructed basins with macrophytes to purify wastewater

• US researchers in the 1970s examined use of natural wetlands to treat wastewater

• EPA provides strong support for treatment wetlands

Approaches

• Natural wetlands – dump wastewater into existing wetlands “nature’s kidneys”– Before legal protection of wetlands

• Constructed wetlands – built to mimic natural wetlands, not part of natural systems– Surface-flow – standing water most of the year– Subsurface-flow – water flows through porous

substrate supporting one or two macrophytes

Classification by Vegetation

• 1. Free-floating macrophyte systems – water hyacinth, duckweed

• 2. Emergent macrophyte – Phragmites, Typha

• 3. Submerged macrophyte

• 4. Forested

• 5. Multispecies algal systems

Early Studies• Max-Planck Institute, Germany – 1950s –

created gravel bed macrophyte system, reduced bacteria, inorganic and organic chemicals, led to subsurface constructed wetlands across Europe

• University of Florida – early 1970s – secondarily treated wastewater added to cypress domes at 2.5 cm/week. Lowered nutrients, heavy metals, microbes and viruses. Productivity increased.

• University of Michigan – mid-1970s – dumped up to 5,000 m3/d of secondarily treated wastewater into a fen. Lowered ammonia N and total dissolved P, Cl didn’t change

Wetland Types by Source

Municipal wastewater Mine drainage – low pH, high iron, sulfate,

aluminum, and trace metalsStormwater and nonpoint source – seasonal,

sporadic, variable flows.Landfill leachate – collect and treat runoff from lined

landfills, to reduce ammonium and CODAgricultural wastewater – wastewater from

concentrated animal feeding operations (CAFOs),

Ohio State Wetland Research Center

Treatment for Arizona CAFO (Feedlot)

Wetland Design

– to integrate natural processes as much as possible

Hydrology – basis for biological and chemical conditions response

Hydroperiod and depth

Seasonal pulses

Hydraulic loading and detention rate Optimum detention time from 5-14 days for municipal water

Basin morphology

slopes of 6:1 to 10:1 Variety of depths allows multiple treatments

Deep – denitrification, increase sediment retentionShallow - allows for more soil/water interaction and emergent vegetationSeries of cells can be used to enhance treatment

Other Wetland Design Factors

• Chemical loading– Important for nutrients and other chemicals – Fe, Selenium

• Substrate/Soils – Organic matter important due to cation exchange capacity

– Texture important in determining if it will be subsurface or overflow

• Vegetation – few plants thrive in high nutrient conditions– Typha, Scipus, Phragmites, Lemna, Eichhornia crassipes

Contaminant

Site Process

BOD5 Stems and LeavesRootsBed media (gravel/sand)

Microbial respirationMicrobial respirationMicrobial respirationSettling

Nitrogen LeavesAlgae in water columnRootsSoilBed media

Volatilization (as N2 and N2O)NO3 and NH4+ -> Soluble Organic NitrogenAmmonium -> NitrateNitrate -> N2, N20, or NH4+Settling

Phosphorus Stems and LeavesRoots\Bed media (gravel/sand)

Microbial RespirationMicrobial RespirationUptakeSedimentation/BurialAdsorption

Management

Plant removal – several times a year increases nutrient/chemical removal, stimulates growth

Mosquito control – use of mosquito fish (Gambusia affinis) and bacterial insecticides (Bacillus thuringensis (Bt), Bacillus sphaericus and Lagenidium giganteum)

Pathogen transmission – chlorination of municipal water, sampling

Other benefits

Surface flow increases wildlife, may help in land building

CostsCost/ha decreases as size of wetland increases ($200,000/ha for 1-ha, $60,000/ha for 10-ha, $19,000/ha for 100-ha)Generally cheaper than chemical treatments

Release much less CO2 than chemical treatment (Table 20-10)

Developing country model

Wetlands in Arizona

LinksFor treatment of manure waste• www.epa.gov/seahome/ manure/src/wetlands.htmConstructed wetland CADD drawings• www.sc.nrcs.usda.gov/ technical/constwet.htmlImages from Purdue• www.ces.purdue.edu/ onsite/alternatives.htmWetlands for farm waste• msa.ars.usda.gov/.../ nsl/wqe_unit/wetlands.html For fecal sludge treatment in Thailand• www.sandec.ch/FaecalSludge/ pages/FSM-construc...In Arizona• http://ag.arizona.edu/OALS/ALN/aln45/wetlands.html#wetlands6anchorRemediation of mine tailingswww.uc.edu/news/ wetlands.htm www.enviromine.com/ wetlands/Welcome.htm