Constructed Wetlands

Chris Akers

Constructed wetlands are engineered wetlands that have saturated or unsaturated substrates, emergent/floating/submergent vegetation, and a large variety of microbial communities that are purposely built for water pollution control.

Constructed wetlands have four main parts: • the liner • distribution media • plants • an underdrain system

The liner is in place to keep the wastewater inside the system while allowing groundwater to remain uncontaminated. The distribution media is usually drainified rock that spreads the wastewater across the width of the wetland. Plants that can grow at optimum efficiency should be used, cattails, bulrushes, reeds, and sedges. The underdrain system can simply be a slotted pipe covered with distribution media and moves the treated effluent out of the wetland. • Depending on plant species used, substrate like sand may be required for plants to take root. • As always, it is recommended that locally adapted species be used for remediation

There are two basic types of constructed wetlands, surface flow and subsurface flow. In a surface flow constructed wetland, water flows over a vegetated surface from inlet to outlet. Can be constructed on variety of soils including mud and clay. Water flows through system slowly, allowing suspended particles to be removed by sedimentation Advantages: • better habitat for wildlife Disadvantages: • require a large amount of land • inefficient purification in winter • attract a lot of mosquitoes • smell bad Some of these can be prevented by constructing only in milder climates where mosquitoes are less of an issue and freezing over winter is less likely.

In subsurface flow constructed wetlands, the water traveling through the system is below the gravel substrate. Advantages: • Limits exposure risk to people and animals • Reduces attraction for mosquitoes Disadvantages • More expensive

(remember a couple advantages/disadvantages for quiz)

Subsurface flow constructed wetlands • Can be divided into horizontal and vertical systems. • In a horizontal flow system, water continuously flows horizontally through substrate from inlet to outlet

• A vertical flow system is watered intermittently and air is allowed to refill the wetland.

Water • Large surface area and shallow depth, strong interaction with atmosphere through rainfall and evapotranspiration • Hydrology effected by density of vegetation

Vegetation • Root system provides habitat for remediating bacteria • Transfer of O2 through the rhizomes. • Assimilate nutrients • Hydraulic pathways in the substrate • Plant litter provides thermal insulation and substrate to microorganisms, carbon source. • aesthetics

Substrate • Permeability affects water movement • Where chemical/biological transformations take place • Storage for many contaminents

Mechanisms • Organic matter broken down by micro-organisms, fermentation, or respiration and used as energy source for wetland or assimilated into biomass • Sediments may settle to the bottom of constructed wetlands. • Nitrogen can be converted into different forms depending on oxidation state of wetland. • Denitrifiers that convert nitrate to N2 are heterotrophic and anaerobic while microbes that perform ammonification and nitrification are autotrophic and aerobic.

Mechanisms • Particulate phosphorus can be removed through sedimentation while soluble phosphorus can be taken up by wetland plants. • Metals: Settling, precipitation, adsorption, ion exchange, phytovolatization, phytoaccumulation • Some wetlands can even hold onto and remove pathogens

Uses of wetlands: • Remove nitrogen from wastewater that causes eutrophication of lakes and rivers. • Remove organic materials that can deplete dissolved oxygen in open water channels. • Removal of heavy metals from drinking water. Constructed wetlands may be part of a larger technique. • Decomposition of organic matter or removal of nutrients after primary treatment of a septic tank.

Site Selection • • • • •

Near the source of pollution A lot of space Sloping, water can flow with gravity Above water table Not a floodplain

(maybe remember a couple of these too)

Concerns • The efficiency of constructed wetlands in removing all of these types of pollutants is widely contested. • Some report that attraction of wildlife and livestock makes constructed wetlands a source rather than treatment of pathogens and nutrients. • Possible sites for bioaccumulation of metals.

Things to consider: • pH drops in pH can hinder nitrification, denitrification and anaerobic organic degradation. • Loading rate Wetlands are effective at higher loadings, however a large increase of hydraulic load decreases the contact time between polluted wastewater and biofilms. Loading rate should be based on soil morphology, structure, and density. • Matrix In order to maximize nitrogen and organic removal in constructed wetlands, wetland media should have both aerobic and anaerobic pores • Carbon If denitrification is a part of your remediation, then an internal carbon source may be needed • Oxygen Limited in wetland environments, but aerobic treatment is faster so active aeration component may be added to system. Air will diffuse more freely through soil with high permeability, low bulk densities, and low moisture.

Agriculture and pollution • The United States has more than 330 million acres of agricultural land. • In 2000, National Water Quality Inventory cited agricultural nonpoint source pollution as leading source of water quality impacts on surveyed rivers and lakes, second largest on wetlands, and a major contributor to contamination of groundwater. • Excess nutrients such as phosphorus, nitrogen, and potassium from fertilizers can runoff into aquatic ecosystems.

Examples Ackerman et. al. tested 7 different constructed wetlands on agricultural tailwater, both continous flow (horizontal) and flood-pulse (vertical) and found that nitrate and suspended solids were consistently removed by all constructed wetlands. Some other constituents measured included E. coli, phosphorus, Ammonium and dissolved organic carbon but all these were varied in results with some having a larger concentration in effluent than influent • Conclusions: – – – –

Performance of constructed wetlands variable depending on many conditions Continous flow-through wetlands more effective than flood-pulse wetlands Flow-through wetlands with low vegetation cover promote growth of algae Slower loading rates in flow-through wetlands lead to organic carbon production

Beutel et. al. focused on nitrogen removal in the lower Yakima River Basin. Catttails and soft stem bulrush. 2003-2006 Conclusions: • Warm temperature and low O2 promote biological denitrification • Effective at removing nitrogen, 90% for nitrate and 60% for total N. Average 100-200 mg N/m2/d • Nitrate removal efficiency followed seasonal trends, best in warm summer months

Cirelli et. al. measured remediation of horizontal subsurface flow wetland in eastern Sicily to reuse small community wastewater for irrigation of olive orchards (secondary treatment) 5 year Removal efficiencies: 77-92% TSS, 37-72% BOD, 51-79% COD, 97-99.5% E.coli, 100% Salmonella and helminth eggs

Kim et. al. treated effluent from publicly owned wastewater treatment plants in Korea for reuse in irrigation. Secondary treatment from wastewater treatment plant Conclusions: • Coliform bacteria and organic compounds did not reach water quality guidelines • Nutrient removal was great, 85% nitrogen, 89% phosphorus.

Bibliography • Agudelo C, Ruth Marina et. al. “Comparison of the removal of chlorpyrifos and dissolved organic carbon in horizontal sub-surface and surface flow wetlands.” Science of the Total Environment Vol. 431 Aug. 2012 pp 271-277 • Beutel, Marc W., Newton, Crystal D., Brouillard, Elaine S. and Watts, Richard J. “Nitrate removal in surface-flow constructed wetlands treating dilute agricultural runoff in the lower Yakima Basin, Washington.” Ecological Engineering Vol. 35 Oct. 2009 pp 1538-1546 • Cirelli, G. L., Consoli, S., Di Grande, V., Milani, M. and Toscano, A. “Subsurface constructed wetlands for wastewater treatment and reuse in agriculture: five years of experiences in Sicily, Italy.” Water Science & Technology Vol. 56 2007 pp 183-191 • Constructed Wetlands. Innovative Onsite Sewage Treatment Systems. University of Minnesota. 2001 http://www.extension.umn.edu/distribution/naturalresources/DD7671.html • Constructed Wetlands. Katholiede Hogeschool Kempen. Department Agro- and Biotechnology. 11/10/2012 http://www.constructedwetlands.net/index.html • Constructed Wetlands. Wikipedia. 11/7/2012 http://en.wikipedia.org/wiki/Constructed_wetland • Davis, Luise. A Handbook of Constructed Wetlands Vol. 1 • Diaz, Francisco J., O’Geen, Antrhony T. and Dahlgren, Randy A. “Agricultural pollutant removal by constructed wetlands: Implications for water management and design.” Agricultural Water Management Vol. 104 Feb. 2012 pp 171-183 • Eautarcie Homes. Eautarcie. http://www.eautarcie.org/en/maisons-en-eautarcie.html • Hydraulic Loading. Washington State Department of Health. Wastewater Management Program 1/30/02 http://www.doh.wa.gov/Portals/1/Documents/Pubs/337-099.pdf • Kim, Y., Lee, D.-R. and Giokas D. “Agricultural reuse of the secondary effluent polished by an algal pond system coupled with constructed wetland.” Water Science and Technology Vol. 50 2004 pp 79-86 • Lee S. Y. et. al. “Phosphorus mass balance in a surface flow constructed wetland receiving piggery wastewater effluent.” Water Science & Technology Vol. 66 2012 pp 712-718 • Liang, Wei et. al. “Roles of substrate microorganisms and urease activities in wastewater purification in a constructed wetland system.” Ecological Engineering. Vol. 21 Dec. 2003 pp191-196 • Sun, Guangzhi et. al. “Enhanced removal of organic matter and ammoniacal-nitrogen in a column experiment of tidal flow constructed wetland system.” Journal of Beiotechnology Vol. 115 Jan. 2005 pp 189-197 • Saeed, Tanveer and Sun, Guangzhi “A review on nitrogen and organics removal mechanisms subsurface flow constructed wetlands: Dependency on environmental parameters, operating conditions and supporting media.” Journal of Environmental Management Vol. 112 Dec. 2012 pp 429-448

Quiz 1) What are 2 reasons a sub-surface flow wetland may be favored over a surface flow wetland? 2) What are 2 things to consider when choosing a site for a constructed wetland?

Answers 1) Limits exposure risk to people and animals does not attract as many mosquitos freezing over the winter time is not a problem requires less land doesn’t smell as bad

2)

Near the source of pollution A lot of space Sloping, water can flow with gravity Above water table Not a floodplain