Free-Water Surface Constructed Wetland

A free-water surface constructed wetland aims to replicate the naturally occurring processes of a natural wetland, marsh or swamp. As water slowly flows through the wetland, particles settle, pathogens are destroyed, and organisms and plants utilize the nutrients. This type of constructed wetland is commonly used as an advanced treatment after secondary or tertiary treatment processes.

Unlike the Horizontal Subsurface Flow Constructed Wetland, the free-water surface constructed wetland allows water to flow above ground exposed to the atmosphere and to direct sunlight. As the water slowly flows through the wetland, simultaneous physical, chemical and biological processes filter solids, degrade organics and remove nutrients from the wastewater.

Raw blackwater should be pre-treated to prevent the excess accumulation of solids and garbage. Once in the pond, the heavier sediment particles settle out, and this also removes the nutrients attached to them. Plants and the communities of microorganisms that they support (on the stems and roots), take up nutrients like nitrogen and phosphorus. Chemical reactions may cause other elements to precipitate out of the wastewater. Pathogens are removed from the water by natural decay, predation from higher organisms, sedimentation and UV irradiation.

Although the soil layer below the water is anaerobic, the plant roots exude (release) oxygen into the area immediately surrounding the root hairs, thus, creating an environment for complex biological and chemical activity.

Design Considerations

The channel or basin is lined with an impermeable barrier (clay or geo-textile) covered with rocks, gravel and soil and planted with native vegetation (e.g., cattails, reeds and/or rushes). The wetland is flooded with wastewater to a depth of 10 to 45 cm above ground level. The wetland is compartmentalized into at least two independent flow paths. The number of compartments in series depends on the treatment target. The efficiency of the free-water surface constructed wetland also depends on how well the water is distributed at the inlet. Wastewater can be fed into the wetland, using weirs or by drilling holes in a distribution pipe, to allow it to enter at evenly spaced intervals.

Appropriateness

Free-water surface constructed wetlands can achieve a high removal of suspended solids and moderate removal of pathogens, nutrients and other pollutants, such as heavy metals. This technology is able to tolerate variable water levels and nutrient loads. Plants limit the dissolved oxygen in the water from their shade and their buffering of the wind; therefore, this type of wetland is only appropriate for low-strength wastewater. This also makes it appropriate only when it follows some type of primary treatment to lower the BOD. Because of the potential for human exposure to pathogens, this technology is rarely used as secondary treatment. Typically, it is used for polishing effluent that has been through secondary treatment, or for stormwater retention and treatment.

The free-water surface wetland is a good option where land is cheap and available. Depending on the volume of the water and the corresponding area requirement of the wetland, it can be appropriate for small sections of urban areas, as well as for peri-urban and rural communities. This technology is best suited for warm climates, but can be designed to tolerate some freezing and periods of low biological activity.

Health Aspects/Acceptance

The open surface can act as a potential breeding ground for mosquitoes. However, good design and maintenance can prevent this.

Free-water surface constructed wetlands are generally aesthetically pleasing, especially when they are integrated into pre-existing natural areas.

Care should be taken to prevent people from coming in contact with the effluent because of the potential for disease transmission and the risk of drowning in deep water.

Operation and Maintenance

Regular maintenance should ensure that water is not short-circuiting, or backing up because of fallen branches, garbage, or beaver dams blocking the wetland outlet. Vegetation may have to be periodically cut back or thinned out.

References

Further Readings

  • Cover image of a reference book or miscellany.

    HOFFMANN, H.; PLATZER, C.; WINKER, M.; MUENCH, E. von; GIZ (Editor) (2011): Technology Review of Constructed Wetlands. Subsurface Flow Constructed Wetlands for Greywater and Domestic Wastewater Treatment. Eschborn: Deutsche Gesellschaft fuer Internationale Zusammenarbeit (GIZ) GmbH. URL [Accessed: 01.07.2013]. PDF

    This publication intends to help spread awareness and knowledge about the technology of subsurface flow constructed wetlands in developing countries. Constructed wetlands (CWs) can be used as part of decentralised wastewater treatment systems, due to their “robust”, “low-tech” nature with none or few moving parts (pumps) and relatively low operational requirements. CWs can be used for the treatment of domestic and municipal wastewater or greywater, and play an important role in many ecological sanitation (ecosan) concepts.

  • Cover image of a reference book or miscellany.

    KUSCHK, P. ; WIESSNER, A.; MUELLER, R.; KAESTNER, M. (2005): Constructed Wetlands – Treating Wastewater with Cenoses of Plants and Microorganisms. Leipzig-Halle: UFZ Centre for Environmental Research. URL [Accessed: 12.10.2011]. PDF

    The underlying philosophy of phytoremediation research at UFZ (Centre for Environmental Research) is to exploit and to optimise the processes in the rhizosphere. Low-cost, simple systems will be developed to control the environmental problems of different countries in several continents irrespective of their industrial capabilities and conditions – without losing sight of the key principle of cleaning up polluted environmental media in a natural, ecologically balanced way.

  • Cover image of a reference book or miscellany.

    MOREL, A.; DIENER, S. (2006): Greywater Management in Low and Middle-Income Countries, Review of Different Treatment Systems for Households or Neighbourhoods. Duebendorf: Swiss Federal Institute of Aquatic Science (EAWAG), Department of Water and Sanitation in Developing Countries (SANDEC). URL [Accessed: 19.05.2010]. PDF

    This report compiles international experience in greywater management on household and neighbourhood level in low and middle-income countries. The documented systems, which vary significantly in terms of complexity, performance and costs, range from simple systems for single-house applications (e.g. local infiltration or garden irrigation) to rather complex treatment trains for neighbourhoods (e.g. series of vertical and horizontal-flow planted soil filters).

  • Cover image of a reference book or miscellany.

    UN-HABITAT (Editor) (2008): Constructed Wetlands Manual. Kathmandu: UN-HABITAT, Water for Asian Cities Program. URL [Accessed: 15.02.2012]. PDF

    This manual has been prepared as a general guide to the design, construction, operation and maintenance of constructed wetlands for the treatment of domestic wastewater as well as introduction to the design of constructed wetland for sludge drying.

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    U. S. EPA (Editor) (1998): Design Manual – Constructed Wetlands and Aquatic Plant Systems for Municipal Water Treatment. Washington D.C.: United States : Environmental Protection Agency (EPA). URL [Accessed: 24.08.2011]. PDF

    This document is a very complete design manual about constructed wetlands and aquatic plant systems for municipal water treatment. It describes different designs, application, performance and it includes several case studies.

  • Cover image of a reference book or miscellany.

    U.S. EPA (Editor) (1999): Manual – Constructed Wetlands Treatment of Municipal Wastewater. Washington D.C.: United States: Environmental Protection Agency (EPA). URL [Accessed: 24.08.2011]. PDF

    This manual discusses the capabilities of constructed wetlands, a functional design approach, and the management requirements to achieve the designed purpose. The manual also attempts to put the proper perspective on the appropriate use, design and performance of constructed wetlands. Furthermore, the document contains two case studies.

  • Cover image of a reference book or miscellany.

    WSP (Editor) (2008): Technology Options for Urban Sanitation in India. A Guide to Decision-Making. pdf presentation. New Delhi: Water and Sanitation Program (WSP). URL [Accessed: 26.03.2010]. PDF

    These guidance notes are designed to provide state governments and urban local bodies with additional information on available technologies on sanitation. The notes also aid in making an informed choice and explain the suitability of approaches.

  • Cover image of a reference book or miscellany.

    KADLEC, R. H.; WALLACE, S. D. (2009): Treatment Wetlands. 2nd Edition. Boca Raton: CRC Press, Taylor & Francis Group. URL [Accessed: 18.06.2014]. PDF

    This book supports in making informed decisions regarding wetland design.

  • Cover image of a reference book or miscellany.

    VYMAZAL, J.; SENGUPTA, M. (Editor); DALWANI, R. (Editor) (2008): Constructed Wetlands for Wastewater Treatment: A Review. Czech Republic: ENKI, o.p.s. and Institute of Systems Biology and Ecology, Czech Academy of Sciences. URL [Accessed: 18.06.2014]. PDF

    Presentation of different types of constructed wetlands for various types of wastewater.

Case Studies

  • Cover image of a reference book or miscellany.

    BOJCEVSKA, H. (n.y.): Treatment performance of a free water surface constructed wetland system receiving sugar factory effluents in the Lake Victoria region. Linkoeping: University of Linkoeping. URL [Accessed: 24.08.2011]. PDF

    The aim of the project was to investigate strategies for improving the wastewater treatment of the factory with constructed wetlands. As part of that project, eight pilot scale free-water surface wetlands were constructed to facilitate experiments with such systems. Those pilot scale wetlands will be used in the proposed study.

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    GAUSS, M.; WSP (Editor) (2008): Constructed Wetlands: A Promising Wastewater Treatment system for Small Localities. Experiences from Latin America. Washington D.C.: The World Bank. URL [Accessed: 12.12.2011]. PDF

    This report provides an overview of how constructed wetlands serve as natural wastewater treatment systems. It focuses especially on the subsurface horizontal flow type—a technology that has high potential for small and medium-size communities because of its simplicity, performance reliability, and low operation and maintenance requirements. The ability of this wetland to reduce pathogens renders the effluent suitable for irrigation of certain crop species if additional health and environmental protection measures are taken. This report describes several experiences with constructed wetland schemes in Central and South America: a full-scale pilot plant in Nicaragua, a community-managed constructed wetland scheme in El Salvador, and other systems in Colombia, Brazil, and Peru.

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    GREENWAY, M. (n.y.): The role of constructed wetlands in secondary effluent treatment and water reuse in subtropical and arid Australia. Nathan: Griffith University. URL [Accessed: 24.08.2011]. PDF

    This paper addresses the role of constructed wetlands in nutrient and pathogen removal in Queensland’s, wetlands, and presents three case studies with respect to effluent reuse.

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    ROBBINS, D.; STRANDE, L.; DOCZI, J. (2012): Opportunities in Fecal Sludge Management for Cities in Developing Countries: Experiences from the Philippines. North Carolina: RTI International . URL [Accessed: 15.01.2013]. PDF

    In July 2012, a team from RTI International deployed to the Philippines to evaluate four FSM programs with the goal of reporting on best practices and lessons learned. The four cases—Dumaguete City, San Fernando City, Maynilad Water for the west zone of metro Manila, and Manila Water from the east zone of metro Manila—were chosen to highlight their different approaches to implementing FSM.

  • Cover image of a reference book or miscellany.

    U.S. EPA (Editor) (1993): Constructed Wetlands for Wastewater Treatment and Wildlife Habitat. Washington DC: Environmental Protection Agency (EPA). URL [Accessed: 22.09.2011]. PDF

    This document provides brief descriptions of 17 wetland treatment systems from that are providing significant water quality benefits while demonstrating additional benefits such as wildlife habitat. The projects described include systems involving both constructed and natural wetlands, habitat creation and restoration, and the improvement of municipal effluent, urban stormwater and river water quality. Each project description was developed by individuals directly involved with or very familiar with the project in a format that could also be used as a stand-alone brochure or handout for project visitors.

Awareness Material

  • Cover image of a reference book or miscellany.

    HORWITZ, P.; FINLAYSON, M.; WEINSTEIN, P. (2012): Healthy Wetlands, Healthy People: A Review of Wetlands and Human Health Interactions. Ramsar Technical Report No. 6. Gland and Geneva: Secretariat of the Ramsar Convention on Wetlands and The World Health Organization (WHO). URL [Accessed: 05.03.2012]. PDF

    Despite the production of more food and extraction of more water globally, wetlands continue to decline and public health and living standards for many do not improve. Why is this – and what needs to change to improve the situation? If we manage wetlands better, can we improve the health and well-being of people? Indeed, why is this important? This report seeks to address these questions.