Horizontal Subsurface Flow Constructed Wetland

A horizontal subsurface flow constructed wetlandis a large gravel and sand-filled basin that is planted with wetland vegetation. As wastewater flows horizontally through the basin, the filter material filters out particles and microorganisms degrade the organics.

The filter media acts as a filter for removing solids, a fixed surface upon which bacteria can attach, and a base for the vegetation. Although facultative and anaerobic bacteria degrade most organics, the vegetation transfers a small amount of oxygen to the root zone so that aerobic bacteria can colonize the area and degrade organics as well. The plant roots play an important role in maintaining the permeability of the filter.

Design Considerations

The design of a horizontal subsurface flow constructed wetland depends on the treatment target and the amount and quality of the influent. It includes decisions about the amount of parallel flow paths and compartmentation. The removal efficiency of the wetland is a function of the surface area (length multiplied by width), while the cross-sectional area (width multiplied by depth) determines the maximum possible flow. Generally, a surface area of about 5 to 10 m2 per person equivalent is required.

Pre- and primary treatment is essential to prevent clogging and ensure efficient treatment. The influent can be aerated by an inlet cascade to support oxygen-dependent processes, such as BOD reduction and nitrification.The bed should be lined with an impermeable liner (clay or geotextile) to prevent leaching. It should be wide and shallow so that the flow path of the water in contact with vegetation roots is maximized. A wide inlet zone should be used to evenly distribute the flow. A well-designed inlet that allows for even distribution is important to prevent short-circuiting. The outlet should be variable so that the water surface can be adjusted to optimize treatment performance.

Small, round, evenly sized gravel (3 to 32 mm in diameter) is most commonly used to fill the bed to a depth of 0.5 to 1 m. To limit clogging, the gravel should be clean and free of fines. Sand is also acceptable, but is more prone to clogging than gravel. In recent years, alternative filter materials, such as PET, have been successfully used. The water level in the wetland is maintained at 5 to 15 cm below the surface to ensure subsurface flow. Any native plant with deep, wide roots that can grow in the wet, nutrient-rich environment is appropriate. Phragmites australis (reed) is a common choice because it forms horizontal rhizomes that penetrate the entire filter depth.

Appropriateness

Clogging is a common problem and, therefore, the influent should be well settled with primary treatment before flowing into the wetland. This technology is not appropriate for untreated domestic wastewater (i.e. blackwater). It is a good treatment for communities that have primary treatment (e.g., Septic Tanks).

The horizontal subsurface flow constructed 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. It can also be designed for single households.

This technology is best suited for warm climates, but it can be designed to tolerate some freezing and periods of low biological activity. If the effluent is to be reused, the losses due to high evapotranspiration rates could be a drawback of this technology, depending on the climate.


Health Aspects/Acceptance

Significant pathogen removal is accomplished by natural decay, predation by higher organisms, and filtration. As the water flows below the surface, any contact of pathogenic organisms with humans and wildlife is minimized. The risk of mosquito breeding is reduced since there is no standing water compared to the risk associated with Free-Water Surface Constructed Wetlands. The wetland is aesthetically pleasing and can be integrated into wild areas or parklands.

Operation & Maintenance

During the first growing season, it is important to remove weeds that can compete with the planted wetland vegetation. With time, the gravel will become clogged with accumulated solids and bacterial film. The filter material at the inlet zone will require replacement every 10 or more years. Maintenance activities should focus on ensuring that primary treatment is effective at reducing the concentration of solids in the wastewater before it enters the wetland. Maintenance should also ensure that trees do not grow in the area as the roots can harm the liner.

References

Further Readings

  • Cover image of a reference book or miscellany.

    TILLEY, E.; ULRICH, L.; LUETHI, C.; REYMOND, P.; SCHERTENLEIB, R.; ZURBRUEGG, C. (2014): Compendium of Sanitation Systems and Technologies (Arabic). 2nd Revised Edition. Duebendorf, Switzerland: Swiss Federal Institute of Aquatic Science and Technology (Eawag). PDF

    This is the Arabic version of the Compendium of Sanitation Systems and Technologies. The Compendium gives a systematic overview on different sanitation systems and technologies and describes a wide range of available low-cost sanitation technologies.

  • Cover image of a reference book or miscellany.

    CRITES, R.; TCHOBANOGLOUS, G. (1998): Small and Decentralized Wastewater Management Systems. New York: The McGraw-Hill Companies Inc.

    Decentralised wastewater management presents a comprehensive approach to the design of both conventional and innovative systems for the treatment and disposal of wastewater or the reuse of treaded effluent. Smaller treatment plants, which are the concern of most new engineers, are the primary focus of this book.

  • Cover image of a reference book or miscellany.

    ENPHO (Editor) (n.y.): Decentralised Wastewater Management Using Constructed Wetlands. Kathmandu: Environment and Public Health Organization (ENPHO). URL [Accessed: 17.08.2011]. PDF

    This paper describes the importance of small-scale decentralised wastewater treatment using reed bed treatment systems (constructed wetlands) in Nepal. It shows how public/community participation can support small-scale construction work while ensuring checks on quality and price of construction, including examples.

  • 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 journal article.

    NATURE (Editor); MORGAN, P.; OTTERPOHL, R.; PARAMASIVAN, S.; HARRINGTON, E. (2012): Ecodesign: The Bottom Line. In: Nature: International Weekly Journal of Science 486, 186.URL [Accessed: 19.06.2012]. PDF

    There is no single design solution to sanitation. But there are universal principles for systematically and safely detoxifying human excreta, without contaminating, wasting or even using water. Ecological sanitation design — which is focused on sustainability through reuse and recycling — offers workable solutions that are gaining footholds around the world, as Nature explores on the following pages through the work of Peter Morgan in Zimbabwe, Ralf Otterpohl and his team in Germany, Shunmuga Paramasivan in India, and Ed Harrington and his colleagues in California.

  • Cover image of a reference book or miscellany.

    SASSE, L. ; BORDA (Editor) (1998): DEWATS. Decentralised Wastewater Treatment in Developing Countries. Bremen: Bremen Overseas Research and Development Association (BORDA). PDF

    Exhaustive report on technological, operational and economic aspects of decentralised waste water treatment systems. Spreadsheet examples support the reader in designing and planning waste water treatment systems components.

  • Cover image of a reference book or miscellany.

    TILLEY, E.; ULRICH, L.; LUETHI, C.; REYMOND, P.; ZURBRUEGG, C. (2014): Compendium of Sanitation Systems and Technologies. 2nd Revised Edition. Duebendorf, Switzerland: Swiss Federal Institute of Aquatic Science and Technology (Eawag). URL [Accessed: 28.07.2014]. PDF

    This compendium gives a systematic overview on different sanitation systems and technologies and describes a wide range of available low-cost sanitation technologies.

  • Cover image of a reference book or miscellany.

    VYMAZAL, J. (2010): Constructed Wetlands for Wastewater Treatment. Prague: Department of Landscape Ecology. URL [Accessed: 17.08.2011]. PDF

    This document explains how constructed wetlands work and there is a collection of different wetlands all over the world.

  • Cover image of a reference book or miscellany.

    WHO (Editor) (2006): Guidelines for the safe use of wastewater excreta and greywater. Volume IV. Excreta and Greywater Use in Agriculture. Geneva: World Health Organisation. URL [Accessed: 26.02.2010]. PDF

    Volume IV of the Guidelines for the Safe Use of Wastewater, Excreta and Greywater recognizes the reuse potential of wastewater and excreta (including urine) in agriculture and describes the present state of knowledge as regards potential health risks associated with the reuse as well as measures to manage these health risks following a multi-barrier approach.

Case Studies

  • Cover image of a reference book or miscellany.

    ENPHO (Editor) (n.y.): Decentralised Wastewater Management Using Constructed Wetlands. Kathmandu: Environment and Public Health Organization (ENPHO). URL [Accessed: 17.08.2011]. PDF

    This paper describes the importance of small-scale decentralised wastewater treatment using reed bed treatment systems (constructed wetlands) in Nepal. It shows how public/community participation can support small-scale construction work while ensuring checks on quality and price of construction, including examples.

  • Cover image of a reference book or miscellany.

    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.

  • Cover image of a reference book or miscellany.

    LIPKOW, U.; MUENCH, E. von (2010): Constructed Wetland for a Peri-urban Housing Area Bayawan City, Philippines. Eschborn: Sustainable Sanitation Alliance (SuSanA). URL [Accessed: 10.01.2011]. PDF

    Case study on constructed wetlands for a peri-urban housing area. Septic tanks are used to pre-treat the sewage. The pre-treated wastewater is transported through a small-bore sewer system.

  • Cover image of a reference book or miscellany.

    ECOSAN CLUB (Editor) (2013): Selected contributions from the 1st WATERBIOTECH conference, 9-11 October 2012, Cairo, Egypt. Vienna: Ecosan Club. URL [Accessed: 29.01.2013]. PDF

    This issue publishes selected contributions from the 1st WATERBIOTECH conference. WATERBIOTECH („Biotechnology for Africa‘s sustainable water supply“) is a coordination and support action funded within the Africa call of the EU 7th Framework Programme.

  • 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.

  • Cover image of a reference book or miscellany.

    MUELLEGGER, E. (Editor); LANGERGRABER, G. (Editor); LECHNER, M. (Editor) (2012): Treatment Wetlands. Vienna: EcoSan Club. URL [Accessed: 18.07.2012]. PDF

    Issue 12 of Sustainable Sanitation Practice (SSP) on „Treatment wetlands“ includes 6 contributions: (1.) the Austrian experience with single-stage sand and gravel based vertical flow systems with intermittent loading (the Austrian type is for treating mechanically pre-treated wastewater), (2.) the French experiences with two-stage vertical flow systems treating raw wastewater. (3.) EcoSan Club‘s experiences with TWs in Uganda, (4.) results from multi-stage TW treating raw wastewater in Morocco. (5.) results from horizontal flow experimental systems from Egypt, and (6.) experiences from Denmark and UK on reed beds treating excess sludge from activated sludge plants.

  • Cover image of a reference book or miscellany.

    MUELLEGGER, E. (Editor); LANGEGRABER, G. (Editor); LECHNER, M. (Editor) (2009): Greywater . Treatment and Reuse. Vienna: Ecosan Club. URL [Accessed: 01.07.2013]. PDF

    This Sustainable Sanitation Practice (SSP) issue contains the following contributions: 1. Combined Greywater Reuse and Rainwater Harvesting in an Office Building - Austria, 2. Household Greywater Treatment for Peri-Urban Areas - Kenya, 3. Greywater Use in Peri-Urban Households - Uganda, 4. Greywater Treatment in Apartment Building - Austria, 5. Combined Greywater Treatment Using a Membrane Bioreactor.

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.

Training Material

  • Cover image of a reference book or miscellany.

    EAWAG/SANDEC (Editor) (2008): Sanitation Systems and Technologies. Presentation. Duebendorf: Swiss Federal Institute of Aquatic Science (Eawag), Department of Water and Sanitation in Developing Countries (Sandec). PDF

    PDF presentation on the technical and non-technical aspects of sanitation systems in developing countries.

  • 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.

  • 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.

    WAFLER, M. (2008): Small-scale Constructed Wetlands for Greywater and Total Domestic Wastewater Treatment. Vienna: seecon international gmbh. PDF

    This training material quantifies and characterises grey- and total domestic wastewater production and exemplifies designing of small-scale horizontal and vertical flow constructed wetland system.

  • Cover image of a reference book or miscellany.

    WAFLER, M. (2008): Technical Lecture Greywater Management. Vienna: seecon international gmbh. PDF

    This PDFPresentation quantifies and characterises grey- and total domestic wastewater production and exemplifies designing of small-scale horizontal and vertical flow constructed wetland system.