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'''A Horizontal Subsurface Flow Constructed Wetland is a large gravel and sand-filled channel that is planted with aquatic vegetation. As wastewater flows horizontally through the channel, the filtermaterial filters out particles and microorganisms degrade organics.'''
Pathogen removal is accomplished by natural decay, predation by higher organisms, and sedimentation.
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{{procontable | pro=
- Requires less space than a Free-Water Surface Constructed Wetland. <br> - High reduction in BOD, suspended solids and pathogens. <br> - Does not have the mosquito problems of the [[Free-Water Surface Constructed Wetland]]. <br> - Can be built and repaired with locally available materials. <br> - Construction can provide short-term employment to local labourers. <br> - No electrical energy required. | con=
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===Adequacy===
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 waste water (i.e. blackwater). This is a good treatment for communities that have primary treatment (e.g. [[Septic Tank|Septic Tanks]] or [[Waste Stabilization Pond|WSPs]]) but are looking to achieve a higher quality effluent. This is a good option where land is cheap and available, although the wetland will require maintenance for the duration of its life.
Horizontal Subsurface Flow Constructed Wetlands are best suited for warm climates but they can be designed to tolerate some freezing and periods of low biological activity.
===Health Aspects/Acceptance===
The risk of mosquito breeding is reduced since there is no standing water compared to the risk associated with [[Free-Water Surface Constructed Wetland|Free-Water Surface Constructed Wetlands]]. The wetland is aesthetically pleasing and can be integrated into wild areas or parklands.
===Maintenance===
With time, the gravel will clog with accumulated solids and bacterial film. The filter material will require replacement every 8 to 15 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===
* Crites, R. and Tchobanoglous, G. (1998). Small and Decentralized Wastewater Management Systems. WCB and McGraw-Hill, New York, USA. pp 599–609. (Comprehensive summary chapter including solved problems.)
* Reed, SC. (1993). [http://water.epa.gov/type/wetlands/restore/upload/2003_07_01_wetlands_pdf_sub.pdf Subsurface Flow Constructed Wetlands For Wastewater Treatment, A Technology Assessment]. United States Environmental Protection Agency, USA. Comprehensive design manual.
===Acknowledgements===
{{:Acknowledgements Sanitation}}