Free Water Surface Constructed Wetlands

Constructed wetlands are essentially a horizontal flow, submerged trickling filters built to treat and remove contaminants from water utilizing the same principals and multiple mechanisms as natural wetlands. They are a series of shallow ponds that have a vast surface area comprised of plant material, living and dead, which are heavily colonized by microbial, invertebrate, and vertebrate populations. They purify the water that flows through them including persistent organic pollutants such as DDT, pharmaceuticals, personal care products and endocrine disrupting compounds (hormones). 

Constructed wetlands differ from natural wetlands in that they do not have short circuits through them and being intensively planted at construction they develop the desired diversity of plants and associated organisms rapidly. They are very inhospitable places for human enteric bacteria. Their life span can exceed a century. So long as they are undisturbed they are a sink for heavy metals and carbon. (Temperate latitude constructed wetlands have been demonstrated to sequester more carbon dioxide that tropical rain forests due to the latter extreme nutrient deficiency.)

Compared to conventional treatment methods, they tend to be simple, inexpensive, and environmentally friendly. They utilize a far smaller ecological footprint than other advanced wastewater treatment mechanisms. Capital costs approximate those of municipal recreational parklands, and operational and maintenance costs that can be ten per cent of that of an advanced physio-chemical treatment plant. Unlike conventional engineered treatment plants, the ancillary benefits such as parkland, education, wildlife habitat, groundwater recharge, temperature amelioration, storm water attenuation, and a number of other benefits can exceed the wastewater treatment values.

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 Free Water Constructed Wetland at Lighthouse Pub, Port Renfrew, BC.
The pond is covered with duckweed, which is a green vegetation that helps aerate the water.

Constructed wetlands have multiple pathways for contaminant removal. An incomplete list:

  • sedimentation
  • filtration
  • heterotrophic and chemotropic microbial decomposition
  • precipitation
  • chemical binding
  • re-oxygenation through photosynthesis
  • gaseous exchange with the atmosphere, submerged surfaces areas, and soils
  • sunlight
  • alternating oxic and anoxic zones with differing microbial communities

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An example of a free water constructed wetland in Prince George, BC

Free Water Constructed Wetland at Arcata Marsh and Wildlife Sanctuary, Arcata, California

Another example of a free Water Constructed Wetland at Arcata Marsh and Wildlife Sanctuary, Arcata, California

Constructed wetlands may be used to treat water from many different sources:

  • Domestic wastewater from cities, small communities, individual homes, and businesses
  • Storm water runoff including streets and highways
  • Mine drainage, even if acidic
  • Agricultural wastewater (including livestock waste, cropland runoff, and drainage water)
  • Landfill leachate
  • Industrial wastewater
  • Removes pharmaceuticals, endocrine disrupting compounds and personal care products

Free Water Constructed Wetland at Lighthouse Pub, Port Renfrew, BC

Free Water Constructed Wetland at Lighthouse Pub, Port Renfrew, BC

Municipal/commercial/residential/industrial wastewater can be effectively treated with free water surface constructed wetlands, which are suitable for a wide variety of fairly dilute wastewater flows. They are the least expensive to build, however, they occupy the most land area.

Landfill Leachate is a result of precipitation flowing through solid wastes, leaching harmful substances, possibly for many decades even after closure. Landfill leachate is a complex and ever-changing wastewater stream that can be comprised of some of the most ecologically troublesome substances known such as heavy metals, high ammonia levels, and virtually any other of society’s discarded chemicals. Free Water Surface Constructed Wetlands are largely passive processes that are powered by sunlight. The inherently sustainable processes employ biomimicry to remove virtually all inimical substances producing water that is biochemically indistinguishable from a natural wetland. Potential for long-term sustainability and significant cost savings are attractive features of this eco-technology.


Knockholt Landfill Constructed Wetland for Septage and Landfill Leachate

The following pictures are of the Free Water Constructed Wetland at Knockholt Landfill, Region of Bulckley/Nechako, located near Houston, BC. The senior designer for the project was Curt Kerns, president of WetlandsPacific.

Construction August 2007

Construction-Knockholt-1 Construction-Knockholt-2 

Planting September 2007

Planting-Knockholt-1 Planting-Knockholt-2Planting-Knockholt-3 Planting-Knockholt-4

Summer 2008

sn850215 2 sn850216 sn850214 3


Summer 2009










Summer 2012

P8140124 P8140125 P8140130

 Note: The local moose have been enjoying the wetland, the city is attempting to dissuade their interest.


Spring 2013

 Knockholt 2013-1 Knockholt 2013-2