Constructed wetlands are a series of shallow ponds built to treat and remove contaminants from water utilizing the same principals and multiple mechanisms as natural wetlands.

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

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

Constructed wetlands are essentially a horizontal flow, submerged trickling filters. They 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). 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 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.

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

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.)

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