From: Public Services and Procurement Canada
Constructed wetlands are specially designed marshes that receive and remove or filter various types of contaminants that may be present in surface water, groundwater or runoff. They are designed to recreate the structure and function of a natural wetland, to act as a filter or purifier. In this self-sustaining system, abiotic (physical and chemical) and biotic (microbial and phytological) mechanisms can act alone, sequentially or simultaneously on pathogenic contaminants or microorganisms. Examples of effluents that can be treated using this technology include runoff water, mine drainage water and municipal, industrial or agricultural wastewater.
The use of this technology allows the treatment of a wide variety of contaminants such as hydrocarbons, nitrogenous and phosphoric compounds, metals and pathogenic microorganisms. The technology also makes it possible to filter suspended solids and restore oxygen levels. Some contaminants are converted into less harmful or dangerous substances, while others are transported, immobilized or concentrated in the substrate. When designing an artificial marsh, it is possible to optimize the parameters in order to enhance the mechanisms needed to treat specific contaminants.
Sources:
Constructed wetlands are classified into two main groups: surface flow wetlands and subsurface flow wetlands.
Surface-flow wetlands best mimic natural wetlands. They consist of shallow basins in the ground, or any other support capable of supporting the roots of plants. They generally consist of a base made of soil and an emergent vegetation. Surface water is exposed to the atmosphere and moves through the wetland (containing a substrate consisting of soils, gravels, sediments, etc.) at low speeds. The plants in this system are adapted to aquatic environments and able to withstand continuously saturated soil conditions, as well as anaerobic conditions that can be encountered below the surface of the water and in sediments at the bottom of the wetland.
Subsurface flow wetlands generally consist of a pond containing a porous substrate composed of rocks, gravel or sand. These wetlands can be designed for horizontal or vertical flow, allowing water to flow through permeable root media below the soil surface.
In all cases, constructed wetlands must be constructed in such a way as to ensure the periodic saturation necessary for the development of the plants, as well as to allow water retention for a sufficient period of time to remove inorganic and organic compounds, nutrients and/or pathogens.
The implementation of such a system may include:
Construction work generally has minimal impact on the site and requires the use of few materials. The main materials to consider for the construction of an artificial marsh are:
Storage is primarily related to the compounds used in the system and the application processes.
In general, constructed wetlands eliminate or filter contaminants. Some residues such as sludge and solids can accumulate during this treatment.
Soils or sludge resulting from the treatment must be recovered and may be returned to the site or disposed off-site. The nature of these discharges will have to be determined in order to make an adequate disposition.
Soils or sludge resulting from the treatment must be salvaged and may be returned to the site or disposed off-site. The nature of these discharges will have to be determined in order to make an adequate disposition.
After the end of treatment, plant residues should be handled, stored and disposed of properly.
The treated water inside the constructed wetland is rejected after its passage in the wetland. It must meet the criteria applicable to the point of exit of the wetland. Otherwise, in the presence of by-products or an unacceptable pH that may pose danger to the receptors, the water must be pumped and appropriately disposed.
Constructed wetlands may remain operational in norther regions, however, development and installation must be adapted to avoid problems related to freezing. In addition, the system cannot be used during the winter months if the temperatures are too low. The recommendations for a more efficient system in a northern region are:
Following treatment and at the end of use of the constructed wetland, litter, vegetation and solids accumulated in the constructed wetland may need to be removed. In this case, analyses must be carried out to determine how the residues will be managed, in order to dispose of them appropriately, depending on the concentrations of contaminants present.
The biological processes under anaerobic conditions may lead to disagreeable odours. Proper design and control of organic matter inflow may minimize odour concentrations.
A minimal pretreatment of wastewater and groundwater prior to discharge to a wetland can reduce capital and operating costs. A constructed wetland may be joined in series to various processes such as settling ponds, oil and water separators and physical (filtration, etc.) and chemical (chemical addition for phosphorus reduction, etc.) treatment methods. Constructed wetlands can be used as a polishing process for ex situ treatments.
Many factors affect the performance of the constructed wetland, including the concentration, solubility, toxicity and other chemical properties of the contaminants. Constructed wetland systems can significantly reduce biological oxygen demand, total suspended solids, nitrogen and metal concentrations, trace of organic content and presence of microbial pathogens. Although removal rates are highly variable and site dependent, removal efficiency observed for biochemical oxygen demand and suspended solids usually range from 70 to 90%; for nitrogen, from 60 to 86%; and between 97 and 99% for copper, zinc and cadmium. Constructed wetlands show long-term performance with low maintenance and modifications, and consequently low operation costs.
Main Exposure Mechanisms
Applies or Does Not Apply
Monitoring and Mitigation
Dust
Applies (only during construction)
Monitoring conditions conducive to dispersion of soil particles
Atmospheric/Steam Emissions—Point Sources or Chimneys
Does not apply
N/A
Atmospheric/Steam Emissions—Non-point Sources
Applies (potentially via phytovolatilization)
According to the particularities of each site: sampling and analysis of plant tissues and transpiration gases
Air/steam—by-products
Runoff
Applies
Water level monitoring
Groundwater—displacement
Groundwater—chemical/ geochemical mobilization
Groundwater—by-product
Applies for subsurface flow wetlands
Water quality monitoring
Accident/Failure—damage to public services
File checks and licensing prior to excavation, development of excavation and emergency procedures
Accident/Failure—leak or spill
In the case of pond overflow or potential flood: risk review, development of accident and emergency response plans, monitoring and inspection of conditions conducive to a spill or leak
Accident/Failure—fire or explosion
Other—Direct contact with sludge that may contain pathogenic bacteria (in the case of municipal effluents)
Risk review, development of accident and emergency response plans, monitoring and inspection of unsafe conditions
Other—Recovery of soil and / or contaminated sludge
Composed by : Claudie Bonnet, M. Sc. , National Research Council
Updated by : Karine Drouin, M.Sc., National Research Council
Updated Date : November 26, 2013
Latest update provided by : Nathalie Arel, P.Eng., M.Sc., Christian Gosselin, P.Eng., M.Eng. and Sylvain Hains, P.Eng., M.Sc., Golder Associés Ltée
Updated Date : March 22, 2019
