Fact sheet: Constructed wetlands

From: Public Services and Procurement Canada

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Description

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:

Implementation of the technology

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:

  • Mobilization, access and preparation of the site.
  • Excavation of the soil at the planned location for the basin.
  • The waterproofing of the wetland base (clay or synthetic barrier), if required, to prevent percolation into the underlying soils, and the installation of substrates (soils, sediments, gravel, etc.).
  • The installation of substrates necessary to promote the treatment and growth of selected plants according to the type of treated effluent, the climatic environment of the site, as well as the treatment objectives;
  • The installation of an oxygenating element if necessary.

Materials and Storage

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:

  • Basins
  • Permeable substrates
  • Site-specific vegetation
  • Waterproof membrane
  • Water inlet/outlet system
  • Addition of nutrients, if necessary

Storage is primarily related to the compounds used in the system and the application processes.

Residues and Discharges

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.

Recommended analyses for detailed characterization

Biological analysis

  • Total heterotrophic and specific bacterial counts (according to the contaminants of interest)

Chemical analysis

  • pH
  • Alkalinity
  • Oxidation reduction potential (Eh)
  • Metals concentrations
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved
    • free
  • Nutrient concentrations including:
    • ammonia nitrogen
    • total Kjeldahl nitrogen
    • nitrates
    • nitrites
    • total phosphorus
  • Electron acceptor concentrations/reaction by-products including:
    • dissolved oxygen
    • nitrate
    • sulfate
    • ferric and ferrous iron
    • methane
    • dissolved manganese
  • Total suspended solids

Physical analysis

  • Temperature
  • Contaminant physical characteristics including:
    • viscosity
    • density
    • solubility
    • vapour pressure
    • etc.

Recommended trials for detailed characterization

Biological trials

  • Microcosm mineralization trial
  • Mesocosm
  • Greenhouse trials
  • Toxicity testing

Other information recommended for detailed characterization

Phase II

  • Detailed topography
  • Regional climatic conditions (precipitation, temperature, etc.)

Phase III

  • Hydrological assessment
  • Residence time

Applications

  • Applies to contaminants that can be degraded or transformed under either aerobic or anaerobic conditions.
  • Applies for treatment of on-site wastewater.
  • Applies for treatment of industrial wastewater such as pulp and paper, petroleum refineries, paint production, textile mills and starch production industries.
  • Applies for treatment of agricultural wastewater including livestock waste, runoff and drainage water.
  • Applies for treatment of municipal and domestic wastewater such as sewage.
  • Applies for treatment of acid mine drainage produced by mining operations.
  • Applies for treatment of stormwater runoff.
  • Applies for treatment of landfill leachate.

Applications to sites in northern regions

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:

  • Raise the water level of surface flow wetlands to maintain a minimum thickness under the ice cover
  • Perform a rigorous follow-up of the installations
  • Promote the construction of subsurface flow wetlands
  • Select plant species that are tolerant of expected weather conditions. For example, duckweed (lemna sp.) can tolerate minimum temperatures of 5° C.

Treatment type

Treatment type
Treatment typeApplies or Does not apply
In situ
Does not apply
Ex situ
Applies
Biological
Applies
Chemical
Applies
Control
Does not exist
Dissolved contamination
Applies
Free Phase
Does not exist
Physical
Applies
Residual contamination
Does not exist
Resorption
Applies
Thermal
Does not exist

State of technology

State of technology
State of technologyExist or Does not exist
Testing
Does not exist
Commercialization
Exist

Target contaminants

Target contaminantsApplies, Does not apply or With restrictions
Aliphatic chlorinated hydrocarbons
Applies
Chlorobenzenes
With restrictions
Explosives
With restrictions
Metals
Applies
Monocyclic aromatic hydrocarbons
Applies
Non metalic inorganic compounds
Applies
Pesticides
With restrictions
Petroleum hydrocarbons
Applies
Phenolic compounds
Applies
Policyclic aromatic hydrocarbons
Applies
Polychlorinated biphenyls
Applies

Treatment time

Treatment time
Treatment timeApplies or Does not apply
Less than 1 year
Does not apply
1 to 3 years
Does not apply
3 to 5 years
Applies
More than 5 years
Applies

Long-term considerations (following remediation work)

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.

Secondary by-products and/or metabolites

The biological processes under anaerobic conditions may lead to disagreeable odours. Proper design and control of organic matter inflow may minimize odour concentrations.

Limitations and Undesirable Effects of the Technology

  • Land of the appropriate size must be available for the development of the constructed wetland. Depending on the water flow to be treated, the space required can be important.
  • Efficiency relies on environmental conditions and meteorological conditions.
  • Sensitivity of the biological components of the system to toxic chemicals, such as ammonia and pesticides.
  • Treatment efficiency can be temporarily reduced following transient influxes of contaminants or water flow surges greater than expected.
  • Minimum water levels must be maintained, as a complete drought can be lethal to the system.
  • Oxygen levels may vary depending on atmospheric diffusion, wind and the amount of algae or macrophytes available to introduce oxygen into the system.

Complementary technologies that improve treatment effectiveness

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.

Required secondary treatments

  • Aeration of the wetland effluent may be required if dissolved oxygen levels are too low; a cascading drop at the wetland exit point may be enough.
  • A cascade aerator may be required to oxidize and precipitate iron if the concentration in extracted groundwater is high.
  • pH adjustment may be necessary at the exit of the system.

Application examples

Performance

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.

Measures to improve sustainability or promote ecological remediation

  • Optimization of the calendar to promote the sharing of resources and reduce the number of days of mobilization.
  • Use of renewable energy and low-energy equipment, such as aeration from cascades rather than electrical equipment.
  • Choice of amendments requiring less energy for production.
  • Optimization of air and water flow to reduce equipment size and energy consumption.
  • Limit the number of field visits using telemetry for remote monitoring of site conditions.

Potential impacts of the application of the technology on human health

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

Applies (potentially via phytovolatilization)

According to the particularities of each site: sampling and analysis of plant tissues and transpiration gases

Runoff

Applies

Water level monitoring

Groundwater—displacement

Does not apply

N/A

Groundwater—chemical/ geochemical mobilization

Does not apply

N/A

Groundwater—by-product

Applies for subsurface flow wetlands

Water quality monitoring

Accident/Failure—damage to public services

Applies

File checks and licensing prior to excavation, development of excavation and emergency procedures

Accident/Failure—leak or spill

Applies

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

Does not apply

N/A

Other—Direct contact with sludge that may contain pathogenic bacteria (in the case of municipal effluents)

Applies

Risk review, development of accident and emergency response plans, monitoring and inspection of unsafe conditions

Other—Recovery of soil and / or contaminated sludge

Applies

Risk review, development of accident and emergency response plans, monitoring and inspection of unsafe conditions

References

Author and update

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

Version:
1.2.1