Brian Barton
Constructed wetlands are engineered systems that treat wastewater and stormwater runoff. They are designed to remove water pollutants such as suspended solids, organic matter, and nutrients like nitrogen and phosphorus. Constructed wetlands are not meant for pathogen removal, but they can reduce bacteria and viruses to some extent. The vegetation in these wetlands provides a substrate for microorganisms to break down organic materials, and these microbes are responsible for most of the pollutant removal and waste breakdown. The overall performance of a constructed wetland depends on its characteristics, such as size, design, type of vegetation, and local conditions. They are a nature-based solution for water management and can improve water quality from various sources, including industrial wastewater and agricultural runoff.
| Characteristics | Values |
|---|---|
| Purpose | To treat sewage, greywater, stormwater runoff, industrial wastewater, and acid mine drainage |
| Pollutants Removed | Suspended solids, organic matter, nutrients (nitrogen and phosphorus), pathogens, heavy metals, hydrocarbons, pesticides |
| Types | Subsurface flow and surface flow wetlands |
| Vegetation | Provides a substrate for microorganisms to grow and break down organic material; different species have different rates of heavy metal uptake |
| Performance | Pollutant removal and waste breakdown are achieved through natural chemical processes (90%), vegetation (7-10%), and filter beds (usually sand and gravel) |
| Advantages | Self-sustaining, lower lifetime and capital costs compared to conventional treatment systems, act as a habitat for wildlife, improve water quality |
| Disadvantages | Require significant space, may accumulate pollutants that affect the survival of larval amphibians |
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What You'll Learn
Constructed wetlands can remove suspended solids, organic matter, and nutrients
Constructed wetlands are artificial wetlands designed to treat wastewater and remove pollutants. They are engineered systems that use the natural functions of vegetation, soil, and organisms to provide secondary treatment to wastewater. Constructed wetlands are of two basic types: subsurface flow and surface flow wetlands. Subsurface flow wetlands are further divided into horizontal and vertical flow wetlands.
Nutrients, such as nitrogen and phosphorus, are removed through a combination of physical, chemical, and biological processes. Denitrification is the dominant removal process for nitrogen, facilitated by heterotrophic bacteria that require a carbon source for growth and energy. These bacteria are more active during higher temperatures, so wetlands work harder to remove nitrogen during the summer months. Nitrogen is released as a harmless gas to the atmosphere. Phosphorus, on the other hand, is removed primarily through chemical processes, such as co-precipitation with iron, aluminium, and calcium compounds in the root-bed medium.
While constructed wetlands are not specifically designed for pathogen removal, they are expected to remove all types of pathogens to some extent. Subsurface wetlands provide greater pathogen removal than surface wetlands. The importance of sunlight exposure in removing viruses and bacteria is minimized in constructed wetlands due to the presence of vegetation that assists in removing other pollutants.
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They can also remove heavy metals
Constructed wetlands are engineered sequences of water bodies designed to treat wastewater or stormwater runoff. They can remove a range of pollutants, including organic matter, nutrients, pathogens, and heavy metals. They are designed to mimic the natural functions of vegetation, soil, and organisms to provide secondary treatment to wastewater.
Constructed wetlands have two basic types: subsurface flow and surface flow wetlands. The choice of plant species in constructed wetlands is essential for efficient heavy metal removal. Different species of aquatic plants have varying rates of heavy metal uptake, and specific plants are better suited for removing certain heavy metals. For example, a study screened the capacity of 34 wetland plant species to remove metals dissolved in water, specifically Cd, Cu, Pb, and Zn, which are common problematic heavy metals in stormwater. The results showed that the plant species with the highest removal capacity could remove up to 98-100% of these metals after 5 days of exposure.
Floating treatment wetlands (FTWs), which consist of rafts supporting emergent plants growing hydroponically, have been proposed as a solution to increase the metal removal efficacy of stormwater ponds. FTWs provide direct contact between the plant roots and the polluted water, and they can be placed in existing ponds without requiring additional land use.
Constructed wetlands are a promising strategy for eco-remediation, especially in industrial catchments. However, further research is needed to optimize their design and the selection and management of wetland plant species to ensure their effectiveness in metal removal.
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Constructed wetlands are not designed to remove pathogens
Constructed wetlands are engineered systems that use the natural functions of vegetation, soil, and organisms to provide secondary treatment to wastewater. They are designed to remove water pollutants such as suspended solids, organic matter, and nutrients (nitrogen and phosphorus). While constructed wetlands can remove a range of pollutants, they are not specifically designed for pathogen removal.
Constructed wetlands are of two basic types: subsurface flow and surface flow wetlands. Subsurface flow constructed wetlands, also known as horizontal flow constructed wetlands, have either horizontal or vertical flow of water through the gravel and sand bed. They are more effective at removing pathogens compared to surface flow wetlands. In a subsurface wetland, the expected removal of pathogens includes 1 to 3 log10 for bacteria, 1 to 2 log10 for viruses, 2 log10 for protozoa, and 2 log10 for helminths.
On the other hand, surface flow wetlands, also known as free water surface constructed wetlands, have a horizontal flow of wastewater across the roots of the plants. They are less effective at pathogen removal due to the presence of vegetation that assists in removing other pollutants like nitrogen and phosphorus. In a free water surface flow wetland, one can expect a 1 to 2 log10 reduction of pathogens, while bacteria and virus removal may be less than 1 log10 reduction in heavily planted systems.
While constructed wetlands are not specifically designed for pathogen removal, all types of pathogens (bacteria, viruses, protozoans, and helminths) are expected to be removed to some extent. The removal of pathogens in constructed wetlands is influenced by various factors, including the type of wetland, the presence of vegetation, sunlight exposure, and the design and operation of the system.
In summary, constructed wetlands are effective in removing a range of pollutants, including organic matter, nutrients, heavy metals, and suspended solids. While they are not primarily designed for pathogen removal, they do contribute to the reduction of pathogens in water, with subsurface flow wetlands being more effective than surface flow wetlands in this regard.
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They can be used to treat industrial wastewater
Constructed wetlands are engineered systems that use the natural functions of wetland vegetation, soil, and their associated microbial assemblages to assist in treating wastewater. They are designed to mimic the processes that occur in natural wetlands, but within a more controlled environment. Constructed wetlands have been used to treat industrial wastewater for more than fifty years, with applications in various industries.
Constructed wetlands can be used to treat industrial effluents from a range of sectors. For example, they have been applied to wastewaters from the petrochemical, abattoir, meat processing, dairy, and pulp and paper industries. During the 1990s, constructed wetlands were also utilised for treating effluents from the textile and wine industries, as well as water from recirculating fish and shrimp aquacultures. More recently, they have been employed for treating brewery, tannery wastewaters, and olive mill effluents.
Constructed wetlands are effective in removing various pollutants commonly found in industrial wastewater. They can reduce or eliminate organic and inorganic pollutants, including nutrients such as nitrogen and phosphorus, heavy metals, hydrocarbons, and pesticides. Constructed wetlands also remove suspended solids and organic matter, such as biochemical oxygen demand and chemical oxygen demand.
The design of a constructed wetland is crucial for effective pollutant removal. The wetland hydrology, which includes free water surface and subsurface flow, plays a significant role. Subsurface flow constructed wetlands, for instance, can be further classified into horizontal and vertical flow systems. Additionally, the type of vegetation is important, with different species of aquatic plants exhibiting varying rates of heavy metal uptake.
Constructed wetlands offer several advantages over conventional treatment systems. They require very low or zero energy input, resulting in lower operation and maintenance costs. Additionally, they can serve as a habitat for native and migratory wildlife, enhancing biodiversity. However, constructed wetlands have a significant spatial requirement, making them less feasible in areas where real estate costs are high.
In summary, constructed wetlands provide a reliable and versatile solution for treating industrial wastewater. By utilising natural processes in a controlled environment, they effectively remove a wide range of pollutants. With their low energy requirements and positive ecological impact, constructed wetlands offer a cost-effective and environmentally friendly option for wastewater treatment in various industrial sectors.
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Constructed wetlands can remove pesticides
Constructed wetlands are engineered sequences of water bodies designed to treat wastewater or stormwater runoff. They are designed to remove water pollutants such as suspended solids, organic matter, and nutrients (nitrogen and phosphorus). Constructed wetlands can also remove pesticides from agricultural runoff and drainage.
Constructed wetlands have been used to treat various types of wastewater since the 1950s, but pesticide removal in constructed wetlands was evaluated much later. The first attempts to use wetland macrophytes for pesticide removal were carried out as early as the 1970s, with numerous experiments carried out during the late 1990s and early 2000s. However, only in the last decade have constructed wetlands for pesticide mitigation become widespread.
A survey of 47 studies from 35 constructed wetlands in 13 countries revealed that constructed wetlands with free water surfaces are the most commonly used type for pesticide removal. The survey also found that removal of pesticides is highly variable, with the highest removal rates for pesticides of the organochlorine, strobilurin/strobin, organophosphate, and pyrethroid groups. The lowest removal rates were observed for pesticides of the triazinone, aryloxyalkanoic acid, and urea groups.
The presence of plants enhances pesticide retention in constructed wetlands. The combination of different types of constructed wetlands in a staged manner, known as a hybrid CW, is also used for pesticide removal. The most common system is the combination of VF and HF, which provides different redox conditions at each stage.
Constructed wetlands are self-sustaining and have lower lifetime and capital costs compared to conventional treatment systems. They can also provide habitat for native and migratory wildlife, enhance flood resilience, and safeguard water availability during droughts.
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Frequently asked questions
Constructed wetlands are artificial wetlands designed to treat wastewater, stormwater runoff, sewage, greywater, and industrial wastewater.
Constructed wetlands can remove or reduce organic and inorganic pollutants such as suspended solids, organic matter, nutrients (nitrogen and phosphorus), heavy metals, hydrocarbons, and pesticides.
Constructed wetlands use the natural functions of vegetation, soil, and microorganisms to break down and remove pollutants from water. The vegetation provides a substrate for microorganisms to grow and break down organic materials, while the natural chemical processes in the wetland also contribute to pollutant removal.
Constructed wetlands are self-sustaining and have lower lifetime and capital costs compared to conventional treatment systems. They also provide habitat for native and migratory wildlife and can improve water quality and regulate hydrological fluxes.
Constructed wetlands take up significant space and are therefore not suitable in areas where real estate costs are high. They are also not designed for pathogen removal, with subsurface wetlands providing greater pathogen removal than surface wetlands.
