A constructed wetland is a man-made wetland used to clean wastewater, including sewage, greywater, stormwater runoff, or industrial waste. It may also help restore land after mining or reduce harm to natural areas affected by development. These wetlands are designed systems that use plants, soil, and living organisms to treat wastewater. The way a constructed wetland is built depends on the type of wastewater it needs to handle. They are used in both large, shared systems and smaller, individual systems. Primary treatment is suggested when wastewater contains a lot of solid particles or organic material, measured by biochemical oxygen demand and chemical oxygen demand.
Like natural wetlands, constructed wetlands act as filters to remove pollutants such as organic matter, nutrients, harmful germs, and heavy metals from water. They are built to remove substances like solid particles, organic matter, and nutrients like nitrogen and phosphorus. All types of germs, including bacteria, viruses, and parasites, are reduced in constructed wetlands. Subsurface wetlands remove more germs than surface wetlands.
There are two main types of constructed wetlands: subsurface flow and surface flow. Plants in the wetland help remove contaminants. A layer of sand and gravel, called a filter bed, also plays a key role. Some wetlands may support wildlife, but this is not their main goal. Subsurface flow wetlands are designed so water moves either horizontally or vertically through the sand and gravel. Vertical flow systems take up less space than horizontal flow systems.
Terminology
Constructed wetlands are sometimes called reed beds, soil infiltration beds, treatment wetlands, engineered wetlands, or man-made wetlands. A biofilter is similar to a constructed wetland, but it typically does not include plants.
The term "constructed wetlands" can also describe land that has been restored or rebuilt after being damaged in the past. This damage may have happened because the land was drained and turned into farmland or used for mining.
Overview
A constructed wetland is a man-made system of water areas designed to clean wastewater or stormwater runoff.
Plants in wetlands provide surfaces, such as roots, stems, and leaves, where microorganisms can grow as they break down organic materials. This group of microorganisms is called the periphyton. The periphyton and natural chemical processes remove about 90% of pollutants and break down waste. Plants remove about 7% to 10% of pollutants and serve as a food source for microbes when they decay. Different types of aquatic plants absorb heavy metals at different rates, which is an important factor when choosing plants for water treatment in constructed wetlands. There are two main types of constructed wetlands: subsurface flow and surface flow wetlands.
Constructed wetlands are an example of nature-based solutions and a method called phytoremediation.
Constructed wetland systems are carefully controlled environments that copy the natural processes of soil, plants, and microorganisms found in wetlands to help treat wastewater. They are built with specific water flow patterns, types of microbes, and plants to create the most effective treatment process.
Constructed wetlands can treat raw sewage, stormwater, agricultural waste, and industrial waste. They copy the functions of natural wetlands to collect stormwater, reduce nutrient levels, and support wildlife. They are used for treating wastewater or greywater.
Many regulatory agencies recommend treatment wetlands as one of their preferred "best management practices" for managing urban runoff.
Removal of contaminants
Wetlands use physical, chemical, and biological processes to remove harmful substances from wastewater. Understanding these processes is important for designing wetland systems and learning what happens to chemicals once they enter wetlands. In constructed wetlands, wastewater treatment happens as water moves through the wetland soil and the area around plant roots. A thin layer around each root releases oxygen, creating an area where oxygen is present. Both oxygen-rich and oxygen-poor microorganisms help break down organic matter. Microbes convert nitrogen into gas, which is released into the air. Phosphorus combines with iron, aluminum, and calcium in the soil. Solid particles settle in surface flow wetlands or are filtered by the soil in subsurface flow wetlands. Harmful bacteria, fungi, and viruses are reduced by filtering and sticking to biofilms on gravel or sand in subsurface and vertical flow systems.
Important nitrogen forms in wetlands include organic nitrogen, ammonia, ammonium, nitrate, and nitrite. Total nitrogen refers to all these forms combined. Removing nitrogen is important because ammonia can harm fish if released into water. Too much nitrate in drinking water may cause health issues in infants, such as reduced oxygen transport in blood. Excess nitrogen from sources like factories or farms can lead to overgrowth of algae, low oxygen levels, and harm to aquatic life in rivers, lakes, and oceans.
Ammonia is removed in constructed wetlands through a two-step process: nitrification and denitrification. First, bacteria convert ammonia to nitrite and then to nitrate. Under oxygen-poor conditions, nitrate is changed into harmless nitrogen gas that enters the air.
Nitrification is the process where bacteria convert organic and inorganic nitrogen compounds into nitrate. This process requires oxygen and results in nitrate as the final product. Ammonium from wastewater is first turned into nitrite, then into nitrate.
Denitrification is when bacteria reduce nitrate and nitrite into gases like nitric oxide, nitrous oxide, and nitrogen gas. These gases return to the atmosphere, and organic matter is used as fuel for this process.
Constructed wetlands are used to remove ammonia and other nitrogen compounds from polluted water, such as mine water containing cyanide and nitrate.
Phosphorus exists in both organic and inorganic forms. The amount of phosphorus that can be used by living organisms is measured as soluble reactive phosphorus. Other forms of phosphorus, like dissolved organic or insoluble forms, are not easily used until they become soluble.
In freshwater ecosystems, phosphorus is often the main nutrient that limits plant growth. Natural conditions usually have low phosphorus levels, so adding too much phosphorus can cause rapid algae growth. Unlike nitrogen, phosphorus does not enter the atmosphere, so its cycle is closed. Phosphorus removal in wetlands happens through: (1) combining with organic matter in living plants, and (2) forming insoluble compounds with iron, calcium, and aluminum in wetland soils.
Aquatic plants help remove phosphorus and can extend the life of wetlands by slowing phosphorus buildup in soil. Plants release oxygen near their roots, improving wetland conditions. They also increase water flow through soil by creating open spaces when roots grow and decay.
Constructed wetlands are used to remove dissolved metals and metalloids from water, such as mine drainage, stormwater, and landfill waste. They are also used to treat acid mine drainage from coal mines.
Constructed wetlands are not designed to remove pathogens, but they help remove other pollutants like suspended solids, organic matter, and nutrients. Pathogens may be reduced in wetlands, with better removal in subsurface flow systems. In surface flow wetlands, pathogen removal is less than 1 to 2 log10 (90% to 99% efficiency). In subsurface flow systems, removal is 1 to 3 log10 (90% to 99.9% efficiency) for bacteria, 1 to 2 log10 for viruses, and 2 log10 for protozoa and helminths.
Types and design considerations
Constructed wetland systems can be surface flow systems with free-floating, floating-leaved, or submerged plants. However, most surface flow systems use emergent plants. Subsurface flow wetlands, which have vertical or horizontal water movement, are also common. These systems can fit in urban areas because they need less space.
The three main types of constructed wetlands are:
• Subsurface flow wetlands – these can have vertical flow (water moves up and down through the planted layer and substrate) or horizontal flow (water moves side to side).
• Surface flow wetlands – these have horizontal water movement.
• Floating treatment wetlands – these use plants that float on water.
Subsurface and surface flow wetlands are built in basins with a solid base to support biofilms that break down waste. Floating wetlands use plants that float until their roots and stems form a thick mat. The bottom of the basin is often lined with materials like polymer, concrete, or clay to protect the ground and water table. The substrate (material used in the wetland) can be gravel (like limestone or volcanic rock), sand, or a mix of different materials.
Constructed wetlands can be used after a septic tank or other systems to separate solid waste from liquid. Some designs skip this step.
In subsurface flow wetlands, wastewater moves between plant roots and stays below the gravel layer. This makes the system more efficient, less smelly, and less likely to attract mosquitoes. It also uses less space. However, the system’s intake areas can clog easily, but using larger gravel often solves this.
Subsurface flow wetlands are divided into horizontal and vertical flow types. In vertical flow systems, water moves up and down through the substrate with the help of air pumps. In horizontal flow systems, water moves side to side with gravity. Vertical flow systems are more efficient and use less space, but they need periodic loading and more technical design. Horizontal flow systems can handle wastewater continuously and are easier to build.
Vertical flow wetlands require about 3 square meters of space per person, or 1.5 square meters in hot climates.
The "French System" uses a series of filters with progressively smaller grains (from gravel to sand) to treat wastewater.
Subsurface flow wetlands can treat many types of wastewater, including household waste, agricultural runoff, paper mill waste, mining water, tannery waste, and stormwater.
The quality of treated water depends on the wetland’s design and its intended use, such as irrigation or toilet flushing.
Wastewater passes through gravel or sand where plants grow. Gravel, such as limestone or volcanic rock, is often used. Volcanic rock can reduce the needed surface area by about 20% compared to limestone, but it is less efficient than sand, which clogs more easily.
Subsurface flow wetlands are used for secondary treatment, meaning wastewater must first go through a primary treatment to remove solids. Primary treatments can include sand and grit removal, grease traps, septic tanks, or anaerobic reactors. After this, biological and physical processes like filtration and nitrification clean the water. Microbial growth on coarse sand helps with these processes, and oxygen must be available for the microbes.
In warm and dry climates, evaporation and rainfall affect wetland performance. Vertical flow systems are better for water loss because they have a dry upper layer and shorter water retention time, though they need external energy. Horizontal flow systems account for rainfall and evaporation in their design.
Treated wastewater may appear yellow or brown if it includes domestic waste. Greywater (wastewater from sinks and showers) usually does not have color. Treated greywater meets safety standards for surface water discharge. Treated domestic wastewater may need further treatment depending on its use.
Reedbeds are commonly used in European subsurface wetlands, but other plants like cattails, sedges, and certain trees can also be used.
Overloading the system with too much waste can reduce its effectiveness.
Subsurface wetlands need regular maintenance, such as checking pretreatment steps, pumps, and the amount of wastewater entering the system.
Subsurface wetlands are less likely to attract mosquitoes than surface flow wetlands because no water is exposed. Surface flow wetlands may support wildlife better but take up more space.
In cities, subsurface wetlands may need more space than traditional wastewater treatment plants. Systems like activated sludge or membrane bioreactors use less space. However, subsurface wetlands are more reliable and do not produce sludge, which is an advantage in developing countries.
The cost of subsurface wetlands depends on the price of sand and land.
Surface flow wetlands, also called free water surface systems, can be used for tertiary treatment or to improve water quality.
Plants and other organisms
Typhas and Phragmites are often used in constructed wetlands because they work well. However, they can spread too much in areas where they are not originally from.
In North America, cattails (Typha latifolia) are common in constructed wetlands because they are found in many places, can grow in different water depths, are easy to move and plant, and can handle various water conditions (including pH, salt levels, oxygen levels, and contaminant amounts). In other regions, Common Reed (Phragmites australis) is often used in both blackwater and greywater treatment systems to clean wastewater.
Plants used in constructed wetlands are usually native to the area for ecological reasons and to ensure the system works best.
- A newly built constructed wetland for blackwater treatment (Lima, Peru)
- The large roots of this uprooted plant in a constructed wetland show it is healthy (Lima, Peru)
- A system that combines Flowforms with a treatment pond (Norway)
Locally grown fish that do not hunt other animals can be added to surface flow constructed wetlands to help reduce pests, such as mosquitoes.
Stormwater wetlands provide homes for amphibians, but the pollutants they collect can harm the survival of their young, possibly making the wetlands act as "ecological traps."
Costs
Constructed wetlands can take care of themselves, so their lifetime costs are much lower than those of traditional treatment systems. Often, the cost to build them is also less than the cost to build traditional systems. However, they need a lot of space, which makes them less suitable in areas where land is expensive.
History
For many years, wastewater from primary clarifiers was sent directly into natural wetlands. However, environmental rules later made this practice less common. Wetlands with sand filters that use underground water flow were first created in China and are now used in small cities across Asia.
Examples
In 2015, there were 5,450 constructed wetlands in Austria. Because of legal rules about removing nitrogen from water, only vertical flow wetlands are used there. Vertical flow wetlands work better for this task than horizontal flow wetlands. About 100 of these wetlands are large enough to serve 50 people or more. The other 5,350 wetlands are smaller than that.
As part of efforts to clean up pollution at CFB Goose Bay, one of the waste dumps there was changed into an engineered wetland.
In 2023, Clearwater County built the first wastewater treatment system in the province that uses biofilters. The system includes two types of subsurface constructed wetlands to treat wastewater in both oxygen-rich and oxygen-poor conditions.