A constructed wetland is a man-made wetland used to treat wastewater, such as sewage, greywater, stormwater runoff, or industrial wastewater. It may also be created to restore land after mining or to replace natural areas lost due to development. Constructed wetlands are man-made systems that use plants, soil, and living organisms to help clean wastewater. The design of the wetland depends on the type of wastewater being treated. These wetlands are used in both large, shared systems and smaller, individual systems. Primary treatment is often needed when wastewater has a lot of solid particles or organic matter, which can be measured using biochemical oxygen demand and chemical oxygen demand.
Like natural wetlands, constructed wetlands act as natural filters and can remove many types of pollution from water, including organic matter, nutrients, harmful germs, and heavy metals. They are specifically designed to remove suspended solids, organic matter, and nutrients like nitrogen and phosphorus from water. All types of germs, such as bacteria, viruses, and parasites, are reduced in constructed wetlands. Wetlands where water flows underground are better at removing germs than those where water flows on the surface.
There are two main types of constructed wetlands: subsurface flow and surface flow. Plants in the wetland help remove contaminants from the water. The sand and gravel layers beneath the water also play a key role in filtering pollution. Some wetlands may also support wildlife, but this is not their main purpose. Subsurface flow wetlands can be designed with water moving either horizontally or vertically through the sand and gravel layers. 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 and artificial 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 repaired after being damaged in the past by draining, turning it into farmland, or mining.
Overview
A constructed wetland is a man-made system of water areas created 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 reactions 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 a constructed wetland used in water treatment. Constructed wetlands are divided into two main types: subsurface flow and surface flow wetlands.
Constructed wetlands are an example of nature-based solutions and phytoremediation.
Constructed wetland systems are carefully controlled environments designed to copy the natural processes of soil, plants, and microorganisms in wetlands to help treat wastewater. They are built with specific water movement patterns, types of microorganisms, and suitable plants to achieve the most effective treatment.
Constructed wetlands can treat raw sewage, stormwater, agricultural waste, and industrial waste. These systems copy the functions of natural wetlands to capture stormwater, reduce nutrient levels, and create habitats for wildlife. Constructed wetlands 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 after 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 hair is oxygen-rich because oxygen leaks from the roots. Both oxygen-rich and oxygen-poor microorganisms help break down organic matter. Microbial processes called nitrification and denitrification change nitrogen into gas that enters the atmosphere. Phosphorus combines with iron, aluminum, and calcium in the wetland soil. In surface flow wetlands, solid particles settle out of the water. In subsurface flow wetlands, these particles are filtered by the soil. Harmful bacteria, fungi, and viruses are reduced by filtering and sticking to biofilms on gravel or sand in subsurface and vertical flow systems.
Nitrogen in wetlands important for wastewater treatment includes organic nitrogen, ammonia, ammonium, nitrate, and nitrite. Total nitrogen refers to all these forms. Removing nitrogen is important because ammonia can harm fish if released into water. Too much nitrate in drinking water may cause health problems in infants, such as reduced oxygen transport in the blood. Excess nitrogen from sources like factories or farms can lead to eutrophication, which causes problems like algae blooms, low oxygen levels, and loss of aquatic life.
Ammonia is removed in constructed wetlands through a two-step process: nitrification and denitrification. This works like in sewage treatment plants but without needing to add oxygen. First, ammonia is converted to ammonium. Then, bacteria called Nitrosomonas change ammonium to nitrite. Another bacteria, Nitrobacter, changes nitrite to nitrate. Under oxygen-poor conditions, nitrate becomes harmless nitrogen gas that escapes into the air.
Nitrification is the process where bacteria change organic and inorganic nitrogen into more oxidized forms. This happens only in oxygen-rich environments and results in nitrate. Ammonium is first turned into nitrite, then into nitrate.
Denitrification is the process where bacteria reduce nitrate and nitrite into gases like nitrogen, nitric oxide, and nitrous oxide. These gases return to the atmosphere, and organic matter is broken down.
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 those in organic matter or soil, are not easily used until they become soluble.
In freshwater ecosystems, phosphorus is often the main nutrient that limits growth. When too much phosphorus enters water, algae can grow rapidly. Unlike nitrogen, phosphorus does not cycle through the atmosphere. Wetlands remove phosphorus by either storing it in plant material or combining it with iron, calcium, and aluminum in the soil.
Aquatic plants help remove phosphorus and can extend the life of wetlands by preventing phosphorus buildup in soil. Plants release oxygen near their roots, which helps other processes. Their growth also makes soil more porous, allowing water to move more easily. When roots decay, they leave behind channels that help water flow through the soil.
Constructed wetlands are used to remove dissolved metals and metalloids from water. These contaminants are found in mine drainage, stormwater, and other sources like landfill waste. Wetlands are also used to treat acid mine drainage from coal mines.
Wetlands are not designed to remove all pathogens, but they can help reduce other pollutants like suspended solids, organic matter, and nutrients. In surface flow wetlands, pathogen removal is limited, with only 1 to 2 log10 reductions for bacteria, viruses, protozoa, and helminths. In subsurface flow wetlands, removal is higher, with 1 to 3 log10 for bacteria, 1 to 2 log10 for viruses, and 2 log10 for protozoa and helminths.
Log10 removal can be understood as percentages: 1 log10 equals 90% removal, 2 log10 equals 99%, 3 log10 equals 99.9%, and so on.
Types and design considerations
Constructed wetland systems can be surface flow systems with free-floating plants, floating-leaved plants, 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 into 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 through plants and down through gravel and soil) or horizontal flow (water moves side to side through gravel and soil).
• Surface flow wetlands – These have horizontal water movement across the surface.
• Floating treatment wetlands – These use floating plants that grow roots into water to help clean it.
Subsurface flow wetlands are placed in basins with gravel or other materials to support biofilms that break down waste. Floating treatment wetlands use plants that float on water and grow roots into it. The bottom of these systems is often lined with materials like plastic, concrete, or clay to protect the ground and water table. The gravel or sand used in the system can vary based on local resources.
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 through plant roots and stays below the gravel surface. This makes the system more efficient, less smelly, and less likely to attract mosquitoes. It also works better in cold weather and needs less space. However, the system’s intake areas can clog easily, though using larger gravel often helps.
Subsurface flow wetlands are divided into vertical and horizontal types. In vertical flow systems, water moves up through plants and down through gravel, requiring air pumps for oxygen. In horizontal flow systems, water moves side to side through gravel, avoiding mosquito breeding. Vertical flow systems are more efficient and need less space but require more careful management. Horizontal flow systems are easier to build and can handle continuous wastewater flow.
Vertical flow systems need about 3 square meters (32 square feet) of space per person, or as little as 1.5 square meters in hot climates.
The "French System" uses multiple filter beds with progressively smaller materials (like gravel to sand) to treat wastewater.
Subsurface flow wetlands can treat various types of wastewater, including household, agricultural, industrial, and stormwater.
The quality of treated water depends on the system’s design and its intended use, such as irrigation or toilet flushing.
Wastewater in subsurface systems moves through gravel or sand where plants grow. Gravel made of limestone or volcanic rock is often used, with volcanic rock reducing surface area by about 20% compared to limestone. Sand is less efficient but can clog more easily.
Subsurface flow wetlands are used for secondary treatment, meaning wastewater must first pass through a primary system (like a septic tank or sand filter) to remove solids. After this, biological and physical processes like filtration and bacterial growth help clean the water. Oxygen is needed for these bacteria to work effectively.
In warm, dry climates, evaporation and rainfall affect system performance. Vertical flow systems are better for water loss because they have a drier top layer and faster water movement, though they need external energy. Horizontal systems consider these factors in their design.
Treated wastewater may appear yellow or brown if it contains domestic or blackwater, but greywater (like from sinks) usually stays clear. Treated greywater meets safety standards for surface water discharge. Treated domestic wastewater may need extra cleaning if used for specific purposes.
Reeds are commonly used in European subsurface wetlands, but other plants like cattails, sedges, and fast-growing trees can also be used.
Too much wastewater at once can harm the system, but continuous overloading reduces its ability to clean water.
Subsurface wetlands need regular checks on pretreatment steps, pumps, and wastewater distribution.
Subsurface systems are less likely to attract mosquitoes than surface systems because no water is exposed. Surface systems can support wildlife better but need more space.
In cities, subsurface wetlands may require more space than traditional treatment plants. However, they are more reliable and do not produce sludge, which is a benefit in areas with limited resources.
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 final cleaning of wastewater.
Plants and other organisms
Typhas and Phragmites are often used in constructed wetlands because they work well, even though they can spread too much in areas where they are not native.
In North America, cattails (Typha latifolia) are commonly used in constructed wetlands because they are plentiful, can grow in different water depths, are easy to move and plant, and can survive in various water conditions, such as pH levels, salt content, oxygen levels, and contaminant amounts. In other places, Common Reed (Phragmites australis) is often used, both in systems that treat blackwater and systems that treat greywater to clean wastewater.
Plants used in constructed wetlands are usually native to the area for ecological reasons and to ensure the wetlands work effectively.
- A newly planted constructed wetland for blackwater treatment (Lima, Peru)
- The large roots of this uprooted plant in a constructed wetland show that the plant is healthy (Lima, Peru)
- A hybrid system using Flowforms in a treatment pond (Norway)
Fish that are not predators and are grown locally can be added to surface flow constructed wetlands to help control pests like mosquitoes.
Stormwater wetlands offer homes for amphibians, but the pollution they collect can harm the survival of young amphibians, possibly turning them into "ecological traps."
Costs
Constructed wetlands can take care of themselves, which means their lifetime costs are much lower than those of traditional treatment systems. Their initial costs are often also less than those of traditional systems. However, they require a large amount of space, so they are not usually chosen in areas where land is expensive.
History
For many years, wastewater from primary clarifiers was sent directly into natural wetlands. This practice became less common after environmental rules were put in place to stop it. Wetlands with sand filter beds that are built below the surface were first developed in China. These types of wetlands are now used in small cities across Asia.
Examples
In 2015, Austria had 5,450 constructed wetlands. Because of legal rules requiring effective nitrogen removal, only vertical flow wetlands are used there. These wetlands work better for removing nitrogen than horizontal flow wetlands. About 100 of the wetlands in Austria are large enough to serve 50 people or more. The other 5,350 wetlands are smaller than that.
As part of clean-up efforts at CFB Goose Bay, one of the waste dumps was changed into an engineered wetland to help remove pollution.
In 2023, Clearwater County installed the first biofilter-based wetland wastewater treatment system in the province. This system uses two types of subsurface constructed wetlands to treat wastewater through both aerobic and anaerobic processes.