What Are Constructed Wetlands? Overview and Benefits

Wetlands like freshwater ponds, swamps, and marshes are excellent natural water purifiers. They are also uniquely equipped to improve water clarity and quality through the removal of kicked-up sediments, toxins, and pollutants. Freshwater wetlands also provide habitat for 20-40% of the world's flora and fauna.

However, the amount of freshwater wetlands around the world is rapidly declining. Recognizing the special capabilities of the world's natural wetlands, people have worked to construct new wetlands where their unique qualities are needed.

The Benefits of Constructed Wetlands

Since the 1960s, the number of constructed wetlands around the world has increased between 5 and 50%. Unlike natural wetlands, which are kept clean thanks to environmental regulations, constructed wetlands are often built to aid in the treatment of wastewater. By carefully monitoring the constructed wetlands' biological and chemical processes, wetlands used in wastewater treatment can help further clean wastewater before it is returned to natural waterways.

To ensure proper functionality, wetlands constructed for wastewater treatment purposes are designed to be manually adjusted. The management of wetlands constructed for use in wastewater treatment typically involves manual adjustments to the amount of water in the system to ensure the wetlands achieve the proper characteristics for breaking down and removing unwanted substances from the water.

Wetlands also naturally accumulate sediment. The texture created by wetland plants slows down water flows, allowing kicked-up sediment time to fall out of the water like snow in a snow globe. By slowing down the flow of water, wetlands can also collect pollutants. By placing constructed wetlands near known sources of pollutants, like near farmlands where pesticides often end up in rainwater runoff, constructed wetlands can help prevent pollutants from spreading across entire ecosystems.

Constructed wetlands also provide habitat for a variety of plants and wildlife. While these built wetlands do not provide habitat that's quite as high-quality as natural wetlands, they can still greatly benefit wildlife.

How Are Constructed Wetlands Built?

Constructed wetlands for use in water treatment generally fall into one of two categories: subsurface flow systems and free water surface systems.

Subsurface Flow Systems

Constructed wetlands designed as subsurface flow systems are used to keep the water being treated below the water's surface. The design is meant to prevent the development of unwanted odors and other nuisances.

There are two forms of subsurface flow systems: horizontal and vertical.

Constructed wetlands that use horizontal subsurface flow typically have beds made of gravel or rock sealed with an impermeable layer. Vegetation is planted among the rock. The freshwater source is placed above the water's outflow location, causing the water to flow below the surface. As the water flows horizontally from inlet to outlet on the floor of the constructed wetland, microbial and chemical processes degrade and remove contaminants from the water.

Constructed wetlands that use vertical subsurface flow have a more intricate design and require greater operation and maintenance effort. The vertical subsurface flow design was originally engineered to help add oxygen to oxygen-lacking water before it flows out of septic tanks. Instead of flowing continuously, like most horizontal subsurface flow systems, vertical subsurface flow wetlands receive water for treatment in large batches. Each water batch is then left to percolate through a sandy layer below. The wetland receives the next batch of water once the last batch has completely percolated and the bed is free of water.

The stepwise operation of vertical subsurface flow constructed wetlands allows for improved oxygenation of the wetland bed. Vertical flow systems also require far less land than horizontal flow systems to treat an equivalent volume of water.

Today, some places use constructed wetlands with a hybridized design that uses elements of both horizontal and vertical subsurface flow systems. Such hybrid designs are particularly useful in removing ammonia and total nitrogen from the inflowing water. In addition to being applied to sewage, hybrid systems have been built to purify water coming out of aquaculture facilities, wineries, and compost facilities.

Free Water Surface Systems

Constructed wetlands designed as free water surface systems most closely match the way in which natural wetlands function. Unlike constructed wetlands designed as subsurface flow systems, water treated by the free water surface wetlands comes in direct contact with the air above.

Most free water surface constructed wetlands are designed to be marsh ecosystems, but occasionally swamps and bogs are created, too. These constructed wetlands are usually shallow and have a sealed basin or series of basins. A submerged soil layer allows plants to root. Most of the wetland surface is usually covered in vegetation, which aids in the filtration of pollutants. Free water surface systems are not as effective at removing phosphorus from wastewater as subsurface flow systems. However, free water surface flow systems can be designed to have a variety of depths to improve nitrogen removal and to generate higher-quality wildlife habitat.