Natural wetlands are some of the most diverse ecosystems on our planets which serve a critical role in terms of water management and specifically flood prevention, water purification and groundwater replenishment[sc:1]. Despite the tremendous benefits these ecosystems bring to our planet, UN research indicates that environmental degradation is more prominent within wetland systems than any other ecosystem[sc:2]. While efforts are ongoing to reverse the declining state of our natural wetlands, human ingenuity is at work to create man-made or constructed wetlands that would be able to deliver similar benefits, particularly for managing and treating wastewater.
How constructed wetlands clean wastewater?
Constructed wetlands make use of the natural functions of plants and vegetation, soil and other organisms to treat different water streams such as municipal or industrial wastewater or storm-water run-off. Man-made or engineered wetlands, as they are otherwise known, can be designed to emulate the features of natural wetlands, such as acting as a bio-filter or removing sediments and pollutants such as heavy metals from the water. Some engineered wetlands may also provide a habitat for different species of wildlife, although that is usually not their primary purpose.
While some concerns remain regarding the potential long-term effects resulting from toxic materials and pathogens present in natural wetland ecosystems; in the case of constructed wetlands, the treatment of wastewater happens in a much more controlled environment. This is why, in some ways, constructed wetlands draw upon nature’s powers, which they enhance through human design and deploy in a fixed environment that limits negative externalities[sc:3].
The efficiency of constructed wetlands
Overall, constructed wetlands are considered an effective and reliable water reclamation and treatment technology. They can remove most pollutants associated with municipal and industrial wastewater and storm-water; they are usually designed to remove contaminants such as biochemical oxygen demand and suspended solids. Constructed wetlands have also been used to remove metals such as cadmium, chromium, iron, lead, manganese, selenium, zinc, and toxic organics from wastewater.
Academics divide constructed wetland treatment systems into two categories:
- Subsurface Flow Systems: these are designed to allow water to flow below the surface by penetrating a permeable medium. This helps avoid the development of unpleasant odours and other nuisance problems. Such systems are commonly referred to as “root-zone systems”, “rock-reed-filters” or “vegetated submerged bed systems.” The permeable media typically used are soil, sand, gravel or crushed rock. They provide limited opportunity for benefits other than water quality improvement.
- Free Water Surface Systems: these are designed to simulate natural wetlands, as the water flows over the soil surface at shallow depths. In this respect, they are frequently designed to maximize the potential for recreating wetland habitats, while also providing water quality improvement.
It is important to note that free waster surface systems, do not only deliver on wastewater treatment, but they are also operated as wildlife refuges or parks. In some cases, they also provide an area for public education and recreation.
Space for improvement
According to the US Environmental Protection Agency, our experience with constructed wetlands and relevant research results to date suggest that managing constructed wetlands systems as a part of wastewater treatment and habitat creation/maintenance efforts offers considerable opportunities for the future. At the same time, while the technical feasibility of implementing such projects has been clearly demonstrated in various parts of the US and elsewhere, it is also clear that more effort needs to be put before such systems are considered for routine use. There is a further need to improve our understanding of the internal components of these systems, their responses and interactions, in order to promote optimum project design, operation and maintenance.