World Wetlands Day is celebrated every February 2nd to raise awareness of wetlands. This day also marks the anniversary of the Wetlands Convention, adopted as an international treaty in 1971. Its enactment is because nearly 90% of the world's wetlands have been degraded since 1700, decimated three times faster than the forests. However, they are essential ecosystems that contribute to biodiversity, climate mitigation and adaptation, freshwater availability, world economies, and much more.
Motivated by this moment of awareness and celebration, we present the characteristics and advantages of the effluent treatment system known as artificial or constructed wetlands. As the name implies, this method simulates the aforementioned natural environments, which are permanently or seasonally flooded — swamps, mangroves — and which, through a natural process, clean and filter the water.
Consisting of lakes or shallow artificial channels that house aquatic plants, constructed wetlands were initially used in Germany by Käthe Seidel, from the Max Planck Institute, in the mid-1950s, for removing phenol and reducing the organic load of dairy effluents. Today, more than 70 years later, the scope of the system includes the treatment of domestic and industrial effluents, gray water or stormwater runoff. In addition, they can be designed for land recovery after mining or as a mitigation step for natural areas suppressed by land use. In general terms, wetlands act as a biofilter, removing a series of pollutants such as organic matter, pathogens or heavy metals.
In practice, wetlands promote the proliferation of microorganisms that act in the degradation of complex organic matter through chemical, physical and biological systems. The macrophytic aquatic plants are the ones responsible for the functioning of this system. They can be of the emerging or floating type. The floating type is used in designs with relatively shallow channels where most suspended solids are removed by sedimentation or absorption into the plant's root system. On the other hand, the emerging type represents the dominant form in natural wetlands, with its root system attached to the sediment.
In a context in which 884 million people do not have access to drinking water and 2.6 billion people do not have access to basic sanitation, artificial wetlands have gained prominence, according to UNESCO data. Wetlands are seen as ideal tools, associated with low costs, simplicity of operation and maintenance, and the possibility of landscape interaction, besides being well adapted to tropical climates. It is also worth mentioning that these systems do not produce bad odors since they are aerobic treatments with relatively low loading rates and do not encourage the proliferation of mosquitoes since there is no apparent water layer. Two myths that end up discouraging its use.
Over the years, many constructed wetland techniques have been developed and applied, according to the characteristics of the effluent to be treated, the final desired efficiency in nutrient removal, the interest in using the biomass produced and the landscape result. For every 160 liters of sewage flow per day (corresponding to the average production of sewage per inhabitant), 1 to 2 m2 are needed. In addition, it is necessary to foresee 40% of the area destined for support spaces, accesses, etc.
As an icon of sustainability in technical, economic and environmental terms, the constructed wetland stands out for being an SBN – Nature-Based Solution. A strategy seen as an alternative to urban planning aimed at consolidating sustainable cities. In addition, the importance of the topic is reinforced by being present in one of the 17 SDGs (Sustainable Development Goals), more precisely SDG 6, which provides for equal access to drinking water and sanitation by 2030. As an example, it is worth mentioning the project known as Between the Waters, a community garden formed by an artificial swamp that collects, treats and cleans the effluent from the toilets and the Emscher river, the most polluted in Germany. Another example is the Xuhui Runway Park, an innovative urban regeneration project that remits to Shanghai's urban development history and artificially applies several patches of wetlands.
