Learn more about constructed wetlands
Water has always represented an asset of primary importance for the life of all living beings. The growth of the population and the productive activities related to it have led to a strong conflict for the use of water resources, also accentuated by its progressive degradation in quality because of the pollution processes. The alteration of the environmental quality of water bodies involves the alteration of biodiversity, less availability of the water resource for human consumption, and, sometimes, also a threat to the health of living species, including humans. At the national and European level, water policy aims to ensure the satisfaction of human and environmental water needs through the maintenance of high-quality standards. It should be considered that, according to the national summary data for surface waters, only 43% of the rivers achieve the quality goal for the ecological status and 75% for the chemical status.
Among the origins of water pollution, it is important to distinguish the nonpoint and point sources. In this regard, all activities related to the agricultural, forestry and livestock sectors may release pollutants into the environment in a variable way in space and time (nonpoint sources of pollution); instead, all productive activities, including industrial and civil settlements, contribute to pollution in a precise way in space and time (point source of pollution).
Bioremediation is a general term identifying a set of natural processes of wastewater treatments due to the triple interaction water-biosphere-soil system. In this context, constructed wetlands are a natural water purification technique that is based on the principle of reproducing the same chemical, physical and biological self-purification processes that characterize aquatic habitats, swamps and natural wetlands. In most cases, constructed wetlands represent a secondary or tertiary treatment, downstream of the traditional purification processes, even if in some cases excellent purification performances are obtained after a pre-treatment suitable to eliminate only coarse materials.
Constructed wetlands are usually classified based on the following criteria: i) hydrology (surface or sub-surface flow), ii) flow path (horizontal or vertical), iii) type of plant species (emergent, submerged or free-floating). By combining the above information, constructed wetlands systems are divided into:
- surface flow constructed wetlands, that is dug basins, characterized by 20-30 cm of soil or substrate and 30-40 cm of water, in which wastewater flows slowly, passing through non-vegetated areas suitable for the sedimentation of the particulates and vegetated bands with macrophytes arranged perpendicular to the direction of the flow;
- floating surface flow constructed wetlands, made by exploiting floating hydrophyte species such as aquatic lettuce hyacinth or, much more often, traditional perennial herbaceous macrophytes supported by a floating support;
- sub-surface flow constructed wetlands, conceptually different from those with surface flow, in which the wastewater is never brought to the surface. In horizontal flow systems wastewater flows parallel to the structure, in vertical flow systems wastewater percolates, moving from the surface of the substrate into depth;
- hybrid constructed wetlands, born as a combination of two or more depuration systems (sub-systems), connected in series.
Constructed wetlands were developed as an eco-technology for the treatment of different types of waste, including agricultural drainage and zootechnical wastewater, urban run-off water, by-products deriving from industrial activities (food industries, wine production, slaughterhouses, paper mills, textile sectors, car washing), but the prevalent use concerned municipal wastewater. The most adapted plant solutions concerned the use of hybrid systems, capable of exploiting the specific purification performance of each sub-system. These systems proved to be quite promising, reducing the concentrations of total nitrogen up to 95%, nitric nitrogen up to 89%, ammonia nitrogen up to 99%, total phosphorus up to 94% and organic matter up to 95%.