Los Artificial wetlands have become one of the great nature-based solutions to address serious problems such as water pollution, biodiversity loss, or drought. They are not simply ponds with plants: behind them lies hydraulic design, complex biogeochemical processes, and extensive scientific research that supports their effectiveness.
Today, these systems are used for purify urban wastewater, treat agricultural runoff, improve the quality of lagoons and rivers and even transform waste such as slurry or water treatment plant sludge into useful resources. Furthermore, they provide shelter for wildlife and act as small green lungs that capture carbon and help mitigate the effects of climate change.
What are constructed wetlands and how do they work?
When we talk about artificial wetlands, we are referring to aquatic systems specifically designed to mimic the functioning of natural wetlandsThey utilize physical, chemical, and biological processes to remove contaminants from the water. They are built in a controlled manner, but their internal logic is that of a living ecosystem.
In essence, these systems They reproduce the decomposition of organic matter and the nutrient cycle (nitrogen, phosphorus, carbon) that occurs in marshes, salt marshes, shallow lagoons and other wetlands. The main objective is usually the treatment of wastewater or agricultural runoff, although they are also used as green infrastructure to improve rivers, lagoons and coastal areas.
Its use can range from a scale from domestic or small rural community to facilities of several hectaresespecially when used as polishing (refining) treatments after a conventional wastewater treatment plant or as green filters that receive water from agricultural or urban sources.
The typical structure of an artificial treatment wetland consists of a bed of sand and gravel placed on top of a waterproof barrier (of compacted clay or synthetic geomembrane), to prevent unwanted infiltration into the subsoil. Aquatic macrophytes are planted on this bed (reeds, reeds, rushes, etc.), whose roots partially oxygenate the substrate and facilitate the formation of bacterial biofilms responsible for purification.
These systems are designed to eliminate a large part of the pathogens, nutrients (nitrogen and phosphorus), organic matter and suspended solids contained in the water. In many cases, they function as the final stage before the treated water is discharged into natural wetlands, lagoons or bays, thus drastically reducing the risk of eutrophication.
Key elements of an artificial wetland
Constructed wetland technology acts as a complex ecosystem in which water, substrate, microorganisms and vegetation interactEach of these components fulfills a specific role and all are necessary for the system to function efficiently.
First, there is the water to be treated, which can come from urban wastewater, agricultural irrigation return flows, urban runoff, or even slurry and other agricultural and livestock wasteThis water circulates through the filter bed and/or around the stems and roots of the plants, where most of the removal of contaminants takes place.
The granular substrate (usually sand, gravel or pebbles of different particle sizesIt serves as a support for plant rooting and, above all, as a surface for the microbial population to attach. These bacterial biofilms are the true "workers" of the system: they carry out processes of organic matter degradation, nitrification, denitrification, adsorption, and transformation of various compounds.
The vegetation is mainly composed of emergent aquatic plants, the so-called helophytes (Reeds, rushes, cattails, bulrushes, etc.). These species are adapted to waterlogged, oxygen-poor soils, as they possess internal tissues (aerenchyma) that allow the transport of oxygen from the aerial parts to the roots. In addition to their ecological function, they contribute significantly to the landscape integration of the wetland.
Types of constructed wetlands: surface and subsurface flow
From a hydraulic point of view, artificial wetlands are mainly classified into of free surface flow and subsurface flowIn turn, subsurface flows can be horizontal flow or vertical flow, with important differences in their operation and in the quality of the effluent obtained.
In surface flow or free-surface wetlands, the Water circulates above the substrate, visible and in direct contact with the atmosphereThese are shallow ponds or channels (usually with water levels below 0,4 m), on the bottom of which emergent plants grow. From a functional point of view, they can be considered a variant of classic lagoon systems, with less depth and vegetation rooted in the bed.
This type of facility usually occupies several hectares and is frequently used as refining treatment after secondary purification processesThe purified water circulates among the stems and roots of the vegetation, and it is the processes of sedimentation, filtration, adsorption and microbial activity that allow further improvement of the effluent quality, which can then be reused in an environmentally friendly way or discharged into sensitive ecosystems.
On the other hand, in subsurface flow wetlands, water moves underground, through the pores of the granular medium (sand, gravel, pebbles), without usually being visible on the surface. This design provides extra protection against odors, insects, and direct contact, and also improves the system's thermal insulation.
These wetlands are constructed within enclosures waterproofed areas containing the filter material and vegetationwith substrate depths of around 0,6-1,0 m. They usually require less surface area than surface flow systems and, in many cases, function as secondary treatment of wastewater from small population centers.
Among its main advantages is the reduction of bad odors, less exposure to mosquitoes and protection against the cold (thanks to underground circulation and the accumulation of plant remains on the bed). In return, they present some drawbacks, such as higher construction costs per unit area due to the granular material, risk of clogging (especially in horizontal flow systems) and a somewhat lower value as a habitat for wildlife, since the water is not accessible.
Within this category, horizontal subsurface wetlands are normally fed continuously (although they can operate intermittently if pumping is necessary), and the Water moves horizontally through a gravel-pebble bed of about 0,6 mAn outlet pipe allows the flood level to be regulated, which is kept a few centimeters below the surface of the substrate, so that the water is not visible.
In vertical subsurface wetlands, on the other hand, feeding occurs intermittently. They are used Pumping controlled by timers or level floats, or discharge siphons if the topography allows it, to distribute the water evenly over the entire filter surface through a network of pipes with discharge points spread throughout the wetland.
Water enters from the top and circulates vertically through a filter substrate made up of layers of sand, gravel and pebbles approximately 1 m thickAt the bottom is a drainage network that collects the treated effluent. Additionally, pipes are installed that protrude from the aggregate bed to promote natural ventilation and increase substrate oxygenation through the chimney effect.
The oxygen supply through plant roots in these vertical wetlands is relatively small compared to that achieved through alternating filling and emptying periods and ventilation. Therefore, Vertical systems operate primarily under aerobic conditionsThey accept higher organic loads and generate well-oxygenated and odor-free effluents, while horizontal ones operate mainly under anaerobic conditions and produce water with very low dissolved oxygen.
In terms of hydraulic retention times, horizontal subsurface wetlands require several days of water remaining in the systemwhile vertical systems operate with flow times of just a few hours, allowing for the treatment of higher flow rates per unit area.
Pretreatment and purification schemes with wetlands
For an artificial wetland to function well in the long term, it is essential incorporate an appropriate pretreatment stageThe goal is to retain coarse solids, sand, and settleable matter that would otherwise cause rapid clogging of the filter bed and drastically reduce the system's lifespan.
The typical scheme begins with a roughing of the wastewater, preferably by grates or automatic cleaning equipment whenever feasible. In combined sewer systems (which collect wastewater and rainwater), a grit removal stage is usually added to eliminate sand and gravel carried by runoff.
Next, the water undergoes primary treatment, which is usually carried out in a septic tank or Imhoff tankThere, a large part of the settleable and floating particles are separated and stabilized, reducing the load of solids that will reach the wetland and decreasing the risk of clogging of the filter bed.
The effluents from the primary treatment are then directed to the horizontal flow wetlands (continuous feed) or towards vertical flow wetlands (intermittent feed)Depending on the chosen configuration. In some countries, such as France, it is very common to combine several vertical wetland cells arranged in parallel and in series, achieving high levels of purification with very compact designs.
These combined schemes allow us to take advantage of the complementary advantages of each typeFor example, using vertical wetlands as the main treatment with high aeration and good removal of organic matter, followed by horizontal wetlands as a polishing and denitrification stage, or finishing with a surface flow system to maximize the ecological value of the whole.
Artificial wetlands as green filters against eutrophication
Wetlands, whether natural or constructed, are extremely productive ecosystems, with a high capacity to process carbon and nutrientsHowever, this same productivity has led to many wetlands being transformed into agricultural crops, especially throughout the 20th century, with the consequent ecological deterioration.
Agricultural intensification, the massive use of fertilizers, and changes in land use have generated alterations in hydrology, increased river and coastal flooding, increased salinity, and frequent eutrophication episodesThe latter occurs when excess nitrogen and phosphorus triggers algae growth, depletes dissolved oxygen, and degrades the quality of water and aquatic habitats.
To counteract these impacts, artificial wetlands have been constructed that act as green filters located between agricultural fields and receiving lagoons or baysReturn water from, for example, rice paddies, arrives loaded with nutrients and other pollutants, and passes through these wetlands before mixing with natural water bodies.
Experience shows that these green filters are capable of significantly reduce average concentrations of ammonium, nitrite, nitrate, and phosphateThis significantly improves water quality. At the same time, they help retain suspended solids through sedimentation and reduce pesticides and other substances through adsorption, degradation, and biogeochemical transformation processes.
In this way, artificial wetlands serve a dual purpose: on the one hand, They protect lagoons, estuaries, and deltas from eutrophicationOn the other hand, they provide additional habitat for fauna and flora associated with aquatic environments, reinforcing biodiversity in landscapes highly transformed by agriculture.
Wetlands, decomposition of organic matter and the carbon cycle
Although its purifying effectiveness is increasingly well documented, research is still being conducted on how Natural and constructed wetlands influence decomposition processes and the carbon cycleThe decomposition of leaf litter and plant biomass is key to understanding carbon storage and soil formation in these systems.
Recent experiments have evaluated, for example, the decomposition of leaves of reed (Phragmites australis) and bulrush (Typha angustifolia) in different types of wetlands in the Ebro Delta. For this purpose, nets with leaf litter were placed in different areas of natural and artificial wetlands, left exposed for a certain period of time and, subsequently, the weight loss was measured as an indicator of the amount of degraded matter.
The results suggest that the Surface water runoff from agricultural activities decomposes leaves in a very similar way in both types of wetlandsThis suggests that both natural and built ecosystems play an important role in carbon processing, provided they are exposed to comparable flux regimes.
In the natural wetlands analyzed, it was estimated that the time needed to decompose 95% of the reed litter ranged approximately between 58 and 150 days, while that of cattails required much longer periods, between 288 and 856 daysIn artificial wetlands, decomposition was generally somewhat slower.
This slower behavior in the constructed systems implies that organic matter remains available for a longer timeThis promotes the accumulation of detritus, soil formation, and carbon sequestration in the sediment. From a climate change perspective, this additional carbon storage represents a highly significant co-benefit of constructed wetlands.
Furthermore, these studies confirm that when plant species native to natural wetlands are incorporated into artificial installations, The ecological functions of the system are enhanced, bringing its behavior closer to that of a well-preserved humid ecosystem and strengthening its capacity to adapt to scenarios of warming and environmental crisis.
Research on water quality and hydraulic design
Recent research at various universities and research centers has shown that Artificial wetlands can work as true buffer systems against pollution spikesespecially when they receive highly variable inputs of urban and agricultural water.
An illustrative case is that of the open-surface wetland known as Tancat de la Pipa, in the Albufera Natural Park of ValenciaWithin the framework of specific research projects, its hydrodynamic behavior and purification capacity have been analyzed when treating both urban runoff and water from agricultural activity.
Modeling and monitoring work has shown that these systems are capable of retain approximately 80% of the suspended solids thanks mainly to natural sedimentation processes controlled by the internal hydrodynamics of the wetland.
Likewise, a notable reduction has been observed in ammoniacal nitrogen due to the combination of dilution, retention and biogeochemical transformation processessuch as nitrification and, in certain areas, subsequent denitrification. These mechanisms are essential to protect aquatic ecosystems from the effects of eutrophication.
Another key aspect that has been highlighted is the Importance of hydraulic design and cell configurationHaving several units in parallel, with carefully calculated residence times, allows for improved treatment efficiency, better distribution of flows and increased operational flexibility, for example, in the face of heavy rain events or seasonal load variations.
These results do not remain in the theoretical realm: They directly inform the design of new green infrastructures intended for the treatment of contaminated water, helping to optimize the sizing of the cells, the arrangement of inlets and outlets and the selection of plant species adapted to each context.
Transforming waste into resources: slurry and sludge from water treatment plants
Beyond conventional wastewater treatment, constructed wetlands are demonstrating enormous potential for to revalue agricultural waste and water treatment sludgecontributing to circular economy models in the water sector and rural areas.
In some demonstration projects, wetlands are being built in small wastewater treatment plants (WWTPs) in rural areasThe aim is to improve effluent quality and facilitate compliance with the Water Framework Directive. The unique aspect is that they use sludge generated in the drinking water treatment processes as an absorbent material.
Thus, a waste product that was previously considered a liability to be managed becomes become a useful resource within the urban water cycle itselfTaking advantage of their capacity to adsorb and retain certain pollutants. In practice, small surface flow wetlands are constructed as shallow lagoons where different purification schemes are compared.
In parallel, initiatives such as the VALPURIN project focus on the Development of nature-based solutions for the sustainable treatment of slurry and the subsequent valorization of its fractions. The objective is to minimize the environmental impact of these wastes on the soil and water, transforming them into usable products (for example, fertilizers or organic amendments of agronomic value).
In this context, constructed wetlands are used as part of innovative treatment chains that reduce the pollutant load and allow for nutrient recoveryIn this way, progress is being made towards a more circular economy in the agricultural and livestock sector, and a contribution is being made to mitigating greenhouse gas emissions associated with the inadequate management of slurry.
Artificial wetlands and biodiversity: the example of La Albufera and the Charco de Tamujo
In addition to their purifying role, many constructed wetlands have proven to be authentic refuges for fauna and floraespecially in territories where natural ecosystems have been greatly transformed by agriculture or urbanization.
In the Albufera Natural Park (Valencia), a large freshwater lake of more than 2.800 hectares that It is home to some 300 species of birds, many of them aquatic.Three “tancats,” or artificial wetlands, have been created on former rice paddies. These spaces are designed to improve the lake's water quality and, at the same time, promote biodiversity.
The most emblematic one is the Tancat de la Pipa, some 40 hectares of former rice paddies transformed into a mosaic of aquatic habitatswith dense vegetation that acts as a natural filter to retain pollutants. For their part, the Mília and L'Illa enclosures maintain restricted public access to maximize tranquility and the protection of wildlife.
The work carried out in these wetlands has allowed them to become key refuge areas for the birdlife of the Albuferaincluding such unique species as the bittern, an extremely discreet heron that camouflages itself among the reeds. Its presence is an indicator that the appropriate conditions of calm and habitat quality are being achieved.
In the province of Ciudad Real, the Charco de Tamujo has undergone a restoration process in the last decade This project has involved planting more than 12.000 specimens of up to 24 different species of trees and shrubs. The creation of this artificial wetland has contributed to the recovery of the Mediterranean woodland and riparian zones.
This space has become established as a place of refuge and breeding for numerous animal speciesFrom otters to various aquatic and riparian birds such as the red-crested pochard, common pochard, water rail, European bee-eater, great reed warbler, and penduline tit. Organizations like the Global Nature Foundation emphasize that these wetlands reproduce, in a controlled manner, the physical, chemical, and biological processes of natural wetlands, allowing for the removal of dissolved pollutants and providing valuable habitats.
Taken together, experiences like those of La Albufera and the Charco de Tamujo demonstrate that Renaturalization through artificial wetlands helps to mitigate the effects of drought and the climate crisisIt promotes the conservation of native vegetation and creates authentic refuges for birds and other faunal groups.
To date, the knowledge accumulated by the scientific community and field management experiences make it clear that Constructed wetlands are highly versatile tools for improving water quality, protecting soils and aquatic ecosystems, and promoting a more circular economy. in the use of water resources and agricultural waste. When properly designed and integrated into the territory, these systems not only purify, but also restore landscapes, store carbon and give a second life to degraded spaces, offering environmental and social benefits that are difficult to match with other conventional technologies.
