Nature-based solutions - Sustainable water management

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Constructed Wetlands

Constructed wetland is a nature-based solution for the wastewater treatment, which recreates the removal processes developed in natural wetlands, exploiting complex biochemical, physical, and physiological removal processes.

The IRIDRA experience allow to propose the better solutions in this field, from the classical constructed wetlands to intensified constructed wetland (called by us constructed wetlands 2.0).

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Constructed wetlands have been utilized for centuries for the treatment of wastewater produced by residential settlements and productive activities. In most cases, however, wetlands were considered merely as storage basins before the discharge to the final receiving hydric element, not as real depuration plants; consequently, natural wetlands quality has been irreversibly degraded due to uncontrolled discharges and incorrect valuations of the environmental impact of wastewater. In common culture, in fact, wetlands have historically been considered insane and not proper for human life, thus, till anthropocentric vision of the world has prevailed, they have been completely set aside also by the scientific world.

Instead, in the last 30 years, we have assisted to a real increase of the interest and to a radical change in their consideration (Williams 1990). In fact, the numerous different benefits of wetlands have been identified, such as possibility of water supply (refill of underground waters, potable an irrigation use), good work for hydraulic control (expansion basins for prevention of floods), exploitation for extraction activities (sand and gravel), utilization of plants present in the wetlands (prime materials for alimentary, cosmetic and medical products, forage, timber, production of paper, fertilizers), presence of free animals (migratory birds, beverage for many species), presence of fishes and invertebrates, possibility of utilization for integrated productions (for example fish raising combined with rice cultivation), control of erosion and desertification, and a great contribution to bio-diversity, possibility of utilization as energy sources (hydroelectric, sun, heat pumps, gas, biomasses) and finally educative and recreative activities (Mitsch & Gosselink 1986, Sather et al 1990, Whigham & Brinson 1990).

Natural wetlands

Natural wetlands are characterized by an extreme variability of their functional components, so to make virtually impossible to foresee the consequences of the discharge of wastewater and to move results from one geographic zone to another. Though can be observed good improvements in the quality of wastewater after a transition in natural wetlands, it is not possible to give a precise quantification of their depurative capacities (Brix 1993). So, starting from the middle 1970s, on this basis have developed numerous experiences of planned and controlled utilization of the auto-depurative capacity of some natural wetlands to obtain precise water quality goals, and, most of all, numerous experiences of "reconstruction" or "creation" of wet systems designed to treat wastewater. The tendency, in fact, has been to preserve existent natural wet areas and to design and realize appropriate wetlands for the purification.

Constructed wetlands

Application of constructed wetlands (rebuilt natural systems) for treatment of wastewater represents by now a diffused solution in many parts of the world. Many research activities have been realized by universities and agencies in UK, Denmark, Germany, USA, Austria, France, etc.: since 15 years ago they have studied pilot and real scale plants and determined models and process kinetics, utilizing data from monitoring that take in account climatic condition of the area, characteristic of wastewater and technical solutions implemented.

Constructed wetlands, on the opposite, offer a greater grade of control, allowing a precise valuation of their efficiency based on the knowledge of the type of substrate, vegetative typologies and hydraulic paths. Furthermore, constructed wetlands offer additional advantages in respect to natural ones, like the possibility to choose the site, the flexibility in dimensioning and geometric solutions, and most of all the control of hydraulic paths and retention times.

In these systems, pollutants are removed by a combination of chemical, physical an biological processes, like sedimentation, precipitation, adsorbing, assimilation by plants and microbial activity. (Brix 1993).

Some natural wetlands are still utilized to treat wastewater (Kadlec & Tilton 1979, Chan et al 1982, Olson 1993), but, at the moment, result more diffused and more efficient the utilization in all the world of constructed wetlands (Reddy & Smith 1987, Hammer 1989a, Cooper & Findlater 1990, Moshiri 1993, Bavor & Mitchell 1992, Kadlec & Brix 1995, Kadlec & Knight 1996, Vymazal et al. 1998).

Systems for treatment of wastewater by artificial wetlands are engineered systems that have been designed and realized with the aim of reproducing the natural auto-depurative processes in a more controllable environment.

The first experience of this kind is dated 1952 when Seidel began to experiment at the Max Planck Institute of Plon (Seidel 1955). After 20 years of researches in 1977 was realized the first real scale "constructed wetland" plant, at Othfresen in Germany for the treatment of civil wastewater (Kickuth 1977).

Constructed wetlands typologies

Constructed wetlands typologies can be classified according to the prevalent form of life of the present macrophytes (Brix 1993):

Floating macrophytes systems (Lemna,...);
Submerged rooted macrophytes systems (Elodea,...);
Emergent rooted macrophytes systems (Phragmites, Typha,...);
Multi-stage systems (combinations of the precedent typologies or with low-tech systems as lagoons or sand-filter).

Emergent rooted macrophytes systems can be again classified according to the hydraulic path of wastewater, i.e. one of the most common classification for conventional constructed wetlands:

Free surface flow or free water systems (FWS);
Horizontal subsurface flow systems (SFS-h or HF);
Vertical subsurface flow systems (SFS-v or VF).

Most recent technical solutions are classified as intensified constructed wetlands (which we like to call constructed wetland 2.0, since they represent an improvement in comparison to conventional solutions), i.e. CWs in which some improvement has been introduced to increase the performances and/or reduce the costs: for instance, French reed beds for raw wastewater, which avoid primary treatments; or aerated wetland systems (Forced Bed AerationTM), which decrease the areal footprint in comparison to conventional solutions.

Constructed Wetlands vS conventional technological solutions - Investment and O&M costs

Investment costs of constructed wetland (CW) systems are usually comparable with those of conventional technological solutions (e.g. activated sludge) up to the threshold of 2000 PE; above this value, scale effect leads to less investment costs for compact technological solutions. On average, and on the basis of our Italian Experience, investment costs for CW is about 100 €/mq of net surface; this is only an indicative value, since they can vary according to with different water quality standards to be met, type of used CW system, WWTP size, and local material costs. For instance, the investment costs for small settlement can reach up to 150-200 €/mq. 

 costi di realizzazione  web

Comparison among investment costs of CW and activated sludge WWPTs on the basis of Italian experience (IRIDRA's analysis published in Masotti, 2009)

Operational and Maintenance (O&M) costs are significantly lower for nature-based solutions in comparison with activated sludge systems, due to the almost negligible costs of consumed energy. Usually, O&M costs of CW for wastewater treatment is on average equal to 14 €/PE per year (even lower if french systems for raw wastewater treatment are adopted, and equal to 7-8 €/PE per year), therefore about 5-10 less than O&M costs for activated sludge systems (50-100 €/PE per year). Moreover, CW WWTPs do not require specialized and permanent personnel to be maintained, reducing, even more, the O&M costs.

costi di gestione   1  web

Comparison of energy consumption for different conventional and nature-based treatment technologies (Kadlec&Wallace, 2009)

costi di gestione   2  web

O&M cost comparison between CW and activated sludge WWTPs on the basis of Italian experience (IRIDRA's analysis)


Kadlec R.H., Wallace S.D. (2009), “Treatment wetlands – Second Edition”, Lewis, Boca Raton.

Masotti L. (2011), Depurazione delle acque, Calderini.