CONSTRUCTED WETLANDS FOR THE TREATMENT OF WASTEWATER

Constructed wetlands have been utilized for centuries for the treatment of waste water 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 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 assist 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 are characterized by an extreme variability of their functional components, so to make virtually impossible to foresee the consequences of the discharge of waste water and to move results from one geographic zone to another. Though can be observed good improvements in the quality of waste water after transition in natural wetlands, it is not possible to give a precise quantification of their depurative capacities (Brix 1993). So, starting from middle 1970's, 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 waste water. The tendency, in fact, has been to preserve existent natural wet areas and to design and realize appropriate wetlands for the purification.

Application of constructed wetlands (rebuilt natural systems) fro treatment of wastewater represent 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 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 waste water (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 waste water 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 fir the treatment of civil wastewater (Kickuth 1977).

In Italy since only a few years have been realized constructed natural systems for the treatment of wastewater, designed with the application of American and European models, or, sometimes, improvising; this has create from one side new prospective of approach to the problem of depuration, from the other side some perplexities caused by bad working and low efficiencies.

From surveyings of realized plants in Italy, in fact, emerges the frequent absence of scientific approach during designing and the data from monitoring of the plants are often scarcely documented and, when present, are realized uncontinously.

Considered this, is very necessary, most of all during design, to choose technical solutions with a multidisciplinary approach (chemical, biological, hydraulic, landscaping) without approximations and standardizations.

So, it is wished that in a near future there will be more attention by committants, both publi and private, and by managers of integrated hydric cycle in the verification of affidability of chosen solutions and a greater engagement in realization and monitoring of the plants.

This last passage results fundamental to obtain data that can be compared, elaborated and utilized to calculate new constants for modelling of kinetic processes and to comprehend dynamics of functionment necessary for the publication of guide lines for the Mediterranean basin.

A correct valuation of real possibilities of application of natural depuration techniques, both for the secondary treatment of wastewater and for the post-treatment of effluents of technological plants, is the ideal instrument to reach a precise estimation of costs of realization, together with the prevision of management costs.

More, in a legislative context that is receiving the UE directive 91/271 and is reconsidering qualitative standards of discharges, it is necessary to consider the greater utilization for treatment of wastewater and the adaptation of existing plants to new goals, without excessive costs.

Constructed wetlands present the characteristic to answer to these necessities.

Furthermore, it has to be considered the principle of the “sustainable sanitation”, that is based on the implementation of water saving, wastewater reuse and respect of natural hydrologic and biogeochemical cycles.

Considered this, natural depuration systems, both for secondary treatment and post-treatment, represent good plant solutions capable to offer very good depurative efficiency (above all for COD, BOD suspended solids and nitrogen), through easiness of management, low costs, low energy demand and low environmental impact.

Natural depuration systems of wastewater can be applied to different typologies of waste water, as shown in the following table, both as secondary and tertiary treatment (post-treatments).

Fields of application of constructed wetlands

Tertiary treatments are generally applied to wastewaters, previously purified with chemical-physic or oxidation plants (activated sludges, biomasses), that have characteristics that do not respect the limits imposed by existent laws. Principal aims of tertiary treatments are in fact:

• Phosphorus reduction;
• Nitrogen reduction;
• Heavy metals removal;
• Removal of organic substances that have slow biodegradation times and therefore need long retention times;
• Ensure a good buffer action in case of bad working of technological plants;
• Improve microbial and chemical quality of effluents.

It has not to be forgotten that to preserve water resource, most of all in the areas considered sensible according to EU directive 91/271, it is necessary to make compatible the effluent with the hydric receptor, so to not compromise the auto-depurative capacities of the natural system. In this sense the application of tertiary treatments takes a role of fundamental importance.

Then, if we take in consideration the real difficulties in operation and management of “traditional” plants, due to the often great variations of hydraulic and organic loads connected to heavy rain events and tourist fluxes, we see how tertiary treatments can play a buffer action able to minimize negative effects (reduction of depurative efficiencies) consequent to these facts.

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

1. Floating macrophytes systems (Lemna,…);
2. Submerged rooted macrophytes systems (Elodea,…);
3. Emergent rooted macrophytes systems (Phragmites, Thypha,…);
4. 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:

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

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