Greening the City

Biodiversity, the city’s ally

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The Dragonfly zone

Since August 2009, an additional innovative system has been helping to treat wastewater from the water treatment plant at Saint-Just in the Hérault department (France). Dubbed the ‘Zone Libellule’ (‘libellule’ is the French word for dragonfly, and an acronym for ‘LIberté Biologique Et de LUtte contre les poLluants Emergents’—Biological freedom and fight against emerging pollutants), it is made up of a series of wetlands containing over 7000 plants from 48 different species. These local plants have been chosen for their ability, demonstrated in the laboratory, to rid water of some emerging pollutants present in trace amounts and which are subject to European regulations, such as pesticides, drug residues, metals, etc. For instance, there’s water-mint, which can absorb zinc, compounds of which are found in many cosmetics, and reeds, which are able to eliminate some pesticides. “Plants and trees interacting with bacteria, varying flow rates and depths, a residence time for water of around ten days, the action of sunshine, etc: it’s the combination of the various factors in this ecosystem, which is one and a half hectares in area and rich in biodiversity, that is expected to make it an effective purifier, and it doesn’t need any specific maintenance either,” explains Eric Blin, who’s in charge of this project managed by SDEI, a subsidiary of the Lyonnaise des Eaux (a French firm). The Zone is overseen by a scientific committee that includes Christian Drakidès, an ecologist at the Hydrosciences laboratory1 in Montpellier. “During the three years that we’ll be monitoring it, we’ll be using our expertise to study and model the treatment of water by plants and microalgae,” Drakidès adds.

1 - CNRS Laboratory/IRD/Université Montpellier 1 and 2