The result is a very simple operating system

In these times of sustainable development, nature gradually resumed its rights in the industrial field. The image of the filter lagoon researchers of the Lyonnaise des Eaux were put in place this summer at the plant in Périgny-sur-Yerres (Val-de-Marne), which provides the production of drinking water for more than 1 million inhabitants to the South of the Paris region. As all the sites that are not can be connected to the network because of their isolation and using an active carbon filtration system, this plant must treat the filter wash water prior to discharge into the natural environment.

As a first step, the researchers of the Group had opted for another solution. It required the achievement of a concrete storage basin, the implementation of a mobile filtration unit and rejection tarps equipped with a low-power pump installation.

Simplified maintenance

More classic, this initial project was reliable technically, but required the construction of special foundations. Because this plant is, indeed, located in a flood zone, on the edge of a small river, the Yerres. With another black point: this place is also listed in natural area of ecological interest, fauna and Flora (ZNIEFF). Because of the cost of this project, of the order of 400,000 euros, "he was released on the table to find less onerous in terms of investment solutions", explains Pierrick Labat, head of agency work at the Lyonnaise des Eaux.

For overcoming technical barriers, it took a month of previous studies at hydraulic and geotechnical level. First, "the site should not require heavy earthmoving equipment, because this site is difficult to access and full residential area," said Pierrick Labat. Then, the dismissal of clarified water should conform to the regulations in force, the process for the protection of persons and operations simplified to minimize maintenance.

To find the correct solution that meets both the requirements and be respectful of the environment, researchers are inspired lagoon systems already used in sanitation. So they led to the development of an innovative system of filter Lagoon which has two advantages: a much lighter investment, of the order of 100,000 euros, and a rapid implementation in only six weeks. The principle Stretch over an area of 500 square metres, the lagoon is planted of familiar plants of aquatic environments, such as phragmites (Reed filter), the iris of water or the loosestrife (perennial) that preserve its ability to filter by avoiding any clogging their root system.

Rejection in nature

To ensure perfect sealing of the lagoon, two layers of geotextile surround a geomembrane, a kind of dense polyethylene felt that prevents any risk of punching that may cause a leak. To facilitate the feeding of the lagoon without pumps, energy-consuming, researchers have relied on the topography of the land. The result is a very simple operating system. Once a week, for ten minutes, the water to be treated is rejected at a rate of 600 cubic metres per hour on the entire surface of the lagoon. Microparticles of activated carbon grains in the wash water, forming a finer dust that the icing sugar, are retained by filtration on layers of natural substrates. Thus treated water is stored deep in the lagoon, and then gradually rejected to the environment via a pump with a flow rate of 5 litres per second. It is the only position of energy expenditure.

Implemented for the first time in France by the Lyonnaise des Eaux, "this system is interesting insofar as there is no nearby sewerage network because treat wash-water on the spot," said Pierrick Labat. A second similar to Périgny filtration Lagoon is being study on groundwater station in Champigny sheet (Seine-et-Marne). Its commissioning is expected in 2009.