The filter basins
New water supply system
As a man-made canal, the Canal de Roubaix has to be regularly supplied with water. Since it was opened in 1877, the canal has been supplied with water from the river Deûle by a pumping station in Lille, elegantly named the “usine élévatoire”.
This system is now life-expired. The pumping equipment itself is antiquated, the flow of the river Deûle is inadequate during the summer months, and the water quality itself is below the required standard. The Blue Links partners therefore had to find an alternative solution to supply the canal with water.
The filter basins, a water reserve of good quality
The solution consists in using part of the outflow from the Wattrelos waste water treatment plant. To guarantee the required water quality, complementary treatment of the outflow is effected by a series of filters planted with reeds, laid out on the banks of a series of basins, commonly referred to as “filter basins”.
This infrastructure complements the system of back-pumping at each lock. This permanent reserve of water is designed to compensate for the permanent losses of water, and will make up the canal’s water supply, particularly during the peak periods of boat traffic (coinciding with low natural flows).
The basins are located on a relatively isolated strip of land between the Espierre stream close to the treatment plant and the Roubaix Canal, within the commune of Leers.
How does the system work?
Water is taken from the outflow of the Grimonpont water treatment plant, and pumped into a first basin planted with reeds, iris and other natural pollution removal species. The water then descends by gravity from basin to basin and is naturally filtered in the process.
After 7 days flowing slowly through the tortuous meandering path of the successive filters, the water finally arrives at the ninth basin, called the “storage basin”, which acts as a reservoir. At this stage the water has been completely purified and is now of a very high quality. It spills by gravity into the canal.
There are manual shut-off valves which allow each basin to be isolated in the case of accidental pollution. They will also be used for maintenance of the site.
As well as fulfilling its primary function as water supply, the filter basins meet another objective: they will form valuable “breathing space” around the canal, a natural conservation area covering 8 hectares, which will be open to the public and ideal for environmental education.
Particular care is taken to ensure accessibility of the site to the physically disabled. LMCU hopes to obtain the “Tourism and handicap” label for this site.
Vertical and horizontal filtration
The filters are lined with successive layers of gravel and sand of various grain sizes, depending on the quality of the water to be treated. The water is distributed directly over the entire surface area of the filter, and is subjected simultaneously to physical, chemical and biological treatment.
The first filtration area works vertically, with three units, each unit being supplied in succession. The filtrate is drained to the bottom of the filter then directed to the next stage. At the end of the vertical filter, the water is separated into two parallel channels, so that flow does not have to be interrupted when one channel is closed for maintenance.
The system also has a series of horizontal filters with plants - the channels – which are laid out around several of the basins. In each channel, the water flows horizontally through a bed of fine gravel. At the end of the process, the clean, treated water is stored in a single basin with a capacity of 9000 m³, enabling the canal to be “topped up” during periods of drought.
A new natural area bordering the canal
The filter basins are designed to provide an attractive natural area. The new wetland zone thus created will favour the development of various ecological habitats, and will increase biodiversity in the ecological corridor formed by the Roubaix Canal. Overall, the area comprising the filter basins and the canal will take on a whole new dimension for environmental education.
The landscaping of the basins is essentially focused on the planting of aquatic species contributing to water purification (cattails, reeds, bullrushes, iris….), but also has flower meadows in the flood expansion areas.
Around the various basins, compacted stone paths make the site fully accessible to the public, including people with impaired physical ability. Thus, the linear path along the canal widens here into an attractive natural area.
Works on the site have now reached the final stages. All the earthworks to form the various basins and channels have been completed, the filters have been laid out and planted. It remains to complete the paths and to plant the shrub beds projected on the site.
The works should be completed early in the new year. The old water supply system will however remain in operation for several months, to give the new system time to become fully operational, with the required level of performance.