Impressive with all these basins, the Tougas wastewater treatment plant is one of the largest in France.
Its mission is to clean up wastewater and rainwater.
A lot of solid waste is collected in the process.
What a funny idea to visit a wastewater treatment plant! “It's ugly”, “it's dirty”, “it stinks”, one might imagine before even setting foot on it. However, these wastewater treatment plants regularly receive school children and the general public. “There is a great lack of knowledge of what is happening there when they are essential to our daily lives. Our visitors appreciate and generally come away with a completely different perspective on the water cycle, ”emphasizes Marielle Pouliquen, deputy director of the Suez agency in Nantes.
This Saturday morning, one of the largest wastewater treatment plants in France opens its doors as part of the European Heritage Days: the Tougas station, west of Nantes.
Spread over twenty hectares not far from the Loire, its mission is to treat the discharges of around 600,000 inhabitants.
“When you flush your toilet, empty your sink, it happens here,” confirms Kevin Boucaron, site process manager, with a smile.
But the particularity of Tougas is that it also receives, through the same pipes, rainwater from downtown Nantes.
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Cans, cotton swabs, masks, wipes ...
Even though it operates with a few staff, the factory never stops. The water, which therefore arrives contaminated, must come out clean, less than 24 hours later. A process that goes through four main stages. The first is “screening”: the raw effluents pass through increasingly fine screens in order to block the solid waste. “We find cans, pieces of plastic, packaging, bits of wood that are carried by the rain and fall into street drains. We also have large quantities of cotton swabs, wipes and even hygienic protections or masks. They should never get there. This is problematic, because they tend to press against the equipment and can cause traffic jams, ”says Marielle Pouliquen.
After the screening comes the phase of "sand removal-degreasing", which consists in extracting, by a movement of air, the grease and grains of sand.
Each time, the odors are captured and treated “not to inconvenience the residents”.
At this stage, the water is already quite loose.
She then heads for the huge "biological treatment" basins.
A spectacular third stage, in the open air.
“In these ponds, different types of bacteria do the work,” explains Kevin Boucaron.
They feed on pollutants, mainly nitrogen, carbon and phosphorus, which they dissolve.
The deposit falls to the bottom of the basins and only clear water remains on the surface.
" The hardest thing is done.
"We send it directly to the Loire"
The fourth step then consists of sucking up the "mud" from the bottom of the basins and drying it by centrifuge. Once collected and completed with lime, this sewage sludge can be reused as a fertilizer in agriculture. Sometimes the biological treatment is "out of order", a sign of a greater presence of chemical pollutants. “The wild discharges of industrialists on the public network, it still exists. In general, avoid dumping chemicals and detergents in the sink. It does not all magically disappear. "
And then the sanitized water, what becomes of it in the end?
“We send it directly to the Loire,” says Marielle Pouliquen.
Its quality is continuously measured and regularly checked.
It is clean enough to be rejected but still not clean enough to be drinkable, contrary to some popular belief.
We have extremely secure drinking water treatment plants for that.
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The Tougas treatment plant treats nearly 4 million m3 of gray water per month, the equivalent of 1,100 Olympic swimming pools.
Its capacity will have to be further increased over the next few years to support demographic growth.
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