Students of the project Erasmus and eco-delegates of Seconde of the Jean-Joly High School visiting the Grand Prado wastewater treatment
On Friday 4th November 2022, the students of the project Erasmus+ and eco-delegates of Second of the Jean-Joly High School had the opportunity to visit the Grand Prado wastewater treatment plant to Sainte-Marie. It is a plant which collects wastewaters of the Saint-Denis and Sainte-Marie municipalities and treats it so they can join natural environment with a «bathing water» quality. During this visit, students were first informed about the average water consumption of the Reunionese population, and how to decrease this consumption in order to preserve the very important resource that is water. Afterwards, they had a guided tour where the facilitator showed them how the wastewater treatment plant work and its purpose.
A consumer who want to save the water is expected to use an average of 7 liters per day to meet their needs, but today, a reunionese consume an average of 200 liters of water per day. This is because many people do not really pay attention to their consumption. For example, during a carwashing, a reunionese use an average of 400 liters of water. This wastewater will return to the groundwater and may pollute it, while we could use the car wash station which use only about 80 liters of water for one wash. Moreover, the wastewaters will flow into the sewer and go straight to the treatment plant. The purpose of a treatment plant is to clean the water to make it more viable for the environment. The water released from a treatment plant is not drinkable. But it’s no more dangerous for the environment. To make the water drinkable, it must go through a water treatment plant. The Grand Prado treatment plant project had, in addition of its initial purpose to treat the water, an objective visual and olfactive. It was necessary to decrease the view of the treatment plant as much as possible. This is why several buildings are partly underground and why there is a large garden area.
It was also necessary to reduce the smell released by the station as much as possible to not disturb the peoples around. Some «electronic noses» are set so the station is alerted when the smell released is too strong. Around the treatment plant, the neighborhood is also formed to recognize the smell of the station, and if this one is unusual, to send a message to the director of the station.
Wastewater treatment is not easy, especially when not everyone is willing to do their part to try not to pollute the water we use. There is a whole process to clean up the wastewater.
When the water leaves our home, his path continues to the treatment center (yogurt jar, cotton swab…) is above all, removed and discarded as water then reaches a basin where fine bubbles are injected. This allows the greases to rise to the surface while the sand falls to the bottom it is the step of deoilling.
Then, it’s the turn of the lamellar decanter to continue the work. Its operation is based on inclined slats that will allow suspended particles to regroup. Theses particles, resembling mud, gradually fall to the bottom of the basin. Then, a rotary scraper will collect the mud and evacuate it.
Another process, biological treatment, is being put in place. Water arrives in the aeration basin
where it is brewed and injected with air. This process will promote self-puration. The bacteries
already present in the water will feed on organic matter and then eliminate it.
After the biological treatment, the water became clear. It will then move to ultraviolet radiation that
kills the germs and viruses it could contain. Following these steps, the water is of « bathing »
quality and can be discarded at sea.
After these explanations, a question could probably come to the mind of the readers of this article : Where are sent the sludge that was recovered during wastewater treatment ? Are they rejected anywhere ? The answer is quite simple and always ecological. They are rich in organic matter and can therefore be used after treatment. They are rich in organic matter and can therefore be used after treatment. They are first drained and then sent to tanks called « digesters », the sludge is heated and deprived of oxygen to promote the digestion of organic matter by the natural action of bacteria. By digesting , these bacteria produce gas (mostly methane). It feeds the boilers on site and allows the production of electricity by cogeneration. So it creates renewable energy. The sludge no longer contains germs, is dried and dried to become a powder. This powder is then mixed with lime and compacted into pellets for use as agricultural fertilizer. Nothing is lost, everything is reused !
