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Producing energy from wastewater

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There are currently 22,300 wastewater treatment plants (WWTPs) in France. Each year, more than 8 billion m³ of wastewater are treated, but only 1.1 billion of this water is reused. In France, wastewater is generally collected for cleaning before being discharged into the natural environment. With the arrival of new environmental issues, it was necessary to rethink the recovery of this wastewater. The recovery of sewage sludge, its hydrolysis, its methanization and its transformation into biogas, make it possible to meet this challenge.

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Activity in wastewater treatment plant


A WWTP is a system whose purpose is to clean domestic, storm or industrial water, before discharge into the natural environment. It is necessary to dissociate the water from potential harmful substances for the environment. In general, the WWTP is located at the end of a sewer line.

The wastewater treatment in the WWTP generates a main waste called sludge. The different treatment stages are thought out according to the nature of the wastewater collected and the types of pollutants to be treated.

Definition :

Wastewater Treatment Plant (WWTP): Installation allowing the depollution of domestic urban wastewater.

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How a WWTP works at Runéo

On a daily basis, the French use water that is discharged. Whether it is domestic wastewater, rainwater or industrial wastewater, it is collected by the sewers, which take it to one of the country's treatment plants. Before this, the wastewater passes through a "storm overflow", which allows, if it rains, to choose the flow that will be redirected to the WWTP.

1. Pre-treatment

The wastewater first undergoes mechanical and physical pre-treatment.

- Screening: The wastewater passes through screens (bar screens) that retain coarse solid matter (branches, leaves, various plastics, cans, etc.).

- Sand and oil removal: In a first basin, the water is cleaned of its grease and oil content (recovered on the surface) and of its sand, shale and clay which settle at the bottom of the structure.

2. Biological treatment

This treatment is based on the natural self-purification mechanism of the river.

- The biological reactor Wastewater is brought into contact with microorganisms ("activated sludge") in an aerated tank in order to "digest" biodegradable pollution.

- The clarifier The mixture is directed to a second tank where the separation between the biological sludge and the treated water is carried out by decantation. The treated water is then discharged into the receiving watercourse. The biological sludge recovered at the bottom of the clarifier is mainly reinjected into the biological basin to ensure the continuity of the treatment. The excess sludge produced is recovered and reused.

3. Sludge recovery

The excess sludge is dehydrated and then follows one of the two existing channels: the recovery of sludge in agriculture or the thermal recovery.

4. Controls

At the various stages of the treatment process, the water and sludge are subject to multiple controls and analyses by control services to ensure their compliance with French and European environmental standards.

Particularity of the Grand Prado WWTP


Like all WWTPs, the Grand Prado WWTP works around the collection and treatment of wastewater.

Thanks to the plant's two methanizers, the sewage sludge is partially transformed into biogas. This biogas is then used to produce energy in the form of electricity and heat, by cogeneration.

The sludge is then treated in a specific process and transformed into a fertilizer approved by the ANSES, called Fertilpéi. From an environmental point of view, the WWTP has also been designed to limit its consumption, its carbon footprint and its greenhouse gas emissions. 

It therefore responds to the major challenges of our time, while preserving our environment to pass it on to future generations in peace.

The importance of WWTPs in Reunion

The WWTPs in Reunion Island allow several things :

- The creation of jobs for the construction and operation of the stations

- The use of local subcontractors, which supports the independence and self-management of the island

- Participate in ecological actions while supporting the agricultural world

- Soliciting all local actors in the valorisation of the station's by-products

Clarisse AARON - QSE Manager at Runéo

Clarisse AARON (on the left in the photo above), talks to us about the risks present at Runéo and the prevention/safety measures put in place.

Who are you?

"My name is Clarisse AARON and I have been QSE Manager at Runéo since 2016."

What is your daily routine at Runéo?

"Since 2007, the QSE department has been centralized with a drive, for all reports and monitoring tables. The company has 290 employees and several sites on Reunion Island."

What are the risks of your activity in STEP?

"At Runéo, we have many risks, among which we can find: The risk related to gas leaks (H₂S, CO, Chlorine, etc), the risk in ATEX zones (presence of gas and dust), or the risk of falling from height."

What safety equipment and training is in place at Runéo?

"The use of portable gas detectors in ATEX zones and respiratory protection masks when handling chlorine bottles. The application of CATEC (Confined Spaces) procedures for cleaning drinking water tanks or cleaning tarpaulins, which includes the fall protection device for working at height or self-rescue masks."



grand prado

BE ATEX intervenes in different WWTP for the maintenance of respiratory protection masks, protective suits and gas detectors, fixed or portable. We intervene for the control of the material, since it is important to guarantee the safety and the protection of the workers. They must always be equipped with fall protection, respiratory protection, PTI (Protection of the Isolated Worker) and gas detector.

Our teams visit all the stations to ensure the safety of the personnel, through the verification and calibration of the protective equipment:

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