At approximately 7:00 a.m., wastewater from a slaughterhouse overflowed into the stormwater connection system. The operator shut down the stormwater basin pumps immediately and checked that the discharge valve to the natural environment was closed. From 7:15 a.m. to 8 a.m., the site’s maintenance team unclogged the system. However, at 10:30 a.m., the gendarmerie, accompanied by the fire brigade and the mayor’s representative, reported pollution in the BRUNELIERE River. This pollution then spread to the VILAINE River, turning the water red. The operator noticed that a leak had caused the pollution under the discharge valve caused by a stone that was preventing it from closing completely, and managed to close the valve fully to the point where the flow was nearly zero. At around 11 a.m., a sanitation company pumped and cleaned the area upstream of the valve. The valve was then opened to remove all foreign bodies and was closed again. The hermetic seal of the valve was also checked. The operation was completed by noon.

The pollutant turned out to be blood from the slaughter of animals, diluted to 90%. Approximately 30 m³ had been released, although no effect was observed on fauna and flora. The VILAINE River was then monitored for three days. No visual inspection of this valve before the pollution was carried out.

The operator took the following measures:

• reinforcement of training among the technical teams, including a presentation of the incident;
• a periodic visual inspection was implemented, initially quarterly;
• operational verification of the valve in the event of heavy rains;
• this scenario was added to the internal contingency plan, which was in the process of being reviewed.