A 1,000 l drum of black dye being handled by a forklift truck tipped over in a dyeworks located on a chemical site; 800 l of dye passed through the rainwater network and entered a 2,000 m³ “emergency” basin which progressively filled with black effluent. A waste treatment company that discharges its cooling water into the same system increased the flow rate of its discharge to flush out the pipes, as the dyeworks was unable to achieve this flow rate. However, the basin filled within 2 hours and the cooling water from the waste treatment company could not be diverted to avoid soiling the pipe with the dye. Furthermore, this company only had 3 hours of capacity in its own basin for retaining its cooling water. Given the characteristics of the pollutant (harmless, irritant), the flushing implemented, the impossibility of shutting down the waste treatment company’s furnaces in such a short time and therefore the need for this company to evacuate its cooling water, it was planned to channel it towards the RHONE river when the various retention capacities were full. As there was no certainty on whether the water collected would be clear again within the 2 hours, the Mayors in the area were informed of the risk of discolouration of the river. At around 8.40 pm, the water entering the emergency basin was no longer coloured. The dyeworks therefore stopped its discharges intended only for cleaning the rainwater network. As for the waste treatment plant, after treatment, it discharged its process water into the RHONE river. A solution had to be studied for the water collected in the emergency basin (800 l of dye diluted in 1,000 m³ of water).

A classified facility inspector went on site the next morning and the waste treatment company’s laboratories analysed the effluent collected in the emergency basin to decide how to dispose of it.