In the water detoxification station of a fastenings surface treatment workshop, a lime milk preparation tank overflowed following the failure of a high-level float that normally interrupted the tank’s water supply. The effluent spread through the station, then flowed into the LA PLANCHETTE river below the building via a hole in the building’s wall. The excessive dilution of lime milk, used for precipitating nickel, zinc and iron oxides before injection of a flocculant, also caused the station to malfunction and the formation of flocs in the watercourse. Firefighters installed 3 successive filtering dams and absorbents 150 m downstream of the site’s discharge point. A specialist company pumped the floating products and cleaned the river’s banks. Several failures caused the pollution: malfunction of the lime milk preparation tank’s float, failure of the alarm on detection of liquid in the tank’s retention system and no station wall sealing. The investigation carried out by the inspection authorities for classified facilities also showed that the employee responsible for monitoring the detoxification station’s operation was on holiday at the time. As no other employee had received the necessary training to replace him, the risk of a fault on the effluent treatment system following the variation in pH of the lime milk, caused by dilution, had not been considered. The hole in the wall was plugged. The operator finally considered doubling certain alarms and training a second operator to monitor the detoxification station.