Accidentally spilled by a surface treatment workshop inside a vehicle manufacturing plant, 100 m³ of a phosphating bath polluted the Seine River through the storm drain network. A defective start-up floater on the treatment bath circulating pump towards the filtration unit sludge tank caused this accident. Abnormal operations of the continuous pump feed triggered a sludge tank overflow, followed by overflow of the retention basins connected to the filtration and treatment installations. The investigation performed by the Classified Facilities Inspectorate also revealed: obstruction of the sludge tank overflow weir leading to a buffer tank, malfunction of the bath level variation detector inside the phosphating tank, and the lack of liquid detection at the low points of retention facilities. The Inspectors recorded these findings and proposed that the Prefect issue an official warning regarding retention basin detection devices.