Around 5 pm in a paper manufacturing plant, employees detected the whitish colouring of a watercourse and immediately sounded the alarm. The site foremen were duly informed and localised the leak 15 min later on a tank containing calcium carbonate; the tank’s retention basin overflowed. These staff proceeded to block the wastewater disposal network and set up a dam around the sewer outlet, whose wastewater contents were assumed to be spilling into the river. The employees then installed the tank pump station with output to the onsite sewer plant’s 600-m³ buffer basin.

At 6 pm, another discharge into the river was observed; the plug had proved inefficient due to poor use of inflator gas bottles; the plug was placed back online 45 minutes later. Between 2 and 3 am, technicians pumped into the wastewater network and removed the plug; cleaning continued until 9:30 am.

An analysis of this event revealed that the leak had occurred subsequent to significant corrosion (2-cm diameter hole) of a drainage tap at the tank base. This tap as well as the entire tank assembly had been covered by rock wool heat insulation. This protection, associated with a design flaw at the level of the raft, contributed heavily to corrosion of the tap as well as the tank base. Since the tank retention had not been operational (insufficient volume, broken seal), calcium carbonate quickly spread over the transfer zone where the discharge vents were found to be connected to a sump, thus making it possible to channel effluent to the buffer basin. Despite this system device, a quantity of product estimated at 25 m³ was still identified in the river. The hypothesis of a poor connection between the stormwater drainage network and the transfer network for water potentially polluted into the buffer basin was forwarded.

Following this incident, the primary action plan measures adopted by the operator consisted of: definitively shutting the tap where the leak began; reinforcing the tank bottom and sides by installing steel sheets; using a glass-epoxy resin laminate (reinforcement + protection) both inside and outside the tank; filling the voids below the sheet metal bottom with concrete; conducting a thorough investigation of all networks by injecting a dye; introducing a suspended matter treatment system; and enhancing treatment at the sewer plant combined with an updated internal response plan and a scheduled personnel training phase.