At 9 am during a transfer operation of 60% nitric acid from a lorry tanker to one of the tanks at an industrial dairy, a break on this tank’s flap connection triggered the spraying and spill of 3 m³ of acid. The product poured out over the transfer zone’s impermeable surfaces and storage tank retention basins before reaching by gravity, through the collection network, the site’s water treatment plant buffer tank. This plant’s manager (outsourced activity) temporarily isolated the treatment system’s buffer tank (bacterial bed) and then attempted to neutralise buffer tank contents; at the same time, dairy personnel were cleaning the networks with a highly basic product, which was ultimately captured in the buffer tank. Recordings available from the treatment plant’s control room indicated that the pH of the stored effluent rose from 0 to 14 within a few hours, during which time the treatment system was placed back into service, with continued discharges into the Vesle River. The appearance of a foam on the biological aeration basin was observed at 8 pm. The next morning, after recognising the relative inefficiency of ferric chloride and antifoam injections, the decision was made to request the intervention of a specialist firm to pump the thick foam formed on the overflow basin, the clarifier and around the metering channel (outfall leading to the Vesle).