A Seveso-rated metallurgical facility specialised in zirconium production was required to reroute the acidic washing water (500 m³ at pH = 5) from a workshop into its retention basin so as to conduct pressure equipment testing. One month prior, the basin had received 2,000 m³ of water generated during a fire drill, executed as part of the site’s internal emergency plan. The very high basin level (2,400 m³) was reached at 4:30 pm, and the effluent spilled into the main channel via an overflow pipe. The pH detectors installed near the outfall in the natural environment triggered the alarm, which in turn caused the outfall valve to close automatically. The site operator shut down the equipment, except for the machinery where reactions were ongoing, before proceeding to clog the overflow pipe with an inflatable plug. Then, the initiates pumping (40 m³/h) of basin contents to the site’s treatment plant, where the acidic effluent was treated for future discharge along with effluent compliant with discharge standards. As long the basin was not empty, site technicians were assigned to visually monitor the plug’s efficiency, while other control room personnel inspected the pH and suspended solids alarm readings at the treatment plant outlet. A metal control sample was withdrawn twice a day at the outfall to the environment. The basin level dipped below the high mark 36 hours after first exceeding it. Moreover, the basin was only completely drained 4 days later since the batch production units, responsible for releasing 160 m³/h of effluent, were restarted for 48 h once the high level was no longer being surpassed. The operator introduced a retention basin management procedure to permanently ensure maintaining the lowest level and blocked the basin’s overflow pipe. Production resumed 5 days hence.