A leak from the valve joint was detected on a sulphuric acid (H2SO4) tank in a starch products manufacturing plant. At 1 pm, a maintenance team drained 60 l of acid from the 110 m³ pit in order to replace the tank bottom valve. At 8 pm, the regeneration phase (100 m³) of the soft water line was started up with regenerates directed towards that same pit. At around 8.40 pm, the sump pump started up on detection of a high level in the pit. The pH meter recorded the rapid drop in pH below the automatic shutdown threshold (pH = 5), but the low pH safety mechanism did not trigger following malfunction of the control relays; 20 l of acid then spilled into the INGON river. At 8.55 pm, the treatment plant’s technician, warned of too low a pH on the screen, alerted the utilities technician whose control screen indicated that the pump had shut down. Seeing that the pH was still low, this technician went on site to check the pump in order to see whether it was still running; he shut it down and recorded this at 9.04 pm. The operator then went to check the Morlemont discharge point to observe a number of fish, but around fifteen were dead. The team leader of the Starch manufacturing station then contacted the production official who in turn warned the permanent management. A crisis unit was activated the next day at 7.35 am before a new round was carried out to inspect the INGON river. Between 9.10 am and 10.20 am, a new inspection from the discharge point up to the confluence with the Canal du Nord made it possible to note the impact in terms of fish mortality, and take samples, as well as record the pH and dissolved O2 levels. Between 10 am and 10.45 am, the firefighters collected 11.6 kg of dead fish on the banks at 100 m downstream of the discharge point. The pump’s control relay was replaced and the 110 m³ pit was drained towards the treatment plant via the containment basin.