In a chemical factory, a defective wastewater pump was removed from a tank for repairs. The tank was first drained for proper cleaning and then completely emptied by opening the foot valve located in the retention basin. The wastewater collected in the sump was channelled to the facility’s treatment plant. The tank was rinsed with water and declared to be clean. After repairs, the pump was reinstalled. Following a shift rotation, the foreman and 2 crew members attempted to close the foot valve that had remained open. One worker entered the retention basin to evaluate the task. He bent over the valve to assess the condition. Shortly after standing up, he suddenly fainted and collapsed in the basin. Two fellow employees notified fire-fighters.

The injured crew member had inhaled a stream of noxious air containing CO. The pump used had been poorly adapted to conveying wastewater, although repairs requiring entry into the retention basin had actually been quite frequent. The authorisation order procedure had not been properly followed when working on the tank. Consequently, workers did not know that the noxious air current discharged from production was capable of reaching the tank should the ducts not be distinct. The air supply ducts had not been separated. When the injured worker was focusing on the tank, pressure rose inside the collection duct. It could thus be surmised that the noxious air flow, via the washing installation overflow outlet, had reached the tank and then the foot valve. The filter worn by the injured worker did not absorb CO. The other crew members entering the retention basin to administer first aid to their stricken colleague were not wearing any protection and, out of precaution, were placed under hospital observation. The smell of xylene regularly noticed in the wastewater collection pipe first suggested xylene intoxication. A new type of pump (dedicated to wastewater) was installed in order to reduce the need for repairs and, with it, human presence in the retention basin.