At around 12:15 p.m., 150 l of sulphuric acid and 150 l of sodium bisulphite were accidentally mixed at an electroplating plant that manufactures caps for fragrances (application of a metal coating to plastic). The mixture released toxic fumes into the air.

On 6 November, three containers of sodium bisulphite and three containers of sodium bisulphite were received for the wastewater treatment plant. The WWTP’s manager was not present at the time. The labelling was correct, but all the containers were identical. With no authorised persons around to put them away, they were unloaded by the warehouse clerk and stored on the receiving quay. Only the label on the first container was visible. On 12 November, the WWTP’s manager came back and took an inventory of the containers. He counted five containers of bisulphite and one of acid instead of three of each. On 13 November, an employee noticed that the level in the container of bisulphite was low. He emptied it and went to change the container. He placed what he believed to be four of the five containers of bisulphite in the bisulphite room. In actual fact, they were three containers of bisulphite and one container of acid. He positioned the last of the five containers (which actually contained sulphuric acid) next to the bisulphite tank and connected it without checking the label. At 11:45 a.m. on 15 November, the technician saw that the level of bisulphite was low and opened the container connected to the tank. Gas immediately began coming out of the tank and the container. The manager went to the WWTP to identify the problem and noticed a strong odour of sulphur. The gaseous sulphur dioxide spread through the WWTP, moved outdoors, and was drawn into an assembly shop through a warm-air recovery grille. The manager moved toward the container of acid, breathing in a significant amount of gas. Unable to breathe in the assembly shop, the employees moved to the plant’s car park and gave the alert at 12:10 p.m. The ambulance service and firefighters were notified at 12:18 p.m. The chemical reaction was neutralised by draining a portion of the contents of the bisulphite tank into the chromic-acid tank and adding water to the bisulphite tank to dilute the remaining mixture and lower the temperature. The emergency services arrived and immediately assisted the employees in monitoring this neutralisation. They assisted the doctors in checking the employees for breathing difficulties and signs of respiratory tract irritation. At 2:00 p.m., the plant’s manager decided to shut down all operations until Monday 18 November in order to ventilate the facilities. Eleven people were evacuated to hospital at around 2:30 p.m. The inspection authorities for classified facilities arrived at 5:00 p.m.

In its report, the operator indicated that the containers of bisulphite and sulphuric acid are usually of different colours. It also noted the most significant causes of the incident:

•  Technicians no longer perceived the hazards involved in this routine operation.
•  Similar containers had been used.
•  The containers had been received by technicians who were not authorised to store chemicals and who had stored them at the sealed area.
•  The failure to read the container labels while the containers were being put away resulted in two containers of acid being placed in the bisulphite room.
•  The labels had not been checked before the containers were emptied into the tank. Also, too much trust had been placed in the inventory drawn up when the containers were stored.
•  The technician inadequately understood the risks involved.

The operator proposed an action plan to be implemented in 2014 which improves the internal procedures in the event of an accident. This plan also provides for contacting the supplier in order set up a safer procurement procedure, revising the receipt and storage procedures and the WWTP filling procedures, and overhauling the way absences and emergencies are handled.