Pollution
Humain
Environnement
Economique

The wastewater treatment plant of a chemical plant began to emit odorous whitish vapours at around 11:00 a.m. The white plume could be seen from outside the plant. The operator implemented the internal emergency plan and shut down the plant, stopping the vapours. The plant’s 40 employees were ordered to shelter in place until midday. Local residents and the authorities were alerted. Air samples were negative for chlorine, ammonia and hydrochloric acid. Water samples collected from the discharge pipe were normal. After getting the okay from the firefighters, operations partially resumed at around noon. However, the wastewater treatment plant was not restarted. The internal emergency plan was lifted at 4:00 p.m.

Analysis of the causes:

The release was caused by the addition of an over-concentrated bleach solution to the washing water network from the basic mineral chemistry work area to the neutralisation station. Residues from tanks and packaging containing concentrated bleach were sent to the neutralisation station. A pneumatic valve in the washing area had remained open.

At the effluent treatment plant, neutralisation is carried out by adding sodium bisulphite, which is suitable for weak concentrations. The influx of bleach led to a large addition of sodium bisulphite, which is regulated by a redox probe. The probe is less reactive in basic environments medium than in those with a neutral pH. It regulates the influx of bisulphite and shuts off the transfer pump between the neutralisation tank and a 70 m³ tank. The bisulphite feed pump’s threshold was lower than that of the transfer pump. As a result, bisulphite was fed into the neutralisation tank while the transfer pump was operating. As solubilisation had not yet occurred, a mixture of bleach and sodium bisulphite was pumped to the 70 m³ tank. Bubbling began in the tank, promoting the reaction between the two substances and the release of a whitish cloud of chlorinated derivatives with an acrid yet non-toxic odour. The acidic pH in the tank amplified the reaction.

According to the packaging procedure, technicians must alert their shift leader of residues in the containers in order to determine the type of effluent and whether it may be recycled or disposed of. The inertia of the redox probe caused the reaction mixture to flow into the 70 m³ tank. The incompatible mixture of sodium bisulphite and bleach was not described in the safety report.

Corrective actions proposed and implemented by the operator:

  • the trip threshold of the bisulphite feed pump and the transfer pump were made the same and an alarm was installed;
  • the frequency at which bisulphite is added was reduced to avoid adding too much;
  • the 70 m³ tank and the effluent reduction tank were completely cleaned;
  • temporary measure before the installation of an automatic system for interlocking the drain valve with a redox probe: the valve was locked and tagged out and effluent was pumped into intermediate bulk containers (IBCs) to be analysed before being transferred to the neutralisation station and a work procedure was created.

Three days after the incident, the inspection authorities for classified facilities authorised the wastewater treatment plant to resume operations. They required the operator to add incompatible-mixture scenarios to his safety report as well as the experience feedback available in the ARIA database. They also required him to ensure that the experience feedback from this event would be taken into account by the other sites in the group.