An excessive amount of phenol was released at a treatment plant whilst a chemical plant was being shut down. The operator stopped the flows after identifying the problem, but not before 250 kg of phenol flowed into the Rhône canal. The release lasted for two days and totalled 0.25 t of phenol and 1 t of acetophenone. The operator stated that no environmental impact was observed.

The accident was caused by an inversion of the aqueous and organic phases in an effluent stripper. The organic stream was sent to the effluent treatment plant, decreasing its efficiency and causing the discharge (phenol concentration above the regulatory limit). This inversion can occur during shutdowns because the stripper’s operating parameters are adjusted manually by technicians. The last liquid-liquid settler, which serves as the final trap for organic matter from the strippers, was shut down. In addition, the technical safeguard consisting of saturating the effluent with hot cumene was also not working due to the shutdown and the absence of cumene. The technicians failed to heed the alarms on the TOC analyser and phenol meter. This event highlighted issues in the plant’s design and monitoring during shutdowns.

The operator subsequently revised the operating procedure for managing effluent during shutdowns. He changed the monitoring system that allowed the stripper parameters to be manually adjusted, considered making the results of effluent analyses available more quickly, and made retrofits so that the settler and the cumene saturation line remain operating during scheduled shutdowns.