At the time of a chemical facility’s mandatory shutdown once every three years, a settling tank overflowed (mix of water, alumina sludge and benzene-containing hydrocarbons) during the cleaning operations of a styrene workshop. The settling tank (covered for limiting hydrocarbons vapours) comprised 3 compartments: a 60 m³ sedimentation basin, a 2nd compartment for collecting hydrocarbons, and a 3rd that collected decanted water. The overflow caused the caused discharge to both the water treatment plant operated by the chemical facility (WTP) and the final treatment plant (FTP) of another chemical installation.

Effluent remained contained in the platform’s treatment plant retention basins. However, due to hydrocarbons evaporation, high benzene concentrations were measured by the air quality monitoring network (up to 1,504 µg/m³ ). A second incidence of over-concentration was observed during the evening of 6th September. No complaint was filed by any neighbours: 4.8 tonnes of volatile organic compounds, including 4.4 tonnes of benzene, were released into the atmosphere.

Analysis of this incident indicated that upon request submitted by the treatment plant operator, the technician responsible for the setling tank reduced water flow into the plant (from 35 to 15 m³/h), yet failed to prevent flow from facility drains into the settling tank. The presence of sludge in the 2nd compartment triggered the drainage pump, and this default was not relayed to technicians. The compartment high-level alarm was acknowledged without any associated action, while the 3rd compartment alarm was deactivated. Levels climbed to a point when all 3 compartments at the same height spilled over. The alarms installed at the final plant finally sounded the alert.

The operator took the following measures: shutdown of all drains leading to the settling tank and channelling of water flow into the treatment plant, hydrocarbon skimming and recovery at the final plant, pumping of treatment plant hydrocarbons and skimming of the 3rd compartment, deployment of floating booms at the final plant, and lastly skimming and pumping of hydrocarbons in the stormwater basin at the final plant trapped by the booms. Subsequent to this incident, personnel were made aware of the need to acknowledge and react to alarms, the 2nd compartment alarm parameters were modified, a study was undertaken on the risks of an unprimed pump, and the 3rd compartment alarm was calibrated.