An unusual intake of hydrocarbons was detected at around 11 pm at the entrance of an industrial waste water treatment plant on a petrochemical site. Suspecting pollution of the blow-off water from the steam cracking workshop’s dilution steam generators of one of the site’s plants, and not having succeeded in stopping these discharges using the process controls, at around 1.30 am, the site’s operator diverted these effluents to the steam cracker’s oil-water settling tank.

However, disruptions to the treatment plant ended only the next day at around 2 pm after the unit’s naphtha heat exchangers were isolated. These heat exchangers continued to be isolated in order to carry out cleaning and leak tests. Following these checks, the hypothesis of a joint leak on one of the heat exchangers was retained. This leak produced the formation of a water/hydrocarbon emulsion causing intermittent pollution of the effluents discharged into the natural environment. Due to the lack of continuity of this pollution, random checks by technicians did not highlight this leak and the need to rapidly divert the flow to the settler.

The accident was caused by an excessive concentration and flow of total hydrocarbons discharged into the natural environment (respectively 25 mg/l and 458 kg/d instead of 10 mg/l and 100 kg/d permitted).

After a quality control check, the steam blow-off water was redirected according to the normal circuit. The heat exchangers in question were isolated and repaired.

For feedback purposes, a TOC meter was installed on the effluents sent to the treatment plant.