Pollution
Humain
Environnement
Economique

Around 7:30 pm, air emissions from a fertiliser manufacturing plant intoxicated three employees at a neighbouring site, necessitating their hospitalisation due to headaches. Alerted by the police, the operator verified the site’s superphosphate production unit. At 10:30 pm, police and fire-fighters carried out pollution measurements in the facility, and these turned out to be negative. The next day, new odours were reported at 6:50 am by the neighbouring factory. The superphosphate unit was shut down at 8 am, thus stopping the foul-smelling emissions.

The unit’s odour treatment installation was checked: the three venturis were drained, and the pH and redox probes replaced as a precaution. Unit restart did not cause any new observation of odours.

Disruption to the pH and redox probes during the previous afternoon was the cause of this accident. A leak on the recirculation line of the washing tank, supplying water to the measuring bowl of the pH/redox probes, prompted a maintenance intervention and loss of calibration of the probes due to an absence of water in the measurement vessel. The malfunctioning sensors, used to control the injection of sodium hydroxide and bleach into the quench tower, caused a decrease in efficiency of the gas treatment apparatus in the superphosphate unit, which was loaded with sulphur compounds (SO2, H2S, mercaptans, etc.). Moreover, a lack of training, the absence of definition for operating ranges on odour treatment unit parameters and inadequate alarms prevented unit operators from detecting and correcting the problem based on the reported supervisory parameters.

The operator put into place corrective measures: hourly inspections of the water level in the measuring bowl of the pH/Redox probes; detection of water in the bowl; broader training of plant technicians to run the gas treatment installation; definition of operating procedures and operating ranges for each parameter; and actions to be implemented in the event of exceeding the limits.