In a chemical plant, operators were preparing an aqueous passivation solution for a hexamethylenetetramine (C6H12N4, highly flammable) treatment column. The excess volume (9 m³) of the preparation was transferred to a tank. An operator took the initiative to drain 6 m³ from the tank before introducing a hydrochloric acid solution (HCl, toxic and corrosive). For 15 minutes, the solution was drained from the tank’s low point via a hose into a purge collection pit of the HCl loading facility, before being released into the natural environment via the stormwater network. Given the low concentration of hexamethylenetetramine in the release (20 mg/l maximum for a PNEC (Predicted No Effect Concentration, value defining the threshold used for evaluating the environmental risks of chemical substances) of 30 mg/l), the effluents released into the natural environment (a pond) during the accident had no environmental impact.

The cause of the accident was attributed to the operator’s inappropriate initiative when confronted with an overflowing tank as a result of a water meter failure. The operator also had a poor understanding of the plant’s wastewater systems: he drained the product into the pit, unaware that its contents were being released to the natural environment via the stormwater network.

The site operator had the water meter replaced and ensures that it is regularly calibrated. The workers were reminded of the rules governing drainage operations.