At around 10:45 a.m., a smell of sulphur was noticed in the vicinity of a waste sorting, transit and consolidation centre. A neighbouring company, located just 150 m from the centre, alerted the emergency services. In the early afternoon, while opening the pipes at the sorting centre, the fire brigade noticed an increased olfactory sensation with the characteristic smell of rotten eggs. Their gas detector detected an abnormally high concentration of hydrogen sulphide (H2S) in the pipes. The site was shut down and the personnel evacuated.

The investigations showed that the lift pump at the outlet of the runoff retention basin (470 m³) was continuously discharging water accompanied by H2S. The operator closed the lift pump to stop any discharge into the water system. With the agreement of the emergency services, the activity resumed in a closed circuit with the lift pump closed. The basin was pumped out and cleaned: the 65 t of sludge extracted was sent for treatment. An employee of the neighbouring company fell ill.

The operator assumed that the degradation of organic matter (fine residues and wood dust, fine residues and respirable dust, leaves, etc.) accumulated in the retention basin was the cause of this H2S production. The snowfall that preceded the incident caused the lifting pumps at the outlet of the retention basin to start up, setting the aqueous layer containing the organic matter into motion. The water discharged into the public network contained the dissolved H2S which had been released into the air when the fluid circulated in the pipes.

The operator had not pumped out the muddy sediment of the retention basin periodically. There was also no means for monitoring the level of fouling in this tank. Following the event, the operator implemented a periodic inspection of the tank’s level of fouling. The tank was pumped out as often as necessary to prevent the accumulation of organic matter.