At around 1:10 p.m., ash waste shot out of the safety valve of a silo while a truck was being unloaded in the stabilisation unit of a hazardous waste treatment and disposal facility. A tonne of ash waste fell to the ground around the silo for four minutes before the driver intervened. The unit’s fire detection system went off. All the employees at the site evacuated the buildings and went to the canteen, passing through the cloud of ash on their way. The firefighters, who arrived at around 2:00 p.m., saw that there were no flames. They examined the dozen or so employees who said that they had inhaled dust and had an irritated throat. Inspectors found that dust had fallen within a 20-m radius of the silo. The area was sprayed and then cleaned by a street sweeper. All the water used was recovered as leachate and retreated in the stabilisation process.

The silo was empty at the time of the incident. As per his usual routine, the driver unloaded at a pressure of 1 bar using a hose connected between the tank and the bottom of the silo. However, the silo was not the usual silo, which was not available that day for technical reasons. The driver had been directed to this one by the site’s employees. The amount of waste to be unloaded was greater than the silo’s available capacity. In addition, the volatility of the ash and the lack of an elbow that could direct the ash to the bottom of the silo at the infeed pipe clogged the silo’s filter. Pressure in the silo increased, causing its safety valve to open. Under the effect of the residual pressure, the safety valve remained open until the unloading valve was closed. The high-level sensor in the silo did not trip on account of the volatility of the ash. Contrary to procedure, the driver first cut the compressor and then closed the unloading valve. As he was not wearing a mask, he went to fetch one from another driver before doing so.

The silo was removed from service and the valve was replaced. The following corrective measures were implemented:

• the silos and their capacities are now better identified;
• the unloading protocol was changed;
• the fire/dust detection systems were modified;
• all the safety valves were checked by an approved company;
• safety meetings have been introduced;
• a new meeting point has been defined;
• an elbow was fitted on the waste inlet pipe;
• feedback from the incident has been incorporated in the emergency drills.