An explosion occurred in a wastewater treatment plant at around 2:20 p.m. in the sludge drying workshop. The auger conveyor at the outlet of a dry sludge silo was at the origin of the explosion and smoke was billowing out. The unit was secured. The internal emergency plan was activated and the employees were evacuated. After studying the situation, the firemen emptied the contents of the silo onto the ground and continued to spray it down. An examination from the top of the silo showed that a block of sludge in pyrolysis (hot spot) was stuck in the bottom. It was removed at around 5 p.m. by injecting water into the silo. The sludge on the ground was pumped and reintroduced into the process. The damaged line was restarted four days later.

The explosion was caused by a cloud of dust

The drying line had been shut down for 2.5 days following a technical failure on equipment downstream from the conveyor. The operating procedures enabled the silo to be kept off-line and full for up to 3 days. The silo, being 60% full, was not emptied. On the morning of the incident, an increase in the concentration of carbon monoxide (CO) was observed in the silo. During a security round, a hot point was detected in the silo at around 1 p.m. Assuming that a self-heating phenomenon was in progress in the silo, the technicians decided to empty its contents onto the ground and spray down the outgoing sludge with water. At around 2 p.m., the silo was only 6% full. The CO concentration was below the alarm threshold. Assuming that all the hot points had been extinguished, the dumping of the sludge onto the ground was stopped. The sludge was then directed toward the conveyor in order to conduct the restarting tests. The explosion occurred just a few minutes later.

According to the operator, the explosion was caused by a cloud of fine dust suspended in the auger. The ignition source was certainly incandescent particles coming from the blocks of sludge in pyrolysis. Moreover, the fine dust, the particle size of which is conducive to the formation of explosive atmospheres, is particularly present at the end of the silo emptying process.

The risk of self-heating was identified for a shut-down period over 72 hours. The blockage of sludge agglomerates in the distribution cone at the bottom of the silo could explain the early self-heating phenomenon.