At 7:04 p.m., the low-level safety of a gas holder at a wastewater treatment plant halted the compression of gas and its transfer to consumers. The plant’s on-call staff were contacted to assist the on-site crews in managing and troubleshooting the event. The internal emergency plan was implemented. Problems with resetting the safety devices and restarting the flare stack resulted in biogas being released via the gas holder’s liquid seals before the excess was sent to the flare stacks. The flare stacks were put back in service at 7:17 p.m. At 8:08 p.m., firefighters arrived at the plant after local residents called to tell them of unusual flames exiting the tops of the flare stacks. The firefighters cut the wire-mesh fence. All told, 268 kg of biogas was released but without any environmental impacts.

The incident was caused by a rapid failure on the low-level safety device, itself caused by a faulty connection.

Over the following weekend, compression without degasification stopped repeatedly. On 2 January, normal compression resumed and all the plant’s safety devices were checked.

The operator implemented the following corrective actions:

• short term: retightening of terminals and replacement of coils, reminder of the procedure in place for on-site responses by firefighters, verification of the telephone numbers of on-call staff with the neighbouring town halls;
• medium term: check of the tightness of all electrical connections in the biogas area;
• long term: inclusion of feedback in the upgrade project underway, particularly regarding the flame detection system.