At 4.30 a.m., one of the 4 vents of a wastewater anaerobic digester opened at a potato processing plant. An alarm was triggered. The ullage pressure, normally between 20 and 40mbar, indicated a value of zero as a result of the vent opening. The recording of the equipment’s operation showed that the booster pumps stopped one minute before opening of the vents, which is normal in the event of sending to the flare. However, the flare did not start, due to a malfunction. The pressure rose from 40 to 55 mbar, the pressure at which the vent opened. Opening ought to have occurred only at 100 mbar, but the vent has a 20% tolerance, i.e. 80 mbar. To this can be added a 70% coefficient to be allowed for, which allows opening as of 56 mbar. Normally, there are two other protective barriers before vent opening: the presence of a relief valve at 50 mbar and a liquid seal limiting the pressure to 60-65 mbar, but these two barriers could not have acted. The event occurred during a week of extreme cold.

A ‘no access’ sign was put in place at the top of the anaerobic digester. The biogas present in the reactor evaporated naturally with the existing wind. The manual valves of the biogas circuit and the booster pumps were closed. The biogas was no longer sent to the boiler, and only the pretreatment part remained operational. Two reactor pumps were isolated. Eight days later, the operator replaced the 4 anaerobic digester vents with vents having a lower uncertainty (10% tolerance and 70% coefficient, which ensures opening only above 63 mbar). The valve supplier performed checks before restoring pressure. Thirteen days later, the operator was using its anaerobic digester again.

Following the event, the operator planned the following actions:

• installation of a gasometer making it possible to have a more constant pressure and to temporarily store the biogas produced if it is not sent immediately to the boiler (start-up in January 2022);
• updating of its hazard analysis to take into account feedback from the incident.