At 7:27 pm, a malfunctioning meshing valve caused the fan on the biogas extraction network to automatically switch off from an urban treatment plant gas-holder. The maintenance crew was unable to quickly resolve this locking defect, and the gas-holder (spherical tank) underwent a pressure rise (incoming flow rate > outgoing flow rate). Since the maximum height had been reached, the site operator began degassing the biogas (a flammable gas composed of 65% CH3, 34% CO2 and various impurities including H2S at 50 ppm) by opening the seal to the atmosphere. The defective valve was repaired in an hour, after which time a 2nd locking defect (erroneous measurement) appeared on the programmable controller, requiring the operator to extend seal removal for another 90 minutes, while waiting for an automation expert. Biogas transfers resumed around 10 pm, when seal removal ceased; in all, 2,275 m³ of biogas, i.e. 2.3 tonnes, were released into the atmosphere. A safety perimeter was set up around the biogas unit while the seal was off. Winds had picked up during the day, making it possible to dilute the discharged biogas, thus reducing the hazardous zone area.

The flawed measurement appearing in the controller for biogas levels exceeding 8 m in the gas-holder was removed, while the programming of all controller operating modes was verified and the modifications seen as necessary to eliminating locking defects were introduced. A similar accident had occurred 2 years prior (ARIA 38485).