Biogas was discharged from the sludge digester of a municipal wastewater treatment plant for three consecutive days. The discharges occurred when the facilities’ safety devices unexpectedly shut down. Each time, the WWTP’s workers turned the facilities back on to stop the discharges. A total of 4000 Nm³, or 4.2 t (of which 60% was methane), of biogas was released.

The automatic shutdowns, all of which occurred at midday, were triggered by the failure of two methane detectors on the double walls of the gas holder. They may have been caused by overheating of the detectors, which were directly facing the sun and the weather during the three days had been hot.

Each time the devices downstream of the digesters shut off, the pressure in the digesters rose, opening the safety valves. The biogas was therefore sent to a flare stack. However, the automated system controlling the facilities, which triggered the emergency stops, strangely also shut down the flare stack. So, the biogas was released to the atmosphere unburnt.

After the accident, the operator changed the automated system so that it would no longer shut down the flare stack during an emergency stop. The flare stack must operate using at least its own pressure sensors when both of the gas holder’s sensors are not working. The operator also looked into the possibility of either protecting the methane detectors from direct sunlight or replacing them by detectors that use a different technology and are less sensitive to temperature.

Accidental releases of biogas occur very frequently at this wastewater treatment plant (ARIA 49450, 48799, 47812, 47809, 47808, 47807, 47805).