Following accidental damage of a biogas storage area network (ARIA 42731), an urban wastewater treatment plant has to flare for 5 days the biogas being continuously produced by the digestion of “primary” settling sludge. At 5:14 am during a biting cold spell, the alarm indicating an extinguished flare pilot flame sounded in the plant’s pipeline control room; the technician on-call was unable to reignite the flare. A few minutes later, pressure rose in the digesters’ expansion space, reaching 49 mbar, without opening their safety valves. Inspection of the flare revealed that its supply valve and burners were frozen: a hot air blowing system was installed to thaw these elements before proceeding with their insulation. Without the capacity to burn biogas at the flare, 180 N-m³ of biogas were released into the atmosphere for 30 min via the digesters’ relief valves, which required preliminary thawing. The operations team was called to force open the biogas network’s shut-off valves that had closed again, hence making it possible to resupply the flare at 5:45 am once the digester pressure alarms had been deactivated. The pressure surge inside the digester housing had broken the housing on a sludge mixing booster pump.

The investigation conducted by the plant operator revealed that pressure probes installed inside the sludge digesters had frozen, thus leading to compromised pressure measurements. The remote control unit triggered the biogas supply cut-off at the gas-holder and flare from the digesters, thereby extinguishing the flare, gradually freezing its burners and locking its supply valve in the closed position.

The operator introduced a dedicated monitoring of pressure curves in the digesters and established operating guidelines in the event of pressure fluctuations in the digesters during cold spells (i.e. thawing of probes) and for flare malfunctions (i.e. isolating the flare and shutting down the digesters). Over the following days, automatic reheating systems were installed for sensitive flare components (tracing), in addition to insulation, the digester relief valves are secured help prevent locking due to freezing. Moreover, measures to reduce the output sludge volumes were anticipated should new problems arise on the digesters, resulting from shutdown of the primary settlement basin, which in turn would cause the discharge of non-compliant treated water into the aquatic environment (containing in particular excess nitrogen).