At around 8 p.m., the biogas sampling pipes on 2 digesters in an intercommunal wastewater treatment froze during a period of low winter temperatures (-6 °C). The continuous oxygen, methane and hydrogen sulphide analyzers were no longer supplied by these pipes and failed to perform the measurement. As no oxygen measurement was available (safety chain), the sludge digestion unit’s automatic control system automatically isolated the digesters by cutting off sludge feeds and the process of moving biogas to the storage gasometers. The pressure in the digesters increased since the thermophilic digestion process of the sludge continued. When the internal pressure exceeded 35 mbar, the 2 safety valves on each digester opened, allowing biogas to be released into the open air. The on-call teams were called and were able to restore power to the analyzers at 7 a.m. The digesters were then put back into operation and the valves closed once the pressure returned to normal. For an hour and a half, 3,975 Nm³ of 65% methane biogas (4.34 t) was released into an urban area classified as sensitive in terms of air quality.
The sampling lines, located outside, had not been protected against the cold weather. The high water content of the biogas produced generated condensation in the sampling line, which became blocked when the gas froze. The operator shunts the oxygen analyzers during cold weather and performs measurements manually to monitor the oxygen levels in the digesters. A solution is also being studied to protect these pipes against the cold (insulation). Solutions to render the oxygen measurement more reliable (safety chain) are currently being studied: elimination of the digester’s automatic isolation system with the organisation of verification rounds to be performed with a portable analyzer in the event of abnormal oxygen measurements. An on-call alarm system must ensure a rapid response time.
The same type of accident occurred 48 hours prior on the station’s 3rd digester (ARIA 47805) and 13 days after on one of the two digesters involved (ARIA 47809).
