Pollution
Humain
Environnement
Economique

At around 6:45 a.m., in an intercommunal wastewater treatment plant, a biogas sampling line on a digester froze during a period of low winter temperatures (-4 °C). The continuous oxygen, methane and hydrogen sulphide analysers were no longer supplied by this pipe 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 digester, cutting off the sludge feeds and directing the biogas to the storage gasometers. The pressure inside the digester increased as the thermophilic digestion process of the sludge continued. When the internal pressure exceeded 35 mbar, the two safety valves opened, releasing biogas into the open air. The on-call teams were called and were able to restore power to the analysers at 8:15 a.m. The digester was put back into operation and the valves closed when the pressure returned to normal. For an hour and a half, 757 Nm³ of 65% methane biogas (0.827 t) was released into an urban area classified as sensitive in terms of air quality.

The sampling line, 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 is studying a solution to protect the pipe 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 later on the station’s two other digesters (ARIA 47807) and then 23 days later on one of the two digesters involved in the previous accident (ARIA 47809).