An oxygen analyzer in a sewage sludge digester at an intermunicipal wastewater treatment plant malfunctioned at around 3 p.m., and then at 6:30 p.m. the following day. In the absence of an oxygen measurement (safety chain), the PLC on the sludge digestion unit automatically isolated the digester, shutting off the sludge feeds and directing the biogas to the storage gasometers. The pressure inside the digester began to rise because the thermophilic digestion process of the sludge continued. The 2 safety valves opened when the internal pressure exceeded 35 mbar, releasing biogas into the open air. After each malfunction, the on-call crews were called to put the oxygen analyzer back into operation. The digester was restarted and the valves closed when the pressure had returned to normal. For a total of 2 hours and 15 minutes, over a period of 2 days, 1,023 Nm³ of 65% methane biogas (i.e. 1.18 t) was released into an urban area classified as sensitive to terms of air quality.

The oxygen analyzer went out of calibration twice, losing its zero value. The measurement of negative values automatically generated an error. The analyzer had been installed for less than 2 months and had been correctly calibrated. The analyzer is not redundant, so when a failure was detected, the digester was automatically isolated. All the analyzers of this type undergo semi-annual preventive maintenance with cleaning, calibration, and verification of the transmission chain to the PLC’s central control unit. The operator has considered installing a backup system for the oxygen analyzers on each digester and increasing the calibration frequency.

The same type of accident occurred less than a month before on the station’s other digesters (ARIA 47805 and 47807).