Biogas was discharged when the valves on three digesters at a wastewater treatment plant were opened intermittently. The operator stopped the discharge, which released 3834 t of biogas to the atmosphere.

The accident occurred while one of the biogas holders had been isolated and locked out. The valves opened due to a rise in pressure in the digesters following the isolation of various downstream equipment. The non-locked-out gas holder automatically isolated itself following an alarm triggered by the low water level switch on its non-return flow device. This produced the unusual result of isolating the three production lines leading from the three digesters. When both gas holders are closed, the flare stack is supposed to turn on to discharge and eliminate the biogas. However, the valve on the locked-out gas holder, which had been placed offline by the operator (and padlocked), did not send a ‘closed’ signal to the control PLC (which registered an “indeterminate” state). The PLC did not open access to the flare stack because it only received one closed gas holder signal. As a result, the pressure rose in the domes of the digesters and biogas was discharged.

After the accident, the operator modified the process control system so that the flare stack can now be opened by its internal pressure sensors. It also modified the gas holder’s valve so that it sends a ‘closed’ signal when the gas holder is locked out.