Around 10:45 a.m., in a sewage plant, the malfunction on the bottom valve resulted in the partial discharge of the sludge contained in a digester. The digester’s vapour space was thus connected with the discharge tank of the digested sludge via the overflow pipe, releasing 2.3 t of biogas into the atmosphere for 2.5 h. The internal emergency plan was activated. The manual valve downstream of the installation was closed, enabling the digester to be filled again, thereby isolating the vapour space from the atmosphere.

The incident originated when the digester’s bottom flush valve broke due to mechanical fatigue. The low-pressure alarms in the control room failed to identify the mechanical failure of the valve’s remote control as the cause of this abnormal situation. As such, a certain amount of time was needed to understand and solve the problem. The valve’s position on the mimic panel in the control room, displayed the activation of its opening and not on its actual position at the end of travel.


The operator evaluated the condition of similar valves used in the station. Three valves were potentially affected and rewiring was scheduled. The limit switches had been positioned on the valves and not on the actuators. The position display of these valves was modified to be based upon arrival at the end of travel stop and not simply on their activation.

The operating modes for managing valve positioning and low-pressure alerts in the digester were completed to take into account the mechanical failure of the valve control system.

The valve lubrication procedure was updated to include a new lubrication interval.

The operating personnel underwent training in these new operating modes.