Starting at 7:13 a.m., the pressure inside the double membrane roof of a digester at an anaerobic digestion plant began rising quickly, increasing from 4 mbar to 80 mbar in just one hour. Alerted at around 7:30 a.m. after checking the remote control system, the site manager asked the operating technician to intervene. At 8:15 a.m., the membrane burst. Two of its angle clips were ripped off, carrying pieces of the concrete skin with them. One of these angle clips flew into the flexible liquid digestate storage tank, puncturing it and causing its contents to spill out. The operator closed the stormwater control valve to contain part of the liquid digestate. The storage tank was placed on a spill containment berm and the liquid digestate was pumped out. A company cleaned out the networks as the stormwater was polluted by 3–4 m³ of low-laden liquid digestate (2.5% dry matter). The incident damaged several pieces of equipment and material damage was estimated at €70,000. The site had to be shut down for one month for repairs, resulting in operating losses of €150,000.
Causes of the accident
When the incident occurred, the facility was at the end of its ramp-up phase and its rated load capacity had just been reached. In accordance with the day’s schedule, the digester had not been loaded for one hour. The membrane burst due to emulsifying material, probably from organic overload. The materials emulsified in the digester overflowed into the raw digestate buffer tank via the biogas pipe between the two tanks. At around 3:00 a.m., the raw digestate buffer tank’s high-level alarm went off. All the gas relief lines were blocked, causing the safety equipment (flare and safety valves) to fail. The overpressure alarms on the raw digestate buffer tank and the digester went off at 7:15 a.m. and 7:36 a.m., respectively. However, the digester’s high-level alarm did not go off. The build-up of gas caused the pressure inside the membrane to quickly rise and lead to the bursting of the membrane at 8:15 a.m.
Scenario not considered by the manufacturer
In terms of the choice of safety devices (valves not suited to the production of foam), the design of the instrumentation and control system (does not discharge materials when the raw digestate buffer tank’s high-level alarm is triggered), and the operating instructions given, the manufacturer had not considered this hazard scenario. Other causes — such as the digester’s high-level sensor (which was not properly connected) and the fact that alarm signals were not received from the supervision system — also played a role.
Measures implemented by the operator
Following the accident, the operator:
- slaved the buffer tank’s drain pump to the tank’s high-level sensor;
- installed new safety valves to automatically regulate the liquid level;
- installed goosenecks to raise the gas intake at the valves and reduce their risk of being flooded with material in the event of emulsion;
- checks the operation of the level sensors more frequently;
- reinforced the alert system (alarm linked by telephone) and set up an on-call system under the terms of its maintenance contract;
- now has materials added more progressively to reduce the risk of foaming;
- had the stormwater control valve checked by the supplier.
Employees are now instructed to look into the digester’s sight glasses when checking the valves and a specific procedure is now to be followed in the event of overpressure in the biogas storage tank.
