At around 10:30 a.m., a mixture of biogas containing 33% methane (which has a lower explosive limit [LEL] of 5%) ignited in a 30 m³ water tank connected to a cogeneration boiler at a hazardous waste treatment plant. A contractor was taken to hospital to be treated for severe burns. The facility was placed in a safe state. The boiler was shut down for 30 days. The financial loss was estimated at €50,000.

The event occurred while the boiler’s water tank was being serviced and retrofitted with new nozzles. The boiler had been shut off, drained, and locked out beforehand. While this was happening, filters on the nearby biogas compressors were being serviced. The electricity had been shut off, causing the site’s biogas plant to shut off as well.

During normal operation, the contents of the biogas filters are dumped to a wastewater tank. The volume of residual biogas is then discharged into the wastewater tank, which has a breather valve fitted with an activated carbon filter and a connection leading to the site’s biogas collection pipes. The purpose of this connection is to continuously scavenge the headspace to remove all remaining traces of biogas. The boiler’s water tank (which was being serviced) was also connected to the wastewater tank and the drain valve at the bottom was left open.

Just before the accident, the shutdown of the biogas plant for maintenance resulted in the scavenging of the wastewater tank headspace. It is likely that the biogas released during the dumping of the filter travelled along the sewer system, up to the biogas compressor, and ultimately on to the boiler water tank being serviced. The amount of released biogas was probably sufficient to create an explosive atmosphere inside the 30 m³ tank. Sparks from grinding probably ignited the gas mixture inside the tank, resulting in a flash fire. The contractor was burnt by hot gases exiting the nozzles that were being retrofitted.

The operator identified multiple root causes:

• defective design:
  • a single sewer system was used for highly different types of water (boiler water, water drained during maintenance of the biogas filters);
  • no traps or liquid seals were used to prevent gases rising via the sewer system;
• flawed analysis of risks during maintenance operations.

To prevent the accident recurring, the operator:

• separated the sewer systems;
• changed the maintenance procedures for replacing the biogas filters and shutting down the biogas plant;
• monitors the proper operation of the wastewater tank headspace scavenging system.