A leak of biogas (composed primarily of methane, CH4, a flammable and explosive gas) was detected around 11:45 am by a mobile beacon inside the sludge digestion zone of a municipal treatment plant, at the spot where a section of underground high-pressure pipe connected 2 spherical storage tanks containing newly produced biogas (nominal diameter: 300 mm, service pressure: 3 bar). A safety perimeter was set up at 11:55, and the pipe was isolated at 2 pm by a “panhandle” type configuration (reversible lens joint) upstream and the closure of a manual valve downstream, followed by depressurisation. Around 3 pm, a vacuum lorry cut a trench in order to inspect the pipe, thus making it possible by 6:30 pm to locate the leak adjacent to a flange. Upon analysis, this breach of seal was due to slow internal corrosion caused by the inadequate biogas circulation speed at this spot (biogas is corrosive and toxic owing to the presence of H2S). The plant operator replaced the cast iron pipe by a stronger material compound (HDPE) and decreased its diameter to boost fluid circulation speed.