Pollution
Humain
Environnement
Economique

A series of incidents on a methanation site led to odour nuisances for local residents. The grapple conveying the manure (the digester’s primary input means) to the feed hoppers broke down. A biological imbalance occurs as the digester is also supplied with vegetable fats. The operator noticed a rise in volatile fatty acids (VFA) in the digester due to too much fat. The biogas production and the biogas’ CH4 content were decreasing while the digester’s temperature was rising abnormally. The operator stopped the input of fat, and then, ten days later, as the operating parameters did not improve, the introduction of material into the digester was stopped completely. The reseeding of the digester, with slurry and digestate from another site, began two weeks later. Following this, the biological evolution was favourable, and the digester resumed its regular operation.

Several odour problems developed during the downgraded phase when the digester was no longer being fed, and then during the restart phase:

  • the inputs (slurry and grease) continued to be received and accumulated on-site in buildings and outdoors (prolonged storage over several weeks instead of the usual few days). At this point, the material started to ferment, generating odours;
  • Following the stoppage of the digester’s supply of input materials, the biogas produced, which was not in compliance, could not be conveyed to either the boiler or the scrubber. The biogas could not be flared owing to work being performed on the flare. Biogas was therefore released into the atmosphere during the digester’s shutdown phase and, to a greater extent, during the reseeding phase. A large number of local residents were mobilised owing to the odour nuisance;
  • before the digester’s reseeding phase, part of the unsuitable digestate, rich in VFAs and therefore more odorous than normal digestate, had to be extracted.

Following this episode, which lasted almost two months, the operator implemented several measures:

  • alternative ways of managing inputs were sought to avoid long-term storage on the site;
  • afforestation of the area was undertaken on the side closest to the houses;
  • manure was sheltered from the wind, and no manure should be delivered at the end of the day;
  • a system was developed with the municipal council to raise the alert in case of olfactory discomfort perceived by the residents. An informational meeting was also organised.

A prefecture issued formal notice on 27/08/2019 to ensure compliance with specific regulatory requirements governing the prevention of bad smells.