Public dossier The risks 7 topics
The risks
"We do not want to spread alarm, or to claim that every biomethane project is dangerous. We want the risks to be assessed seriously." For each risk: what research says, what reduces it, what is known about Guarda and what to ask.
How serious these risks are depends on the location, the technology, the management and the distance to homes. That is why we ask for an environmental impact assessment before any decision.
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Air and odours
Odour noticed up to 600 m
In field inspections at biogas plants, odour was noticed 250–600 m away. Waste reception is the main source of odour.
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Water
Spills have killed thousands of fish
Digestate spills into rivers are the best-documented serious risk. At Châteaulin (France, 2020), about 180,000 people had tap-water restrictions for about 5 days.
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Soil
The digestate goes on fields
Digestate is spread as fertiliser. Digestion does not remove heavy metals, and digestate from packaged food waste can carry microplastics.
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Traffic and noise
How many trucks? Not disclosed
At comparable plants, traffic studies forecast dozens of trucks a day. In Guarda the number has not been disclosed.
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Leaks, fires and explosions
Over 800 accidents in 10 years
Across 13,171 European biogas stations there were more than 800 accidents in 10 years, most without serious consequences. Modelled effects usually stay within tens of metres.
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House values
About −5% within 1 km
In France, rural methanisation units lowered nearby house prices by about 5% within 1 km. No effect was found beyond 1 km.
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Health: what the evidence says
The strongest evidence is nuisance
The strongest evidence is on odour nuisance and its effect on quality of life. Evidence of direct toxic harm to neighbours during normal operation is weak.
Sign the petition How close is too close?
Sources on this page
- Carta aberta à população
- Flash ARIA: Méthanisation, comment développer la filière sans développer les risques ?
- Faulques, Bonnet e Bourdin (2025), Energy Economics 145: 108488: efeito das unidades de metanização no preço das casas em França
- Odour emissions impact of on-farm biogas production with co-digestion: a case study in four European farms
- Bayle et al. (2018), Chemical Engineering Transactions, vol. 68, pp. 79–84: emissões de odor em três unidades de metanização em França
- 400 m³ de digestat dans l'Aulne à Châteaulin : Engie Bioz condamnée par la justice
- Frontiers in Sustainable Food Systems (2024): revisão sobre o uso agrícola do digerido
- Weithmann et al. (2018), Science Advances 4: eaap8060: fertilizantes orgânicos como via de entrada de microplásticos no ambiente
- Development Control Committee report P/14/700/FUL (digestão anaeróbia de Stormy Down)
- Ata da reunião ordinária da Câmara Municipal da Guarda de 19 de junho de 2026 (Ponto 18, Proposta VPCM n.º 10/2026)
- Kotek, Trávníček e Blecha (2015), Chemical Engineering Transactions 43: 1933–1938: acidentes em estações de biogás europeias
- Scénarios accidentels et modélisation des distances d'effets associées pour des installations de méthanisation de taille agricole et industrielle (rapport Ineris-201652-2437679-v2.0)
- Guadalupe-Fernandez et al. (2021), Environmental Health 20: 108: poluição industrial por odores e saúde, revisão sistemática e meta-análise
- Bayle et al. (2016), Chemical Engineering Transactions, vol. 54: projeto EMAMET sobre emissões de unidades de metanização
Carta aberta à população
- No public link