Public dossier The risks Odour noticed up to 600 m
Air and odours
In field inspections at biogas plants, odour was noticed 250–600 m away. Waste reception is the main source of odour.
What is known
At three French methanisation plants, the waste reception and preparation area was the main source of odour nuisance at every one of them, and emissions varied with the season. At a Swiss plant, biogas leaks increased the odour impact and the biogas smell was rated "definitely unpleasant".
In field inspections at co-digestion plants in Belgium, Luxembourg and Germany, odour was noticed up to 250–600 m away; the largest distances occurred during digestate drying and manure handling. At a large German food-waste site, next to other odour sources, odour hours were measured at homes more than 700 m away. In the field measurements, odour reached further when the wind was stronger.
Raw biogas contains hydrogen sulphide (H2S) in the thousands of ppm. Above certain concentrations people stop smelling it, so smell is not a reliable warning.
Methane leaks (climate)
Measured methane losses at biogas plants average between 2% and 5% of production, with some cases far higher. In Denmark the national average was 2.8%, with no improvement since 2021 despite new regulation. A 2026 study estimates that 59% of these emissions could be eliminated at no net cost.
What reduces the risk
Enclosed reception under negative pressure with a biofilter, covered storage and leak detection. Castilla-La Mancha's draft decree requires enclosed reception with biofilters, fully covered lagoons and an odour study.
In Guarda
Especially in summer, winds come from the west and north-west (about 50–56% of hours), so odour tends to travel east and south-east; in winter they also blow from the north-east and south. The farm to the east is 654 m from the parcel and Pêra do Moço church 1,391 m. No odour study for this project is known.
What to ask the promoter
- Is waste reception enclosed and under negative pressure, with air treatment?
- Where is the odour study, and with what emissions and winds was it done?
- Will there be an odour monitoring plan, like the one required at Sousel?
- Air and odours
- Water
- Soil
- Traffic and noise
- Leaks, fires and explosions
- House values
- Health: what the evidence says
Sources on this page
- 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
- Odour Impact by Field Inspections: Method and Results from an Agricultural Biogas Facility
- Geruchsimmissionsmessung (Rasterbegehung) ReFood Marl, Bericht Nr. 17 0830 14-2
- Energies 12: 4605 (2019): sulfureto de hidrogénio no biogás de digestores
- NIOSH Pocket Guide to Chemical Hazards: hydrogen sulfide
- Outlook for Biogas and Biomethane: key issues affecting projects
- Bakkaloglu et al. (2021), Waste Management 124: 82–93: emissões de metano de 10 centrais de biogás no Reino Unido
- Scheutz e Fredenslund (2019), Waste Management 97: 38–46: emissões fugitivas de metano em 23 centrais dinamarquesas
- Danske biogasanlæg udleder i snit 2,8 pct. af det producerede metan
- Methane emissions from biogas plants are higher than thought, but cost-neutral avoidable (Olczak et al. 2026, Communications Sustainability)
- Proyecto de Decreto sobre las plantas de transformación en biometano (documento de participación pública)
- Rosa-dos-ventos entre Avelãs de Ambom e Pêra do Moço, 2015–2024: cálculo próprio a partir das reanálises ERA5 e ECMWF IFS (Open-Meteo)
- Climatologia do vento no local, 2021–2025: reanálise ERA5 através da Open-Meteo Historical Weather API (16 setores, velocidade, dia/noite)
- MOV.AP: medições sobre imagem de satélite, outubro de 2026 (distâncias do limite da parcela à zona de exploração de água, quintas e aldeias)
- Participa: portal de consultas públicas (pesquisa de consultas para este projeto)
- Parecer da Comissão de Avaliação, procedimento de AIA n.º 3857: Unidade de Biometano de Sousel
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
Odour Impact by Field Inspections: Method and Results from an Agricultural Biogas Facility
Geruchsimmissionsmessung (Rasterbegehung) ReFood Marl, Bericht Nr. 17 0830 14-2
Energies 12: 4605 (2019): sulfureto de hidrogénio no biogás de digestores
NIOSH Pocket Guide to Chemical Hazards: hydrogen sulfide
Outlook for Biogas and Biomethane: key issues affecting projects
Bakkaloglu et al. (2021), Waste Management 124: 82–93: emissões de metano de 10 centrais de biogás no Reino Unido
Scheutz e Fredenslund (2019), Waste Management 97: 38–46: emissões fugitivas de metano em 23 centrais dinamarquesas
Danske biogasanlæg udleder i snit 2,8 pct. af det producerede metan
Methane emissions from biogas plants are higher than thought, but cost-neutral avoidable (Olczak et al. 2026, Communications Sustainability)
Proyecto de Decreto sobre las plantas de transformación en biometano (documento de participación pública)
Rosa-dos-ventos entre Avelãs de Ambom e Pêra do Moço, 2015–2024: cálculo próprio a partir das reanálises ERA5 e ECMWF IFS (Open-Meteo)
Climatologia do vento no local, 2021–2025: reanálise ERA5 através da Open-Meteo Historical Weather API (16 setores, velocidade, dia/noite)
MOV.AP: medições sobre imagem de satélite, outubro de 2026 (distâncias do limite da parcela à zona de exploração de água, quintas e aldeias)
- No public link