Facts
The appellant, an agricultural operator mainly engaged in cattle fattening, owns a plot located in the agricultural zone in the municipality of Chessel (VD), with a surface area of 47'144 m2. He applied for a building permit for a biogas installation comprising two concrete tanks (a 780 m3 digester and a 1880 m3 digestate storage tank), an intermediate room housing electromechanical components, technical energy production premises (combined heat and power unit, wood-fired boiler room, chipped wood silo, office, control room), and a shed including a manure store of approximately 230 m2 for receiving and storing solid substrates (manure, cereal waste), as well as a chipped wood storage area of approximately 55 m2.
The nearest dwellings, located in a villa neighbourhood, are approximately 100 m away (103 m from the manure store and 100 m from the covered slurry pit). The project, put to public inquiry in early 2017, gave rise to several objections. On 7 June 2017, the CAMAC Centre issued a summary of special authorisations and cantonal opinions, including in particular a positive opinion from the Directorate-General for the Environment (DGE) on air protection, subject to conditions. On 7 March 2018, the municipal council dismissed the objections and issued the building permit.
By judgment of 16 May 2019 (AC.2018.0132), after a local inspection, the Administrative and Public Law Division of the Vaud Cantonal Court (CDAP) annulled the contested decisions. It acknowledged that the project complied on most points, but considered that the distance between the installation and the dwellings did not allow compliance with air protection legislation, by strictly applying the minimum distance calculated under the Agroscope 2018 study (220 m).
The appellant lodges a public law appeal with the Federal Supreme Court and requests confirmation of the permit or, in the alternative, remittal of the case to the cantonal court. The municipality requests dismissal of the appeal. The objectors also request dismissal. The Federal Office for the Environment (FOEN) considers the cantonal judgment compliant with federal law, while the Federal Office for Spatial Development (ARE) requests remittal of the case.
Main question: could the cantonal court, by strictly applying the Agroscope 2018 recommendations on minimum distances, refuse authorisation for a biogas installation located 100 m from dwellings, in breach of art. 12 EPA and Annex 2 no. 512 OAPC?
Considerations
The Federal Supreme Court first examines the admissibility of the appeal, which it accepts as a public law appeal against a final decision rendered in last cantonal instance in a building law matter (art. 90, 86 para. 1 let. d, 82 let. a, 89 para. 1 FSCA). The appellant's requests for investigative measures (consultation of the FOGE and information on complaints) are deemed irrelevant to the outcome or already satisfied.
On the merits, the Federal Supreme Court recalls the regime of preventive limitation of atmospheric emissions (art. 11 para. 2 and 12 EPA, art. 3 OAPC). Number 512 of Annex 2 OAPC requires compliance with minimum distances from residential areas, according to livestock farming rules, in particular the recommendations of the federal research station Agroscope (formerly FAT). A biogas installation linked to a livestock operation constitutes a component of it. The distance rules aim to maintain the quality of living environments without making the construction of agricultural installations in the agricultural zone excessively difficult, the project owner having to demonstrate, under art. 34 para. 4 let. b SPO, the absence of a more appropriate alternative site.
The Federal Supreme Court holds that the reference in number 512 is dynamic and that the Agroscope recommendations of 2018 should be given preference as the most recent document. They do not, however, have strictly binding value: they guide the administration, which has a margin of manoeuvre and must take into account widespread and relevant criticism as well as particular circumstances not captured by the formula. The application of the rules must be accompanied by an assessment of the concrete circumstances, without authorities being able to rely solely on the result of mathematical formulas.
The Federal Supreme Court then notes the shortcomings of the 2018 study: the distance for a biogas installation is set by a single factor (f Biog) alone yielding a distance of 201 m, regardless of size, processing capacity and technology; the type of substrates is not taken into account;…