Facts
[A.] has operated a dental practice in Geneva since 1992 as a sole proprietorship, employing between ten and nineteen staff according to the Geneva companies register as at 29 June 2018.
By decision of 26 March 2018, the City of Geneva classified this practice as a medium waste producer within the meaning of art. 5 para. 2 RCGD (municipal regulation of 20 December 2017 on waste management) and not as a micro-enterprise. Accordingly, it invited the practice to enter into a contract with a private service provider for the disposal of household and similar waste, in accordance with art. 12 para. 1 RCGD.
By judgment of 2 November 2018, the Administrative Court of First Instance dismissed the appeal of the party concerned. By decision of 10 September 2019 (ATA/1367/2019), the Administrative Chamber of the Court of Justice allowed the appeal, annulled the judgment and remitted the matter to the City for a new decision. According to the cantonal court, the City could not rely on the number of employees without an indication in full-time equivalents (FTE); it was required to classify as micro-enterprises all entities with fewer than eight FTE, which was the case for the practice.
The City of Geneva lodges a public law appeal with the Federal Supreme Court, principally seeking annulment of the cantonal decision and confirmation of its own decision, and in the alternative remittal to the cantonal court. The cantonal court did not file observations. The respondent seeks dismissal of the appeal as inadmissible or, in the alternative, its rejection. The Federal Office for the Environment (FOEN) did not file observations.
The central question is whether the practice must be subject to the micro-enterprise regime (collection and disposal ensured by the municipality) or to the medium waste producer regime (contract with a private service provider), having regard to municipal law and federal law (art. 31b and 31c EPA, art. 3 let. a ADWO).
Considerations
On admissibility, the cantonal decision is an interlocutory decision because of the remittal to the lower authority. It is, however, capable of causing irreparable harm within the meaning of art. 93 para. 1 let. a FSCA where it contains precise instructions leaving the municipality no margin of discretion, which is the case here, as the City is required to classify the practice as a micro-enterprise. The appeal is therefore admissible on this basis.
The City has standing to appeal under art. 89 para. 1 FSCA: it is affected in its public authority prerogatives and has a public interest worthy of protection, given the costs and organisational tasks entailed by the need to determine the FTE of more than 9200 companies, and the precedential value of the case. In the alternative, its standing may also be based on art. 89 para. 2 let. d FSCA in conjunction with art. 57 EPA, the decision resting in part on the EPA and the ADWO.
On the merits, the Federal Supreme Court applies the law of its own motion (art. 106 para. 1 FSCA) and may substitute its own reasoning. Arts. 31b and 31c EPA allocate responsibilities: the cantons must dispose of urban waste (a state monopoly, which may be delegated to municipalities), while other waste falls to its holder. Art. 3 let. a ADWO, in force since 1 January 2016, defines urban waste as waste from households, from companies with fewer than 250 full-time positions and from public administrations, the composition of which is comparable to that of household waste.
The threshold of 250 full-time positions is decisive in distinguishing urban waste subject to the municipal monopoly from other waste. The Federal Council certainly…