Analysis of the legal nature of the treated sewage contained in sanitary facilities. First, it reviews the subject matter in the laws, jurisprudence and doctrine; furthermore, it shows the changing administrative praxis. Then, it offers a dogmatic development on the legal nature of treated sewage, analyzing the various phases of sanitation activity and the legal links of sanitation companies with their facilities and the volumes of water that drain into them. Finally, it analyzes the current regulation that assigns the legal status of spillage to abandoned treated sewage.