Office National de l'Assainissement du Sénégal - Senegal - Sanitation authority managing wastewater systems and promoting public hygiene infrastructure in Senegal. Office National de l'Assainissement du Sénégal - Senegal - Sanitation authority managing wastewater systems and promoting public hygiene infrastructure in Senegal. ONAS is the national sanitation agency responsible for developing and managing sewerage and wastewater infrastructure. It ensures hygiene standards and promotes sustainable urban sanitation in Senegal.