ONAS was established by Law N° 73/1974, dated 3 August 1974, and was entrusted with the mission of management of the sanitation sector.

The law establishing ONAS was amended by virtue of Law N° 41/1993, dated 19 April 1993, thus raising ONAS to the status of a main operator in matter of protection of the water environment and of combating pollution sources.

ONAS is a public institution of an industrial and commercial character, enjoys civil status and financial autonomy and is placed under the authority of the Ministry of Environment and Sustainable Development.

ONAS actions include the following fields: 

Studies: These relate to sanitation master plans in cities and Governorates (regional administrative departments), feasibility studies, prospective studies and final design studies related to sewerage networks, pumping stations and wastewater treatment plants;

Works: These relate to the implementation of sanitation projects and supervision of the projects conducted by such other parties as public or private developers . . .;

Operation and maintenance (O & M) of sanitation networks and facilities: ONAS intervenes fully and directly in the zones annexed by decree to its action zones;

 Technical assistance: ONAS provides technical assistance and guidance to local government and other public or private institutions or enterprises in the field of water pollution.

Combating the sources of water pollution under ONAS purview;
Management, operation, maintenance, rehabilitation and construction of all facilities intended for the sanitation of the towns assigned to ONAS by decree;  
Promoting the sector of the sale and distribution of the sub-products of wastewater treatment plants, such as treated wastewater and sludge;
Planning and implementation of the sanitation sector programmes;
Planning and implementation of integrated projects related to the treatment of wastewater and disposal of storm water.