Biological aerobic treatment - NAS® technology

Hydroitalia – Colsen proposes an innovative and unique active sludge system named NAS® where a part of the biomass is constituted by the Annamox bacteria.
The NAS® system is the alternative solution to the traditional activated sludge process for the COD and nitrogen removal from wastewater. The principal differences from the conventional denitrification – nitrification treatment lie in the activity of the Annamox bacteria able to convert ammonia nitrogen into nitrites and gaseous nitrogen. The Annamox process reduces the oxygen needs, the secondary sludge production and cancel the external carbon source requirement.

Therefore, the advantages of the NAS® technology installation are the following:

  • Nitrogen removal yield > 95%
  • Small footprint of the aerobic treatment plant;
  • Low operational costs
  • Reduced aerobic sludge production


The NAS® system applications are often combined with the MBR systems in order to obtain an higher final effluent quality and to reduce the footprint of the plant to comply with the restricted available area for the industrial applications.