Consorzio Aquarno provides the cleansing of  municipal wastewater of Santa Croce sull'Arno, Fucecchio and Castelfranco di Sotto.

This kind of sewage is completely different from the industrial one, both for qualitative and quantitative characteristics. All reference indicators (such as COD, BOD5, Total Nitrogen, Ammonium, Salinity, etc.), at the entrance of this treatment section, have lower value than in industrial wastewater. This condition has unavoidable consequences on types and dimensions of each treatment stage. The incoming flow of municipal wastewater is generally lower than industrial one, representing about the 40-45% of the total annual incoming flow. Although this quantitative similarity, depuration issues are very different. This consideration is clearly and directly observable looking at the plant organization and the dimension of each treatment section.

Depuration of municipal wastewater, within the WWTP, occurs across three main stages:

  1. Primary sedimentation
  2. Biological Oxidation
  3. Secondary sedimentation

1) Primary sedimentation
Influent municipal wastewater flow, after a coarse screening through a mechanically cleaned bar screen, is settled in a rectangular tank with a capacity of about 900m3. A travelling-bridge-type collector equipped with scraper blades removes continuously settled solids, which are subsequently discharged.

3) Secondary sedimentation
The aerated mixed liquor outcoming from the biological oxidation process, is a dispersion of activated sludge in purified water. Simply exploiting the gravity force, it is possible to separate these two phases, which are poured into the sedimentation stage, where no chemicals are added and the spontaneous settling of sludge occours.
After a first degasation step, the mixed liquor is distributed between 4 rectangular tanks  of about 900m3 each and  equipped with chain-and-flight solids collectors. In these tanks gravity sedimentation of the biological sludge take place. The clarified stream flow outcomes from each tank by the effluent channel, whereas settled sludge is scraped to solid hoppers by the chain and flights collector, at the same time .
Treated wastewater is continuously monitored by the internal laboratory and, on the basis of specific analysis and results, it can be addressed to two different stages:
  • denitrification step of the industrial wastewater section
  • clariflocculation step of the industrial wastewater section
Settled activated sludge are mainly returned to the biological oxidation step, while excess sludge is discharged together with the primary sludge into a thickening tank, or it is returned to the second biological oxidation of the industrail wastewater treatment section. 

2) Biological oxidation

The overflow stream of the primary settlement tank is transferred to the biological oxidation tank where, thanks to its 9000m3 capacity and the use of pure oxygen, a coarse depuration is accomplished.

Some particular characteristics of this tank like the circular geometry, the utilization of pure oxygen and the inner flow management, enable the gathering of two distinct phases such as denitrification and oxidation in only one stage. The oxigenated mixed liquor, containing the activated sludge, continues to the secondary sedimentation, where a substantial separation from activated sludge occours and the clarified streams drop out the tanks by the overflow channels.