The settling process is one of the oldest known unitary operations used in wastewater treatment, whether urban or industrial, as well as one of the most widely used today, being a purely physical process of elimination of suspended solids by density difference, in such a way that the particles with higher density than water are separated by the exclusive action of gravity.
These units do not separate suspended solids with a very fine particle size or those in a colloidal state, as well as those with a density close to or lower than that of the water to be treated.
The settling process is also known as sedimentation.
As, in general, part of the suspended solids are constituted by organic matter, a consequence of primary wastewater treatment is usually the reduction of BOD, and a reduction of bacteriological contamination can also be achieved.
The degree of treatment or reduction of these pollution rates achieved in primary treatment depends on the process used and the characteristics of the wastewater.
Good treatment selection and design affects the economics of the entire treatment process. The resources invested in primary treatment often give very high economic returns in terms of cost per unit of pollutant removed; it is the process that removes the most BOD at the lowest cost.
In addition, the decanters exert an important effect of equalization of the raw water, protecting the downstream units.
