The decanting process is one of the oldest known unit operations used in wastewater treatment—whether municipal or industrial—as well as one of the most widely used today. It is a purely physical process for removing suspended solids based on differences in density, such that particles with a higher density than water are separated solely by the force of gravity.
These units do not remove suspended solids with very fine particle sizes, those in a colloidal state, or those with a density close to or lower than that of the water to be treated.
The decanting process is also known as sedimentation.
Since, in general, a portion of the suspended solids consists of organic matter, one result of primary wastewater treatment is usually a reduction in BOD; a reduction in bacteriological contamination can also be achieved.
The extent to which these pollution levels are treated or reduced during primary treatment depends on the process used and the characteristics of the wastewater.
A good choice of treatment process, as well as a well-designed system, affects the cost-effectiveness of the entire treatment process. The resources invested in primary treatment often yield 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 play an important role in equalizing the raw water, thereby protecting the downstream units.
