The thickening is a process that increases the solids matter of the sludge, removing part of the water associated with it, and consequently reduces its volume. This treatment is an aid for the next processes of the sludge, such as digestion, dehydration, drying and combustion, in many points of view:
- Smaller required tanks and equipment
- Lower quantity of chemical reagents required for conditioning the sludge
- Lower quantity of heat to be supplied to the digester or the quantity of auxiliary fuel required for the processes of drying or thermal incineration.
The thickening by gravity is the most used system; it is conceptually similar to the conventional clarifier process and it requires a dedicated tank, where the sludge settles and compacts. Once thickened, the sludge is conveyed to the conical pit on the bottom of the tank, through an agitator with scraping blades. Generally, this agitator has also some arms, which keep homogeneous the sludge, avoiding the formation of more or less concentrated areas and, thus, optimizing the feeding to the dehydrator machine.
Il surnatante viene estratto dalla parte superiore della vasca e ricircolato in testa alla linea di trattamento acqua.
Often, to facilitate and speed up the thickening of sludge, it is provided the dosage of polymer (polyelectrolyte) in the tank, which have the additional advantage of mechanically preparing the sludge to the next stages of dehydration.