Sludge thickeners are used to decrease the water quantity that sludge contains. Thickening through settling is based on the difference in specific weight between solid particles and the liquid in which they are.
It is especially effective in primary sludge treatment and when the sludge comes from a chemical precipitation. It is rarely used for activated excessive sludge.
Thickening is conventionally conducted in cylindrical tanks, tapered tank bottom and it may or not have a scraper bridge.