Clarifiers separate solids from the liquid stream. In wastewater treatment the terms clarifier and sedimentation tank are synonymous. The purpose of the scraper mechanism mounted inside the tank, is to collect the settled solids for removal from the tank by pumping. In circular sedimentation tanks the clarifier mechanism has floor scrapers attached to a rotating arm scraping the sludge towards a central hopper or an overhead trough.
In rectangular clarifiers scrapers are carried along the tank bottom collecting the sludge into a trough or hopper at the influent end of the tank.
Sedimentation tank scraper mechanisms may take different forms depending on whether the mechanisms are for primary, secondary or tertiary treatment. In circular clarification tanks the equipment may be of the peripheral drive type, fixed bridge or fixed half bridge (cage drive). In rectangular clarification tanks the mechanism may be of the chain and flight type or have scrapers and scum removal equipment suspended from a travelling bridge.
Circular settling tanks and clarifiers are generally preferred, as they require less maintenance, sludge removal is faster and higher removal efficiencies can be obtained. Rectangular tanks are predominantly used in very large treatment plants or in confined spaces, making maximum use of the area available.
Variations of the sedimentation tank scraper mechanism are also found in sludge thickeners and fermenters.