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 sludge scrapers attached to a rotating arm scraping the sludge towards a central hopper. 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.

The three types of clarifiers are:
Primary tank
Secondary tanks
Tertiary tanks

Variations of the sedimentation tank scraper mechanism are also found in sludge thickeners and fermenters.

There are a variety of materials and equipment types available to suit your installation, refer to options .

Primary Tanks

After coarse solids have been eliminated and grit removed from the sewage stream, the primary process begins. In the sedimentation tank the solids are settled out to be removed by mechanically driven scrapers for further displacement. In the case of the circular tanks the scraper blades scrape the settled sludge to a central sludge removal hopper and in rectangular tanks to an end hopper.

Scraper mechanisms for primary tanks will also be fitted with surface scum removal equipment to remove floating matter, scum and grease etc. Approximately 65% of organic solids and 35% of the BOD in the sewage is removed in the primary process.

From the primary settling tank, the liquid goes through an aeration and or bio filtration process before entering the secondary tank.

Primary Clarifier Elevation View

Primary Clarifier Plan View

Secondary Tank

Following the primary settling phase and the
biological treatment stage, secondary sedimentation takes place. This process separates mixed liquid suspended solids (MLSS) or humus sludge from the purified effluent stream. Secondary tanks may be fitted with scraper blades as for primary tanks or can be of the suction tube type as illustrated below. Secondary mechanisms are also normally fitted with scum skimming systems.

Suction Tube Clarifier Elevation

Tertiary Tank

These tanks may be fitted with any of the above scraper types or simple sweeper chains. Scum skimming is usually omitted.

Sludge Thickeners and Fermenters

These mechanisms are designed to scrape heavier sludges and are considerably smaller than that of the other clarifiers and may be fixed bridge or peripheral drive. Scum skimming is usually omitted.

Picket Fence Sludge Thickener

Fixed Bridge Mechanisms

Fixed bridge clarifiers of the circular type are being used in larger tanks than was previously the case particularly when the sedimentation tank is covered to facilitate odour removal. They may be for primary, secondary or tertiary sedimentation.


Outlet launder cleaning by either trailing brush or high pressure spray.
Submerged scum boxes.
Flocculating double compartment feedwells.
Stamford Baffles.
Centre pocket stirring.
Variable speed drive.
Mild Steel, Stainless Steel or Aluminium construction

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