Wastewater Treatment Clarifiers

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.

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

Primary Tanks

From the primary clarifier the liquid goes to the biological treatment stage, either biofiltration (trickling filters) or an activated sludge process. In the sedimentation tank the solids settle out to be removed by mechanically driven scrapers for further processing. In the case of circular tanks the scraper blades scrape the settled sludge to a central sludge removal hopper and in rectangular tanks to a hopper at one end of the tank.

Cessnock Wastewater Treatment Plant - Primary Clarifier

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.

Secondary Tanks or Final Tanks

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. These mechanisms are also normally fitted with scum skimming systems. They may also be fitted with options such as Stamford Baffles.

	Cleveland Bay Wastewater Treatment Plant  - Suction Tube Clarifier
	Pimpama Wastewater Treatment Plant - 45m full bridge primary clarifiers with launder covers.

Tertiary Tanks

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

Fixed Bridge Mechanisms

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

Subiaco Wastewater Treatment Plant - Fixed bridge clarifier with odour covers

Sludge Thickeners

These mechanisms are designed to promote agglomeration of the sludge particles, thus producing a sludge having a higher solids concentration than would be achieved in a sedimentation tank. They are usually fixed bridge devices and scum skimming is usually omitted.

The thickened sludge proceeds to further dewatering processes, whilst the supernatant liquor continues through the treatment process.

Fermenters used in certain activated sludge processes also use picket fence thickener type mechanisms for continuous stirring as the sludge in the fermenter can have a concentration range to 6% DS or more

	Longreach Water Treatment Plant - picket fence sludge thickener
Options 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. Launder covers in lieu of cleaning brushes.   Please contact us for further information or a quotation.