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 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:
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.
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| Primary
Clarifier Elevation View |
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| 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.
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.
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Picket
Fence Sludge Thickener
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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.
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.
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