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Sedimentation (water treatment)

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Sedimentation is a physical water treatment process using gravity to remove suspended solids from water.[1] Solid particles entrained by the turbulence of moving water may be removed naturally by sedimentation in the still water of lakes and oceans. Settling basins are ponds constructed for the purpose of removing entrained solids by sedimentation.[2] Clarifiers are settling basins built with mechanical means for continuous removal of solids being deposited by sedimentation.[3]

Circular clarifier with surface skimmer visible in the lower right. As the skimmer slowly rotates around the clarifier, skimmed floating material is pushed into the trap visible above the fenced enclosure at the lower left.

Basics

Suspended solids (or SS), is the mass of dry solids retained by a filter of a given porosity related to the volume of the water sample. This includes particles of a size not lower than 10 μm.

Colloids are particles of a size between 0.001 µm and 1 µm depending on the method of quantification. Due to Brownian motion and electrostatic forces balancing the gravity, they are not likely to settle naturally.

The limit sedimentation velocity of a particle is its theoretical descending speed in clear and still water. In settling process theory, a particle will settle only if:

  1. In a vertical ascending flow, the ascending water velocity is lower than the limit sedimentation velocity.
  2. In a longitudinal flow, the ratio of the length of the tank to the height of the tank is higher than the ratio of the water velocity to the limit sedimentation velocity.

Removal of suspended particles by sedimentation depends upon the size and specific gravity of those particles. Suspended solids retained on a filter may remain in suspension if their specific gravity is similar to water while very dense particles passing through the filter may settle. Settleable solids are measured as the visible volume accumulated at the bottom of an Imhoff cone after water has settled for one hour.[4]

There are four types of sedimentation processes:

  • Type 1 - Dilutes, non-flocculent, free-settling. (Every particle settles independently.)
  • Type 2 - Dilute, flocculent. (Particles can flocculate as they settle.)
  • Type 3 - Concentrated Suspensions, Zone Settling, Hindered Settling. (Sludge Thickening).
  • Type 4 - Concentrated Suspensions, Compression (Sludge Thickening).

Applications

Potable Water Treatment

Sedimentation in potable water treatment generally follows a step of chemical coagulation and flocculation, which allows grouping particles together into flocs of a bigger size. This increases the settling speed of suspended solids and allows settling colloids.

Waste Water Treatment

Sedimentation tanks have been used to treat wastewater for millennia.[5]

Primary treatment of sewage is removal of floating and settleable solids through sedimentation.[6] Primary clarifiers reduce the content of suspended solids as well as the pollutant embedded in the suspended solids.[7]: 5–9  Due to the large amount of reagent necessary to treat domestic wastewater, preliminary chemical coagulation and flocculation are generally not used, remaining suspended solids being reduced by following stages of the system. However, coagulation and flocculation can be used for building a compact treatment plant (also called a "package treatment plant"), or for further polishing of the treated water.[8]

Sedimentation tanks called Secondary clarifiers remove flocs of biological growth created in some methods of secondary treatment including activated sludge, trickling filters and rotating biological contactors.[7]: 13 

Technology

Rectangular sedimentation tanks with effluent weir structure visible above the fluid surface.
Drained circular sedimentation tank showing central inlet baffles on the right with solids scraper and skimmer arms visible under the rotating bridge.

Although sedimentation might occur in tanks of other shapes, removal of accumulated solids is easiest with conveyor belts in rectangular tanks or with scrapers rotating around the central axis of circular tanks.[9] Mechanical solids removal devices move as slowly as practical to minimize resuspension of settled solids. Tanks are sized to give water an optimal residence time within the tank. Economy favors using small tanks; but if flow rate through the tank is too high, most particles will not have sufficient time to settle, and will be carried with the treated water. Considerable attention is focused on reducing water inlet and outlet velocities to minimize turbulence and promote effective settling throughout available tank volume. Baffles are used to prevent fluid velocities at the tank entrance from extending into the tank; and overflow weirs are used to uniformly distribute flow from liquid leaving the tank over a wide area of the surface to minimize resuspension of settling particles.[10]

Tube settlers

High efficiency tube settlers use a stack of parallel tubes, rectangles or flat pieces separated by a few inches (several centimeters) and sloping upwards in the direction of flow. This structure creates a large number of narrow parallel flow pathways encouraging uniform laminar flow as modeled by Stokes' law.[11] These structures work in two ways:

  1. They provide a very large surface area onto which particles may fall and become stabilized.
  2. Because flow is temporarily accelerated between the plates and then immediately slows down, this helps to aggregate very fine particles that can settle as the flow exits the plates.

Structures inclined between 45°  and 60°  may allow gravity drainage of accumulated solids, but shallower angles of inclination typically require periodic draining and cleaning. Tube settlers may allow the use of a smaller sedimentation tank and may enable finer particles to be separated with residence times less than 10 minutes.[11] Typically such structures are used for difficult-to-treat waters, especially those containing colloidal materials.

See also: Water purification and Wastewater treatment

See also

Sources

  • Franson, Mary Ann Standard Methods for the Examination of Water and Wastewater 14th edition (1975) APHA, AWWA & WPCF ISBN 0-87553-078-8
  • Goldman, Steven J., Jackson, Katharine & Bursztynsky, Taras A. Erosion & Sediment Control Handbook McGraw-Hill (1986) ISBN 0-07-023655-0
  • Hammer, Mark J. Water and Waste-Water Technology John Wiley & Sons (1975) ISBN 0-471-34726-4
  • Metcalf & Eddy Wastewater Engineering McGraw-Hill (1972)
  • Steel, E.W. & McGhee, Terence J. Water Supply and Sewerage (5th edition) McGraw-Hill (1979) ISBN 0-07-060929-2
  • Weber, Walter J., Jr. Physicochemical Processes for Water Quality Control John Wiley & Sons (1972) ISBN 0-471-92435-0

Notes

  1. ^ "IngentaConnect Coagulation and sedimentation in lakes, reservoirs and water treatment". www.ingentaconnect.com. Retrieved 2008-03-27.
  2. ^ Goldman, Jackson & Bursztynsky pp.8.2&8.12
  3. ^ Hammer pp.223-225
  4. ^ Franson pp.89-98
  5. ^ Chatzakis, M.K., Lyrintzis, A.G., Mara, D.D., and Angelakis, A.N., 2006, Sedimentation Tanks through the Ages, Proceedings of the 1st IWA International Symposium on Water and Wastewater Technologies in Ancient Civilizations, Iraklio, Greece, 28–30 October 2006, pp. 757-762
  6. ^ Steel & McGhee pp.469-475
  7. ^ a b U.S. Environmental Protection Agency (EPA). Washington, DC (2004). "Primer for Municipal Wastewater Treatment Systems." Document no. EPA 832-R-04-001.
  8. ^ EPA. Washington, DC (2000). "Package Plants." Wastewater Technology Fact Sheet. Document no. EPA 832-F-00-016.
  9. ^ Metcalf & Eddy pp.449-453
  10. ^ Weber pp.128-131
  11. ^ a b Weber p.130
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