SEDIMENTATION

SEDIMENTATION

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  SEDIMENTATION
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  Overview of theProcess Location in the Treatment Plant  After the source water has been coagulated and flocculated, it is ready for sedimentation.
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  Objective of Sedimentation toseparate solids from liquid using the force of gravity. In sedimentation, only suspended solids (SS) are removed. Use Sedimentation is used in water and wastewater treatment plants
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  Applications in WaterTreatment: 1. settling of coagulated and flocculated waters prior to filtration 2. settling of coagulated and flocculated waters in a softening plant 3. settling of treated waters in an iron and manganese removal plant Applications in Wastewater Treatment: 1. grit removal 2. suspended solids removal in primary clarifier 3. biological floc removal in activated sludge
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  Sedimentation Basins Shapes − Circular,Rectangular, and Square Sizes •Circular – 15 to 300 ft (diameter) and 6 to 16 ft (depth) – Typical sizes are 35 to 150 ft (diameter) and 10 to 14 ft (depth) •Square – 35 to 200 ft (width) and 6 to 19 ft (depth) •Rectangular (depends on sludge removal mechanism) •Freeboard – 1 to 1.25 ft for circular and square tanks
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  SETTLING of particlesfrom suspension depends on: Characteristics of the Particles Concentration of Particles in Suspension DISCRETE PARTICLES particles whose size, shape and specific gravity do not change with time. FLOCCULATING PARTICLES particles whose surface properties are such that they aggregate upon contact Thus, changing in size,shape, and perhaps specific gravity with each contact. DILUTE SUSPENSIONS suspensions in which the concentration of particles is not sufficient to cause significant displacement of water as they settle or in which the particles will not be close enough for velocity field interference to occur CONCENTRATED SUSPENSIONS suspensions in which the concentration of particles is too great to meet the conditions mentioned for dilute suspensions
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  TYPE 1 (discrete particle settling) · settlingof discrete particles in dilute suspensions · particles have no tendency to flocculate · they settle as individual entities and there is no significant interaction with neighboring particles Example: removal of grit and sand in wastewater treatment TYPE 2 (flocculant settling) · settling of flocculant particles in dilute suspensions · as particle settle and coalesce with other particles, the sizes of particles and their settling velocity increases Examples: • removal of SS in primary sedimentation tanks of WWTP • settling of chemically coagulated waters TYPE 3 (hindered settling) or (zone settling) · settling of intermediate concentration of flocculant particles · particles are so close together that interparticle forces are able to hold them in fixed positions relative to each other and the mass of particles settles as a zone at a constant velocity Example: biological floc removal in secondary settling basins of WWTP TYPE 4 (compression settling) · settling of particles that are of such a high concentration that the particles touch each other and settling can occur only by compression which takes place from the weight of particles Examples: · occurs in the bottom of deep secondary clarifiers · in sludge thickening facilities
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  Sedimentation Tanks Sedimentation tankscan be divided into 4 different functional zones: 1. Inlet zone 2. Settling zone 3. Sludge zone 4. Outlet zone
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  •should disseminate influentenergy •distribute the flow •mitigate density currents •minimize sludge blanket disturbance Inlet structures are designed to uniformly distribute the influent suspension over the cross section of the settling zone. For Rectangular Basins full width inlet channels - effective spreading of flow introduce a vertical velocity component into sludge happen that may resuspend sludge. INLET ZONE
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  inlet channels withsubmerged orifices For sedimentation tank followed by flocculation (width of flocculation basin – width of settling tank) Depth of inlet channel = depth of flocculator basin Pipe connection between flocculation unit & sedimentation Low velocity in pipe - settling of floc High velocity in pipe - breakage of floc Permissible flow velocity to maintain floc suspension 0.15 – 0.6 m/sec If sedimentation tank does not adjoin a flocculator inlet channels with submerged orifices do not extend down the full depth of the tank
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  For circular tanks circulartanks -radial flow to achieve a radial flow, pattern influent is introduced in the center of the tank or around the periphery of the tank Central feed water enters a circular well designed to distribute the flow equally in all directions D of feed well = 15-20 % of tank diameter Depth= 1- 2.5m Velocity through the orificies on feed well 0.075- 0.15 m/sec Entrance Pipe suspended from bridge OR encased in concrete beneath the tank floor
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  It depends onthe following design parameters: Settling characteristics of the suspended matter Surface loading (over flow rate) Width / length ratio OR diameter Detention time SETTLING ZONE
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  Rectangular tanks -the bottom is slightly sloped to facilitate sludge scraping a pair of endless conveyor chains bridge – type mechanism continuously pulls the settled material into a sludge hopper where it is pumped out periodically. Motion of scraper momentarily resuspend lighter particles a few cm above the scraper blades Excessive horizontal velocity (for the case of rectangular basins) move these materials towards outlet zone. SLUDGE ZONE
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  To prevent this, HorizontalVelocity 9 m/hr for light flocculant suspensions 36 m/hr for heavier discrete suspensions Bridge type mechanism travels up and down the tank one or more scraper blades are suspended from the bridge
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  Circular tanks The bottomof the tank is sloped to form an inverted cone and the sludge is scraped to a relatively small hopper located near the center of the tank. Velocity or scraper Important Very high velocIty resuspension of settled particles (<5mm/sn) Travelling bridge with sludge suction headers and pumps not very good
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  Weir channels areused. Large weir loading resuspension of particles settled near to effluent launders Effluent weirs placed as far from the inlet as possible To increase weir length (i.e to decrease weir loading) double-sided weirs can be used Typical weirs 90˚ V notch metal plates bolted onto the effluent collection through OUTLET ZONE
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  May be placedat the opposite end of the rectangular basins through the entire width of tank through the length of the tank If the weir loading causes the required weir length to be greater than tank width the channel may be extended to a length of 1/3 the basin length (Reynolds) around the perimeter of center – feed circular tanks at the center of peripheral feed circular tanks