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PARTECH Report
Carla Felix
Leanne Tan
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Contents of Report:
Gravity Sedimentation
Thickeners
Clarifiers
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GRAVITY
SEDIMENTATION
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Gravity Sedimentation
Simple Gravity Settling Tank
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Gravity Sedimentation
Two Types of Classification
Equipment:
Simple Gravity Settling Classifier Spitzkasten Gravity Settling Chamber
Principle: The slower the settling
velocity, the farther it will travel.
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Gravity Sedimentation
Simple Gravity Settling Classifier
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Gravity Sedimentation
Spitzkasten Gravity Settling Chamber
Adjustable flow rates
between vessels to
change degree ofseparation
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Gravity Sedimentation Equipment
Corrugated Plate Separator (CPS)
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Factors Affecting the Effectiveness
of Gravity SedimentationFloc which is too small or too large, is irregularly shaped, or hasa low density will not tend to settle out in the sedimentationbasin.
Disintegration of floc
Short circuiting
Temperature Differences
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Disintegration of floc
sharp bends at the inlet high water velocity
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Short circuiting
occurs when water bypasses the normal flowpath through the basin and reaches the outlet inless than the normal detention time
floc does not have enough time to settle out ofthe water, influencing the economy of the plantand the quality of the treated water
If areas of water in the basin do not appear to becirculating, or if sludge build up on the bottom ofthe basin is uneven
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Short circuiting
a basin in which the water
is flowing primarilythrough the left half of the
basin. (Flowing water is
shown as green blobs.)
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Temperature Differences
Differences of temperature can causestratification of the water - separation of waterinto bands of different temperature
Incoming water will tend to flow through the bandof water which corresponds to its own temperature,and will not spread throughout the rest of the basin
Cold water prevents floc from settling, so thatlonger settling times or larger doses of coagulantchemicals are needed
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Temperature Differences
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THICKENERS
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Thickeners
Concentrate suspended solids by gravity
settling.
Rates of solids withdrawn and solidssupplied are equal.
Inventory of pulp - maintained in order to
achieve the desired concentration. Used
for storage of solids when rates change.
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Thickeners
Important consideration in plant design
since thickeners take up a large amount of
space.
In plant design, there are usually more
than one thickener because if the
thickener is broken, the whole process will
stop since it cannot be by-passed.
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Thickeners Top View
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Thickeners Front View
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Aerial View of Actual Thickener
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Types of Thickeners based on
Rake driving mechanism
Bridge-Supported Thickeners
Center-Column-SupportedThickeners
Traction Thickeners
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Bridge-Supported Thickeners
Diameters up to 30to 45 meters.
Can transfer loadsto the tank
peripherally.
Give a denser andmore consistent
underflowconcentration.
Less complicatedlifting device.
Fewer structuralmembers subjectto accumulation.
Access to drivefrom both ends.
Lower cost forsmaller diameters.
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Center-Column-Supported Thickeners
20 meters ormore in diameter.
Mechanism issupported by asingle steel or
concrete column.
Raking arms are attached to drivingcage which rotates around the column.
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Traction Thickeners
60 meters ormore in
diameter.
High installationcost but low
maintenancecost.
No lifting device
can be used.
Not suitable in
cold climates.
More torque is required since theforce comes from the periphery to
the center.
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Components of Thickeners
Contains the slurry.
Sizes range from 2 to 150 meters.
Tank basins are constructed using steel, concretewood, compact earth, plastic sheeting or soil cement.
Tank
Where feed enters the tank.
The feed pipe is where the slurry passes through toget to the feed well.
The feed well distributes the slurry to the tank.
Feed pipingand
Feedwell
Moves solids to withdrawal points
Aids in thickening since the rakes break the solids torelease any liquid trapped to escape.
Design can be changed based on application.
Rakemechanism
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Components of Thickeners
Lifts and lowers the rake based on the torque.
There is little lift in the center.
It is difficult to return the rakes when the solidsare compact.
Rake-Lifting
Mechanism
Provides force for the rakes and force to move
solids to the discharge. Supports rotating mechanism.
Has reserve capacity for temporary overloads.
Protects mechanism during overloads.
DriveAssembly
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Components of Thickeners
Where clarified effluent is removed.
Hydraulic capacity is taken intoconsideration to prevent flooding.
Overflow
Uses centrifugal or gravity
displacement to remove thickenedsolids.
Recycling underflow is used when thesolids are to be stored.
Underflow
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Feedwell and Feed piping
Using the Feedwell, the flow of theslurry to the tank can become
more laminar.
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Feedwell and Feed piping
The feed is diluted so that there ishigh settling rates in the tank.
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Rake Mechanism
Rakes the entire tank floor twice per revolution.
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Rake Mechanism
Rakes the entire tank floor twice and the
inner circle four times per revolution.
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Raking Mechanism
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Rake Lifting Device
Thickeners
BridgeType
Centrallymounted
Platformmounted
ColumnType
TelescopicCage
TelescopicColumn
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Rake Lifting Device
Centrally Mounted Lifting Device
Drive head is fixed to the bridge.
A screw attached to the shaft of the arms lifts and lowers
the arms depending on the torque.
Platform Mounted Lifting Device
A lower platform carries the drive head and moves up ordown and an stationary upper platform and carries the
lifting motor. The platforms are connected by screws that move the
lower platform up or down.
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Rake Lifting Device
Telescopic Cage Type
Drive head remains stationary on the column.
Internal cage is bolted to the rotating drive rim. External cage is lifted with raking arms by
screws.
Telescopic Column
Drive head is mounted on a base raised andlowered by screws using electric motor.
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Drive Assembly
Used forBridge TypeThickener
Used forBridge TypeThickener
Used forColumn TypeThickener
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Basic Underflow Arrangements
Cheap but is susceptible to plugging.
More than one pipes are installed and valvesare used to remove blockage.
Pump Adjacent to Thickener withBuried Piping
More expensive but has operational andmaintenance advantage.
Used when slurry is known to plug the pipes.
Tunnel
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Basic Underflow Arrangements
Used for reduced installation cost for flat-bottomtanks.
Not suitable for coarse solids or solids that becomenon-fluid at high concentration.
Peripheral Discharge
Alternative to tunnel. Pump selection is important.
Most common: bridge-mounted pump with suction linethrough a wet or dry center column.
Center-Column Piping
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Types of Thickeners
The use of flocculant results to
a greater capacity
High-Rate orConventional ThickenersHigh-Rate orConventional Thickeners
Produces high viscosityunderflow
High-Density ThickenersHigh-Density Thickeners
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Discharge
Actualdischarge
from thetank
Bridgetype
thickenerdischarge
cone
Columntype
thickenerdischarge
trench
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CLARIFIERS
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ClarifiersRemove relatively
small quantities ofsuspended
particles and
produce a cleareffluent.
Primary purposeis to produce arelatively clear
overflow.
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CoagulationCoagulation, the first
step in complete
clarification, is theneutralization of theelectrostatic charges
on colloidalparticles.
Most of the smaller
suspended solids insurface waters carry
a negativeelectrostatic charge,
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Coagulation
Inorganic Coagulants
The most common inorganic coagulants are:
1. Alum-aluminum sulfate-Al2(SO4)3
2. Ferric sulfate-Fe2(SO4)3
3. Ferric chloride-FeCl3
4. Sodium aluminate-Na2AI204
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Coagulation
The positive chargeof the metals serves
to neutralize thenegative charges on
the turbidity particles
The metal salts alsoform insoluble metal
hydroxides
Aluminum salts are
most effective ascoagulants in a 5.5-8.0 pH range
Iron salts, on theother hand, are most
effective ascoagulants at higher
pH ranges (8-10)
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Flocculation
Charge reduction increases theoccurrence of particle-particle
collisions, promoting particleagglomeration
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Clarifying Equipments
Conventional Clarifying Equipments
An initial period of turbulent mixing is necessary
for contact between the coagulant and the
suspended matter, followed by a period of gentlestirring to increase collisions between particles
and increase floc size.
Typical retention times are 3-5 minutes for rapid
mix, 15 to 30 minutes for flocculation. and 4-6
hours for settling
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Clarifying Equipments
Conventional Clarifying Equipments
The coagulant is added to the wastewater in the rapid mix chamber or justupstream
Water passes through the mix chambersinto the settling basin
Flocculated particles settle to the bottomand are scraped into a sludge collectionbasin for removal and disposal
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Clarifying Equipments
Rectangular Clarifiers
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Clarifying Equipments
Rectangular Clarifiers
Raking mechanism consists of a chaintype drag
Drag moves the deposited pulp to a sludge hopperlocated on one end by means of scrapers fixed toendless chains
Skimming devices for removal of surface scum
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Clarifying Equipments
Circular Clarifiers
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Clarifying Equipments
Circular Clarifiers
Used for mechanical removal of settleable solids from waste
Often equipped with a surface-skimming device, whichincludes a rotating skimmer, scum baffle, and scum-boxassembly
Floating matter is skimmed by a skimmer arm that is supportedby the sludge rake and rotates with it around the tank
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Clarifying Equipments
Circular Clarifiers
Floating matter is pushed over the beach plateby the wipers attached to the skimmer arm andinto a scum box attached to the tank wall
Scum baffle prevents floating matter fromreaching the effluent launder
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Clarifying Equipments
Circular Clarifiers
Appropriately sized V-notch weir assures
uniform laminar flow throughout theclarifier
Upflow Clarification - water flows upwardthrough the clarifier as the solids settle tothe bottom
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Clarifying Equipments
Vertical/Lamella Clarifiers
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Clarifying Equipments
Vertical/Lamella Clarifiers
Plates inclined at 45 to 60 from the horizontal
Influent passes into each inclined channel at about one-third of the vertical height from the bottom
The clarified liquid passes in the opposite directionbeneath the ceiling of each channel to the overflowconnection
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Clarifying Equipments
Vertical/Lamella Clarifiers
The influent enters the lower area of thelaminar plate sections from the channelsthrough side slots.
The waste liquid is then distributed acrossthe entire width of the plates, and flowsupward under laminar flow conditions
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Clarifying Equipments
Vertical/Lamella Clarifiers
Principal advantage of the tilted-plate clarifieris the increased capacity per unit of plane area
Disadvantages are an underflow solidsconcentration that generally is lower than inother gravity clarifiers and difficulty of cleaningwhen scaling or deposition occurs
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Clarifying Equipments
Vertical/Lamella
Clarifiers
Lamella plates
increase the settlingefficiency and speed
in sedimentation
basins
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Clarifying Equipments
Solids Contact Clarifiers
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Clarifying Equipments
Solids Contact Clarifiers
The influent is contacted with previously settled solids in arecirculation draft tube within the reaction well
All chemical reactions are more rapid and more nearly complete, andflocculation is improved.
The mixture passes out of the contacting and reaction well into the
clarification area, where the flocculated particles settle out.
They are raked to the center to be used again in the recirculationprocess, with a small amount being discharged through the sludgepump
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Clarifying Equipments
Solids Contact Clarifiers
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Clarifying Equipments
Sludge Blanket Clarifier
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Clarifying Equipments
Sludge Blanket ClarifierThe inverted cone within the clarifier producesan increasing cross-sectional area from bottomto top
Upward velocity of the water decreases as itapproaches the top
As the water containing flocculated solids passes upthrough this blanket, the particles are absorbed onto
the larger floc, which increases the floc size anddrops it down to a lower level.
It eventually falls to the bottom of the clarifierto be recirculated or drawn off
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Parts of a Clarifier
Inlet Zone
Settling Zone
Outlet Zone
Sludge Zone
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Parts of a Clarifier
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Parts of a Clarifier
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Inlet Zone
distributes the waterand to control the
water's velocity as it
enters the basin
prevent turbulence ofthe water
inlets control thevelocity of theincoming flow.
If the water velocity isgreater than 0.5 ft/sec,then floc in the waterwill break up due to
agitation of the water.
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Inlet Zone
Water leaves the inlet and enters thesettling zone of the sedimentation
basin by flowing through the holesevenly spaced across the stilling wall.
stilling wall (perforatedbaffle wall)
allows water to enter the basin by firstflowing through the holes evenlyspaced across the bottom of thechannel
Channel
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Settling Zone
where water velocity is greatlyreduced and actual
sedimentation occurs
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Outlet Zone
controls the water flowing out ofthe sedimentation basin and can
also be used to control the waterlevel in the basin
designed to ensure that the waterflowing out of the sedimentation
basin has the minimum amount offloc suspended in it
An effluent structure
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Outlet Zone
The best quality water is usually found atthe very top of the sedimentation basin, sooutlets are usually designed to skim thiswater off the sedimentation basin.
A typical outlet zone begins with a baffle in
front of the effluent and then comes theeffluent structure
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Outlet Zone
A trough which collects the water flowingout of the sedimentation basin and directs it
to the effluent piping.
Effluent launder
Walls preventing water from flowinguncontrolled into the launder and serve toskim the water evenly off the tank.
Weirs
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Sludge Zone
Found across the bottom of thesedimentation basin where the
sludge collects temporarily
Velocity in this zone should be
very slow to preventresuspension of sludge
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Sludge Zone
If sludge is not removed
the effective(useable) volume
of the tank willdecrease,
reducing theefficiency of
sedimentation
sludge built up onthe bottom of thetank may becomeseptic, meaningthat it has begun
to decayanaerobically
sludge may alsobecome
resuspended inthe water and be
carried over tothe filters
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Sources
www.solidliquid-separation.com
Perrys Chemical Engineering Handbook