partech report

8/7/2019 partech report

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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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Simple Gravity Settling Tank


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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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Simple Gravity Settling Classifier


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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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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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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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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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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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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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Rectangular Clarifiers


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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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Circular Clarifiers


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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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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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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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Vertical/Lamella Clarifiers


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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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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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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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Vertical/Lamella

Clarifiers

Lamella plates

increase the settlingefficiency and speed

in sedimentation

basins


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Solids Contact Clarifiers


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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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Sludge Blanket Clarifier


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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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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

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