Plain sedimentation

This PPT is meant for UG course of Civil Engineering

1. Sedimentation
Theory, settling tanks, types, design, concept
of plate and tube settlers.
Dr. Dayananda H S
Professor & Head
Department of Civil Engineering
VVCE, Mysore
4/14/20201

2. Sedimentation Defined
Theprocessofseparatingthesuspendedparticlesfrom
waterbygravitationalsettlingisknown as
“Sedimentation”
Inotherwords,undertheinfluenceofgravity,the
particleswithmassdensityhigherthanthatofwater
startsmovingdown
Principleinvolved
•Thespecificgravityofsuspendedparticlesinwater
rangesbetween1.03forflocculatedmudparticles
and2.65fordiscreteparticles
•Theseparticlesremaininsuspensionandwhenthe
velocityofwaterflowisreducedorretarded,they
tendtosettledownatthebottom
•Thebasininwhichflowofwaterisretardedisknown
assedimentationtankorsettlingtankorclarifier
4/14/20202

3. •Sedimentationisusuallyemployedasapreliminary
processtoreduceheavysedimentloadsfor
subsequenttreatmentprocesssuchascoagulation
andfiltration
•Sedimentationisthemostcommon separation
techniquefallingundertheunitoperation“Solids
transfer”
•Sedimentationtanksarealsoknownassettlingtank
orclarifier
•Sedimentationtanksaredesignedtoreducethe
velocityofflowofwatersoastopermitthe
suspendedsolidstosettleoutofwaterbygravity
4/14/20203

4. TYPES OF SEDIMENTATION
1.Plain sedimentation (Unit operation) and
2. Chemically assisted sedimentation or
clarification (Unit process)
•Plainsedimentationisaprocessofsettling
downofsolidsandimpuritiesintheraw
watertothebottomofsedimentationbasin
bynaturalgravityforcealone(physicalforce
predominates),nochemicalisadded
•Sedimentationfallsunderthecategory-Type
IsettlingorDiscretesettling
•Thismethod-cheaperandcommonlyused
inallfiltrationandpurificationsystemof
water
4/14/20204

5. 2. Chemically assisted sedimentation or
clarification
•Inthisunitprocess,chemicalsareaddedto
waterandthoroughlymixed
•Aftermixing,thesuspendedsolidsandother
impuritiessticktogetherandformflocs
•On slow agitation,microflocs get
agglomeratedtomacroflocsandsettles
downatbottomofthebasin
•Ingeneral,thistypeofsedimentationis
mostlyadopted
4/14/20205

6. Factors influencing sedimentation
•Velocity of flow: more length and less velocity
-more efficiency
•Viscosity, density & temperature of water
-inversely proportional
•Size, shape, & density nature of particle –
discrete or flocculent
•Inlet and outlet arrangements –short
circuiting
•Detention period and
•Effective depth of settling zone
4/14/20206

7. Terminology
Detentionperiod:Itisdefinedastheaverage
theoreticaltimerequiredforthewatertopass
betweentheinletandoutletofasettlingtank
Or
Inotherwords,itistheaveragetimeforwhich
thewaterisdetainedinthetank
4/14/20207

8. •If ‘V’ is the volume of the tank
Q is the discharge rate and
t
ois the detention period,
•We have, t
o= V/Q = LBH/Q
•Detention period depends upon the type of
suspended impurities in water
•Varies between 4-8 hours for plain
sedimentation tank
•Unless otherwise specified, it is based on
average flow
4/14/20208

9. Surface Overflow rate or Overflow rate or
Surface loading
•Thequantityofwaterpassingperhour(or
perday)perunithorizontalareaisknownas
theSurfaceoverflowrateoroverflow
rateorsurfaceloading
•Surfaceloadingrateortankloadingandthe
settlingvelocityarenumericallyequal
(providedadjustmentforunitsismade)
•V
s=Q/A=Surfaceloading
4/14/20209

10. Types of Sedimentation Tanks
Nature of working Shape
Fill & draw type Continuous flow type
Intermittent
H
le
flow tank V
le
flow tank
Rectangular tank Circular tank with Hopper bottom
with H
le
flow radial flow or tank with V
le
flow
spiral flow
4/14/202010

11. TYPES OF SEDIMENTATION BASIN
•Sedimentationtankmayfunctioneitherintermittent
(fillanddraw)orcontinuously
•Theintermittenttypetankarethosewhichstore
waterandkeepitatcompleterestforacertain
period
•Whereas,incontinuoustypetank,theflowvelocity
isonlyreduced
•Sedimentationtankmaybeeitherrectangularor
circularinplan
•Longnarrowrectangulartankswithhorizontalflow
aregenerallypreferredtothecirculartankofradial
orspiralflow
4/14/202011

12. Intermittenttypesedimentationtank
•Waterisfilledandallowedtorestinasettlingtank
•Duringrestperiod,suspendedparticlessettlesdown
•Usualdetentionperiod-24hr
•Clarifiedwateriswithdrawnthroughoutletvalve
•Filling,emptyingandcleaningoperation-6to12hr
•Completecycleofoperation-30to36hr
•Inlet&Outletarrangementsprovided@opposite
endstoavoidshortcircuiting
•Provisionforaccumulationofsediment@bottom
•Minimumnumberoftanks–Three
•Twotanksinoperation&oneasstandbyunit
4/14/202012

13. Fig: Intermittent (Fill & Draw ) Type Sedimentation tank
4/14/202013

14. Continuous flow type Sedimentation
Tank
•Waterkeepscontinuouslymovingthroughthetank
•Flowvelocityisreducedbyprovidingbafflewalls&
increasinglengthoftravel
•Thisenablessuspendedparticlestosettledown@
bottombeforeitreachestheoutlet
•Velocityissoadjustedthattimetakenbyparticleto
travelfromoneendtootherendoftankisslightly
morethanthatrequiredforsettlementofparticle
•Permissiblemaximumvelocity-0.30m/sec
•Flushvalveprovidedforcleaningandremovalofsilt
@regularintervals
4/14/202014

15. ‘Up and down’ Baffle tank
Fig: ‘Around the end’ Baffle tank
4/14/202015

16. Inlet & Outlet arrangements
•Inlet&Outletarrangementsaresodesignedtocause
minimumdisturbanceatinletandoutletofthetank
•Asfaraspossible,thewaterisuniformlydistributedacross
thewidthanddepthoftank
•Freshwaterismixedwithwateralreadyinthetankto
preventdensitycurrents
•Minimizelargescaleturbulence
•Initiatelongitudinalorradialflowsoastoachievehigh
removalefficiency
•Eachinletopeningmustfaceabaffle
•Velocityofflowthroughslots-0.2–0.3m/sec
4/14/202016

17. 4/14/202017
Types of Inlet arrangement
a. Perforated Baffle
b. Submerged orifice
c. Influent channel with bottom openings and
d. Overflow weir with Baffle

18. Fig: Inlet arrangements
4/14/202018

19. Outlet arrangements
•Consists of i) Weirs, notches or orifices
ii) Effluent trough or launder and outlet
pipe
•Weir loading rates limited to prevent high approach
velocity near the outlet
•Weirs consists of V-notches 50mm in depth &
placed 150 –300 mm c/c with baffle provided in
front of weir to avoid floating matter escaping into
outlet
•Proper design of inlets and outlets is necessary to
avoid short circuiting through basin
4/14/202019

20. Fig: Outlet arrangements
4/14/202020

21. DesignelementsofcontinuousflowSedimentation
tank
i)Detentiontimeanddisplacementefficiency
ii)Overflowrateandsurfaceloading
iii)Basindimensions
iv)Inletandoutletarrangements and
v)Sludgeremovaldevices
4/14/202021

22. Design criteria for sedimentation tanks
•Detentiontime-3to4hr(basedondepth&
overflowrate
•Depthofwaterinbasin-3to4.5m
•Displacementvelocityofwater-0.3m/sec
•Surfaceloadingrate-24to36m
3
/m
2
/day
•Weirloadingrate-200to300m
3
/m
2
/day
•Sludgestoragecapacity-25%oftotalvolume
•Floorslope-1in12formechanicalscraper
•Timetakenforonerevolution–45to80min
•Velocityofwaterinoutlet-≤0.4m/s
•V-notch:50mmdeepand150-300mmc/c
•Diameterofcirculartank-≤30m
•LengthtoBreadthratioforrectangulartank-4:1
4/14/202022

23. Short Circuiting
Shortcircuitingiscauseddueto
•Eddycurrentssetupbytheinertiaoftheincoming
fluid
•Surfacecurrentsinducedduetowindinopentanks
•Verticalconvectioncurrentsduetothermalgradient
alongthedepthofthetank
•Densitycurrentscausingcold&heavywaterto
underrunthebasin&warmorlightwatertoflow
acrossit’ssurface
•Duetodifferenceinvelocity,higherinletlevel,
differenceindensityduetochangeintemperature
andsaltconcentration
4/14/202023

24. •InidealSettlingbasin,displacementissteady
&uniform
•Eachunitvolumeoffluidisretainedfortime,
t
o=C/Q
•Eveninwelldesignedtank,howeversomeof
theinflowreachestheoutletinlessthanthe
theoreticaldetentiontime&sometimesmuch
longeralso
4/14/202024

25. RECTANGULAR v/s CIRCULAR SEDIMENTATION
BASIN
•Rectangularbasinsarethesimplestdesign&
usuallyfoundinlarge-scalewatertreatmentplants
•Rectangularbasinshaveofadvantages -
predictability,cost-effectiveness&lowmaintenance
•Itistheleastlikelytoshort-circuit,especiallyifthe
lengthisatleasttwicethewidth
•Onlydisadvantage-largeextentoflandarea
required
•Circularsedimentationbasinswithhorizontalflow
areoftenknownasclarifiers
•Circularbasin disadvantage-short-circuiting
problems
4/14/202025

26. Plate settler (Shallow depth sedimentation)
•Theplateandtubesettlertechnologywas
developedtoimprovethesettlingefficiencyof
smallerparticles
•ItisclearfromHazen’stheorythatthe
removalofsettleablesolidsisprimarilya
functionofthesurfaceareaofthebasinand
notdependentondepth
•Shallowdepthsedimentation-introducing
falsebottomsorhorizontaltraysinthe
sedimentationtank,dividingthetankheight
intocompartments
•Itisdonetoreducethedepthandtoprovide
multiplesurfaceareas
4/14/202026

27. •Such an arrangement increases the surface
area, thereby reducing the surface overflow
rate (V
H) and resulting in more particle
settlement
Drawbacks
•The introduction of wide shallow trays at
regular intervals gives rise to problems of
unstable hydraulic conditions and
•Difficulties in operation of mechanical sludge
removal equipment
4/14/202027

28. Tube settlers
•Tubesettlersusetubesof25-50mm
diameter,havinglargewettedperimeter
relativetothewettedarea
•Itprovideslaminarflowconditionsandlow
surfaceloadingrateforsedimentation
•Twoconfigurationsoftubesettlersareused
i)Horizontaltubesettlerand
ii)Steeplyinclinedtubesettler
4/14/202028

29. Fig: Horizontal and inclined Tube settler
4/14/202029

30. Horizontal tube settler
•Blackcolored,1.5mmthickACpipesor
plastictubesofsquare,rectangular,circular
orhexagonalshapesareused
•Alargenumberofsuchtubesof50mmx
50mmorsmallerinsizeofheight1mare
placedtogethersidebysidetomakea
bundleofrectangularorcircularorsquare
shapetoformamodule
•Theheightofthemodulemaybeofthe
orderof1m
•Figureinthepreviousslideshowsa
horizontaltubesettlerinwhichthetubes
areinclinedslightlyby5°inthedirectionof
flow
4/14/202030

31. •Thesludgesettledinthetubeisdrainedbythelast
portionofthefilterbackwash
•Secondconfigurationshowsdeeplyinclinedtube
settlermadeofplasticbafflesinclinedat60°tothe
horizontallocatedbetweenthinsheetsofplastics
•The60°angleinclinationpermitsaccumulatedsludge
toslidedownthetubes
•Thesettledsludgeisconveyedtoasludgecollector
zone
•Tubesettlersprovideexcellentclarificationwith
detentionperiodof≤10minutes
4/14/202031

32. 4/14/202032
FAQ
1. Define overflow rate and detention period for
plain sedimentation tank.
2. Explain the sedimentation process in water
treatment.
3. Show mathematically that the surface loading
and not the depth is a measure of the efficiency
of a settling basin.
4. Sketch the various types of inlets and outlets
adopted in sedimentation basins.
5. Mention the design criteria for continuous flow
sedimentation tanks.
6. Write a note on plate settlers and tube settlers.