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Wastewatertreatment:UnitProcesses

Wastewater

treatment:

Unit

Operations/Processes,

Their

Functions

and

Units

Used

for

Domestic

Wastewater

Treatment

Phase of treatment Unit Operations /Processes Functions Treatment Devices

Preliminary treatment

Screening Removal of large floating,

suspended and settleable solids

Bar racks and screens of

various description

Primary treatment

Grit Removal Removal of inorganic

suspended solids

Grit chamber

Primary Sedimentation Removal of organic/inorganic

settleable solids

Primary sedimentation tank

Secondary/Biological

treatment

Aerobic Biological Suspended

Growth Process

Conversion of colloidal,

dissolved and residual

suspended organic matter into

settleable biofloc and stable

inorganics

Activated sludge process units

and its modifications, Waste

stabilisation ponds, Aerated

lagoons

Aerobic Biological Attached

Growth Process

same as above Trickling filter, Rotating

biological contactor

Anaerobic biological growth

processes

Conversion of organic matter

into CH4 & CO2 and relatively

stable organic residue

Anaerobic filter, Fluid bed

submerged media anaerobic

reactor, Upflow anaerobic

sludge blanket reactor,

Anaerobic rotating biological

contactor

Anaerobic Stabilization of

Organic Sludges

same as above Anaerobic digestor

Advanced treatmentNitrification-denitrification

Preliminary

wastewater

treatment

Screening

Ascreenisadevicewithopeningsforremovingbiggersuspended

orfloatingmatterinsewagewhichwouldotherwisedamage

equipmentorinterferewithsatisfactoryoperationoftreatment

units.

1. CoarseScreens: Coarsescreensalsocalledracks,areusuallybar

screens,composedofverticalorinclinedbarsspacedatequal

intervalsacrossachannelthroughwhichsewageflows.Bar

screenswithrelativelylargeopeningsof75to150mmare

providedaheadofpumps,whilethoseaheadofsedimentation

tankshave

smaller

openings

of

50

mm.

Bar

screens

are

usually

handcleanedandsometimesprovidedwithmechanicaldevices.

Thesecleaningdevicesarerakeswhichperiodicallysweepthe

entirescreenremovingthesolidsforfurtherprocessingor

disposal.Handcleanedracksaresetusuallyatanangleof45to

thehorizontal toincreasetheeffectivecleaningsurfaceandalso

facilitatetherakingoperations.Mechanicalcleanedracksare

generallyerectedalmostvertically. Suchbarscreenshave

openings25%inexcessofthecrosssectionofthesewage

channel.

2. MediumScreens: Mediumscreenshaveclearopeningsof20to

50mm.Barareusually10mmthickontheupstreamsideand

taperslightlytothedownstreamside.Thebarsusedforscreens

arerectangularincrosssectionusuallyabout10x50mm,

placedwithlargerdimensionparalleltotheflow.

3. FineScreens:Finescreensare

mechanicallycleaneddevicesusing

perforated plates,wovenwirecloth

orverycloselyspacedbarswithclear

openingsoflessthan20mm.Fine

screensarenotnormallysuitablefor

sewagebecauseofclogging

possibilities.

Equalization

tanks

Equalization

Tanks

Theequalizationtanksareprovided(i)tobalancefluctuatingflowsorconcentrations,(ii)toassist

selfneutralization,or(iii)toevenouttheeffectofaperiodic"slug"dischargefromabatchprocess.

TypesofEqualizationTanks

Equalizationtanksaregenerallyofthreetypes:

Flowthroughtype

Intermittentflow

type

Variableinflow/constant dischargetype

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Grit

Chamber

GritChambers

Gritchambersarebasintoremovetheinorganicparticlestopreventdamagetothepumps,andto

preventtheiraccumulationinsludgedigestors.

TypesofGritChambers

Gritchambersareoftwotypes:mechanicallycleanedandmanuallycleaned.Inmechanically

cleanedgritchamber,scraperbladescollectthegritsettledonthefloorofthegritchamber.Thegrit

socollectediselevatedtothegroundlevelbyseveralmechanismssuchasbucketelevators,jet

pumpandairlift.Thegritwashingmechanismsarealsoofseveraldesignsmostofwhichare

agitationdevicesusingeitherwaterorairtoproducewashingaction.Manuallycleanedgrit

chambersshouldbecleanedatleastonceaweek.Thesimplestmethodofcleaningisbymeansof

shovel.

Aerated

Grit

Chamber

Anaeratedgritchamberconsistsofastandardspiralflowaerationtankprovidedwithairdiffusion

tubesplacedononesideofthetank.Thegritparticlestendtosettledowntothebottomofthe

tankatratesdependantupontheparticlesizeandthebottomvelocityofrollofthespiralflow,

whichinturndependsontherateofairdiffusionthroughdiffusertubesandshapeofaeration tank.

Theheavierparticlessettledownwhereasthelighterorganicparticlesarecarriedwithrollofthe

spiralmotion.

Aeratedgritchamber

Aeratedgritchamber

AnAeratedGritChamberoffersalowcost,mechanically

simplewaytoremovegritfromawastewaterstream.Airis

introducedintoaneductortubewhichislocatednearthe

bottomofagritchambertherebycreatingatoroidalflow

pattern. Thisflowpatterncausesthegrittosettletothe

bottomofthechamberwhilekeepinglighterorganicmaterial

insuspensiontobeprocessedfurtherdownstream.Sincethe

drivingforceisablower,thistypeofgritremovalhasno

movingpartsbelowthesurfacewhichmeansthatmaintenance

issignificantlyreduced.Oncethegrithassettled,eithera

recessedimpellergritpumpor,morecommonly,anairlift

pumpisusedtoremovethegritslurryandsenditonfor

dewatering.Theairliftpumpoptionisparticularly

advantageous sincetheairsupplyneededforoperationofthe

aeratedgritchamberisalreadyinplace.

ADVANTAGES

Extremelysimplemechanicaldesign

No

moving

parts

below

the

water

surfaceCanusetheblowerairforairliftpumpingaswell

Possiblesepticconditionsoftheplantinfluentmaybe

alleviatedthroughprepreparation

inthegritchamber

Vortexgritchamber

Thevortexgritseparatorefficientlyremovesgrit

fromthetreatmentplantinfluentstream.Avortex

gritseparator usuallyfollowsscreeningequipment.

Thescreenedinfluententerstangentiallyandflows

aroundtheupperchamber.Adjustable,rotating

paddlesaugmentthespiralingflowtocreatea

mechanicallyinducedvortexwhichsettlesthegrit,

transportsittothecentreopeningofthefixedfloorplateforcollectioninthelowerchamber,andlifts

andreturnsthelighterorganicparticlestothemain

flow.Thegritsolidsareremovedfromthelower

chamberbyanairliftorrecessedimpellerpumpfor

furtherwashinganddewatering.

Advantages

Efficientgritremovalovervariableflowrates

Ruggedprecisionbearingdrive

Compactdesignreducescivilworkexpenses

Lowenergyconsumption

Simpleandinexpensivemaintenance

requirements

Principle

of

grit

chamber Principle

of

Working

of

Grit

Chamber

Gritchambersarenothingbutlikesedimentationtanks,designedtoseparate

theintendedheavierinorganicmaterials(specificgravityabout2.65)andto

passforwardthelighterorganicmaterials.

Hence,theflowvelocityshouldneitherbetoolowastocausethesettlingof

lighterorganicmatter,norshoulditbetoohighasnottocausethesettlement

ofthesiltandgritpresentinthesewage.Thisvelocityiscalled"differential

sedimentationanddifferentialscouringvelocity".Thescouringvelocity

determinestheoptimumflowthroughvelocity.Thismaybeexplainedbythe

factthatthecriticalvelocityofflow'vc'beyondwhichparticlesofacertainsize

anddensityoncesettled,maybeagainintroduced intothestreamofflow.It

shouldalways

be

less

than

the

scouring

velocity

of

grit

particles.

The

critical

velocityofscourisgivenbySchield'sformula:

V=3to4.5(g(Ss1)d)1/2

Ahorizontalvelocityofflowof15to30cm/secisusedatpeakflows.Thissame

velocityistobemaintainedatallfluctuationofflowtoensurethatonlyorganic

solidsandnotthegritisscouredfromthebottom.

TypesofVelocityControlDevices

Asutroweirinachannelofrectangularcrosssection,withfreefalldownstream

ofthechannel.

Aparabolicshapedchannelwitharectangularweir.

Arectangularshapedchannelwithaparshallflumeattheendwhichwouldalso

helpeasyflowmeasurement.

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PrimarySedimentation

PrimarySedimentation

Primarysedimentationinamunicipalwastewatertreatmentplantisgenerallyplain

sedimentationwithouttheuseofchemicals.Intreatingcertainindustrialwasteschemically

aidedsedimentationmaybeinvolved.Ineithercase,itconstitutesflocculentsettling,and

theparticlesdonotremaindiscreteasinthecaseofgrit,buttendtoagglomerateor

coagulateduringsettling.Thus,theirdiameterkeepsincreasingandsettlementproceedsat

anoverincreasingvelocity.

Thesettlingtankdesigninsuchcasesdependsonbothsurfaceloadinganddetentiontime.

Industrialwastewater:Longtubesettlingtestscanbeperformedinordertoestimate

specificvalueofsurfaceloadinganddetentiontimefordesiredefficiencyofclarificationfora

givenindustrialwastewaterusingrecommendedmethodsoftesting.Scaleupfactorsusedin

thiscaserangefrom1.25to1.75fortheoverflowrate,andfrom1.5to2.0fordetentiontime

whenconverting laboratory resultstotheprototypedesign.

Municipalordomesticsewage:Forprimarysettlingtankstreatingmunicipalordomestic

sewage,laboratory testsaregenerallynotnecessary,andrecommendeddesignvaluesgiven

intablemaybeused.Usinganappropriatevalueofsurfaceloadingfromtable,therequired

tankarea

is

computed.

Knowing

the

average

depth,

the

detention

time

is

then

computed.

Excessivelyhighdetentiontime(longerthan2.5h)mustbeavoidedespeciallyinwarm

climateswhereanaerobicitycanbequicklyinduced.