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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.