3. grit chamber

Purpose, Types and design of grit chamber

1. by
Prof V. V. Sasane(Assistant Professor)
Department of Civil Engineering
SanjivaniCollege of Engineering, Kopargaon, 423603
Subject: Environmental Engineering-II
B.E. Civil
Unit-II: Sewage Treatment
2.4 Grit Chamber
SanjivaniRural Education Society’s
SanjivaniCollege of Engineering, Kopargaon, 423603
An Autonomous Institute Affiliated to SavitribaiPhulePune University, Pune

2. Contents
1.Introduction to Grit Chamber
2.Types of Grit Chambers
3.Design of Grit Chamber
4.Cleaning and Disposal of Grit

3. Introduction to Grit Chamber
1.Gritchamberisusuallythesecondunitoperationand
islongnarrowtanksthataredesignedtoslowdown
theflowforremoveofgritfromthewastewater.
2.Gritistheheaviestmaterialinwastewaterandincludes
substancessuchassand,coffeegrounds,gravel,
cinders,crockerypieces,glasspieces,eggshellsetc.
(specificgravitybetween2.4‐2.65,muchhigherthan
organicsolids).
3.Gritisnon‐putrescibleandpossessesahigher
hydraulicsubsidencevaluethanorganicsolids,
thereforecanbeseparatedfromorganicsolidsby
differentialsedimentationinagritchamberand
sedimentationtank.

4. Agritchamberisanenlargedchanneloralongbasininwhichcrosssectionisincreasedsoastoreduce
theflowvelocityofthesewage,sothatheavyinorganicsolidsdosettledownbygravity.
Gritremovalisnecessarytoprotectthemovingmechanicalequipmentandpumpelementsfromabrasion
andabnormalwearandtear.Removalofgritalsoreducesthefrequencyofcleaningofdigestersand
settlingtanks.
ObjectiveofGritChamber
•Toprotectthepumpsandotherequipment
•Toavoidoverloadingonfurthertreatments.
•Toreducethefrequencyofcleaningofsludgedigestersandsettlingtank.
•Toreducethecloggingofsludgepipes.
•Agritchamberisusuallyinstalledbeforeprimarysedimentationtanks.Therearebasically
threedifferenttypesofgritchambers.
1.Horizontalflowtype2.Aeratedgritchamber3.Vortextype.

5. Types of Grit Chamber
•Horizontal Flow Type Grit Chamber
•Inahorizontalflowtypeofrectangulargrit
chamber,thewastewaterflowsinhorizontal
directionatacertainvelocity(0.3m/s).
•Particleswillstarttosettleatthebottomof
channelbeforereachingtheoutletpoint.
•Thesearenormallydesigned/constructedto
removeparticlesof0.20mmsizeandmore.
•Oncethegritsettlestothebottomofthe
chamber,itcanthenberemovedoutfromthe
system,usingconveyorwithbucketsorplows.

6. •Aerated Grit Chamber
Aeratedgritchamberconsistsofanaerationtank,which
isdesignedtocreateaspiralflowofwastewaterasit
movesthroughthechamber.
•Airisintroducedalongonesideofarectangulartankto
createaspiralflowpatternperpendiculartotheflow
throughthetank.
•Ifthevelocityistoogreat,gritwillbecarriedoutofthe
chamber;ifitistoosmall,organicmaterialwillbe
removedwiththegrit.
•Normallydesignedtoremove0.21-mm-diameteror
larger,with2-5-minutedetentionperiodsatthepeak
hourlyrateofflow.
•Airdiffusersarelocatedabout0.45to0.6mabovethe
normalplaneofthebottom.

7. VortexTypeGritChamber
•Inthistypeofgritchamber,thesewageisappliedin
thetangentialdirectiontoinduceavortextypeof
flow,whichwillfunnelthegrittowardsthecenter,
andhencecanbedrawndownatthebottom
chamber.
•Anagitatorisusedtokeepthegritinsuspension.
•Duetothevortextypeofflow,thegritwhichhasa
heavierspecificgravityof2.4to2.65settlesdown
whichislaterdrainedontoagritfilterbedbygravity
orpumpedtothebeds.
•Thisunitiswidelyusedwheresewageflowratesare
varywidely.

8. Design of Grit Chamber
•Settling velocity: Grit chamber may be designed on a rational basis by considering it as sedimentation basin having
discrete settling. Velocity id governed by the size and specific gravity of grit particles minimum size of the grit to be
removed is 0.2 mm. the settling velocity is given by Stokes’slaw applicable for particles size 0.1 mm
??????
�=
??????
18
??????
�−??????
??????
�
2
??????
�=
??????
18
??????
�−1
??????
�
2
•Where ??????
�= settling velocity (cm/sec)
•d= size of particle(cm)
•??????= dynamic viscosity of liquid (centipoise)
•??????=??????/??????= kinematic viscosity (centistokes)
•??????
�= mass density of liquid (gm/�??????
3
)
•??????= mass density of particles(gm/�??????
3
)
•??????
�= ??????
�/??????= specific gravity of particles
•g= acceleration due to garvity

9. •For particles larger than 1 mm the flow will be turbulent and velocity is given by Newton’s
??????
�=3.33??????
??????
�−??????
??????
�
•However grit particles lies between 0.1 to 1 mm therefore velocity is given by Hazen’s
modified formula
??????
�=60.6??????
�−1�
3??????+70
100
The flow velocity should be enough to scour out the settled organic matter and reintroduce
it in to the stream.
The critical velocity of scour may be calculated by modified Shield’s Formula
??????
�=3????????????4.5????????????
�−1�
For grit size =0.2 mm ??????
�= 17.1 to 25.6 m/s

10. Other Design Parameters
•Detention period = may vary from 45 to 90 sec but 60 sec usually adopted=
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����????????????�????????????�??????��??????�??????
•Number of units = Atleast2 units should be provided
•Free board = 150 to 300 mm is provided
•Loss of head = 0.06 to 0.6 m
•Horizontal velocity = 15 to 30 cm/sec

11. Clearing of the Grit
1.Gritscanberemovedmanuallyormechanically,howevermanualclearingshouldbeavoided
exceptinthecaseofverysmallSTPs(<1MLD)wherevelocitycontrolledchannelscanbe
clearedbytheoperatorusingashovel.
2.Inmechanicalclearing,equipmentareprovidedforcollectionaswellaswashingofgrit
(mostlybyagitationmechanisms),andcanbeoperatedoneitheracontinuousorintermittent
basis.
3.Thesettledgritontheflooriscollectedbyscrapperbladesorploughsandelevatedtothe
groundlevelbyvariousmechanismssuchasbucketelevators,jetpump,screwsandairlift.
4.Inintermittently(normallyonceortwiceaday)operatedtype,sufficientstoragecapacityto
holdthegritbetweenintervalsofgritelevationshouldbeprovided.

12. Disposal of Grit
1.Clean grit is odourless and may be disposed by dumping or burying or by sanitary
landfill.
2.If not washed, grit particles may contain organic matter which adds odour and is not
suitable for disposal.
3.However, the ultimate method of disposal depends on the quantity and other
characteristics of the particles and further on the availability of the land for dumping or
burial.
4.The odorous grit particles are preferably buried, unless

13. Thank you