Lecture 4: Sedimentation

1 Islamic University of Gaza Environmental Engineering DepartmentWaterTreatmentWater TreatmentEENV 4331 Lecture 4: SedimentationDr. Fahid Rabah14. Sedimentation in water Definition of Sedimentation :It is the process of removing solid particles heavier than waterby gravity force. Particles that will settle within a reasonable period of timecan be removed using a Sedimentation tank (also calledclarifiers). Sedimentationisusedinwatertreatmentatthe locationsindicated in Figures 1 through Sedimentation in water Applications of Sedimentation in water treatment:pp1. Plain settling ( or pre Sedimentation ) of river surfacewater .2. In filtration treatment plants treating surface water toremoves flocculated solids.

2 The Sedimentation tank comesafter the flocculation In Softening treatment plants treating hard water toremoves flocculated solids. The Sedimentation tank comesftthfl l In aeration treatment plant removing iron and manganesefrom ground water34. Sedimentation in water Geometryof Sedimentation tanks:y Sedimentation tanks are either rectangular or circulartanks. Figures 5 to 10 show typical Sedimentation tanks used inwater WaterPre SedimentationScreenCoagulationFlocculati onSedimentationDistributionDisinfectionS torageFiltrationFigure 1: Filtration Treatment Plant(River Water)5 ScreenSurface waterSdi ttiCoagulationFlocculationSedimentationD istributionDisinfectionStorageFiltration Figure 2 : Filtration Treatment Plant6 Ground WaterWaterRapid Mixing FlocculationSedimentationRecarbonationDi stributionDisinfectionStorageFiltrationF igure 3: Softening Treatment Plant Single stage softening7 Ground Water wellFiltrationAerationDistributionDisinf ectionStorageFigure 4 : Aeration Treatment Plant(ironandmanganeseremovalplant)( iron and manganeseremoval plant)89 Figure 5.

3 Rectangular Sedimentation Tank10 Figure 6 : Rectangular Sedimentation Tank11 Figure 7 : Circular Sedimentation Tank12 Figure 8 : Circular Sedimentation Tank13 Figure 9 : Circular Sedimentation TankSolid contact typeFigure 10 : Circular Sedimentation Tank Solidcontacttype14 Solid contact typeFigure 11 : Rectangular Sedimentation Tank sludgecollectionsystem15sludge collection systemFigure 12 : Rectangular Sedimentation Tank ldll tit16sludge collection systemFigure 13 : Rectangular Sedimentation Tank ldll tit17sludge collection systemFigure 14 : Rectangular Sedimentation Tank ldll tit18sludge collection system4. Sedimentation in water Theoretical background on Sedimentation431 Type of particles :1) Discrete / individual particle- Size, velocity constant during the settlingDit20002200k/3-Density 2,000 2,200kg/m32) Flocculent particles- Size, velocity fluctuates during the settling- Particles flocculate and grow bigger in size-Density1,030 1,070kg/m319 Density 1,0301,070kg/m4.

4 Sedimentation in water Three classes of particles settling : Type1 Type 1i) Particles settle discretely at a constant velocityii) Settle as individual particles and do not ) :Sand,gritmaterialiii) : Sand, grit material iv) Occurs during :i) Pre Sedimentation for sand removalii) Settling of sand during rapid sand filter cleaningv) Concentration : very low204. Sedimentation in water TreatmentType 2i)Flocculateduringsedimentationi)Floccu late during sedimentationii) Size constantly changingiii) Settling velocity is changingiv) Settling velocity increase with depth and extent of ) Occurs during : i) Alum or iron coagulationvi) Concentration : low214. Sedimentation in water TreatmentType 3 or Zone i) Settle as mass and form a layer blanket ii)Concentrationhigh(greaterthan1000mg/L )ii)Concentration high (greater than 1000 mg/L)iii) Distinct clear zone and sludge zone are present.

5 Iv) Occurs during :i) Lime softening sedimentationii) Sludge thickeners in water Sedimentation in water Sedimentation theory234. Sedimentation in water Treatment Settling velocity (vs) must be determined to assure good Sedimentation tank design. Overflow rate (vo) must be set at some value LESS THAN or EQUAL to VstHvs ssAQHWlHQVHtHv **tsQsQ sAQv 0 ButWhereQ= flow rate (m3/h)f(2)svv 0 SoAs= surface area (m2)H= depth of water, mW = tank width, mL=tanklengthm24L = tank length, mt= detention time, hr4. Sedimentation in water Design of Sedimentation tanks451 Plain Sedimentation :Particles settle separately: ssCdgV3)(4 DC3Vs=settlingvelocityVs settling velocityD = particle diameter = water density s= particle density spyCD= drag coefficients254.

6 Sedimentation in water Plain Sedimentation :Drag coefficient is calculated as:Re< 1, Laminar Flow e,aao 1<R<104 TransitionalFlow RCD24 1 < Re< 10, Transitional Flow Re>104, Turbulent Flow RRCDW here R = Reynolds DC = dynamic viscosity , DVRs 26 4. Sedimentation in water Treatment45 DifSdi Design of Sedimentation Plain Sedimentation :At laminar flow, settling velocity equation is simplified to Stocks law: 18)(2 Dgvss Vs=settlingvelocity 18Vs settling velocityD = particle diameter = water density s= particle density spyCD= drag coefficients274. Sedimentation in water TreatmentExample41:Example :Find the settling velocity (vs) for sand particles with a diameter of = 2650 kg/m3, = 20 0C, = , = what is vs for particles withD=05mm?

7 With D = mm?Solution:Assume first the flow is laminar and check for Reynolds number:)10*020)(10002650(809423 R<1 * *18) )(10002650( 00715010* *10* *100034 DVRsR<1 , so itslaminar * RForparticleswithD= :For particles with D : * *18)10* )(10002650( 11010* * * *100033 DVRsR>1 , so its transitional flow28 4. Sedimentation in water DC110110D310*50)10002650(819*4 *110*310* )10002650( *4 Solve again for Re:55 eRR>1 , so its transitional again fir CD: Solve again for vs :sdmmdmsmvvTake/31/31/10*593234 4. Sedimentation in water Design tanks451 Plain Sedimentation :Example :Design pre Sedimentation tanks to be used to remove grit and sand from a river hidd200003/ddi kiUhfldwater that is used to produce 20000 m3/d drinking water.

8 Use the overflow rate and horizontal velocities calculated in example Use two lwQAQvs*0 *Flow/tank = (Q/2)= 20000/2 = 10000 m3/dhAs = 10000/31 = m2 Select Width to length ratio 1:4AW*4W32252W898L3592(t kW9L36) A = W* 4W = m2 W= m , L = (take W= 9 m, L= 36 m) Assume detention time = 3 hrsH = t*V0= (3*31 )/24= m m VQ/(W*H)10000/(24*60*9*39)0198m/min30 Vh= Q/(W*H) = 10000/(24*60*9* ) = m/min Take weir loading rate = 250 m3 :L weir= Q/Wload= 10000/250 = 40 m , Use suspended troughs inside the tank. 31 Figure 15 : Rectangularsedimentation Tank4. Sedimentation in water TreatmentExample :Repeat example using circular tank. Note: the maximum tank diameter is 40 sAQv 0*Flow/tank = (Q/2)= 20000/2 = 10000 m3/dAs = 10000/31 = m2 As= D2/4= m2 D = m m < 40 m Assume detention time = 3 hrsH = t*V0= (3*31)/24= m mCheck horizontal velocity at the beginning and end of settling zones: Vh= Q/( Din H) = 10000/(24*60* * * ) = m/min (End of inlet zone) Vh= Q/( DoutH) = 10000/(24*60* *24* ) = m/min (beginning of outlet zone)Tkildit2503/d Take weir loading rate = 250 m3 :L weir= Q/Wload= 10000/250 = 40 m , Use suspended troughs inside the tank.

9 Available Length = D total= (2H + D) = *(2* + ) = m > 40 m <250m3/mdOK32 Available W load= Q/L = 10000 = m3 < 250 m3 side wall height (tank depth)Din= HDtotalinDout=H+DDtotal= 3H+DD= diameter calculated from DinDoutthe Over flow rate D/2 OtltHDtotalOutlet zone =HDtotalD/2 DinDoutHFigure 16 : CircularsedimentationTankinlet zone = H33sedimentation TankDimensions definition34 Figure 17 : Circularsedimentation Tank4. Sedimentation in water Design of Sedimentation tanks452 FlocculentSedimentation(type2and3) Flocculent Sedimentation ( type 2 and 3):The design procedure for Sedimentation tanks of type 2 and 3 are the same as type 1. The difference is mainly in the overflow rate (v0) .The following table gives the design criteria of these two types.

10 ProcessSettlingtypeDetentionSurfaceWeirl oadingProcessSettling typeDetention time(h)Surface loading rate (m3 )Weir loading rate(m3 )Coagulation24 820 40250softening32 640 6025035