Water Treatment Lecture 5 - Islamic University Of Gaza

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Islamic University of Gaza ‐Environmental Engineering DepartmentWater TreatmentEENV 4331Lecture 5: FiltrationDr. Fahid Rabah1

5. Filtration in water Treatment5.1 Definition of Sedimentation:Filtration is a solid –liquid separation process in which theliquid passes through a porous medium to remove asmuch fine suspended solids as possible5.2 Locations of filtration tanks in water treatment:Filtration tanks are used in all types of water treatmentplants except for disinfection treatment plants. SeeFigures 5.1 through 5.4 illustrating the location of filtrationtanks.2

DistributionFigure 5.1: Filtration Treatment Plant (RiverWater)3

nFlocculationStorageS diSedimentationt tiDistributionFigure 5.2 : Filtration Treatment Plant4

GroundWaterRapid ionStorageRecarbonationDistributionFigure 5.3: Softening Treatment Plant Single stagesoftening5

geDistributionFigure 5.4 : Aeration Treatment Plant( iron and manganese removal plant)6

5. Filtration in water Treatment5 3 Need for filtration:5.3 Settling is not sufficient to remove all particles andflocs from water. Filtration Needed for fine particles not removed bysedimentation.dit ti Filters can also capture Giardia cysts, viruses, andasbestos fibers Typical overflow qualities from sedimentation tanksrange from 1 to 10 NTU. Filtration, usually rapid sand filtration, is thenemployed for further “polishing”, i.e. to get theturbidity to lower than 0.50 5 NTU (as required bylegislation). Rapid sand filtration after prior sedimentation is themost common configuration worldwide7

5. Filtration in water Treatment5.4 Types of filters used in water treatment:Granular material filters are the most used types of filtersin water treatment. Usuallyy sand, anthracite, and Garnet.There are three types of granular filters:1. Single –medium filters :one type of media is used: either sand or anthracite2.Dual‐media filters: two types of media is used usually sandand anthracite3. Multimedia filters: three types of media are used usuallysand , anthracite , and GarnetMost famous filters in water treatment are Rapid Sand Filters.8

5. Filtration in water Treatment5.5 Geometryy and componentspof Rapidp Sand Filter: Rapid sand filters are always rectangular tanks. Figures 5.5 to 5.10 show typical Rapid sand filters used inwater treatment.Main components of Rapid sand filter are:1. A concrete tank2. Filter media3 Under drain system3.4. Backwash system: pressurized water and air lines 9

Figure 5.5 : Rapid sand filter components10

Figure 5.6a :Rapidp sand filter componentsp: with ggravel andperforated pipes under drain system11

Figure 5.6bFi5 6b :Rapid sand filter components : with gravel andperforated pipes under drain system12

Figure 5.7 :Rapidp sand filter componentsp: with ggravel andperforated pipes under drain system13

Figure 5.8:Fi58Rapid sand filter components: with ducts under‐ drain system14

Figure 5.9:Rapid sand filter components: with nozzle under‐ drain system15

5. Filtration in water TreatmentFigure 5.10 :Rapid sand filter perforatedslab and nozzle under‐drainsystem16

5. Filtration in water TreatmentFigure 5.11 :Nozzle used in Rapid sand filterunder‐drain system17

5. Filtration in water Treatment5.6 Operation of Rapid Sand Filter:There are two modes of operation of Rapid sand filter Filtration mode ( see Figure 5.12 ) Backwashing mode ( see Figure 5.13 )18

5. Filtration in water Treatment5 7 Filtration mode:5.7 Water flows downward through a bedof sand and gravel Particles are captured on and betweensand grains Filtered water is collected in the underdrain, sent to disinfection19

5. Filtration in water Treatment5 8 Backwash mode:5.8 Sand is backwashed when– It becomes clogged, or– Turbidity of filtered water gets too high During backwash, water is pumpedupwards through the sand bed20

5. Filtration in water Treatment Sand becomes “fluidized”, and particlesare flushed from the sand Dirty backwash water is pumped into asettling pond,pond and either– Recycled back into plant, or– DisposedDid Backwashing can consume 1% to 5% of aplant’s production21

Figure 5.12 : Rapid sand filter duringfiltration22

Figure 5.13 : Rapid sand filter during backwashing23

5. Filtration in water Treatment5.9 Filter media propertiesFigure 5.14 : filter media grain distribution24

5. Filtration in water Treatment‐Thesehffiltersluse sandd andd crushedh d anthracitehcoall on agraded gravel base.‐Media layers are arranged in a course to fine gradation in thedirection of flow, which allows greater depth of penetration offloc particles.p‐Multimedia filters are selected with specific gravities so thatmoderate intermixing between media layers occurs duringbackwashing.25

5. Filtration in water Treatment5.10 Filter media propertiesThe filter media is characterized by two mainparameters: the effective size and the uniformitycoefficient.Effective size of the filter mediaThe effectiveThff ti sizei off theth mediadi isi theth diameterditthat 10% of the filter media is less than it size andis denoted as d10.Uniformity coefficient of the filter mediad 60U d10U Uniformity coefficientd60 sieve size that passes 60% by weightd10 sieve size that passes 10% by weight‐ d60 and d10 are found by sieve analysis of themedia to be used in the filter.‐Another important sieve size is d90 that is usedto calculate the backwash rate.26

5. Filtration in water TreatmentFigure 5.15 :Rapid sand media layers27

Figure 5.16 :Rapid sandmedia layers28

5. Filtration in water TreatmentTable 5.129

5. Filtration in water TreatmentTable 5.25230

5. Filtration in water TreatmentTable 5.331

5. Filtration in water TreatmentFigure 5.17 :Head loss and effluent turbidityy increase with time duringg filtration32

5. Filtration in water TreatmentFigure 5.14 : Head loss in rapid sandfilter during filtration cycle33

Figureg5.14 : Head loss in rapidp sand filter34

Head loss in a clean filterCarmen –Kozeny equation:h k (1 ) V v3Lg A 2V6 A d2Qv AsWhere,k dimensionless coefficient , 5 for sand, 6 for anthracite;v filtration rate m3/m2.dd , or filtration velocity m/dm/d.A the grain surface area;As surface area of the sand filter;V the grain volume;ε filter porosity; around 0.40 for sand filterФ shape factor ; 1 for spherical particles, 0.70 for sand;µ dynamic viscosity; NN.s/ms/m2ρ water density; kg/m3h head loss in clean filter,m35

Example 5.1:A dual media filter is composed of 0.30 m anthracite (mean particle size0.20mm) that is placed over a 0.60 m layer of sand (mean particle size 0.70mm)with a filtration rate of 9.78 m/h./ Assume the grain sphericity ф 0.75 andporosity (ε) 0.40 for both. Estimate the headloss in the clean filter at 150C.A HeadA.H d lossli theinth anthraciteth it layer:l0.00113 * (1 0.40) 2 6 h 0.30 * 6 * * 0.00272 0.0508m39.81*1000 * 0.40 0.75 * 0.002 2B. Head loss in the sand layer:0.00113* (1 0.40) 2 6 h 0.60 * 5 ** 0.00272 0.6918 m 39.81*1000* 0.40 0.75* 0.007 2B. Head loss in the sand layer:yhtotal 0.0508 0.6918 0.743 m36

Head loss during filtration(None clean filter) hl t v a bV filterd Where,Wherev filtration rate m3/m2.d , or filtration velocity m/d.a ,b coefficients depending on the filter media properties;32Vfilteredfilt d filtered volume per unit area of filter since last backwash; m /m(hl)t head loss at any time (t), m37

Example 5.2:A filter has a head loss of 0.30 m when clean ( newly washed), and 1.30 m after24 hrs of filtration at a rate of 1.5 L/s.m2 . Estimate the head loss bothimmediately after backwash and 10 hrs later, if the filtration rate is changed to 2L/s.m2 .A. Estimate the values of a and b :1.5 a b * 0 0.30 10001.5 1.5 1.30 a b** 24 * 3600 1000 1000 By solving the 2 equations simultaneously ,a 200, b 5.14B. Calculate head loss for the new flow rate:2 200 b * 0 0.40 mH0 10002 2 H 10 * 10 * 3600 1.88 m 200 5.14 *1000 1000 38

Filtration hydraulics Calculations39

Filtration hydraulics Calculations40

Filtration hydraulics Calculations41

Calculations of filter backwash rateThe backwash flow rate is calculated using the following equations:vb d 90Gn 1135.69 0.0408Gn 0.5 33.7 d 90d 903 s g 2 vb design 1.3vbWhere,vb backwashb kh ratet m3/m/ 2.dd,d90 sieve size that passes 90% by weightµ dynamic viscosity; N.s/m2ρ water density; kg/m3ρs filter particles density; kg/m3Gn Galileo number, dimensionlessg gravitational acceleration,acceleration m/s242

Calculations of filter expansionThe expansion during backwash is calculated using the followingequations:q 1 Le L 1 e vb design e vs 0.22Where,L bed depth during filtration, mLe expanded bed depth,depth mεe expanded bed porosity, dimensionlessε bed porosity during filtration , dimensionlessvs settling velocity of the filter particlesparticles, m/s43

Calculations of headloss during filter backwashHeadloss during backwashing is calculated using the followingequation:qh Le 1 e s Where,Le expanded bed depth, mεe expanded bed porosity, dimensionlessρ water density;dkg/mk / 3ρs filter particles density; kg/m344

Backwash hydraulics Calculations45

Backwash hydraulics Calculations46

Backwash hydraulics Calculations47

Backwash hydraulics Calculations48

Filtration hydraulics Calculations49

Filtration hydraulics Calculations50

Filtration hydraulics Calculations51

Water Treatment EENV 4331 Lecture 5: Filtration Dr. Fahid Rabah 1. 5. Filtration in water Treatment 5.1 Definition of Sedimentation: Filtration is a solid –liquid separation process in which the liquid passes through a porous medium to remove as .

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