Sedimentation. Treatment Processes. Sedimentation. Sedimentation. Sedimentation. Sedimentation. CIVL 1112 Water Treatment - Sedimentation 1/7

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1 CIVL 111 Water Treatment - 1/7 Treatment Processes is te donards movement of an object relative to its surrounding medium, due to te force of gravity. Screening Aeration Preclorination locculation Coagulation Dissolved-air flotation (DA) is a metod ereby bubbles are roduced by te reduction of ressure in a ater stream saturated it air. Te urose of sedimentation is to remove reexisting solids, as ell as te reciitates formed in coagulation and flocculation. Te urose of sedimentation is to remove reexisting solids, as ell as te reciitates formed in coagulation and flocculation. Model of a circular settlement tank it sludge scraers as used to estimate te distribution of articulate concentration over time

2 CIVL 111 Water Treatment - /7 Click HERE for animations about sedimentation tt://tecalive.mtu.edu/meec/module0/wasteaterandwildlife.tm is te accumulation troug gravity of articulate matter at te bottom of a fluid. Tis natural rocess is frequently used to searate contaminants from air, ater, and asteater. Tere are four tyes of settling: discrete flocculant indered comression Discrete - Individual articles settle indeendently, neiter agglomerating nor interfering it te settling of te oter articles resent. Tis occurs in ater it a lo concentration of articles. locculant - Particle concentrations are ig enoug tat agglomeration occurs. Tis reduces te number of articles and increases average article mass. Te eavier articles sink faster. Hindered - Particle concentration is sufficient tat articles interfere it te settling of oter articles. Comression - In te loer reaces of clarifiers ere article concentrations are igest, articles can settle only by comressing te mass of articles belo.

3 CIVL 111 Water Treatment - /7 If te V > V ten settling can occur V V z V Pat of smallest consistently settled article V sludge layer sludge layer L L If te V < V ten sort-circuiting can occur V V sludge layer L Te orizontal velocity, V, of a article can be aroximated by considering te florate,, and te cross-sectional flo area of te tank. V V A z V A Te residence time of ater in te sedimentation tank can be aroximated as: V z Lz t V L t Estimate of te residence time of ater in a small sedimentation tank ere = 1 liter/min, L = 6 in., = 6 in., and z = 10 in. (dimensions of a tank in te lab). Lz t 60in. min t 1000ml 6in.(6in.)10in. 1, 000 ml min 16.9ml in. 5.9 min

4 CIVL 111 Water Treatment - 4/7 Discrete settling, can be analyzed by calculating te settling velocity of te individual articles contained itin te ater. Te forces acting on a settling article are: b d Te forces acting on a article are: gravity in te donard direction, drag acting in te uard direction as te article settles uard buoyancy due te ater dislaces by te article g d b is te force due to gravity is te drag force is te buoyant force g g = d + b Te gravitational force can be exressed as: g m g Using te density and volume of te article yields: ere: is te density of te article, lb-mass/ft., V is te volume of te article, ft., and g is te gravitational constant, ft./s V g g Te drag on te article can be calculated by te drag equation from fluid mecanics ere 1 d CdAv C d is te drag coefficient, dimensionless, A is te article cross-sectional area, ft., is te density of ater, lb-mass/ft., v is te velocity, ft./sec. Te buoyant force acting on te article is: b m g Substituting te article volume and density of ater, yields: V g b By balancing te forces acting on a settling article and using te relationsis for g te force due to gravity, d te drag force, and b te buoyant force, te folloing relationsi can be develoed: 1 Vg CA d v Vg ere: is te density of ater, lb-mass/ft,

5 CIVL 111 Water Treatment - 5/7 Solving for te settling velocity, v, results in: v ( ) Vg C A d If te article is assumed to round and te formulas for area and volume of a sere are used: v 4( ) dg C d ere d is te diameter of te article At lo ynolds numbers (for N, < 1) C d, can be aroximated by: 4 C d N or ynolds Numbers is transition flo, 1 < N < 10,000, te drag coefficient for seres is: C d or turbulent flo, N > 10,000, te relationsi for te drag coefficient for seres is: N N Cd 0.4 Te ynolds Number is: or N, < 1 te article settling velocity can be estimated as a function of te roerties of te article and ater, and te article diameter, or v N vd ere u is te absolute viscosity of te ater, lb-forcesec./ft. (at 50 0, μ =.7(10-5 ) lb.-sec./ft. ). ( ) 18 Tis relationsi is knon as Stokes' la, and te velocity is knon as te Stokes velocity. v ( ) 18 Te vertical velocity of ater in a settling basin is often described as te overflo rate (OR). It is usually exressed as gal./ft. -day (m /m -day). Te overflo rate is calculated in te folloing ay: ere: OR A OR is te overflo rate, gal./ft. -day, is te florate, gal./day, and A is te clarifier area, ft.. Examle 1 Estimate te settling velocity of sand ( =,650 kg/m ) it a mean article diameter of 0.1 mm. Assume te sand is aroximately serical. Using a safety factor of 1.4 to account for inlet and outlet losses, estimate te area required for a camber to remove te sand if te florate is 0.10 m /sec (1,000 liters = 1 m ).

6 CIVL 111 Water Treatment - 6/7 Examle 1 Te density of ater at 0 0 C is 998 kg/m and te viscosity of ater at 0 0 C is 1.01(10 - ) N-s/m (Neton = kg-m/s ). Te Stokes settling velocity is: ( ) v vs OR 18 4 m kg kg m m m s kg ms Examle 1 Knoing te overflo rate, te area required is: 0.10m A ( S) s (1.4).6m OR 0.09m s ere S is te safety factor, 1.4 = 0.09 m/s =.9 cm/s Examle Estimate te settling velocity of te floc articles e ave seen in lab - esecially te jar test results. Use Stokes' la to estimate te settling velocity. Wat are good estimates of te article density and diameter? Ho does your estimate comare to at you ave seen in te lab? Examle Wat are good estimates of te article density and diameter? Let s assume te folloing values: Particle density = 1,100 kg/m Particle diameter = 10-4 m Examle ( ) v vs OR 18 kg kg 4 1, m m 9.81 m m s kg ms = 5.5 x 10-4 m/s = cm/s Examle OR = 5.5 x 10-4 m/s = cm/s 0.055cm cm 86,400 s 1 gal 0.48 cm OR s cm day cm 1ft OR 1,166. gd ft or ferric cloride tyical ORs are in te 700-1,000 gd/ft.

7 CIVL 111 Water Treatment - 7/7 Examle If te settling velocity of te floc articles is cm/s, determine te area of te sedimentation tank. Assume a factor of safety of 1. Assume te system florate can varying from 750 ml/min to 1,50 ml/min Ho does your estimate comare to at you ave seen in te lab? Examle Knoing te overflo rate and te minimum florate, te area required is: 750ml 1cm A ( S) (1.) min m l OR cm 60s s min 1in. A 95.5 cm.54 cm In lab, eac tank is 6 in. by 6 in. or 6 in in cm Terefore, for tis estimate of article velocity e need 1.7 tanks or sedimentation tanks Examle Examle Knoing te overflo rate and te minimum florate, te area required is: 1,50ml 1cm A ( S) (1.) min m l OR cm 60s s min 1in. A 49.4 cm.54 cm In lab, eac tank is 6 in. by 6 in. or 6 in in cm Grou uestions Wat if te settling velocity of te floc articles is greater tan te comuted cm/s? Wat if te settling velocity of te floc articles is less tan te comuted cm/s? Ho do tese estimates comare to at you ave seen in te lab? Terefore, for tis estimate of article velocity e need.1 tanks or sedimentation tanks Treatment Processes Any uestions?