ENVE 411 Water Engineering Design

Transcription

1 ENVE 411 Water Engineering Design Design of Coagulation & Flocculation Units Fall Nov 2012 Assist. Prof. A. Evren Tugtas

2 Orhaniye WTP 2

3 Mixing Mixing liquids is used to: Blending of two immiscible liquids (ethyl alcohol/water) Dissolving solids in liquids Dispersing a gas in a liquid as fine bubbles Agitation of the fluid to increase heat transfer Mixing in water treatment is used to: achieve coagulation achieve flocculation 3

4 Mixing Mixing can occur in following locations; Water intake (Pumps, pipes) Flash mix tanks Flocculation tanks Other 4

5 Mixing Three phenomena contribute to mixing; 1) Molecular diffusion Diffusion: Random motion of molecules from high concentration to low conentration Molecular Diffusion: Moving molecules self propelled by thermal energy, not affected by concentration (Brownian motion) Ref: 5

6 Mixing Three phenomena contribute to mixing; 2) Eddy Current (Circular flow): Water flows opposite to the original flow (whirlpools function of a degree of turbulance) Ref: 175/Default.aspx Munson BR, Young DF, Okiishi TH. Fundamentals of Fluid Mechanics ISBN:

7 Mixing 3) Non-uniform flow: At any given time, velocity is not same at every point of the flow. Ref: 7

8 Factors that affect mixing Number of particles Size of particles Mixing time Water temperature Chemical dosage 8

9 Mixing Power input per unit volume of liquid can be used as a rough measure of mixing effectiveness. More input power creates more turbulence, and greater turbulence leads to better mixing Power imparted to the water can also be measured by the Velocity Gradient (Camp, 1955) 9

10 Velocity Gradient (G) for mechanical or pneumatic mixing G = P μv Rate of particulate collision is proportional to G G must be sufficient enough to achieve desired rate of collisions P: Power imparted to the water (Nm/s OR W) : Absolute viscosity of water (Ns/m 2 ) V: Basin volume (m 3 ) G: Velocity gradient (s -1 ) Velocity gradient Ratio of relative velocity of two particles to the distance between the particles 10

11 Coagulation / Flocculation It is diffucult to separate colloids from water, colloids do not settle by gravity are too small, they pass through the pores of most common fitration media Natural organic matters, disinfection byproducts, bacteria, cysts of protozoa, clay, silt, mineral oxides may be classified as colloidal particles. 11

12 Coagulation / Flocculation Coagulation and flocculation consist of adding a floc-forming chemical reagent to a water or wastewater. to enmesh or combine with non-settleable colloidal solids and slow-settling suspended solids to produce a rapid-settling floc. Floc is then removed by sedimentation 12

13 Coagulation & Flocculation Coagulation is the addition and rapid mixing of a coagulant to achieve, destabilization of the colloidal and fine suspended solids initial aggregation of the destablized particles Flocculation is the slow stirring or gentle agitation to aggregate the destabilized particles form a rapid settling floc 13

14 Coagulation & Flocculation Coagulation and Flocculation Water Treatment Wastewater Treatment Principle Coagulants Aluminum Iron salts Polyelectrolytes ( anionic, cationic, nonionic) Principal Coagulants Aluminum Iron salts Lime Polyelectrolytes 14

15 Colloidal Characteristics Electrostatic forces Colloidal particles have electrostatic forces. Electrostatic forces maintain the dispersion of the colloid. Ionization of surface groups and adsorption of ions from the surrounding solution cause electrical charge on the surface of a colloid. Also colloidal minerals such as clays, have an electrostatic charge due to ion deficit within the mineral lattice /similar.htm 15

16 Coagulants Principle use of coagulants is to; destablize particle suspensions increase the rate of floc formation Ref; al_treatment.htm 16

17 Characteristics of Coagulants Inorganic coagulants used in water treatment should exhibit following characteristics; They are non-toxic at the dosage they are supplied They have high charge density They are insoluble at neutral ph 17

18 Coagulants Most commonly used coagulants are Aluminum sulfate (Alum) Al 2 (SO 4 ) 3.xH 2 O Iron salts Hydrolyzing metal salts (HMS) Coagulants 18

19 Ref: Davis M.L. Water and Wastewater Treatment: Design Principles and Practice McGrawHill 19

20 Hydrolyzing Metal Salt (HMS) Coagulants Most water treatment plants using alum operate at; ph 0f Alum dossage of 5-50 mg/l 20

21 Coagulant Aids Coagulant aids are sometimes used to produce quick-forming, dense, rapid-settling flocs. Coagulant aids are; Alkalinity addition Polyelectrolytes Turbidity addition Adjustment of ph 21

22 Rapid Mixing - Coagulation G, s -1 Detention Time Reference min Peavy sec AWWA 22

23 Slow Mixing - Flocculation Dimensionless Gt number is used to determine mixing efficiency. Gt t 10 to 30 min 23

24 Mixers 1) Hydraulic mixing devices a) Venturi sections, Orifices b) Hydraulic jumps c) Parshall flume d) Weirs e) Baffled mixing devices f) Static mixers 2) Mechanical mixing devices a) Propeller mixer b) Turbine mixer c) Paddle mixer 3) Pneumatic mixers a) Air diffusers 24

25 Coagulation Coagulation unit is used to achieve: Complete mixing of the coagulant and water Destabilization of colloidal particles and Early stages of floc formation 25

26 Coagulation Mixing is achieved by: Hydraulic mixing devices P = γqh L = ρgq h L Mechanical mixing units h L = C D V 2 2 2g Ref: htm 26

27 Coagulation units may be single or double compartment Coagulation Single compartment basins are usually circular or square Liquid depth: times the basin diameter or basin width Vortexing can be minimized by baffles (10% of tank diameter) Metcalf & Eddy, Inc. (2003). Wastewater Engineering- Treatment and Reuse, 4 th ed., McGraw-Hill, New York, NY. 27

28 Mechanical Mixers Turbine or Propeller Mixers Vortexing Vortexing may occur: Liquid to be mixed may rotate with the impeller Vortexing causes the difference between the impeller velocity and water velocity to decrease, which decreases effectivenes of mixing Ref: 28

29 Mechanical Mixers Turbine or Propeller Mixers Vortexing To eliminate vortexing: Four baffles can be placed vertically at the tank wall. Each baffle width = 10% - 12% of the tank diameter Baffle width = 1/10 WL W Baffle width = 1/10D L 29

30 Mechanical Mixers Turbine or Propeller Mixers Vortexing To prevent vortexing in small tanks Impeller should be mounted off-center Impeller can be mounted at an angle Impeller can be mounted to the side of basins at angle Turbine or propeller mixers are usually constructed with a vertical shaft driven by a speed reducer and electric motor Types of impellers: 1. Radial flow impellers Generally have flat or curved blades located parallel to the axis of shaft 2. Axial flow impellers 3. Make an angle of less than 90 o with drive shaft 30

31 Mechanical Mixers Turbine or Propeller Mixers Power Requirement Laminar Flow; P = K L μn 2 D i 3 Power imparted by baffled or unbaffled tank Turbulent Flow; P = K T ρn 3 D i 5 Power imparted by baffled tank P=Power requirement (Nm/s) K L =Impeller constant for laminar flow K T =Impeller constant for turbulent flow n=rotational speed (rps) D i =Impeller diameter (m) =density of the liquid (kg/m 3 ) =Specific weight of the liquid (N/m 3 ) =dynamic viscosity (Ns/m 2 ) Re = D i 2 nρ μ 31

32 Mechanical Mixers Turbine or Propeller Mixers Power Requirement In laminar flow power imparted is independent of the presence of baffles In turbulent flow Power imparted in an unbaffled tank = 1/6 of the power imparted in the same tank with baffles Power imparted in an unbaffled square tank = 75% of the power imparted in a baffled square or a baffled circular tank Power in a baffled vertical square tank = Power in a baffled vertical circular tank having D=width of square tank 32

33 Flocculation Destabilized colloids may still settle very slowly Flocculation is a slow mixing process to bring the desabilized particles in contact to promote their agglomeration. 33

34 Flocculation Degree of flocculation depends on Floc characteristics Velocity gradient GT value (dimensionless parameter) Magnitute of Gt is related to total number of collisions High Gt value large number of collisions 34

35 Flocculation If G is too great; Shear forces will prevent the formation of a large floc If G is too insufficient; Adequate interparticular collisions will not occur Proper floc will not form If the water is difficult to coagulate, floc will be fragile and a final G < 5mps/m may be required. If the water coagulates easily, final G as high as 10 mps/m can be used 35

36 Flocculation Mixing in an individual flocculator basin hydraulic flow regime approaching complete mix condition. Plug-flow conditions are desirable to minimize short-circuiting of the flow Short circuiting a portion of the incoming flow traverses the chamber in a much shorter time than the nominal detention period nominal detention period 36

37 Flocculation Flocculation units are usually designed to provide for taperred flocculation In tappered flocculation flow is subjected to decreasing G values as it passes through the flocculation basin Taperred flow; promotes plug flow through the system (ensure that all particles are exposed to mixing for a significant amount of the total detention time allows the G value to be decreased from one compartment to next as the average floc size increases. 37

38 Taperred Flocculation Rapid build up of small dense floc, which subsequently aggregates at lower G values into larger, dense, rapid settling floc particles. High G provided during the first third of the flocculation period Lower G value during the next third Much lower G value during the last third Ref: n_math2.html 38

39 Tappered Flocculation Typical Series of G values 50, 20, 10 mps/s Optimum flocculation requires tappered flocculation Power input can be changed using variable speed motors. Compartments of a flocculation unit is often separated by baffles 39

40 American Water Works Association. Water Treatment Plant Design. 4th ed. McGraw Hill,

41 Paddle Mixers Paddle mixers consists of series of appropriately spaced paddles mounted on either a horizontal or vertical shaft Generally rotate slowly Paddles are commonly used as flocculation devices Ref: 41

42 Vertical Paddle Wheel Flocculators Look at this web site: ent.com/jms_floccul ators.html Ref: 42

43 Vertical Paddle Flocculator

44 Horizontal Paddle Wheel Flocculators Ref: Look at this web site: ent.com/jms_floccul ators.html 44

45 HorizontalPaddle Flocculator

46 Cross Flow Pattern In cross flow pattern, blades are perpendicular to flow. Taperred flocculation can be achieved by varying the paddle size the number of paddles diameter of the paddle wheels on the various horizontal shafts the rotational speed of the various horizontal shafts Ref: Reynolds, T. D., and P. A. Richards. Unit Operations and Processes in Environmental Engineering. 2nd ed. Boston, MA: PWS Publishing Company,

47 Axial Flow Pattern Blades are parallel to the flow Taperred flocculation may be achieved by varying the paddle size number of paddles on each paddle wheel Ref: Reynolds, T. D., and P. A. Richards. Unit Operations and Processes in Environmental Engineering. 2nd ed. Boston, MA: PWS Publishing Company,

48 Paddle Mixers The diameter of a paddle impeller is usually 50-80% of the tank diameter or width Width of a paddle is usually 1/6 to 1/10 of the diameter Paddles are mounted ½ of a paddle diameter above the tank bottom The paddle speeds range from 20 to 150 rpm Paddles do not produce turbulance 48

49 Power imparted to water by a paddle impeller F D = C DAρV p 2 2 F D =Drag force (N) C D =Coefficient of drag of paddle moving perpendicular to fluid A=Cross sectional area of paddles (m 2 ) =density (kg/m 3 ) V p =Relative velocity of paddles with respect to the fluid (m/s), usually assumed to be 0.6 t o0.75 times the paddle tip speed P=Power requirement (W) 49

50 Paddle Flocculator

51 3.) P, power Paddle Flocculator t = V/Q = 51,780.9ft 3 /12x10 6 gpd x x 1440 minutes/day t = minutes Gt = 25s -1 x minutes x 60s/minute Gt = 69,720 between 50, ,000 OK velocity of the water,v = 75% of the maximum peripheral velocity The distance traveled is D or 2 r per revolution, rev/s x D/rev = D/sec v =.75 x 2 r x R(revolutions per second) v 1 (first compartment) =.75 x 2 (5.25 ) x R v 1 (first compartment) = 24.74R v 2 (second compartment) =.75 x 2 (3.75) x R v 2 (second compartment) = 17.67R v 3 (third compartment) =.75 x 2 (2.25 ) x R v 3 (third compartment) = 10.60R

52 Paddle Flocculator P=.97C D Av 3 =.97C D A 1 v C D A 2 v C D A 3 v 3 3 =.97C D A(v v v 33 ), A 1 =A 2 =A 3 P =.97(1.50)(.5 x10 board dim.)(2 boards,1up,1down)[ ]R 3 P=317,976R 3 first compartment P= VG 2 = = 2.73x10-5 lb.s/ft 2 x 51,780.9 ft 3 /3(3 compartments)x 45 2 P=950.7 ft.lb/s x 1hp/550ft.lb/s P 1 =1.73hp ft.lb/s / 7wheels = 317,976R 3 R =.075 rps RPM(max) =.075 rps x 60s/min RPM(max) = 4.50rpm 1:4 turndown) = 4.50rpm/4 1:4 turndown) = 1.13rpm

53 Peripheral speed of outside blade v = circumference x RPM v 1 (actual v as opposed to 75%) = R x 2 r v 1 =.075 x 2 (5.25) v 1 = 2.47fps Paddle Flocculator second compartment P= VG 2 = = 2.73x10-5 lb.s/ft 2 x 51,780.9 ft 3 /3(3 compartments)x 20 2 P=187.8 ft.lb/s x 1hp/550ft.lb/s P 2 =.34hp ft.lb/s / 7wheels = 317,976R 3 R =.044 rps RPM(max) =.044 rps x 60s/min RPM(max) = 2.64rpm 1:4 turndown) = 2.64rpm/4 1:4 turndown) =.66rpm

54 Paddle Flocculator third compartment P= VG 2 = = 2.73x10-5 lb.s/ft 2 x 51,780.9 ft 3 /3(3 compartments)x 10 2 P=46.95 ft.lb/s x 1hp/550ft.lb/s P 3 =.085 hp ft.lb/s / 7wheels = 317,976R 3 R =.0276 rps RPM(max) =.0276 rps x 60s/min RPM(max) = 1.66 rpm 1:4 turndown) = 1.66 rpm/4 1:4 turndown) =.42 rpm

55 Baffled Chanelled Flocculators 55

56 Baffled Chanelled Flocculators Baffled channel flocculators operate under plugflow conditions Short-circuiting is prevented by the use of baffled passages Baffled structures cause headlosses. Therefore, baffled flocculators should be used for large treatment plants with flow rates higher than 10000m 3 /d. 56

57 Velocity Gradient (G) for baffle basin G = γh L μt : specific weight of water (kgm 2 /s 2 OR kn/m 3 ) : Absolute viscosity of water (Ns/m 2 ) h L : head loss (m) T: detention time (s) G: Velocity gradient (s -1 ) 57

58 Flocculation in different Structures Pipe flocculation Laminar Turbulent Baffle Filters Paddle Flocculators Floc Blanket Tanks 58

59 Flocculation in different Structures 59

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61 Velocity should be greater than 0.3 m/s Increase baffle number area decreases velocity increases Decrease opening area by lowering the baffle velocity increases 61

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65 Orhaniye WTP - Design 65

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69 Perforated walls Port velocity m/s Holes m in diameter m apart Lowest port should be 0.6 m above the basin floor Ref: agement/drinking-watertreatment1/lectures/lectures/ 69

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