CT4471 Drinking Water 1

Transcription

1 CT4471 Drinking Water 1 Coagulation & flocculation Dr.ir. J.Q.J.C. Verberk Room September,

2 Contents 1. Introduction 2. Coagulation: theory 3. Coagulation: practice 4. Flocculation: theory 5. Flocculation: practice 6. Special constructions 25 September,

3 Introduction 25 September,

4 Why coagulation and flocculation? Removal of turbidity (clay) and colour (humic acids) public health and aesthetics Public health: Removal of heavy metals and organic compounds Aesthethics: Attractiveness of water 25 September,

5 Classification Colour Turbidity dissolved compounds colloids suspended solids m Settling solids Organic material Salts Gases Clay Silt Sand Gravel 25 September,

6 Settling velocity of particles particle diameter (mm) particle ρ = 2,650 kg/m 3 gravel coarse sand fine sand silt bacteria clay colloids Sedimentation time (over 30 cm) 0.3 seconds 3 seconds 38 seconds 33 minutes 35 hours 230 days 63 years 25 September,

7 Principle of coagulation & flocculation Colloids and humic acids are negatively charged stability Adding coagulant (coagulation) destabilisation Flocculation Growth of aggregates After coagulation and flocculation removal of floc aggregates by sedimentation and/or filtration 25 September,

8 Quality of surface water Suspended matter [mg/l] Turbidity Colour [FTU] [mg Pt/l] DOC [mg/l] Rhine Meuse Biesbosch reservoirs Lake IJssel Drentse Aa Tropical river 10,000 5,000 1, Standards for drinking water < 0.05 < 0.1 < 20 (<10) 25 September, (3)

9 Quality of surface water, worldwide 25 September,

10 Coagulation: theory 25 September,

11 Turbidity and humic acids Turbidity clay particles/colloids size μm charge = negative Color Humic compounds size 0.01μm charge of humic acids depends on the ph C n H 2n OH + H 2 O C n H 2n O - + H 3 O + 25 September,

12 Coagulants The appearance of iron salts in water depends on ph. Calculation of Fe 3+ concentration Fe(OH) 3 Fe OH - K = H 2 O H 3 O + + OH - K = K w [Fe = 3+ [H 3 O + ] [OH ] [OH ] 3 = 10 ] 38 [OH [Fe ] = [H ] = 3 O (10 ] ) 3 [H 3 O + ] 3 = [H 3 O + ] 3 log[fe 3+ ] = log( ) + 3 log(h 3 O + ) = 4 3 ph 25 September,

13 Coagulants A concentration of Fe 3+ ions of 0.05 mg/l is desired Only at ph of the water is 3.3 [Fe 3+ ] = 0.05 mg/l = mmol/l log[fe 3+ ] = log[ ] = 4-3 ph ph = 3.3 ph < 3.3 then more Fe 3+ ph > 3.3 then less Fe 3+ Surface water has a ph of approximately 7. The consequence is that mol/l Fe 3+ can maximally be dissolved. If there are more Fe 3+ ions in the water, they will precipitate with OH- ions and form Fe(OH) September,

14 Coagulants 25 September,

15 Coagulants dosing in practice 10-4 mol/l Fe = 5.6 mg/l Fe = 27 mg/l FeCl 3 6H 2 O dosing of iron is done with FeCl 3 FeCl 3 6H 2 O Fe Cl H 2 O Fe OH - Fe(OH) 3 Result of dosing coagulants = ph decrease, thus conditioning 25 September,

16 Coagulants Reaction coefficients Solubility products of iron salts Fe H 2 O Fe(OH) 2+ + H 3 O + K = Fe(OH) H 2 O Fe(OH) 2+ + H 3 O + K = Fe(OH) H 2 O Fe(OH) 3 + H 3 O + K = Fe(OH) 3 + 2H 2 O Fe(OH) 4- + H 3 O + K = September,

17 Coagulants 25 September,

18 log(al x (OH) y 3x-y ) [mol/l] Coagulants Al(OH) 2+ Al 3+ Al(OH) Al 13 (OH) ph 25 September,

19 Coagulants: Pre-polymerized Aluminium Chloride (PAC) Part of the coagulation reaction already finished Very good results at low temperatures OH Cl - OH OH Cl Cl - +Al-OH-Al-OH-Al-O-Al-O-Al-OH + OH OH OH Cl - OH 25 September,

20 Destabilisation Three mechanisms electrostatic coagulation adsorptive coagulation precipitation coagulation 25 September,

21 Electrostatic coagulation Dosage mmol/l Fe 3+ ph 3 25 September,

22 Adsorptive coagulation < 1 sec Fe 3+ + H 2 O Fe(OH) 2+ Adsorptive coagulation occurs at a low ph, because positive hydrolysis products are needed. restabilisation: - under-dosage of coagulants - over-dosage of coagulants 25 September,

23 Precipitation (sweep) coagulation At low turbidites Fe(OH) 3 floc is neutral, flocs can collide 25 September,

24 Polyelectrolytes (flocculant aid): long organic polymers Stronger flocks larger flocks Used at low temperatures 25 September,

25 Conclusions coagulation electrostatic coagulation: not of importance in drinking water treatment adsorptive coagulation colour at low ph, dosing proportional with removal of organic compounds low dosing high dosing results in re-stabilisation optimum with low ph mostly for colour (organic compounds) precipitation coagulation no re-stabilisation high dosing for turbidity removal evident optimum ph 8 with iron ph 6 with aluminum 25 September,

26 Coagulation: practice 25 September,

27 Jar test apparatus Variation in: ph, dosage, flocculation time, sedimentation time, stirring energy 25 September,

28 Optmising dosage 25 September,

29 Optimising ph 25 September,

30 Coagulation Braakman polder water = humic acids 25 September,

31 WRK I-II WRK III Rhine water <-> Lake IJssel water 25 September,

32 Rapid mixing - mechanical mixers -static mixers parameters: - residence time (T) -velocity gradient (G c ) 25 September,

33 Weir mixer 25 September,

34 Example weir mixing ΔH static mixer G c = ρ w g ΔH τ μ c Example: ΔH = 0.75 m; Q = 4500 m 3 /h; V = 2 m 3 T = 20 C μ = N s/m 2 Q = 4500 m 3 /h = 1.25 m 3 /s τ c = 2/1.25 = 1.60 sec G c = = s 1 25 September,

35 Variation in velocity gradient G c 25 September,

36 Construction forms of static mixers Coagulant dosing v v Coagulant dosing Main flow coagulant dosing through 6 holes 25 September,

37 Construction forms of static mixers 25 September,

38 Coagulation: practice mechanical mixer G c P = μ V 25 September,

39 Mixing: Dutch practice WRK I-II WRK III Braakman Type mixer constriction hydraulic jump cascade Energy loss over mixer [m] Mixing time [s] G c value at 20 C [s -1 ] Type of coagulant FeCl 3 Fe SO 4 Al 2 (SO 4 ) 2 Dosing [mg/l] September,

40 Flocculation: theory 25 September,

41 Perikinetic flocculation driving force = Brownian movement Von Smoluchowski: dn 4 k T n 2 =α dt 3 μ solution: n n0 = α k T 3 μ 25 September,

42 Orthokinetic flocculation driving force = turbulence dn 4 = n n R G dt v Plug flow n n o = e k c G t a v v Complete mixing n 1 = n 1 + k c G t o a v v 25 September,

43 25 September, dx dy dz dz dz dv v + v V P G V G dz dy dx dz dv dv dy dx dz dv dv F P dy dx dz dv F 2 2 = = = = = = μ μ μ μ μ Velocity gradient

44 Flocculation: practice 25 September,

45 Construction forms of flocculation tanks parameters: - residence time T - residence time distribution n -velocity gradient - floc volume concentration c v G v residence time: seconds 25 September,

46 Residence time (distribution) Plug flow Completely mixed no mixing, thus particle concentration differences no residence time distribution particles perfectly mixed, thus no particle concentration differences residence time distribution particles 25 September,

47 Residence time distribution complete mixing: J( Θ) = 1 exp ( Θ ) plug flow: t < T J(Θ) = 0; t T J(Θ) = 1 mixers in series: J( Θ) = 1 exp(-n Θ) n i= 1 i 1 (n Θ) (i 1)! Θ = t/t t = time T = calculated residence time 25 September,

48 Residence time distribution cumulative frequency distribution [%] 1 0,8 0,6 0,4 0, plugflow perfect mixing 4 in serie 20 in serie 25 September,

49 Optimal design flocculators L/B ratio (L/B >= 3) Vertical, deep and narrow or horizontal, long and narrow 25 September,

50 Non optimal design 25 September,

51 Velocity gradient Energy input turbulence collision of particles flocs Degree of energy supply = velocity gradient G v P = V η Velocity gradient in practice: s -1 Considering floc break up: tapered flocculation 25 September,

52 Variation in velocity gradient G v 25 September,

53 Flocculators P = V η P = ρ π (1 k ) N C L r r ( 4 4 w 2 d blad u i ) G v G = constant N 3 Design parameters: L blade, C d, r u, r i, k 2 L blade Operating parameters: N r u 25 September, r i

54 Tip velocity v = 2 π r N N = 4 rotations/min v max = 1 m/s v 1 r < < < 2 π N 4 2 π m 25 September,

55 Short circuit flow due to flocculators flow parallel to stirring axis = no short circuit flow 25 September,

56 Short circuit flow due to flocculators flow perpendicular to stirring axis = short circuit flow flow velocity = 0.03 m/s, tip velocity = 1 m/s water velocity 0.97 tot 1.03 m/s 25 September,

57 Floc break-up avoiding floc break-up: - more compartments with different G v value - tip velocity less than 1 m/s results in maximum width of 5 m approaching plug flow: - compartments with large L/B ratio (L/B >= 3) - water flow parallel to axis of stirring device 25 September,

58 Well designed flocculator 25 September,

59 Well designed flocculator 25 September,

60 Flocculation: practice transport rapid filtration floc removal Lek flocculation 25 September,

61 Flocculation: practice Dutch practice WRK I-II WRK III Braakman Flocculation time minimum [min] Flocculation time maximum [min] Number of compartments Width [m] Depth [m] Length [m] Direction of flow vertical horizontal vertical Rotations of stirring device [rpm] G v value [s -1 ] per comp Direction axis to flow perpendicular parallel parallel 25 September,

62 Optimal design parameters flocculation installations Coagulation mixing time 1-10 sec velocity gradient mixing > 1,500 sec -1 Flocculation flocculation time = 30 minutes 3-4 compartments Length/width ratio = 3 to 6 axis stirring device parallel to water flow velocity gradient flocculation sec -1 rotations: 1-8 rpm maximum tip velocity 0.9 m/s 25 September,

63 Special constructions 25 September,

64 Sludge blanket installation Berenplaat 25 September,

65 Sludge blanket installation Bombay 25 September,

66 Sludge blanket installation Bombay 25 September,

67 Hydraulic flocculation 25 September,