Module 5 Water Treatment On completion of this module you should be able to:

1. Module 5 Water TreatmentOn completion of this module you should be able to: • Be aware of the objectives of water treatment • Have an appreciation of the location, layout of a plant • Describe the processes involved in water treatment • Discuss the types of separation processes • Design a simple sedimentation tank Module 5

2. Basic Methods for Correcting Water Quality Deficiencies • The processes and extent of required treatment are dependent on the nature and degree of quality deficiencies to be corrected. • There is virtually no water that cannot be treated to potable standards. Cost effectiveness is one of the guiding principles • The basic methods are physical and chemical processes and to a lesser extent, biological Module 5

3. Water Treatment matrix Module 5

4. Module 5

5. Plant Layout and Headloss Through the Plant • Planning and environmental constraints • Selected source • Plant design factors • Site factors • Environmental factors • Unit processes should lie on the system gravity hydraulic grade line Module 5

6. Preliminary TreatmentDepending on the source, the following unit processes are likely • Intake screens • Aeration • Preliminary settling tanks • Pre-chlorination and algal control Module 5

7. Intake screen Module 5

8. Aeration • Increase dissolved oxygen in ‘stale’ water • Remove or reduce dissolved CO2 and other gases • Precipitate out dissolved ferrous and manganese compounds • Reduce volatile impurities and odour • Various methods of aeration e.g. spray, cascade, tray and diffused air Module 5

9. Preliminary settling tank Module 5

10. Chemical treatment through coagulation • Coagulants are chemicals that react with colloidal matter to form absorbent bulky precipitates (flocs) • Destabilisation of colloidal particles (10-3 - 1 m), hydrophilic or hydrophobic in nature • Salts of aluminium and iron form insoluble hydroxides • Reaction is pH dependent (6 - 7 optimum range) Module 5

11. Aluminium salts are commonly used • Aluminium sulfate; sodium aluminate • Natural or added alkalinity is required • Al2(SO4)3 + 3Ca(HCO3)2 2Al(OH)3 + 3CaSO4 + 6CO2 • Reaction is sensitive to pH • May revert to soluble for if pH increases/decreases • Some recent concerns relating to health issues Module 5

12. Aluminium sulfate In the absence of alkalinity • Al2(SO4)3 + 6H2O 2Al(OH)3 + 3H2SO4 • H2SO4 + Ca(OH)2 CaSO4 + 2H2O • Al2(SO4)3 + 3Ca(HCO3)2 2Al(OH)3 + 3CaSO4 + 6CO2 Natural alkalinity Module 5

13. Iron salts are more difficult to control • Ferric chloride/iron(III) chloride; ferric sulfate • 2FeCl3 + 3Ca(HCO3)2 2Fe(OH)3 + 3CaCl2 + 6CO2 • Natural or added alkalinity is required • Wider operating pH range • Cheaper material and forms heavier floc • Iron salts cake and are dirty to handle, difficult sludge to dispose Module 5

14. Coagulant aids • They assist difficult coagulant processes and result in dramatic improvement with increased floc formation and faster settling • Polyelectrolytes of organic synthetic high molecular weight material with electrical charges • Clays, lime, soda ash and activated silica are other examples of coagulant aids Module 5

15. Optimum coagulant dosage • Use of laboratory jar test • Determine least cost of chemicals that remove turbidity, colour in an shortest possible time • Comparison of first floc appearance, floc size, dosage and settling time • Optimum dosage also tested against pH Module 5

16. Optimum coagulant dosage using the jar test Module 5

17. Flash/Rapid MixingTo cause rapid dispersion at minimum power input • Use of various devices e.g. bends, baffles, can result in energy losses • Energy for good mixing requires 3 - 15 kW.s/m3 • 30 - 60 sec detention time at maximum flow • Rate of chemical diffusion is quantified by the shear velocity gradient, G = [P/(V)]0.5 Module 5

18. Flash/Rapid Mixing (cont) • G = 500 - 600 s-1 at 30 - 60 s residence time • Mechanical power for head loss, P = Q  g h watt • Head loss from hydraulic mixing varies 0.15 - 0.5 m • Excessive G values can be harmful • Increased contact time of 120 s or more achieve little Module 5

19. Flash/Rapid Mixer Module 5

20. FlocculationGentle stirring following rapid mixing so that floc particles can coalesce and agglomerate • Two phases are involved; initial perikinetic, orthokinetic > 1 m • Shear velocity gradient, G = 20 - 75 s-1 • Detention time, t = 20 - 60 minutes • Camp No, G t of (12 to 270) x 103 • Mechanical flocculation power input • Tapered flocculation using high G values and progressively lower as floc size increase Module 5

21. Relationship between Shear Gradient and time, t Module 5

22. A Mechanical Flocculator Module 5

23. SedimentationRemoval of suspended particles in an aqueous medium through gravity settling • Class I Unhindered settling of discrete particles • Class II Settling of dilute suspension of flocculent particles • Class III Hindered settling and zone settling • Class IV Compressive settling (compaction) Module 5

24. Class I settlingFor discrete particles settling freely, the terminal velocity is reached when gravitational force is balanced by frictional drag force • vs = gd2 (1 - )/(18 ) • As particle size increases, vs increases • As CD increases vs decreases • CD varies inversely as NR Module 5

25. Module 5

26. Class I settling (cont) • Detention time, t = Volume/Q • Depth of tank is not relevant, vs = Q/surface area • Performance is influenced by overflow rate and detention time • High water temperature decreases CD and thus increases vs Module 5

27. Drag coefficient Module 5

28. Shallow depth sedimentation • Proposed as early as 1904 with initial failure • Obvious inherent advantages • Tube clarifiers with high surface loading rates achieve 9 m/h • Plated tanks in zig-zag pattern, vh = 44 m/h, HRT of 22 min • Lamella separator with vs = 20 m/h Module 5

29. Tube clarifier at Mt Kynoch settling tank Module 5

30. Shallow depth sedimentation Plate settler tank Module 5

31. Shallow depth sedimentation Lamella separator tank Module 5

32. Difficult settling operation conditions • Excessive suspended solids • High colloidal content • Coincidence of peak demand and high turbidity • Low coefficient of fineness < 1 • Low temperature, overturn • Persistent wind condition • Streaming caused by density currents, temperature gradients Module 5

33. Settlement in horizontal flow tanks • Overflow rates varies from 18 to 54 m/d • Typically 28 m/d for a 3.5 m depth and 3 h HRT • In tropical countries with more turbid water, 18 m/d with 4 h HRT is appropriate with depths of 3 - 3.5 m • In practice, particles are not wholly discrete and there is merit in depth • As a preliminary guide use HRT x (TSS/900)0.5 h to adjust for varying TSS in water Module 5

34. A typical horizontal flow sedimentation tank Module 5

35. Settlement in upward flow tanks • Area of tank to ensure vs > v = Q/A • In practice, vs 2 v • vs = 3 m/h for well formed floc • = 6 - 10 m/h with coagulant aids • = 8 m/h in water softening plants • Types: hopper bottomed sludge blanket square tanks, solid contact clarifiers, pulsator Module 5

36. Vertical flow tank Module 5

37. Pulsator Module 5

38. Solids contact clarifier Module 5

39. FlotationAn effective means of removal of particles of density less than the liquid medium • Use of air bubbles to separate solids/particulates from a liquid phase • Air bubbles (20 - 100 m) generated by dissolved air flotation, diffused air flotation and vacuum filtration • Attachment of solids to bubbles in a 3 phase system; size of flocs less important • Solids separation through a floating scum and removed by a skimmer Module 5

40. Flotation (cont) • Advantage of high surface loading rates 5 - 12 m/h, and the ability to remove oils, grease and algae • Short HRT of 40 - 80 minutes; bubbles rise at 1 - 1.5 mm/s • Flotation units are smaller in size than normal clarifiers • Saving in chemical costs • Optimum amount of air is determined from pilot studies • Disadvantage of additional equipment cost, high operating cost and energy use Module 5

41. Dissolved Air Flotation (DAF) Module 5

42. Dissolved Air Flotation (DAF) Dissolved air flotation Module 5

43. FiltrationA process of passing water through a sand bed or other suitable medium at low speed to remove suspended solids • Removal of non-settleable flocs after coagulation and sedimentation • Properties of the medium (effective size, hardness etc) • More than a mechanism of straining Module 5

44. Mechanisms of filtration • Straining • Sedimentation • Interception • Adhesion • Flocculation • Adsorption Module 5

45. Rapid sand filterA process of depth filtration as solids are removed within the granular medium • Sand bed 0.6 - 0.75 m deep of 0.4 - 0.7 mm effective size and a uniformity coefficient  1.7 • Supporting gravel layer 0.3 - 0.5 m (graded 2 - 60 mm) • Underdrain system to collect filtered water and to discharge air scour and backwash water uniformly • Filtration rate varies from 4 - 15 m/h Module 5

46. Rapid sand filter (cont) • Backwash when head loss  2 m • Application of backwash water assumes practical importance in the design of filters • Some problems associated with rapid sand filters are mud balls, air-binding, surface cracks and shrinkage • Other forms are direct filtration, and up-flow filtration Module 5

47. clogs up readily ideal but unattainable Arrangement of filter media Module 5

48. Typical rapid sand filter Module 5

49. Rapid sand filter isometric view (Droste 1997, p. 418) Module 5

50. Types of filter underdrain system (McGhee, 1991, p.212) Module 5