Water Treatment On completion of this segment you should be able to:

1. Water TreatmentOn completion of this segment you should be able to: • Be aware of the objectives of water treatment • Describe the processes involved in water treatment • Discuss the types of separation processes

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

3. Basic TreatmentDepending on the source, the following unit processes are likely • Screening • Aeration • Coagulation and flocculation • Sedimentation and filtration • Disinfection

4. A belt screen

5. Coagulation and Flocculation • Coagulants are chemicals that react with colloidal matter to form absorbent bulky precipitates (flocs) • Destabilisation of colloidal particles (10-3 - 1 m) • Salts of aluminium and iron form insoluble hydroxides • Reaction is pH dependent (6 - 7 optimum range) • Flocculation is gentle stirring that increases the size of the flocs

6. Aluminium salts are commonly used • Aluminium sulfate • Al2(SO4)3 + 3Ca(HCO3)2 2Al(OH)3 + 3CaSO4 + 6CO2 • Natural or added alkalinity is required • Reaction is sensitive to pH • May revert to soluble for if pH increases/decreases

7. Iron salts are more difficult to control • Ferric chloride/iron(III) chloride • 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

8. Flash/Rapid Mixer

9. FlocculationGentle stirring following rapid mixing so that floc particles can coalesce and agglomerate • Two phases are involved; initial periknetic, orthokinetic > 1 m • Detention time, t = 20 - 60 minutes • Mechanical flocculation power input • Tapered flocculation as floc size increase

10. A Mechanical Flocculator

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

12. Class I settling – unhindered discreteFor discrete particles settling freely, the terminal velocity is reached when gravitational force is balanced by frictional drag forcve • vs = gd2 (1 - )/(18 ) • As particle size increases, vs increases • Detention time, t = Volume/Q • Depth of tank is not relevant, vs = Q/surface area • Performance is influenced by overflow rate and t

13. Idealised unhindered discrete settling

14. Drag coefficient

15. 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

16. A typical horizontal flow sedimentation tank

17. Vertical flow tank

18. Solids contact clarifier

19. 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

20. Mechanisms of filtration • Straining • Sedimentation • Interception • Adhesion • Flocculation • Adsorption

21. 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

22. 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

23. clogs up readily ideal but unattainable Arrangement of filter media

24. Typical rapid sand filter

25. Typical rapid sand filter (Droste 1997, p. 418)

26. Slow sand filtersThese are the oldest and effective method for removing pathogenic microorganisms in water. Cake filtration when solids are removed on entering the face of the granular material • No pre-treatment or chemicals are required but turbidity < 30 JTU • Filter media 0.7 - 1.2 m layer of 0.2 - 0.4 mm effective size with a uniformity coefficient  3 • Supported on gravel layer 0.1 m (graded 5 - 25 mm) • Relies on surface straining and microbial action (schmutzdecke) • Slow filtration rates of 350 - 700 L/s.ha (3 - 6 m/d)

27. Slow sand filters (cont) • 1 - 3 months filter run or when head loss  1 m • Surface renewal by removing 12 - 25 mm of surface layer each time until 600 mm of sand layer is left • Requires large land area • Labour intensive to remove and clean the sand • Suitable for reservoir-fed supply and small communities requiring no technical supervision but not effective for turbidity > 40 JTU • Does not remove colour but is able to deliver bacteriologically superior water

28. Slow sand filter

29. Pressure filtration • No different from rapid sand filters • Filter lies on the HGL

30. Chlorine disinfectionIt is presently the most cost-effective disinfection method but it has some adverse effects • Properties of chlorine • Reaction is highly pH dependent • Cl2 + H2O HOCl + HCL • As pH increase the hypochlorous acid (HOCl) will further dissociate to H+ and OCl- (hypochlorite ions) • HOCL and OCl represent the free available chlorine

31. Chlorine disinfection (cont) At 20o C pH %HOCL %OCl 6 97 3 7 79 21 8 21 73 9 4 96 • Chlorine:ammonia reaction • Breakpoint chlorination • Superchlorination

32. Chlorine - ammonia reactionFormation of monochloramine (NH2Cl)HOCl + NH3 H2O + NH2Cl Cl2:NH3 < 5:1; pH  7Formation of dichloramine (NHCl2)NH2Cl + HOCl H2O + NHCl2 Cl2:NH3 < 10:1 Formation of trichloramine (NCl3)NHCl + HOCl H O + NCl3 Cl2:NH3 < 20:1; pH < 4 Monochloramine and dichloramine represent the combined available chlorine, with less disinfecting power compared with the free available chlorine

33. Breakpoint chlorination • Oxidation of chloramines until appearance of free available chlorine • At this point the free available chlorine residual is lowest • Taste, odour are reduced through oxidation • Some colour may also be removed • Good control required to ascertain that breakpoint is reached

34. Breakpoint chlorination

35. Superchlorination • How is it used? • When is it applied? • Need to de-chlorinate using sulfur dioxide, sodium bisulfite, sodium thiosulfate or granular activated carbon

36. Other forms of disinfection • Ozone • Ultra violet light (UVL) • Halogens (bromine, iodine) • Metal ions (silver) • Ultra-filtration • Simple retention time

37. End of Water Treatment segment