Water purification - large scale

An overview of methods used for large scale water purification in public health

1. Water purification on a large scale Dr. S. A. Rizwan, M.D., Assistant Professor Department of Community Medicine VMCHRI, Madurai

2. Learning objectives Define the aim of water purification methods in public health List out the various methods for water purification in large scale Describe the steps of each method D escribe the advantages and disadvantages of each method

3. Water pollution Natural Man made: Urbanization and industrialization Sources of pollution Sewage Industrial and trade waste Agricultural pollutants Physical pollutants, viz heat, radioactive substances Indicators of pollution Total suspended solids, biochemical oxygen demand (BOD) at 20 deg. C, concentration of chlorides, nitrogen and phosphorus

4. Water related diseases Biological (Water-borne diseases) Caused by infective agent Viral, Bacterial, Protozoal , Helminthic , Leptospiral Caused by aquatic host Snail, Cyclops Chemical Dental Cyanosis in infants Cardiovascular Inadequate use of water Insect breeding

5. Water pollution law Water (Prevention and Control of Pollution) Act , 1974 Central and State Water Boards and Joint Water Boards endowed with wide powers for controlling pollution

6. Water purification Large scale Storage Filtration Slow sand filter Rapid sand filter Disinfection Chlorination Ozonation Other agents Membrane processes

7. Water purification Small scale Household purification Boiling Chemical disinfection: Bleaching powder, Chlorine solution , High test hypochlorite ( HTH), Chlorine tablets, Iodine, Potassium permanganate Household filtration Disinfection of well By adding bleaching powder Double pot method

8. 1. STORAGE

9. 1. Storage In natural or artificial reservoirs Effects of storage: Physical: gravity – 90% suspended impurities settle down in one day Chemical: oxidizing action Biological: only 10% bacteria remains at the end of 1 week Optimum period of storage: 2 weeks

10. 2. FILTRATION

11. 2. Filtration Water passed through porous media Slow sand filter: biological Rapid sand filter: mechanical

12. 2A. SLOW SAND FILTER

13. Slow sand (BIOLOGICAL) filters Used first in 19th century in Scotland Elements of slow sand filter Filter Box Supernatant water Sand bed Under drainage system Filter control valves

14. Slow sand (BIOLOGICAL) filters General plan and layout

15. Slow sand (BIOLOGICAL) filters Parts in general

16. Slow sand (BIOLOGICAL) filters Cross-sectional view of the filter bed

17. ELEMENTS of Slow sand filter Supernatant water Sand bed Under drainage system Filter control valves

18. ELEMENTS of Slow sand filter Supernatant water Depth: 1 to 1.5 m Promotes downward flow of water through the sand bed Waiting time of 3-12 hours for raw water to undergo partial purification by sedimentation and oxidation

19. ELEMENTS of Slow sand filter Sand bed Depth, 1 m (sand of diameter 0.2-0.3 mm), 0.3m (gravel with 0.2 - 1 cm diameter ) Sedimentation The supernatant water acts as a settling reservoir. Settle-able particles sink to the sand surface. Mechanical straining Particles too big to pass through the gap between the sand grains are retained

20. ELEMENTS of Slow sand filter Vital/ Biological/ Zoogleal / Schumtzdecke layer Slimy, gelatinous layer over sand bed containing threadlike algae, bacteria and diatoms ‘Heart’ of the slow sand filter Ripening of filter: Formation of vital layer S uspended particles are retained by adhesion to the biological layer Removes organic matter, holds back bacteria and oxidizes ammoniacal nitrogen in to nitrates

21. SchMUTZDECKE

22. ELEMENTS of Slow sand filter Under drainage system Depth: 0.15 m At the bottom of filter bed Porous pipes: Outlet for filtered water as well as support to the filter media above Rate of filtration 0.1-0.4 m3/ hr /m3

23. ELEMENTS of Slow sand filter Filter control valves To regulate the flow of water in and out Filter cleaning Increased bed resistance -> Necessary to open the regulating valves fully -> Scrapping top portion of sand bed up to 2 cm depth -> Time for cleaning the filter After 3-4 years new filter bed is constructed

24. Advantages of slow sand filter Simple to construct and operate C onstruction is cheaper than rapid sand filters P hysical, chemical and bacteriological quality of filtered water is very high (99.9 to 99.99 per cent and E. Coli by 99 to 99.9 per cent)

25. 2B. RAPID SAND FILTER

26. Rapid sand filter First in1885 in USA Gravity type (Open)/ Paterson’s Pressure type (Closed)/ Candy’s

27. Rapid sand FILTER

28. Overview of rapid sand filter

29. STEPS OF Rapid sand filter 1. Coagulation Addition of Alum (5-40 mg/ litre ) 2. Rapid mixing Mixing chamber Violent mixing of alum (minutes )

30. STEPS OF Rapid sand filter 3. Flocculation Flocculation chamber Slow stirring of water by paddles ( 30 minutes) Flocculent ppt. of Aluminium Hydroxide entangles all particulate, suspended matter along with bacteria 4. Sedimentation Sedimentation chamber Flocculent ppt. settle down (removal is done from time to time) Clear water above goes for filtration

31. STEPS OF Rapid sand filter 5. Filtration Filter bed “Effective size” of the sand particles is 0.4-0.7 mm Graded gravel, 30 to 40 c m Depth of the water on the top of the sand bed is 1.0 to 1.5 m Rate of filtration is 5-15 m3/m2/ hr Remaining alum floc forms a slimy layer over sand bed, it holds back bacteria, oxidize organic matter Back washing: by air bubbles or water when floc layer becomes very thick, takes about 15 min

32. Rapid sand filter Cross-sectional view

33. ADVANTAGES OF RAPID SAND FILTER Rapid sand filter can deal with raw water directly. No preliminary storage is needed The filter beds occupy less space Filtration is rapid, 40-50 times that of a slow sand filter The washing of the filter is easy There is more flexibility in operation

34. Differences between slow and rapid filters Properties Rapid sand filter Slow sand filter Area Little space Large area Rate of filtration( m.g.a.d ) 200 2-3 Sand size (diameter) 0.4-0.7 mm 0.2-0.3 mm Pretreatment Coagulation & sedimentation Sedimentation Filter cleaning Backwashing Scraping Operation More skilled Less skilled Removal of colour Good Better Removal of bacteria 98-99% 99.9%-99.99%

35. 3. DISINFECTION

36. 3. Disinfection Criteria for satisfactory disinfectant Not influenced from properties of water within short time Should not be toxic and colour imparting or leave the water impotable Available, cheap, easy to use R esidual concentration to deal with recontamination Detectable by rapid, simple techniques in small concentration

37. ACTION of chlorination Kills pathogenic bacteria (no effect on spores and viruses) Oxidize iron, manganese and hydrogen sulphide Reduces taste and odours Controls algae Maintains residual disinfection

38. MOA of CHLORINATION H 2 O+Cl 2 ( at pH 7 )  HCl + HOCl (main disinfectant ) HOCl ( at pH > 8.5 )  H + + OCl - (minor action ) NH 3 + Cl 2  NH 2 Cl/ NHCl 2 / NCl 3 + H 2 O (Mono, Di, Tri Chloramines)

39. Principles of chlorination Water should be clear, free from turbidity Chlorine demand: Chlorine needed to destroy bacteria, to oxidize organic matter and to neutralize the ammonia in water Free residual chlorine for a contact period of 1 hour is essential Breakpoint: Point when chlorine demand of water is met and free residual chlorine appears Breakpoint chlorination: Chlorination beyond the breakpoint . The principle of break point chlorination is to add sufficient chlorine so that 0.5 mg/L free residual chlorine is present in the water after one hour of contact time Dose of Chlorine = Chlorine demand + Free residual chlorine M inimum recommended concentration of free chlorine is 0.5 mg/L for 1hr

40. METHODS of Chlorination C hlorine gas ( Paterson's chloronome ) Chloramine Perchloron or high test hypochlorite ( HTH)

41. Super chlorination Method of choice for highly polluted waters High dose of chlorine is added After 20 minutes of contact, dechlorination is done with sodium sulphate / sodium thiosulphate to reduce the taste of excess chlorine

42. Tests to measure Residual Chlorine Orthotolidine Test Yellow colour In 10 seconds - free chlorine In 15 min - both free and combined chlorine Orthotolidine Arsenite (OTA) Test Yellow colour Tests both free and combined chlorine separately Yellow colour due to nitrites, iron, manganese are overcome

43. Other disinfection methods Ozone Used in Europe and Canada Strong oxidizing agent Strong virucidal No residual effect Should be used with chlorination

44. Other disinfection methods UV Rays Used in UK Water should be clear No residual effect Expensive Chloramine Chlorine + Ammonia - Chloramine Less effective than chlorine

45. MEMBRANE PROCESSES High-pressure processes Lower-pressure processes

46. MEMBRANE PROCESSES High-pressure processes Reverse osmosis Rejects monovalent ions and organics of molecular weight >50 daltons Pore sizes <0.002 μm Desalination of brackish water and seawater Nanofiltration A llow monovalent ions such as sodium or potassium to pass but reject a high proportion of divalent ions such as calcium and magnesium Pore sizes are typically 0.001-0.01 μm Effective for the removal of colour -forming organic compounds

47. MEMBRANE PROCESSES Low-pressure processes Ultrafiltration Reject organic molecules of molecular weight above about 800 daltons P ore sizes 0.002 - 0.03 μm Microfiltration Pore sizes 0.01-12 μm capable of sieving out particles greater than 0.05 μm used for water treatment in combination with coagulation

48. REVIEW

49. Review 1 All are true for Rapid Sand Filter except (All India) No preliminary storage of water is required Operation requires skilled workers Frequent washing is not required Can be gravity or pressure type

50. Review 2 All are true for Rapid Sand Filter except (All India) No preliminary storage of water is required Operation requires skilled workers Frequent washing is not required Can be gravity or pressure type

51. Review 3 Disinfection action of chlorine in water is due to (All India) Hydrogen chloride Hypochlorous acid Hypochlorite ions Hydrogen ions

52. Review 4 Which of the following have residual germicidal effect in water disinfection? (AIIMS) Chlorine only Chlorine and ozone gas Chlorine and UV radiation Chlorine, ozone gas and UV radiation

53. Review 5 What is used to find the dose of bleaching powder required for disinfection of water? (AIIMS) Chloroscope Chloronome Horrock’s apparatus Winchester Quart Bottle

54. Review 6 Minimum recommended of dose of free residual chlorine for routine chlorination? (AIIMS) 0.5 ppm for 1 hr 0.5 ppm for 30 min 1.0 ppm for 1 hr 1.0 ppm for 30 min

55. Review 7 True statement regarding chlorination is (DPG) Orthotoulidine test measures combined chlorine separately Chlorine acts best when pH is 7 It kills bacteria, viruses and spores Hypochlorite ions are mainly responsible for disinfection

56. Review 8 Slow sand filter is differentiated from rapid sand filter by (PGI) Bacteria removed more effectively Skilled person is needed Cost of construction is cheaper Sand particle are smaller size Longer duration is needed

57. Review 9 Orthotoulidine test is done for Free chlorine Combined chlorine Fluorine Iodine

58. Review 10 Schmutzdecke refers to Suspended matter in drinking water Algae in drinking water Alum flocculate in surface of sand filter Algae, plankton, diatoms, bacteria on surface of sand filter

59. Thank you Email your queries to [email protected]