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
• Describe 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
1. Storage
2. Filtration
• Slow sand filter
• Rapid sand filter
3. Disinfection
• Chlorination
• Ozonation
• Other agents
• Membrane processes

7. WATER PURIFICATION
• Small scale
1. Household purification
• Boiling
• Chemical disinfection: Bleaching powder, Chlorine solution , High test hypochlorite
(HTH), Chlorine tablets, Iodine, Potassium permanganate
• Household filtration
2. 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

12. 2. FILTRATION
• Water passed through porous media
1. Slow sand filter: biological
2. Rapid sand filter: mechanical

13. 2A. SLOW SAND FILTER

14. SLOW SAND (BIOLOGICAL) FILTERS
• Used first in 19th century in Scotland
• Elements of slow sand filter
1. Filter Box
a) Supernatant water
b) Sand bed
c) Under drainage system
2. Filter control valves

15. SLOW SAND
(BIOLOGICAL)
FILTERS
General plan and
layout

16. SLOW SAND
(BIOLOGICAL)
FILTERS
Parts in general

17. SLOW SAND
(BIOLOGICAL)
FILTERS
Cross-sectional view
of the filter bed

18. ELEMENTS OF SLOW SAND FILTER
• Supernatant water
• Sand bed
• Under drainage system
• Filter control valves

19. 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

20. 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

21. 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
• Suspended particles are retained by adhesion to the biological layer
• Removes organic matter, holds back bacteria and oxidizes ammoniacal
nitrogen in to nitrates

22. SCHMUTZDECKE

23. 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

24. 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

25. ADVANTAGES OF SLOW SAND FILTER
1. Simple to construct and operate
2. Construction is cheaper than rapid sand filters
3. Physical, 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)

26. 2B. RAPID SAND FILTER

27. RAPID SAND FILTER
• First in1885 in USA
• Gravity type (Open)/ Paterson’s
• Pressure type (Closed)/ Candy’s

28. RAPID SAND FILTER

29. OVERVIEW OF RAPID SAND FILTER

30. STEPS OF RAPID SAND FILTER
1. Coagulation
• Addition of Alum (5-40 mg/litre)
2. Rapid mixing
• Mixing chamber
• Violent mixing of alum (minutes)

31. 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

32. 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 cm
• 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

33. RAPID SAND FILTER
Cross-sectional view

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

35. 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%

36. 3. DISINFECTION

37. 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
• Residual concentration to deal with recontamination
• Detectable by rapid, simple techniques in small concentration

38. 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

39. MOA OF CHLORINATION
• H2O+Cl2 (at pH 7)  HCl + HOCl (main disinfectant)
• HOCl (at pH > 8.5)  H+ + OCl- (minor action)
• NH3 + Cl2  NH2Cl/ NHCl2/ NCl3+ H2O (Mono, Di, Tri
Chloramines)

40. PRINCIPLES OF CHLORINATION
1. Water should be clear, free from turbidity
2. Chlorine demand: Chlorine needed to destroy bacteria, to oxidize organic matter
and to neutralize the ammonia in water
3. Free residual chlorine for a contact period of 1 hour is essential
4. Breakpoint: Point when chlorine demand of water is met and free residual
chlorine appears
5. 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
6. Dose of Chlorine = Chlorine demand + Free residual chlorine
7. Minimum recommended concentration of free chlorine is 0.5 mg/L for 1hr

41. METHODS OF CHLORINATION
• Chlorine gas (Paterson's chloronome)
• Chloramine
• Perchloron or high test hypochlorite (HTH)

42. 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

43. 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

44. OTHER DISINFECTION METHODS
• Ozone
• Used in Europe and Canada
• Strong oxidizing agent
• Strong virucidal
• No residual effect
• Should be used with chlorination

45. OTHER DISINFECTION METHODS
• UV Rays
• Used in UK
• Water should be clear
• No residual effect
• Expensive
• Chloramine
• Chlorine + Ammonia - Chloramine
• Less effective than chlorine

46. MEMBRANE PROCESSES
• High-pressure processes
• Lower-pressure processes

47. 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
• Allow 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

48. MEMBRANE PROCESSES
• Low-pressure processes
• Ultrafiltration
• Reject organic molecules of molecular weight above about 800
daltons
• Pore 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

49. REVIEW

50. REVIEW 1
• All are true for Rapid Sand Filter except (All India)
a) No preliminary storage of water is required
b) Operation requires skilled workers
c) Frequent washing is not required
d) Can be gravity or pressure type

51. REVIEW 2
• All are true for Rapid Sand Filter except (All India)
a) No preliminary storage of water is required
b) Operation requires skilled workers
c) Frequent washing is not required
d) Can be gravity or pressure type

52. REVIEW 3
• Disinfection action of chlorine in water is due to (All India)
a) Hydrogen chloride
b) Hypochlorous acid
c) Hypochlorite ions
d) Hydrogen ions

53. REVIEW 4
• Which of the following have residual germicidal effect in water
disinfection? (AIIMS)
a) Chlorine only
b) Chlorine and ozone gas
c) Chlorine and UV radiation
d) Chlorine, ozone gas and UV radiation

54. REVIEW 5
• What is used to find the dose of bleaching powder required for
disinfection of water? (AIIMS)
a) Chloroscope
b) Chloronome
c) Horrock’s apparatus
d) Winchester Quart Bottle

55. REVIEW 6
• Minimum recommended of dose of free residual chlorine for routine
chlorination? (AIIMS)
a) 0.5 ppm for 1 hr
b) 0.5 ppm for 30 min
c) 1.0 ppm for 1 hr
d) 1.0 ppm for 30 min

56. REVIEW 7
• True statement regarding chlorination is (DPG)
a) Orthotoulidine test measures combined chlorine separately
b) Chlorine acts best when pH is 7
c) It kills bacteria, viruses and spores
d) Hypochlorite ions are mainly responsible for disinfection

57. REVIEW 8
• Slow sand filter is differentiated from rapid sand filter by (PGI)
a) Bacteria removed more effectively
b) Skilled person is needed
c) Cost of construction is cheaper
d) Sand particle are smaller size
e) Longer duration is needed

58. REVIEW 9
• Orthotoulidine test is done for
a) Free chlorine
b) Combined chlorine
c) Fluorine
d) Iodine

59. REVIEW 10
• Schmutzdecke refers to
a) Suspended matter in drinking water
b) Algae in drinking water
c) Alum flocculate in surface of sand filter
d) Algae, plankton, diatoms, bacteria on surface of sand filter

60. THANK YOU
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