The document discusses various methods for purifying water on large and small scales. For large scale purification, it describes the steps of storage, filtration, and disinfection. It compares slow sand and rapid sand filtration methods. For small scale purification, it outlines boiling, chemical disinfection using chlorine or iodine, filtration using ceramic filters, ultraviolet irradiation, and reverse osmosis.

1. Purification of water
Dr. Moumita Pal
MBBS,DPH, MD
Dept. of Community Medicine
College of Medicine and Sagore Dutta Hospital

2. Purification on a large scale
1. Storage
2. Filtration
3. Disinfection

3. Storage
• Natural/artificial reservoirs
• Purification in terms of-
1. Physical: 90% suspended impurities settle
down by gravity in 24 hrs
2. Chemical: aerobic bacteria oxidize organic
matter. From free ammonia to nitrates.
3. Biological : drop in bacterial count
90% bacteria die out in 5-6 days.

4. Filtration
• 98-99% bacteria + impurities removed
• 2 types are there:
1. Slow sand/ Biological filter
2. Rapid sand /mechanical filter

5. Slow sand /Biological filter
• Elements are: 1. Supernatant water
2. Bed of graded sand
3. Under drainage system
4. System of filter control valves

6. Supernatant water
• Above the sand bed
• 1-1.5 meter deep
• Constant head of water to overcome
resistance
• 3-12 hrs waiting period for partial purification

7. Sand bed
• Most important part
• Thickness 1 mt
• Sand grain rounded with effective diameter B/T
0.2-0.3 mm
• Clean, free from clay/organic matter
• Supported by gravel layer ( 30-40 cm)
• Water percolates very slowly ( 0.1-0.4 m3/hr/sq
mt)
• Purification process: mechanical straining
sedimentation, adsorption, oxidation,
bacterial action

8. Vital layer
• Surface of sand bed get covered
• Slimy growth/ gelatinous: consist of algae, plankton,
diatoms, bacteria
• Known as SCHMUTZDECKE /vital/zoogleal/biological
layer
• Formation of vital layer: Ripening of filter
• 2-3 cm
• Heart of the filter
• Removes organic matter, hold back
bacteria, oxidize ammoniacal nitrogen into
nitrates

9. Under drainage system
• Bottom of filter bed
• Consists of perforated pipes
• Outlet for filtered water & supports medium
FILTER BOX: open box, 2.5-4 mt deep, walls made
up of bricks/cement
Supernatant water:1-1.5 mt
Sand bed: 1.2 mt
Gravel: 0.30 mt
Filter bottom: 0.16 mt

10. Filter control
• Valves/ devices in outlet system
• Maintain constant rate of filtration
• Venturi meter: measures resistance or Loss of
head
• Resistance built up= opening of valves
• Loss of head > 1.3 mts = uneconomical to run
filter

11. Slow sand Filter

12. Cleaning
• Normally run weeks/ months without cleaning
• Bed resistance increases= fully opening of valve=
time to clean
• Supernatant water is drained off
• Sand layer cleaned by SCRAPING off top layer (1-2
cm)
• Done by unskilled laborers
• After 20-30 scraping sand bed reduced
0.5-0.8 mts= time to close the plant

13. Advantages
• Simple to construct
• Cheaper
• Quality of water is very high ( Total Bacterial
count reduced by 99.9 to 99.99% with E.coli)

14. Rapid / Mechanical Filter
• 2 types: 1. Gravity type (Paterson’s filter)
2. Pressure type (Candy’s filter)
Steps involved: 1. Coagulation
2. Rapid Mixing
3. Flocculation
4. Sedimentation

15. Flow diagram of rapid sand filter plant
River
Alum
Consumption
Chlorine
Mixing
Chamb
er
Floccul
ation
Chamb
er
Sedime
ntation
Tank Filters
Clear
water
storage

16. • Coagulation: raw water treated with alum. Dose 5-40
mg/lit.( depending upon turbidity/colour/ pH/ temp)
• Rapid mixing: subjected to violent agitation in mixing
chamber
• Flocculation: slow stirring in a flocculation chamber
for 30 mins. Formation of thick Aluminium hydroxide
• Sedimentation: detained in sedimentation chamber
for 2-6 hrs. flocculent precipitate with
impurities.
• Filtration: partly clarified water is now
subjected to rapid sand filtration

17. Filter beds
• Sand is filter medium
• Effective size 0.4-0.7 mm
• Depth is 1 mt
• Below sand bed layer of graded gravel
• Depth of water is 1-1.5 mt
• Rate of filtration: 5-15 m3/m2/hr

18. Filtration
• Alum floc, not removed by sedimentation :
held back on sand bed. form slimy layer
comparable to zoogleal layer.
• Adsorb bacteria, oxidation of ammonia
• Loss of head up to 7-8 feet: filter subjected to
wash named BACKWASHING need
daily/weekly washing, by reversing
the flow of water ( dislodges the
impurities). Takes 15 mins

20. Advantage
• Can deal with raw water directly
• Occupies less space
• Filtration is rapid
• Washing is easy

21. Disinfection
Chemical having the criteria:
• Capable of destroying pathogenic organism within the
contact time
• Not unduly influenced by range of physical /chemical
properties of water
• Not leave products of reaction which make water toxic
• Available and reasonable cost
• Leaving residual concentration to deal with
possible contamination
• Amenable to detection by rapid/simple tests

22. Comparison of rapid and slow sand
filter
Features Rapid sand filter Slow sand filter
Space Occupies less space Occupies large space
Rate of filtration 200 m.g.a.d 2-3 m.g.a.d
Effective size of sand 0.4-0.7 mm 0.2-0.3 mm
Preliminary treatment Chemical coagulation and
sedimentation
Plain sedimentation
Washing Back washing Scraping the sand bed
Operation Highly skilled Less skilled
Loss of head allowed 6-8 feet (2-2.5 m) 4 feet (1.5 m)
Removal of turbidity Good Good
Removal of colour Good Fair
Removal of bacteria 98-99 % 99.9-99.99 %

23. Chlorination
• Supplement and not substitute of sand filtration
• Kills pathogenic bacteria but no effect on spores and
viruses (except high dose)
• Oxidizes iron/manganese /hydrogen sulphide
• Controls algae
• H2O + Cl2  HCl + HOCl
• HOCl H + OCl
• HCl is neutralized by alkalinity of water
• Disinfection action due to HOCl and OCl
• Most effective formHOCl
• Acts best at pH 7 as more HOCl

24. Principles of Chlorination
1. water should be clear/ free of turbidity
2.Chlorine demand should be estimated.
• Amount of Cl added- residual Cl at end of
contact period (60 Min) at a given temp/pH
• At which point the Cl demand of water is met
called Break point
• If further Cl added after this it will
appear as free Cl

25. 3. Presence of free residual Cl for a contact
period of 1 hr is essential to kill bact/viruses.
4. Min recommended free Cl is 0.5 mg/L for 1 hr.
Gives margin of safety for subsequent
contamination.
5. correct dose of Cl = Cl demand+ 0.5 mg/L
residual Cl

26. Calculation of Chlorine demand
• By Horrock’s apparatus
• To find out dose of Bleaching powder required for
disinfection
• Contents: 1. 6 white cups (200 ml each)
2. one black cup with a circular mark inside
3. 2 metal spoons (2 g when level filled)
4. 7 glass stirring rods
5. One special pipette
6. Two droppers
7. starch-iodide indicator solution
8. Instruction folder

28. Procedure
1. one level spoonful (2g) bleaching powder in black cup 
make a thin paste with little water  add water up to
circular mark & stirring  allow to settle  Stock solution
2. Fill 6 white cups with water to be tested
3. With special pipette add stock solution 1 drop to 1st cup, 2
drops to 2nd, 3 to 3rd…
4. Stir the water with different rods
5. Wait 30 mins for action of Cl
6. Add 3 drops of starch iodide indication to each cups.
Development of Blue colour  free Cl
7. Note the 1st cup which show distinct blue colour.
Suppose 3rd cup show blue colour 1st 3 level
spoonfuls/6 g of bleaching powder would be
required to disinfect 455 lit of water.

29. Method of Chlorination
• Cl added as :
1. Chlorine gas (Cheap/easy/Chlorinating
equipment: Paterson’s chloronome)
2. Chloramine (less chlorinous taste/slower action)
3. Perchloron ( Ca compound with 70 %
available Cl.)

30. Break point Chlorination
• Point at which free residual Cl appears after
entire combined Cl residual has been
completely destroyed  Break point
• The point when Cl demand of water is met 
Break point Chlorination
• If Cl is added further it will only increases the
free Cl

31. Superchlorination
• Super-chlorination followed by de-chlorination
• Addition of large dose of chlorine and removal
of excess of it after disinfection
• Applicable for heavily polluted water

32. Orthotolidine (OT) Test
• Both free n combined Cl can be determined
• reagent= Orthotolidine (desolved in 10% sol. Of
HCl)
• Water containing Cl+ reagent= yellow
colour(intensity varies with conc. )
• Reaction with free Cl is fast
• 0.1 ml reagent+ 1 ml water=yellow color matched
with color disc.
• Reading: within 10 sec=free Cl
after 15-20 min=both free and
combined Cl

33. Orthotolodine-arsenite (OTA) Test
• Modification of OT test to identify free and
combined Cl separately
• Error caused by iron/nitrites/ manganese by
producing yellow color can be overcome

34. Ozonation
• Powerful oxidizing agent
• Removes undesirable colour/ odour/ taste/
organic matter
• Can destroy viruses
• Dosage 0.2-1.5 mg/L
• No residual effect. So used in combination of
Chlorination ( minimal dose)
• High cost
• Onsite generation due to instability

35. Purification of water on a small
scale

36. Household purification of water
1. Boiling – rolling boil for 10-20 min
Kills bact/ spores/ cysts/ ova
Removes temporary hardness
Taste can be altered
No residual protection
Should be stored in same
container where boiled

37. 2.Chemical disinfection:
A. Bleaching powder: Ca(OCl)Cl, if freshly
made=33% available Cl, unstable(on
exposure to air/light/moister losses Cl
content. Stored in dark, cool, dry place in
closed container)

38. B. Chlorine solution : prepared from bleaching
powder. 4 kg of bleaching powder with 25 %
available chlorine mixed with 20 lit of water
gives 5% solution of chlorine.
C. High test hypochlorite: perchloron. More
stable.
D. Chlorine Tablets: (Halazone Tablet).
Good but costly. Single tablet of 0.5 g
is suficient to disinfect 20 lit of water.

39. E. Iodine:- emergency disinfection of water.
- 2 drops of 2% ethanol solution is suffice
for 1 lit of water.
- contact time needed 20-30 min.
- High cost
- Physiologically active (thyroid activity)
F. Potassium permanganate: No longer used.
Changes color/smell/taste of water.

40. 3. Filtration: ceramic filters –Pasteur
Chamberland filter, Berkefeld filter, Katadyn
filter.
- Main part is candle ( porcelin /infusorial earth)
- In Katadyn: Surface covered with silver
catalyst, bact. destroyed in contact with silver
ion (oligodynamic action)
- can remove bact. Not virues

41. 4. Ultraviolet irradiation: can destroy bact. Viruses,
yeast, fungi, algae, protozoa
- Mercury vapor arc lamps emitting UV rays at a
wave length of 254 nano mt
- Water should be free from turbidity/ colloidal
suspended constituents
- Short exposure required, no foreign
matter added, no taste /odour change
- No residual effect

42. 5. Multistage Reverse osmosis purification of
water:
- Remove total dissolved solid, hardness, heavy
metals, bacteria, viruses, protozoa, cysts.
- Clarity cartridge removes suspended particles
( dust/mud/sand)
- The reverse osmosis cartridge removes
dissolved solid/hardness/heavy metals/
micro organism.