Seminar on purification of water on large scale and small scale(small scale - Less Detail) - Ref: K. Park

1. By Sanjay George
PURIFICATION OF WATER

2. INTRODUCTION
• Purpose of purification of water is to produce water that is safe and wholesome.
• Purification of water on large scale
• Purification of water on small scale

3. PURIFICATION OF WATER ON LARGE SCALE
• The method of treatment to be employed depends upon nature of raw water and desired
standards of quality.
• Components of water purification system
- Storage
- Filtration
- Disinfection

4. STORAGE
• Water is drawn out from the source and impounded in natural or artifical reservoirs.
• Optimum period of storage : 10 – 14 days
Advantages
• Storage provides a reserve of water from which further polution is excluded.
• By mere storage about 90% of suspended impurities settle down in 24 hours by
gravity, the water becomes clear allowing penetration of light.
• Aerobic bacteria oxidize organic matter present in the water with the aid of dissolved
oxygen. Free ammonia and nitrates drop.
• Pathogenic organisms eventually die out. Decrease in bacterial count by 90% within 5 – 7
days.

5. • If water is stored for long periods there is likelihood of development of algae which
imparts foul odor and color to water.

6. FILTRATION – SLOW SAND FILTERS
• Comprises of :
-Supernatant (raw) water
-A bed of graded sand
-An under-drainage system
-A system of filter control valves

8. SUPERNATANT WATER
• Depth : 1 – 1.5 meter
• Provides constant head of water so as to overcome the resistance of the filter bed and
thereby promote down flow.
• Provides waiting period (3- 12 hours) for raw water to undergo purification by
sedimentation, oxidation and particle agglomeration.
• Level of supernatant kept constant.

9. SAND BED
• Thickness: 1 meter
• Sand grains : Preferably rounded and have an effective diameter between 0.2 to 0.3 mm.
- Should be clean
• Sand bed supported by layer of graded gravel. (30 – 40 cm deep)
• Water percolates slowly.
• Purification processes : mechanical straining, sedimetation, adsorption, oxidation,
bacterial action.
• Rate of filtration: 0.1 – 0.4 m3/hour/sq. meter of sand bed surface.

11. Vital Layer:
• Schmutzdecke, zoogleal layer or biological layer
• Contains threadlike algae and numerous lifeforms like plankton, diatoms and bacteria.
• Process of formation called ripening of the filter.
• Takes several days to form
• Extends 2 – 3 cm into top portion of sand bed.
• Removes organic matter, holds back bacteria and oxidizes ammoniacal nitrogen into
nitrates.

12. UNDER-DRAINAGE SYSTEM
• Located at bottom of filter bed
• Consists of porous or perforated pipes.
• Serves dual purpose of providing outlet for filtered water and supporting filter medium
above.

13. FILTER BOX
• Rectangular open box 2.5 to 4m deep built wholly or partly below the grond.
• Supernatant water 1.5m
• Sand Bed 1.2m
• Gravel support 0.30m
• Filter Bottom 0.16m

14. FILTER CONTROL
• Venturi meter
• Measures bed resistance or “loss of head”
• When loss of head exceeds 1.3 meter its uneconomical to run the filter.

15. ADVANTAGES OF SLOW SAND FILTER
• Simple to construct and operate
• Cost of operation is cheap
• Quality of filtered water is high.
• Known to reduce bacterial counts by 99.9 – 99.99%

16. RAPID SAND FILTER
River
Mixing Chamber
Flocculation chamber
Sedimentation Tank
Filters
Clear water storage
Consumption
Alum
Chlorine
Coagulation

17. ADVANTAGES OF RAPID SAND FILTERS
• Can deal with raw water directly no preliminary storage required
• Filter beds occupy less space
• Filtration is rapid 40 – 50 times that of a slow sand filter
• Washing of filter is easy
• More flexibility in the operation

18. DISINFECTION
Criteria for disinfectant in water supplies:
• It should be capable of destroying pathogenic organisms present within contact time
available and should not be influenced by constituents present in water.
• Should not leave products of reaction which render water toxic or impart color.
• Should have ready and dependable availability at reasonable cost.
• Possess the property of leaving residual concentration to deal with possible
recontamination
• Be amenable to detection

19. CHLORINATION
• Kills pathogenic bacteria, no effect on spores and certain viruses.
• Also oxidizes iron, manganese and hydrogen sulphide, destroys some taste and odor
producing constituents
• Also controls growth of algae and slime organisms and aids coagulation.

20. PRINCIPLES OF CHLORINATION
• Water to be chlorinated should be free of turbidity
• Chlorine demand should be estimated:
Chlorine demand = Amount of chlorine added – residual chlorine
Point at which chlorine demand is met is called break point chlorination
• Contact period of at least one hour
• Minimum recommended concentration of free chlorine : 0.5mg/L for one hour
• The sum of chlorine demand of specific water plus the free residual chlorine of 0.5mg/L
constitutes the correct dose of chlorine to be added.

21. METHODS OF CHLORINATION
• Chlorine Gas
• Chloramine
• Perchloron

22. ADVANTAGES
Advantages
• Low cost
• Ease of application
• Kills almost all bacterial contaminants
Disadvantages
• Formation of halogenated compounds which are suspected carcinogens.

23. ALTERNATIVES
• Ozonation
• UV irradiation

24. PURIFICATION OF WATER ON SMALL SCALE
• Boiling
• Chemical disinfection
Bleaching powder
Chlorine solution
High test hypochlorite
Chlorine tablets
Iodine
Potassium permanganate
• Filtration
Pasteur Chamberland filter, Berkefeld filter, Katadyn filter

25. DISINFECTION OF WELLS
• Wells are main source of water supply in rural areas
• Need arises to disinfect them during epidemics of cholera, gastroenteritis etc.
• Most effective method is by bleaching powder.