Purification of water

purification of water on a large and small scale

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 Storage Filtration D isinfection

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 m 3 /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 Chamber Flocculation Chamber Sedimentation 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 A luminium 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

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

20. 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

21. 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 %

22. 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 H 2 O + Cl 2  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

23. 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

24. 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

25. 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

26. 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 1 st cup, 2 drops to 2 nd , 3 to 3 rd … 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 1 st cup which show distinct blue colour . Suppose 3 rd cup show blue colour 1 st  3 level spoonfuls/6 g of bleaching powder would be required to disinfect 455 lit of water.

27. 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.)

28. 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

29. 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

30. 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

31. 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

32. 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

33. Purification of water on a small scale

34. Household purification of water 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

35. 2.Chemical disinfection: 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)

36. 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.

37. 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.

38. 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

39. 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

40. 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.