Lecture on Purification of Water by Dr. Fazal MahmoodM.B.B.S(Pb), M.C.P.S(Pak)
Purification on Large scale When sources of water are Rivers,Streams,Lakes etc. then water is Purified by Storage Sedimentation Filtration(rapid sand & slow sand) Disinfection(chlorination)
Purification on Medium Scale When sources of water are wells,Springs,Tanks etc.Then water is purified by the addition of Bleaching powder/Chlorinated Lime as it is Cheep Easy to use Reliable and safe
Purification on Small Scale(Domestic Methods) BOILING: for 5-10 minutes kills almost all organisms & removes temporary Hardness. DISTILLATION: not commonly used due to higher cost, used in Labs. etc. ADDITION OF CHEMICALS Bleaching Powder: 5% solution is used Dose: 3-6 drops/L contact time of ½ hour.
Contnd.----------- Chlorine Tablets/Halazone Tablets. one tablet/litre. Iodine Solution: 02 drops of 2% Soln./litre KMnO4: an amount that gives just pink coloration to the Water. Alum: used for turbid water in a dose of 0.1-0.4 grains/5 litres of Water.
Purification of water in Emergency/Epidemic Boiling: for 5-10 minutes. High Test Hypo chlorite: used in granular form Dose: 01gm./10 litres of water. Liquid Bleach/5% Sodium Hypo chlorite solution: Dose: 14 ml./1000 litres of water. Bleaching Powder: used in a dose of 2.3gm./1000 litres of water.
Purification of water on Large Scale On large scale purification, water is disinfected by addition of Chlorine after filtration. On large scale, water is purified through the following processes.
Contnd.---------A.STORAGE: Not a sole measure of purification but valuable as a preliminary to other processes. There is fall in ammonia & rise in nitrates due to oxidation of organic matter by aerobic bacteria.
Contnd.---------- Sedimentation, may be hastened by the addition of Alum in a dose of 35 mg./ litre for a detention period of 4-6 hrs. Sedimentation will prevent clogging of the filters. Limitation to storage includes algae growth which can be controlled by addition of CuSo4 in a dose of 1-5 Kg. / 5000 cu.m.
Contnd.---------B. FILTRATION: Filtration through sand is the oldest and universally, most accepted method through out the world. 98-99% bacteria are removed along with other impurities through filtration.
Slow Sand Filtration Firstused in 1804 in Scotland & subsequently in London. During 19th.Century used through out the world. Slow sand filter consists of concrete/brick work rectangular basin containing carefully selected graded sand supported on gravel and stones.
Contnd.----------- ELEMENTS:1. Supernatant (raw) water: Upper most layer of raw water(1-1.5M) It provides a constant head of water It provides a waiting period of 3-12 hours
Elements contnd.-----2. Sand Bed: It is 1 M thick supported by a layer of fine and then coarse gravel and finally by the stones.3. Drainage System for filtered water: It consists of perforated pipes4. Filter Control Valves: Valves & Devices incorporated in the outlet- pipe system to maintain a constant rate of filtration Venturi meter measures bed resistance or loss of head.
Actions of slow sand filter Physical: Mechanical straining/sedimentation Chemical: Oxidation of organic matter by aerobic bacteria. Biological: Occurs through “Vital layer” Efficiency of the filter depends upon the “Vital layer”
Vital Layer Slimy growth known as “Schmutzdecke”, Vital layer, zoogleal layer or biological layer The layer is slimy & gelatinous,It consists of threadlike algae & numerous forms of life plankton, diatoms and bacteria Ripening of the filter Vital layer is the “heart” of slow sand filter
ADVANTAGES Simple to construct and operate Cost of construction cheaper than rapid sand filter Process: physical, chemical and biological Bacterial count reduction is 99.9% to 99.99% and E.coli reduction is 99% to 99.9%
DISADVANTAGES Old fashioned and outdated method of water purification (but still in use) Initial cost is low but maintenance cost is much more than rapid sand filter These filters need a lot of space
RAPID SAND FILTERS In 1885, first rapid sand filters were installed in USA, since that time they are gaining considerable popularity in highly industrialized countries as well as developing world.
Contnd.--------- TYPES:1. Open / Gravity type (Peterson’s Filters)2. Closed / Pressure type (Candy’s Filters)
Contnd.---------- STEPS / PROCESSES IN RAPID SAND FILTERS1. COAGULATION: Raw water is first treated with Alum in a dose of 5-40 mgs./L pH has to be adjusted by addition of lime or soda ash as Alum needs CaCo3 to form the flock.
Contnd.---------2. RAPID MIXING: Water is mixed rapidly with Alum for a few minutes in mixing chamber.3. FLOCCULATION: From mixing chamber water is moved into the Flocculation chamber where it rests for half an hour.
Contd.--------4. SEDIMENTATION: The coagulated water is now led into the sedimentation tank where it is detained for 2-6 hours. Flocculant precipitate together with impurities & bacteria settle down in the tank
Contnd.--------5. FILTRATION Partially clarified water is now subjected to the rapid sand filtration. Depth of the sand bed is 1 M & each rapid sand filter unit has a surface of about 80-90 Sq.M. Oxidation of NH3 also takes place in these filters.
ADVANTAGES Rapid sand filters deal with raw water directly Occupies less space Filtration is rapid about 40-50 times as compared to slow sand filters Washing of filter is easy More flexibility in operation
DISADVANTAGES Preliminary treatment with Alum is required Operation is highly skilled Removal of color material is less than slow sand filter Reduction of bacterial count is less than slow sand filter
Comparison of Rapid and Slow sand filters Rapid.S.F. Slow.S.F.1. Space: Occupies little space-----Occupies large area2. Rate of filtration: 5-15 cu.m./sq.m./hr.----- 0.1-0.43. Effective sand size: 0.4-0.7mm-----------------0.2-0.3mm4. Preliminary treatment: Coagulation& sedimentation--plain Sediment5. Washing: By back washing--------By scraping the sand6. Loss of head allowed: 6-8 feet--------------------4feet7. Removal of turbidity: Good-----------------------Good8. Removal of color: Good------------------------Fair9. Removal of bacteria: 98-99 %--------------------99.9-99.99
DISINFECTION OF WATER CHEMICAL METHODS / GERMICIDES1. CHLORINE: is used for purification of water on large scale2. KMnO4: used for disinfection of water in wells especially during epidemics3. CuSO4/CaCl2: used for the removal of algae4. Lime: used for muddy water5. Ozonization & U.V. irradiation: both methods are used for disinfection of water in swimming pools6. Katadyn Silver: sand coated with silver used in slow sand filters instead of plain sand7. Charcoal: used as adsorbent to remove color,odor & taste
Disinfection of Wells The most effective & cheapest method of disinfecting wells is by bleaching powder STEPS IN WELL DISINFECTION :1. Find the volume of water in a well:a. Measure the depth of water column..(h) Mb. Measure the diameter of well………(d) Mc. Substitute h and d in:
Cotnd……. 3.14 x (d)2 x hVolume(litres)=------------------------------ x 100 4d. One cubic meter = 1,000 litres of water Find the amount of bleaching powder required for disinfection: Estimate the Chlorine demand of well water by HORROCK’S APPARATUS
Contnd……..3. Dissolve Bleaching Powder in water4. Delivery of Chlorine solution into the well5. Contact period6. Orthotolidine arsenite test
Chlorination of water It is the most widely used method for water purification, both on large as well as on small scale. It is the most reliable and cheapest method. It is supplement but not the substitute to sand filtration.
Properties of Chlorine Killing of pathogenic bacteria by inhibiting their protein synthesis. Oxidizes Iron, Manganese & Hydrogen Sulphide. Destroys taste & odor producing constituents. It controls algae growth. It helps in coagulation and suspension of the organic matter.
Action Of Chlorine When chlorine is added to water it forms HCl & HOCl. The disinfecting action of chlorine is mainly due to Hypochlrous acid & to a small extent due to Hypochlorite Ions. Hypochorous acid is 70-80 times more effective than Hypochlorite Ions.
Actions of Chlorine depends upon the…….. Organic matter Metals Bacterial content Temperature & pH of water. Suspended matter and, Amount of free residual chlorine in water
Contnd……… Efficiency of chlorine increases, when it combines with Ammonia and…… Itdecreases, when it combines with Sulphides, Ferrous salts & inert matter in suspension form
Principles of Chlorination Water to be chlorinated must be free from turbidity Chlorine demand of water should be estimated Contact period of one hour is essential to kill the bacteria and viruses Minimum recommended concentration for free chlorine is 0.5mg. / liter
Chlorine demand of water Is the difference between the amount of chlorine added to water and the amount of residual chlorine remaining at the end of a specific period of contact (usually 60 min.) at a given temperature and pH of water. ACUAL DOSE OF CHLORINE: It is the sum of chlorine demand of water and free residual chlorine
Methods of Chlorination1. Chlorine gas2. Chloramine3. Perchloron (H.T.H.) TYPES OF CHLORINATION:a. Superchlorinationb. Break Point Chlorination
Tests for the presence of Chlorine in Water Orthotolidine (O.T.) Test Orthotolidine-Arsenite (O.T.A.) Test Starch Iodide Test
Surveillance of drinking Water1. Sanitary survey2. Sampling3. Bacteriological surveillance4. Biological examination5. Chemical surveillance
Sampling of Water Collection, Storage & Transportation of water samples from…… Taps Rivers Streams Tanks WellsA. For Physical & Chemical ExaminationB. For Bacteriological Examination PERTICULARS OF A SAMPLE
Standards of Water PHYSICAL STANDARDS Turbidity Color Odor Taste
Contnd…….. Toxic substances and a specific group of chemicals causing Non-Specific diseases of water Poly-nuclear Aromatic Hydrocarbons and Radioactive substances ( a group of chemical substances of water affecting the acceptability of water.
Bacteriological Standards1.BACTERIAL STANDARDS : W.H.O. recommended these standards in 1984 for Public water supplies Ideally all samples taken from the distribution system should be free from Coliform organisms In Practice this standard is not always attainable
Recommended Standards for Water samples collected Throughout any year 95 % samples 100 ml. each should not contain any Coliform organisms No samples should contain E.coli No sample should contain more than 03 Coliform Any two consecutive samples should not contain Coliform
Contnd……… Incase of Individual / Small water supplies such as from household Tap, Well, Spring etc. Coliform count should not exceed 10 /100 ml. and E.coli should be nil
Contnd………..2. VIRAL STANDARDS : W.H.O. Standards fix the limit for viruses such as Eneroviruses, Retroviruses & Adenoviruses in water supplies at 01 Plaque forming unit Faecal Bacteroiphage & Enteropathogenic viruses should be completely absent
Bacteriological Indicators These are based on organisms indicative of faecal pollution, These organisms include!1. E.coli and Coliform group as a whole2. Faecal Streptococci3. Clostridium Perfringens4. Examination for Protozoa, Parasitic worms & Larvae is also recommended
Contnd…….. Reasons why Coliform organisms are chosen as indicators of Faecal Pollution1. Constantly present in human intestine an average person excretes 200-400 billions / day of these organisms in faeces2. They are foreign to potable water supplies3. Easily detectable by ordinary cultural methods4. Survival time is much better than other organisms5. Have greater resistance to natural forces of purification
Contnd…….. FAECAL STREPTOCOCCI : Faecal Strepococci occur regularly in much smaller number than E.coli Their presence is regarded as confirmatory evidence of recent faecal contamination of water in doubtful cases
Contnd……… CLOSTRIDIUM PERFRINGENS : They are spore bearing organisms Their presence along with Coliform indicate recent water contamination While the presence of their spores but the absence of Coliform confirms remote contamination of water