waterpurification method-student copy 23-24
Note
Water Consumption & Purification
Purpose of water treatment is to produce safe and wholesome water.
Water purification methods depend on the nature of raw water and desired water quality.
Components of water purification system include storage, filtration, and disinfection.
Storage
Storage is a method of natural purification.
Storage reduces suspended impurities and bacterial count.
Optimum storage of river water is 10-14 days.
Excessive storage results in excessive algae growth, bad smell, and color to water.
Copper sulfate is used to control excessive growth of algae.
Purification of Water on Large Scale
Storage is followed by sedimentation to prevent rapid clogging of filters.
Filtration removes 98-99% of bacteria and impurities.
Slow sand filters use fine sand, while rapid sand filters use coarse sand.
Disinfection can be done through physical methods (boiling, UV radiation) or chemical methods (chlorination, ozonation).
Slow Sand Filters
Slow sand filters have four main elements: water head, sand bed, drainage system, and filter control valves.
The vital layer or "Schmutzdecke" is a slimy, gelatinous layer that forms on top of the sand bed and removes organic matter and holds back bacteria.
Slow sand filters are simple to construct and operate, and they have high-quality filtered water.
Rapid Sand Filters
Rapid sand filters can deal with raw water directly without preliminary storage.
They have a higher rate of filtration compared to slow sand filters.
The filter beds occupy less space and are easier to clean.
Rapid sand filters are more flexible in operation.
Comparison of Rapid and Slow Sand Filters
Rapid sand filters occupy less space and have a higher rate of filtration.
Slow sand filters occupy a larger area and have a lower rate of filtration.
Rapid sand filters require chemical coagulation and sedimentation as preliminary treatment, while slow sand filters only require plain sedimentation.
Rapid sand filters are cleaned by back-washing, while slow sand filters are cleaned by scraping the sand bed.
Water Chlorination
More than 98% of U.S. supply systems use chlorine-based disinfectants.
The effectiveness of chlorine-based disinfectants depends on the temperature, pH level, and clarity of water.
Chlorine acts more rapidly in acid water and its efficiency increases with higher temperatures.
Chlorine is not effective if the pH of water is above 7.2 or below 6.8.
Principles of Chlorination
Water should be clear and free from turbidity for efficient chlorination.
The chlorine demand of water should be estimated to determine the correct dose of chlorine.
The contact period for chlorine to kill bacteria and viruses is ½ to 1 hour.
The minimum free residual chlorine should be 0.3-0.5 mg/L or 0.5 ppm for safety against microbial contamination.
Solid forms of chlorine should never be added directly to the water supply. A paste should be made first.
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Various forms of Chlorine
Chlorine gas: chloronomes, chlorine gas cylinders
Chlorine powder, Bleaching powder or WSP(Water Sterilizing Powder): chlorine content = 33% + quick lime or calcium oxide
Chlorine tablets: Puritabs or Halazone tablets
Chlorine Stock solution: 3 tablespoons of WSP + 1 liter of water
Conversion rates and equivalents
One teaspoonful of WSP = 5 gm
Small safety box of WSP = 15 gm
One tablet of 5-mg sufficient for 20 litres of water
One scoopful of WSP when added to 100 gallons of water = dose of 1 ppm of chlorine
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Methods of Chlorination
Simple Chlorination
Chloramination
Breakpoint Chlorination
Super chlorination
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Simple chlorination
Dose determined by Horrock's apparatus
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Chloramination
Ammonia + chlorine ratio = 1:4 or 1:5
Long action of 2 hours
Germicidal action of 2 hours
Germicidal power less than chlorine
More stable, long acting
No objectionable odour or taste
Used in swimming pools
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Breakpoint Chlorination
Chlorine + natural ammonia = chloramines
Destruction of chloramines
Breakpoint: point at which free residual chlorine is available for continuous disinfection
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BREAK POINT CHLORINATION
FORMATION OF CHLORAMINE
DESTRUCTION OF CHLORAMINE
BREAK FREE RESIDUAL POINT CHLORINE
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Super chlorination
Addition of excess chlorine than requirement
Dechlorination: sulphur dioxide 1:1 or sodium thiosulphate 1.8:1
Each one gm of sodium thiosulphate removes 1 ppm of chlorine from 100 gallons or 455 liters of water
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SUPER CHLORINATION – CONT’D
Epidemics, heavily polluted waters
Kills viruses, ova, cysts
Provides a chlorine residual of 3-5 mg/L, 10 times the recommended minimum breakpoint chlorine concentration
Retention time: 5 minutes
Activated carbon filtration removes the high chlorine residual.
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Tests For Free Chlorine
Orthotolidine test: yellow colour matched with standard or colour discs
Orthotolidine – arsenite test: determines free and combined chlorine residuals separately
Starch Iodide test: blue colour indicates presence of chlorine
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OZONATION
Ozone is produced on site by the corona discharge of high-voltage electricity in to dry air or oxygen
Unstable gas
Powerful oxidizing agent, disinfectant than chlorine
No by-products
Strong virucidal effect
Eliminates odour, taste, colour
Employed in combination with chlorination usually
Disadvantage: ozone cannot be purchased, must be generated on-site
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OZONATION (cont’d)
Ozone as pretreatment of water is used to destroy viruses, bacteria & organic compounds
Dosage: 0.2 – 1.5 mg/L
More than 1000 municipal water treatment plants are utilizing ozonation
Drawback of Ozonation: no residual germicidal effect, expensive, energy intensive
Used in Europe and US to reduce the level of THMs in finished water
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Ultraviolet Irradiation
Uses light, UV rays, to kill microorganisms
Used in individual or institutional systems
Water should be free from turbidity or suspended impurities
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ULTRAVIOLET IRRADIATION – CONT’D
Advantages: exposure for shorter period, no taste or odour produced, no harmful effects, no requirement for addition of chemicals, no toxic by-products
Disadvantages: no residual effect, lack of rapid field test for assessing treatment efficiency, apparatus / maintenance expensive, does not kill giardia, cysts, worms, cloudy or turbid decreases effectiveness
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Purification of water on a small scale or Emergency Disinfection
Boiling: rolling boil for 5-10 minutes
Bleaching powder: dose determined by Horrock’s Test or 2-3 gm/1000 liter of clear water
Chlorine stock solution: Add 3 tablespoons (33gms) of WSP to one liter of water. Add 3 drops (0.6ml) of this solution to one liter of water.
Chlorine tablets: Halazone, Puritbas, one tablet in 1 liter of water.
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PURIFICATION OF WATER ON A SMALL SCALE OR EMERGENCY DISINFECTION
Iodine solution or tablets: 2 drops of 2% solution in 1 liter of water or one tablet in one liter of water
Filtration: Portable Water Filters for Emergency Relief
Pasteurization
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Disinfection of Wells
Find the volume of water in a well
Find the amount of Bleaching Powder required for disinfection by Horrock’s test
Dissolve Bleaching powder in water bucket
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Disinfection of Wells
Deliver chlorine solution to water
Contact period: one hour
Test for residual chlorine (0.5 ppm)
Wells best disinfected at night
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HORROCKS TEST
Used to determine the smallest dose of Water Sterilizing Powder (WSP) in standard scoopfuls needed to sterilize 455 liters (100 gallons) of a water sample
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HORROCKS TEST
Contents of Horrocks Apparatus: Six white cups, One black cup, Two metal scoops, Seven glass sterilizing rods, One special pipette, Two droppers, Cadmium Iodide starch indicator solution
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HORROCKS TEST (cont’d)
Procedure:
Prepare stock solution by taking one level of scoopful of WSP in black cup
Fill six white cups with water to be tested
Add one drop of stock solution to first cup, two drops in second cup, 3 drops to third cup & so on
Stir the water & wait for half an hour
Add 3 drops of starch iodide indicator solution to each of white cups
Development of blue colour indicates free residual chlorine
Dose for super chlorination: H+1
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HORROCKS TEST (cont