Raw water should be treated to make it potable/fit for drinking. So a line of treatments should be followed to treat the water. After Coagulation and sedimentation the process of filtration and disinfection are followed.
Plant: In this plant, the atmospheric air is dried.Generator: The function of a generator is to create ozone in the dry airInjector: the injector mixes water with the ozonised air.Tank: a verical tank with baffles is provided to keep water and ozonized air in contact for a long period.The bacteria are destroyed and the water of a sufficiently high standard of bacterial purity for public supply purposes is obtainedThe colour of water which is not removed by other methods of treatment is reducedThe decomposition of ozone into oxygen indicates the absence of any foreign subtance in water
Invisible light rays beyond the violet spectrum are very effective in killing all types of bacteria.Does not develop any taste or colour.There is no danger or overdose.Unsuitable for large scale treatment plants.Can be adopted for private institutions especially of swimming pools.
Filtration and disinfection of water
FILTRATION ANDDISINFECTION OF WATER By, M.Vinod kumar, 09011BB047.
FILTRATION OF WATER“In order to remove or to reduce the contents of impurities stillfurther, the water is filtered through the beds of fine granularmaterial like sand”THEORY OF FILTRATION:• Mechanical straining• Sedimentation• Biological metabolism• Electrolytic changes
FILTER SANDFilter sand is classified on the basis of effective size and uniformitycoefficient.• The effective size is the size of sieve through which 10% of the sample of sand by weight will pass.• Uniformity coefficient: sieve size/ effective sizeCLASSIFICATION OF FILTERS: Slow sand Gravity filters filters Rapid sand Filters Pressure filters filters
SLOW SAND FILTERS:Purpose:The water is allowed to pass slowly through a layer of sand placedabove the base material and thus the purification process aims atsimultaneously improving the biological, chemical, and physicalcharacteristics of water.
Essential parts: Enclosure tank: -stone masonry/brick masonry -waterproof material -bed slope is 1 in 100 to 1 in 200 -depth of the tank is 2.5 m to 3.5 mUnderdrainage system: Central drain Lateral drain
Lateral drain:• Placed at a distance of about 2.5 to 3.5 m.• Stopped at a distance of 500 mm to 800 mm from walls of the tank.• Open joint pipesBase material:• Placed on top of the underdrainage system• Varies from 300 mm to 700 mm depth• Graded and laid in layers of 150 mm LAYER DEPTH OF SIZE OF LAYER GRAVEL Topmost 150mm 3mm- 6mm Intermediate 150mm 6mm- 20mm 150mm 20mm- 40mm Lowest 150mm 40mm- 65mm Total 600mm depth
Filter media:• Sand layer is placed above the gravel of layer depth 600 mm to 900 mm.• Size of the sand varies from 0.2 mm to 0.3 mm.• Uniformity coefficient of sand is 2 to 3.• The finer the sand, the better will be the filtration.Working:• The water is allowed to enter the filter through the inlet chamber. It descends through the filter media and during this process, it gets purified.• These filters are usually worked for a maximum filtration head of 750 mm.
Cleaning:• Top layer of the sand is removed to a depth of 15 to 25 mm• Effective depth of filter media is reduced• Fresh layer of 150mm depth of graded sand is addedRate of filtration:100 to 200 liters per hour per m² of filter areaEfficiency:• Bacterial load: 99.50 to 99.9 percent is removed•Colour: less efficient in removal of colour(20-25%)•Turbidity: can remove turbidity of 50 p.p.m
Filter media of sand:• Filtering medium: sand ( effective size of sand particles 0.4-0.7 mm, coarse sand)• Depth of sand bed: 1-1.5 feet deep• Clogging of filters by suspended impurities and bacteria: Loss of Head• Cleaning by back-washing daily or weekly for 15 minutes
Layout of typical rapid sand filter(gravity type)Advantages:• Rapid sand filter can deal with raw water directly• No preliminary storage is needed• The filter beds occupy less space• Filtration is rapid, 40 – 50 times that of a slow sand filter• The washing of the filter is easy• There is more flexibility in operation
COMPARISON OF RAPID & SLOW SAND FILTERSProperties Rapid sand filter Slow sand filterArea Small area Large areaRate of 4000-7500 100-400filtration(L/m2/hr)Sand size (diameter) 0.4-0.7 mm 0.2-0.3 mmPretreatment Coagulation and Sedimentation sedimentationFilter cleaning Backwashing ScrapingOperation More skilled Less skilledRemoval of colour Good BetterRemoval of bacteria 98-99% 99.9%-99.99%Prior water storage Storage needed No need
PRESSURE FILTERS:“Filter is enclosed in space and the water passes under pressuregreater than atmospheric pressure”
Construction:•Closed steel cylinders.•Diameter varies from 1.5 to 3.0 m.•Length or height varies from 3.5 to 8.0m.•Manholes are provided at the top for inspection.Working:•The water mixed with coagulant is directly admitted to the pressure filter.•In working condition all valves are closed except those for raw water andfiltered water.Cleaning•The compressed air may be used to agitate sand grains.•Valves for wash water and wash water drain are opened during washing.Rate of filtration: 6000-15000litres/hr/sq.m.Efficiency: Less efficient than the rapid sand filters.
DISINFECTION OF WATER• Disinfection is the process of removing harmful bacteria.• The substances or materials which are to be used for disinfection are called the disinfectants.• When water leaves the filter plant, it is till found to contain some of the impurities. These impurities can be grouped as: -Bacteria, -dissolved inorganic salts, -colour, odour and taste, -iron and manganese.
METHODS OF DISINFECTION:BOILING METHOD• This is the most effective method of killing bacteria but impracticable in large scale.• Most of bacteria are destroyed when the water has attained of about 80˚C temperature.• Prolonged boiling is unnecessary and wasteful.EXCESS LIME TREATMENT• Treatment of lime is given to the water for the removal of dissolved salts.• Excess lime added to water works as disinfecting material.• When pH value is about 9.50, bacteria can be removed to the extent of 99.93 per cent.• Lime is to be removed by recarbonation after disinfection.
IODINE AND BROMINE TREATMENT• Use of iodine or bromine is limited to small water supplies such as swimming pools, troops of army, private plants, etc.• Dosage of iodine or bromine is about 8 p.p.m.• Contact period with water is 5 minutes.• Available in the form of pellets or small pills.SILVER TREATMENT• Colloidal silver is used to preserve the quality of water stored in jars.• Metallic silver is placed as filter media. Water get purified while passing through theses filters.• Dosage of silver varies from 0.05 to 1 p.p.m.• Contact period is about 15 minutes to 3 hours.• It is costly and limited to private individual houses only.
OZONE TREATMENT(3O2 =2O3)• Nascent oxygen is very powerful in killing bacteria.• Ozone is unstable and doesnot remain in water when reaches the consumer.• Ozoniser:• Dosage of ozone is about 2 to 3 p.p.m. to obtain residual ozone of 0.10 p.p.m• Contact period is about 10 minutes
POTASSIUM PERMANGANATE TREATMENT(KMnO4)• It is a powerful oxidising agent, effective in killing cholera bacteria• Restricted to disinfection of water of village wells and ponds• Dosage is about 2.1 p.p.m• Contact period of 3 to 4 hours• The treated water produces a dark brown coating on porcelain vessels and this is difficult to remove except with scratching or rubbing.ULTRA-VIOLET RAY TREATMENT• For generating these rays, the mercury is enclosed in one or more quartz bulbs and electric current is then passed through it.• The water should be passed round the bulbs several times .• Depth of water over the bulbs should not exceed 10 cms.
CHLORINATIONUsed as a disinfecting material as,• It is easy to apply due to relatively high solubility of about 7000 mg per litre.• Readily available as gas, liquid or powder.• Very toxic to most of the micro-organisms.• Cheap and reliable.• Chlorine can be applied in water in one of the following ways: as bleaching powder, as chloramines, or as free chlorine gas.Mechanism of action:• 1. H2O+Cl2 (at pH 7) = HCl + HOCl (main disinfectant) HOCl (at pH > 8.5) H+ + OCl- (minor action)• 2. NH3+ HOCl = NH2Cl/NHCl2/NCl3 + H2O (Mono, Di ,Tri Chloramines)
These processes help the sanity of public health butit is up to people to make consciousness of theimportance of avoiding contamination andpollution of our water streams. These processes costmore money day by day as water pollutionincreases.