FILTRATION AND DISINFECTION OF WATER

FILTRATION AND DISINFECTION OF WATERBy,M.Vinod kumar,09011BB047.WATER TREATMENT PROCESS

FILTRATION OF WATERIn order to remove or to reduce the contents of impurities still further, the water is filtered through the beds of fine granular material like sand

THEORY OF FILTRATION:Mechanical strainingSedimentationBiological metabolismElectrolytic changes

FILTER SANDFilter sand is classified on the basis of effective size and uniformity coefficient.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 size

CLASSIFICATION OF FILTERS:

SLOW SAND FILTERS:Purpose:The water is allowed to pass slowly through a layer of sand placed above the base material and thus the purification process aims at simultaneously improving the biological, chemical, and physical characteristics 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 m

Underdrainage system:

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 systemVaries from 300 mm to 700 mm depthGraded and laid in layers of 150 mm

LAYERDEPTH OF LAYERSIZE OF GRAVELTopmost150mm3mm- 6mmIntermediate 150mm150mm6mm- 20mm20mm- 40mmLowest150mm40mm- 65mmTotal600mm depthFilter 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 mmEffective depth of filter media is reducedFresh layer of 150mm depth of graded sand is added

Rate of filtration: 100 to 200 liters per hour per m of filter area

Efficiency: Bacterial load: 99.50 to 99.9 percent is removedColour: less efficient in removal of colour(20-25%)Turbidity: can remove turbidity of 50 p.p.mRAPID SAND FILTER OF GRAVITY TYPE:

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 deepClogging of filters by suspended impurities and bacteria: Loss of HeadCleaning 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 directlyNo preliminary storage is neededThe filter beds occupy less spaceFiltration is rapid, 40 50 times that of a slow sand filterThe washing of the filter is easyThere is more flexibility in operation

COMPARISON OF RAPID & SLOW SAND FILTERSPropertiesRapid sand filterSlow sand filterAreaSmall areaLarge areaRate of filtration(L/m2/hr)4000-7500100-400Sand size (diameter)0.4-0.7 mm0.2-0.3 mmPretreatmentCoagulation and sedimentationSedimentationFilter cleaningBackwashingScrapingOperationMore skilledLess skilledRemoval of colourGoodBetterRemoval of bacteria98-99%99.9%-99.99%Prior water storageStorage neededNo needPRESSURE FILTERS:Filter is enclosed in space and the water passes under pressure greater 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 and filtered water.

CleaningThe 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.

Cleaning:

DISINFECTION OF WATERDisinfection 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 METHODThis 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 80C temperature.Prolonged boiling is unnecessary and wasteful.

EXCESS LIME TREATMENTTreatment 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 TREATMENTUse 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 TREATMENTColloidal 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.mContact period is about 10 minutes

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

20POTASSIUM PERMANGANATE TREATMENT(KMnO4)It is a powerful oxidising agent, effective in killing cholera bacteriaRestricted to disinfection of water of village wells and pondsDosage is about 2.1 p.p.mContact period of 3 to 4 hoursThe treated water produces a dark brown coatingon porcelain vessels and this is difficult to removeexcept with scratching or rubbing.ULTRA-VIOLET RAY TREATMENTFor 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.

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.

21CHLORINATIONUsed 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 but it is up to people to make consciousness of the importance of avoiding contamination and pollution of our water streams. These processes cost more money day by day as water pollution increases.

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