UNIT 3FILTRATION & DISINFECTION

FILTRATION OF WATER“ To remove or to reduce impurities still further, water is filtered through the beds of fine granular material like sand, anthracite coal etc.

THEORY OF FILTRATION: 4 types namely

• Mechanical straining

• Sedimentation

• Biological metabolism

• Electrolytic changes

FILTER SAND MEDIA

Filter sand is classified on the basis of effective size as well as uniformity coefficient.

CLASSIFICATION OF FILTERS:

Filters

Gravity filters

Slow sand filters

Rapid sand filters

Pressure filters

SLOW SAND FILTERS:Purpose:Water is allowed to pass slowly through a layer of sand placed above the base material.Purification process aims at improving the biological, chemical, and physical characteristics of water.

Essential components:

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:• 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 LAYER SIZE OF GRAVEL

Topmost 150mm 3mm- 6mm

Intermediate 150mm150mm

6mm- 20mm20mm- 40mm

Lowest 150mm 40mm- 65mm

Total 600mm depth

Filter media:• A layer of sand is placed above the gravel of layer depth 600 mm to 900 mm.• Size varies from 0.2 mm to 0.3 mm.• Uniformity coefficient (Cu) of sand is 2 to 3.Working/ Operation:• 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 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 added

Rate of filtration: 100 to 200 liters per hour per m² of filter area

Efficiency: • Bacterial load: 99.5 to 99.9 percent removal efficiency

•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

Properties Rapid sand filter Slow sand filter

Area Small area Large area

Rate of filtration(L/m2/hr) 4000-7500 100-400

Sand size (diameter) 0.4-0.7 mm 0.2-0.3 mm

Pretreatment Coagulation and sedimentation

Sedimentation

Filter cleaning Backwashing Scraping

Operation More skilled Less skilled

Removal of colour Good Better

Removal of bacteria 98-99% 99.9%-99.99%

Prior water storage Storage needed No need

Pressure filters:

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.

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.

How to clean pressure filters:

Part 2: Disinfection• Disinfection may be defined as the killing or destruction of pathogenic

(disease causing m/o)• Sterilization may be defined as the destruction or killing of pathogenic

as well as non pathogenic (which are not disease causing m/o).

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• Ozone does not remain in water till the time it reaches the consumer while travelling

through the water supply lines.

• Dosage of ozone is about 2 to 3 p.p.m. • Contact period is approx 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 removeexcept 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.

Disinfection by chlorination:Used 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)