This document discusses water filtration and disinfection methods. It covers various filtration methods like slow sand filtration and pressure filtration. Slow sand filtration uses layers of sand, gravel and other media to slowly filter water. Pressure filtration uses enclosed steel cylinders to filter water more quickly under pressure. The document also discusses several disinfection methods including boiling, chlorination, ozonation, and UV light treatment. Chlorination is commonly used because chlorine is inexpensive, effective at killing microorganisms, and can be applied as a gas, liquid or powder to disinfect drinking water.

1. UNIT 3
FILTRATION & DISINFECTION

2. 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

3. 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
filtersPressure
filters

4. 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.

5. 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 pipes
Base material:
• Placed on top of the underdrainage system
• Varies from 300 mm to 700 mm depth
• Graded and laid in layers of 150 mm

6. LAYER DEPTH OF LAYER SIZE OF GRAVEL
Topmost 150mm 3mm- 6mm
Intermediate 150mm
150mm
6mm- 20mm
20mm- 40mm
Lowest 150mm 40mm- 65mm
Total 600mm depth

7. 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.

8. 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

9. 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

10. 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

11. Pressure filters:

12. 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.

13. How to clean pressure filters:

14. 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).

15. 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.

16. 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.

17. 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

18. 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.

20. 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)