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Domestic or Household Purification of Water

Methods available for purifying water on an individual or domestic scale are:
Chemical disinfection
Ultraviolet irradiation
Multi – stage reverse osmosis purification
These methods can be used singly or in combination
It is an excellent method of purifying water but
For being effective:
The water must be brought to a ‘rolling boil’ for
10 – 20 minutes
It kills
All bacteria
Cysts and
Yields sterilized water
It removes temporary hardness (driving off 〖CO〗_2 and precipitating 〖CaCO〗_3 )
Alters the taste of water (though this is harmless)
No ‘residual protection’ against subsequent bacterial contamination
Hence boil the water preferably in the same container in which it is to be stored
Chemical Disinfection
Bleaching powder
Chlorine solution
High Test Hypochlorite (HTH)
Chlorine tablets
Potassium Permanganate

Bleaching powder:
Also k/a chlorinated lime
White amorphous powder
With a pungent smell of chlorine
Freshly made powder has 33% ‘available chlorine’
It is an unstable compound
On exposure to light, air and moisture; it rapidly loses its chlorine content
But, when mixed with excess of lime, it retains its strength
This is called a ‘stabilized bleach’
Bleaching powder should be stored in a dark, cool and dry place and
In a closed container which is resistant to corrosion
The chlorine content of the bleaching powder stocks should be checked frequently
Principle of chlorination:
Highly polluted and turbid waters are not suited for direct chlorination
We can estimate the required amount of dose using readymade charts,
Though the rough estimation is 0.7 mg of applied chlorine per liter of water
Ensure a ‘free’ residual chlorine of 0.5 mg/L at the end of one hour contact
Other Sources of chlorine
Chlorine solution:
May be prepared from bleaching powder
Mixing 4 Kg of bleaching powder (having 25% available chlorine) in 20 L of water will give 5% solution of chlorine
Ready made chlorine solutions are available in different strengths
The chlorine solution is also subject to losses on exposure to light or prolonged storage
Hence the solution should be kept in a dark, cool and dry place and in a closed container
High Test Hypochlorite (HTH or ‘Perchloron’):
It is a calcium compound which contains 60 – 70% available chlorine
It is more stable than bleaching powder, hence deteriorates much less on storage
Solutions prepared from HTH are also used for water disinfection
Ready-made charts can be used to estimate the dose for certain quantities of water
chlorine tablets:
Available under various trade names e.g. ‘Halazone’ tablets
Good for small quantities of water
These are costly
NEERI, Nagpur formulated a type of chlorine tablet which is 15 times better than ordinary halogen tablets
These are available in various strengths
One tablet (0.5gm) is sufficient for 20 Liter of water
May be used for emergency disinfection
Two drops of 2% ethanol solution suffices for one liter of CLEAR water
Contact time of 20 – 30 min is required for effective disinfection
Iodine does not react with ammonia or organic compounds, hence remains in its active molecular form over a wide range of pH values and water conditions and
Persists longer than chlorine
Still unlikely to become a large scale (municipal water supply) disinfectant because:
High in costs and
Is physiologically active (thyroid activity)
Potassium permanganate:
Strong oxidizing agent
Kills cholera vibrio
Not so effective against other disease organisms
Alters the color, taste and smell of water
No longer recommended for water disinfection

Filtration through Ceramic Filters
The essential part is the ceramic “Candle” made of:
-Porcelain in Chamberland filter
-Kieselgurh (infusorial earth) in Berkefeld filter
-Coated with a silver catalyst in Katadyn filter
Silver liberate silver ions into the water which ‘oligo-dynamic’ action and kills the bacteria coming in contact with the surface of the candle
Filter candles having fine pores :
Remove bacteria and protozoa but Not the filter passable viruses
Candles are liable to be clogged with impurities and bacteria
They should be cleaned by scrubbing with a hard brush under running water and Boiled at least once a week
Only clean water should be used with ceramic filter
Not quite suitable for widespread use under Indian conditions
-Not as effective against viruses
-No chlorine residual protection - can lead to recontamination
-Variable quality control for locally produced filters
-Filters can break over time - need for spare parts
-A low flow rate of 1-3 liters per hour for non-turbid waters
Filters and receptacles must be cleaned regularly, especially after filtering turbid water

Ultraviolet irradiation
UV irradiation is effective against most microorganisms known to contaminate water supplies like;
Bacteria, Yeast, Viruses, Fungi, Algae, Protozoa etc.
Method of disinfection
Exposure of a film of water (up to 120 mm thick) to one or more ‘quartz Hg vapor arc’ lamps emitting UV radiation (of 254 nm wavelength )
The water should be free from turbidity, suspended and colloidal constituents
Short period of exposure is required
No foreign matter introduced
No alteration of taste, odor and color
Over exposure does not result in any harmful effect
No residual effect
Lack of a rapid field test for assessing efficiency
The needed apparatus is expensive
Hence limited Application to Individual and Institutional systems

Multi – Stage Reverse Osmosis Purification
The Clarity cartridge removes the suspended particles such as dust, mud, and sand
The Reverse Osmosis (RO) cartridge reduces the dissolved solids, hardness, heavy metals and eliminates micro- organisms
Principle: If two solutions are separated by a semipermeable membrane (membrane that allows the passage of the solvent but not of the solute),
The solvent naturally passes from the lower-concentration solution to the higher-concentration solution. This process is known as osmosis.
It is possible, however, to force the flow of solvent in the opposite direction, from the higher to the lower concentration, by increasing the pressure on the higher-concentration solution.
The required pressure differential is known as the ‘osmotic pressure’ and the process is k/a ‘reverse osmosis’ (RO)
Therefore, RO creates two streams of water; one of treated water and another of waste water which is highly concentrated
RO rejects monovalent ions and organic with higher molecular weight
It reduces:
Total dissolved solids
Heavy metals (arsenic, lead, Hg etc.) and
Bacteria, virus, protozoa and cysts
The water is rendered both chemically and microbiologically potable
These do not provide residual disinfection hence a need to add small amount of persistent disinfectant like chlorine as a preservative during distribution to the community

OT and OTA test:

Membrane processes in water purification:

Super Chlorination and Break Point Chlorination:

Slow sand filter:

Rapid sand filter:

Purification of water at a large scale:

Criteria for a Chemical to be Suitable as Water Disinfectant in Water Supplies:

Water Supplies Disinfection with Chlorination:

Principles of Chlorination:

Methods of Chlorination in Water Supplies:

Horrock's Apparatus:

Disinfection of well during emergency:

Double Pot method:

Disinfection of Wells:

OT and OTA test:

Domestic or Household Purification of Water: