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Water Purification at a LARGE SCALE (for water supply)

Safe & Wholesome Water
Water intended for human consumption should be both safe and wholesome.
This has been defined as water that is –
a. free from pathogenic agents
b. free from harmful chemical substances
c. pleasant to the taste, i.e., free from colour and odour; and
d. usable for domestic purposes
Water is said to be polluted or contaminated when it does not fulfil the above criteria
Without ample and safe drinking water, we cannot provide health care to the community
• The steps in a typical water purification system comprise one or more of the following measures
1. Storage
2. Filtration
3. Disinfection
• The actual steps would depend upon
1. the source of the raw water and
2. the desired standards of water quality
• E.g. ground water (like springs, deep well) may need only disinfection
• On the other hand, surface water tend to be turbid and polluted (e.g. river water) and requires extensive treatment
1. Storage
• Water is drawn out from the source and stored in reservoir
• The reservoir may be natural or artificial
• Storage helps in two ways:
a) Provides a reserve of water from which further pollution is excluded
b) Considerable amount of purification occurs as a result of storing water
c) This purification is a natural process
d) This purification occurs in 3 aspects i.e. physical, chemical and biological
• Physical quality improvement:
By mere storage, 90% of suspended impurities settle down in 24 hours due to gravity
Water becomes clearer physically
This allows penetration of light which further improves water quality (hence reduces the load on filter)
• Chemical:
Certain chemical changes occur during storage
The aerobic bacteria oxidize the organic matter present in the water with the aid of dissolved oxygen
This results in reduction of free ammonia and Rise in Nitrates
• Biological changes
This is one of the greatest benefits of storage
A tremendous drop in bacterial count
The pathogenic organisms gradually die out
It has been seen that when river water is stored the total bacterial count drops by as much as 90% in 5 – 7 days
The optimum period of storage of river water is about 10 – 14 days
If the water is stored for longer periods, algae might grow which imparts a bad smell and color to the water

2. Filtration
In this stage, 98 – 99 % of the remaining bacteria get removed
Other impurities are removed too
One of the two types of filters are may be used
a. Biological or the SLOW sand filter and
b. Mechanical or the RAPID sand filter
SLOW sand filter
Vital Layer (within the sand bed of SLOW sand filter):
• When the filter is newly laid, it acts merely as a mechanical strainer, it is not ‘biological’ at this time
• But soon, the surface of the sand bed gets covered with a slimy growth known as “Schmutzdecke”
• This is a zoogleal layer or biological layer
• It is living layer hence k/a the ‘vital layer’
• This slimy and gelatinous layer consists of:
– Threadlike algae
– Plankton, diatoms and bacteria
• When this layer get formed, this is k/a ‘ripening’ of the filter
• May take several days to form fully
• Once fully formed it extend for 2 – 3 cm into the top portion of the sand bed
• This vial layer is the ‘heart’ of the slow sand filter
• The vital layer:
– Removes organic matter
– Holds back bacteria and
– Oxidizes ammoniacal nitrogen into nitrated and
– Helps in yielding a bacteria free water
• Hence, until the vital layer is fully formed, the first few days filtrate is usually run to waste
RAPID sand filter
Steps in purification of water using rapid sand filter
1. Coagulation
2. Rapid mixing
3. Flocculation
4. Sedimentation
5. Filtration: using the rapid sand filter

3. Disinfection
The final step in water purification, is disinfection of the filtered water
Most practical disinfection at the large scale is chemical disinfection
Chlorine is the most widely used disinfectant at the large level

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: