Water Purification System / পানি পরিশুদ্ধ করন
How to purify water? / কিভাবে পানি পরিশুদ্ধ করা যায় ?
Water purification methods are
- Purification of water on large scale
- Purification of water on small scale
Purification of water on large scale/
How to purify water in large scale?
Large scale water purification system includes three types:- Storage
- Disinfection
- Filtration
(1) Water purification on large scale: Storage:
What is storage method of water purification system?
- Storage means reserve of water, from which further pollution is excluded.
- As a result of storage, a very considerable amount of purification takes place.
- It is natural purification method
- It is easier, comparatively low cost and effective method.
Mechanism of purify water by storage:
In case of storage, purification is considered from
three points of view-
1. Physical:
About 90% of suspended impurities
settle down within 24 hours due to gravity
2. Chemical:
Free ammonia is reduced and rise of
nitrate because of anaerobic bacteria oxidize organic matter
3. Biological:
Numerous bacteria die due to storage
of water
(2) Large scale Water purification System: Disinfection
What is disinfection method?
There are different methods for disinfection of
water in large scale-
- Chlorination
- Ozonation
- Ultraviolae irradiation
Chlorination of water:
- Kills pathogenic bacteria, no effect on spores and certain viruses
- Also oxidizes iron, manganese & hydrogen sulphide
Advantages of chlorination methods:
- Low cost
- Easy to application
- Kill almost all bacteria
Disadvantage of chlorination methods:
- Formation of halogeneted compound which are suspected carcinigens
Methods of chlorination:
- Chlorine gas
- Chloramine
- Perchloron
Principles of chlorination:
- Water to be chlorinated should be free from turbidity
- Chlorine demand should be estimate
- Minimum recommended concentration of free chlorine is 0.5mg/L for 1 hour
- The sum of chlorine demand of specific water plus the free residual chlorine of 0.5mg/L constitutes the correct dose of chlorine to be added
- Contact period is at least one hour
Note:
Chlorine demand:
The chlorine demand of water is the
difference between the amount of chlorine added to the water and the amount of
residual chlorine remaining at the end of a specific period of contact (usually
60 minutes) at a given temperature and pH of the water.
It is the
amount which reacts with the substances in water, leaving behind an inactive
form of chlorine.
Break point chlorine:
Point at which chlorine demand is met is known as break point
chlorination
Super-chlorination:
The addition of large doses
of chlorine to the water, and removal of excess of chlorine after disinfection.
(3) Large scale water purification system: Filtration
Steps of filtration:
- Collection and storage of water
- Bring the water into the filter
- Collection of pure water after purification
There are two type of filters are used for filtration of water in large scale-
- Slow sand filter or biological filter
- Rapid sand filter or mechanical filter
Elements/parts of slow sand filter
 |
| Sand filter |
2. A bed of graded sand and gravel
a) Filter sand, Coarse sand
b) Fine gravel,
Course gravel
3. An under drainage system
4. A system of filter control valves
Filter bed or sand bed:
- The most important part of the filter is the filter bed or sand bed.
- The thickness of the sand bed is about 1 meter.
- Water percolates through the sand bed
- The sand grains are carefully chosen so that they are preferably rounded and have an 'effective diameter between 0.2 and 0.3 mm.
- The sand should be clean and free from clay and organic matter.
- The sand bed is supported by a layer of graded gravel, 30-40 cm deep which also prevents the fine grains being carried into the drainage pipes.
Advantages of Slow sand filter:
- Simple to construct and operate.
- The cost of construction is cheaper than that of rapid sand filters.
- The physical, chemical and bacteriological quality of filtered water is very high.
- It constructed with gravity flow only, no (electrical) pump required
- No necessity to use chemical
- Long lifespan (Usually > 10 years)
- Reduce bacteria count by 99.9 - 99.99%
Disadvantages of slow sand filter:
- Minimum quality and constant flow of water is needed
- Cold temperature may lower the efficiency due to decrease biological activity
- Required large land area
- It is a slow process
Vital Layer/ Biological layer/ Zoogloeal layer/ Schmutzdecke
- Contain Thread like algae, plankton, diatom and bacteria
- Process of formation called ripening of filter
- Takes several days to form
- Expend 2-3 cm into top portion of sand bed
- Removes organic matter, holds back bacteria and oxidized ammoniacal nitrogen to nitrate
Large scale water purification system
Steps of Rapid Sand Filter
Advantages of rapid sand filter:
- Can deal with raw water directly.
- No preliminary storage required
- Filter bed occupy less space
- Filtration is rapid, 40 – 50 time that of a slow sand filter
- Washing of filter is easier
- Easy to operate
Water purification system on small scale
অল্প পরিমান পানি বিশুদ্ধকরন পদ্ধতি
Household purification of water
- Physical (e.g-Boiling)
- Chemical disinfection
- Filtration
- Disinfection of well
Boiling method of water purification system:
- It is a satisfactory method of purifying water for household purposes.
- To be effective, the water must be brought to a "rolling boil" for 10 to 20 minutes.
- It kills all bacteria, spores, cyst and ova
- Boiling also remove temporary hardness of water.
- It offer no residual protection against subsequent microbial contamination
Chemical disinfection/Chemical methods of water purification
- Bleaching powder/ chlorinated lime- Free residual chlorine of 0.5 mg/L for 1 hour contact
- Alum- 2-5 gm for 1 gallon
- Chloride tablet- 1 tablet (0.5gm) for 20 liter of water
- Bromide- 0.06 gm for 1 liter of water for 5 minute
- Iodine- 2 drop of 2% solution for 1 liter of water for 20 – 30 minutes
Filtration of water/ Household filtration of water:
Disinfection of well:
- The volume of the water in a well is calculated
- The amount of the bleaching powder required for disinfection is estimated.
- The bleaching powder is dissolved in water
- Chlorine solution is delivered into the well
- A contact period of 1 hour is allowed before the water is drawn for use.
- Orthotolidine arsenic test- to test for the residual chlorine at the end of 1 hour.
Tips:
Top 5 methods of water purification system:
1. Boiling
2. Filtration
3.
Distillation
4. Chemical
method
5. Rivers
osmosis
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Sources of information:
1.
K.Park 23rd edition
2.
Rashid Kabir Hyder
3.
Internet
4. Blogs
Thank you