Water is purified on large scale for supply to a city or a town. It is also purified on small scale for domestic use.
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In the purification of water on large scale following methods are used:
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l. Slow sand filtration (biological filtration).
2. Rapid sand filtration (mechanical filtration).
1. Slow Sand Filtration:
This system was first introduced in England about more than a century ago and still commonly used. Hence it is also known as English system. In slow sand filtration following stages are involved:
(a) Storage
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(b) Filtration
(c) Chlorination
(a) Storage:
The raw water from the source, usually a river, canal or a stream is collected in natural or artificial large open reservoirs known as settling tanks. The water is allowed to remain there for 1 to 2 days. During this short period of storage natural purification takes place. About 90 percent suspended impurities settle down by gravity.
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The organic matter present in the water is oxidised by bacteria with the help of dissolved oxygen present in water. The number of micro-organisms present also decreases considerably. The turbidity due to mud etc. also decreases. At this stage the quality of water improves to a great extent and water becomes much clearer in appearance.
(b) Filtration:
Filtration is the second stage in the purification of water and is very important stage because about 99 percent bacteria are removed at this stage. During filtration the clarified water from storage tanks is now admitted to the slow sand filters. The filter beds are water tight rectangular tanks made up of concrete.
Usually they are arranged side by side and generally kept open. At least two filtration beds must be constructed so that one must remain in function when the other one is being cleaned. The size of the tanks depends on the requirement of water to be supplied to the community. These beds are generally 2.5 to 4 meters deep.
The filtration bed or sand bed is the most important component because the water is to be filtered through this bed.
They are filled from bottom to upward as follows:
(i) The lowest layer consists of 4 cm size gravel or broken stones.
(ii) Above this layer there is fine gravel. The total thickness of gravel layer is 15-30 cm.
(iii) The third layer above the fine gravel is of course sand. The thickness of this layer should be 15 to 30 cm. The gravel layer gives support to the sand layer.
(iv) The fourth layer above the coarse sand layer is of fine sand. The thickness of this layer should be from 60 to 90 cm. As the particles of sand are very small and large in number, they provide large surface area for the water to pass through. Apart from mechanical straining, oxidation, sedimentation and removal of bacteria also takes place.
(v) On the top of these layers there is layer of clarified water from settling tank, about 1.5 to 1.8 meters in height. The inlet of water is controlled by valves fitted at the top of the filter bed. At the bottom of the bed a number of perforated pipes are fitted to collect the filtered water.
No doubt all the components of filter bed especially the sand play a great role in the filtration of water but the vital layer which is formed at the surface of sand plays the greatest role in the purification of water.
This is a thin green slimy gelatinous layer which consists of numerous forms of plant and animal life e.g. algae, fungi, protozoa and bacteria. The process of formation of vital layer is known as “ripening” of the filter.
This layer is formed within 2-3 days on a new filter bed and when fully formed it is about 2-3 cm in thickness. As the thickness of this vital layer goes on increasing, with the passage of time, the filtration goes on decreasing.
After sometime when the thickness increases to a great extent (about 4 feet) and filtration reduces then cleaning of the filter is done. For cleaning, the top sand layer is scraped off. This process of cleaning the filter is repeated periodically.
When after repeated cleanings the thickness of sand layer reduces to about 30-40 cm, the plant is closed down and a new bed is prepared. This is usually done after an interval of three years.
The new filter takes about 3 days to work effectively because it takes 2-3 days for the vital layer to form. So the water received during first three days should be allowed to go waste and purified water should be supplied only when bacteriological examination of water shows that efficient filtration is taking place.
Advantages of Slow Sand Filters:
Slow Sand Filters have the Following Advantages:
(iii) They are more practicable for filtration of water in developing countries with minimum filtration. They were first used in 1804 in Scotland. Even today they are accepted as standard method of purification of water all over the world.
(iv) They yield 98-99% bacteria free water.
Disadvantages:
(i) More land is required for their construction so initial cost is much more than rapid sand filters.
(ii) They require periodical cleaning.
2. Rapid Sand Filtration (Mechanical Filtration):
Rapid sand filters were first introduced in 1885 in U.S.A. Since then they have gained considerable popularity and are still commonly used.
Types of Rapid Sand Filters:
There are two types of rapid sand filters:
(i) Gravity type e.g. Paterson’s filter.
(ii) Pressure type e.g. Candy’s filter.
The Paterson’s rapid filter is more commonly used. During rapid filtration 5 steps are involved which are discussed below:
(a) Coagulation
(b) Mixing
(c) Flocculation
(d) Sedimentation
(e) Filtration
(a) Coagulation:
The water from the setting tanks is led continuously into the plant. Here the water is treated with a chemical coagulant such as alum to remove turbidity and colour. The amount of alum used is 5 to 40 mg per liter, depending upon the amount of turbidity present in the water.
(b) Mixing:
The alum treated water is then agitated mechanically in a mixing chamber for a few minutes so as to dissolve the alum and the impurities get precipitated.
(c) Flocculation:
The water is then passed into the flocculation chamber where it is stirred at a slow speed for about half an hour so as to form floccules of aluminum hydroxide.
(d) Sedimentation:
The coagulated water is then led to the settlement tank where the precipitates are allowed to settle at the bottom of the settlement tank. The water is allowed to remain there for 2 to 6 hours.
During this time the precipitates along with suspended matter and bacteria settle at the bottom and the supernatant water now looks very clear in appearance. The flocculated material from the settlement tank is removed from below.
(e) Filtration:
Filtration is the most important step in the rapid sand filtration process. The clarified water is led to the rapid sand filter which purifies water from 98-99 percent.
In rapid sand filters the medium of filtration is like that of slow sand filtration i.e. sand and gravel. The filter is made up of concrete chamber about 7 feet deep which contains filtering medium i.e. sand supported by gravel.
The thickness of filtering medium should be 4 to 5 feet. The filtered water is collected through a network of perforated pipes attached at the bottom of the filter.
Cleaning of the Filters:
As the filtration proceeds, after 6-7 days, a layer similar to vital layer in slow sand filters develops due to collection of floccules which were not sedimented; suspended matter and bacteria. As a result of continuous filtration the filter bed becomes dirty and requires cleaning of filters.
For cleaning the filters washing process known as ‘back washing’ is done. During this process the inlet and outlet valves are closed. A backflow of purified water from the clean reservoir is made through the bottom of sand bed with simultaneous stirring of the upper layer of sand by means of rotator metal arms or a blast of compressed air.
In this way the deposited layer of floccules and suspended matter will be dislodged and removed with wash water. When the sand looks clear the washing should be stopped.
The entire process of washing takes about 15 to 20 minutes and the filters are ready for use again within another 20 minutes. This is an advantage over slow sand filtration where the entire bed needs reconstruction.
Advantages:
(i) Very little space is required.
(ii) Initial cost is less.
(iii) They are suitable for turbid water.
(iv) Water is filtered rapidly.
(v) Cleaning the filter is easy.
(vi) There are no chances of contamination by the labourers.
Disadvantages:
(i) Running costs are high.
(ii) A chemical coagulant such as alum is required.
(iii) Results of purification are not good so chlorination of water is required.
