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The following points highlight the top three methods adopted for purification of water. The methods are: 1. Sedimentation 2. Filtration 3. Disinfection.
Method # 1. Sedimentation:
Sedimentation is done when water consists of large sized organic materials such as leaves, and gravels which have run off from the soil. Suspended particles settle down depending on their size and weight and conditions of the stored water. Sedimentation is done in large reservoirs or in restricted area of settling tank.
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The rate of sedimentation is enhanced by adding alum, iron, salts, colloid silicates which act as coagulants (Fig. 31.5A-D). The suspended materials and microorganisms are entrapped by coagulants and settle down rapidly.
This procedure is called coagulation or flocculation. The microorganisms remain viable for some time. Thus, sedimentation provides partial reduction of microorganisms in water due to their settling down on bottom but does not sterilize the polluted water.
Method # 2. Filtration:
It is the second step of purification. After sedimentation the water is further purified by passing to filtration unit (Fig. 31.5 E). It is the effective means of removing microorganisms and the other suspended material from the water.
There are two types of sand filters which are used in water purification such as slow sand filter and rapid sand filter:
(i) Slow Sand Filter:
In slow sand filtration plants the rate of filtration of water is slow; hence the plant requires a considerable area. This plant consists of a concrete floor containing drainage tiles (for collection of filtered water). The tile is covered with first coarse sand and finally 2 to 1 feet of sand at the top of plant (Fig. 31.6).
Water is passed through this plant. Water passes slowly through the filter and collected by tile drain pipes at the bottom which later on is pumped into a reservoir. If water is turbid, slow sand filters are clogged soon. Therefore, turbid water, which is to be filtered, should be clarified first by sedimentation, thereafter, passed through slow sand filters. The capacity of slow sand filter plant is to filter about 5 million of water per acre per day.
Water purification is done not by physical action but by physiological mechanisms supported by microorganisms. In the surface of layers of fine sand a colloidal material, consisting of bacteria, algae and protozoa, is attached. This mucilaginous material makes the pores more effective by closing the pores between the sand grains. Sand grains have positive charges and bacterial cell walls have negative charge.
Therefore, bacteria are adsorbed on the surface of sand. Protozoa ingest bacteria. Due to intense microbial interactions, chemical concentration of water is reduced.
When filtration efficiency of the plant is reduced, due to deposition of thick mucilaginous material, the plant is taken out for cleaning. Through this plant, the pathogenic microorganisms such as Giardia and its cysts which are not removed by any other methods can be filtered from water.
For the first time in 1852, parliament of London required that the entire water supply be passed through slow sand filters before use. This plant was installed in many countries after suffering from cholera epidemics.
(ii) Rapid Sand Filter:
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Similar to slow sand filter, the rapid sand filter is also constructed. This plant consists of layers of sand, gravel and rock (Fig. 31.7). Before filtration, water is treated with alum or ferrous sulphate in a settling tank where precipitates settle down.
Then water is allowed to pass through rapid sand filter plant. This plant depends on physical trapping of fine particles and floes or coagulants. The pores of the plants are soon clogged. It is cleaned by forcing cleaned water backward i.e. back washing through the beds of gravels and sands without disturbing the fine sand.
About 99% bacteria are removed by this plant. But unfortunately the use of coagulants, rapid filtration and chemical disinfection often does not remove Giardia lamblia cysts, Cryptospordium oocysts, Cyclospora and viruses. More consistent removal of these pathogens is possible through slow sand filter. Therefore, water collected after filtration needs further treatment.
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In addition, rapid sand filter plant operates about 50 times, faster than slow sand filter plant, and can delivers about 150 to 200 million gallons of water per acre per day. It requires less land area, less cost and less maintenance. Therefore, many plants are constructed in a chain. If one plant is being cleaned, the others are under operation.
Method # 3. Disinfection:
Some of the bacteria pass through filter even after filtration which must be killed before consumption of water. Therefore, disinfection of public water supply needs to be done. Disinfection is the final step of water purification. Solutions of sodium hypochlorite are treated in small towns but in recent years, chlorination of public water supply has become popular.
Chlorination involves the release of chlorine gas in water which gets readily mixed up with water (Fig. 31.5F). The amount of chlorine required depends on organic matter and number of microorganisms present in water, and duration of time to act upon. High concentration of chlorine quickly acts upon microorganisms and vice-versa. Therefore, the amount of chlorine required for disinfection is called chlorine demand.
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Water is chlorinated to contain about 0.1 to 0.2 ppm of residual chlorine which reaches to this concentration after 20 minutes of its addition. However, if the concentration of chlorine exceeds its demand, peculiar odour and tastes are experienced.
If action of chlorine prolongs in water containing high amount of organic matter, chloramines, are formed. Change in odour and taste of water is due to the formation of chlorophenols. In the presence of high organic matter, chlorine reacts with it and produces halomethanes which are a group of carcinogenic compounds.
The mechanism of action of chlorine on microorganisms is obvious. After reacting with water, chlorine is converted into hypochlorous acid which in turn quickly releases nascent oxygen.
The nascent oxygen soon oxidises the cellular components of microorganisms as well as organic matter. In addition, chlorine fails to kill the microbial spores. The other gas which behaves like chlorine is the ozone. The simplest method to make water free from microbes and for consumption is boiling for 10-15 minutes.
