Water on small scale such as for domestic purposes can be purified as follows:
(i) Boiling
(ii) Distillation
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(iii) Filtration through muslin cloth.
(iv) Three pitcher system.
(v) Chemicals.
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(vi) Domestic filters e.g. Berkefeld filter and Pasteur’s Chamber- land filter.
(i) Boiling:
Boiling is the oldest and satisfactory method of purification of water on small scale. Boiling for 5 to 10 minutes kills bacteria, spores, cysts and ova of intestinal parasites. It also removes hardness of water and soft water is produced.
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Boiling is an excellent method of purification of water provided boiling is done in a neat and clean vessel and after boiling it is stored in clean covered container. Preferably water should be boiled in the same container in which it is to be stored. Only that much amount of water should be boiled which can be used within a few hours.
(ii) Distillation:
Distillation is also a good method of purification of water. During this method all kinds of dissolved impurities can be removed even the volatile one as well. For this purpose first and last portion of the distillate must be rejected because these portions may contain the volatile ingredients which may again contaminate the distilled water. Distillation is not possible for the purification of water to be used for routine household purposes.
(iii) Filtration through Muslin Cloth:
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Muslin cloth acts as a coarse filter which can remove the suspended materials. So water filtered through muslin is not fit for drinking purposes though it can be used for other household purposes like bathing, washing the clothes etc.
(iv) Three Pitcher System:
This is very old system of purification of water. In this system three pitchers are used which are kept one above the other on a wooden stand. The top Picher contains sand, second charcoal and sand; and the lowest collects the purified water. The raw water is filled in the first pitcher from where it percolates through a hole into the 2nd pitcher. From here the water further percolates through the hole to the third pitcher.
(v) Chemicals:
Various types of chemical agents used for disinfection of water are discussed as follows:
(a) Bleaching Powder (Chlorinated Lime):
Chemically it is CaOCl2. A fresh sample of bleaching powder contains 33% of available chlorine but on storage it loses chlorine content. Therefore bleaching powder is stored in dry, air-tight containers and at cool and dark places.
Roughly speaking 2.5 gm of a good quality of bleaching powder could be required to disinfect 1000 liters of water. Bleaching powder will not directly purify the turbid and polluted water. Therefore such water should first be treated with preliminary filtration and then subjected to chlorination.
(b) Chlorine tablets:
These tablets are good for disinfecting small quantities of water. They are available in different strengths for disinfecting various quantities of water. One tablet of 500 mg is sufficient for disinfecting 20 liters of water. These are available in the market under various trade names e.g. halazone tablets manufactured by the Boots company.
(c) Quick Lime (Calcium Oxide):
Some people prefer to use dry slaked lime than ordinary lime. About 360 mg of slaked lime will disinfect 4.5 liters of water. It is cheap, easily available and quite effective.
Therefore it is recommended for disinfecting wells and tanks in cholera outbreak. Disadvantage of quick lime is that large doses of it are required for disinfection of water i.e. 20 times than that of bleaching powder.
(d) High Test Hypochlorite (HTH):
It is a calcium compound and contains about 65 to 75 percent of available chlorine. This is much stable compound and I gm of HTH is needed for one cubic meter of water.
(e) Alum:
Alum is not a germicidal. It is used to purify muddy water and to remove turbidity. 60 to 240 mg of alum cans purity 4-5 liters of water. Calcium carbonate which is present in all kinds of water also gets precipitated as calcium sulphate and aluminum hydrate. The suspended impurities as well as bacteria also get precipitated which are removed after filtration and clear purified water is obtained.
(f) Potassium Permanganate:
It is a strong oxidising agent and can kill cholera vibrios but it does not destroy other disease producing organisms. It is used for disinfecting wells. Its dose is 0.5 parts per million (0.5 ppm). It is not suitable for disinfecting large volume of water.
Its disadvantages are that it alters the taste, smell and colour of water thus treated. Moreover this method is not considered dependable therefore no longer used for disinfecting the water.
(vi) Domestic Filters:
Water for drinking purposes can be purified by means of domestic filters which are discussed below:
(a) Berkefeld Filters:
These are cylindrical filters known as ‘filter candles’ or ‘ceramic candles’. They are made up of unglazed porcelain or kieselguhr and are available in various porosity grades.
When water is purified through these candles the pores get clogged which need cleaning from time to time at least once a week by scrubbing with a hard brush and passing the water under pressure from inside to outside direction which will remove the entangled particles from the interstices.
(b) Pasteur’s Chamber land Filter:
It is made up of unglazed porcelain tubes which can be screwed on to a water tap. They work only under pressure and muddy water cannot be filtered through it because the pores will be immediately blocked.
Therefore such water must be cleaned to remove mud. For cleaning the filters they are scrubbed from outside with a hard brush and water is made to pass under pressure from inside to outside. They are quick and reliable as they make the water free from all kinds of impurities including bacteria.