Water is never ‘pure’ in a chemical sense. It contains various kinds of impurities such as dust particles, dissolved gases, dissolved minerals, microscopic paints and animals, suspended impurities and bacteria. These are natural impurities derived from the atmosphere, catchments area and soil. Besides this, there are various other reasons by which water i9 polluted.
The upland surface water derives its impurities from the catchments area, the sources being human habitations and animal keeping or grazing. It is therefore very necessary to keep the catchments area free from human or animal intrusion.
The general belief of purity in the water of mountain streams is often untrue. Even if there are no human habitations there is still a possibility of contamination caused by wild animals the impurities of river water are derived from surface washings, sewage and silage water, industrial and trade wastes, and drainage from agricultural areas.
The customs and habits of the people like bathing, animal washing and disposal of the dead body all add to the pollution of water. ‘On the bank of the tank the ignorant and dirty people pass motion (stool) and use them as latrines.
In some cases the liquid refuse from latrines, cattle sheds and the foul contents of drains and from similar places are flown in to the tanks. The tank water is polluted very easily than the well water. If the mouth of the well is just below the level of the surface of the ground, then there is enough possibility of water being polluted.
In such type of wells the washing of the street, latrines, the foul contents of drains and the discharges of animals easily enter in to the well and as a result of which water is polluted. In majority of the cases it is a common practice in our country that very often people wash their dirty clothes and bathe themselves while standing over well and use dirty vessels and dirty ropes for the purpose of drawing water.
Moreover, the wells are not cleaned out for years together and mud, broken pots, pieces of ropes and other refuses in consequence collect at the bottom and stop the spring from which the water flows. These are some of the ways how the water is polluted.
While pollution of water seems to be an inevitable sequence of modern industrial technology, the problem, now, is to determine the level of pollution that permits economic and social development without presenting hazards to health. The WHO has been active in fostering research in this field.
Purification of water is of greater importance in preventing diseases. It may be considered under ‘two headings:
(i) Purification of water on a large scale:
(ii) Purification of water on a small scale:
Large Scale Purification:
Water on a large scale, such as an urban water supply, is purified in 3 main stages:
Water is drawn out from the source and impounded in natural or artificial reservoirs. Storage provides a reserve of water from which further pollution is excluded. As a result of storage, a very considerable
Amount of purification takes place. This is a natural Purification and the optimum period of storage of river water is considered to be about 10 to 14 days. If the water is stored for long periods, there is likelihood of development of vegetable growths such as algae which impart a bad smell and color to water.
Filtration is the second stage in purification of water, and quite an important stage because 98 to 99 percent of the bacteria are removed by filtration apart from other impurities. Two types of filters have been used, the Slow sand and Rapid sand filters.
In the Slow sand filters, the falter beds are large water tight beds, square or rectangular in shape and 10 to 12 feet deep. Each unit has a surface of about an acre or more. Several units are needed so that filtration is carried out without closing down the plant.
The beds are laid out from bottom upwards as follows:
(a) A layer of bricks or broken stones at the bottom of the bed for a thickness of 6 to 7 inches.
(b) A layer of graded gravel one foot deep in the middle:
(c) A layer of sand 2 to 3 feet resting on the bed of gravel:
(d) 4 to 5 feet of water at the top :
There are under drains at the bottom of the bed to collect filtered water.
The advantages of a slow sand filter are: –
(1) Here is no need for chemical coagulation of water prior to filtration;
(2) Equipment is simple;
(3) Suitable land is readily secured;
(4) Supervision is simple;
The disadvantages are:
(1) A large area is required;
(2) The filters are open and therefore liable to contamination;
(3) Less flexibility in operation;
(4) Raw water requires sedimentation.
However, slow sand filters are becoming out of date; they are increasingly being replaced by rapid sand or mechanical filters.
In the Rapid sand filters, each unit of filter bed has a surface of about 900 square feet. Sand is the filtering medium. The ‘effective size’ of the sand particles is between 0.4—0.6mm. The depth of the sand bed is usually about 3 feet. Below the sand bed is a layer of gravel 1-1^ feet deep- The gravel supports the sand bed and permits the filtered water to move freely towards the under drains. The depth of the water on their top of the sand bed is 5-6 feet. The under drains at the bottom of the filter beds collect the filtered water. The advantages of a rapid sand filter over the slow sand filter are:
(i) Rapid sand filter can deal with raw water directly. No preliminary storage is needed.
(ii) The filter beds occupy less space.
(iii) Filtration is rapid, 40-50 times that of a slow sand
(iv) The washing of the filter is easy;
(v) There is more flexibility in operation; (iii) Chlorination: Chlorination is one of the greatest advances in water purification. It is a supplement, not a subsume to sand filtration. Chlorine kills pathogenic bacteria and renders water safe from the bacteriological point of view. Being an oxidizing agent it oxidizes iron, manganese and hydrogen supplied and destroys taste and odor producing compounds.
Small Scale Purification:
(1) Household Purification of Water: The methods employed for the purification of water on an individual or domestic scale are:
(b) Chemical disinfection.
Boiling is a satisfactory method of purifying water for household purposes. Boiling for 5-10 minutes kills bacteria, spores, cysts and ova; expels the dissolved gases particularly carbon dioxide, and removes temporary hardness. But the boiled water is not as tasteful as the unboiled water. However, it is safe for drinking.
The chemicals most commonly used for disinfecting water are (i) bleaching powder (ii) potassium permanganate and (iii) certain chlorine tablets. These should be added according to the proportion of the water.
The purification of water by means of filtration is safe only in the hands of persons who possess an intelligent knowledge of the construction of filters. But several simple and effective methods have been devised for this purpose, of which the pot method or Indian Filter, is one which has been used with success. Charcoal and sand are used in that well known simple and cheap Indian Filter, which consists of four earthen ware vessels, arranged one above the other in a wooden frame.