Importance of Water:

Without water there would have been no life. For this reason in early times habitation used to be near rivers, lakes and springs.

          Water constitutes nearly two-thirds of the total weight of the body, 79 % of blood, 80 % of brain and muscles and 10 % even of bones. It is required both for internal and external clean­liness of the body. Usually it is utilised as a solvent and diluent in the body. Its main functions are that it:

(1)Replaces loss of fluids from tissues.

(2)Maintains the fluidity of blood and lymph.

(3)Helps elimination of waste material of the body.

(4)Acts as a vehicle for dissolved food.

(5)Helps in the secretion of digestive juices.

(6)Regulates body temperature and acts as a dis­tributor of body heat.


The population should be provided not only with enough of water, but also with qualitative water. Water should not cause any pathological change in the organism, should not cause of spread of infectious diseases, and also not to cause unpleasant sensations.


Waters used for drink and everyday needs, must correspond to the demands:

·        good organoleptic properties: refreshing temperature, transparence, colorless, no smell and no taste .

·        harmlessness of its chemical composition

·        the absence of pathogenic microorganisms

·        safety in the radiological attitude

 The pollution of water sources represents the important ecological problem. Depending on type of pollution there are:

Ø     chemical,

Ø     physical (radioactive substances, hot water),

Ø     bacterial, virus and biological.

Ø     Industrial wastewater is characterized by considerable quantity of components.

       Major categories of water pollutant

Ø     1. Infections agents  -  Bacteria, viruses

Ø     2. Organic chemical - Pesticides, plastics, detergents, oil, and gasoline

Ø     3. Inorganic chemicals - Acids, caustics, salts, metals

Ø     4. Radioactive materials  -  Uranium, thorium, cesium, iodine, radon


Sources of Water Supply. The chief sources of water suply are:

(a)Rain water or snow water,

(b)Surface water i.e., streams, canals, rivers, lakes, tanks and ponds.

(c)Upland surface water and natural/artificial lakes.

(d)Ground water, i.e., wells and springs.

(e)Sea water.

Water is Essential for Life

It covers 71% of the earth's surface and makes up 65 % of our bodies. Everyone wants clean water-- to drink, for recreation, and just to enjoy looking at. If water becomes polluted, its loses its value to us economically and aesthetically, and can become a threat to our health and to the survival of the fish living in it and the wildlife that depends on it.

Water is the main substance of biosphere, without which the existence of organic nature is impossible. Water is vital to humans. It is needed for food preparation, drinking, washing, and irrigation. In addition, massive quantities are used daily in industrial processes. Any vital process cannot be performed without water, and any cell cannot exist in anhydrous environment. Water has the importance not only for drinking and meal, and also for normal existence of the human. Yet, it is a limited resource that must be collected and distributed with increasing care.

The most important source of water is rain, which may be collected directly in cisterns and reservoirs or indirectly through a watershed system or well. A watershed is the network of rivulets, streams, and rivers by which entire areas are watered. Ground water is rain that has trickled through rock layers, forming pools after many years. If it is under pressure, groundwater may bubble to the surface as a spring. Irrigation canals, reservoirs, wells, and water towers are man-made devices for diverting and collecting water from these natural sources. Because of contamination concerns, water from reservoirs, wells, and rivers is usually processed in a treatment plant before distribution.

Because of enlargement of cities, villages, development of economy and raising of cultural inquiries of the population the consumption of water increases with each year. While choosing a water source for water supply its output and quality of water, and, also stability of parameters, which characterize its reliability that is determined by its origin and conditions of formation of water quality, and also character and intensity of its pollution are taken into consideration

The pollution of water sources represents the important ecological problem. Depending on type of pollution there are: chemical, physical (radioactive substances, hot water), bacterial, virus and biological. Industrial wastewater is characterized by considerable quantity of components.

It is a lot of chemical substances, which come at reservoir, worsen biological and organoleptic properties of water (smell, smack, color, turbidity, formation of a pellicle, foams etc) that not favourably influences on water usage and population’s health.

The output of industries, agriculture, and urban commu­nities generally exceeds the biologic capacities of aquatic systems, causing waters to become choked with an excess of organic substances and organisms to be poisoned by toxic materials. When organic matter exceeds the capacity of those microorganisms in water that break it down and recycle it. the excess of nutrients in such matter encourages rapid growth, or blooms, of algae. When they die, the remains of the dead algae add further to the organic wastes already in the water; eventually, the water becomes deficient in oxygen. Anaerobic organisms (those that do not require oxygen to live) then attack the organic wastes, releasing gases such as methane and hydrogen sulfide. which are harmful to the oxygen-requiring (aerobic) forms of life. The result is a foul-smelling, waste-filled body of water, a situation that has already occurred in such places as Lake Erie and the Baltic Sea and is a growing prob­lem in freshwater lakes of Europe and North America. The process by which a lake or any other body of water changes from a clean, clear condition—with a relatively low concentration of dissolved nutrients and a balanced aquatic community to a nutrient-rich, algae-filled body and thence to an oxygen-deficient, waste-filled condition is known as accelerated eutrophication.


 As a matter of fact all water is primarily derived from oceans. In tropical regions, evaporation of water into air is so great that it has been estimated that about 700 gallons (3182.20 litres) of water is evaporated every minute from each square mile (2.59 square kilometres) of ocean surface.

Water reaches earth in the form of rain, hail, snow, dew or mist, from water vapour in the atmosphere, derived mainly from evaporation of the sea, from lakes, rivers and other waters of the land. Sea water contains sodium chloride and land water contains lot of dissolved and suspended impurities, but it evaporates in the form of pure distilled water which reaches earth again in the form of rain, snow or hail. This condensed water from the air, which is the ultimate source of all our natural water supply, is pure except for a few impurities that are absorbed from the atmosphere.

A part of rain water on reaching earth is evaporated again into the atmosphere and a part of it percolates into the earth. Some part of it gets col­lected in the form of lakes, ponds etc. But a major portion of it runs away at once in the direction of natural slope of ground and gets collected in the form of small streams, which forms rivers and finally it runs again to the sea and thus the cycle goes on repeating. This phenomenon is known as Hydrolog-cal Cycle.

Sources of Water Supply.

The chief sources of water supply are:

(f) Rain water or snow water,

(g)Surface water i.e., streams, canals, rivers, lakes, tanks and ponds.

(h)Upland surface water and natural/artificial lakes.

(iGround water, i.e., wells and springs.

(j)  Sea water.

 Ground Water. It is superior to surface water, because the ground provides water an effective filtering medium. Water gets filtered and purified, while passing through it. Wells and springs constitute important sources of ground water.

Wells. These are artificial holes or pits dug into the earth to reach the underground water level. They constitute a very important source of water supply in Indian villages. There are four varieties of wells:

(1)         Shallow   Wells   are   those   which   do   not penetrate an impermeable stratum. They simply tap the subsoil water, i.e., ground water lying between the surface and first impermeable stratum. The water of these wells gets pollut zd, cither from surface water or from contamination of subsoil water. Their water is moderately hard.

(2) Deep Wells are those which tap some water bearing layer below the first impermeable stratum. They may pass through one or more impermeable layers. They yield comparatively safer water for drinking purposes than shallow wells on account of efficient filtration, because their water travels a greater distance through the eai (h and also gets bet­ter protection from surface contamination by the impermeable strata above. They yield as a rule, more permanent supplies than shallow wells. The water is usually pure and germ free, but is often hard.

(3)Artesian Wells are a variety of deep wells in which water under great pressure comes out to the surface automatically. To accomplish this, the strata, which the well penetrates, must be cup shaped and the upper level of the ground water tapped between the two impervious strata must be higher than the surface of the earth, where the well lies. Thus in such a case the water shoots up. They are named after Artois province in France where they have been in use for a very long time.

 Norton's Abyssinian Tube Wells are really shallow wells which are bored by simply driving iron pipes 1 1/2" to 2" (3.8 to 5 cm.) in diameter and 20-25 ft. (6.096 to 7.62 metres) deep to tap the ground water. A pump is attached to the pipe to draw the water. Chiefly they were used temporarily in the Abyssinian Campaign. Nowadays these are used where the ground water is not many feet deep below the surface of earth. These wells are of use only when a temporary water supply is required. Since water is drawn out by means of pumps, their water is free from most of the dangers manifested in open wells.

Requirements of a Sanitary Well.


A well is a hole in the earth from which a fluid is withdrawn. Water wells are the most common type. Oil and natural gas wells are also common. Mining companies also pump steam and hot water down wells to remove salt and sulphur from deep in the ground (Gunning.)

Water Wells

The underground water that flows into wells is called ground water. Most of this water comes from rain that soaks into the ground and slowly moves down to the ground water reservoir, an area of soil and rock saturated with water. The top of this zone is the water table, the level at which water stands in a well that is not being pumped (Taylor.)

In damp places, the water table may lie just below the surface. It is easily reached by digging. A dug well is usually lined with bricks, stone, or porous concrete to keep the sides from caving in. In drier places, the water table may be hundreds of feet or meters down. It may be necessary to drill the well and sink pipes. Power-driven pumps usually draw the water out of deep wells (Berger, Cossi, Taylor


Impure water may be purified by either of the following methods:


(a) Pounding or Storage.

(b) Oxidation and Settlement.


I. Physical



II. Chemical


(b)Disinfection or Sterilisation.

III. Filtration

(a)"Biological" or "Slow Sand" Filtration.

(b)"Rapid Sand" or "Mechanical" Filtration.

(c)Domestic Filtration.


            Purification of water is of great importance in community medicine. It may be considered under two headings.

Ø     Purification of water on large scale

Ø     Purification of water on small scale


Three main steps in purification of water on large scale:

Storage, Filtration, Chlorination

1. Storage:

Water is drawn out from source and impounded in natural or artificial reservoirs. Storage provides a reserve of water from which further pollution is excluded.


Ø     Physical — About 90% of suspended impurities settle     down in 24 hours by gravity.

Ø     Chemical — The aerobic bacteria oxidize the organic matter present in water with the aid of dissolved oxygen. As a result the content of free ammonia is reduced and a rise in nitrates occur.

Ø     Biological — 90 % of total bacterial count drops in first 5 - 7 days.

 How is water treated?

Coagulation:  Alum and other chemicals are added to water to form tiny sticky particles called "floc" which attract the dirt particles.

Sedimentation:  The heavy particles (floc) settle to the bottom and clear water moves to filtration.

 Filtration:  The water passes through filters that help to remove smaller particles. 

Disinfection:  A small amount of chlorine is added to kill any bacteria or microorganisms that may be in the water. 


 Water is placed in a closed tank or reservoir where it flows through pipes to homes and businesses in the community.

 2. Filtration

Filtration is important because 98 - 99% of bacteria are removed by filtration, a part from other impurities. Two types of filters are in use, they are:

a.          Slow sand filters (biological filters)

b.          Rapid sand filters (Mechanical filters)

a. Slow Sand Filter.

Following are the elements of a slow sand filter.

*          Supernatant (raw) water:

Supernatant water is present above sand bed, its depth varies from (1 – 1,5 m). It serves 2 important functions.

i. It serves as a constant supply of water,

ii.          It provides waiting period of some hours and there is partial purification by sedimentation & oxidation

* Sand bed:

It is most important part of the filter; its thickness is about (1,2 m). The sand grains are of diameter between (0,15 – 0,35 mm). Sand bed is supported by a layer of graded gravel. Water percolates through the sand bed very slowly and during this it is subjected to number of purification processes — mechanical straining, sedimentation, adsorption, oxidation and bacterial action.

Vital Layer

Few days (3 to 5 days) after laying the filter, surface of sand bed gets covered with a slimy, green coloured growth made of fungi, algae, bacteria, diatoms and plankton. The formation of vital is known as "ripening of the filter". The growth present on sand bed is called as "Schmutzdecke". It removes organic matter, holds bacteria and oxidizes ammoniacal nitrogen into nitrates and helps in yielding a bacteria - free water.

 Under - drainage system

It consists of porous or perforated pipes lying at the bottom of the filter bed and provides an outlet for filtered water and also provides support to the filter medium above.

Slow filters clear water well only after they’re "ripening": the diameter of pores in the sand decreases, owing to the keeping of suspended matters in the highest layer. So, small particles, worm eggs and larvae and to 99.9% of bacteria can be detained. The clean sand does not absorb viruses. But after the "ripening", about 50-99% viruses are kept in the slow filters. If such filters are used correctly they free water from cercaria. At the same time the series of biological processes proceed in the ripped highest layer - biological film: the mineralization of organic matters and the death of bacteria. The contaminated highest layer of the sand is changed in 30-60 days.Slow filters are used when water turbidity doesn't exceed 200 mg/l in the small country water-pipes. They are always used after the preceding aeration to remove surplus iron and manganese.

Filter control valves:

The filter is equipped with certain valves and devices which are incorporated in the outlet pipe system maintaining a steady rate of filtration.

When the vital layer becomes dense and resistance to the passage of water is increased the supernatant water is drained off Sand bed is cleaned by scrapping of the top portion of the sand layer to a depth of 1 - 2 cms. Scrapping is done 20 - 30 times. The process is known as Filter Cleaning.


b. Rapid Sand Filter

Rapid sand filters are of two types, the gravity type and the pressure type. Both the types are in use. The following steps are involved in the purification of water by rapid sand filters.

 i. Coagulation:

Raw water is treated with a chemical coagulant such as alum. The dose required is usually 5 - 40 mg/liter.

ii. Rapid mixing:

The treated water is subjected to rapid mixing for few minutes. This allows a quick and thorough dissemination of alum throughout the bulk of water.

iii. Flocculation:

Treated water is gently stirred in a "flocculation chamber" for about 30 minutes. In chamber there are paddles which act as flocculator. They rotate at speed of rate per minute. This slow and gentle stirring results in the formation of a thick, copious, white flocculent precipitate of aluminum hydroxide. The thicker the precipitate or flock diameter, the greater the settling velocity.

iv. Sedimentation:   

The coagulated water is now led into sedimentation tanks where it is detained for periods varying from 2 - 6 hours.

v. Filtration:

The partly clarified water now subjected to rapid and filtration.

 Filter Beds:

Each unit of filter bed has a surface of about 80 - 90 m2. Sand is filtering medium. The effective size of sand particles is between 0.6 - 2.0 mm. The depth of sand bed is usually about 1 m.  Below the sand bed is a layer of graded gravel 30 - 40 cm. The gravel supports the sand bed and the filtered water to move freely towards the under drains. The rate of filtration is 5 - 15 m3/ m2/hour.

Back - Washing:

Rapid sand filters need frequent washing daily or weekly. Washing is accomplished by reversing the flow of water through the sand bed, which is called "back-washing". Back - washing dislodges the impurities and cleans up the sand bed.

The disinfection is one of the most usable methods of water improvement. Usually it is a concluding and very important stage. The most spread methods of disinfection are different methods of chlorination. Sometimes ozonization and UV-irradiation use also.


Chlorination is the process in  which chlorine is added to water for purification. Chlorination-is more effective when pH of water is around 7.

 Effects of Chlorine:

a.          Chlorine kills pathogenic bacteria, it has no effect on spores and certain viruses.

b.          It has germicidal effects.

c.          It oxidizes iron, manganese and Hydrogen sulphide

d.          If destroys some taste and odour producing constituents.

e.          It controls algae and slim organisms

f. It aids coagulation

Action of Chlorine.

When Chlorine is added to water, there is formation of hypochlorous and hydrochloric acid. The hydrochloric acid is neutralised by alkalinity of the water. The hypochlorous acid ionizes to form hydrogen ions and hypochlorite ions as follows.

H2O + CI2              HCI--+HOCI

HOCI                        H++OCI"

The disinfecting action of-chlorine is mainly due to hypochlorous acid and to a small extent due to hypochloriteon.

Principles of Chlorination

a.          First, water should be clear and free from turbidity.

b.         Chlorine demand of water should be estimated.

c.          At least one hour is essential as a contact period of free residual chlorine for killing bacterial and viruses.

d.         Minimum recommended concentration of free chlorine is 0.5 mg/L for one hour.

e.          The sum of the chlorine demand of the specific water plus the free residual chlorine of 0.5 mg/l constitutes the correct dose of chlorine to be applied.

Methods of Chlorination

a.         By means of chlorine gas   It is of first choice because it is cheap, quick in action, efficient and easy to apply. Chlorinating equipment is required to apply chlorine gas to water as chlorine gas is irritant to eyes.

b.         By means of Chloramine: Chloramines are loose compounds of chlorine and ammonia. They have slower action than chlorine. They give more persistent type of residual chlorine. They have a less tendency to produce chlorinous taste.

Chlorine Demand It 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 (1 hour) at a given temperature and pH of water.

Residual Chlorine: Amount of untreated chlorine, remaining in the water after some time as an effective disinfecting agent i.e. 0.3 – 0.5 mg/liter

 Break point chlorination:

The point at which the chlorine demand of water is met and if further chlorine is added free chlorine begin to appear in water

Super Chlorination:

It is addition of large doses of chlorine to the water and removal of excess of chlorine after disinfection.

Agents alternative to Chlorination

Ø     a. Ozonation

Ø     b. U.V. radiation     

The ozonization of water

 Ozone has been used in water treatment since 1903. It is more effective against bacteria and viruses than chlorine and adds no chemicals to the water. Ozone cannot be stored and requires an on-site ozone generator. In general, ozonation equipment and operating costs are higher than other treatment procedures.

     Ozone contains three oxygen atoms. It is destroyed in water, forming atomic oxygen: O3 → O2  → O. ozonization is one of the best methods of disinfection: water is well disinfected, organic admixtures become destroyed, organoleptic features are improved. Water becomes blue and it is equated with spring water.

     Ozone dose is 0,5 - 6 mg/l. Sometimes, higher doses are necessary for the lighting of water and improving other organoleptic features. The time of disinfection is 3-5 min. The remaining ozone should make up 0,1 – 0,3 mg/l. The concentration of the remaining ozone 0.4 mg/l provides the reliable inactivation of 99 % viruses for 5 min.

Ultraviolet Light

        Ultraviolet irradiation will kill bacteria by creating photochemical changes in its DNA. No chemicals are added to the water by this process. Most ultraviolet water treatment units consist of one or more ultraviolet lamps usually enclosed in a quartz sleeve, around which the water flows. The UV lamps are similar to fluorescent lamps and the quartz sleeve surrounding each lamp protects the lamp from the cooling action of water. The killing effect of the lamp is reduced when the lamp temperature is lowered. Ground water is usually a constant temperature year round and so it is possible to set a flow rate that will not lead to excess cooling.

        The effectiveness of UV irradiation depends on the intensity of the light, depth of exposure and contact time. Water passes in a relatively thin layer around the lamp; therefore, water flow must be regulated to ensure that all organisms receive adequate exposure. If the water is at all turbid, or if it contains traces of iron, the effectiveness UV is greatly reduced. In such cases, the water needs to be filtered before it reaches the UV system.

       The maximal bactericidal effect is achieved by the waves 250-260 nm, which pass even through the 25 cm layer of transparent and decolorized water.

            The disinfection proceeds very quickly: vegetative forms of microorganisms die in 1-2 min. The turbidity, colour and iron salts decelerate the disinfection, decreasing the transparence of water. Consequently, it is necessary to light and decolorize water before the disinfection.

            There are some advantages of UV-irradiation over the chlorination:

Ø      bactericidal rays don't denaturate the water and don't change its organoleptic features,

Ø     they have wider biological action.

Ø      Their bactericidal action is spread over the spores, viruses and worm eggs, resistant to chlorine.

            Many investigators consider this method the best for the disinfection.


a.         By Boiling:

Water should be boiled for 5 -10 minutes.

It kills all bacteria, spores, cysts & ova.

It removes temporary hardness

Taste is altered but is harmless

b.         Chemical disinfection

i) Bleaching Powder (CaOCI2)

Bleaching powder is a white amorphous powder.

Produced by action of chlorine on slaked lime.

When freshly made contains 33 % of available chlorine.

It must be stored at dark, cool, dry place in a closed container that is resistant to corrosion.

In practise one cup (250 g) of laundry bleach is mixed with three cups (750 ml) of water to make a litre. Three drops of this solution are added to 1 litre water for disinfection. Contact period is 30 minutes to 60 minutes.

ii.) Chlorine Solution

Chlorine solution may be prepared from bleaching powder.

* If 4 kg of bleaching powder with 25 % available chlorine is mixed with 20 litres of water, it will give a 5% solution of chlorine.

* This solution should be kept in dark, cool and dry place in closed container

iii. Chlorine tablets

Available under different trade name e.g. Halazone

One tablet of 0.5 g is sufficient to disinfect 20 litres of water. Used in camps and during travel.

iv. Iodine:

Two drops of 2 % ethanol solution of iodine is used. A contact period of 20 - 30 minute is sufficient for 1 litre water.

v. Potassium Permanganate.

It is a powerful oxidizing agent but not recommended as it alters colours, smell and taste of water.

C. By Filtration

Water can be filtered and is purified. Different filters are.

The new filtration system uses centrifugal force to spin the untreated water above the filter media (sand). This helps remove suspended solids that accumulate on the inside walls of the tank 

Oddsei - What are the odds of anything.