Water

Water is essential to life. Without it, the biosphere that exists on the surface of the earth would not be possible. It is also the only known chemical compound that occurs in all the three physical states viz., solid (snow, hail, sleet and ice), liquid (rain, water droplets) and gas (water vapors). Water is the chemical substance with chemical formula H₂O, one molecule of water has two hydrogen atoms covalently bonded to a single oxygen atom.

Water covers more than 70 % of the earth’s surface, of which 97% is in the ocean, which is unfit for human consumption and other uses because of its high salt content. Of the remaining three percent, 2% is locked in the polar ice caps and glaciers and only one percent is available as fresh water in rivers, lakes,  streams, reservoirs,  and  ground  water which  is  suitable  for  human  consumption.  A continuous and supply of clean water is essential for the survival and health of all living organisms.

Characteristic of water

Physical characteristics of water

Turbidity of water: Turbidity is the amount of suspended matter in water and expressed in parts per million

Color: The colour of a good water is transparent. The colour of water is measured by tintometer.

Taste and odour: Preferably, the water should be free from taste and odour.

Chemical characteristics of water

Total solids and suspended solids: The suspended solid can be found by filtering the water sample. Total permissible amount of solids in water is generally limited to 500 ppm.

pH of water: pH of water should be 7.

Hardness of water: Hard waters are undesirable because they may lead to greater soap consumption, scaling of boilers, causing corrosion and incrustation of pipes, making food tasteless etc.

Sources of water

Source water refers to bodies of water (such as rivers, streams, lakes, reservoirs, springs, and ground water) that provide water to public drinking-water supplies and private wells. Water on the earth is broadly classified under two categories. They are

Surface water

Ground water

Surface water

Surface water is quite a broad term when we look at it. It consists of any above-ground water which gets collected. For instance, we have ponds, rivers, lakes, oceans and more. Surface water collects on the ground or in a stream, river, lake, reservoir, or ocean. Surface water is constantly evaporating out of water bodies, seeping into ground water supplies, and being replenished by rain and snow.

Surface water is the most used source of water. It accounts to at least 80 per cent of the water used by living beings. Following are the sources of surface water

Rain water: Rain water is the purest form. It is obtained by natural process of evaporation and condensation of water.

River water: A river is a ribbon-like body of water that flows downhill from the force of gravity. A river can be wide and deep, or shallow enough for a person to wade across. A flowing body of water that is smaller than a river is called a stream, creek, or brook. Some rivers flow year-round, while others flow only during certain seasons or when there has been a lot of rain. The largest rivers can be thousands of miles long.

Lake: A lake is a body of water that is surrounded by land. There are millions of lakes in the world. They are found on every continent and in every kind of environment—in mountains and deserts, on plains, and near seashores. Lakes vary greatly in size. Some measure only a few square meters and are small enough to fit in your backyard. Such small lakes are often referred to as ponds. Other lakes are so big that they are called seas. The Caspian Sea, in Europe and Asia, is the world’s largest lake.

Sea water: This is the most impure form of natural water. Rivers throw all the impurities carried with water into sea. The continuous evaporation of water from surface of sea increases concentration  of dissolved impurities. This sea water contains about 3.5% of dissolved impurities, out of which about 2.5% is sodium chloride. Thus, sea water cannot be used directly for domestic or industrial purpose.

Ground water

When we say groundwater, we mean the source of water which is found beneath the layer of soil. It exists in the soil and between rocks and other things. Groundwater contributes to 30% of water which we use in our daily lives.

A part of rainwater which reaches earth’s surface or water from river percolates into the earth. This water travels downwards, during which it comes in contact with various mineral salts present in the soil and dissolves some of them. Water continues its downward flow till it meets hard rock.

Spring and well water is generally clear in appearance as it is filtered through soil layers, but contains considerable quantity of dissolved salts. The water from well and spring contains more hardness. The underground water is suitable for domestic use.

Impurities of water

Water available on earth is not in its purest form. The water is generally contaminated with materials which are often referred to as impurities. The common impurities present in natural water may be classified as follows:

Suspended particles

Dissolved impurities

Colloidal impurities

Biological impurities

Suspended impurities: Suspended impurities are the impurities that are partially soluble in water, and kept suspended in water. These impurities consist of clay, mud, algae, organic matter, etc. Such impurities are referred to as suspended impurities. They are present in most of the surface water. These impurities are generally visible with naked eyes. Suspended impurities in water if consumed can result in a range of medical ailments and illnesses.

Dissolved impurities: The dissolved impurities include salts and minerals. These minerals are not important and are harmful to organisms. Dissolved impurities may also include organic and inorganic salts as well as gases.

Gases: The dissolved gases present in water may include oxygen, carbon dioxide, nitrogen, etc. These gases are soluble in natural water.

Mineral salts: The commonly observed salts in natural water are carbonate, bicarbonates, magnesium, etc. When salts of calcium and magnesium are present in water, it becomes hard water.

Colloidal impurities: Colloidal impurities are such impurities which doesn’t settle down even in standing water and cannot be removed by filtration. Size of colloidal particle is about 10^–4 cm to 10^–7 cm.

Biological impurities: Biological impurities in water are caused by the presence of living organisms. These include algae, protozoa, pathogens, bacteria, viruses, microbes, and parasites along with their cysts (eggs) in contaminated water. Such water is dangerous for human consumption.

Hard and soft water

Soft water

Soft water is the type of water that contains lower amounts or lower concentrations of minerals like calcium and magnesium. However, soft water may include salt like sodium or potassium dissolved in it. Soft water tastes salty while drinking.

Hard water

The water with naturally present minerals like magnesium and calcium with detectable amount is called hard water. Hard water contains more minerals, but that doesn’t mean it’s contaminated. Contamination is different than mineral content and refers to germs and bacteria instead. Minerals are valuable nutrients for your body, although too many may pose health risks.‌‌ Minerals improve water’s taste.

Difference between hard water and soft water

Hard Water	Soft Water
It is rich in minerals	Contains very few elements
Soap is not so effective	Soap is easily effective
No foam and lather from soaps	Bubbly lather from soaps
Leaves spots on the washed dishes after they are dried	Does not leave any spots on dishes after they are dried
Contains minerals like magnesium and calcium	Contains sodium ion
Sometimes preferred drinking water	Sometimes not preferred drinking water
Example: Groundwater like deep wells	Example: Rainwater
Hair and skin become dry	Hair and skin become soft

 

Causes of hardness of water

Hardness of water is due to the presence of calcium and bicarbonates present in it. Deposition of calcium and magnesium salts make the water hard. Water receives such mineral while moving in its examples. Some reasons are mentioned below:

Rainwater absorbs carbon dioxide from air and also from decaying plants on soil. This rain water when flows over the rocks containing calcium and magnesium carbonate reacts slowly with the substances forming bicarbonates. Chloride and sulphate of calcium and magnesium are also present on the surface layer. These salts are soluble in water. Thus, water becomes hard.

Types of hardness

There are two types of hardness. They are as follows

·         Temporary or carbonate hardness

·         Permanent aur non carbonate hardness

Temporary or carbonate hardness

The presence of magnesium and calcium carbonates in water makes it temporarily hard. In this case, the hardness in water can be removed by boiling the water.

When we boil water the soluble salts of Mg(HCO₃)₂ is converted to Mg(OH)₂ which is insoluble and hence gets precipitated and is removed. After filtration, the water we get is soft water.

Permanent or non-carbonate hardness

Permanent or non-carbonate hardness is due to the presence of dissolved chlorides, sulphates of calcium and magnesium. Permanent hardness cannot be destroyed on boiling. It requires special chemical treatment for the removal of hardness. There are several softening methods that is used to treat permanent hard water. Some of them

Difference between temporary and permanent hardness of water

Temporary hardness	Permanent hardness
This type of hardness is due to soluble bicarbonates of calcium and magnesium.	It is due to soluble chloride and sulphate salts of calcium and magnesium.
Since it is due to carbonates, and hence it is known as carbonate hardness.	It is due to salts other than carbonates and hence known as non- carbonate hardness
Temporary hardness can be removed by boiling.	Permanent hardness cannot be removed by boiling. They require different softening techniques

 

Harmful Effects of Hard Water

·         Some of the most common signs of hard water include:

·         Linens and clothes look dull and feel rough.

·         Ugly stains on white porcelain and scale build-up on faucets

·         Low water pressure from showers due to clogged pipes.

·         Chalky, white residue or spots appear on dishes.

·         Strains appearing in the shower.

Degree of hardness

The amount of hardness causing substances in a certain volume of water measures the degree of hardness. The hardness of water is always calculated in terms of calcium carbonate CaCO₃. However, it is never responsible for causing hardness as it is insoluble in water.

The choice of CaCO₃ is due to its molecular weight 100 and equivalent weight 50. The units commonly used to express hardness of water is as follow

Different units used to express degree of hardness

·         ppm (parts per million) = This is weight in milligrams of CaCO₃ equivalent to all hardness causing substance in one million (10⁶) milligrams of water.

·         mg/L (milligram per litre) = weight in milligrams of CaCO₃ equivalents in on litre of water.

·         ⁰Cl (degree Clark) = weight in grams of CaCO₃ equivalents in 70,000 grams of water.

·         ⁰Fr (degree French) = weight in grams of CaCO₃ equivalents in 105 grams of water.

Relationship between these units

1 ppm = 1 mg/L = 0.1⁰Fr = 0.07 ⁰Cl

Boiler and steam generation

The manufacturing industries need water for a great variety of purposes out of which steam generation is the most important one. Hard water has an adverse effect on steam boilers. Hence, water for raising steam in boilers must be soft and must not contain too much impurities. Impure water produces foam, and also deals with several other problems such as corrosion, caustic embrittlement, scale and sludge formation.

A boiler or steam generator is a device used to create steam by applying heat energy to water. It is a closed vessel made of high-quality steel in which steam is generated from water by the application of heat. The water receives heat from the hot gases formed by burning fuel through the heating surface of the boiler. Steam is mainly required for power generation, process heating, and space heating purposes.

Problems associated with hard water in boiler

Corrosion: The most common problem associated with boiler due to the use of hard water is corrosion. Corrosion causes the decaying of boiler. Hard water consists of dissolved gases and salts which chemically reacts with metallic surface of boiler. This leads to decaying of boiler.

Caustic embrittlement: Caustic embrittlement is also a type of corrosion for boiler. This type of corrosion is caused by the use of highly alkaline water in boiler and it is generally observed in the boiler which operates under a high pressure. During water softening process, small amount of sodium carbonate is added. In high pressure boilers, sodium carbonate decomposes to give sodium hydroxide and carbon dioxide.

Due to the form of sodium hydroxide (caustic soda), water becomes more alkaline. There are minute cracks in the inner lining of boiler. The alkaline water flows into the cracks of boiler lining due to capillary action. Water evaporates and dissolved caustic soda is left behind. The alkaline action of caustic soda attacks the surrounding areas of cracks thereby dissolving iron material of the boiler. This causes embrittlement of boiler parts, particularly, at stressed parts like rivets, bends, joints, etc. causing even a failure of the boiler.

Prevention for caustic embrittlement

·         Use sodium phosphate instead of sodium carbonate for softening water.

·         Use lignin as additives to the boiler water which will block the minute cracks thereby preventing infiltration of caustic soda solution.

·         By adjusting the alkalinity of water to optimum level.

Priming and Foaming: When a boiler is steaming (i.e., producing steam) rapidly, some particles of the liquid water are carried along-with the steam. This process of 'wet steam' formation is called priming.  Priming is caused by

·         the presence of a large amount of dissolved solids

·         high steam velocities,

·         sudden boiling

·         improper boiler design

·         sudden increase in steam-production rate.

Foaming is the production of persistent foam or bubbles in boilers, which do not break easily. Foaming is due to presence of substances like oils (which greatly reduce the surface tension of water).

Priming can be avoided by:

(i) fitting mechanical steam purifiers;

(ii) avoiding rapid changing steaming rate;

(iii) maintaining low water levels in boilers

(iv) efficient softening and filtration of the boiler-feed water.

Foaming can be avoided by:

(i) adding anti-foaming chemicals like castor oil

(ii) removing oil from boiler water by adding compounds like sodium aluminates.

Limitation of priming and foaming

(i) dissolved salts in boiler water are carried by the wet steam to super-heater and turbine blades, where they get deposited as water evaporates. This deposit reduces their efficiency

(ii) dissolved salts may enter the parts of other machinery, where steam is being used, thereby decreasing the life of the machinery;

(iii) actual height of the water column cannot be judged properly, thereby making the maintenance of the boiler pressure becomes difficult.

Scale and sludge formation

In a boiler, water is continuously evaporated and converted into steam. As a result, the water becomes saturated due to increase in the concentration of dissolved impurities. Finally, a stage is reached where the ionic products of these salts exceed their solubility product and are thrown out as precipitates on the inner walls of the boiler.

Sludge: It is soft, loose and slimy precipitate formed within the boiler. It is formed at comparatively colder portions of the boiler and are collected at the bends. It is formed by substances which have greater solubility in hot water than in cold water, e.g., MgCO₃, MgCl₂, CaCl₂, MgSO₄, etc

Disadvantages of sludge formation

·         Sludges are poor conductors of heat, so they tend to waste a portion of heat generated.

·         Excessive sludge formation disturbs the working of the boiler.

Sludge can be removed by using

 1) Softened water

 2) by blow down operation i.e., drawing off a portion of the concentrated water.

Scales: It is hard deposits firmly sticking to the inner walls of the boiler. The hardness of scale depends upon the nature of impurities present in the water. The most troublesome scales are formed due to the presence of sulphates and silicates of calcium and magnesium. Such scales are non-porous and non-conductor of heat. Scales are difficult to remove, even with the help of hammer & Chisel.

Scale is formed due to:

i) Decomposition of Calcium bicarbonate:

In high pressure boilers, CaCO₃ is soluble due to formation of Ca (OH)₂

ii) Deposition of CaSO₄

Solubility of CaSO₄ decreases with increase in temperature.

It is completely insoluble in super-heated water.

Hard scale formation takes place in high pressure boilers

iii) Hydrolysis of Magnesium salts:

Soft scale formation due to hydrolysis of Mg salts in high pressure boiler.

Disadvantages of scale formation:

i) Wastage of fuel:

Rate of heat transfer is greatly reduced due to poor conductivity of scales

Overheating is required for steady supply of heat hence fuel consumption increases.

ii) Lowering of boiler safety:

To supply steady heat overheating is required, which makes the boiler material weak &soft. Results in distortion of boiler tube & makes the boiler unsafe to bear the high pressure.

iii) Decrease in efficiency of boiler:

Deposition of scales in valves and condensers choke them partially& decreases the efficiency of boiler

iv) Danger of Explosion:

Due to uneven expansion the thick scales get cracked, results in formation of large steam & develops high pressure. It may cause explosion of boiler.

Removal of scales:

·         Mechanical/chemical method

·         Loosely adhering scales are removed with the help Scraper/wire brush

·         Brittle scales are removed by giving Thermal shocks

·         Loosely adhering scales are removed by frequent blow down operation (frequently removing precipitates)

 

Softening methods of water

The process of removing soluble calcium and magnesium salts from hard water is called softening the water. During the process of softening of hard water, the soluble salts are converted into insoluble salts. These insoluble salts then can be filtered and soft water can be obtained. Several methods are used for the softening of water. Some of them are as follows:

·         Boiling

·         Lime-soda process

·         Zeolite or permutit process

·         Ion exchange process

·         Clark’s method

Boiling

Boiling water only removes temporary hardness. Boiling water is certainly one of the most effective ways to soften it. The boiling will have the effect of draining the hard water minerals to the bottom, like calcium and magnesium. Boiling precipitates the dissolved minerals out of the water. Since boiling removes the water’s calcium content, the result is softer water.

When you boil water, the salts precipitate leaving clean, soft water.

Put some water in a pot and leave it boiling for at least five minutes for the best results. After the water has boiled sufficiently, turn the heat off.

Let the water cool. Allow sufficient time, so the mineral deposits sink to the bottom. Next, pour the soft water from the pot and leave the mineral deposits in the pot.

Boiling is a quick and cheap way to fix hard water for consumption purposes. However, it only addresses temporary hardness and not permanent hardness.

Clark’s method

Clark’s method is one of the solutions for treating hard water by adding slaked lime into the water. Hardness of water can be defined as the dissolved content of divalent metal cations in water. In Clark’s method, hard water is treated with Ca (OH)₂. This method removes hardness by conversion of bicarbonates into carbonates. 

Clark's technique for softening water:

Hard water is softened with in Clark's water softening technique (slaked lime). Clark's reagent is Calcium Hydroxide. Calcium hydroxide is used in Clarke's method to soften water (lime). It eliminates transient hardness. By turning bicarbonates into carbonate, it softens the water.

Slaked lime, either in solid or liquid form, is added to water using this technique. Soluble bicarbonates are changed into insoluble carbonates as a result.

In this method, a calculated amount of lime is added to hard water, it precipitates out calcium carbonate and magnesium hydroxide which can be filtered off.

Chemical reaction involved in Clark’s method

Lime soda method

The water is treated with lime or a combination of lime and soda ash (carbonate ion). These chemicals react with the hardness and natural alkalinity in the water to form insoluble compounds. The compounds precipitate and are removed from the water by sedimentation and, usually, filtration. When water has minimal magnesium hardness, only calcium needs to be removed. Only enough lime and soda ash are added to water to raise pH to between 10.3 and 10.6, and calcium hardness will be removed from the water.

When lime and soda ash are added, hardness-causing minerals form nearly insoluble precipitates. Calcium hardness is precipitated as calcium carbonate CaCO₃. Magnesium hardness is precipitated as magnesium hydroxide Mg (OH)₂. These precipitates are then removed by conventional processes of coagulation/flocculation, sedimentation, and filtration.

Chemical reaction involved in soda ash method

 

Hardness	Lime	Precipitate
+
+
+
+

 

Soda ash method

Soda Ash is the common name for sodium carbonate, which is a chemical compound with the formula Na₂CO₃. This compound is also known as washing soda and soda crystals. In this method water is treated with a calculated amount of soda ash (also referred to as washing soda). Soda ash converts the chlorides and sulphates of calcium and magnesium into their respective carbonates which get precipitated.

Chemical reaction involved in soda ash method

Zeolite or permutit process

The zeolite softening process is used for removing both the temporary and permanent hardness of the water by precipitating the calcium and magnesium present in water as insoluble zeolites.

In this process of softening of water, there is an exchange of Ca²⁺ and Mg²⁺ ion with the help of zeolite. Hence it is called the zeolite softening process. Zeolite is micro porous mineral which is used as catalyst in many industrial purposes such as water purification. The zeolites are hydrated aluminosilicates and general composition is Na₂Al₂Si₂O₈. Zeolites are of two types i.e., synthetic and artificial. The natural zeolite that is used for water softening is gluconites. Permutit is the synthetic zeolite that is most used in water softening. Permutit are more porous, glassy and have higher softening capacity than gluconites.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Zeolite process is commercially accepted for the fact that zeolite can be easily regenerated. When Ca²⁺ and Mg²⁺ ions containing hard water passes through a bed of sodium zeolite, the sodium ions are replaced by the calcium and magnesium ions. When all sodium ions are replaced by calcium and magnesium ions, the zeolite becomes inactive. Then zeolites need to be regenerated. Brine solutions are passing through the bed of inactivated zeolite. The following reactions takes place to form Na₂Ze.

The chemical reactions that take place during softening process are as follows:

Ion exchange method for water softening

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Aside from industry, ion-exchange softeners are widely used in small water systems and individual homes. Ion-exchange resins, exchanges one ion from the treated water for another ion in the resin (sodium is one component of softening salt, with chlorine being the other). Sodium is exchanged for calcium and magnesium by ion exchange resin. Certain organic compounds possess a property like zeolite i.e., they are capable of exchanging ions. Such organic synthetic compounds are known as resins. There are two types of resins

 

Cation exchange resins: These resins are capable of exchanging rapidly cations by H⁺ ions. Cation exchange resins can be represented as RH₂, so their exchange reaction with cations is

Anion exchange resins: These resins are capable of exchanging rapidly anions by OH^– ions. Anion exchange resins can be represented as

 

If hard water is passed first through cation exchanger and then through anion exchanger, the resulting water will be free from both cations and anions and water is said to be deionised or demineralised.

The process of ion exchange is carried out as follows:

It consists of two cylindrical towers, out of which the first tower consists of cation exchanger (RH₂) and the other one consists of anion exchanger[R’(OH)₂]. Hard water is first allowed to pass through a tower containing cation exchanger, which removes all the cations like Ca⁺⁺, Mg⁺⁺, Na⁺⁺ and releases H⁺ ions

Thus, the anions like chlorides, sulphates and bicarbonates are converted into their corresponding acids HCl, H₂SO₄ and H₂CO₃. In other words. This acidic water is then passed through another tower containing an ion exchanger, where acids are converted into water. Consequently, the water thus produced is free from all ions and is virtually distilled water. The water is finally freed from dissolved gases like CO₂ by passing it through a degasifier, which is a tower whose sides are connected to vacuum pump. High temperature and low pressure reduce the quantity of dissolved CO₂ and O₂ in water.

Difference between ion-exchange process and zeolite process

Ion-exchange process	Zeolite process
This process can produce softened water with residual hardness ranging between 0 to 2 ppm.	This process can produce softened water with residual hardness ranging between 0 to 15 ppm.
The resultant water is suitable for all types of boilers, especially high pressure boilers.	The resultant water is not suitable for use in high pressure boilers. Water can be used only in low or medium pressure boilers.
Capital cost is higher	Capital cost is less
It occupies more space	It occupies less space.
This process is useful for acidic as well as alkaline water.	This process is not useful for highly acidic water.

 

Plumbo solvency

All drinking water is plumbosolvent, which means it can dissolve very small amounts of metals if they come into contact with them. Plumbo solvency refers to the dissolution of lead in water. It increases lead concentration in water that passes through lead pipes. This makes water unfit for human consumption and may lead to health hazards. Lead is toxic. It affects the functioning of heart, kidneys, reproductive and nervous system.

Water plumbosolvency can be counteracted by reaching a pH of 7.5 by increasing the pH with lime or sodium hydroxide (lye) or by providing protection on the inside of lead pipes by applying phosphate to water treatment works.

While the optimal pH for plumbosolvency prevention is 7.5, performance in the range pH 7.2-7.6 is still very good. Attaining this pH has been shown to decrease blood lead concentrations in the population.

Additionally, chlorinating water eliminates dissolved lead. With this purpose, if the water could have been in contact with lead, water to be used for drinking or cooking food can never be drawn from a hot-water tank. Water should be taken from a cold water tap and heated in a saucepan or kettle without lead or lead solder.

Water treatment methods

Following methods are generally carried out to make water fit for drinking

·         Sedimentation

·         Coagulation

·         Filtration

·         Sterilization

Sedimentation

Sedimentation is the process of removing suspended impurities by allowing the water to stay undisturbed for some period of time in large tanks when most of the suspended particles settle down due to the force of gravity. The clear water is then taken out from the tank with the help of pumps.

Coagulation

Coagulation is the chemical water treatment process used to remove solids from water, by manipulating electrostatic charges of particles suspended in water. This process introduces small, highly charged molecules into water to destabilize the charges on particles, colloids, or oily materials in suspension. Selecting the right coagulant for a system will enhance overall system performance, and particularly improve solids removal efficiency by enhancing filter and clarifier performance. The commonly used coagulants are the salts of iron and aluminium. These coagulates react with bicarbonates present in water and form bulky gelatinous precipitate called flock. These flocks absorb or catch suspended fine particles from water and form bigger flocks, which settle down quickly. The addition of coagulants to water also removes colour, odour and improves its taste.

Filtration

Filtration is a process of removing insoluble colloidal and bacterial impurities by passing water through a bed of proper sized material. During filtration, the clear water passes through filters that have different pore sizes and are made of different materials (such as sand, gravel, and charcoal). These filters remove dissolved particles and germs, such as dust, chemicals, parasites, bacteria, and viruses. Activated carbon filters also remove any bad odours.

Water treatment plants can use a process called ultrafiltration in addition to or instead of traditional filtration. During ultrafiltration, the water goes through a filter membrane with very small pores. This filter only lets through water and other small molecules (such as salts and tiny, charged molecules).

Sterilization

Sterilisation (also referred to as disinfection in other sources) is the final process to make potable water. It is where all microorganisms are killed or removed. The process of destroying these disease-causing bacteria and microorganisms etc from water is known as disinfection or sterilization of water. The chemicals used for sterilization are known as sterilizers. The disinfection of water can be carried out by any of the following methods

·         Boiling

·         Chlorination

·         Ozonisation

·         Aeration

·         Ultra-violet rays

Boiling: This is the simplest method of sterilization. More often, sterilization is the boiling of water before domestic use. Boiling kill all the pathogenic bacteria like cholera and typhoid within five minutes. But this method is useful only for household purposes, because this process is quite expensive for industrial processes.

Chlorination: The chlorination can be carried by either of the following (i) By using chlorine gas (ii) By adding bleaching powder (iii)

By using chlorine gas Cl₂

Chlorination is the process of adding chlorine to drinking water to kill parasites, bacteria, and viruses. Different processes can be used to achieve safe levels of chlorine in drinking water. Using or drinking water with small amounts of chlorine does not cause harmful health effects and provides protection against waterborne disease outbreaks. It reacts with water to form hypochlorous acid and nascent oxygen; both are germicides.

However, excess of chlorine should be avoided because it produces unpleasant odour, taste and irritating effect on mucous membrane. The treated water should not contain more than 0.1-0.2 ppm of free chlorine.

By adding bleaching powder (CaOCl₂)

Bleaching powder is a good sterilizer for small water works. Bleaching powder which is chemically called as calcium oxychloride (CaOCl₂) is an active agent which is used to kill germs and bacteria in drinking water. The main content of the bleaching powder is the Chlorine element which acts as the main disinfectant material.

Bleaching powder when exposed to moisture will release chlorine and this chlorine kills off the germs and disinfects the area. If this is added to water, the chlorine released will react with water and will cause production of oxygen in the atomic state which is highly reactive.

Disadvantages of using bleaching powder

·         Bleaching powder introduce calcium in water thereby making it more hard.

·         If used in an excess amount, it imparts a bad taste and disagreeable smell to water.

By using chloramine (ClNH₂)

Chloramination is the process of adding chloramine to drinking water to disinfect it and kill germs. It is sometimes used as an alternative to chlorination. Chloramines are a group of chemical compounds that contain chlorine and ammonia. The particular type of chloramine used in drinking water disinfection is called monochloramine which is mixed into water at levels that kill germs but are still safe to drink. Chlorine and ammonia are mixed in ratio 2:1 by volume to produce a compound known as chloramine.

Ozonisation

This is an effective method of sterilization of water. Ozone O₃ is unstable and it decomposes into molecular oxygen O₂ and nascent oxygen O

 

 

 

 

 

 

 

 

 

 

 

 

 

The nascent oxygen thus produced is very effective for killing all the germs and bacteria. In ozonisation, water is allowed to percolate through a tower having perforated partition. Ozone is allowed to enter from the bottom which kills the germs when they come in contact with water. Sterilized water is collected at the bottom of the tank. This method is quite expensive and hence not commercially accepted.

Aeration

Aeration treatment consists of passing large amounts of air through water and then venting the air outside. The air causes the dissolved gases or volatile compounds to release from the water. The air and the contaminants released from the water are vented. This is the most modern method of purifying water for town supply. Water is forced under pressure through a perforated pipe. As water sprays into air, it comes in contacts with the oxygen of air and it is exposed to the ultra-violet rays of the sun. This kills the bacteria and the oxygen oxidises organic matter present in the water. It removes colour and odour also. Natural aeration takes place in streams and rivers when the water flows slowly in its bed or when it falls from a certain height.

Ultra violet rays

The invisible ultra violet rays are very effective in killing all types of bacteria. This method is widely used for disinfection of swimming pool water, because it does not require any chemical to be mixed with water. This method requires focussing of uv rays of water which makes this method quite expensive. Hence this method is not employed for municipal water supply.