Water
Water is the most abundant and a vital natural resource compound which occupies about 75% of the earth’s crust. It has an important role in living organisms like plants and animals and in all aspects of industrial developments. The most important industrial application of water is in steam generation apart from other common uses in industries like paper, textiles, chemicals, leather etc.
Sources of water
Generally there are two sources of water:
(i) Surface water
(ii) Underground water
Surface water
The surface water is again subdivided in to
(i) Rain water
(ii) Lake water
(iii) River water
(iv) Sea water
(i) Rain water: This is the purest form of natural water. This gets contaminated by the polluted atmosphere and contains suspended organic and inorganic impurities.
(ii) Lake water: This has the least dissolved minerals and has other types of organic impurities. Few lakes provide pure water that can be used for drinking purpose after proper treatment by disinfection, i.e. Sasthamcottah Lake in Kerala and Nainital in Uttar Pradesh.
(iii) River water: This origin of water is spring water and it also collects rain during the course of flow over the surface of the land. It dissolves the soluble minerals from soil and it also contains organic impurities.
(iv) Sea water: This is the most impure form of natural water containing about 3.6% dissolved minerals and a major portion of which is common salt
(2.5%). The presence of excess sodium chloride makes the sea water brackish in taste. Other salt usually present in sea water are sulphates of sodium, bicarbonates of potassium, calcium, magnesium, bromides of potassium, magnesium and a quite a number of salts.
Underground water
Underground water is divided in to well water and spring water. Earth absorbs rain water and it reaches the underground rock layer. During this course it dissolves as many soluble minerals present in the soil region. Hence underground water is rich in minerals but least in organic impurities. This water may come out in the form of minerals but least in organic impurities. This water may come out in the form of springs or it can be obtained by digging wells at water deposits underground regions.
Water quality parameters
Colour and odour
Colour of water may be due to certain dissolved minerals as well as free colloidal particles suspended in water. Other factors which attribute to the colour are industrial effluents suspended in water. Even pure water is not colour less. It has a pale blue or green colour when stored in large quantities. Colour can be determined by using Tintometer.
Similarly odour in water may be due to presence of decayed micro organisms like bacteria, algae, fungae etc., sewage or industrial effluents. Presence of chlorine, sulphur, phosphorous and nitrogen compounds are earthly impurities like mud, clay etc. also cause unpleasant odour in water.
Turbidity
Turbidity is the measurement of lack of water clarity. Turbidity is the result of suspended solids in the water. Suspended solids are variable, ranging from clay, silt and plankton to industrial wastes and sewage. Turbidity can be measured accurately with a turbidimeter. Turbidity is measured in NTUs, the abbreviation for nephelometric turbidity unit. Turbidity in drinking water should be less than one NTU. Water treatment is required for drinking water when there is excessive turbidity. High turbidity water will appear to be muddy. Turbidity in excess of five NTUs can be easily detected. Turbidity at that level may not affect your health, but water treatment may be desirable. Excessive turbidity may interface with disinfection processes and is measured by municipalities to monitor the efficiency of public water supply filtration systems used to remove parasites and viruses in the water.
Temperature
Temperature is an important parameter in its effect on the solubility of oxygen in water, the rate of photosynthesis by algae and higher plants, the metabolic rates of aquatic organisms, and the sensitivity of organisms to toxic wastes, parasites and diseases. Colder water can hold more dissolved oxygen. Significant increases in water flow where are operating or soil erosion may eliminate cold-water aquatic species and increase large aquatic growth.
Hardness
Water hardness is a state or quality of being hard caused by various dissolved salts of calcium, magnesium or iron. Hardness is expressed as PPM of calcium carbonate. Hard water precipitates carbonate mineral deposits, scale and incrustations on pipes, hot water heaters, boilers and cooking utensils. Water hardness can cause other problems in the home such as increased in the soap consumption by preventing soap and detergents from lathering by giving rise to an insoluble curdy precipitation. Water hardness can be lessened on a small scale by softening the water. Hard water can be softened by on a large scale by adding just enough lime to precipitate the calcium as carbonate to remove the calcium salts. Home water softeners commonly use natural or artificial zeolite minerals to soften the water. Zeolite minerals are hydrous aluminum silicates of sodium, calcium, potassium or barium.
pH
pH is a general measure of the acidity of alkalinity of a water sample. The symbol pH stands for potential for hydrogen. The pH of water, on a scale of 0 to 14 is a measure of the hydrogen ion concentration. Water contains both H ions and OH ions. Pure distilled water contains equal number of H and OH ions and is considered neutral (pH 7), neither basic nor acidic. If water contains more H than OH ions the water is considered acidic with pH less than 7. If water contains more OH than H ions, the water is considered basic with a pH greater than 7. Stream water usually ranges from pH 6.5 (slightly acidic) to a pH of 8.5, an optimal range for most organisms. Rain water by contrast is naturally acidic at about 5.6.
Electrical conductivity
It is a measure of conductance of water and is the capacity of water to conduct electricity current. It is due to presence of free ions produced by the dissolution of mineral salts. Conductivity of water increases with increase in temperature and it is proportional to the concentration of the dissolved salts. Conductivity water used for analysis purpose should not be less than 2.
Total acidity
It gives the ability of water to neutralize alkalinity caused by hydroxyl ions. Acidity of water may be due to industrial effluents, acid rain and other factors like dissolved carbon dioxide, sulphur compounds etc. The total acidity can be measured by titration of water sample against a standard alkali.
Alkalinity
Alkalinity is a measure if quantity of compounds that shift the pH to the alkaline side of neutrality about 7 or it is a measure of capacity of water to neutralize acids. The pH of a normal stream usually falls between 6.5 and 8.5. Mostly alkalinity is due to the presence of bicarbonates ion which is derived from the dissolution of carbonates by carbonic acid. Minor contributors to alkalinity include carbonate and hydroxide ions. Alkalinity is important because it buffers pH changes that occur naturally during photosynthesis cycles, water exchanges and the addition of acids to water. Raising the alkalinity almost always raises the pH. Alkalinity is measured in part per million (ppm) or milligrams per litres (mg/L). A range to 100 or 250 ppm for river water is considered normal. If the alkalinity of water is too high, the water can be cloudy which inhibits the growth of underwater plants. Too high alkalinity raises the pH level, which in turn harm or kills fish and other river organisms.
Chloride
Chlorides are binary compounds of chlorine. A chloride is made of chlorine chemically combined with a metal. It is formed naturally when hydrochloric acid reacts with any metal in the water. Chloride is common in areas with limestone deposits but it is not found in most other soils, rocks or minerals. The presence of chloride, when it is not found in most other soils, rocks or minerals. The presence of chloride, when it does not occur naturally indicates possible water pollution. Other sources of chloride are septic tank effluent, animal waste and potash fertilizer. Other forms of chloride are iron chloride, calcium chloride and cupric chloride. Chloride contaminates rivers and ground water and can make it unsuitable for humans to drink. High levels of chloride kill plants and wildlife. The normal range for river surface water is 45-155 mg/L and for ground water the normal range is 35-125 mg/L. at concentrations greater than 250 to 400 mg/L the water testes salty. High concentration is corrosive to most metals.
Sulphide
The sulphide of river water and ground water is usually hydrogen sulphide. Hydrogen sulphide is usually produced by the reduction of sulphate. Hydrogen sulphide inputs an objectionable rotten egg odour to water. Hydrogen sulphide concentrations of less than 0.1 mg/L are noticeable to most people. Hydrogen sulphide can also corrode metal pipes and fixtures. Waters with excessive hydrogen sulphide can be aerated to remove the hydrogen sulphide.
Sulphate
Industries and utilities that burn coal release sulphur compounds in to the atmosphere to become a part of the acid rain problem. Dissolved sulphate is derived from the dissolution of gypsum or the oxidation of sulphide minerals such as pyrite. Dissolved sulphate is stable under oxidizing conditions. However, under reducing conditions, it can be converted to hydrogen sulphide. Dissolved sulphate can combine with calcium to form scale heaters and boilers. At concentrations exceeding 500-600 mg/L it imparts a bitter taste.
Dissolved oxygen (DO)
Dissolved oxygen is a measure of the amount of oxygen freely available in water. It is commonly expressed as a concentration in terms of milligrams per liter (mg/L) or ppm. Or as percent saturation which is temperature dependent. Percent saturation is the percent of the potential capacity of the water to hold oxygen that is present. The DO for surface water ranges from 0 in extremely poor water conditions to a high of 15 mg/L in 0 degree Celsius water. Oxygen is the single most important gas for most aquatic organisms; free oxygen (O) or DO is needed for respiration. The colder the water, the more oxygen it can hold. DO levels below 3 ppm are stressful to most aquatic organisms. DO levels below 1 ppm will not support fish; levels of 5 to 6 ppm are usually required for most fish. Oxygen gets in to the river when water crashes over rocks, when wind blows over the water and by any other method which causes air to mix with water. Based on this fact, fast moving rivers have more oxygen content that slow moving rivers.
Biological oxygen demand (BOD)
Biological oxygen demand is a measure of the oxygen in the water than is required by the aerobic organisms. When measured, BOD is the decrease in the oxygen content in milligrams per liter of a sample of water kept in the dark at a temperature of 20 degrees Celsius over a specified period of time. The difference between the initial DO level of the collected water and final DO level is due to the consumption of oxygen brought about by the bacterial breakdown of organic material and the oxidation of chemicals in the water during the storage period. As a rule, BOD is measured after five days (BOD5) at room temperature. River with high BOD have high nutrient levels in the water. Most of the oxygen is consumed by the organisms. River with low BOD have low nutrient levels, therefore, much of the oxygen remains in the water. Unpopulated, natural water will have a BOD of 5 mg/L or less.
Chemical oxygen demand (COD)
Chemical oxygen demand is a measure of oxidisable impurities present in the water. COD may be defined as the amount of oxygen required for the oxygen required for the oxidation of biologically oxidisable and biologically inert organic matters such as cellulose etc. in a sample of water by a strong oxidizing compound potassium cromate. Since BOD measures only oxygen consumed by living organisms, the COD values are generally higher than BOD values. The advantages of COD are that it takes only about 2 hours compared to the larger period of 5 days for the BOD determination.
Fecal coliform
Fecal coliform is defined as those bacteria that ferment lactose with gas formation and grow on specialized media within 24 hours at 44.5 degree Celsius. Fecal coliform can be found in populations ranging from 0 to thousands of colonies per millimeter of water. Fecal coliform are distinguished from other coliform because they are able to ferment lactose at higher temperatures than 35 degrees Celsius. High counts of fecal coliform bacteria in rivers, streams and lakes are caused by contamination from the intestinal tract of humans and other warm-blooded animals. Ecoli index bacteria are not in themselves harmful, but are associated with other bacteria and viruses which are, such as typhoid fever, hepatitis A, cholera, dysentery, shigellosis etc. current coliform bacteria. Current regulatory standards for drinking water are based on the presence or absence of total coliform bacteria. Fecal coliform determinations are required on any sample testing total coliform bacteria. Fecal coliform determinations are required on any sample testing total coliform positive. Coliform bacteria are indicator organism s used to indicate the presence of viruses and other pathogenic organisms. More than 1 fecal coliform colony indicates that the water supply may be vulnerable to contaminated by human or other warm-blooded animals, but will acclimatize to warm water at near-freezing temperature. Sapling water for coliform bacteria in the winter months in northern latitudes is therefore, an exercise in futility.
Iron
Iron is a chemical element and one of the most important metals in the world. The normal range for iron in freshwater is 0.1-0.5 ppm or mg/L. if there is too much iron in water, the water appears dusty. This water has a metallic taste and it stains fistures, utensils and laundry. High concentrations of iron form reddish-brown ferric hydroxide sediments, coatings and strains. Rust occurs when iron combines with the oxygen in water. Despite the problem of rusty drinking and wash water, iron is needed by our body. It helps carry oxygen throughout the body. Iron deficiency in the blood will cause body fatigue. Certain foods provide the human body with the iron that is required.
Nitrates
Nitrogen is a much more abundant element in nature than phosphorous. Nitrogen is known to be an important plant nutrient, thus, it is used often as a fertilizer and is found in high concentrations in agricultural runoff. Nitrate concentrations result from functioning septic systems. As with phosphorous, too much nitrogen also contributes to eutrophication of lakes and streams. Nitrates in river water often ranges from 0.01 to ppm to 3 ppm. The presence of nitrates in ground water indicates an oxidizing condition in the aquifer. An elevated concentration (0.10mg/L) is indicative of the influence of man such as the use of nitrate fertilizer, septic tank failure and the vulnerability of the aquifer by surface drainage.
Total phosphorous
Phosphorous is a non-metallic chemical element. Phosphorous is found in many forms. In water, phosphorous is usually found as phosphates. Phosphates are formed when metallic atoms replace some or all of the hydrogen in phosphoric acid. Generally the lower the total phosphorous values in the water, the better the quality. Total phosphorous includes organic and inorganic phosphate. Organic phosphate is a part of living plants and animals. Inorganic phosphates comprise the ions bonded to sol particles and phosphates present in laundry detergents (polyphosphates). Phosphorous is considered to be a limiting factor in aquatic systems, meaning that it is not phosphorous come from erosion, fertilizers, detergents and the draining of wetlands.
Suspended solids
Suspended solids are pieces of sand, silt and fine inorganic matter of leaves, pieces of wood etc., suspended in a stream or lake. The suspended solids in humid areas range from 0 ppm to 100 ppm. Suspended solids can increase when stream flow increases. The faster flowing water erodes the banks and because the stream is moving so fast, the suspended solids don’t have a chance to settle down. High suspended solids can make the water unusable in many ways. Pesticides and bacteria can attach to the suspended solids making it more readily transportable. This can kill off the plants and animals downstream and make the water undrinkable to humans and wild life alike.
Total dissolved solids (TDS)
Rivers have solids particles in them called dissolved and suspended solids. The total dissolved solids test measures the amount of particles that are dissolved in the river water. The TDS ranges from 20 to 2000 mg/L in rivers and may be higher in groundwater. High levels in drinking water may cause objectionable tastes. Water is tested for TDS because excessive amounts may be unsuitable for aquatic river life and poor for crop irrigation, in addition to being unsuitable for drinking water. It may cause foaming or may corrode some metals. The quantity of TDS in a body of water depends on several factors, including the precipitation contributing to the body of water (rainwater is almost pure with less than 10 ppm TDS), the type of soil and rock the water passes over and human activities. The major dissolved substances found in water can cause the above problems are the positively charged ions of sodium, calcium, magnesium, potassium and iron and the negatively charged ions of chloride, bicarbonate and sulphate.
Total solids
Total solids are the combined weight of both total dissolved solids and suspended solids. All natural waters contain dissolved and suspended inorganic and organic substances. The major dissolved solids are sodium, potassium, calcium, magnesium, chloride, sulphate, carbonate, bicarbonate and silica. The major suspended solids on the other hand include anything from silt and plankton to industrial wastes and sewage.
Hardness and softness of water
Hardness
This is the property of water which prevents the formation of lather or foam readily with soap. Water showing this property is called hard water. This is mainly due to the presence of dissolved salts of calcium and magnesium and other heavy metals in water.
Soap is the sodium or potassium salts of higher fatty acids like oleic, palmitic, stearic etc. When soap is treated with water containing calcium or magnesium salts, these salts react with soap to produce white precipitate of insoluble soap of calcium or magnesium and hence it does not gives lather.
Softness
This is the property of water by virtue of which it gives lather or foam readily with soap. Water showing this property is called soft water. This is due to the absence of calcium, magnesium and other heavy metal salts in water.
Types of hardness
There are two types of hardness
- Total hardness or carbonate hardness
- Permanent hardness or non-carbonate hardness.
Temporary hardness or carbonate hardness
This is due to the presence of dissolved bicarbonates of calcium and magnesium and other heavy metals. Temporary hardness can be removed by boiling water. On boiling water, the soluble bicarbonates of calcium, magnesium etc. are decomposed to form insoluble precipitates of carbonates, hydroxides etc. which can be removed by mechanical means.
Permanent hardness or non-carbonate harness
This is due to the presence of soluble chlorides and sulphates of calcium and magnesium and other heavy metals. Permanent hardness cannot be removed by boiling the water as the soluble chlorides and sulphates of calcium; magnesium etc. cannot be converted in to insoluble compounds on boiling. Total hardness is the sum of temporary hardness and permanent hardness.
Alkalinity
Alkalinity of water is due to presence of hydroxide, carbonate and bicarbonate ions. This can be divided in to two categories (i) caustic alkalinity and (ii) Bicarbonate or temporary alkalinity. The caustic alkalinity is due to the presence of hydroxide and carbonate ions. Bicarbonates or temporary alkalinity is due to the presence of bicarbonate ions. Alkalinity can be estimated by titrimetry method in which water sample is titrated against a standard acid using phenolphthalein and methyl orange indicator. The phenolphthalein end point represents the completion of first two reactions and methyl orange end point represents the completion of all the three reactions. Hence the amount of acid used after the phenolphthalein end point corresponds to one half of the carbonate and all the bicarbonates. The total amount of acid used after the methyl orange end point corresponds to the total amount of hydroxide carbonate and bicarbonate ions, i.e., total alkalinity.
Disadvantage of hard water
In domestic use
If hard water is used for washing or bathing purposes, it won’t give lather freely with soap. Instead it gives white sticky precipitates of insoluble soap of calcium and magnesium. Soap will be first consumed for the precipitation of these salts and then after the complete precipitation, it start giving free lather. This causes wastages of soap. If hard water is used for cooking purpose, it causes wastages of fuel as the dissolved salts will elevate the boiling point of water. Moreover it gives different taste to food material due to the presence of different dissolved salts.
In industrial use
Textile industry: Hard water causes wastage of soaps for the washing of fabrics etc. and the precipitates of calcium and magnesium soap produced during washing will stick to the fabrics. These fabrics will not produce the exact shade of colour when dyed. Presence of iron and manganese also cause coloured spots on the fabrics.
Dyeing industry: The dissolved salts of Ca, Mg, fe, mn present in hard water may react with costly pigments of dyes forming precipitates which cause multiple shading or spots on the fabrics being dyed.
Sugar industry: Hard water contains sulphates, carbonate, nitrate etc. of different metals and causes problems in crystallization of sugar during manufacturing process.
In steam generation in boilers: If hard water is used for the steam generation in boilers, it gives trouble like scale and sludge formation, priming and foaming and corrosion of boilers parts etc.
Boiler feed water or industrial water
The main industrial application of water is that it is used for steam generation. The water used for steam generation should be free from dissolved salts like calcium, magnesium, iron etc. The water suitable for steam generation in boilers is called boiler feed water. If we use naturally occurring hard water for steam generation should be free from dissolved salts like calcium, magnesium, iron etc. The water suitable for steam generation in boilers is called boiler feed water. If we use naturally occurring hard water for steam boilers, this will give rise to problems in boiler operations. Common boiler problems are (i) scale and sludge formation (ii) priming and foaming and (iii) boiler corrosion.
Scale and sludge formation
During boiler operations water evaporates continuously and the concentration of dissolved salts increases progressively. When it reaches a saturation point, it starts precipitating.
Scales: Scales are hard and sticky precipitates formed inside the boiler, which sticks firmly on the inner walls of the boilers like a cement coat. The scales are formed mainly due to precipitation of salts like calcium sulphate, magnesium hydroxide, calcium silicates, magnesium silicates etc.
They are very hard and cannot be removed by mechanical means. It can be removed of EDTA wash or acid wash. Scales so formed are poor conductors of heat, which leads to wastage of fuel and decrease the efficiency of the boiler.
Sludge: Sludges are soft and loose precipitates formed inside the boiler during boiling of water and settle down at the bottom of the boiler. The sludges are formed due to the precipitation of magnesium chloride, magnesium sulphate, calcium chloride, magnesium carbonate etc.
As they are looses or slimy precipitates, they settle down and block or choke the valves, pipelines etc. connected with the boiler and thereby hinder the boiler operation. Hence this also reduces the efficiency of the boiler. Sludges can be removed by blow down operation in which concentrated water from downward portion of the boiler is removed frequently.
Priming and foaming
Priming: During rapid boiler operations, water droplets are carried along with the steam. This process of wet steaming is called priming. Priming is caused by presence of large amounts of dissolved salts, high steam velocities, sudden/rapid boiling, very high boiler water level, improper boiler design etc. Priming can be minimized by using treated water, by using mechanical steam purifiers, maintaining medium water level, proper design and by using uniform heating etc.
Foaming: It is the production of persistent foam bubbles on the surface of boiler water which do not break easily. Foaming is due to the presence of impurities like oils which reduces the surface tension of water, grease, clay organic matters, dissolved salts etc. This can be minimized by using anti-foaming chemicals like castor oils, polymides, sodium aluminates etc. during boiler operation.
Boiler corrosion
Use of natural water in steam boiler makes the boiler parts corroded due to different factors like chemical or electro chemical attack by environment. The corrosion of boilers occurs due to dissolved oxygen, dissolved carbon dioxide, dissolved alkali contents and acids produced by the hydrolysis of salts usually iron, foaming the corrosion products.
Corrosion due to dissolved oxygen can be minimized by chemical or mechanical means. Some of the chemical used for the removal of dissolved oxygen contents are sodium sulphate, sodium hydrazine, sodium sulphide etc.
Dissolved oxygen can be removed by mechanical deaeration also. The carbon dioxide dissolved in water produces carbonic acid which is harmful to the metallic boiler parts like other acids.
This can be produced due to the decomposition of salts.
The dissolved carbon dioxide can be removed by adding required quantity of ammonium hydroxide.
Dissolved carbon dioxide can also be removed by mechanical deaeration. Acids produced during the boiler operation due to the hydrolysis of different salts will be another reason for corrosion of boiler parts.
Caustic embrittlement
Caustic embrittlement is a type of corrosion due to the continued action of caustic alkali and stress. The sodium bicarbonate or sodium carbonate present in the natural water gets hydrolysed during the boiling process and is converted in to caustic soda or sodium hydroxide.
The sodium hydroxide containing water flows through the joints, bends, hairline cracks, rivets, stressed parts etc. and gets concentrated thus producing minute local ized electro chemical cells which result in the corrosion of boiler.
Boiler parts
Metal parts at the anodic regions form sodium ferrate. This makes the appearance of fissures and blisters on the inner boiler surface. Corrosion due to caustic enbrittlement can be avoided by adding required quantity of acids to neutralize the alkalinity of natural water used or by adding sodium phosphate instead of sodium carbonate during softening of water or by adding lignin, tannin etc., which will block the hair cracks, gaps etc.
Treatment of water
In order to make water suitable for boiler operations we have to treat the water to remove harmful hardness causing dissolved salts and other impurities. The process of removing hardness producing salts and other impurities in water is known as softening. There are two types of treatment of water. Internal treatment and external treatment. In internal treatment the water is softened within the boilers and in external treatment water is made soft before feeding in to the boiler.
Internal treatment
In spite of the external treatment of water the concentration of water the concentration of salts go on increasing inside the boiler and if it is not treated then the boiler problems may not be avoided. In internal treatment the soluble problems may not be avoided. In internal treatment the soluble hardness causing metal ions like calcium, magnesium etc. are converted in to insoluble salts or they will be converted in to some other forms of soluble salts which will not interface with boiler operations or they will not give any boiler operation problems.
Different internal treatment or internal conditioning
Colloidal conditioning
Scale formation can be avoided by adding chemicals like tannin, lignin, agar-agar gel, kerosene etc. to boiler operations. When sticky crystalline compounds gets precipitated, these chemicals get coated over them and make them looses and slimy precipitate, settle down easily and can be removed by blow down operation.
Carbonate conditioning
Scale formation can be avoided by adding calculated quantity of sodium carbonate to boiler water during boiler operations, where the salts causing scale formation will be converted in to insoluble loose precipitate of respective carbonates which can be removed by blow down operations. Excess of sodium carbonate added may lead to caustic embrittlement.
Phosphate conditioning
The chances of caustic embrittlement due to the excess of sodium carbonate can be avoided by adding sodium phosphate instead of sodium carbonate. The sodium phosphate reacts with soluble calcium and magnesium salts in to insoluble precipitate of phosphate of calcium and magnesium which can be removed by blow down operations. There are three types of phosphates, namely, trisodium phosphate, disodium hydrogen phosphate and sodium dihydrogen phosphate which are used for the different types of natural water on the basis of its acidity or alkali.
Calgon conditioning
During boiler operation calgon (sodium hexa meta phosphate is added to boiler water which converts the scale forming salts in to soluble complex phosphates which will not interface with the boiler operation.
External treatment
Treatment of water done before feeding in to the boilers in order to remove the harness causing salts is otherwise known as external treatment. Some of the external treatment methods are lime soda process, zeolite process or permutit process and ion exchange method or determination or de-ionosation method.
Treatment of water for domestic supply
The water used for domestic purpose should have the following requirements:
(i) It should be clear and odourless.
(ii) It should be free or turbidity.
(iii) It should be free of objectionable minerals like lead, manganese, chromium, arsenic etc.
(iv) It should have a pH range of 7-8.
(v) it should be free from pathogenic bacteria or any other harmful micro organisms.
(vi) Its total dissolved solids should be less than 500 ppm.
The water obtained from different sources is subjected to screening sedimentation, coagulation, filteration, and sterilization to obtain the above requirements.
1. Screening: The natural water is passed through screens, having large number of perforations which separate the floating matter from the natural water.
2. Sedimentation: In this process water is allowed to stagnate in large tanks for 4-8 hours time undisturbed and by this time the suspended impurities or particles settle down at the bottom automatically due to gravity.
3. Coagulation: This process involves the purification of natural water containing fine clay particles and other colloidal impurities it is necessary to add coagulants like sodium aluminates, alum ferric or ferrous sulphate etc., which will coagulate the colloidal impurities and settle down at the bottom.
4. Filtration: Filtration is another process of purification of water. In this process, colloidal impurities and most of the biological micro organisms are removed by allowing the water to filter through a bed of fine sand and other coarse of less dense anthracite.
5. Sterilization (Disinfection): Sterilization of disinfection is the removal and destruction of pathogenic bacteria and other micro organism by adding chemicals to the natural water and makes it safe for drinking purpose. The chemical substances used for this process are called disinfectants. Chlorine, bleaching powder, ozone, UV light are some of the disinfectant agents.