Water Technology PDF

Unit-2 Water Technology Sources of Water – Impurities and their influence of living systems – WHO Limits – Hardness and its Determination – Boiler Troubles and their removal – Water Softening Methods – Lime-Soda, Zeolite and Ion Exchange - Municipal Water...

Unit-2 Water Technology Sources of Water – Impurities and their influence of living systems – WHO Limits – Hardness and its Determination – Boiler Troubles and their removal – Water Softening Methods – Lime-Soda, Zeolite and Ion Exchange - Municipal Water Treatment-Break Point Chlorination – Desalination of Sea Water – Reverse Osmosis Method, Electro-dialysis Sources of Water The chief sources of water supply for industrial use are:  Rain water - The purest form of water, collected on the roofs. Yet this method is seldom adopted in industry.  Surface waters. Flowing waters, such as rivers, streams etc. ◦ Still waters, such as lakes, ponds etc.  Ground water. ◦ Water from springs. ◦ Water from shallow wells. In the case of shallow wells, the boring is done only through one geological stratum. ◦ Water from deep wells. Here the boring is done through many geological strata.  Sea water. Its use is very limited as its uses entails very great problems of chemical engineering. Source and nature of impurities of water Water in the form of vapour in clouds is said to be pure. Yet when it condenses as rain and flows on the ground it takes many impurities from atmosphere and ground. The major situations in which water gets impure are as followed:  Dissolved gases: during raining water absorbs much of the gases like oxygen, carbon dioxide, hydrogen sulphide etc. from atmosphere. The resultant water will be slightly acidic and on high concentrations of impurities it may result into acid rain.   Soluble crystalloids: water when flow over the surface of the land (like rivers, streams), dissolves soluble minerals. The most of the soluble minerals include chlorides, sulphates, bicarbonates of sodium, calcium, magnesium and iron.  Where as in seawater 3.5% of dissolved salts are seen in which 2.5% include sodium chloride.  Organic matter: This is derived from the decomposition of plants and small particles of sand and rock in suspension. Impurities and their influence of living systems Types of Impurities present in water: The natural water is usually contaminated by different types of impurities. They are mainly three types. 1. Physical impurities Colour: in water is caused by metallic substances like salts. Turbidity: is due to the colloidal, extremely fine suspensions such as insoluble substances like clay, slit, and micro-organisms. Taste: presence of dissolved minerals in water produces taste. Bitter taste can be due to the presence of Fe, Al, Mn, Sulphates and lime. Soap taste can be due to the presence of large amount of sodium bicarbonate. Odour: In water is undesirable for domestic as well as industrial purpose. 2. Chemical impurities Inorganic chemicals: Cations (Al+3, Ca+2, Mg+2, Fe+2, Zn+2, Cu+2 Na+, K+), Anions (Cl−, SO4−2 , NO3−, HCO3−, F− , NO2−) Organic chemicals: dyes, paints, petroleum products, pesticides, detergents, drugs textile materials, and other organic related materials. 3. Biological Impurities  Biological impurities are Algae, pathogenic bacteria, fungi, viruses, pathogens, parasite worms. Some of the other common impurities present in natural waters may be classified as follows.  Dissolved minerals – mostly comprise of carbonates, bicarbonates, sulphates and chlorides of calcium, magnesium, sodium and potassium.  Dissolved gases – mostly air and carbon dioxide.  Suspended matter – mostly mineral matter, giving turbid or muddy water. Organic matter may also be present.  Colloidal impurities – Products from organic wastes, finely divided silica and clay etc.  Microscopic matter – consists mostly of plant and bacterial life giving colour, taste and odour. In general, the removal of impurities from water of classes 1, 2 and 3 form the chief problem for industrial usage, and 3 and 4 for municipal supplies. WHO Limits WHO produces international norms on water quality and human health in the form of guidelines that are used as the basis for regulation and standard setting world-wide. The Guidelines for drinking-water quality (GDWQ) promote the protection of public health by advocating for the development of locally relevant standards and regulations (health based targets), adoption of preventive risk management approaches covering catchment to consumer (Water Safety Plans) and independent surveillance to ensure that Water Safety Plans are being implemented and effective and that national standards are being met. IS specifications of potable water Many developed countries specify standards to be applied in their own country. In europe, this includes the european drinking water directive and in the usa the united states environmental protection agency (epa) establishes standards as required by the safe drinking water act. For countries without a legislative or administrative framework for such standards, the world health organisation (WHO) publishes guidelines on the standards that should be achieved. India has Indian Standard specifications for drinking water IS: 10500. According to these standards, desirable limits in brief, are 1. Colour 5 hazen units and may be extended up to 50 if toxic substances are suspected 2. Turbidity 10 nt units and may be relaxed up to 25 in the absence of alternate 3. Ph 6.5 to 8.5 and may be relaxed up to 9.2 in the absence of alternate 4. Total hardness 300ppm and may be extended up to 600 in the absence of alternate Specifications for drinking water, Bureau of Indian Standards (BIS) and World Health Organization (WHO) standards: Hardness and Causes From the standpoint of an engineer the dissolved salts of calcium and magnesium are of special interest as they are responsible for some of the common and major boiler troubles and therefore will be considered in greater detail. A simple method of detecting these is by the use of soap (not detergent). A sample of water that reacts with soap to form a white scum and does not produce lather is known as hard water. Soft water lathers freely with soap. Calcium and magnesium salts are present mostly as bicarbonates, sulphates and also as chlorides. The difficulty in producing lather arises because of the fact that soaps are sodium and potassium salts of higher fatty acids and are soluble in water but when soap is used in water containing calcium and magnesium salts, insoluble soaps of calcium and magnesium are formed and precipitated, thus destroying the soap. A typical reaction may be illustrated as follows: 2C17H35COONa + CaCl2 (C17H35COO)2Ca + 2NaCl Soap (Sodium stearate) (Precipitate) Soluble Insoluble Disadvantages of Hard water Domestic  Washing & Bathing: Hard water does not form lather easily with soap. As a result, a large amount of soap is wasted.  Drinking: Hard water causes bad effects on our digestive system. Sometimes, stone formation takes place in kidneys.  Cooking: The boiling point of water is increased due to the presence of salts. Hence, more fuel and time are required for cooking. Industrial  Textile industry: Hard water causes wastage of soap. Precipitates of calcium and magnesium soaps adhere to the fabrics and cause problem.  Paper industry: Calcium and magnesium salts in water may affect the quality of paper.  Sugar industry: Water containing sulphates, carbonates, nitrates affects the crystallisation of sugar.  Dyeing industry: The salts of calcium and magnesium in hard water react with dyes and spoil the desired shade.  Pharmaceutical industry: Hard water may form some undesirable products while preparation of pharmaceutical products.  Concrete making: Chloride and sulphates present in hard water will affect the hydration of cement and the final strength of the hardened concrete.  Finally in industries where steam is employed, if hard water is used in steam production, the troubles like corrosion, scale & sludge formation, priming & foaming & caustic embrittlement are seen. Types of Hardness Calcium and magnesium salts are not the only salts that cause hardness. They are mentioned because they are most commonly present. As a general rule, all soluble salts of heavy metals cause hardness because they form precipitates with soap. The bicarbonates of calcium and magnesium are decomposed by boiling the water when carbonates are formed which are less soluble. Thus the hardness due to the bicarbonates can be removed by boiling. The hardness due to these salts is called temporary hardness. As magnesium carbonate is only partially precipitated, the term carbonate hardness is now used instead of temporary hardness. Sulphates and chlorides of calcium and magnesium are not removed by boiling and hardness due to these is called permanent or non carbonate hardness. Temporary hardness is precipitated from water when heated and is responsible for the scaling of heating elements and metal pipes and therefore very bad in industrial point of view. Permanent hardness consists of non-carbonate base metal salts, and does not precipitate out of heating. Carbonate/Temporary hardness is caused by a particular ratio of carbonate to bicarbonate with a certain buffering action in an aquarium or pond and prevents any pH changes, called “pH shock”, to aquatic life and livestock. Permanent hardness on the contrary is bad for biota since it interferes with osmotic processes of biota and is not suitable to plants and fish. In a boiler house, the degree of hardness of feed water forms a simple and important guide to its suitability for boiler feed. Further knowledge of hardness of water helps in proper designing of engineering processes and structures. Units of Expression of Hardness of Water Normally the hardness of water is represented in ppm. ie. Parts per million. Or mg/L 1mg/L = 1ppm. ppm: It is the number of parts by weight of CaCO3 equivalent hardness present per million (106) parts by water. ie. 1ppm=1part of CaCO3 equivalent hardness in 106 parts of water. mg/L: It is the number of milligrams of calcium carbonate equivalent present per liter of water.  ie. 1mg/L = 1 mg of CaCO3 equivalent hardness per liter of water. But it is known that 1lit = 1kg = 1000 g = 1000 x 1000 mg = 10 6 mg. ==> 1 mg/L = 1 mg of CaCO3 equivalent hardness per 106 mg of water. = 1 part of CaCO3 equivalent hardness per 106 mg of water. = 1 ppm. Hardness is also expressed in the following ways.  Degree clark (°Cl): 1 °Cl = 1 part of CaCO3 equivalent hardness per 70,000 parts of water. i.e. 1ppm = 0.07°Cl.  Degree French (°Fr): 1 °Fr = 1 part of CaCO3 equivalent hardness per 105 parts of water. i.e. 1Ppm =0.1 °Fr. Analysis of Water For Boiler Purposes It involves the  Estimation of Hardness.  Estimation of Dissolved oxygen. Determination of Hardness of water The degree of hardness of water is estimated by the following ways:  Soap titration method.  O.Hener’s alkalimetric method.  EDTA method. Soap titration method It is a simple technique. Soft water gives lather with soap where as hard water does not. The dissolved salts in the hard water react with soap and are precipitated. Thus the amount of soap initially consumed before the formation of lather is a measure of the total hardness and indirectly the dissolved magnesium and calcium salts. The hardness is easily estimated by adding a soap solution of a definite strength to a known volume of the sample of water until a permanent lather is formed on shaking. The standard solution of soap can be obtained from the market or otherwise it is prepared and then standardized in the laboratory with standard CaCl2 solution. O.Hehner’s alkalimetric method Temporary hardness of water can be determined by using HCl or H 2SO4, as the calcium or magnesium bicarbonates react directly with dilute acids. Ca(HCO3)2 + 2HCl → CaCl2 + 2H2O + 2CO2 Mg(HCO3)2 + 2HCl → MgCl2 + 2H2O + 2CO2 The water sample is titrated with dilute hydrochloric acid solution taking in a burette after adding methyl orange indicator. The change of colour from yellow to pale pink indicates the end point. From the titre value the amount of temporary hardness in terms of CaCO 3 equivalents is calculated. Estimation of Hardness in water by EDTA Principle: Ethylene diamine acetic acid (EDTA) in the form of sodium salt yield the anion which forms stable complex ions with Ca2+ or Mg2+. In order to determine the equivalent point, indicator Eriochrome black – T (EBT) is used. When EBT is added to hard water at a pH of about 10, using Ammonical buffer solution (NH4OH-NH4Cl), a wine red unstable complex with Ca2+ or Mg2+ ions is formed. When this solution is titrated with EDTA, at the endpoint wine red colour changes to deep blue colour solution i.e. the EDTA has formed stable complexes. M2+ + EBT→M-EBT + EDTA →M-EDTA + EBT  Unstable wine Red Stable deep blue Experimental procedure Preparation of standard solutions Standard Hard Water: Dissolve one gram of pure, dry CaCO3 in little quantity of dil.HCl and evaporate the solution to dryness on the water bath. Dissolve the residue in small amount of water and transfer it into 100ml standard flask. Make up the solution to the mark with the distilled water and shake the flask well for uniform concentration. EDTA solution: Dissolve 4 gms of EDTA crystals along with 0.1gm MgCl2 in little distilled water in one litre standard flask and make it upto the mark with distilled water. Shake the flask for uniform concentration. Indicator: Dissolve 0.5gms of Erichrome Black – T in 100ml of ethanol / methanol. Buffer solution: Add 67.5gms of NH4Cl to 570ml of Conc. Ammonia solution and dilute with distilled water to one liter. Standardisation of EDTA solution: Rinse and then fill the burette with EDTA solution up to the mark and fix it to the burette stand. Pipette out 20ml of standard hard water solution into a conical flask, add 2ml of buffer solution. Add 2 – 3 drops of EBT indicator, the solution turns to wine red. The contents of conical flask are titrated against EDTA. The end point is achieved with the turning of wine red solution to deep blue coloured solution. The volume of EDTA rundown is noted as V1 ml. Standardisation of Hard water: Rinse and then fill the burette with EDTA solution up to the mark and fix it to the burette stand. Pipette out 50ml of hard water sample into a conical flask, add 5ml of buffer solution. Add 2 – 3 drops of EBT indicator, the solution turns to wine red. The contents of conical flask are titrated against EDTA. The end point is achieved with the turning of wine red solution to deep blue coloured solution. The volume of EDTA rundown is noted as V2 ml. Standardisation of Permanent hardness of water: Rinse and then fill the burette with EDTA solution up to the mark and fix it to the burette stand. Pipette out 100ml of hard water sample into a beaker and boil the water till the volume reduces to 50ml. (all the bicarbonates are dissociated in to carbonates or hydroxides). Cool the solution andfilter the water into a conical flask. Wash the beaker and the filter paper twice and add the filtrate to conical flask. Make up the filtrate up to 250ml. pipette 50ml of the sample add 5ml of buffer solution. Add 2 – 3 drops of EBT indicator, the solution turns to wine red. The contents of conical flask are titrated against EDTA. The end point is achieved with the turning of wine red solution to deep blue coloured solution. The volume of EDTA rundown is noted as V 3 ml. With the volumes of the solutions utilized are known, the hardness can be estimated by the following procedure: Volume of standard hard water used = 50ml. Volume of EDTA used for Standardisation of Standard hard water = V 1 ml. Volume of EDTA used for Standardisation of water sample = V2 ml. Volume of EDTA used for Standardisation of water sample subjected to boiling = V 3 ml. Calculations: 50ml of standard hard water ≡ V1 ml of EDTA consumed. i.e. 50 x 1mg of CaCO3 = V1 ml of the EDTA consumed. 50 ⇒ 1 𝑚𝑙 𝑜𝑓 𝐸𝐷𝑇𝐴 ≡ 𝑚𝑔 𝑜𝑓 𝐶𝑎𝐶𝑂3 𝑒𝑞𝑡. ℎ𝑎𝑟𝑑𝑛𝑒𝑠𝑠 𝑉1 Now for 50ml of sample hard water given: 50ml of sample hard water ≡ V2 ml of EDTA consumed. 50ml of standard hard water : V1 ml of EDTA : : 50ml of sample water : V2ml of EDTA 𝑉2 × 50 ⇒ 1 𝑚𝑙 𝑜𝑓 𝑠𝑎𝑚𝑝𝑙𝑒 𝑤𝑎𝑡𝑒𝑟 𝑐𝑜𝑛𝑡𝑎𝑖𝑛 𝑚𝑔 𝑜𝑓 𝐶𝑎𝐶𝑂3 𝑒𝑞𝑡. ℎ𝑎𝑟𝑑𝑛𝑒𝑠𝑠 𝑉1 × 50 Since the hardness is estimated per 1 litre (1000 ml) the equivalent hardness will be 𝑉2 × 50 1000 × 𝑉2 × 1000𝑚𝑔 𝑜𝑓 𝐶𝑎𝐶𝑂3 𝑒𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡 ⇒ 𝑚𝑔 𝑜𝑓 𝐶𝑎𝐶𝑂3 𝑒𝑞𝑢𝑖𝑣𝑞𝑙𝑒𝑛𝑡 𝑉1 × 50 𝑉1 1000 × 𝑉2 𝑚𝑔 ∴ 𝑇𝑜𝑡𝑎𝑙 ℎ𝑎𝑟𝑑𝑛𝑒𝑠𝑠 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 = 𝑜𝑟 𝑝𝑝𝑚 𝑉1 𝐿 Similarly, 1000ml of the boiled water = 1000ml of sample water  1000 × 𝑉3 𝑚𝑔 𝑃𝑒𝑟𝑚𝑎𝑛𝑒𝑛𝑡 ℎ𝑎𝑟𝑑𝑛𝑒𝑠𝑠 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 = 𝑜𝑟 𝑝𝑝𝑚 𝑉1 𝐿 Temporary hardness = Total Hardness – Permanent Hardness 𝑉2 𝑉3 𝑉2 − 𝑉3 ⇒ 1000 ( − ) ⇒ 1000 ( ) 𝑉1 𝑉1 𝑉1 Boiler Troubles and their removal Boiler troubles If the concentration of salts is more in water that is used for boilers, the following troubles may arise:  Carry over: Priming and foaming  Scale and Sludge formation  Caustic embrittlement Corrosion Carry over: The phenomenon of carrying of water by steam along with the impurities is called carry over. This is mainly due to priming and foaming. As the steam rises from the surface of the boiling water in the boiler, it may be associated with small droplets of water. Such steam, containing liquid water, is called wet steam. These droplets of water naturally carry with them some suspended and dissolved impurities present in the boiler water. Priming Priming is the phenomenon of carrying out droplets of water with steam in boilers. Because of very rapid and violent boiling of water inside the boiler, the water particles mix up with steam and pass out of the boiler. This process of wet steam generation is due to the following reasons.  Presence of large quantity of suspended organic matter, oily matter and alkalies.  Very high steam velocity.  Sudden boiling.  Defective boiler design.   High water level in boiler.  Sudden increase in steam – production rate. Priming can be avoided in the following ways  Using mechanical purifiers or purifying the water by effecting softening and filtrationprocess.  Avoiding rapid change in steam rate.  Maintaining the low water level.  Using well designed boiler providing for proper evaporation of water with uniform heat distribution and adequate heating surfaces.  Minimizing foaming.  Blow down of the boiler (replacing the water concentrated with impurities with fresh water). Foaming is the phenomenon of formation small but persistent bubbles on the surface of water inside the boiler. Foams are generally formed when there is a difference in concentration of solute or suspended matter between the surface film and the bulk of the liquid. Causes: Presence of oily or soapy substances.  Presence of dissolved impurities and suspended matter in high concentrations. Preventive measures  Adding anti foaming agents like castor oil. (It should be noted that excess castor oil also produces foam and the amount of castor oil required differs from boiler to boiler)  Use of coagulants like sodium aluminate for sedimenting the suspended oily impurities before feeding the water to boiler.  Blow down of the boiler. Disadvantages of Priming & Foaming: Priming and foaming causes the following boiler troubles  The actual height of water in the boiler cannot be judged.  Wastage of heat results in decrease of steam pressure and efficiency of boiler.  Water concentrated with dissolved salts may deposit on the parts of the machinery, which cause corrosion. Scale & sludge formation In the boiler the continuous steam production results in the concentration of the dissolved impurities. Then the salts start separating out from the solution in order of their solubility, the least soluble one separating out first. The concentrations of the solids that separate in the liquid and form soft and muddy deposits or form suspension are known as sludges. Whereas some of the solids deposit on the surface form sticky and coherent deposits called scales. The most commonest solids that separate from boiler water are the sparingly soluble calcium salts e.g. CaSO4, Ca(OH)2, CaCO3, Ca3(PO4) and magnesium compounds. Disadvantages of Scale formation 1. Scale effect like an insulator coating on the metal, which results in the reduced rate of heat transfer hindering the boiler efficiency. 2. Scale formation on boiler acts as insulator for water and over heats the metal, making it soft and weak. This is unsafe for boiler at high pressures. 3. The overheating of boiler metal and rapid reaction between water and iron at high temperatures, cause additional thinning of the walls. 4. Under high pressure of steam, the metal expands and causes the cracks in the scales. Sudden entry of water through these cracks to the very hot metal causes sudden cooling of the boiler in the metal with the simultaneous conversion of water into steam. The sudden increase inthe pressure of steam may lead to explosion. Prevention of scale formation External treatment This involves removal of hardness causing impurities and silica before the water fed into boiler. Internal treatment The internal treatment consists of adding chemicals directly to the water in the boilers for removing scales and sludges. 1. Colloidal conditioning: organic substances like kerosene and agar-agar are added to water which prevent the scales. 2. Phosphate conditioning: Phosphates react with water and a loose sludge is formed which can be easily removed by blow-down operation. 3. Carbonate conditioning: In low pressure boilers, scale formation can be avoided by adding sodium carbonate to the boiler water. 3CaCl2 + 2Na3PO4→ Ca3(PO4)2↓ + 6 NaCl 4. Calgon conditioning: Sodium hexa meta phosphate(calgon) is added to boiler water to prevent scale formation by formation of soluble complex. 2CaSO4 + Na3(Na4(PO3)6) → Na2(Ca2(PO3)6) + 2Na2SO4 5. Sodium aluminate conditioning: Sodium aluminate gets hydrolysed yielding NaOH and aluminium hydroxide precipitate. The sodium hydroxide reacts with magnesium salts to magnesium hydroxide. These hydroxides entrap finely divided particles and neutralize the charge on colloidal particles, and the loose precipitate can be removed by the blow-down operation. NaAlO2 + 2H2O → NaOH + Al(OH)3↓ Caustic Embrittlement: Caustic embrittlement is a form of corrosion caused by highconcentration of sodium hydroxide in the boiler water. It is characterized by the formation of irregular cracks on the boiler metal, particularly at places of high local stress, such as riveted seams, bends and joints. The caustic embrittlement occurs in boilers at high pressures, where NaOH is produced in the boiler by the hydrolysis of Na 2CO3. Na2CO3 + H2O → 2NaOH + CO2 The extent of the hydrolysis increases with temperature and may reach even 90% of the carbonate present. The rate and extent of corrosion by caustic embrittlement increases with the concentration of NaOH and the temperature and hence with increasing operating pressure. Preventing of caustic embrittlement 1. By using sodium phosphate as softening reagent instead of sodium carbonate. Disodium hydrogen phosphate is the best softening reagent because it not only forms complex with Ca2+ and Mg2+ resulting the softening of water but also maintains pH of water at 9-10. The other phosphates used are trisodium phosphate and sodium dihydrogen phosphate etc. 2. By adding tannin or lignin to the boiler water which block the hair cracks and pits that are present on the surface of the boiler plate thus preventing the infiltration of caustic soda solution. 3. By adding sodium sulphate to boiler water, which also blocks the hair cracks and pits present on the surface of the boiler plate, preventing the infiltration of the caustic soda solution. The amount of sodium sulphate added to boiler water should be in the [𝑁𝑎2 𝑆𝑂4 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛] ratio [𝑁𝑎𝑂𝐻 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛] kept as 1:1, 2:1 and 3:1 in boilers working as pressures upto 10, 20 and above 30 atmospheres respectively. Boiler Corrosion: The decay of boiler material by chemical or electrochemical attack of its environment is called boiler corrosion. The prime reasons of boiler corrosion are  Dissolved oxygen  Dissolved carbon dioxide  Acids from dissolved salts WATER SOFTENING The process whereby we remove or reduce the hardness of water, irrespective of whether it is temporary or permanent is termed as 'softening' of water. It is very essential process since hard water is unsuitable for domestic as well as industrial use. One of the most important applications of water is in steam production for the generation of electricity. For this water need to be fed to industrial boilers. We just cannot feed any water into the industrial boilers because it has been identified that hard water creates large number of problems like scale and sludge formation, priming and foaming etc. The hardness causing salts can be removed from water by following two ways: (a) External treatment and (b) Internal treatment The External treatment of water is carried out before its entry into the boiler. This treatment prevents boiler problems. It can be done by lime-soda, zeolite or ion-exchange processes. All are preventive methods. Illustration of types of water treatment methods and there point of application. In contrast, the Internal treatment of boiler feed water refers to the conditioning of water in the boiler itself by the addition of chemicals. This is essentially a corrective method to remove those salts which are not completely removed by external treatment of water softening. The following conditioning methods are used in the Internal treatment: Colloidal, Phosphate, Calgon and Carbonate conditioning. Differences between Internal and External treatment methods: EXTERNAL TREATMENT: It can be done by the following methods: (i)Lime-soda process, (ii) Zeolite process, and (iii) Ion-exchange process These important methods by which hard water is commonly softened are discussed below: Lime-Soda Process: The basic principle of this process is to chemically convert all the soluble may be removed by hardness causing impurities into insoluble precipitates which settling and filtration. For this purpose, a suspension of milk of lime, Ca(OH)2, together with a calculated amount of sodium carbonate, Na2CO3 (soda) is added in requisite amount. Proper mixing of the chemicals and water is carried out Calcium carbonate, CaCO3 magnesium hydroxide, Mg(OH)2 ; ferric hydroxide, Fe(OH)3, and aluminium hydroxide, Al(OH)3, so precipitated are filtered off. At room temperature, the precipitates formed are very fine. They do not settle down easily and cause difficulty in filtration. If small amount of coagulants like Alum [K2SÓ4. Al2(SO4)3. 24H2O]; Aluminium sulphate (Al2(SO4)3] or Sodium aluminate [NaAIO2] are added, they hydrolyse to precipitate of aluminium hydroxide which entraps the fine precipitate of CaCO3 and Mg(OH)2. Thus coagulant helps in the formation of coarse precipitates. There are a number of variations of the lime-soda process: Like Batch or continuous; Cold or hot lime soda processes which are briefly described below: (1) Cold Lime Soda Process In these processes water is treated with lime and soda at room temperature in the presence of coagulant. There are four types of cold lime soda softeners which work either in batch process or in continuous process. These are briefly discussed below: (a) The intermittent type (Batch process) Cold Lime-Soda Softener. It consists of a pair of tanks which are used in turn for softening water. Each tank is equipped with a mechanical stirrer, inlets (for raw water and chemicals) and outlets (for soft water and sludge). Raw water and calculated quantities of the chemicals are passed simultaneously into the tank from the opposite sides and the stirring is started with the help of stirrer. For accelerating the process, some sludge (from a previous operation) is also added which acts as nucleus for the fresh precipitation. Reaction is complete by the time tank fills up, and thus stirring is stopped. The sludge (ppt.) formed is allowed to settle down, and is then removed from the tank through the sludge outlet. The softened water is taken out through a floating pipe and sent to the filtering unit. By this batch process, continuous supply of softened water can be ensured by using a pair of tanks planned for alternate cycles of reaction and settling. (b) Continuous cold lime-soda softeners. (i) The conventional type. In this process, raw water and requisite amount of lime, soda and coagulants are fed at room temperature from the top into an inner chamber of vertical circular chamber fitted with a paddle stirrer. Vigorous stirring ensures continuous mixing and thus as the raw water and chemicals flow down, softening of water takes place. The softened water is allowed to come into the outer co-axial chamber. The softened water rising up passes through a wood-fiber filter whereby traces of sludges are removed. The sludge settles down in the bottom of outer chamber by the time the softened water reaches up from where it is periodically removed through the sludge outlet. Filtered soft water comes out continuously through the filtered softened water outlet at the top.  Fig: Continuous cold lime-soda softeners (ii) The sludge-blanket Type. This process is similar to the above process with a difference that the treated water is filtered upwardly through a suspended sludge blanket composed of previously formed precipitates. This ensures complete utili- zation of the added lime. But some lime is wasted in the conventional type as it is carried down in the sludge formed by the precipitates before it has time for dissolution and reaction. Silica is removed better in sludge-blanket type. Moreover, the retention periodrequired is just one hour as against four hours with the conventional type softeners. (iii) Catalyst or spiractor type cold lime-soda water softener. This softener consists of a conical tank which may be open (for gravity operation) or closed (for operation under pressure). About two-third of the conical tank is filled with finely divided granular catalyst (0.3 to 0.6 mm diameter) which can be green sand or sand or graded calcite. Inlets for raw water and chemicals are provided near the bottom of the tank. The outlet for soft water is at the top.  Fig: Catalyst or spiractor type cold lime-soda water softener Method. The raw water and chemicals enter the tank tangentially near the bottom and spiral upwards through the suspended catalyst bed. The sludge formed deposits on the granular catalyst in an adherent form and hence the granules grow in size. The softened water rises to the top from where it is drawn off. It is to be noted that sludge is formed in the granular shape which drain and dries rapidly and can be handled easily. (II) Hot Lime-Soda Process In these processes water is treated with chemicals at a temperature of 94° - 100° C. The softeners used may be of the intermittent type or continuous type. These are briefly discussed below: (a) The intermittent type (Batch process) Hot lime-soda softener. It is similar to the intermittent type cold lime-soda softener except that the heating coils are installed in it for heating water. (b) The continuous type Hot lime-soda softeners. This softener essentially consists of three parts: (i) Reaction tank. This tank has three separate inlets, one each for raw water, chemicals and super-heated steam. After all these three are taken in, they are thoroughly mixed. The beginning and completion of reaction occurs in reaction tank. (ii) Conical sedimentation tank. From the reaction tank the contents go to this tank so that sludge settles down. (iii) Sand filter. It has layers of fine and coarse sand which acts as filter and ensures complete removal of sludge from the softened water. Fig: continuous Hot lime-soda softeners A soft water with 15-30 ppm of residual hardness is obtained by this process. Advantages of Hot Lime-Soda processes: (i) These processes are more rapid in operation. The time taken for completion are 15 minutes and several hours for hot and cold lime soda process respectively. (ii) Elevated temperature accelerates the actual chemical reactions and reduce the viscosity of the water. This increases the rate of aggregation of the particles. Hence, both the settling rates and filtration rates are increased. Thus the softening capacity of the hot process is several times higher than the cold process. (iii) The sludge and the precipitate formed settle down rapidly and hence no coagulant is needed. (iv) Quantity of chemicals required for softening is low. (v) At the higher temperatures, the dissolved gases such as CO2 are driven out of the solution to some extent. (vi) The residual hardness in the softened water is less when it is treated by hot process in comparison to cold lime-soda process. Advantages of Lime-soda Process: (i) Lime-soda process is very economical, (ii) Treated water is alkaline and hence has less corrosion tendencies, (iii) It removes not only hardness causing salts but also minerals, (iv)Due to alkaline nature of treated water, amount of pathogenic bacterias in water is considerably reduced. (v) Iron and manganese are also removed from the water to some extent. Disadvantages of Lime-Soda Process: (i) It requires careful operation and skilled supervision for economical and efficient softening. (ii) Sludge disposal is a problem. (iii) Water softened by this process contains appreciable concentrations of soluble salts, such as sodium sulfate, and cannot be used in high pressure boilers. The main differences between cold and hot lime soda processes are summarized Municipal Water Treatment-Break Point Chlorination Potable water The water which is fit for human consumption is known as potable water Municipalities have to supply potable water, i.e., water which is safe to human consumption should satisfy the following essential requirements 1. It should be sparkling clear and odourless. 2. It should be pleasant in taste 3. It should be perfectly cool 4. Its turbidity should not exceed 10 ppm 5. It should be free from objectionable dissolved gases like hydrogen sulphide. 6. It should be free from objectionable minerals such as lead, arsenic, chromium and manganese salts. 7. Its alkalinity should not be high. Its pH should not be above 8.0 8. It should be reasonably soft 9. Its total dissolved solids should be less than 500 ppm 10. It should be free from disease- producing micro- organisms. Purification of domestic water for domestic use: For removing various types of impurities in the natural water from various sources, the following treatment process is employed; Removal of suspended impurities a. Screening: The raw water is passed through screens, having large number of holes, when floating matter are retained by them. b. Sedimentation: It is the process of allowing water to stand undisturbed in big tanks, about 5 m deep, when most of the suspended particles settle down to the bottom, due to gravity. The clear supernatant water is then drawn from the tank with the help of pumps. The retention period in a sedimentation tank ranges from 2-6 hours. c. Filtration: It is the process of removing colloidal matter by passing water through a bed of fine sand and other proper – sized granular materials. Filtration is carried out by using sand filter. Removal of micro-organisms: The process of destroying /killing the disease producing bacteria, micro-organisms, etc., from the water and making it safe for the use, is called disinfectation. a. Boiling: By boiling water for 10-15 minutes, all the disease producing bacteria is killed and the water becomes safe for use. b. Adding bleaching powder: In small water works, about 1 kg of bleaching powered per 1000 kiloliter of water is mixed and allowed to standing undisturbed for several hours. The chemical action produces hypochlorous acid (a powerful germicide). CaOCl2+ H2O → Ca(OH)2+Cl2 Cl2+ H2O → HCl+ HOCl Germs+ HOCl → Germs are killed c. Chlorination: Chlorination (either gas or in concentrated solution from) produces hypochlorous acid, which is a powerful germicide. Cl2+ H2O → HCl+ HOCl Bacteria+ HOCl → Bacteria are destroyed Break point chlorination (or) or free residual chlorination : Chlorination and Disinfection Basic Principles Disinfection is the process designed to kill or inactivate most microorganisms in wastewater, including essentially all pathogenic organisms. Contrast this to sterilization, which is the removal and destruction of all living microorganisms, including pathogenic and saprophytic bacteria, vegetative forms and spores. Chlorine and its various forms are powerful oxidants that will kill or inactivate most pathogenic organism that are harmful to human and animal life. Chlorination is the most commonly used disinfection process for wastewater treatment. Chlorination chemicals are relatively: Easy to obtain Economical Effective Easy to apply Typical forms of chlorine used in wastewater treatment are: Elemental chlorine Hypochlorite Chlorine Dioxide It involves addition of sufficient amount of chlorine to oxidize: (a) organic matter (b)reducing substance and (c) free ammonia in raw water; leaving behind mainly free chlorine, which possesses disinfecting action against disease- producing bacteria. Chlorine Demand Chlorine will react with wastewater and combine with many of its components. These components react and combine with chlorine prior to its reaction with pathogens. The demand by inorganic and organic materials is referred to as the chlorine demand. It is the difference between the amount of chlorine applied to the wastewater and the amount of residual chlorine after a given contact time. Chlorine Residual The chlorine in combined forms (e.g., monochloramine) that have disinfecting properties plus any free chlorine is the chlorine residual. It is the component of the applied chlorine that is available for disinfection. The residual is available in three forms: Chloramines: A form of combined chlorine Chlororganic Compounds: A weak form of combined chlorine Free Chlorine: The strongest form of residual for disinfection. ▪ The sum of the chlorine demand and the chlorine residual is the chlorine dose. Chlorine Dose = Chlorine Demand + Chlorine Residual, where Chlorine Residual = Combined Chlorine Forms + Free Chlorine You can also rearrange the equation to: Chlorine Demand = Chlorine Dose – Chlorine Residual Establishing Dosages Chlorine dosage may be established from either bench scale laboratory testing, or actual measurement of field results from known plant operation. The results are suitable for establishing base feed rates however; real time corrections must be made to adjust for changing conditions. Since field conditions are not as controlled as laboratory tests, the actual dosage will generally be higher than those established in the laboratory. Calculations to determine the chlorine dosage and chlorine demand as established by field conditions are illustrated in the following example: Example 1.1: A chlorinator is set to feed 50 pounds of chlorine per 24 hours; the wastewater flow is at a rate of 0.85 MGD, and the chlorine as measured by the chlorine residual test after thirty minutes of contact time is 0.5 mg/L. Find the chlorine dosage and chlorine demand in mg/L. Breakpoint Chlorination Breakpoint chlorination is related to the chlorine necessary to satisfy the inorganic, ammonia, and organic demands of the wastewater. Once achieved, additional chlorine applied to the wastewater is in the form of free chlorine. This is referred to as free chlorine residual. Breakpoint chlorination reactions are depicted as follows: (1)NH3+HOCl→NH2Cl+H2O(Monochloramine) (2)NH2Cl+HOCl→NHCl2+H2O(Dichloramine) (3)NHCl2+HOCl→NCl3+H2O(Nitrogentrichloride) The initial reaction, after chlorine dosing, involves the formation of monochloramine when chlorine reacts with the ammonia (Equation 1). Continuing with the chlorine dosage will lead to the formation of dichloramine (Equation (2)) and eventually lead tri-chloramine formation and the residual chlorine required (Equation (3)). Tri-chloramine is unstable and decomposes to nitrogen gas. Capodaglio et al. (2015) summarised the breakpoint chlorination equation as follows (Eqn. (4)): (4)NH4++1.5HOCl→0.5N2↑+1.5H2O+2.5H++1.5Cl− https://www.sciencedirect.com/topics/engineering/breakpoint-chlorination https://files.dep.state.pa.us/water/bsdw/operatorcertification/TrainingModules/ww05_disinfe ction_chlorination_wb.pdf The addition of chlorine at the dip or break is called “break point” chlorination. This indicates the point at which free residual chlorine begins to appear. Advantages: (1) It oxides completely organic compounds, ammonia and other reducing compounds. (2) It removes color, odour and taste of water. (3) It removes completely all the disease causing bacteria/micro-organism (4) it prevents the growth of any weeds in water. Using Chloramine (ClNH2): When chlorine and ammonia are mixed in the ratio of 2:1 by volume, chloramine is formed. Cl2+NH3 → ClNH2+ HCl Chloramine is a better bactericidal than chlorine. Disinfection by Ozone: Ozone gas is an excellent disinfectant, which is produced by passing silent electric discharge through cold and dry oxygen. 3O2 → 2O3 O3 → O2 + [O] Desalination of Sea Water – Reverse Osmosis Method, Electro-dialysis The process of removing common salt (NaCl) from the water is known as desalination. Water containing high concentration of dissolved salts with a peculiar salty taste is called brackish water. Sea water is an example containing 3.5% of dissolved salts. The common methods for the desalination of brackish water are; Eletctrodialysis It is a method in which the ions are pulled out of the salt water by passing direct current, using electrodes and thin rigid plastic membrane pair. An Eletctrodialysis cell consists of a large number of paired sets of rigid plastic membranes. Hard water is passed between the membrane pairs and an electric field is applied perpendicular to the direction of water flow. Positively charged membrane and negatively charged membrane repel positively charged ions and negatively charged ions respectively to pass through. So, in one compartment of the cell, the salt concentration decreases while in the adjacent compartment it increases. Thus, we get alternative stream of pure water and concentrated brine. Advantages 1. It is most compact unit. 2. The coast of the plant and its operation is economical. Reverse osmosis When two solutions of unequal concentrations are separated by a semi permeable membrane, flow of solvent takes place from dilute to concentrate sides, due to osmosis. If, however a hydrostatic pressure in excess to osmotic pressure is applied on the concentrated side, the solvent flow is reversed, i.e, solvent is forced to move from concentrated side to dilute sideacross the membrane. This is the principle of reverse osmosis.(RO). Thus in reverse osmosis method, pure solvent is separated from its contaminants, rather than removing contaminants from the water. The membrane filtration is sometimes also calledsuper-filtration or hyper-filtration. Method: In this process, pressure is applied to the sea water or impure water to force the pure water content of it out the semi-permeable membrane, leaving behind the dissolve solids. The principle of reverse osmosis as applied for treating saline/sea water as illustrated in the above figure. The membrane consists of very thin film of cellulose acetate, affixed to either side of a perforated tube. However, more recently superior membranes made of polymethacrylate and polyamide polymers have come into use. Advantages 1. Reverse osmosis possesses distinct advantages of removing ionic as well as non-ionic, colloidal and high molecular weight organic matter. 2. It removes colloidal silica, which is not removed by demineralization. 3. The maintenance cost is almost entirely on the replacement of the semi permeable membrane. 4. The life time of membrane is quite high, about 2 years, 5. The membrane can be replaced within a few minutes, thereby providing nearly uninterrupted water supply. 6. Due to low capital cost, simplicity, low operating cost and high reliability, the reverse osmosis is gaining grounds at present for converting sea water into drinking water and for obtaining water for very high –pressure boilers.