UNIT-II: WATER TECHNOLOGY

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1 UNIT-II: WATER TECHNOLOGY Introduction: Water is the principal (about 75%) constituent of earth s surface. It is an important component of animal and vegetable matter and plays a vital role in their life processes. It ranks next to oxygen in importance for our existence. It constitutes about 65% of human body and about 95% by weight of some plants. Of the total estimated global water supply of 1.4 x 10 9 km 3, the fresh water amounts to only.7%. Unfortunately most of the fresh water is locked up in frozen lakes, glacier or under the ground and is not readily accessible. The fraction of water available for human use is only 0.003% of the total global water supply. In nature, water is found in all the three states i.e., solid, liquid and gaseous. Water is a colourless, tasteless and odourless liquid. Sources of water: The main sources of water are: i) rain ii) lake, river and sea water (surface water) and iii) wells and spring underground water. i) Rain water is the purest of all natural waters, because it is obtained by the evaporation of water. It may contain impurities depending upon the region of collection. The surface water contains dissolved CO, derived from the atmosphere or from the biological oxidation of organic matter. When water containing dissolved CO comes into contact with basic materials such as lime stone (CaCO 3) these goes into solution as bicarbonates. CaCO H O CO Ca (HCO ) 3 3 River water is likely contain several impurities which are of organic and inorganic in origin. More is the contact time with soil, greater the percentage of inorganic impurities. The presence of organic impurities is due to the decomposition of vegetative matter and dead animals. ii) The composition of lake water is generally constant. It usually has less dissolved organic and inorganic compounds. Surface water when percolates into the deeper part of the earth, underground water is formed. iii) When water going deep touches a rock, may comeup again which is called as spring. If it settles at the inner side it is well water. This well water is relatively safe for consumption as it is free from organic impurities and pathogens. Spring water may contain colloidal sulphur and hence can have some medical properties.

2 Water from wells and springs is rich in mineral salts because of increased solubility at the increased temperatures prevailing under ground. Sea water is also a source of natural water. It is highly concentrated with salts (3.5%). Sodium chloride is the main salt (.7%), others are calcium carbonate, magnesium chloride, magnesium sulphate, calcium chloride, calcium sulphate etc., so it is the most impure form of natural water. Hardness of Water: Water is classified into two categories depending upon its behaviour towards soap solution. a) Soft water: Water which produces lather with soap solution readily is called soft water. Distilled water and rain water are common examples of soft water. b) Hard water: Water which does not produce lather with soap solution easily is called hardwater. River water and sea water are common examples of hard water. Hardness of water is due to the dissolved impurities of the salts like bicarbonates, chlorides and sulphates of calcium and magnesium. Water gets the contamination of these salts when it passes over the ground and rocks. Hardwater does not produce lather with soap solution readily because the cations (Ca + and Mg + ) present in hard water react with soap to form precipitate of calcium and magnesium salts of fatty acids. M + + C 17H 35COONa > (C 17H 35COO) M + Na + from Hardwater sodium stearate metal stearate Where M = Mg or Ca Thus, no lather is produced until all the calcium and magnesium ions have been precipitated. This leads to the increased consumption and hence, wastage of lot of soap. Hard water is, therefore not fit for washing purposes and also for many domestic and industrial applications. Differences between Hardwater and Softwater: Sl.No. Hardwater Softwater 1 Water which does not produce lather with soap solution readily, but form a white curd is called hardwater. Hard water contains dissolved Calcium and Magensium salts in it. 3 In hard water, cleansing quality of soap is depressed and a lot of it is Water which lathers easily on shaking with soap solution is called softwater. Soft water does not contain dissolved calcium and magnesium salts in it. In softwater, cleansing quality of soap is not depressed and

3 wasted during Washing and Bathing. 4 Due to the presence of dissolved hardness producing salts, the boiling point of water is elevated. Consequently, more fuel and time are required for cooking. so soap is not wasted during washing and bathing. Less fuel and time are required for cooking in soft water. Advantages and Disadvantages of Hardwater: Sl.No. Advantages Disadvantages 1 The taste of hardwater is usually better than soft water. The label on Hard water produces scum with soap. Thus, the washed the bottle of mineral water shows clothes look dull. Efficiency that it contains Mg+ and Ca+ ions and it tastes good. of soap decreases in hard water so economy decreases. The dissolved calcium in hard water can help to produce strong Boiler feed water should be free from hardness otherwise teeth and healthy bones in even explosion can occur. children. 3 In old houses, lead piping is used for distribution of water. Hardwater coats these with a layer of insoluble CaCO3. This prevents any of the poisonous lead dissolving in the drinking water. TEMPORARY AND PERMANENT HARDNESS: Hardness of water is of two types. hardness: Temporary hardness and Permanent i) Temporary hardness: It is due to the presence of soluble bicarbonates of calcium and magnesium and also due to carbonate of iron. Such water is also said to possess carbonate hardness. The term temporary indicates that most of the hardness can be removed by simply boiling the water. The bicarbonates of calcium and magnesium are formed in water by dissolution of carbonates of calcium and magnesium in the presence of atmospheric carbon dioxide. It is also called as carbonate hardness. The methods employed for removing temporary hardness are the following. 1. By boiling: Temporary hardwater contains calcium and magnesium bicarbonates. These salts are decomposed on boiling the water, forming insoluble calcium and magnesium carbonates which can easily be removed by filteration or decantation and thus the water is softened.

4 Ca(HCO 3) > CaCO 3 + H O + CO Calcium bicarbonate Calcium Carbonate (Soluble) (Insoluble) Mg(HCO 3) > Mg(OH) + CO Magnesium bicarbonate Magnesium Carbonate (Soluble) (Insoluble) This method is effective only on domestic scale since it is not possible to soften the whole of a towns water supply by the process of boiling.. Clark s Method: This is the cheapest method for removing temporary hardness of water and is, therefore, widely used. In this method sufficient amount of lime (Calcium hydroxide) is added to the water which converts all the soluble bicarbonates into insoluble carbonates as shows below: Ca(HCO 3) + Ca(OH) > CaCO 3 + H O (Soluble) (lime) (insoluble) Mg(HCO 3) + Ca(OH) > CaCO 3 + MgCO 3 + H O (Soluble) (lime) (insoluble) (insoluble) Too much excess of lime should not be added otherwise it would harden and not soften the water. The permanent hardness of water cannot be removed by above methods. ii) Permanent Hardness: It is due to the presence of chlorides and sulphates of calcium and magnesium. Such water is said to possess Non- Carbonate Hardness. The term permanent indicates that such type of hardness cannot by removed by mere boiling of water. Chlorides and sulphates of some heavy metals also can cause permanent hardness. The permanent hardness can be removed by addition of calculated quantity of sodium carbonate (washing soda) by which calcium and magnesium carbonates are precipitated as shows below: CaSO 4 + Na CO > CaCO 3 + Na SO 4 (Soluble) washing soda (insoluble) MgCl + Na CO > MgCO 3 + NaCl (soluble) washing soda (insoluble) The temporary hardness is also removed according to the following equation. Ca(HCO 3) + Na CO > CaCO 3 + NaHCO 3

5 UNITS OF HARDNESS AND IT S INTERCONVERSION: The hardness causing ions are usually expressed in terms of an equivalent amount of CaCO 3. Units of Expression of Hardness: 1) Parts per million (ppm) ) Milligram per litre 3) Degree French ( 0 Fr) 4) Clarke s degree ( 0 Cl) 1) Parts per million (ppm): It is defined as the number of parts of hardness causing substance in CaCO 3 equivalent hardness per 10 6 parts (one million) of water. ) Milligram per litre (mg/litre): The number of milligrams of hardness causing substance in CaCO 3 equivalent hardness present per litre of water. 3) Degree Clark ( 0 Cl): The number of parts of hardness causing substance in CaCO 3 quivalent hardness per 70,000 parts of water. (or) the number of grains (1/7000 lb) of CaCO 3 equivalents of hardness causing salt present in one gallon (10 lb) of water. 4) Degree French ( 0 Fr): The number of parts of hardness causing substance in CaCO 3 equivalent hardness per 10 5 parts of water. The hardness of water can be converted into all the four units by making use of the following interconversion formula. 1ppm = 1mg / litre = Cl = Fr 1 0 Cl = Fr = 14.3 ppm = 14.3 mg/litre Estimation of hardness by EDTA method: Procedure: 1. Preparation of standard hardwater: 1g of pure dry CaCO 3 should be dissolved in minimum quantity of dilute HCl and then evaporated to dryness on a water bath. The residue is dissolved in distilled water and made upto 1L. The concentration is 1mg/mL.. Preparation of EDTA solution: 4grams of pure EDTA crystals should be dissolved in 1L of water. 3. Preparation of EBT Indicator: 0.5g of EBT should be dissolved in 100mL of alcohol. 4. Preparation of buffer solution: It can be prepared by mixing 67.5g of NH 4Cl in 570mL of liquid ammonia and then diluted to 1L with water. 5. Standardisation of EDTA solution: 50mL of standard hardwater is pipetted into clean conical flask. To this 10mL of buffer and to 3 drops

6 of indicator is added. Then it is titrated against standard EDTA solution. Let this volume be V 1 ml. 6. Titration of unknown hardwater (Total hardness): 50mL of unknown sample of water as described above. Let the volume of standard EDTA solution be V ml. 7. Titration for Permanent hardness: Take 50ml of water sample in a large beaker, boil it till volume is reduced to about 50ml [when all bicarbonates are decomposed to insoluble CaCO 3 + Mg(OH) ]. Filter, wash the residue with distilled water, collect filtrate and washings in 50ml, measuring flask. Finally makeup the volume of the filtrate to 50ml by adding distilled water. Then, titrate 50ml of the filtrate against EDTA solution. Let the titre value of solution be V 3 ml. Structure of EDTA Salt It can form a complex with Ca+ or Mg+ (M) ions as shown below: Calculations: 50ml of standard hardwater = V 1 ml of EDTA 50 x 1 mg of CaCO 3 = V 1 ml of EDTA 1ml of EDTA = 50/V 1 mg of CaCO 3 equivalent Now 50ml of given hardwater = V ml of EDTA = V X50/V 1mg of CaCO 3 equivalent 1L (1000ml) of given hardwater = 1000 V /V 1 mg of CaCO 3 equivalent Total hardness of water = 1000V /V 1 mg/l=1000v /V 1 ppm

7 Now 50ml of boiled water = V 3 ml of EDTA = V 3 X 50/V 1 mg of CaCO 3 equivalent 1000 (ml) = (1L) of boiled water Permanent hardness = 1000 V 3/V 1 mg of CaCO 3 equivalent Permanent hardness = 1000 V 3/V 1 ppm and Temporary hardness = Total hardness Permanent hardness Advantages of EDTA method: V V 3 = 1000 ppm V V ( V V ) 3 = ppm V This method is definitely preferable over the other methods, because of the (i) greater accuracy (ii) convenience and (iii) more rapid procedure. DISADVANTAGES OF HARDWATER: 1. In the domestic use, hardwater is not suitable for washing as it will not give lather with soap. So no cleaning takes place. It causes wastage of lot of soap.. Hardwater is not suitable for bathing also. Because cleaning is not proper. 3. Due to the impurities in water, the boiling point is elevated. So more fuel and time are required when hard water is used for cooking. The food will have unpleasant taste. It may also injurious to health. 4. Drinking hardwater disturbs the digestive system and also calcium oxide crystals may accumulate in urinary tract. Hence excretion becomes difficult. 5. Even in industries also, use of hardwater is not recommended. i) In the textile industry, cleaning of raw textile becomes difficult. There may be fading of colour of the textile. Colours may not be fixed properly. ii) In sugar industry, the crystallization of sugar will be difficult. Quality of sugar will be poor. iii) In paper industry, the dissolved salts in water may react with constituents of paper and may decrease the quality. iv) In the Pharmaceutical industry hard water should not be used at all as it can change the composition of drugs. Consumption of such drugs leads to further problems. v) In boilers if hard water is used scales and sludges may be formed. This leads to several complications. vi) Water containing chlorides and sulphates, if used for concrete making, affects the hydration of cement and the final strength of the hardened concrete. 1

8 WATER FOR DOMESTIC USE AND TREATMENT OF WATER FOR DOMESTIC PURPOSE (OR) MUNICIPAL SUPPLY: The following are the specifications of water for drinking purpose. 1) The water should be clear, colourless and odor less. ) The water must be free from pathogenic bacteria and dissolved gases like H S. 3) The optimum hardness of water must be 15 ppm and ph must be 7.0 to 8.5 4) The turbidity in drinking water should not exceed 5 ppm. 5) The recommended maximum concentration of Total Dissolved Solids in Potable water must not exceed 500 ppm. Natural water does not meet the above requirements. So it must be purified as described below. At first instance, the treatment of water for drinking purposes mainly includes the removal of suspended impurities, colloidal impurities and harmful pathogenic bacteria. The following is the flow diagram of the water treatment for domestic purposes and various stages involved in purification are given. 1) Screening: The water is passed through screens having large number of holes in it, to remove floating impurities. ) Sedimentation with coagulation: The suspended and colloidal impurities are allowed to settle under gravitation. The basic principle of this treatment is to allow water to flow at a very slow velocity, so that the heavier particles settle under gravitation. For setting of fine particles, coagulants like alums, sodium aluminate and salts of iron are added, which produces floc. Floc attracts and helps accumulation of the colloidal particles rsultingin setting of the colloidal particles. 3) Filtration: Filtration helps in removal of the colloidal and suspended impurities not removed by sedimentation. Usually sand filters are employed. There are two types of sand filters, slow sand filters and rapid sand filters or pressure filters. In slow sand filter the filter bed consists of three layers of sand of different particle size. A fine sand layer on the top supported by coarse sand layer, which is supported by gravel. The colloidal impurities are retained by the fine sand layer resulting the very slow filtration of water. The top layers of the fine sand layer is scraped off, washed, dried and introduced into thte filter bed for reuse. Rapid sand filters make use of compressed air for fast filtration. 4) Sterilisation and disinfection: Destruction of harmful pathogenic bacteria from drinking water is carried out by sterilization and disinfection. The following are the methods adopted for sterilization and disinfection. a) Boiling: By boiling water for 15-0 minutes, harmful bacteria are killed. This is not possible for the municipal supply of water. This method of sterilization is adopted for domestic purpose.

9 b) Passing ozone: Ozone when passed into water acts as disinfectant. Ozone is an unstable isotope of oxygen, produces nascent oxygen which is a powerful disinfection. O > O +(O) This treatment is costly and ozone is unstable and cannot be stored for a longtime. c) Chlorination: The process of utilizing chlorine as a powerful disinfectant is called chlorination. There are three types of chlorinating reagents. i) By passing chloramines: Chlorine is mixed with ammonia in the ratio :1 by volume to form a stable chloramine which generates hypochlorous acid, a powerful disinfectant. Cl NH ClNH HCl 3 ClNH H O HOCl NH 3 Hypochlorous acid (HOCl) inactivates the enzymes of bacteria and kills bacteria. Chloramine is useful for disinfecting swimming pools. ii) By bleaching powder: Bleaching powder contains 80% chlorine. When bleaching powder is used as disinfectant, it is called hypochlorination because the disinfection is due to hypochlorous acid. CaOCl Bleaching powder H O Ca OH ) ( Cl Cl H O HOCl HCl Disinfectant iii) Chlorination: The process of applying calculated amount of chlorine to water inorder to kill the pathogenic bacteria is called chlorination. Chlorination also reactswith water and generates hypochlorous acid, which kills bacteria. Chlorine is a powerful disinfectant than chloramine and bleaching powder. Calculated amount of chlorine must be added to water because chlorine after reacts with bacteria and organic impurities or ammonia, remains in water as residual chlorine which gives bad taste, odour and toxic to human beings. Break point chlorination: The amount of chlorine required to kill bacteria and to remove organic matter is called break point chlorination. From graph it is clear that: a grams of chlorine added oxidises the reducing impurities of water. b grams of chlorine added forms chloramines and other chloro compounds. c grams of chlorine added causes destruction of chloroamine and chlorocompounds. d grams of chlorine is residual chlorine. Hence c grams is the break point for the addition of chlorine to water. This is called break point chlorination.

10 Advantages of break-poing chlorination: 1. It removes taste, colour, completely oxidises organic compounds, ammonia and other reducing impurities.. It destroys completely (100%) all disease producing bacteria. 3. It prevents growth of any weeds in water. Dechlorination: Overchlorination after breakpoint produces unpleasant taste, odour, toxicity to water. The over chlorination is removed by passing the water through a bed of granular carbon and also by the addition of SO and sodium thiosulphate. SO Cl H O H SO HCl 4 Na S O Cl H O Na SO HCl 3 4 Problems in calculating hardness: 1) A sample of water gives on analysis 13.6 mg/l of CaSO 4, 7.3 mg/l of Mg(HCO 3). Calculate the total hardness and permanent hardness. ) Calculate the total hardness of 1000 litre of a sample of water containing the following impurities 16. mg/l of Ca(HCO 3), 11.1 mg/l of CaCl, 60 mg/l of MgSO 4 and 19 mg/l of MgCl. 3) A sample of hardwater contains the following dissolved salts per litre. CaCl = 111 mgs, CaSO 4 = 1.36 mgs, Ca (HCO 3) = 16. mgs, Mg(HCO 3) = 14.6 mgs, Silica = 40 mgs, Turbidity = 10 mgs. Calculate the temporary, permanent and total hardness of water in ppm, degree Clark and degree French. 4) Calculate the temporary and permanent hardness of water in 0 Cl, containing the following dissolved salts. CaCO 3 = 50 mg/l, MgCl = 9.5 mg/l, CaCl =. mg/l and MgSO 4 = 1 mg/l. 5) A sample of water contains 100 ppm of total hardness and 5 ppm of temporary hardness. Calculate the permanent hardness of water in degree Clark and degree French.

11 TREATMENT OF WATER FOR INDUSTRIAL PURPOSE (OR) INDUSTRIAL WATER TREATMENT: Water to be used for various industrial purposes such as steam generation, brewing, dyeing etc., should have no hardness or some specific hardness and should be free from certain impurities. For industrial purposes softening of water, removal of iron, silica and other impurities are all practical treatment methods which have become essential. The methods of water softening and removal of other impurities are given below. 1) Desalination (Electrodialysis & Reverse Osmosis) ) Lime-Soda Process 3) Zeolite Process 4) Ion-Exchange Process 1) DESALINATION: Water containing high concentrations of dissolved solids with a peculiar salty or brackish taste is called brackish water. Sea water is an example of brackish water containing about 3.5% of dissolved salts. This water cannot be used for domestic and industrial applications unless the dissolved salts are removed by desalination. Commonly used methods are: a) Electrodialysis, b) Reverse Osmosis. a) Electrodialysis: Electrodialysis is based on the principle that the ions present in saline water migrate towards their respective electrodes through ion selective membranes. Under the influence of applied e.m.f. The unit consists of a chamber, two electordes (the cathode and the anode). The chamber is divided into three compartments with the help of thin, rigid, ion-selective membranes which are permeable to either cation or anion. The anode is placed near anion selective membrane. While the cathode placed near cation selective membrane. The anion selective membrane is containing positively charged functional groups such as R 4N + and is permeable to anions only. The cation selective membrane consists of negatively charged functional groups such as RSO 3 - and is permeable to cations only. Under the influence of

12 applied e.m.f. across the electrodes the cations move towards cathode through the membrane and the anios move towards anode through the membrane. The net result is depletion of ions in the central compartment while it increases in the cathodic and anodic compartments. Desalination water is periodically drawn from the central compartment while concentrated brackish water is replaced with fresh sample. Advantages of electrodialysis: 1) The unit is compact. ) The process is economical as far as capital cost and operational expenses are concerned. b) REVERSE OSMOSIS: When two solutions of unequal concentration are separated by a semipermeable membrane which does not permit the passage of dissolved solute particles (i.e., molecules and ions). Flow of solvent takes place from the dilute solution to concentration solution, this is called Osmosis. If a hydrostatic pressure in excess of osmotic pressure is applied on the concentrated side, the solvent is forced to move from higher concentration to lower concentrated side across. Thus the solvent flow is reversed hence this method is called reverse osmosis. Thus in reverse osmosis pure water is separated from the contaminated water. This membrane filtration is also called super filtration or hyper-filtration. Method of purification: The reverse osmosis cell consists of a chamber fitted with a semipermeable membrane, above which sea water / impure water is taken and a pressure of 15 to 40 kg/cm is applied on the sea water/impure water. The pure water is forced through the semipermeable membrane which is made of very thin films of cellulose acetate. However, superior membrane made of polymethylmethacrylate and polyamide polymers have come to use. Advantages of Reverse Osmosis: 1) Both ionic and non-ionic, colloidal and high molecule weight organic matter is removed from the water sample. ) Cost of purification of water is less and maintenance cost is less. 3) This water can be used for high pressure boilers. ) LIME-SODA PROCESS: If temporary and permanent hardness is present together then water can be softened by lime-soda method which is oldest method of water softening. It is the combination of Clark s method and washing soda method. Though both types of hardness can be removed by the latter method the excess of washing

13 soda will then be expensive. Moreover by the addition of only sodium carbonate the hardness due to magnesium salt will not be completely removed because the precipitate of magnesium carbonate formed is some what soluble which can be converted into insoluble magnesium hydroxide by the addition of extra lime. The process involves through mixing of the calculated quantities of hydrated lime (calcium hydroxide) and soda ash (sodium carbonate) reagents with the water to be softened which is analysed for temporary and permanent hardness. The chemistry of the method is indicated by the following reactions. For calcium and magnesium bicarbonates, only lime is required which reacts with these salts according to the equations given below: a) Ca(HCO 3) + Ca(OH) > CaCO 3 + H O b) Mg(HCO 3) + Ca(OH) > CaCO 3 + MgCO 3 + H O MgCO 3 + Ca(OH) > CaCO 3 + Mg(OH) Mg(HCO 3) + Ca(OH) > CaCO 3 + Mg(OH) + H O For MgSO 4 and MgCl are present, then lime and soda ash both are required. The following reactions take place and their quantities can be calculated from these equations. c) MgSO 4 + Na CO > MgCO 3 + Na SO 4 MgCO 3 + Ca(OH) > CaCO 3 + Mg(OH) MgSO 4 + Na CO 3 + Ca(OH) > Mg(OH) + CaCO 3 + Na SO d) MgCl + Na CO > MgCO 3 + NaCl MgCO 3 + Ca(OH) > CaCO 3 + Mg(OH) MgCl + Na CO 3 + Ca(OH) > Mg(OH) + CaCO 3 + NaCl If CaSO 4 and CaCl are present, then only sodium carbonate is required. The quantity of Na CO 3 is calculated according to the reaction shown below: e) CaSO 4 + Na CO > CaCO 3 + Na SO 4 f) CaCl + Na CO > CaCO 3 + NaCl The process of softening hardwater by use of lime and soda ash reagents is carried out either in cold or hot water. If it is carried out in cold water, the process is called cold lime-soda method and if in hot water, the process is called hot lime-soda process.

14 Cold lime-soda process: In this method the calculated quantities of lime are mixed with water at room temperature, and precipitates formed are finely divided. They cannot settle down easily. Filtration cannot bedone easily. Hence small amounts of alums were added. The addition of sodium aluminate can act as a coagulant and also helps in the removal of silica and oil. Raw water alongwith calculated amounts of chemicals (lime+soda+coagulant) are fed from the top into the inner chamber, fitted with a vertical rotating shaft carrying a number of paddles. As the water and chemicals flow down the chamber vigorous mixing and softening of water takes place. As the water comes out into the outer chamber and rises up, the settling of the sludge takes place. The water passes through the wood-fibre filter and flows out continuously from the outlet at the top. The sludge settling at the bottom of the outer chamber is drawn off occasionally. The water sample contains residual hardness ppm. Hot lime-soda process: In this method the raw water is treated with softening chemicals at C. Because the hot limesoda process is carried at the temperature of the boiling point. (1) The reaction proceeds faster, () The precipitates formed settle down rapidly, (3) No coagulant is required, (4) Filtration is fast and (5) dissolved gases like CO are driven out. The residual hardness of water will be 15 to 30 ppm.

15 Hot lime-soda process contains essentially 3 parts. 1) A reaction tank in which; water + chemical + steam are thoroughly mixed. ) A conical sedimentation vessel in which sludge settle down. 3) A sand Filter, which ensures complete removal of sludge from the softened water. Advantages of Lime-Soda process: 1) It is economical. ) Lesser amount of coagulants are required. 3) The process increases the ph value of water, so corrosion of pipelines will be reduced. 4) Due to alkaline nature, presence of pathogenic organisms are reduced. Disadvantages of Lime-Soda process: 1) Skilled supervision is required. ) Disposal of large amounts of sludge is a problem. 3) The residual water has hardness upto 15 ppm, which is not good for boilers. Problems on limesoda method: Formula to determine the amount of Lime and Soda: Amount of lime required for softening = 74 = 100 temp. Ca hardness 1 HCl H SO 4 ( xtemp 1 NaHCO. Mg 3 hardness 1 KHCO 3 ) permanent FeSO 4 (3 xal. Mg ( SO hardness 4. CO 1 ) ) NaAlO 3

16 Amount of Soda required for softening = permanent. Ca hardness permanentm g HCl H SO NaHCO KHCO hardness FeSO 4 (3 xal ( SO 4 ) ) 3 1) Calculate the amount of lime and soda required for the treatment of 10,000 litres of raw water containing the following dissolved salts per litre. CaCO3 = 50 mgs, CaCl = 11.1 mgs, MgSO4 = 1 mgs, NaHCO3 = 7.5 mgs, Silica = 10 mgs. ) A sample of water contains the following dissolved salts in mgs/litre. CaSO 4 = 6.8, MgCO 3 = 8.4, Al (SO 4) 3 = 34., CO = 4.4, HCl = Calculate the amount of lime and soda required for the treatment of 5000 litres of water. 3) Calculate the amount of lime and soda required for softening 50,000 litres of hardwater containing MgCO 3 = 144 ppm, CaCO 3 = 5 ppm, MgCl = 95 ppm, CaCl =111 ppm and Na SO 4=15 ppm. Differences between the Cold and Hot Lime-Soda processes: Sl.No. Cold Lime Soda Process Hot Lime Soda Process 1 It is done at room temperature. It is done at elevated temperatures ( C) It is slow process. It is a rapid process. 3 The use of coagulants is must. Coagulants are not needed. 4 Filtration is not easy. Filtration is easy as the viscosity of water becomes low at elevated tempreatures. 5 Softened wter has residual Softened water has residual hardness around 60 ppm. hardness of ppm. 6 Dissolved gases are not removed. Dissolved gases such as CO are removed to some extent. 7 Low softening capacity. High softening capacity. 3) ZEOLITE PROCESS (OR) PERMUTIT PROCESS: The term zeolite stands for boiling stones (zeo-boining, olite-stone). Zeolites are porous, when water passes through it. The chemical composition of zeolites is hydrated sodium aluminium silicates, represented as Na O.Al O 3.xSiO.yH O where x=-10 and y=-6. Zeolite are capable of exchanging reversibly its sodium ion for hardness causing Ca + and Mg + in water.

17 Zeolites are of two types. 1) Natural zeolites which are natural and non-porous having the composition. For eg: Natrolite Na O. Al O 3. 4SiO.H O ) Synthetic zeolites are porous, possess gel structure and prepared from china clay, feldspar and soda ash. The synthetic zeolites possess higher exchange capacity. Process: The hardwater is passed through a zeolite bed fixed in a cylinder at a specific rate. The hardness causing ions, Ca +, Mg + etc., are retained by the zeolite as CaZ and MgZ respectively. An equivalent amount of sodium salts were introduced into water. The reaction taking place during the softening process are: Ca(HCO 3) + Na Z > CaZ + NaHCO 3 Mg(HCO 3) + Na Z > MgZ + NaHCO 3 CaSO 4 + Na Z > CaZ + Na SO 4 MgSO 4 + Na Z > MgZ + Na SO 4 CaCl + Na Z > CaZ + NaCl MgCl + Na Z > MgZ + NaCl Regeneration: After sometime, the zeolite bed is completely converted to calcium and magnesium zeolites and no purification of raw water takes place i.e., the zeolite bed gets exhausted. At this stage the purification ofhardwater is stopped and the zeolite bed is regenerated by treating the bed with 10% brine (NaCl) solution. CaZ + NaCl > Na Z + CaCl MgZ + NaCl > Na Z + MgCl The washings containing CaCl and MgCl are discarded.

18 Limitations of zeolite process: 1) Raw water should not contain turbidity. Turbidity will block clog the process of zeolite bed and makes it inactive. Turbidity of water must be removed by coagulation, filtration etc., ) Raw water must be not contain any coloured ions like Mn + and Fe +, because they form manganese zeolite and ferrous zeolite which cannot be regenerated. 3) Mineral acids if present in water will destroy the zeolite bed permanently. Water must be neutralized with soda before it is fed into the zeolite bed. Advantages of Zeolite process: 1) It removes hardness almost completely and the treated water contains hardness upto 10 ppm. ) No precipitation of the products takes place. Hence no disposal of sludge is required. 3) The equipment is compact and requires less skilled assistance. 4) The process adjusts itself for variation of hardness of incoming water. Disadvantages of Zeolite process: 1) The treated water contains more sodium salts than in lime-soda process. ) The method replaces only Ca+ and Mg+ ions by Na+ ions, leaves acidic ions like HCO 3 - and CO 3 - as NaHCO 3 and Na CO 3. 3) These NaHCO 3 salt decompose and liberate CO which causes corrosion. 4) The Na CO 3 salt produced in water decomposed due to the high temperature maintained in the boiler to NaOH, which causes caustic embrittlement. 4) ION-EXCHANGE PROCESS: Ion-exchange process includes the exchange of the cations and anions of the dissolved salts with H+ and OH- ions respectively. For this two types of ionexchangers are used, which are insoluble, cross-linked long chain organic polymers with microporous structure. 1) Cation exchangers are capable of exchanging their H+ ions with cations of the dissolved salts, which comes in their contact. The cation exchangers are represented by general formula RH, are mainly styrene-divinyl benzene copolymers containing the functional groups COOH or SO 3H. R is the general structure of resin and H is exchangeable with cation. ) Anion exchangers are phenol-formaldehyde or amine formaldehyde copolymer resins which exchange their OH- ion with any anion present in the dissolved salts. The anion exchangers are represented by the formula R OH. R is the representation of the general structure of the resin and OH is exchangeable with anion. Thus all the cations and anions present in the dissolved salts are exchanged with cation exchange and anion exchanger.

19 Process: This process involves the following steps. Step-1: The hardwater is passed through a bed of cation-exchange resin (RCOOH or R-SO 3H) of tank A. The Ca + and Mg + ions are exchanged with H + ions of the resin. RCOOH + Ca > (RCOO) Ca + H + RCOOH + Mg > (RCOO) Mg + H + Thus, hardness producing cations (Ca+ & Mg+) are removed. The water coming out contains H +, Cl -, SO 4 - and HCO 3 - ions. Step-: The hardwater is then passed through a bed of anion exchange resin (R-OH or R-NH ) of tank B. The Cl -, SO 4 - and HCO 3 - ions are exchanged with OH - ions of the resin. R-OH + Cl > RCl + OH - R-OH + SO > R SO 4 + OH - R-OH + HCO > RHCO 3 + OH - Thus, hardness producing anions (Cl -, SO 4 - & HCO 3 -) are removed. Step-3: The H + ions produced in tank A combine with OH - ions produced in tank B to form water. H + + OH > H O Thus, this process removes all types of hardness producing cations and anions present in water. The resulting water is known as demineralized or deionized water. Regeneration of Resins: With constant use, the resins get exhausted. This can be regenerated as follows: i) The exhausted cation-exchange resin can be regenerated by passing dilute HCl. (RCOO) Ca + HCl > RCOOH + CaCl (RCOO) Mg + HCl > RCOOH + MgCl CaCl and MgCl are removed as wash.

20 ii) The exhausted anion-exchange resin can be regenerated by passing dilute NaOH. RCl + NaOH > R-OH + NaCl R SO 4 + NaOH > R-OH + Na SO 4 RHCO 3 + NaOH > R-OH + NaHCO 3 NaCl, Na SO 4 and NaHCO 3 are removed as wash. Comparison of Limesoda Process, Permutit Process and Ion-Exchange Process: Limesoda Process Permutit Process Ion-exchange Process Lime and soda are the reagents, exchangers used for purification. Zeolites are used for purification. Cation and Anionic exchangers are used for purification. The dissolved salts are Ca + and Mg + are precipitated as CaCO 3 and Mg(OH) Alongwith hardness causing salts CO, Acids, alums and NaAlO are removed from water. The treated water contains hardness upto ppm. The process following by filtration to remove sludge. The cost of plant and materials are lower. The operational expenses are high. No limitation to suspended impurities and silica in raw water. Only Ca + and Mg + ions are exchanged with Na+ as CaZ and MgZ, Hence, no precipitates are formed. Only Ca + and Mg + ions are removed from water. The treated water contains hardness upto ppm. No sludge is produced hence no filtration. Higher Low No turbidity (or) No suspended impurities must be there in raw water. All the cations and anions were exchanged by Cation and anion exchangers. Hence no precipitates are formed. All cations and anions are removed from water hence this is deionization process. The treated water contains hardness upto ppm. No sludge is produced hence no filtration but degasification. Higher Low No turbidity (or) No suspended impurities must be there in raw water.