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UNIT 1: WATER TECHNOLGY Introduction:Water is the major creation of the nature. It covers about three-fourth part of the earth’s surface out of this 99.4% is locked up into seas, oceans, glaciers and polar ice caps. However about 1% is available for human requirements, plants and animals. Also water is essential materials in the industries for steam generation, power generation, construction materials and textile. Source of water: The main sources of water are as follows. 1) Ground water or Surface water: - It includes Rain water, River water, lake water and sea water. a) Rainwater: It is the purest form of natural water. Still it contains impurities such as CO2, NO2, SO2, etc. b) River water: It contains soluble minerals of the soil such as chlorides, sulphates, bicarbonates of sodium, calcium, magnesium and iron. It also contains organic matters and suspended impurities. c) Lake water: It contains much lesser amount of dissolved minerals but the quantity of organic matter is higher. d) Sea water: It is the most impure from of natural water. It contains about 2.6% sodium chloride and other salts like sodium sulphate, bicarbonates of potassium, magnesium and calcium etc. 2) Underground water or well water: It contains more amounts of dissolved salts. Hence hardness is more than surface water. It is of high organic purity and clear in appearance. Impurities in water: The impurities present in water may be broadly classified as follows. 1) Suspended impurities: These are due to presence of suspended particles of clay and organic matter which can be removed by sedimentation process. 2) Colloidal impurities: These are due to presence of very fine suspended particles of clay and organic matter which can be removed by coagulation process. 3) Mineral impurities: These are due to presence of dissolved mineral salts of Ca, Mg & other heavy metals which can be removed by special chemical treatment process like lime-soda, zeolite or ion-exchange process. 4) Biological impurities: These are due to presence of Micro-organisms, bacteria and viruses etc. these pathogens are harmful to human being, which can be removed by sterilization of water such as boiling, using chemicals like bleaching powder, UV-rays, ozonization. Hardness of Water: Definition of hardness: It is a property of water which prevents lathering of soap or it is the soap consuming capacity of water. The property of hardness in water is due to presence of certain salts of calcium, magnesium and other heavy metals in dissolved form. When a sample of hard water treated with soap (sodium or potassium salt of higher fatty acids), it does not produce lather. On the other hand, it forms a white scum or precipitate. This is due to formation of insoluble soaps of calcium & magnesium as following. 2C17H35COONa + CaCl2 (C17 H35 COO)2Ca ↓+ 2NaCl Sodium stearate Calcium stearate (Insoluble) (C17 H35 COO)2Mg↓ + Na2SO4 2C17H35COONa + MgSO4 Magnesium stearate (Insoluble) It seems from above reactions that soap forms white precipitate or scum instead of lather. Thus, soap is wasted. The water, which does not produce lather readily with soap but forms a white scum, is called hard water. The hard water contains more than 60 ppm hardness. The water which produces lather easily with soap is called soft water. Usually it has hardness upto 60 ppm. Types of hardness: Hardness may be divided into two types. 1) Temporary or carbonate hardness: It is caused by the presence of dissolved bicarbonates of Ca, Mg and other heavy metals and the carbonate of iron. It can be removed by mere boiling. On boiling, bicarbonates convert into insoluble carbonates or hydroxides. The reactions are as follow. Ca(HCO3)2 Mg(HCO3)2 CaCO3↓+ H2O+ CO2↑ Mg(OH)2↓+ CO2↑ 2) Permanent or Non- carbonate Hardness: It is caused due to the presence of sulphates, chlorides and nitrates of Ca, Mg, Fe and other heavy metals. It is also known as Non alkaline hardness. It can not be removed by simple boiling. It can be removed by softening methods such as lime soda, zeolite or ion exchange process. Units of Hardness 1) Parts per million (ppm): It is the part of CaCO3equivalent hardness per 106 parts of water. 2) Milligrams Per liter (mg/dm3): It is the no of milligrams of CaCO3 equivalent hardness per liter of water. 1mg/L =1 mg CaCO3 equivalent per 106 mg of water 3) Degree Clarkes (°Cl): It is the no. of grams of CaCO3 equivalent per gallon of water. 1°Cl = 1 part of CaCO3 per 70,000 parts of water. 4) Degree French (°Fr): It is the part of CaCO3equvalent hardness per 105 parts of water. 1°Fr = 1 part of CaCO3equvalent hardness per 105 parts of water. Inter-conversion between various units of hardness: 1ppm = 1mg/L = 0.1°Fr = 0.07°Cl =0.02meq/L. Determination of Hardness: Hardness of water can be determined by any of the following methods. 1) O’Hehner’s method 2) Soap titration method 3) EDTA Method. EDTA Method or Complexmetric method: Principle: In this method EDTA acts as a complexing agent. It forms complex with various metal ions present in hard water. Hence this method is also known as complexometric method. The method includes the titration of standard solution of EDTA (disodium salt of EDTA) with hard water sample using Erichrome Black–T (EBT) indicator. EBT forms unstable wine-red complex with metal ions Ca2+ and Mg2+ in hard water at 10 PH. M2+ + EBT (M2+= Ca2+ or Mg2+) [M-EBT] red complex (Unstable) During the course of titration, EDTA decomposes the above complex. It forms stable complex with metal ions and EBT becomes free. [M-EBT] + EDTA Wine red complex (Unstable) [M- EDTA] + EBT Blue complex (Stable) Thus end point is indicated by the change of wine-red colour to blue colour. Knowing the volume of EDTA, hardness can be determined. The structure of disodium salt of EDTA and M-EDTA complex is as follows. Requirement of Solution: In this process, following solutions are required. 1) Standard Hard Water (SHW): It is prepared by dissolving 1.0g of pure and dried CaCO3 in concentrated HCl. This solution is heated to obtain precipitate by removal excess of HCl. The precipitate is then dissolved into distilled water to make1 liter solution. 1 ml of this solution contains 1mg CaCO3 (1ml SHW = 1 mg CaCO3) 2) EDTA solution: It is prepared by dissolving 4 g of pure EDTA + 0.1g Mg Cl2 in 1 liter of distilled water. 3) EBT indicator solution: It is prepared by dissolving dissolve 0.5g of Eriochrome black-T in 100 ml of alcohol. 4) Buffer solution: It is prepared by dissolving 67.5g of NH4Cl in 570 ml of concentrated ammonia solution and dilute to 1 liter with distilled water. Procedure: 1) Standardization of EDTA solution: Take 50 ml of standard Hard water (SHW) in a conical flask with the help of pipette. Add 10-15 ml of buffer solution and 4-5 drops of EBT indicator. Titrate with EDTA solution till wine-red colour changes to deep blue. Let volume of EDTA required is V1 ml. 2) Titration of Unknown hard water: Titrate 50 ml of unknown water sample as above. Let volume of EDTA required is V2 ml. 3) Titration of boiled water: Take 250 ml of hard water sample in a large beaker. Boil it till the volume remains upto 50 ml. Filter obtained precipitate from the water. Finally make the volume of filtered water upto 250 ml by distilled water. Titrate 50 ml of this water sample as above. Let volume of EDTA required is V3 ml. Calculations: The hardness of water is calculated as follow. 1) Standardization of EDTA: V1 ml of EDTA = 50 ml of SHW V1 ml of EDTA = 50 mg of CaCO3 ( 1ml SHW = 1 mg CaCO3) 50 1 ml of EDTA = 𝑉1 mg of CaCO3 2) To find total hardness: 50 1 ml of EDTA = 𝑉1 mg of CaCO3 V2 ml EDTA = 𝑉2×50 𝑉1 mg of CaCO3 We know that, V2 ml of EDTA = 50 ml of hard water sample 50ml of hard water = 𝑉2×50 𝑉1 mg of CaCO3 𝑉2 1000 ml of given hard water = 1000× ( 𝑉1 ) mg of CaCO3 Total hardness of water = 1000× ( 3) To find permanent hardness: 𝑉2 𝑉1 ) mg/L or (ppm) 50 1 ml of EDTA = 𝑉1 mg of CaCO3 V3 ml EDTA = 𝑉3×50 𝑉1 mg of CaCO3 We know that, V3 ml of EDTA = 50 ml of boiled water sample 50ml of Boiled water = 𝑉3×50 𝑉1 mg of CaCO3 1000 ml of given Boiled water = 1000×( Permanent hardness =1000 × ( 𝑉3 𝑉1 𝑉3 𝑉1 ) mg of CaCO3 ) mg/L or (ppm) Temporary hardness = (Total permanent) Hardness 𝑉2 𝑉3 = 1000 𝑉1 − 𝑉1 𝑝𝑝𝑚 = 1000(𝑉2−𝑉3) 𝑉1 𝑝𝑝𝑚 Advantages of EDTA method: In comparison with other methods, this method is 1) Very accurate, 2) Convenient, and 3) more rapid. Hence this method is most commonly used for the determination of hardness. Softening of hard water: The process of removing hardness producing salts from water is known as Softening of hard water. The hardness causing salts can be removed from the water by two methods. 1) Internal treatment: This process is carried out inside the boiler. It includes calgon, phosphate, carbonate and aluminate conditioning. 2) External treatmentThis process is carried out outside the boiler. It includes lime soda process, Zeolite process and ion-exchange process. Lime-soda process: Principle: In this process, required amount of Lime (Ca(OH)2) and Soda (Na2CO3) are added to the hard water. The hardness causing salts react with lime and soda. During the reactions, insoluble precipitates of carbonate and hydroxides (e.g. CaCO3, Mg(OH)2 etc. are formed. These precipitates may be removed by settling and filtration. It involves following chemical reactions. i) Lime removes temporary hardness Ca (HCO3)2 + Ca(OH)2 Mg (HCO3)2 + 2Ca(OH)2 2 CaCO3 + 2H2O Mg (OH)2 + 2 CaCO3 + 2H2O ii) Lime removes permanent magnesium hardness MgCl2 + Ca (OH)2 Mg (OH)2 + CaCl2 MgSO4 + Ca (OH)2 Mg (OH)2 + CaSO4 Mg (NO3)2 + Ca (OH)2 Mg (OH)2 + Ca(NO3)2 Mg2+ + Ca (OH)2 Mg (OH)2 + Ca2+ iii) Lime removes dissolved iron and aluminium salts FeSO4 + Ca (OH)2 Al2 (SO4)3 + Ca(OH)2 Fe(OH)2 2Al(OH)3 + CaSO4 + 3 CaSO4 iv) Lime removes free mineral acids 2HCl + Ca (OH)2 CaCl2 + 2H2O H2SO4 + Ca (OH)2 CaSO4 + 2H2O i.e., 2H+ + Ca (OH)2 Ca2+ + 2H2O v) Lime removes dissolved CO2 and H2S Ca (OH)2 + CO2 CaCO3 + 2H2O Ca (OH)2 + H2S CaS vi) Lime also reacts with bicarbonate ions + 2H2O 2HCO3 + Ca (OH)2 CaCO3 + 2H2O + CO3 vii) Soda removes permanent hardness CaCl2 + Na2CO3 CaCO3 + 2NaCl2 CaSO4 + Na2CO3 CaSO4 + Na2SO4 Ca2+ + Na2CO3 CaCO3 + 2Na2+ Role of Coagulant: During the softening of hard water by L.S. Process, various insoluble compounds (such as magnesium hydroxide and calcium carbonate) are formed. In order to increase the rate of precipitation of these compounds as well other impurities (clay particles, etc.), the coagulants are added to water for the quick settling. Some of the commonly used coagulants are FeSO4.7H2O, NaAlO2, K2SO4.Al2 (SO4)3.24H2O (alum), etc. The coagulants react with water and form the precipitate of hydroxide as follows. NaAlO2 + H2O Al2 (SO4)3 + 3Ca (HCO3)2 Al(OH)3 + NaOH 2Al(OH)3 + CaSO4 + 6CO2 As shown in above reactions the precipitate of aluminium hydroxide is formed. It is flocculant and gelatinous. It helps for settling of dust, oil, etc. Cold lime-soda process:In this method, calculated quantity of chemicals (lime + soda) is mixed with water at room temperature. The small amount of coagulant (like alum, sodium aluminate or aluminium sulphate) is added for the easy settling of precipitates. Sodium aluminate removes silica and oil also. Hardness can be removed upto 50-60 ppm. This process can be carried out in the following ways: a) Batch process or intermittent process:It consists of a set of two tanks, which are used alternately. Each tank is provided with inlets for raw water and chemicals, outlets for softened water and sludge and a mechanical stirrer. Raw water and calculated quantities of chemicals are added through inlet. They are mixed thoroughly with the help of mechanical stirrer. The chemicals react with the salts present in water. The precipitate of metal hydroxide and carbonates are formed. It is allowed to settle. Thus clear softened water is collected through a float pipes which is further sent to filtering unit. b) Continuous process:It consists of inner vertical circular chamber, fitted with paddle stirrer. Raw water and calculated quantities of chemicals (lime + soda + coagulant) are fed from the top into the inner chamber. Due to continuous mixing by paddle stirrer, softening reactions take place. The heavy sludge (precipited) settles down in outer chamber and the clean softened water reaches upwards. It passes through the filtering media (wood fiber). Thus filtered soft water is collected through the outlet at top. Sludge is removed from the bottom at the intervals. Fig:- Cold lime-soda process Hot lime soda process:In the process the raw water is treated with chemicals (lime + soda) at a temperature of 80-1500C. It is maintained by passing hot stream. Hot lime soda plant consists of three parts. a) A reaction tank- in this tank raw water, chemicals and stream are thoroughly mixed. b) Sedimentation vessel- It is conical in shape at the bottom of the plant. Sludges settle down in this tank. c) A sand filter- It is the separate chamber in which filter is arranged (gravels, course sand and fine sand). Thus softening reactions takes place easily in reaction tank. Sludge settles at the sedimentation vessel. Soft water is passed through the filter. Thus clean soft water is obtained. Diagram: Hot lime soda continuous process Advantages hot lime soda process: 1) In hot lime-soda process, the hardness is removed upto 15-30 ppm. 2) Due to high temperature reaction takes place easily and the process becomes fast. 3) No coagulants are needed in this process. 4) Stirrer is not required because the mixing of chemicals take place by blowing steam. Advantages of L-S process:1) It is most economical. 2) Lime and soda increases pH value of water therefore corrosion of pipes is reduced. 3) Fe and Mn ions are also removed to certain extent. 4) Coagulants are not needed in hot L-S process. 5) Amount of pathogenic bacteria’s also reduce due to alkaline nature of treated water. Disadvantages of L-S process:1) Hardness can be removed up to 15 ppm only. 2) Careful operation and skilled supervision is required. 3) Disposal of large amount of sludge poses problem. 4) Softener requires a large space. Hot L.S. Process In this process, reactions proceed faster. Cold L.S. Process In this process, reactions proceed slowly. Stirrer is not needed, because mixing of water Stirrer is needed for the mixing of water and and chemical take place by blowing steam. chemicals. Coagulants are not needed, as the precipitate Coagulants are needed, for quick steeling the settles rapidly at high temperature. sludge/ precipitate. Dissolved gases such as CO2, air, etc. are Dissolved gases are not removed in this removed. process. Hardness of water is removed upto 15-30 ppm Hardness of water is removed upto 50-60ppm. Zeolite or Permutit process: Principle: In this process hard water is passed through zeolite bed. When zeolite comes in contact with hard water, its sodium ions are exchanged by hardness causing metal ions (Ca2+, Mg2+, etc.) from hard water. Thus hardness of water is removed by using zeolite. Zeolite is hydrated sodiumaluminosilicate i.e. Na2OAl2O3xSiO2yH2O (where x is 2−10 and y is 2−6). Zeolite is also known as permutit. It is represented as Na2Ze. There are two types of Zeolite, 1) Natural Zeolite: They are obtained from natural rocks, for example natrolite ( Na2OAl2O34SiO22H2O) 2) Synthetic zeolite: They are prepared by heating together china clay, feldspar and soda ash. They possess high exchange capacity than natural zeolite. Process: Zeolite softner consists of vertical cylindrical tank. It is packed with a bed of zeolite (Na2Ze). The hard water is allowed to pass through zeolite bed at specified rate. The hardness causing ions (Ca2+, Mg2+, etc.) are exchange with Na+ ions of zeolite. Thus, Ca2+, Mg2+, etc. ions are retained by zeolite and Na+ ions flow in outgoing water. Following reactions take place during the softening process. Na2Ze + Ca(HCO3)3 Na2Ze + Mg(HCO3)3 CaZe + 2NaHCO3 MgZe + 2NaHCO3 Na2Ze + CaCl2 CaZe + 2NaCl Na2Ze + MgCl2 MgZe + 2NaCl Na2Ze + CaSO4 CaZe + Na2SO4 Na2Ze + MgSO4 MgZe + Na2SO4 Diagram: Zeolite process Regeneration of exhausted zeolite: When zeolite is completely exhausted, it can be regenerated by washing with 10% NaCl solution (brine Solution). Following chemical reactions take place during regeneration CaZe + 2NaCl Na2Ze + CaCl2 MgZe + 2NaCl Na2Ze + MgCl2 Thus zeolite bed can be reused for further softening. Advantages of zeolite process: 1) Hardness of water removed upto 10 ppm. 2) The equipment is compact and occupies less space. 3) The process does not produce any sludge and hence it is clean process. 4) It requires less time for softening. 5) It is for operation and maintenance. Disadvantages of zeolite process: 1) The softened water contains more amounts of sodium salts. 2) Acidic water destroys the zeolite bed. 3) If water is highly turbid, suspended particles are deposited on the zeolite bed. Thus, it clogs the pores of zeolite bed and affects softening process. 4) Anions like HCO3−, CO3−, Cl, SO42, etc. in hard water do not exchange with zeolite bed. These anions remain in soft water in the form of sodium salts. It causes alkalinity to soft water. They hydrolyze into NaOH at high temperature in boilers and causes caustic embrittlement. 5) The hardness water is removed upto 10 ppm only. 6) The cost of equipment and materials is expensive. Limitations of zeolite process: 1) If the raw water is turbid, the suspended matters block the pours of zeolite bed. Hence these impurities must be removed before treatment. 2) The coloured ions like Fe2+, Mn2+, etc. react with zeolite. Such zeolite cannot be easily regenerated. Hence these ions must be removed before treatment. 3) Mineral acids destroy the zeolite bed. Hence they must be removed with the help of soda before treatment by zeolite process. Ion Exchange or de-ionization or de-mineralization process: Principle: In an ion exchange process, both cations (Ca2+, Mg2+, etc.) and anions (Cl, SO42, etc.) are removed from hard water by using ion exchange resins. These resins are cross-linked, long chain organic polymers with microporous structure. They are of two types. Cation Exchange resins (RH): These resins are co-polymers of styrene-divinylbenzene containing acid groups like –SO3H, - COOH, etc. They are represented by RH or RH+. They are capable of exchanging their H+ ions with cations like Ca2+, Mg2+, etc. from hard water. Anion Exchange resins (ROH): These resins are co-polymers of styrene-divinylbenzene or amine-formaldehyde containing basic groups like amino (NH3+), quaternary ammonium [(CH3)3N+], etc. in hydroxide form. They are represented by ROH or ROH. Process: In practice, cation exchange resins are packed in separate columns. Raw water is first passed through cation exchange column. In this column, cations Ca2+, Mg2+, etc. from hard water are exchanged with H+ ions of resin. The reactions are as follow. 2RH + Ca2+ R2Ca + 2H+ 2RH + Mg2+ R2Mg + 2H+ After the removal of cations from the hard water, it is then passed through anion exchange column. In this column, all the anions (Cl, SO42, etc.) from hard water are exchanged with OH ions of resin. The reactions are as follow. ROH + Cl RCl + OH R2SO4 + 2OH 2ROH + SO42 H+ and OH ions released during above reactions, combine to produce water. H+ + OH H2O Thus, water obtained by this process is free from cations as well anions which are responsible for hardness. Hence this soft water is known as de-ionized or de-mineralized water. Diagram: Ion exchange resins Regeneration of ion exchange resins: Both cation and anion exchangers lose their ion exchange capacity after working for certain period. At this stage, they are said to be exhausted. The exhausted cation exchange resin is regenerated by passing dilute HCl or dilutes H2SO4. During the regeneration, cation exchanger gets back its H+ ion. The reactions are as follow. R2Ca + 2HCl 2RH + CaCl2 R2Mg + 2HCl 2RH + MgCl2 The exhausted anion exchange resin is regenerated by passing dilute NaOH solution. During the regeneration, anion exchanger gets back its OH ion. The reactions are as follow. RCl + NaOH ROH + NaCl R2SO4 + 2NaOH 2ROH + Na2SO4 After the regeneration, both columns are washed with de-ionized water and washing is passed to sink or drainage. Advantages of ion exchange resins: 1) Hardness of water is removed up to 0 to 2 ppm. Hence this soft water is fit for use in high pressure boilers. 2) Both acidic and alkaline water can be softened by this process. 3) No coagulants are required. 4) All types of ions can be removed by this process. Disadvantages of ion exchange resins: 1) Cost of equipment is very high. 2) More expensive chemicals are needed. 3) Turbidity of raw water must be below 10 ppm. If it is more, it has to be removed by coagulation, filtration, etc.