water analysis and purification notes

 Module 3:Unit V: Water Analysis and Purification

 Water: Main sources of water on earth are 1) Rain water 2) Surface water 3) Underground water 1) Rain water: It is regarded as the purest form of naturally occurring water. But during its passage through air, it dissolves gases and carries dust particles. Dissolved gases such as CO2 , SO2 , Nitric oxide, chlorine, hydrogen sulphide etc. get dissolved in rainwater. 2) Surface water: There are three main sources of surface water, i) River water ii) Sea water iii) Lake water. i) River water: River water contains impurities like dissolved salts, dissolved gases and organic matter, derived from plants, animals etc. along with suspended matter like clay. ii) Sea water: Sea water is the most impure form of water. It contains a very high percentage of dissolved salts and organic matter. iii) Lake water: Lake Water has constant composition of impurities. It contains dissolved salts (with fixed composition) and more organic matter as compared to river water. Classification of Impurities from Water: The common impurities present in natural water are classified into following types: A) Suspended impurities: e.g. Clay, mud, organic matter, industrial waste. They cause turbidity and give smell or odor to water. They can be removed by filtration. B) Dissolved impurities: They are subdivided into two types - 1. Dissolved gases like CO2 , Cl2 , H2S, SOx , NOx 2. Dissolved salts like bicarbonates, sulphates, chlorides, nitrates of Ca, Mg, Fe, Al, Na, K etc. They are removed by special softening procedures. C) Colloidal impurities: Colloidal particles of clay, mud, organic matter etc. which make the water turbid. They cannot be removed by filtration but are usually removed by coagulation followed by filtration. Coagulating agents commonly used are potash alum, sodium aluminate. D) Biological impurities: These impurities include microscopic algae, fungi, bacteria etc. They can be removed by the sterilization process. Common sterilizing agents used are liquid chloride, ozone and UV light. Types of Water: Water received from different resources is of two types: a) Hard water and b) Soft water. To confirm the type, usually water is tested with soap solution. Hardness in water is that characteristic which prevents the lathering of soap. This is due to the presence of certain salts of calcium, magnesium and other heavy metals in water. A sample of hard water when mixed with soap solution (Sodium or potassium salt of higher fatty acid like oleic, palmitic or stearic) does not produce lather, but forms a white scum or precipitate. This precipitate is formed as a result of insoluble soaps of calcium and magnesium. Typical reactions of soaps with calcium chloride and magnesium sulphate are as follows.1 A] C17H35COONa + CaCl2→(C17H35COO)2 Ca + 2NaCl (Sodium stearate) (Hardness) calcium stearate B] C17H35COONa + MgSO4 → (C17 H35COO)2Mg + Na2SO 4 (Sodium stearate) (Hardness) Magnesium stearate • Thus, water which does not produce lather with soap solution but forms precipitate is called hard water. • Water which lathers easily with soap solution is called soft water. Causes of Hardness of Water: Rain water dissolves a number of gases from the atmosphere during its journey through air and then falls on the ground. The surface water thus is acidic in nature. When this acidic water flows over the rocks, various minerals from it slowly get dissolved in water as a result of following reactions. a) Dissolution: Some minerals from rocks are readily soluble in water such as NaCl, CaSO 4 etc. b) Hydration: Some rock minerals like anhydrides and silicates readily undergo hydration forming soluble salts. c) Oxidation: Dissolved oxygen present in water brings about oxidation of some rock minerals which then undergo hydration and get dissolved in water. d) Action of dissolved carbon dioxide: i) It converts insoluble carbonates of Ca, Mg and Fe into soluble bicarbonates. ii) Rock forming silicates and aluminosilicates of Na, K, Ca and Fe reacts with dissolved CO2 and get converted into soluble carbonates, and bicarbonates and silica. Thus, hard water mainly contains bicarbonates, chlorides, sulphates and nitrates of Ca, Mg along with heavy metals like Fe, Al and Mn. Hardness of water is classified into two types based on nature of impurities present, as I) Temporary hardness II) Permanent hardness I) Temporary Hardness of Water: dissolved impurities present in water like bicarbonates of Ca, Mg, Fe, Al, and Mn can be removed from water by filtration after mere boiling. On boiling soluble bicarbonates get decomposed and form insoluble carbonates or hydroxides, CO 2 which escapes from water at boiling temperature. Ca (HCO3 )2 → CaCO3↓ + H2O + CO2 Mg (HCO3 )2 → Mg (OH) 2 ↓ + 2CO2 Temporary hardness is also known as carbonate hardness or alkaline hardness. II) Permanent Hardness of Water: Dissolved impurities present in water like chlorides, sulphates, and nitrates of Ca, Mg, Fe, Al, Mn contribute to permanent hardness. These dissolved salts cannot be removed easily and therefore contribute to permanent or non-carbonate or non-alkaline hardness. Units of Hardness: Hardness of water is the net amount of hardness causing impurities present in water in a finite volume. The concentration of dissolved impurities is usually expressed in terms of calcium carbonate equivalent. The choice of CaCO3 equivalent is accepted universally as its molecular weight is 100 (and equivalent weight is 50) and is the most insoluble salt in water. The CaCO3 equivalent for anydissolved salt from water can be calculated, if its concentration in water is known as follows CaCO3 Equivalent weight of salt = Equivalent wt. Of CaCO3 × Amt. of salt given Equivalent wt. of the salt If the dissolved salt has only bivalent cations, then the above formula can be modified as, CaCO3 Equivalent of bivalent salt = Molecular wt. Of CaCO3 × Amt. of salt given Molecular wt. Of CaCO3 Units used of expressing CaCO3 equivalent hardness: 1) Milligrams per liter (mg / lit): 1 mg/lit hardness is 1 mg of CaCO3 equivalent hardness present in 1 liter of water. 2) Parts per million (ppm): 1 ppm hardness is 1 part of CaCO3 equivalent hardness present in a million liters of water i.e. 10 6parts of water. Relation between mg / lit and ppm: 1 mg / lit = 1 ppm Other units of CaCO3 equivalent are degree Clark (Cl) and degree french (Fr). Chemical Analysis of Water I] Hardness of Water: Hardness in water is mainly contributed by bicarbonates, sulphates, chlorides, nitrates, of Ca, Mg and other heavy metals. In order to find out total hardness or even temporary and permanent hardness, EDTA method is used in water analysis. Structure of M-EDTA complex: (n – 4) Metal-EDTA complex EDTA method is a complexometric method of determining hardness of water. The complexing agent used in this method is Ethylene Di-amine Tetra acetic Acid (EDTA). EDTA reacts with cations like Ca, Mg and also other metal ions from water and gives rise to coordinate complexes, which aresoluble and stable. EDTA is partially soluble in water. Therefore, disodium salt of EDTA is used in the preparation of EDTA solution. Determination of Hardness of Water: Hardness producing constituents like Ca, Mg and other heavy metal salts can be estimated by titrating water samples with EDTA solution at pH 10 using a specific indicator. Most widely used indicator is Eriochrome black T (EBT). Principle: Di-sodium salt of EDTA (Na2EDTA) forms a soluble complex with Ca, Mg ions present in water. The titration is carried out in the presence of indicator - Eriochrome black T. The indicator when added to hard water at pH 10, combines with Ca +2and Mg +2 ions to form a weak, soluble complex which are wine red in colour. These complexes are unstable. When wine-red colour complexes are titrated with EDTA solution, unstable complexes of Eriochrome black T are quickly converted into more stable complexes of EDTA. At this stage Eriochrome black T is released from the metal - complexes, which gives or discharges blue colour. Thus in EDTA titration, using Eriochrome black T indicator, colour change is observed from wine red to blue. Chemical Reactions involved in the titration: A] Ca +2 / Mg +2 + Eriochrome Black T → Buffer pH=10 [Ca +2 /Mg +2 - E.B.T.] (Unstable Wine red complex) B] Ca +2 /Mg +2 Eriochrome Black T + Na2EDTA →Buffer pH=10 [Ca +2 /Mg +2– EDTA] + EBT (Stable colourless complex) blue Procedure: The titration with EDTA to determine hardness of water is divided into three parts. Part I] Standardization of EDTA solution EDTA solution (Na2 EDTA) can be standardized by titrating against ZnSO4 .H2O solution. Preparation of primary standard solution of ZnSO4 .7H2O: Weigh out about 0.143 g of Zinc Sulphate heptahydrate, ZnSO4 .7H2O into 100ml of volumetric flask. Make up the volume with distilled water. The solution will be about 0.01N. Procedure: 25 ml of 0.01N ZnSO4 soln. is pipetted out in conical flask to which 10 ml buffer solution (with pH 10) is added and 4-5 drops of Eriochrome black T indicator. Wine red colour obtained is titrated with EDTA solution from burette till blue colour is obtained. Burette reading is recorded as 'V1 ' ml. To find exact concentration of EDTA solution 1. N1V1 EDTA = N2V2 ZnSO4.7H2O Now we can calculate the exact normality of the EDTA solution.Use this normality value for further calculations. Part II] To find Total hardness of the water sample: 1. 25 ml water sample is pipetted out in a conical flask. 2. Add 1 ml buffer solution and indicator EBT to the hard water sample to get wine red colour. 3. Titrate against the ‘standardised’ EDTA solution taken in burette till wine red colour changes to blue. 4.Note the burette reading as ‘Y’ ml.III] Permanent hardness of water sample: 1. 25 ml water sample is pipetted out in a conical flask and is boiled. The precipitate obtained after boiling is filtered and filtrate is used for titration. (As temporary hardness was removed on boiling i. e. it gets decomposed forming a precipitate) 2. Add buffer and indicator EBT to filtrate, wine red color obtained is titrated with the standardised EDTA solution from burette till blue colour develops. Burette reading is recorded as 'Z' ml. Calculations: N1YEDTA = N2V2HARD WATER N1 x YEDTA =N2 HARD WATER 25 Convert N2 HARD WATER to mg CaCO3 equivalent hardness by multiplying by eq wt of CaCO3 i.e. 50 and by 1000 to get hardness in terms of mg CaCO3 equivalent hardness. ∴ Total Hardness of Sample water is expressed as = ……….CaCO3 eq mg / lit or ppm. PART III: Calculation for Permanent hardness of water sample 25ml Boiled Sample water → reacts with Z ml of EDTA soln. N1ZEDTA = N2V2HARD WATER N1 x ZEDTA =N2 HARD WATER 25 Convert N2 HARD WATER to mg CaCO3 equivalent hardness by multiplying by eq wt of CaCO3 i.e. 50 and by 1000 to get hardness in terms of mg CaCO3 equivalent hardness. ∴ Permanent Hardness of Sample water is expressed as = ……CaCO3 eq mg / lit or ppm. Temporary Hardness of Sample water= Total Hardness – Permanent Hardness Note: In case of molar solutions of EDTA taken, convert to mg CaCO3 equivalents by multiplying molarity with 10 5 .Ill Effects of using Hard Water in Steam Generation The most important use of water in industry is for steam generation using boilers. The boiler feed water should be free from impurities in order to avoid troubles inside the boiler. Depending on the operating pressure of the boiler, the feed water should satisfy following requirements of hardness. Type of boiler Permitted hardness in feed water Low pressure < 15 kg/cm 2 40 - 80 ppm Medium pressure 15 - 30 kg/cm 2 10 - 40 ppm High pressure > 30 kg/cm 2 0 - 3 ppm If the boiler feed water is not upto the standard limit, it gives rise to number of problems in boiler like : I. Priming and foaming II. Boiler corrosion III. Scales and sludge formation IV. Caustic embrittlement I) Priming and Foaming Priming: Violent or vigorous boiling which lead to formation of wet steam is known as Priming. Priming phenomena results into wet steam formation i.e. steam contaminated with water droplets. Causes of Priming: i) Presence of large amount of dissolved salts, ii) High steam velocities, iii) Improper boiler design, iv) Sudden increase in steaming rate. Prevention of Priming: i) Fitting mechanical steam purifiers. ii) Avoiding rapid change in steaming rate. iii) Maintaining low water levels in boilers iv) Efficient softening and filtration of boiler feed water. Foaming: Production of persistent foam or bubbles on the surface of water in boilers which do not break easily is known as foaming. Causes of foaming: i) Presence of substances like oils, soaps (which reduce surface tension of water).Prevention: i) Adding antifoaming chemicals like castor oil. ii) Removing oil impurities by adding compounds like Na-aluminates. iii) The priming and foaming processes usually occur together and are objectionable. Disadvantages of Priming and Foaming i) Wet steam produced during these processes carry dissolved salts which get deposited on turbine blades and on heaters. These deposits reduce their efficiency. ii) Dissolved salts from wet steam may enter the parts of other machinery, thereby decreasing the life of the machinery. iii) Actual height of the water column cannot be judged properly, thereby the operation and maintenance of adequate quantity of water inside the boiler becomes difficult. II) Boiler Corrosion Boiler corrosion is the most serious problem caused by using unsuitable water. Boiler corrosion can be defined as the destruction of boiler metal by a chemical or electrochemical attack by its environment. Corrosion in boilers is due to the following reasons: 1) Dissolved Gases - a) Dissolved oxygen: Dissolved oxygen is the main corrosion causing impurity in water. Dissolved oxygen in water at high temperature attacks boiler metal. 2 Fe + 2 H2O + O2→ 2Fe(OH)2 4 Fe(OH)2 + O2 → 2 [Fe2O3 .2H2O] Prevention: Dissolved oxygen can be removed by adding calculated quantities of sodium sulphite (Na2SO3 ) or sodium sulphide (Na2S) or hydrazine (N2H4 ) or by mechanical deaeration. 2 Na2SO3 + O 2 →2 Na2SO4 Na2S + 2 O2 → Na2SO4 N2H4 + O2 → N2+ 2H2O Hydrazine reagent is preferred as it does not release any salt in the water. Graphical representation of a mechanical deaerator to remove dissolved gases from waterb) Dissolved carbon dioxide: Dissolved carbon dioxide (CO 2 ) in water forms carbonic acid. CO 2 + H2O → H2CO3 Carbonic acid is a weak acid, has a slow corrosive effect on boiler metal. Carbon dioxide is also released inside the boiler if water contains bicarbonates. Mg(HCO3 )2 → ∆ Mg(OH)2 ↓+ 2CO2 Prevention: Dissolved carbon dioxide may be removed by adding calculated amounts of liquid ammonia (NH4OH) or by de-aeration. 2NH4OH + CO2 →(NH4 )2CO 3 + H2O 2) Acids from Dissolved Salt - Water used in the boiler, if it contains dissolved magnesium salts, they liberate acid on hydrolysis. MgCl2 + 2H2O →Mg(OH)2+ 2HCl The liberated acid reacts with iron metal of the boiler in a chain reaction, producing HCl again and again as follows, Fe + 2HCl→ FeCl2 + H2 FeCl2 + 2H2O →Fe(OH)2 + 2HCl Hence, even a small amount of Mg-salts can cause considerable corrosion of boiler metal. Prevention: · Corrosion by acids can be avoided by adding alkalinity externally to neutralize the produced acidity. · Mg-salts can be removed by using zeolite or ion-exchange processes. III) Scale and Sludge Formation In boilers, water evaporates continuously and the concentration of the dissolved salts increases progressively. When their concentration reaches saturation point, these dissolved salts are thrown out of water in the form of precipitate. Sludge When boiler is steaming rapidly, dissolved salts from boiler feed water are precipitated out, in the form of loose and slimy precipitate after saturation point is reached, they are known as sludges. Sludge is a soft, loose and slimy precipitate formed within the boiler. Sludge is generally formed at comparatively colder portions of the boiler and get accumulated or collected in areas where the flow rate is slow. Causes: Sludge is formed by substances which have greater solubility in hot water than in cold water, for example MgCO3 , MgCl2 , CaCl2 , and MgSO4 . Disadvantages of sludge: i) Sludge is poor conductors of heat, so they will waste a portion of heat generated. ii) If sludge is formed along with scales, then it gets entrapped in scales and gets deposited as scales. iii) Excessive formation of sludge decreases the efficiency of the boiler. Sludge settles down in the regions of poor water circulation such as pipe connection, plug opening etc. thereby causingchoking of pipes. Prevention of sludge formation: i) By using soft water ii) By frequent blow down operation (i.e. drawing off a portion of concentrated water and replacing it by adding fresh water). iii) Sludge can be scraped off with mechanical means like wire brush, scrappers etc. Scales Scales are hard; adherent deposits produced when dissolved salts are thrown out of boiler feed water as precipitate after saturation point is reached during boiler operation. Scales are hard deposits, which adhere very firmly to the inner walls or surface of the boiler. Scales are so hard and adherent that they are very difficult to remove even with the help of hammer and chisel. Therefore scales are the main troubles in the boilers. Causes of scale formation 1. Decomposition of bicarbonates At high temperature (boiling temperature) bicarbonates undergo decomposition to form precipitate as scale. Ca(HCO3 )2 → CaCO3+ CO2 + H2O Mg (HCO3 )2 → Mg(OH)2+ 2CO2 2. Decrease in the solubility of salt like CaSO4 There are some salts present in hard water which are insoluble in superheated water. e.g. CaSO4 and CaCO3 . These salts usually are precipitated at hot regions of the boiler forming a hard scale. 3. Hydrolysis of magnesium salts dissolved magnesium salts undergoes hydrolysis forming magnesium hydroxide precipitate which forms a soft type of scale. MgCl2 + 2H2O → Mg(OH)2 + 2HCl MgSO4 + 2H2O → Mg(OH)2 + H2SO4 Mg(NO3 )2 + 2H2O → Mg(OH)2+ 2HNO3 4. Presence of silica (SiO2 ) Small quantity of silica present reacts with soluble salts of Ca and Mg, forming CaSiO3 or MgSiO3 . These silicate deposits stick very firmly on inner walls of the boiler surface and are very difficult to remove. Disadvantages of scale formation: 1. Wastage of fuel - Scales have low thermal conductivity. So they act as partial heat insulators. The rate of heat transfer from boiler to inside water is greatly decreased. In order to get steady supply of heat, overheating is done and this causes an increase in fuel consumption. 2. Lowering of boiler safety - Due to scale formation, overheating of boilers is done in order to maintain a constant supply of steam. The overheating of the boiler makes the boiler material softer and weaker and makes the boiler unsafe to bear the pressure of the steam, especially in high pressure boilers. 3. Decrease in efficiency –Sometimes scales may deposit in valves, condensers of the boiler and choke them partially. This leads to decrease in efficiency of the boiler. 4. Danger of explosion - Due to overheating boiler metal undergoes expansion. The scale deposits present on the boiler metal surface being bad heat conductors, suffer cracking. Through these cracks, red hot metal comes in contact with boiling water, which causes sudden increase in steam formation. As a result suddenly high steam pressure is developed inside this may cause an explosion of the boiler. Sr. No. Sludge Scale 1. Sludge is loose and slimy precipitates. Scales are hard and adherent precipitates. 2. Sludge are formed by the salts which are less soluble in cold water but soluble in hot water. Scales are formed by the salts which are less soluble in hot water but may be soluble in cold water. 3. Sludge is formed at cooler parts of the boiler. Scales are formed at hot parts or regions of the boiler. 4. Sludge is formed due to increase in concentration of the salt. For e.g. MgCO3 , Mg Cl2 , CaCl2 . Scales are formed due to decomposition of bicarbonates, hydrolysis of Mg-salts, less solubility in hot water shown by CaSO4 , presence of silica. 5. They can be removed by blow-down operation. They can be removed by EDTA treatment. Removal of scales: 1. Soft scales can be removed with the help of scraper or piece of wood or wire brush. 2. By dissolving them, by adding some chemicals like EDTA solution, if they are hard and adherent, which react with hard scales and form soluble complexes. 3. Soft scales also can be removed along with sludge by blow down operation. 4. By giving thermal shocks to the boiler if scales are very hard. Prevention of scale formation The scale formation can be prevented or at least minimized by giving certain chemical treatments to boiler feed water, before supplying to the boiler. Such treatments are known as External treatments. Whereas when boiler feed water is treated during the steaming process i.e. inside the boiler, such treatment is known as internal treatment. Difference between Scale and Sludge IV) Caustic Embrittlement: It is a type of boiler corrosion caused by using highly alkaline water in the boiler. It is most likely to take place in boilers which operate under high pressures.Causes: During the softening process by lime – soda, free soda (Na2CO3 ) is usually present in a small portion in the softened water. (i.e. excess soda is always present). In high pressure boilers, soda decomposes to give sodium hydroxide and carbon dioxide. Na2CO3 + H2O → High NaOH + CO2 Pressure Formation of caustic soda makes the boiler water alkaline. Caustic soda (NaOH) containing water flows into the minute cracks always present in the inner walls of the boiler by capillary action. As the boiler is steaming rapidly, water from cracks evaporates and dissolved caustic soda gets deposited in the cracks. The concentration of caustic soda increases progressively and slowly attacks surrounding areas, thereby dissolving boiler metal. Caustic soda (NaOH) reacts with boiler metal i.e. iron, forming sodium ferroate and hydrogen as follows. 2Fe + 2NaOH + O2→2 NaFeO2 + H2 The products of the reaction, sodium ferrate and hydrogen also tend to penetrate along grain boundaries. This causes brittlement of the boiler parts. Therefore this trouble is known as caustic embrittlement. Prevention: Caustic embrittlement can be avoided by - i) Using sodium phosphate as a softening agent instead of sodium carbonate. ii) By treating the boiler walls with tannin or lignin which blocks the cracks, thereby preventing accumulation of caustic soda. Water Treatment Treatment of water for industrial purposes primarily aims at reducing the hardness present in received water or raw water. These treatments are of two types I) External treatments II) Internal treatments I] External Treatments: The treatments in which scale forming and corrosive impurities are removed from water before it enters the boiler are known as External treatments there are different methods to carry out external treatment which include, A. Zeolite/ Permutit process. B. Ion-exchange process. C. Soda-lime process. In order to obtain good results, sometimes combinations of two methods are applied. A. Zeolite / Permutit Process The name zeolite came from Greek word, ‘Zein’ - boiling, lithos - stone, meaning ‘boiling stone’. The zeolite was first used in 1756 by Axel Cronsted (Swedish geologist). The chemical structure of sodium zeolite may be represented as, Na2O. Al2O3 . xSiO2 . yH2O where x = 2 to 10 and y = 2 to 6 Thus, zeolite is hydrated sodium aluminosilicate, capable of exchanging reversibly their sodium ions for multivalent cations present in water. Therefore, zeolites find application in softening of water. Zeolites are also known as ‘permutit’. Two types of zeolites are commonly used in water softeninga) Natural zeolites (nonporous, green sand) b) Synthetic zeolites (porous, gel structure) Extra Reading: Natural zeolites are derived from green sand by washing, heating and treating with caustic soda (NaOH). They are nonporous and are more durable. Synthetic zeolites are prepared by heating china clay, feldspar and soda ash together followed by cooling and granulating the resultant mass. They can be prepared by heating solutions of sodium silicate, aluminium sulphate and sodium aluminate. Such zeolite is porous and possesses gel structure and has higher exchange capacity than natural zeolites. Theory Zeolites hold sodium ions loosely and can easily exchange their sodium ions with other cations like Ca 2+ , Mg 2+ etc. Thus if water containing dissolved calcium and magnesium salts is passed over a bed of sodium zeolite, Ca and Mg are exchanged for sodium as follows, (Abbreviation given to sodium zeolite as Na2Z or Na2P) Na2Z + CaCl2 → CaZ+ 2NaCl Na2Z +MgSO4 →MgZ + Na2SO4 From the above exchange reaction, sodium zeolite is converted into calcium and magnesium zeolite, whereas water becomes free from Ca and Mg salts but becomes richer in sodium salt. When zeolite is exhausted i.e. when it loses its sodium exchanging capacity, it can be regenerated by washing the bed with a concentrated solution of sodium chloride (brine) as follows. CaZ + 2NaCl → Na2 Z + CaCl2 MgZ + 2NaCl →Na2Z + MgCl2 The regenerated zeolite (Na2 Z) can be used again for softening of water. Process For softening of water by the zeolite process, hard water is percolated at specified rate through a zeolite bed .The hardness causing impurities are retained by zeolite crystal as CaZ or MgZ, whereas outgoing water contains sodium salts. Na2Z + Ca(HCO3 )2→CaZ + 2 NaHCO3 Na2Z + MgSO4→ MgZ + Na2SO4 Na2Z + Ca(NO3 )2 →CaZ + 2 NaNO3 Na2Z + MgCl2→ MgZ + 2 NaCl Regeneration After sometime, when zeolite is completely converted into calcium and magnesium zeolite, it is unable to soften water further i.e. it gets exhausted. At this stage, the supply of hard water is stoppedand exhausted zeolite is reclaimed by treating the zeolite bed with concentrated brine solution (10 % NaCl solution) CaZ + 2NaCl →Na2Z + CaCl2 MgZ + 2NaCl →Na2Z + MgCl2 (Exhausted (Brine) (Reclaimed (Washings) zeolite) zeolite) The washings containing CaCl2 and MgCl2 are drained and the regenerated zeolite bed is used again for softening purposes. Advantages 1. Hardness is completely removed. 2. Equipment used is compact and occupies less space. 3. No impurities are precipitated, so there is no danger of sludge formation. 4. The process automatically adjusts itself for variation in hardness of incoming water. 5. It is quite a clean process and requires less time for softening. 6. Soft water obtained has hardness between 5 - 10 ppm. Limitations 1. Water having turbidity and suspended matter should not be directly fed to the zeolite softener because the pores of the zeolite bed will be clogged and the rate of flow will be unduly decreased. Thus, turbidity and suspended matter from the water should be removed before subjecting it to the zeolite treatment. 2. Water containing excess acidity or alkalinity may destroy zeolite crystals. Therefore, it is preferable to have pH 7 of the water passing through zeolite softener. 3. Water containing Fe +2and Mn +2cannot be used as their respective zeolite cannot be regenerated easily with brine. B. Ion Exchange or Demineralization Process The boiler feed water, as far as possible should be free from all types of impurities and if possible, it should be as pure as distilled water. Water of such quality can be obtained by demineralization or deionization process. The demineralized water is soft and it does not contain any dissolved salts. In this process, ion exchange resins are used as softener. Ion exchange resins are insoluble, cross linked, long chain organic polymers with micro-porous structure and the 'functional groups' attached to the chain are responsible for the ion-exchanging properties. Resins containing acidic functional groups (–COOH, ---SO3H etc.) are capable of exchanging their H + ions with other cations, which comes in their contact whereas, resins containing basic functional groups (--NH 2 , – OH etc.) are capable of exchanging their anions with other anions, which comes in their contact. Cation exchange resins: Resins which can easily exchange their (H + ) ions with cations are known as cation exchange resin. These are mainly styrene-di-vinyl benzene copolymers which on sulphonation or carboxylation become capable of exchanging their hydrogen ions with the cations in the water. Amberlite IR 120, Dowex 50, Calcite-HCR are the resins of this type. These resins can be represented as RH2 . Their exchange reactions with cations are as follows.Acidic or cation exchanger 2RH + Ca +2→R2Ca + 2H + 2RH + Mg +2→R2Mg + 2H + Ca 2+ Cation exchange resin: Resins which can easily exchange their (H + ) ions with cations Anion exchange resins: Resins which can easily exchange their (OH – ) ions with anions. These are styrene-di-vinyl benzene or amine formaldehyde copolymers, which contain amino or quaternary ammonium or tertiary sulphonium groups as an integral part of the resin. These resins after treatment with dilute NaOH become capable to exchange their OH anions with the anions in the water. Example - Amberlite 400, Dowex 3, Zeolite FF are the resins of this type. These resins can be represented as R' (OH) Cl Their exchange reactions with anions like SO4 2- , Cl - , NO3 - , and HCO3 -can be illustrated as follows. 2R’(OH) + SO4 2-→R'2SO4 + 2 OH R'(OH) + Cl -→R'Cl + OH From the above reactions, it is clear that if hard water is passed first through cation exchanger bedand then through anion exchanger bed, the resulting water will be free from both cations and anions. Process The hard water is passed through cation exchanger; it removes all cations like Ca +2 , Mg +2etc. from hard water and an equivalent amount of H + ions are released from this exchanger to water. Thus, water received from cation exchangers is acidic in nature. This acidulated water is then passed through anion exchanger, which removes all the anions like SO4 2- , Cl - , NO -3etc. present in the water and release equivalent amounts of OH - from this exchanger to water. Suppose hard water contains impurities like Ca (HCO 3 ) 2 , MgCl2 CaSO4 , then they are removed easily in the ion exchange process. The reactions in cation and anion exchanger are as follows. Reactions in cation exchanger: Ca(HCO3 )2 + 2RH →R2Ca + 2H2CO3 MgCl2 +2RH→ R2Mg + 2HCl CaSO4 + 2RH →R2Ca + H2SO4 Reactions in anion exchanger: 2R'(OH) + H2CO3→R'2CO3 + 2H2O R'(OH) + HCl →R’Cl + H2O 2R'(OH) + H2SO4→R'2SO4 + 2H2O In this way, the water coming out from anion exchanger is free from cations and anions. Then, water is passed through a degasifier in which water is allowed to pass over hot plates present in the degasifier so that dissolved gases in water are expelled out and water obtained from the degasifier is free from dissolved gases along with cations and anions. In the process, the sequence of water flow is important. Always cation exchange resin need to be used first, because if water is passed first through anion exchange resin, alkali is produced which can harm the cation exchange resin. Regeneration: When all H +and OH – ions from resins are replaced by various cations like Ca, Mg ions and anions like Cl – , SO4 –2 resin is said to be exhausted. The exhausted cation exchange resin is regenerated by passing a solution of dil. HCl or H 2 SO4 . The regeneration reaction can be written as, R2Ca + 2HCl → 2RH + CaCl2 R2Mg + 2HCl → 2RH + MgCl2 The column is washed with water and washings are drained out.The exhausted anion exchange resin is regenerated by passing a solution of dil NaOH. The regeneration reaction can be written as, R'2 SO4 + 2NaOH → 2R'(OH) + Na 2SO4 R'Cl + NaOH → R'(OH) + NaCl The column is washed with water and washings are drained out. Advantages 1. Process can be used for highly acidic or alkaline water samples. 2. It produces water with 0 to 2 ppm hardness which is very low hardness. Disadvantages 1. Equipment is costly and chemicals used are expensive. 2. Water sample, if it contains turbidity cannot be used (turbidity should be removed by coagulation and filtration and then used in the process.) Sr. Zeolite process Ion-exchange process 1. Softener used in the process is zeolite crystals. Softener used in the ion exchange process is (synthetic resin) ion exchange resin. 2. Zeolite crystals can exchange sodium ions with multivalent cations present in water. Synthetic resins exchange either their cation or anion for impurities present in water. 3. On treatment with zeolite crystals, soft water obtained contains sodium salt. On treatment with cation exchanger resin and anion exchanger resin, soft water obtained is free from mineral salts. 4. Zeolite process cannot be used for water containing iron, manganese salts and highly acidic or alkaline water. Ion exchange processes can be used for all types of water samples. 5. Exhausted zeolite can be regenerated by NaCl solution. Exhausted resins can be regenerated, cation exchange resin using dil HCl and anion exchange resin using dil KOH. 6. Soft water has hardness 5 - 10 ppm. Soft water has hardness 0 - 2 ppm.Internal Treatment of Water The treatments in which various substances are added to boiler feed water to remove residual, non carbonate hardness in order to prevent scale forming tendency or overcome corrosive tendency is known as internal treatment of water. General Principle: Internal treatment consists of adding chemicals directly to the water in the boilers for removing scale forming salts which were not completely removed in the external treatment. Internal treatment is mainly used as a corrective treatment to remove the residual hardness still left in water after external treatment. It also takes care of scale forming salts, by converting them into loose, soft precipitate or soluble complexes. The internal treatment is carried out inside the boiler i.e. boiler feed water is treated with softening reagents. These treatments are generally followed by blow down operation, so that accumulated sludge is removed. Important internal treatment methods are: I) Colloidal Conditioning: Principle: By adding certain organic substances colloidal particles of scales are converted into sludge. In low pressure boilers, scale formation can be avoided by adding organic substances like tanin, agar-agar etc. These substances get coated over scale forming precipitates, thereby converting them into nonsticky and loose deposits which can be easily removed by blow down operation. II) Phosphate Conditioning: Principle: Soluble phosphates are added to water to convert scales into loose sludge. In high pressure boilers, scale formation can be avoided by adding sodium phosphates. The phosphates reacts with Ca and Mg salts from boiler water, forming non-adherent and loose precipitate or soft sludge of Ca/Mg phosphate which can be removed by blow down operation. The main phosphates employed in this treatment are A] Orthophosphates like a. Trisodium phosphate: Na3PO4 b. Disodium hydrogen phosphate: Na2HPO4 c. Sodium dihydrogen phosphate: NaH2PO4 B] Sodium pyrophosphate: Na4P2O7 C] Sodium meta-phosphate: NaPO3 The typical reactions of various ortho, meta and pyro phosphates with hardness causing impurity like CaCl2 may be as follows, A] Orthophosphates: i) 2 Na3 PO4 + 3 CaCl2→Ca3 (PO4 )2 + 6 NaCl This orthophosphate is alkaline in nature hence is used when feed water is acidic in nature. ii) 2 Na2 HPO4 + 3CaCl2 → Ca3 (PO4 )2 + 4 NaCl + 2HCl This orthophosphate is slightly acidic in nature hence is used when feed water is weakly alkaline in nature. iii) 2NaH2PO4 + 3CaCl2→Ca3 (PO4 )2 + 2 NaCl + 4HClThis orthophosphate is comparatively higher acidic in nature hence is used when feed water is highly alkaline in nature. From the above reactions, the choice of specific orthophosphate can be made on the basis of the nature of boiler feed water or pH of the boiler feed water. B] Sodium metaphosphate: NaPO3 Metaphosphate undergoes hydrolysis in water to form orthophosphate as: NaPO3 + H2O→ NaH2PO4 (Sodium dihydrogen phosphate) This orthophosphate then react with scale forming impurity to convert it into sludge. C] Sodium pyrophosphate: Na4P2O7 Pyrophosphate undergoes hydrolysis in water to form orthophosphate as: Na4 P2 O7 + H2O→ 2 Na2HPO4 (Disodium hydrogen phosphate) This orthophosphate then react with scale forming impurity to convert it into sludge. III] Calgon Conditioning : Principle : By adding calgon, scales are converted into soluble complexes. It involves the addition of calgon or sodium hexametaphosphate (NaPO)6 to boiler water at pH 10. It prevents scales and sludge formation by forming soluble complex compounds. (NaPO3 )6⇆ 2 Na + [Na4P6O18 ] –2 → 2 CaSO4 + [Na4 P6O18 ] 2– → [Ca2 P6O18 ] 2–+ 2Na2SO4 Calgon Soluble complex Desalination of Brackish Water For domestic, industrial and agricultural use, water containing a high percentage of dissolved salts is not suitable. Sea water or brackish water contains 3.5% dissolved salts. Thus sea water can be converted into fresh water or potable water, after removing dissolved salts. The process of removing dissolved salts from sea water to make it potable or suitable for drinking and suitable for agricultural purposes is called desalination. This technique is employed in order to overcome water shortage caused due to scarcity of water in arid regions and depletion of water due to excessive human usage. There are many techniques which can be used for treatment of seawater. Desalination can be performed by two processes, namely membrane process and thermal process. Reverse Osmosis (RO) and electro dialysis are two membrane processes and distillation is a thermal process. Even though distillation is one of the best techniques, it is very costly. Membrane Processes A] Osmosis The movement of solvent between two solutions having different concentrations which are separated by semi-permeable membrane is known as osmosis. The solvent moves from the solution of lower concentration to that of higher concentration untilboth the solutions have the same concentration as shown in figure. B] Reverse osmosis When the direction of normal osmotic flow of water across the membrane is reversed by applying pressure over the solution having high concentration is known as reverse osmosis. Then the solvent flows from the compartment containing higher concentration of the solute to that having lower concentration. This results in concentration of solute in one compartment and dilution in the other. Reverse osmosis water treatment method has been extensively used to convert brackish or seawater to drinking water, to clean up waste water and to recover dissolved salts from industrial processes. Reverse osmosis treatment reduces the concentration of dissolved solids, which include a variety of ions and metals and very fine suspended particles like asbestos which may be found in water. RO also removes certain organic contaminants, some detergents and specific pesticides. Reverse osmosis is known to be the finest hyper filtration technique which can remove particles as well as dissolved individual ions from solution. Advantages of RO technique for water treatment: 1. RO is capable of removing/rejecting dissolved salts as well as organic substances along with bacteria and viruses present in water. 2. The process is simple and requires a few minutes. 3. It can convert seawater to drinking water at low cost and low capital