UNIT I WATER TECHNOLOGY

Transcription

1 UNIT I WATER TECHNOLOGY Characteristics alkalinity types of alkalinity and determination hardness types and estimation by EDTA method (problems) Domestic water treatment disinfection methods (Chlorination, Ozonation, UV treatment) Boiler feed water requirements disadvantages of using hard water in boilers internal conditioning (Phosphate, calgon and carbonate conditioning methods) external conditioning deminerlization process desalination and reverse osmosis.

2 UNIT I WATER TECHNOLOGY Introduction: Water is nature s most wonderful, abundant and useful compound. It is most important compound for the existence of human beings, animals and plants. Without food, human can survive for a number of days, but water is such an essential that without it one cannot survive. Besides these, water has great applications in industries. Water is mainly used in power generation industry for the production of electricity through steam generation. Water is also used as a coolant in nuclear reactor and chemical plants. In addition to it, Water is widely used in other fields like production of steel, rayon, paper, atomic energy, textiles, chemicals, ice and for air-conditioning, drinking, bathing, sanitary, washing, irrigation, fire-fighting, etc. SOURCES OF WATER: There are two main sources of water 1. Surface water 2. Underground water ICE BERGS LAKE SEA SPRING

3 SURFACE WATER: WATER FALLS 1. Rain water: It is probably the purest form of natural water. However, during the journey downwards through the atmosphere, it dissolves a considerable amount of industrial gases like SO 2, CO 2, NO 2,. and suspended solid particles. 2. River water: Rivers are fed by rain and spring waters. Water from these sources flow over the surface of land dissolves the soluble minerals of the soil and finally fall in rivers. River water thus contains dissolved minerals of the soil such as chlorides, sulphates, bicarbonates of Na, Ca, Mg and Fe. River water also contains the organic matter obtained from the decomposition of plants, animals, small particles of sand and rock in suspension. Thus the river water contains considerable amount of dissolved minerals. 3. Lake water: It has constant chemical composition. But it contain very small amount of dissolved minerals than well water. It contains more amount of organic matter. 4. Sea water: It is the most impure form of natural water. Rivers throws the impurities to the sea at joining. Moreover, continuous evaporation of water from the surface of sea water more in dissolved salts. It contain around 3.5 % of dissolved salts, out of this 2.6 % is NaCl. Other salts present are Sodium sulphate, Potassium bicarbonate, Magnesium bicarbonate, Calcium bicarbonate, Potassium bromide and Magnesium bromide etc. (Na 2 SO4, KHCO3, Mg (HCO3) 2, Ca(HCO3) 2, KBr and MgBr 2 )

4 UNDERGROUND WATER: The rain water reaches the surface of the earth, percolates into the earth and travels downward; it dissolves the number of mineral salts present in the soil. Finally it meets a hard rock and come out in the form of spring. Generally spring and well water are very clear in appearance due to the filtering action of the soil, but contain more amounts of dissolved salts. CHARACTERISTICS IMPARTED BY IMPURITIES IN WATER: PHYSICAL IMPURITIES: Imparts, 1. Colour 2. Turbidity 3. Taste 4. Odour CHEMICAL IMPURITIES: Imparts, 1. Acidity 2. Alkalinity 3. Nitrogen content 4. Fluoride content COLOUR: Colour is a shade imparted by organic or inorganic material, which change he appearance of the water. The change in colour of water is not harmful, unless it is associated with any chemical or toxic nature. The colour in water is caused by the inorganic sources like salts of Fe, Mn and chemicals from dye industry, and the organic sources like algae, tannin, peat, humus material, weeds, protozoa and industrial effluents from paper, textile, sugar industries, etc. The range of colour varies from pale straw to dark brown.

5 Sanitary significance: The variation in colour of water from the same source with time often serves as indices of quality. Usually, yellowish tinge due to the presence of Chromium; and more amount of organic matter, yellowish red colour due to the presence of Fe and reddish brown due to the presence of peaty matter. Removal of colour: The colour and colour producing substances are removed by coagulation, settling, adsorption and filtration, etc. TURBIDITY: Turbidity is the process of reduction in clarity of water due to the presence of finely divided, insoluble impurities suspended in water. It is caused by the inorganic sources like clay, silt, silica, Fe(OH)3, CaCO3, etc and the organic sources like finely divided plant, animal matter, oils, fats, greases, micro organisms like plankton, etc. Sanitary significance: The amount of turbidity in water decides the grade of product being manufactured in industries and efficiency of boilers. i.e. the turbidity creating substances in water are affects the boilers, power plants and cooling water systems. Removal of turbidity: It can be removed by sedimentation, followed by coagulation, filtration, etc. TASTE: Usually, taste is interlinked directly with odour. The presence of dissolved mineral in water produces taste. For example, Bitter taste is due to the presence of Fe, Al, Mn salts and CaCO3, Soapy taste is due to the presence of more amount of NaHCO3, Brackish taste is due to the presence of unusual amount of salts and palatable taste is due to the presence of dissolved CO2 and nitrates. Removal of taste: Taste can be removed by desalination, de-aeration and boiling followed by filtering, etc ODOUR: The unwanted odour in water may be caused by the presence of algae, fungi, bacteria, weeds and sulphides. The contamination of industrial effluents,

6 containing aldehydes, phenols, ester, ketones etc.. and sewage containing organic matters, creates unwanted odour in water bodies. Removal of odour: The odour can be removed by boiling or disinfection process. ACIDITY: It is a measure of base neutralizing ability. It may be caused by the presence of dissolved CO2, minerals and Mineral acids like HCl, H2SO4, etc. Mineral acids are formed by the oxidation of S and FeS2. 2 S + 3 O H2O 2 H 2 SO4 2 FeS 2 + O H2O 2 FeSO4 + 2 H 2 SO4 CO2 is formed from the industrial smoke and smokes from automobiles. Removal of acidity: It can be removed by adding calculated quantity of NaOH or Na 2 CO3 ALKALINITY: It is a measure of acid neutralizing ability. It is caused due to the presence of hydroxides, carbonates and bicarbonates of alkali metals. These salts are released from the industries like fertilizer, detergent, leather and paint, etc. Sanitary significance: 1. As the amount of alkalinity increases in water bodies, the aquatic organisms are greatly affected. 2. Alkalinity in boiler feed water causes caustic embrittlement of pipes Removal of alkalinity: It can be removed by adding a small amount of HCl. NITROGEN: It is an inert gas, and relatively unimportant as far as water treatment is concerned. The plant matters, fertilizers are the main sources of Nitrogen. Sanitary significance: It has no corrosion action on metals, since it is inert, it cannot be determined practically.

7 Removal of nitrogen: It can be removed by boiling the water. FLUORIDE: Fluorides are found in ground water due to the dissolution from geologic formulations. Generally, ground water contains more concentration of fluoride than the surface water.the main sources are Fluorapatite Ca 10 F 2 (PO4) 6, Cryolite Na3AlF 6, igneous rocks containing fluosilicates and phosphate fertilizers. Sanitary significance: 1. The optimum fluoride concentration should be mg/l 2. If the fluoride concentration is high, it causes fluorosis. 3. If the fluoride concentration is low, it causes dental problems in children. Removal of fluorides: Fluorides can be removed by adsorption on activated carbon, using basic anion exchange resin and using Al salts in alkaline medium. WATER: All living things need an adequate supply of potable water. Water is necessary for the survival of all living things humans, animals, and plants on earth. Available water 3% Fresh water 97 % saline water (Oceans & seas) % 0.036% Ice at poles Lakes, rivers and reservoirs It is the fact that proper maintenance, conservation, and use of water resources needed to avoid scarcity of water for the future generation. CLASSIFICATION OF WATER: Based on the reaction with soap, the water is classified into soft and hard water. The water which does not produce lather with soap solution readily, but forms a white precipitate is called hard water, the water which produce lather easily

8 on shaking with soap solution, is called soft water. Such water does not contain dissolved Ca and Mg salts in it. HARDNESS OF WATER: Hardness is the characteristic property of water, which prevents the lathering of soap. This is due to the presence of bicarbonates, sulphates, chlorides of Ca and Mg and other heavy metals dissolved in it. A sample of hard water, when treated with soap (Na or K salt of higher fatty acid) does not produce lather, but produces a white scum or precipitate. This precipitate is formed, due to the formation of insoluble soaps of Ca and Mg. 2 C 17 H35COONa + CaCl 2 (C17H35COO) 2 Ca + 2 NaCl Sodium stearate Hardness Calcium stearate 2C17H35COONa + MgSO4 (C17H35COO) 2 Mg +2 Na 2 SO4 Sodium stearate Hardness Magnesium stearate Differences between Hard and soft water: Sl.No HARD WATER SOFT WATER 1 It does not give lather with soap, but gives white scum or precipitate 2 It contains chlorides, sulphates and bicarbonates of Ca & Mg 3 It gives wine red colour with Eriochrome Black T It gives lather with soap readily It does not contain chlorides, sulphates and bicarbonates of Ca & Mg It does not give wine red colour with Eriochrome Black T Hardness of water can be classified into two types: 1. Temporary or Carbonate hardness 2. Permanent or non-carbonate hardness 1. TEMPORARY HARDNESS: Hardness of water is due to the presence of dissolved bicarbonates of Ca and Mg, other heavy metals and carbonates of Fe is called temporary hardness. It can be

9 removed by either boiling the water (or) adding lime to the water. On boiling; the bicarbonates are converted into insoluble carbonates and hydroxides. Ca (HCO3) 2 CaCO3 + H2O + CO2 Mg (HCO3) 2 Mg (OH) CO2 2. PERMANENT HARDNESS: Hardness of water due to the presence of chlorides and sulphates of Ca and Mg, Fe and other heavy metals is called permanent hardness. It cannot be removed by boiling, but can be removed by Lime-soda process, Zeolite process and De- Ionization process, etc. DIFFERENCES BETWEEEN TEMPORARY & PERMANENT HARDNESS: Sl. No TEMPORARY HARDNESS PERMANENT HARDNESS 1 Due to the presence of dissolved bicarbonates of Ca and Mg, other heavy metals and carbonates of Fe 2 It can be removed by either boiling the water (or) adding lime to the water Due to the presence of chlorides and sulphates of Ca and Mg, Fe and other heavy metals It cannot be removed by boiling, but can be removed by Lime-soda process, Zeolite process and De-Ionization process, etc. EQUIVALENTS OF CaCO3: The equivalent of CaCO3 = { Mass of hardness producing substance } X {Chemical equivalent of CaCO3} Chemical equivalent of hardness producing substance = { Mass of hardness producing substance } X 50 Chemical equivalent of hardness producing substance The concentration of hardness and non-hardness expressed as equivalents of CaCO3. The choice of CaCO3 in particular is due to its molecular weight is 100 (equivalent weight = 50) which is found to be convenient for the calculation. Moreover, it is the most insoluble salt that can be precipitated in water treatment.

10 UNITS OF HARDNESS: Hardness of water can be expressed in different units. (i) Parts per million (ppm): It is the parts of CaCO3 equivalent hardness per 10 6 parts of water. 1 ppm = i.e. 1 part of CaCO3 equivalent hardness in 10 6 parts of water. (ii) Milligrams per litre (mg/l): It is the number of milligrams of CaCO3 equivalent hardness per litre of water. 1 mg/l = 1 mg of CaCO3 equivalent hardness of 1 litre of water = 1 mg of CaCO3 equivalent hardness per 10 6 mg parts of water. = 1 part of CaCO3 equivalent hardness per 10 6 parts of water = 1 ppm (iii) Clarke s degree ( Cl): It is the number of parts of CaCO3 equivalent hardness per 70,000 parts of water. 1 Cl = 1 part of CaCO3 equivalent hardness per 70,000 parts of water (iv) Degree French ( Fr): It is the number of parts of equivalent hardness per 10 5 parts of water 1 Fr = 1 part of CaCO3 equivalent hardness per 10 5 parts of water (v) Milli equivalent per litre (meq/l): It is the number of milli equivalent of hardness present per litre of water. 1 meq/l = 1 meq of CaCO3 per litre of water = 10-3 X 50 g of CaCO3 equivalent per litre = 50 mg per litre of CaCO3 equivalent = 50 ppm Relation between various units of hardness: 1 ppm = 1 mg/l = 0.1 Fr = 0.07 Cl = 0.02 meq/l DISADVANTAGES OF HARD WATER: 1. Hard water is unfit for various domestic purposes like washing, bathing and so on. The hardness producing Ca 2+ and Mg 2+ ions convert soaps into insoluble precipitates. This causes wastage of soaps in washing and bathing. 2. When hard water is used for drinking, it affects the digestive system and it forms stones in the kidney (as Calcium oxalate)

11 3. When hard water is used for cooking, more fuel and time consumption are required. Because of the presence of salts of Ca and Mg, this increases the boiling point of water. 4. Hard water is not useful for industries like textile, sugar and paper. The dissolved salts like Ca, Mg, Fe and Mn affect the following properties. (a) Smooth and glossy finish to paper in paper industry (b) lather in laundry (c) shades and colour to fabrics in textile industry. 5. When hard water is used for steam production, the boiler affected by the problems like scale-sludge formation, priming and foaming, corrosion and caustic embrittlement. 6. Hard water should not be used for laboratory analysis, because the hardness producing Ca 2+ and Mg 2+ ions interfere in reactions. 7. When hard water is used for concrete making, the hydration of cement and the strength of the concrete are affected. ESTIMATION OF HARDNESS OF WATER: It is essential to estimate the hardness of water prior to its use in boilers and industries. Hardness of water can be estimated by the following methods. 1. EDTA method 2. O. Helmer s method 3. Soap titration method. EDTA METHOD: It is a complexometric method. The structure of Ethylene Diamine Tetra Acetic acid (EDTA) is HOOC-H 2 C CH 2 -COOH N CH2 CH2 N HOOC-H 2 C CH 2 -COOH Since EDTA is insoluble in water, its disodium salt is used as a complexing agent.

12 The structure of disodium salt of EDTA is HOOC-H 2 C CH 2 -COONa N CH2 CH2 N NaOOC-H 2 C CH 2 -COOH This forms complex ions with Ca 2+ and Mg 2+ ions of hard water. PRINCIPLE: Estimation of hardness by EDTA method is based on the principle that EDTA forms metal complexes with Ca 2+ and Mg 2+ ions in water. These complexes are stable when ph range is 8 10 and to maintain the ph, buffer solution (mixture of NH4Cl and NH4OH) is added. The completion of the complexation reaction is indicated by Eriochrome Black T indicator. When this indicator is added to the sample water, it forms metal indicator complex of wine red or purple red colour. [ Ca 2+ and Mg 2+ ] + EBT [Ca, Mg EBT complex ] Unstable When this solution is titrated against EDTA in the burette, EDTA replaces the indicator from metal indicator complex. When all the Ca 2+ and Mg 2+ ions are complexed by EDTA, the indicator is set free and the end point is the sharp colour change from wine red to steel blue. [Ca, Mg EBT complex]+ EDTA Unstable [Ca and Mg EDTA complex] + EBT (stable blue) Thus the total hardness is determined. The temporary hardness is removed by boiling and after the removal of precipitate by filtration; the permanent hardness in the filtrate is determined by titration with EDTA as before. Temporary hardness = Total hardness Permanent hardness EXPERIMENTAL PROCEDURE: Pipette out 50 ml of the hard water sample into a clean conical flask. Add 10 ml of buffer solution and a pinch of Eriochrome Black T indicator. It is titrated against standard EDTA taken in the burette. The end point is the colour change from wine red to steel blue. Repeat the titration for concordant values. Let the volume of EDTA consumed be V 1 ml

13 ESTIMATION OF PERMANENT HARDNESS: Take 250 ml of hard water sample in a beaker and evaporate it to 50 ml. Filter the solution into a 250 ml standard flask. Make up the filtrate again to 250 ml by adding distilled water. Pipette out 50 ml of made up solution into a clean conical flask. Add 10 ml of buffer solution and a pinch of Eriochrome Black T indicator. It is titrated against standard EDTA taken in the burette. The end point is the colour change from wine red to steel blue. Repeat the titration for concordant values. Let the volume of EDTA consumed be V 2 ml CALCULATIONS: TOTAL HARDNESS: 1 ml of EDTA = 1 mg of CaCO3 V 1 ml of EDTA = V 1 mg of CaCO3 This hardness is present in 50 ml of hard water. Therefore, total hardness = V 1 x 1000 ppm of CaCO3 equivalent 50 PERMANENT HARDNESS: 1 ml of EDTA = 1 mg of CaCO3 V 2 ml of EDTA = V 2 mg of CaCO3 This hardness is present in 50 ml of boiled, filtered hard water sample. Therefore, permanent hardness = V 1 x 1000 ppm of CaCO3 equivalent 50 TEMPORARY HARDNESS: Temporary hardness = Total hardness Permanent hardness ALKALINITY: The acid neutralizing ability of water is called alkalinity of water. PRINCIPLE: Alkalinity in water is due to the presence of OH -, CO3 2- and HCO3 - ions. There are five alkalinity conditions possible in a given water sample, 1. OH- only 2. CO3 2- only 3. HCO3 - only

14 4. Combination of CO3 2- and OH - 5. Combination of CO3 2- and HCO3 -. The various alkalinities of the given water sample can be estimated by titrating it against standard acid (H2SO4) using phenolphthalein and methyl orange indicators successively. Hydroxide alkalinity is completely neutralized and carbonate alkalinity is partially neutralized during phenolphthalein end point. OH - + H+ H 2 O CO H + HCO3 - Bicarbonate alkalinity is neutralized during methyl orange end point. HCO3 - + H+ [H2CO3] H2O + CO2 The possibility of OH - and HCO3 - ions together is ruled out, because they combine instantaneously to form carbonate ion. OH - + HCO 3 - CO H 2 O Eg., NaOH + NaHCO3 Na 2 CO 3 + H 2 O Thus, OH - and HCO3 - cannot exist together in water. On the basis of same reasoning, all the three ions (OH -, CO3 2- and HCO3 - ) cannot exist together. From the two titre value (P and M) the different alkalinities are identified and calculated using the following table Calculation of alkalinity of water: Sl. No. Alkalinity OH - (ppm) CO3 2- (ppm) HCO3 - (ppm) 1. P=0 0 0 M 2. P= M/2 0 2P 0 3. P<M/2 0 2P M-2P 4. P>M/2 2P-M 2(M-P) 0 5. P=M P=M 0 0

15 PROCEDURE: 20 cc of the given hard water sample is pipetted out into a clean conical flask. To this, 2-3 drops of the phenolphthalein indicator is added and is titrated against H2SO4 solution taken in the burette. The end point is the disappearance of pink colour. The volume of H2SO4 consumed is noted as P (Phenolphthalein end point). Then in the same solution, add 2 to 3 drops of methyl orange. Continue the titration, till the pink colour reappears. Now the volume of H2SO4 is noted as M (Methyl orange end point). CALCULATION: Volume of acid used to phenolphthalein end point=p cc Volume of acid used to methyl orange end point = M cc Thus, phenolphthalein alkalinity = P x Normality of H2SO4 x 50 x 1000 Volume of water sample Methyl orange alkalinity = M x Normality of H2SO4 x 50 x 1000 ppm Volume of water sample ppm DOMESTIC WATER TREATMENT: Municipalities have to supply potable water, i.e. water which is safe to drink. Essential requirements of drinking water: 1. It should be colourless and odourless 2. It should be pleasant in taste 3. It should be perfectly cool 4. It s turbidity should not exceed 10 ppm 5. It should be free from objectionable dissolved gases like H 2 S, CO 2 and NH 3 6. It should be free from objectionable minerals like Pb, As, Cr and Mn salts 7. It s alkalinity should not be high 8. It s ph should be about It should be reasonably soft 10. It s total dissolved salts (TDS) should be less than 500 ppm 11. It should be free from pathogens (Pathogens disease producing bacteria) 12. It should not have more than ppm of free chlorine Purification of water for domestic use: Natural water from rivers, canals, etc does not satisfy the basic requirements of drinking water. For removing various types of impurities, the following treatment processes are employed:

16 Sl.No Process Impurities removed 1 Screening Floating matter like wood pieces and leaves 2 Sedimentation Suspended impurities like clay and sand 3 Coagulation Finely divided matter 4 Filtration Bacteria, colour, taste, odour and finely suspended particles 5 Sterilization/Disinfection Pathogenic bacteria 1. Screening: Screening is the process of removing floating materials like wood pieces and leaves from water. Raw water is allowed to pass through a screen having a large number of holes which removes the small and large floating matter. 2. Sedimentation: Sedimentation is the process of removing suspended impurities by allowing the water to stand undisturbed for 2 to 6 hours in big tanks. Due to force of gravity, most of the suspended particles settled down at the bottom and they are removed. Sedimentation can remove 60 to 70 % of the suspended matter. 3. Coagulation: Finely dived silica, clay, colloidal particles etc do not settle down easily and hence cannot be removed by sedimentation. Such impurities are removed by coagulation method. Coagulation is the process of removing colloidal matter from water with the addition of requisite amount of coagulants. Chemical substances like alum K 2 SO 4. Al 2 (SO4)3. 24H 2 O, ferrous sulphate FeSO4. 7H 2 O, sodium aluminate NaAlO 2, etc are used as coagulants. These chemical substances react with carbonate and bicarbonate ions present in water and form precipitates which settles down. Al 2 (SO4)3 +3Ca(HCO 3 ) 2 2 Al(OH)3 + 3 CaSO4 + 6 CO2 (Coagulant) (Flocculant ppt) FeSO4 + Mg (HCO3) 2 (Coagulant) Fe (OH) 2 + MgCO3 +CO2 + H 2 O

17 4Fe (OH)2 + O2 + 2 H2O (D.O) 4 Fe(OH)3 (heavy floc) NaAlO2 + 2 H2O Al (OH)3 + NaOH (Gelatinous floc) The sodium hydroxide, thus produced, precipitates Mg salts as Mg (OH)2. MgSO4 + 2 NaOH Mg(OH) 2 + Na 2 SO 4 4. Filtration: Filtration is a process of removing colloidal and bacterial impurities by passing water through sand filters. The sand filter consist of thick layer of fine sand placed over several beds of coarse sand and gravels and water is allowed to percolate through the bed. The water flows through the various beds slowly due to gravity. The rate of filtration slowly decreases due to the clogging of impurities in the pores of the sand bed. When the rate of flow becomes very slow, filtration is stopped and the fine sand bed is replaced. The sand filter method is an effective and efficient process to treat municipal water supply. Bacteria s are partly removed by this filtration process. SAND FILTER 5. Sterilization: The process of killing/destroying the disease producing bacteria s from the water and making it safe for use is called sterilization or disinfection. The chemical substances, which are added to the water for killing the bacteria, etc, are

18 known as sterilizers or disinfectants. There are various methods to sterilize the water. (i) Sterilization by boiling: Boiling the water for 10 to 15 minutes, all the pathogens are killed and water become safe for use. CHLORINATION PLANT (ii) Chlorination: Chlorine is the most important sterilizing agent in water treatment. It can be added in the form of bleaching powder or directly as gas or in the form of concentrated solution in water or chloramines. BEFORE CHLORINATION (a) Sterilization by adding bleaching powder: When bleaching powder is added to water, the following chemical reaction takes place

19 CaOCl 2 + H 2 O Ca (OH) 2 + Cl 2 Cl2 + H2O HOCl + pathogens HCl + HOCl pathogens are killed The HOCl acts as a powerful germicide, it reacts with pathogens and all the pathogens were killed. (b) Sterilization by adding chlorine: When chlorine gas added to the water, it produces the powerful germicide HOCl, which kills all the pathogens. Cl2 + H2O HOCl + pathogens HCl + HOCl pathogens are killed (c) Sterilization by adding chloramines: Chloramines prepared as follows: Cl 2 + NH3 ClNH 2 + HCl It is added to the water and produces a powerful germicide HOCl which kills the pathogens. ClNH 2 + H 2 O HOCl + NH3 (iii) Sterilization by Ozone: Ozone (O3) is an excellent disinfectant, which is produced by passing silent electric discharge through cold and dry oxygen. 3 O2 2 O3 It is highly unstable and decomposes to give nascent oxygen which is a powerful oxidizing agent and kills all the pathogens and also oxidizes the organic matter present in the water. O3 O2 + [O]

20 AFTER OZONATION Advantages: 1. This method removes colour, odour and taste simultaneously without giving any residue. 2. The presence of excess ozone is not harmful as it decomposes into oxygen. Disadvantages: This process is relatively expensive. (iv) Sterilization by UV treatment: Sterilization of water can be done by using UV rays. In this process, UV rays are concentrated on flowing water. The UV radiation kills bacteria. But it is expensive process, when applied to large quantities of water.

21 UV PLANT UV REGION BREAK POINT CHLORINATION: Chlorination provides the sterilization for public drinking water systems and swimming pools, etc. Insufficient chlorination of ammoniacal water leads to the formation of a mixture of chloramines (NH2Cl, NHCl2 & NCl3) which irritate the skin, eyes and produces unpleasant dour to the water. At sufficiently high concentration of active chlorine (hypochlorite, Hypochlorous acid & molecular chlorine) a phenomenon known as break point chlorination occurs. In break point chlorination, ammonaceous & ammoniacal materials are completely oxidizes to nitrogen and active chlorine is simultaneously reduced to chloride. Break point of chlorination is the application of chlorine to produce residual free available chlorine with no combined chlorine present. As chlorine is added to water, it reacts with NH3 and forms chloramines. Cl 2 + NH3 ClNH 2 + HCl

22 Cl 2 + NH 2 Cl NHCl 2 + HCl Cl 2 + NHCl 2 NCl3 + HCl Hence, chlorine in the combined form gradually increases. After a particular limit, oxidation of chloramines & other impurities start and there is a fall in the combined chlorine state. When all combined chlorine has been oxidized by reaction with free available chlorine, the residual now consisting of only free available chlorine, rises again and continues to increase in direct proportion to increased dosage. The point at which the residual again begins to increase is the break point chlorination. BREAK POINT OF CHLORINATION The break point chlorination curve has four stages: Stage: 1. It shows a typical break point of water containing a considerable amount of NH3. During the initial upward rise, chloramines are first formed. The curve rises until sufficient free available chlorine is developed to react with chloramines, and then it falls until a point where all NH3 compounds have been oxidized. While less organic matter in the water, as in stage 2 and 3, free available chlorine is formed sooner, destroying chloramines formed at the early stage. This results in lower combined chlorine residuals and flattened curves before break point. In stage 4, the chloramines are neutralized at an early stage by the upswing of the curve. Advantages: 1. It destroys all the disease producing bacteria 2. It removed colour and odour from the water 3. It completely oxidizes the organic compounds present in the water.

23 BOILER FEED WATER: In industry, one of the main uses of water is generation of steam by boilers. The water fed into the boiler for the production of steam is called boiler feed water and it should be free from dissolved salts, gases, suspended impurities, silica and oil. BOILER FIRE TUBE BOILER BOILER FEED WATER TREATMENT If the water containing these impurities is used in boilers, the following problems may arise Scale and sludge formation, Boiler corrosion, Caustic embrittlement and Priming and foaming.

24 ESSENTIAL REQUIREMENTS OF BOILER FEED WATER: Boiler feed water should be free from the following, 1. Hardness producing Ca 2+ and Mg 2+ ions to avoid, scale and sludge formation 2. Dissolved oxygen and CO2 to prevent boiler corrosion 3. Turbidity, oil and non-scaling salts to reduce priming and foaming 4. Caustic alkali (NaOH) to remove caustic embrittlement I. SCALE AND SLUDGE FORMATION: As water evaporates continuously in boilers for steam production, the concentration of dissolved salts increases. When their saturation points are reached, the salts are precipitated on the inner walls of the boilers. If the precipitate formed is soft, loose, slimy and non adherent; it is called sludge. Sludge s are formed by the substances like MgCl 2, MgCO3, MgSO4 and CaCl 2. These have greater solubility in hot water than in cold water. Sludge s are generally formed at comparatively colder parts of the boiler and get collected at places where flow rate is low. They can be easily removed by scraping off with a wire brush. These are poor conductors of heat and also cause choking of pipes. Sludge formation can be prevented by using soft water and can be removed by blow down operation which consists of withdrawal of portion of concentrated water from the boiler and replacing it with fresh water. If the precipitate is hard and adherent on the inner walls, it is called scale. These are formed by the substances like CaSO4 and Mg (OH) 2. Scales are so hard and adherent that they are difficult to remove them even with the help of hammer and chisel. Scales may be formed due to the following reasons: 1. Decomposition of CaCO3 2. Formation of CaSO4 precipitate 3. Hydrolysis of Mg salts 4. Presence of small quantity of SiO 2

25 SLUDGES SCALES Scales are poor conductors of heat. Hence, they act as partial heat insulators and the transfer of heat from the inner boiler surface to the water inside is greatly reduced, which causes wastage of fuels. When the thick scales cracks due to uneven expansion may cause explosion of boiler. Brittle scales may be removed by giving thermal shocks and loosely sticking scales that of by scraping. Firmly sticking scales can be removed by chemical reactions. DIFFERENCES BETWEEN SLUDGES AND SCALES: Sl.No SLUDGES SCALES 1 Sludge s are soft and non adherent deposits 2 It can be removed easily scraping off with a wire brush 3 Sludge s can transfer heat to some extent and are less dangerous 4 Sludge s are formed by substances like MgCl 2, MgCO3, MgSO4 and CaCl 2. Scales are hard and adherent deposits on the inner wall of the boiler It is difficult to remove even with the help of hammer and chisel, but can be removed by chemical reactions Scales are poor conductor of heat and are more dangerous Scales are formed by the substances like CaSO4 and Mg(OH) 2

26 II. BOILER CORROSION: Boiler corrosion is the decay of boiler material by a chemical or electrochemical attack by it environment. Boiler corrosion is mainly due to the presence of, 1. Dissolved oxygen 2. Dissolved CO2 3. Dissolved salts like MgCl 2 Therefore the removal of these prevents corrosion in boilers. (i) Removal of dissolved oxygen : Water usually contains about 8 ml of dissolved oxygen per litre at room temperature. The dissolved oxygen in water attacks the boiler material at higher temperatures. 2Fe + 2H 2 O + O2 2Fe (OH) 2 4Fe(OH) 2 + O2 + 2H 2 O 2[Fe 2 O3.3H 2 O] (Rust) Dissolved oxygen can be removed from water by chemical or mechanical methods. CHEMICAL METHOD: Dissolved oxygen can be removed from water by the addition of chemicals like sodium sulphite, hydrazine, etc 2Na 2 SO3 + O 2 N 2 H4 + O2 2Na 2 SO4 N 2 + 2H 2 O Hydrazine is the best chemical for removing dissolved oxygen since the products are water and inert nitrogen gas. Also it removes oxygen without increasing the concentration of dissolved salts. MECHANICAL DE-AERATION METHOD: Oxygen along with CO2 can be removed by this method. In this method water is allowed to fall slowly on the perforated plates fitted inside the tower. To reduce the pressure inside the tower, the deaerator is connected to a vacuum pump. The sides of the tower are heated by means of steam jacket. The water flows down through a number of perforated plates and this arrangement exposes a large surface of water for deaeration. High temperature, low pressure and large exposed surface, reduces the dissolved gases (O 2 and CO 2 ) in water.

27 MECHANICAL DE-AERATOR (a) Removal of dissolved CO 2: Dissolved CO 2 in water produces Carbonic acid which has a slow corrosive effect on the boiler material. CO 2 is released inside the boiler, if water used for steam generation contains bicarbonates. Mg (HCO3) 2 MgCO3 + H 2 O + CO 2 Dissolved CO2 can be removed by the addition of NH4OH 2NH4OH + CO 2 (NH4) 2 CO3 + H 2 O (b) Removal of dissolved salts: Water containing Mg salts liberate acids on hydrolysis. MgCl 2 + 2H 2 O Mg(OH) 2 + 2HCl The liberated acid reacts with Iron boiler in a series of reactions producing HCl again and again which corrodes boiler severely. Thus,

28 Fe + 2HCl FeCl2 + H2 FeCl2 + 2H2O Fe (OH)2 + 2HCl Corrosion by HCl can be avoided by the addition of alkali to the boiler water. HCl + NaOH NaCl + H 2 O II. CAUSTIC EMBRITTLEMENT: Caustic embrittlement means intercrystalline cracking of boiler metal. It is a type of boiler corrosion, caused by using highly alkaline water in the boiler. Boiler water usually contains small amounts of Na 2 CO3. In high pressure boilers, Na 2 CO3 undergoes hydrolysis to produce NaOH. Na 2 CO3 + H2O 2NaOH + CO 2 This NaOH flows into the minute hair cracks and crevices usually present on the boiler material by capillary action and dissolve the surrounding area of Iron as Sodium ferroate, Na2FeO2. This type of electrochemical corrosion occurs when the concentration of NaOH is above 100 ppm. This causes brittlement of boiler parts, particularly stressed parts like bends, joints, rivets etc. Prevention of caustic embrittlement: 1. Using Na 2 SO4 as softening agent instead of Na 2 CO3 2. Adding chemicals like tannin, lignin to the boiler water 3. Adjusting the ph of the boiler feed water around 8 9 IV. PRIMING AND FOAMING: Priming: When a boiler is producing steam rapidly, some particles of the liquid water are carried along with the steam. This process of Wet steam formation is called priming. Priming is caused by: 1. The presence of large amount of dissolved salts 2. High steam velocity 3. Sudden boiling 4. Improper boiler design 5. Sudden increase in steam production rate Priming can be prevented by: 1. Fitting mechanical steam purifiers

29 2. Controlling the velocity of steam 3. Maintaining low water levels in boilers 4. Using treated water 5. Good boiler design Foaming: The production of stable bubbles above the surface of water is called foaming. Priming and foaming, usually, occur together. Foaming is caused by, 1. Presence of air 2. Presence of grease 3. Presence of finely divided sludge particles Foaming can be prevented by, 1. Adding anti foaming agents like Castor oil 2. Adding coagulants like Sodium aluminate (NaAlO2), Ferrous sulphate (FeSO4) etc. to remove the finely divided sludge particles and oil. SOFTENING OF WATER: Water used for industrial purposes (steam generation) should be sufficiently pure and free from hardness producing salts to avoid the problems like boiler corrosion, scale and sludge formation, and caustic embrittlement etc. The process of removing hardness producing salts from water is called softening of water. In industry, the following methods are mainly used for softening of water. 1. Ion-Exchange or De-Mineralization or De-Ionization process 2. Desalination process (i) Electro-dialysis (ii) Reverse Osmosis DE MINERALIZATION METHOD: The boiler feed water should be free from all types hardness producing salts. Water of such quality can be obtained through demineralization process. It is also called Ion exchange method and De ionization method. The process, in which the hardness producing cations like Ca 2+ and Mg 2+, and anions like Cl - and SO 4 2- are removed from the hard water using cation and anion exchange resins respectively, is called Ion-exchange process. Ion exchange resins are insoluble, cross linked, long chain organic polymers with a micro porous structure. The functional groups attached to the chains are responsible for ion exchanging properties. Resins containing acidic groups (--COOH, --SO3H) are capable of exchanging their H + ions with cations of hard

30 water. Resins containing basic groups (--NH 2, --OH) are capable of exchanging their anions with the anions of hard water. PROCESS: In this process hard water is allowed to pass through two kinds of columns. 1. The first column is packed with a cation exchange resin which exchanges H+ ions with Ca 2+, Mg 2+ and all other cations present in the water. 2 RH + Ca 2+ R2Ca + 2 H + 2 RH + Mg 2+ R2Mg + 2 H + RH + Na + RNa + H + 2. The cation free water is now passed through the second column packed with an anion exchange resin which exchanges OH - ions with Cl -, SO 2-4 and all other anions present in the water. ROH + Cl - RCl + OH - 2 ROH + SO 4 2- ROH + HCO 3 - R 2 SO4 + 2 OH - RHCO3 + OH - The H + and OH - ions released from cation and anion exchanger combine to give water molecules which are free from all the cations and anions present in the hard water. The ion free water is called de ionized water or de mineralized water. REGENERATION: Cation exchange resins are regenerated by passing dil.hcl

31 R 2 Ca + 2 HCl 2 RH + CaCl 2 R 2 Mg + 2 HCl 2 RH + MgCl 2 RNa + HCl RH + NaCl Similarly, the anion exchange resins are regenerated by passing dil.naoh. RCl + NaOH R 2 SO4 + 2 NaOH ROH + NaCl 2 ROH + Na 2 SO4 Advantages: 1. This process produces water of hardness nearly 2 ppm 2. Highly acidic or alkaline water can be softened Disadvantages: 1. The equipment is costly and more expensive chemicals are needed. 2. Highly turbid water (more than 10 ppm) cannot be softened. DESALINATION: Depending upon the quantity of dissolved salts, the water is graded as follows: 1. Fresh water: It contains, <1000 ppm of dissolved salts. 2. Brackish water: It contains ,000 ppm of dissolved salts. 3. Sea water: It contains, >35,000 ppm of dissolved salts The water containing dissolved salts (high percentage of NaCl) with a peculiar salty taste or brackish taste is called brackish water. It is totally unfit for drinking purposes. Sea water and brackish water van be made available as drinking water through desalination process. The process of removing common salts (NaCl) from water is known as desalination. The need for such a method arises due to the non availability of fresh water. Desalination is carried out by electro dialysis and desalination methods. ELECTRO DIALYSIS: Electro dialysis is a process in which solutions are desalted or concentrated electrically. It is based on the fact that the ions present in saline water migrate towards oppositely charged electrodes under the influence of an applied emf. The

32 movement of ions takes place through ion selective membranes. The cation permeable membranes are generally polystyrene based polymers and the anion permeable membranes are resins containing tetra alkyl ammonium chloride. An electro dialysis cell consists of alternate cation and anion permeable membranes. The cathode is placed near the cation permeable membrane (C) and the anode is placed near the anion permeable membrane (A). Under the influence of an emf applied across the electrodes, the anions (Cl-) move towards the anode and the cations (Na+) move towards the cathode. The net result is the depletion (reduction) of ions in the even numbered compartments and concentration of ions in the odd numbered compartments with even number are filled with pure water and the compartments with odd number are with concentrated brine solution. Thus the salinity is removed from salt water. REVERSE OSMOSIS: When a semi permeable membrane separates two solutions of different concentrations, flow of solvent takes place from dilute to concentrated solution due to Osmosis. If pressure in excess of osmotic pressure is applied, the water flows in the reverse direction on the concentrated solution side. This process is called reverse osmosis. This process is also known as hyper filtration or super filtration.

33 R.O. Plant R.O. FLOW CHART

34 Reverse osmosis is used in conjunction with one or more activated carbon filters. This combination removes 99.9 % of undesirable water contaminants. Using reverse osmosis method pure water is separated from sea water. The membranes used are cellulose acetate, cellulose butyrate, polysulphone, polyamides, etc. REVERSE OSMOSIS Advantages of electro dialysis: 1. It is most compact unit 2. The cost of installation of the plant and its operation is economical 3. If electricity is easily available, it is best suited. REVERSE OSMOSIS PURIFIERS 4 stage R.O. purifier 5 stage R.O. purifier

35 5 stage R.O. purifier 6 stage R.O. purifier with UV *************

36 SOLVED PROBLEMS 1. A sample of water contains 204 mg of CaSO4 and 73 mg of Mg(HCO3)2 per litre. What is the total hardness in terms of CaCO3 equivalent? Solution: Salt Amount M.Wt Hardness CaSO (204/136) X 100 = 150 Mg(HCO3) (73/146) X 100 = 50 Permanent hardness due to the presence of CaSO4 = 150 mg/l Temporary hardness due to the presence of Mg(HCO3)2 = 50 mg/l Total hardness = = 200 mg/l ************************* 2. Calculate the temporary and permanent hardness of water sample containing the following. Mg(HCO3)2 = 146 mg/l, Ca(HCO3)2 = 81 mg/l, CaSO4 = 68 mg/l, CaCl2 = 11.1 mg/l and MgCl2 = 9.5 mg/l. Solution: Salt Amount M.Wt Hardness Mg(HCO3) (146/146) X 100 = 100 Ca(HCO3) (81/162) X 100 = 50 CaSO (68/136) X 100 = 50 CaCl (11.1/111) X 100 = 10 MgCl (9.5/95) X 100 = 10 (i) Temporary hardness is due to the presence of Mg(HCO3)2 & Ca(HCO3)2. Temporary hardness = = 150 mg/l (ii) Permanent hardness is due to the presence of CaSO4,CaCl2 & MgCl2. Permanent hardness = = 70 mg/l (iii) Total hardness = = 220 mg/l ***********************

37 3. Calculate the carbonate and non-carbonate hardness of a water sample containing dissolved solids (mg/l) given as follows: Mg(HCO3)2 = 7.3, Ca(HCO3)2 = 40.5, CaSO4 = 13.6, MgCl2 = and NaCl = 60. Solution: Salt Amount M.Wt Hardness Mg(HCO3) (7.3/146) X 100 = 5 Ca(HCO3) (40.5/162) X 100 = 25 CaSO (13.6/136) X 100 = 10 MgCl (21.75/95) X 100 = 22.8 NaCl 60 Do not consume any hardness (i) Temporary hardness (Carbonate hardness ) is due to the presence of Mg(HCO3)2 &Ca(HCO3)2.Carbonate hardness= = 30 mg/l (ii) Permanent hardness (Non-carbonate hardness) is due to the presence of CaSO4 & MgCl2 Non-carbonate hardness = = 32.8 mg/l (iii) Total hardness = = 62.8 mg/l *********************** ml of hard water required 15 ml of EDTA solution for a titration. Calculate the hardness of the sample of water. Solution: We know that 1 ml of EDTA = 1 mg of CaCO3 equivalent Therefore 15 ml of EDTA = 15 mg of CaCO3 equivalent i.e. 50 ml of hard water contain 15 mg of CaCO3 equivalent Then the hardness = (15/50) X 1000 = 300 mg/l ******************

38 5. 50 ml of standard hard water containing 1 mg of CaCO3 per ml consumed 17 ml of EDTA solution. 50 ml of sample of hard water consumed 12 ml of EDTA solution. Calculate the total hardness in ppm. Solution: 50 ml of standard hard water = 50 mg of CaCO3 equivalent Therefore 17 ml of EDTA = 50 mg of CaCO3 equivalent 1 ml of EDTA = (50/17) mg of CaCO3 equivalent 12 ml of EDTA = 50 ml of water sample 12 ml of EDTA = 12 X (50/17) mg of CaCO3 equivalent Thus, 50 ml of water sample contain 12 X (50/17) mg of CaCO3 equivalent. Therefore hardness = 12 X (50/17) X1000 /50 = mg/l ****************** ml of a sample of water required 15 ml of 0.01 M EDTA solution for the titration using Eriochrome Black-T indicator. In another experiment, 100 ml of the same sample was boiled to remove the Carbonate hardness, the precipitate was removed and the cold solution required 8 ml of 0.01 M EDTA solution using Eriochrome Black-T indicator. Calculate (i) Total hardness (ii) Non-carbonate hardness (iii) Carbonate hardness, in terms of mg/l of CaCO3. Solution: Total hardness: We know that 1 ml of 1 M EDTA = 100 mg of CaCO3 Then 1 ml of 0.01 M EDTA = 1 mg of CaCO3 equivalent 15 ml of 0.01 M EDTA = 15 mg of CaCO3 equivalent i.e. 100 ml of water sample contain 15 mg of CaCO3 Total hardness = (15/100) X 1000 = 150 mg/l Non-carbonate hardness (Permanent hardness): 1 ml of 0.01 M EDTA = 1 mg of CaCO3 equivalent

39 8 ml of 0.01 M EDTA = 8 mg of CaCO3 equivalent i.e. 100 ml of water sample contain 8 mg of CaCO3 Non-carbonate hardness = (8/100) X 1000 =80 mg/l Carbonate hardness (Temporary hardness): Carbonate hardness = Total hardness Non-carbonate hardness = = 70 mg/l **************** PROBLEMS FOR PRACTICE 1. A sample of water contains 120 mg of MgSO4 per L. Calculate the hardness in terms of CaCO3 equivalent. (Answer: 100 ppm) 2. If a sample of water contain 50 mg of Ca 2+ ion per L. Calculate its hardness in terms of CaCO3 equivalent. (Answer: 125 mg/l) 3. Calculate the temporary and permanent hardness of a water sample containing the dissolved salts in 1 L of hard water as given below: Ca(HCO3)2 = 64.8 mg, Mg(HCO3) 2 = 14.6 mg, CaSO4 = 13.6 mg and MgCl2 = 50 mg. (Answer:T= 50 mg/l, P= 62.6 mg/l & Total= 112.6mg/L) 4. Calculate the temporary and permanent hardness of a water sample having the following data. Mg(HCO3) 2 = 73 mg/l, Ca(HCO3)2 =162 mg/l, CaSO4 = 136 mg/l, MgCl2 = 95 mg/l and CaCl2 = 111 mg/l. (Answer: T= 150 mg/l, P= 300 mg/l) 5. A sample of water contain the following dissolved salts in mg/l. Mg(HCO3) 2 = 73, Ca(HCO3)2 = 81, CaCl2 = 111 and MgSO4 = 40. Calculate the temporary and permanent hardness of the water (Answer: T= 100 mg/l, P= mg/l) ml of water sample needs 20 ml of EDTA solution for a titration. a ml of EDTA solution is equivalent to 1.1 mg of CaCO3. Calculate hardness in ppm. (Answer: Hardness = 220 mg/l) 7. A water sample of 100 ml requires 18 ml of EDTA solution for a titration. 22 ml of the same EDTA solution was required for the titration of 100 ml of standard hard water containing 1 g CaCO3 per litre. Calculate hardness of water sample in ppm. (Ans:Hardness= mg/l)

40 ml of a sample of water required 20 ml of 0.01 M EDTA solution for the titration with Eriochrome Black-T indicator. 100 ml of the same sample after boiling and filtering required 10 ml of 0.01 M EDTA solution. Calculate the total, carbonate and non-carbonate hardness of the sample. Answer: Total= 200 ppm, CH = 100 and NCH = 100 ppm) References: Engineering chemistry 15 th Edition By Jain and Jain, Dhanpat Rai Publishing Company, New Delhi. Engineering chemistry S.S. Dara, S.Chand & Co., New Delhi. Anna University Model question papers and Previous semester Examination question papers (Theoretical problems) ******************

41 UNIT I (QUESTION BANK) PART A 1. Name the different sources of water 2. What is the cause for alkalinity of natural water? 3. List out the hardness producing salts in water. 4. Define: Turbidity, and name the sources for turbidity of water. 5. How do acidity and alkalinity of water are caused? 6. What is the sanitary significance of fluorides in water? 7. How does colour imparted in water? Give example. 8. How will you remove turbidity and colour from water? 9. Mention the common units used for expressing hardness of water. 10. Define: ppm 11. Distinguish between hard water and soft water. 12. What are the salts responsible for the temporary and permanent hardness of water? 13. Why does hard water consume lot of soap? 14. The simultaneous presence of OH -, HCO3 - and CO3 2- in water is not possible. Give reason. 15. Give a test to identify the hard water. 16. Every soft water is not a demineralized water where as every demineralized water is a soft water. Justify 17. Define: Hard water and soft water. 18. Define: Hardness of water. 19. Distinguish between Temporary and permanent hardness of water. 20. Define: Standard hard water. 21. What happens when temporary hard water is boiled? 22. Why is ammonia buffer solution added during determination of hardness of water by using EDTA? 23. What is the indicator used in EDTA method?

42 What is the end point? 24. Write the structure of EDTA and disodium salt of EDTA. 25. What is boiler corrosion? How it can be prevented? 26. What is caustic embrittlement? How it can be prevented? 27. What is meant by priming and foaming? List out their effects in boilers. 28. What are the problems encountered in boiler feed water? 29. Name the gases dissolved in water that cause corrosion. 30. Differentiate scale from sludge. 31. Why should natural water not be fed in boiler? 32. Why boiled water is not always 100 % safe for drinking purposes? 33. What is meant by softening of water? Mention any two methods of Water treatment. 34. Name any two coagulants. 35. In the deionization process, water is usually first passed through the cation exchanger and then through the anion exchanger. Give reason. 36. How will you regenerate the column in demineralization process? 37. Give few examples for cation and anion exchangers. 38. What is brackish water? 39. What is desalination? Name few methods of converting sea water into fresh water? 40. What is electro dialysis? 41. What is reverse osmosis? 42. What are the advantages of reverse osmosis? 43. Chloramines are better than chlorine for sterilization of water. Why? 44. What is meant by break point chlorination? 45. Calgon conditioning prevents scale formation in boilers. Give reason.