17 Water Treatment ‘The nature's most abundant supply i.e, water is essential for the survival ofall the living beings on earth ie., man, animals and plants. Water is net only essential for the survival of life, but it is also used for the operation in a large number of industries as coolant, solvent, for steam generation, for air conditioning, fire-fighting ete. Water is also used for all domestic Purposes like bathing. drinking. washing, sanitary ivigation ete. Sources of Water 1, Rainwater. Itisthe purest form of natural water. But unfortunately it dissolves the toric gases like CO, SO,, NO, te. and other solids, which pollute the atmosphere, 2, Sea water, It is the most impure form of water containing about 3.5% dissolved salts of which about 26% is sodium chloride, Other salts present include sulfates, bicarbonates, bromides of sodium, potassium, magnesium etc. Other impurities are carried to sea through 8. River water. The scurces of river water are the springs and the rainwater. River water while lowing through the land collects lots of organic matters from falling trees and nearby habitats and also other soluble and suspended matters from the lands, soils ete. The dissolved mattersinclude the salts like sulfates bicarbonates and bromides f sodium, potassium and magnesiut, 4. Lake water. It is much purer than river water, dissolved impurities are less but cantains lots of organic matter. 5. Underground water. The rainwater and other surface water percolate down through the soil and rocks and get filtered and finally collected on rocky surface or again come out as spring. Though it contains less suspended matter but the dissolved mineral content is quite high and is of high organic purity Thus we can say that the surface water collects lots of suspended materials, micro- organisms and other pellutants from the habitats and become unsuitable for direct human consumption and other usage. Effect of Water on Rocks Water, while percolation, ets contaminated by the following processes: (c) Dissolution. Sedium chloride, gypsum (CaSO,2H,0) and other saluble salts get directly dissolved in the pereolated water. (b) Hydration. Whilepercolation, water causes hydration of the minerals like anhydrite, olivine which lends to increase im velume and disintegration of such rocks, viz. CaSO, + 2H,0 —> Ca80,2H,0 Achy Capen bttou/Keralatechnologi®ifiniversity blogspot.comWATER TREATMENT 351 Mg,Si0,+ xH,0 —> Mg, SiO ,xH,0 Olivine Serpentine (©) Oxidation. Dissolved oxygen brings about oxidatien alongwith hydration. 2Fe0,-" , oFe,0, ™", 2F 0, 2H,0 Manette Haesatite Limenite (d) Action of carbon dioxide. Insoluble carbonates of Cs, Mgand Fe alongwith silicates ‘of Na, K,Ca,andFeare converted inta soluble bicarbonates and soluble carbonates, vz, CaCO, + CO, +H,0 —> CalHC0,), Astle ‘Goluhie) K,0.A,0,.680, + C0, + 2,0 —+ ALO, 28i0, 2H,0 + K,CO, + 48i0, Hardness of Water Hardness of water is the characteristic of preventing lather formation of water with soap. Generally salts like chlorides, bicarbonates and sulfates of Ca'*, Me** and Fe make water hard. This hard water on treatment with scap which is steavic or palmitic acid salts of sodium or potassium causes white precpitate formation of calcium or magnesium stearate or palmitate. 2€ Hl COONa + CaCl, —+ (C,,Hy,COO),Ca + aNaCI feta simcie Calum cowate ‘cn nahi ‘Thus the cause offhardness isthe precipitation ofthe soap and hence prevents lathering st Bist. When the harinee canting ans are removed as insoluble sveps, water bectmee oof and forms lather. The hardness eof two types: (© Temparary harness ia de ts the biarbonatos af Ca and Mel* and carbonnte of Pe? Since bicarbonates readily get precipitated on boiling the water, the temporary hardness can be easily removed, wiz. cagico), “, caco,t + H,0+0, inl (ii Permanent hardiness is due to the presence of chlorides and sulfates of Ca, Mg, Fe, ote, Permanent harness cannot be removed easily on beiling. Units of Hardness Beth temporary and permanent hardnesses are expressed in ppm as CaCO. The choice of CaCO, is due tothe fact that its mel. wt. is 100 and equivalent weight i 50 and itis the most insoluble salt in water. Bquivalent of CaCO, (Mass of hardness producing substance) x (Chemical equivalent of CaCOs) 5 ‘Chemical equivalent of hardness producing substance Mass of hardness producing substance x 50 = Chemical equivalent of hardness producing substance Hardness is principally expressed in ppm unit. Other limits include French degree of hardness, English degree of hardness or Clark, USA degree of hardness and German degree of hardness, [part of hardness 10° part of water 1 ppm: http//keralatechnologicaluniversity. blogspot. comENGINEERING CHEMISTRY ‘partof hardness 10° parteof water Iprainf hardness 1 4 Clatk = eallon oF 70.000 grains ” 7% 10" igsin | ipain TUSA gallon ” 58,300 genine ~ 5.83% 10" Ageain | grain Terman grain "35000 grime" SOx10" Relation between various units of hardness 1 Fr. degree of hardness 1 USA degree of hardness = 1 German degree of hardness = 1ppm =I mg 201° Pr s007 cl Impl =1 ppm 01 Fr 007 Cl VOL SLA Fr 3pm = 14.3 mal Fr = 10ppm 10m = 0G Disadvantages of Hard Water (a) In domestic uses. For washing and bathing, hard water creates difficulties, since it does not form lather freely with scap. It also creates sticky precipitates that deposit on bath tub, bed, clothes etc, until all the Ca/Mg salts get precipitated, Thus a lot of soap get wasted also. For cooking hard water creates similar difficulties by producing scum on the bottom of the vessels. Due to the presence of hardness producing salts in hard water, beiling point gets clevated and during cooking alot off! is wasted. Pulses et. do not cook in hard water, Taste oftea, coffee becomes unpleasant. Drinking of hard water is also problematic sinc it affects the digestive system and at the same time the possibility of deposition of cakium oxalate crystals in the urinary trac is alarming. (b) In industrial ses, For teetile industry and dyeing industry, haed water causes the ‘usual preblem ef deposition of insoluble sats that interfere with the proper dyeing and printing ofthe fabrics. The stains of iron sata also are undesirable on fabrics. Hard water also hampers the ecenomy by wastage of soap as it does nat form good lather, © Forsugar industry, the salts responsible for hardness create dificutics in sugar refining and crystallization of sugar and the sugar becomes deliquescent, * Calcium and magoesium salts also interfere with the smooth and glossy finish of the papers in the paper industry. Iron salts intexfere with the ccleur of the paper. + In laundry, hard water causes wastage of costly soap and also interferes with the + The hydration of cement and final hardening of coment are affected by use of hard ‘water in concrete making. © Hard water is net suitable for preparing drug solutions in pharmaceutical industry + For steam generation in boilers, hard water creates many problems like (i) scale formatien, (i) corrosion, (i) priming and foaming and (ix) caustic embrittlement Sludge and Scale Formation in Boilers ‘When hard water is used far beilers, on continucus evaporation, the salts present in the hard water gets saturated ancl are finally deposited in the areas where the flow is slow. When htip//seralalechnologicaluniversity, blogspot.comWATER TREATMENT 383 these precipitates are loose and slimy in nature, it is called sludge whereas when these precipitates are hard and they adhere strongly to the inner surface of the boilers, they are called seale, Sludge can be easily removed by serapping with a brush. Sludge is formed by the presence ‘of MgC0s, MgS0,, MgCl,, CaCl, ete, These salts are more soluble in hot water. Disadvantages of sludge formation are () poor heat conduction dve te the presence ef sludge on the surface; (i) difficulty in the operation ofthe boils; (ii if sludge is formed along ‘with the scale and is trapped within the scale formed and so it is difficult to remove and (iv) it clogs the pipe lines and other connections to the vessel near the places where water circulation rate is slow. Remeval of sludges. Shudge formation can be prevented by (i) using soft water for boiler ‘operation and (i) removing the concentrated salty water from time te time so that deposition of sludge is prevented, Scales are the hard deposits on the inner surface of the boilers which are difficult to remove. This scale formation takes place due to the fllowing reasons: (a) In low pressure boilers seale formation occurs due to the formation of CaCO, from Ca(HCO,) ie, Ca(HCO,), — CaC0,! + H,0 + CO, Seale (6) In high pressure boilers this CaCO, gets converted to sokible Ca(OH),, But here CaSO, forms the hard scale, Since the solubility of CaSO, decreases with increase in temperature, and at high temperature the precipitated CaSO, forms hard scale, Similar hard scales are formed when SiO, is present in the hard water. It deposits as CaSiO, or MgSiO,, These calcium or magnesium silicate scales are very difficult to remove. Dissolved magnesium salts also precipitate as Mg(OH), forming soft type of scale. Disadvantages of scale formaticn are similar to sludge formation but the severity is more, since its removal is more difficult. Disadvantages include @ Poor heat transfer from boiler to water Ieading to increase in fuel consumption, The inerease in thickness of the scale from 1.25 mm to 12 mm leads te inerease in fuel ‘consumption from over 50% to 150%. (ti) Due ta the overheating of the beiler, different parts of the boiler become weak and distorted and s0 the operation of the boiler becomes unsaje, particularly the high pressure boilers, ‘The thick scales may sometimes lead to explosion due to sudden development of high pressure. (iii) Valves and condensers of the boilers are chocked due to scale formation and boiler efficiency decreases. Removel of scales cas be done by: ( Wooden scraper or wire brush, suitable for removing loose scales. (ii) Blow down operations for loose scales, The operation actually involves the re- moval of very hard water from a tap at the bottom of the boiler and replenishing the water with scftened water called ‘make up' water. (ii) Giving thermal shocks, which involve alternate heating and cooling to make the scales brittle, (iv) Chemical treatment with 5-10% HCI fer carbonates and EDTA treatment for CaM salts forming complexes, hittpy/keralatechnologicaluniversity blogspot.com354 ENGINEERING CHEMISTRY ‘These are methods for the removal ofthe scales, when they are formed. There are also some methods forthe prevention of scale formation by internal or external treatment, Intemal teatment Internal treatment involves addition of chemical tothe boiler water either to (i) precipitate the scale forming impurities in the form of sludges, which can be easily removed or (i) convert the impurities to soluble compounds, 40 that scale formation cam be avoided, Important internal treatments involve, (a) Callodat Conditioning: Organic substances like kerosene, tannin, ngar-agar are added to form gels and form loose non-sticky deposits with scal-forming precipitates, which ‘ean be easily removed by blow-down operations in low pressure beilers. (b) Different sodium phosphates like NaH,PO,. Na,HPO, and Na,PO, are added to high ‘pressure boilers to react with the hardness forming impurities to form soft sludge of cakium and magnesium phosphates and finally this can be removed by blow down operation, CuCl, + Na,PO, —+ Cay(PO,), + NaCl (6 Carbunate conditioning: Sodium carbonate ix added to the water of low pressure boiler whereby the scale ferming CaSO, gets converted to loose shudge of CaCO, ‘which can be easly removed by blowdown operation. Ca8O, + Na,CO, = CaCO, + Na,80, (4) Caigen conditioning: Calgon Le. sodium hexa meta phosphate when added to boiler ater, reacts with scale forming CaSO, and forms soluble complex compound, 2Ca80, + [Na,P,0J* —+ [Ca,P,0,)* + 2Na,S0, (6) Sodium aluminate conditioning: Sedium aluminate is hydrclysed yielding NxOH and gelatincus AH), The NaOH fermeil reacts with magnesium salts to precipi tate Mg(OH),. This Mg(OH), and KOH) are focculent and entraps the clloidal as ‘well as the finely divided impurities lke silica in the boiler water and the loose ‘precipitate i finally removed by blow dewn operation, NaAlO, + 2H,0 —+ NaOH + AKOH),t MgCl, + 2NaOH —+ Mg(OH),l + 2NaCl (/) Electrical conditioning: Rotating mercury bulbs on heating by the boiling water emit cletrical discharges that prevent scale formation by the particles (a Radioactive conditioning: Tablets of radiaactive salts placed inside a boiler emit radiations, which prevent scale formation Caustic Embrittiement Embrittloment is the namo that has been given to boiler failures due to development of certain types of erack resulting from excessive stress and chemical attack. In steam boiler operation, the chemicals that are believed to be responsible are NaOH and sien. During softening processes, Na,CO, are added and it gives rise to NaOH at elevated temperature ‘according to the following reaction Na,CO, + H,O— 2Na0H + CO, Steam boilers are made by rivetting constructions rather than welding and are particularly lisble to damage by caustic alkali solutions, This eracking isnot due to corrosion tnd the eracks apy brite fracture and hence called caustic embrittlement, NaOH attacks and dissolves out iron of oiler forming sedium ferroate. This finally causes the stressed parts like bends, joints, rivets to lead tothe boiler failure, hito/keralatechnologicaluniversity blogspot comWATER TREATMENT 355 Concentration cell develops between the boiler and the NaQH of different concentrations as + Feat Concentrated |) Dilute Feat = rivets, joints NoOH NaOH plane te, Salution |] Solution | Surfacor Prevention of Caustic Erbrittlement (@ Addition of sodium phesphate as softening agent instead of Na,CO,, (i) Addition of sodium sulfate te ensure a weight ratio Na,SOJNaOH > 2.5, whereby the deposition of Na,SO, prevents the penetration of NaOiH into the cracks and stops caustic embrittlement in high pressure boilers (ii) Addition of ergeaie agents like tannin, lignin. querbracho ot, also prevents cracking ‘similar to sedium sulfate in lew pressure boilers, (iu) Use of erackeresistng steels: Certain steels containing Al added during manufacture ‘appear tobe resistant against caustic cracking Corrosion in Bollors Boiler corrosion eccurs by chemical or electrochemical attack of the contents of water. Main types are of chemicals (a) Lissolved axygersOxygenis dissolved in water tothe extentof 8 mil atroom tersperature and itis responsible for cerrosicn inthe boiler Greater the pressure, higher the dissolved Q, centent, As the water i heated in the boiler the clinsolved oxygen it iberated and iren i correded 2Fe + 2H,0 +0, —+ 2Fe(OM, 4 4F(OH), 1 + 0, —+ 2fFe,0, 21,0] Dissolved oxygen can be removed by: (i) Mechanical deaeration methovs using distillation, steam scrubbing, desorption flash» ‘ype deaeration, which reduce O, concentration to about 0.01 ppm. (i) Chemical treatment-ozygen concentration is virtually made sero by using reducing ‘agents ike hydrazine, sedis efito ete (ii) Lon exchange techniques also reduce exygen concentration to very lew level (b) Dissolved COs Water contains some CO, and the decomposition product of bicarbo- nates present in water also produces CO, This CO, dissolves in water forming carbonic acid Which causes les! eotrosion called pitting. Fe+CO, + 1,0 —» FoC0, + H, AFCO, +0, + 10110 —+ 4Fe(OH), + 41,0 + 4C0, 4F (OH), —+ 2Fe,0, + 61,0 Removal of CO, is done by: (i) Mechanical deseration (i) Adding Hime or NH,, (ii) Heating. (© Mineral acids: Water from industrial areas contains acilie wastes or inarganie salts which on hydrolysis produce acids. These acids cause corrosion in bilers. The acids react with irenin chain reactions producing acids again and again and also produce H, causing hydrogen embrittlement which leads to boiler failure. MgCl, + 2,0 —+ Mg(OH), + 2HCI Fe + 2HCl— FeCl, +H, FeCl, + 2H,0— Fe(OM), 4+ 2HC1 hitpv/keralatechnologicaluniversity. blogspot.com386, ENGINEERING CHEMISTRY Priming and Foaming During rapid steam production, some liquid water drops are also carried along with the steam, This wet-steam formation is called priming. Priming occurs due te () presence of large amount of dissolved solids, (i) high steam velocities, (i) sudden boiling and (ic) sudden increase in steam production. Priming can be controlled by (i) maintaining low water level in boiler, (i) avoiding rapid change in steam rate, (ii) softening of boiler water and (iu) using mechanical device for steam purification. Foaming is the production of bubbles and foams which do not break easily. Foaming occurs due to the presence of cil in the water. scans aming sabe reduced by () remnving ol from talereed water and (i) ang ani foaming agents. Priming and foaming occur together and they are undesirable since they wet other ‘mechanical parts of the bniler and reduce their efficiency. Actual height ofthe water column cannot be judged due to foaming hence creating difficulty in the maintenance. Highlights: + Chief sources of water: Sea water, rainwater, ground water and surface water. + Impurities present in wa (Suspended, (i) Cellidal, (i) Dissolved impurities + Temporary hardness is due to Ca(HCO,), and Mg(HCO,), and can be removed by boiling. + Permanent hardness is due to chlorides and sulfates of Ca**, Mg, Fe" and other heavy metals. «Hardness expressed in equivalent amount of CaCO, Mass of hardness producing substances x 50, ‘Chemical equivalent of hardness producing substances © 1 ppm hardness = | part of CaCO, equivalent hardness in 10° parts of water. + Hard water cannot be used for steam generation in boilers due te the problems like scale and sludge formation, priming and foaming, beiler corrosion and failure. Softening of Water Softening of water means the removal of calcium, magnesium, iron salts and similar other metallic is, whieh would form insoluble metallic soups. The three important industrial methods employed for softening of water are: 1. Cold and bot lime-soda process. 2, Permutit or zeslite process. 8, Jon-exchange or demineralization process. 1. Lime-soda process. By this process, scluble calcium and magnesium salts are rendered insoluble by adding caleulated amount of lime [(CaOH),] and soda [Na,CO,). The insoluble precipitates ef CaCO, and Mg(OH), are removed by filtration, By this method, both temporary and permanent hardness are removed. For the removal of temporary hardness the reactions are: Ca(HCO,), + Ca(OH), —+ 2CxC0, 4+ 2H,0 MglHCOq), + Ca(OH), —» 2CaC0, 4+ MgCO, + 24,0 ‘MgCO, + Ca(OH), —+ Mg(OH), + + CaCO, + Hence, to remove equivalent quantities of Ca and Mg hardnesses the amount of lime necessary isin the ratio of 1:2 htip:/keralatechnologicaluniversity blogspot. comWATER TREATMENT 387 [Again forthe removal of permanent hardiness, The resetions are: CaSO, + Na,CO, —+ CaCO, 4 + Na,S0, MySO, + Na,CO, —+ MgCO, + Na,SO, MgCO, + Ca(OH), —+ Mg(OH), + CaCO, L Hence, for the removal of permanent hardness due to Cacsalts, lime is not necessary, but it is necessary for Me salts. Extra addition of Ca(OH), causes hardness, So calculated quantities of ime and sada are to be added after the determination of actual hardness In the actual process the water is thereughly mixed with the chemicals and allowed to react for sufficient time. Activated charcoal is added as activator, alum ete. are added as coagulants, To avoid after-precipitation of CaCO,, sludge of the previous operation is added, Which supplies the niclel for the precipitation, Cold Lime-Soda Process Inthis method, calculated quantity of chemicals and water, along with accelerators and congulators are added to a tank fittod with a stirrer (Fig. 17.1). on vigorous stirring, thorough mixing takes place. After softening the soft water rises upwards and the heavy sludges sotile dows. The softened water passes through a filtering media ensuring complete removal ofthe sludge and finally the filtered water ows cut through the top. Cold Kime soda process is sed for partial softening of municipal water, for softening of cooling water etc, In actual purpose, ‘magnesium hardness is brought down to almost zero but calcium hardness remains about 40pm. Fig. 17.1 Continuous cold line-soda softener. hitpi/keralatechnologicaluniversity. blogspot.com358 ENGINEERING CHEMISTRY Hot Lime-Soda Process ‘This process is similar to the cold limessoda process. Here the chemicals alongwith the water are heated near about the boiling point of water by exhaust steam As the reaction takes place at high temperature, there are the following advantages: (the precipitation reaction becomes almast complete, (Gi) the reaction takes place faster. (ii) the sludge settles rapidly. (@v) no coagulant is needed (©) dissolved gases (which may cause corrosion) are removed. (ci) viscosity of soft water is lower, hence filtered easily, (cif) Residual hardness is low compared to the cold process Ht lime-soda process consists of three parts: (@)‘Reaction tank’ in which complete mixing ofthe ingredients takes place, (6 lonical sedimentation vessel where the sludge setiles down and (9 Sand filter’ where sludge is completely removed. ‘The soft water from this provess is uaed fer feeding the boilers (Fig, 17.2), ‘Advantages Indude () Lime soda process is economical (ii) The process improves the corrosion resistance of the water. (iii) Mineral content of the water is reduced. (jv) pH of the water rises, which reduces the content of pathogenic bacteria cones A secinriaion y = y oman mae Z Foes [eaco, Mg(OH,} A sotered Z ‘waler outlet Fig. 172 Continuous hot bime-seda saftner Disadvantages Include: () Huge amount of sludge is formed and disposal is difficult, (Gi) Due to residual hardness, water is not suitable for high pressure boiler. htip//keralatechnol niversiyWATER TREATMENT 359 Permutit or Zeolite Process Zealite is hydrated sodium aluminosilicate capable of exchanging reversibly its sodium ‘ons for Ca®* and Mg, having the general formula Na,0.41,0,.15i0,.)H,0. Common zeolite is Na,O ALO, 8i0,2H,0 and is known as natrolith. Others gluconites, gen sand ete. are used for water softening, Artificial zeolite used for softening purpose is permutit, These are porous and glassy and have greater softening capacity than green sand, They are prepared by heating together With china clay, feldspar and soda ash, Method of Softening: Hardwateris passed through a bed of zeolite at aapecificrate at ordinary temperature; the hardness causing cations ie,, Ca* and Mg” are exchanged for Na ‘and itis comverted to CaZe and MgZe. Reactions taking place are: Na,Ze + Ca(HCO,), = 2Na HCO, + Cae Na,he+ Mg(HCO)), = 2NaHCO, + MeZe NayLe + Ca80,#Na,S0, + CaZe NayZe + CaCl, = 2NaCl+ Cade Regeneration of Zeolite: The process is alvo commercially successful since the Ca/Mg ‘eoltes formed by passing hard water through the bed can be easly regenerated into Na,Ze by passing brine through the bed of inactivated zedite. Cade or Mye + 2NaCl® Na,Ze + CaCl, (Or MgCl) ‘The washings containing CaCl, or MgCl, are wasted, The water softenedby this process can be use for laundry purposes. Fig. 17.3 Scttenng of hard water ty permutt proces, Advantages. () Hardness of water can be removed completely upto about 10 ppm; (i) The ‘equipment used is small and easy to hanale; (ii) It requires less time for softening; (it) There is no sludge formation, hence the process is clean; (x) Easy to regenerate; (i) Any hardness can be removed without any adjustment of the process, Disadvantages, () Coloured water ot water containing suspended impurities cannot be used before filtration; (i) Water containing acid cannot be used for softening since acid may nol ni m360, ENGINEERING CHEMISTRY destroy the zeolite; (i) Since on removal of Ca** and Mg the soft water contains lange amount of NaHCO,, this on heating liberates CO, which causes corrosion inthe beilers and hence this soft water is net suitable for boilers. lon Exchange or Demineralization lon exchange resins are organic polymers which are crosslinked having microporous structure and the functional groups are attached to the chains which are responsible for the jon exchange properties @) Cation exchange resins (RH?) are phenol-sulfonic acid-formaldehyie resin, styrene- divinyl benzene copolymers which exchange their H* ions with the cations present in the water ie, Ca2* and Me". oc, — onc sou] sox aon exchange rst (ii) Anion exchange resins (ROH): The styrene divinyl benzene or amine formaldehyde copolymers contain quaternary ammonium tertiary sulphonium or amino group in the resin, The resin on trestment with NaOH solution is eapable of exchanging the (OIF with different anions of water ie, CI, $0, ete. CH oh —0H, CAH, SHO 09 — Aa cxrmeson’ — CHNME:OH doionexrange ie ‘Method: The hard water is passed first through cation exchange resin similar to the permutit process whereby the cations like Ca*, Mg” are removed from the hard water and exchanged with H* as fellows: RH" +Ca*/Mg"* — R, Ca" Mg + 2H" After this the hard water is again passed through anion exchange column, which exchanges all the anions like $0,*, CI ete. present in the water with OH” ROH-+ Cr — R'C + OH- ‘These H* and OH- combine te form water molecule, Thus the water coming out finally from the two exchangers is ion free and called deiainized or demineralized water. http:/Keralalechnologicaluniversity blogspot com361 WATER TREATMENT Fig. 174 Demineratsatin of water. shyly =e" Me _ ot ery On tr 9 PN 98> 99 Fig. 17 Dornneraisaton of water showing the fon exchanges. aicom362 ENGINEERING CHEMSTRY Regeneration: The inactivated or exhausted cation exchange rosin in regenerated by «dl, H,80 /HCL Ry Cai + 2H —s 2RH" + Cs Similarly, the exhausted anien exchange resin is regenerated by dil. NaOH R, 80 2+ 20H-——+ 2ROH- + 80," ‘The columns are finally washed with deionized water and the washings are discarded, Mixed Bed Deionizer Iti actualy a single cylinder containing and intimate mixture of cation exchanger and strongly basic anion exchanger. Hence, water while passing through the column comes in contact with the two exchangers for a large numberof times and the hardness is reduced to a ‘very low level (1 ppm. Thus, itis actually equivalent toa series ofcation and aninon exchangers, Advantages ofion exchangers include () Easy regeneration; (i) both acidic and alkaline water can be softened; (ii) residual hardness is very low and hence the water is suitable for high pressure boilers also. Disadvantages are () the equipment and the process is costly and (i) turbid water cannot be directly charged for soitening, It needs prier filtration, ‘Treatment of Water for Domestic Use Municipalities supply potable water, which is suitable for drinking and it must satisfy the fellowing requirements: Requirements for Drinking Water: + It should be clear and odourless. + It should be devoid of any unpleasant taste. + Suspended matter should not exesed 10 ppm, + Tt should be devoid of heavy metals and arsenic, «pH should be about § + Content of dissolved salts should not exceed 500 ppin, «+ Lastly, it should be free from pathogenic (disense causing) microorganisms, Natural water from river, lakes, canals aro treated for the domestic purpose. The steps ews: + Acration of the raw water by passing compressed air to remove mainly the obnoxious ‘dour and iron as Fe(OH), 4 Setting ina large tank to remove suspended! impurities, «Sedimentation through congulaticn of the colloidal particles by adding a suitable coagulant. Chemical coagulants are generally (i) Alum (K,80,, Al(SO,)y.24H,0] (i) Given vitriol (PeSO,7H,0) + Coagulants or flacculants are generally added in the form of solution and mixing is carried out by stirring. + The gelatinous precipitates mainly contain aluminium hydroxide, ferrous and ferric hydroxides (ferrous hydrexides are converted to ferric hydroxide by the dissolvedoxygen of water), «These heavy flocculant precipitates contain also some bacterin making the water free cof some of the miroorganisms. + These partially clarified water is filtered through sand gravity filters, hitp.//keralatechnologicaluniversity blogspot.comWATER TREATWENT ES + These filters are rectangular tanks, which contain layers of fine sand at the top of one ameter thick, at middle of caarse sand of 0.5 meter thick and at bottom a gravel of 0.5 meter thick. Sterilization (By addition of bleaching powder (1 per 1000 biloliters). Here sterilization is effected by the HOC! generated by the following reaction: Ca(OCICl + 1,0 —> CwOH), +Cl, C1, +0 —> Hel+ Hoc. and this HOCI kills the germs Disadvantages ofthis process: (@ Bleaching powder makes water unnecessary hard and sometimes imparts bad taste tothe water, (i) By diet chlorination: Here alao the generated HOC kills the pathogenic bacteria Liquid chlorine isthe most effective reagent, .3-0.5 ppm chlorine is sufficient. ‘Advantage of using Chlorine, It is effective and ecenomical; chlorine does not make hard and requires « litle space for storage. isadvantages, Excess chlorine imparts unpleasant taste and odour to the treated water, Distolved chlorine causes irritation to mucus membrane and lowers the pH of water below 6.5. The treated water should contain chlorine less than 0.2 ppm. (i Break-point Chlorination Its seen that when liquid chlcrine is added or chlorine gus is passed through water the consumption of chlorine makes the svailable chlorine less and after some time the available chlorine increases. This is due to the consumption of chlorine in oxidising some oxidisable organic matter present in water at first. Then available chlorine again increases due to decomposition of chloro-orgenic compounds. This free chlorine is responsible for destruction of pathogenic bacteria in water, The point at which the residual chlorine begins to appear is known as break point and at this point water is devoid of bad taste and odours and is bacteria free Advantages of brea point chlerination + Organic matters present in water are completely oxidised leaving behind odour fice ‘water and sometimes colourree water. + Pathogenic bacteria are destroyed completly. # Tt prevents the growth of weed in water in future, (ii Steritzation by Chloramine When chlorine and ammonia are mixed in the ratio 2:1 of volume chloramine is formed. Cl, + NH, —> CLNH, + HCL Chloramine is much mare effective than chlorine in its bactericidal (bacteria killing) action, Here also HOCIis liberated. CLNH, + H,0-—> HOCI+ NH, HOC! itself is bactericidal and hove the liberated nascent axygen is also bactericidal HOCI—+ HCI + [0]. htip:/keralalechnologicaluniversity blogspot com364 ENGINEERING CHEMISTINY The use of chloramine for steriliraton is gaining importance as it does not impart any irritating odour and does not affect the taste ofthe treated water, (iv) Steriization by Ozonization Orone is an unstable gas and decomposes to give nascent oxygen. This nascent oxygen tall bacteria and oxidises the organic matter present in the water, The ozone treatment plant consists of a tower made of enamelled iren, separated by perforated celluloid partition into a large number ef compartments, ozone is passed through bottom and the water is allowed to percolate through the celluloid partition. The perforated partition makes the water into minute bubbles resulting in a counter-current contact with ezene. This makes the treatment most fective Advantages of ozone sterilization are (a) Sterilization, bleaching, decolourisation and deodourisation take place at the same time, (6) Oxene does not impart any unpleasant taste or odour to the treated water and des ‘not change its pH appreciably as it simply decomposes into exygen. So it dees not ‘cause any irritation to the mucus membrane as ie the case with chlorine or Bleaching powder treatment, (6) Time of contact is only 10-15 minuten and dove strength is only 2-3 ppm, ‘The only disadvantage of this proces is that it is comparatively costly. Sterilization through Physical Methods () Boiling, (i) Exposure tothe sunlight, i Dechlorination When during chlorination, break point chlorination is reached, it becomes semetimes necessary to remove the excess chlorine from the treated water, this process is known as dechlorination, Sulphur dioxide is very commen as an antichlor, Other common antichlers tre: sodium bisulphite, sodium thionulphato and hydrogen peroxide, 1, +H,0= Hocl+ HCL Sterilization with UV light, H,0+ 80, = 11,80, 80, + Cl, + 2H,0= 201+ H,80, HOCI + H,80, = HCI + H,80, H,0, + Cl,=2HC1+ 0,¢ Dechlorination is needed to avoid the harmful effects of treated water, Desalination of Brackish Water ‘The water containing dissclved salts and having very salty taste is called brackish water, like the sen water and it is not suitable for drinking. The process of removal of common salt from this brackish water is called desalination. Desalination can be done by the following methods, (a) Blectrodiolysis, This method involves the removal of ions from the brine solution by applying direct current and employing pair of plastic membranes through which ions ean pass, From Fig. 17.6 we ean find that when direct current ia passed through saline water, the Na’ ons move towards the cathode while C- ions move towards the anode through the membrane and collect in the two wide compartments while the central compartment contains pure desalinated water, +htip://keralatechnologicaluniversity, blogspot.comWATER TREATMENT 365 bE Fd Corcontatet Pure Concerts rine ate bare Fig. 17.6 Line diagram of sectodayss ‘To make this process mare effective, ions sensitive membranes are used that allow the passage of either specific entions or specific anions. These membranesare selective na they are ‘made up of materials containing five functional groups. ‘An clectrodialysiscell as shown in Fig, 17.7 contains large number ef pair of membranes ‘and saline water is passed under prossie in electric field applied perpendicular the direction offlowing water, Cations and anions passthrough the membrane pairsand we cbiain alternate streams of pure water and concenteated salt solution Soa tr eee ator ‘ir smonbane morbrane 8 2 amore ‘anode Pure ater outt = Concent «= ‘sa ater ome Fig. 17.7 Electod alysis eal hittp:/keralatechnologicaluniversity blogspot.com366 ENGINEERING CHEMISTRY (b) Reverse osmosis. Osmosis describes the flow of salvent from dilute to concentrated solution through a semipermeable membrane. Whereas Presse reverneontonis describes the flow of solvent in opposite cross a semipermeable membrane by applying hydrostatic ‘The cellulse acetate and the more recently used polymethaerslate and polyamide membranes donet allow the solute pass, while the solvent is ferced thrvtigh and collected as a pure solvent in a direction as shown in Fig. 17.8, There are many advantages ef using reverse oamosis ax the purification procou for water which include () removal of ionic, non-ionic, colloidal and high molecular weight solutes from water, (i) regeneration of tho process involves the easy replacement of the semipermeable membrane, (i) exry maintenance and Fig 174 Reverse oxross cet. cconomical, a8 the membrane lifespan is high, (iv) uninterrupted supply of large volume of water for industrial or domestic purpose can be obtained. Reverse osmosis is largely used for purification of sea water for domestic use Chemical Analysis of Water 1, Estimation of free chlorine, The residual free chlorine remaining after the municipal processing of dmetie water is injurious to health and hence estimation ofthis free chlerine is ‘avential. The principle is the treatment of water containing free chlorine with KI selution ‘The chlorine present liberates an equivalent amount of iodine which ean be estimated with standard thiesulfate solution using starch as an indicate. 1, + 2KI— 2KC1 +1, I, + 2Na,8,0, —+ 2Nal + Na,$,0, I, + Starch —+ Deep-blue complex Method: To 60 mi of water sample in a iodine flask, 10 ml of 10% KI solution added, shaken and waited for some time inthe dark and finally titrated with N50 sodium thiosulfate solution using starch solution as indicator towards the end. The end point indicates a change from deep-blue to just colarlos solution Coleulation: Velume of NI50 Na,8,0, required = V mi Vx35.5x 108 2500x1000” 2, Allealinity. The alkalinity of waters estimated by estimating (a) OH" and CO,2-ions sand (b) HCO; ions, Strength of frve chlorine = @ OH" + HY —» H,0 C0,*+ Ht! —9 HCO; ) HCO; +H’ —+ H,0+ CO, niversiWATER TREATMENT 367 These esti done by (a) titrating against standard acid solution using phenolphthalein as an indicator and (b) using methyl orange as an indicator, Method: 100 ml of water sample is taken in a ask and few drops of phenolphthalein added and titrated against N/S0 H,SO, solution to colerless end point, The same selution ix further titrated with 2 t 3 drops of methyl orange indicator. Caleulation: Ve. of acid upto phenolphthalein end point = V, Exten volume of acid ade to get methyl orange en point —+ V, ml Phenolpbthalein alkalinity (equivalent CaCO,) 2 ¥ix50 x 108 5x 700% 1000 Methyl orange alkalinity (equivalent CaCO) = MLA Ny) 50108 50 100. 1000 P. M 4. Hardness (a) Estimation of temporary hardness. Temporary hardness is due to Ca(HCO,), and Mg(HCO,), which cause alkalinity in water. Tomporary hardness is determined by finding the alkalinity of water before and after boiling, since temporary hardness is removed on boiling cattco), MM, caco, + C0, + 1,0 MgdICO,), —+ Mg(O4, + CO, Method: Inte 50 mi of water sample taken in a conical sk, add 2t 3 drops of methyl orange and titrate with NSO HCL Let the velume of ac be V, Now take 100 lof water sample ina beaker, evaporate to dryness, Add 100ml disilld water ta the beaker and Ghaslve the resid If Then, take 50 ml ofthis water sample ind titrate with N60 HI ting afew drops of methyl orange ns an indestor. atthe volume of ai required be V, ml «allalnity du to temparary hardness in CaCO, equivalent = Wi: V2)x108 ‘5 1000 (0) () Determination of permanent hardness. The estimation is done by adding exceas of standard NaCO, solution to a given volume of boiled water containing permanent hhardness. Th chlrides and sulfates frm insoluble carbonates, The residual Na, CO, istiteated ‘against standard acid and the difference of Na,CO, equivalent gives permanent hardness, Method: 50 ml of the water sample is taken in» beaker. 60 ml of N/60 Na,CO, solution is added and boiled for 15 minutes, Cool and filter the sclution, wash the residue on filter paper and add it tothe filtrate and titrate the NVG0 Na,CO, let in the flask with N/S0 HCI uring methyl orange as indicator, Lat titre vol be V. (50 ~ V) x 10% : an _- (i Complexometric determination of permanent hardness, Disodium salt of ethylene diamine tetra acetic acd (EDTA) is used Permanent hardness =368, EENGNEERING CHENISTRY Erichrome back Tis used as an indicator and pH is maintained at pH 10 using NH,OH! NH,CI buffer. Erchrome black T forms unstable complex with C22", Mg giving wine red colour, When EDTA add and the total Ca™, M* forms complex with it and the indicator becomes free, the colour of the solution changes from wine red to blue at the end point, ‘Method: Indicator solution 0.5 gm. Erichrome black T dissolved in 100 ml alcohol. Standard EDTA solution (N/100); 4 gm of EDTA dissolved with 0.1 gm of MgCl, in 100 mal water Analiquot = 4 “ine acetate solution (primary standard) is taken, to it 5 ml of NHC NH.OH buffer and a few drops of EBT indicator are added and titrated to blue end point with EDEA and the strength of EDTA is calculated. ‘This same procedure is repeated with 50 ml water sample. Let the volume of titrant be Y= () MA00 EDTA solution. Permanent hardness = 200D¥% x10" ppm. ‘The EDTA method for the determination is preferred to other methods since the method hhas greater accuracy, simplicity and rapidness, Bulfer solution: A mixture of 142 ml of cone. NH/OH and 17.5 gm NH,CI is diluted to 250 ml distilled water. Highlights: + Softening methods include: (@) Hot and cold lime seda process (i) Zealite or pernatit proce (Gi) Ion exchange or demineralization. + Micro-organisms from potable water can be removed by: (@ Beiling for 10 to 15 minutes (Gi) Bleaching powder treatment (iii) Chlorination with C1, (jv) Chloramine treatment (0) Ozonization, + Desalination of brackish water dane by: i) Blectrodialysis (i) Reverse osmesi + Exhausted ion-exchange resins are regenerated by: 4 Cation exchangers regenerated by passing strong acid solution through the bed (Gi) Anion exchangers regenerated by passing strong base solution through the bed. + Hardness of water can be estimated by: Permanent harness determined by titrating with standard Na,CO, solution (i) Complexometeie titration by EDTA + Units of hardness are: @ ppm (Gi) mitre (1 ppm = 1 meflitre) (iii) Clark's dogree (1 ppm = 0,07" C) (Gv) degree French (1 ppm = 0.1* Fr). itp:/keralatechnologicaluniversity blogspot.comWATER TREATIENT 369 ‘+ Hard water doos not easily lather with soap, instead it forms a greasy scum. + Water is bard if t contain calcium oF magnesium ions ‘© Scum is a precipitate formed when seap comes in contact with hard water Cat*(ag) + CyHyyCO5(ag)—4 Ca H,,C0,), «Temporary hard water contains bicarbonates er hydtogen caubooates of clei or magoesim, + Toonporary hardness is removed by biling, Ca(HC0,),(aq) = CaCO, L+H,00) +00,7. + The slid cak:ium carbonate is precipitated aa aca, which aflecta heating clement cof boilers and gradually blocks the pipes in heating ayster, + Permanent hardness is not removed by boiling + The minoral gypaum (CaSO,) in slightly soluble in water permanently hard «Jen exchange resin and other methods of softening of water by remo ‘and magnesium jons is known ax water treatment. Problem 1. Calculate temporary hardness and total harduess of a sample of water containing: Mg(HCO,), = 7.5 mg/l. Co(HCO,), = 16 mg/l MgCl, = 9 mg/l. CaSO, = 13.6 mg/ 100 | 4g , 100 Sol. Temporary bandas =| xf 6H | gil 1007 ~) sgl. Problem 2. How many mg. of FeSO, dissolved per litre gives 200 ppm of hardnesst Sol. (66+ 16 + 61)g of FeSO, # 100 ¢ CACO, For 100 ppm hardness FeSO, required per 10# parts of water is 186 parts 1. Por200 ppm Batduess, FeSO, regi = 492%208 = 272 mel Problem 3, A sample of water hasa CO} concentration of 15.6 ppm. What is the molarity, of CO} in the sample of water? Permanent hardness © [ox As x ‘httpu/keralatechnologicaluniversity.blogspat.com370 ENGINEERING CHEMISTRY Sol. 15.6 ppm= 15.6 x 10-¢pil of COP CO g moarty= BOI ay =26%109M. nh sme fee has Rt ead: nnn th foam Co(HCO,), = 10.5 pom: Me(HCO,), * 12.5 ppm: CaCl, = 8.2 ppm; MgSO, = 2.6 ppm; €480,='75 po Coteutate (i) temporary and permanent hardness and (ii) the vol. of Zz EDTA required for titration of the 100 ml ofthe sample ta determine the total hardness ofthe sample. Sol. @) Ca(H1CO,), = 10.5 ppm = 10.5 x x =6481 ppm CaCO, yea, 105 pee 2 sep C00, 100 CaCl, = 82 ppm = 82x TT #7347 ppm CaCO, a 100 MgSO,= 26 ppm = 26% 35> =2167 ppm CaCO, 80,75 ppm = 7.5 x 2 = 5515 ppm CaCO, Temporary hardness = (6.481 + 8.562) ppm = 1503 ppm Permanent hardness = (7.387 + 2.167 + 5.15) ppm = 15.069 ppm © Total hardness += (15,043 + 15.069) = 90.112 ppm (ii) The volume of sample taken = 100 ml 80.112 100 YVoluae of W100 EDTA required = STE Problem . Coleulate the amount of lime ond soda required for softening 50,000 litres of hard water containing MgCO,= HA ppm; CaCO, = 25 ppm: MeCly 95 ppm: CaCl, = 111 ppm; FeO, 25 ppm; Na SO, 15 ppm. aul mb co Sol. MgCO, = 144 pen = 144x = = 171A ppm CaCO, CaC04= 25 ppm. MgCl, = 95 ppm = $4100 «100 ppm CaCO, 111100 CaCl, = 111 ppm= = 100 ppm CxCO, m1 versity blogWATER TREATMENT am + Lime required = EE jaxmgco, + cuC0, + MgC * whane of water um 5 pg 2 * 171A + 2500 + 100.0 » 60,000me = 17,809,000 mg = 17.31 ke, Problem 6. A water sample an onalysis gives the following data : Ca** = 20 ppm, Mg** = 25 ppm, CO, = 30 ppm, HCO, = 150 ppm, K* = 10 ppm. Calculate the lime (87% pure) and soda (91% pure) required to soften 1 million litres of water sample. 20% 100 0 Mg?* = 25 ppm = ae = 104.65 ppm CxCO, Sol. Ca?" = 20 ppm= = 50 ppm CaCO, 0,130 pp = M28 63.18 ppm CaCO, 150% 100 HCO," = 150 ppm = = = 122.95 ppm CaCO, seis i 4 Lime requirement = 7 Me" + CO, + HCO,-] x vl. of water my = Fe oss +6818 + 122.95)» 08 = 218.9% 10 mg = 218.9 ke. Soda requirement = ifew Mg!* - HCO,"] x vol. of water. 106 pp (50+ 108.85 ~ 122.95) » 10° = 33.6 x 104 mg = 33.6kg. Problem 3, An exhausted ot softener was regenerated by passing 180 itres of NaCt sel having strength of 150 g/1of NaCl Ifthe hardness of water is 600 ppm, caleulate the tata volume of water that has been softened in the softener. Sol. 150 litres of NaCI solution contain = 150 * 150 = 22.500 gm NaCl 100 in ca, = 1912 «164, CaCO, [the hardoeas of wate is 600 ppm = 22,500 x Volk of water = litres 600 = 9.129% 104 litres Problem 8. 10 ml of 0.85 (4) 2n (OAC), required mol of an EDTA solution for standardization. 50 ml ofa water sample required 20 ml of the above EDTA solution. Caleulate the total hardness of water in ppm. hite://keralatechnologicaluniversity. blogspot.com372 ENGINEERING CHEMISTRY Sol, Stength of EDTA solution = 422288 w 1000. mt [ 94] epma solution = 1 ¢CaCO, 20m «i i) EDTA solution = aw Foon “20% 084 # CaCO g = 0019 g CaCO, 50 ml ef the water sample contains 0019 g CaCO, 019108 50 10 ml ofthe water sample contains = =3778 gm CaCO, Hardness of water = 377.8 ppm. Problem 8, A sample on wate en analysis hos ben found te contin the ollowing Ca(HCO,), = 10.5 ppm. Mg (HCO), = 12.5 ppm CaSO, = 7.5 ppm CaCl,=82ppm MgSO, = 26 ppm. Coleulate the temperary and permanent hardness in degree French. 105x100 Sol. Ca(HCO,), = 10.5 ppm = = 6.481 ppm 162 1255100 (C0), 125ppm= = £562ppm Me(HCO)),= 125 ppm = TE Pe 481 + 8.569) = 15,043 ppm 143 « 0.1° Fr = 1.508* Fr SHORT QUESTIONS AND ANSWERS Q.1. What is the need for “Desalination of water” programmet ‘Ans. Virtually there is population explosion in the modern world. So for drinking, domestic and irrigation purposes river water and other sweet water sources seem to be insufficient, So for the exploitation ofthe vast source of sea water, desalination programme is required, specially in the seaside area, Q.2. What are the causes of corrosion of boilers? ‘Ans. The causes of corrosion are: ‘¢ Oxygen corresion if boiler water is not properly deaerated. http://Keralatechnologicaluniversity blogspot.comWATER TREATWENT 373 ‘¢ Electrochemical corrosion, also known as out of service corrosion, when boiler is shut down. Q.3, What are the disadvantages of scale formation in a boiler? Ans. 1 Scales and sludges are bad conducters of heat, as a result fuel consumption increases ‘© Tubes and plates are clogged as a result the efficiency ofthe boiler i decrensed @ Tubes of the boiler are corroded by the scales deposited and they may crack and dead to boiler failure MgCl, + Fe + H,O=Mg(OH), + FeCl, +H, Q. 4. Why is demineralisation preferred to zeolite softening? ‘Ans. Demineralized waters practically fee fom all the ions whereas the zelite softened water contains vodium sate which are source of caustic embrittlement, Q.5. Why is chloramine preferred to Cl, for sterilization of drinking water? Ans, It is because chloramine docs not impart any repulsive odour to the treated water, Q.6. What are the criteria for drinking water? Ans. It should be devoid of any suspending matter, and harmful dissolved matter of organic or inorganic matter and pathogenic organisms. Q. 7. Why is UV radiation superior to other methods of sterilization of water? Ans. UV radiation does not impart any unpleasant taste or odour to the water and the process is simple. Q.8. What is pitting? Ans. Pitting is corrosion ofthe boiler. Disselved gases like O,, CO, ,H,S corrode the iron surface of the boiler particularly near the rivet, bends and leads to leak er boiler failure. Q.9. What are boiler compounds? Ans. When the boiler water is difficult to soften or purify, some chemical substances are added to the boiler water that makes the scale in the boiler loose and easily removable. These ‘compounds are called boiler compounds and examples of these are flour, kerosene, tannin, agar-agar ete Q. 10. What is plumbo solvancy? How can it be prevented? Ans, Water transported through lead pipes are contaminated with Pb** due to solubility of metallic lead in water. 2Pb + 2H,0 + 0, = 2Pb(OH), The hydrolysis of Ca?* and Mg"* salts dissolved in water may lead to the formation of lead salts like PbSO, and PbCI,. ‘MgSO, +2H,0=Mg(OH),+H,80, -2Pb + H,80, + 0, = 2Pb80, + 2H,0 MgCl, + 21,0 = MgiOH), + 2HCL 2Ph + AHCl+ 0,=2PbCl, + 2H,0 ‘These lead sats are somewhat soluble in water. This phenomenon is known as plumbe solvaney. ‘The Pt salts are poisonous and is the source of lead poisoning. Lead poisoning can be prevented by treatment of the water with alkali silicate and phosphate followed by filtration. httpu/keralat blogspot.com374 ENGNEERING CHEMISTRY Q.11. Differentiate between sludge and scale. ‘Ans, Sludge is soft, loose, slimy deposit formed inside the beiler, which is easier to remove, while scale isa hard, sticky, adherent deposit onthe inner surface of the boiler which is difficult to remove. Q. 12. What is break-point chlorination? ‘Ans, See text page 362. Q.18. State the harmful effects of silica present in water. ‘Ans. Presence of silica in the water leads to the formation of calcium and magnesium ates and these silicates form sticky scales in the boiler which are very difficult te remove Q. 14. What is the difference between hard water and soft water? ‘Ans, See text page 350-355, Q.15. Alkalinity of water cannot be due to the simultaneous presence of OH", €O,* and HCO;. Why? ‘Ans. OH and HCO; ions react to form CO, ions: OH- + HCO, = CO,- + HO. Q. 16. Why do we express hardness of water in terms of CaCO, equivalent? ‘Ans, This mode permits easy addition and subtraction of concentratien of handness- causing constituents, since its mol. wt. is 100. Moreever, it has also been adopted as standard for expressing hardness Q. 17. What are the salts responsible for temporary and permanent hardness of water? ‘Ans, Temporary hardness = Ca(HCO,), and Mg(HCO,), Permanent hardiness -> Chloride and sulphate of Ca Q. 18. Mention the units of hardness. Define them. ‘Ans. () ppm, (i) degree ‘ppm is defined as the parts of hardness salt present in 10* parts of water expressed in terms of caleium carbonate. Degree hardness is defined as the parts of hardness salt present in 10* parts of water expressed in terms of calcium carbonate Q. 19. A water sample contains lithium chloride or zinc acetate as dissolved solids. Do you think that water will be hard? ‘Ans, Yes, as Lit and Zn both form insoluble soaps and Me". sare ar a san 02000, 120 100g iB ou i | iit a = gy = 2 ea ow 00 PI on So, hardness in ppm 75, x 10 i sage paw Again, hardness in degree = 77> x 10° = 10 htip:/Keralalechnologicaluniversity blogspot comWATER TREATUENT ws Q. 21. The presence of carbon dioxide is avoided in boiler feed water. Explain, Ans, Dissolved carbon dioxide forms carbonic acid and attacks boiler pipes and corredes. Q.22. Why does hard water consume lot of soap? Ans. When hard water comes in contact with soap Ca** and Mg* ions react with soap and scam is formed when the reaction is overconsuming much of stap, lather is then formed, That is why hard water consumes a lt of soap. Q. 28. Why is calgon conditioning hetter than phosphate conditioning? Ans. Calgon conditioning forms soluble complex salt ‘Na,[Na,PO))] + CaSO, = Na,[Na, Ca(PO,) + Na,S0, Cals Sale compe whereas phosphate conditioning Lc, treatment with sodium phosphates, precipitates calcium and magnesium phosphates causing scale in boiler pipes. Q.24. Why is water softened before using in boiler? ‘Ans, Naturalor hard water when fedin eiler produces scale, sludge and eauses priming and foaming and boiler corrosion. That is why water is softened before using in boiler. 25. Why is caustic embrittlement controlled by adding sodium sulphate to boilersfeed water? ‘Ans. Sodium sulphate when used blocks hair cracks thus prevents infiltration of caustic soda through the cracks, Thus seditum sulphate prevents embrittlement in boilers, Q. 26. State two harmful effects of silica in water. ‘Ans, Silica reacts with Ca and Mg® ions forming calcium silicate and magnesium silicate in boiler. This type of scales are diffcalt to remove. EXERCISES: 1 Explain the various steps for the purification of water for municipal supply. 12, What is desalination? Name the diferent methods of desalination ard describe any one ‘3, What are bilertroutlos? Why are they caused? What are the methods of their climination? 4. Distinguish between: (a) Temporary and permanent hardness, (6) Stodge and Seale, (6) Scftening and densineraizatica 45. Write notes on: () Caustie embrittlement, (i) Reverse osmosis, (ii) Priming and foaming, (ie) Hot lime-soda proces, (¢) Boiler earrsin. 6. What is tho principle involved inthe determination of tatsl hardness of water by EDTA method? ‘7. What arethe funetions lime and soda in hot lime soda process? Give equations Why coagulants are not used in hot Hime-soda process? 8 Why does hard water ecneume more snap? 8. During deionization process, water is first passed through eation exchanger and then through anion exchanger, why? Deseribe the methedks of dsinfetion of water. Why is chloramine beter than ehlorine fr stoi lization of water? http:/keralalechnologicaluniversity blogspot com376 10. un. 12 ry “4. 15. 16. 1 18, 19. 20, a. ENGINEERING CHEM STRY OBJECTIVE TYPE QUESTIONS ‘Tompornry hardness of water enn bo removed by (4) Fiteatien () Boiling (6 Coagulation Permanent hardaess of water is caused by (4) Caleium ehlride () Magnesium sulfate (0 Both, Hard water ean be setencd by passing through (4) Calgon (2) Sodium silieste (© lon exchange resin, Calgon i the trade name of (4) Sodium hexame: phosphate (@) Sodium reste (6) Calcium silicate, Pure water ean be abiained by (a) Lime-soda process (2) Boiling (6 Demineralization Brackish wator enn be puriiod by (a) Roverse oxmesis mothod (6 Calgon treatment ‘The chemical that gets dissolved in high pressure boilers (a) Mgsio, () Cac, (@ C080, ‘A somipermeatie mentrane allows the flow of (a) Solute molecules (i Solvent molecules (b) Both solute and solvent molecules, ‘The mont ideal dsinfetant in water works ix (4) Chiorine () Bleaching powder (© Chloranmine. A sample of water centaining NaCl is (4) Hard water () Soft wator Write ashore note with a proper sketch diag brackish water. ‘Why treatment of water is exsential? Write down the posible sources of water? Give reasons for why water becomes hard, (Corteate the various units of the hardnass. ‘Write down the disadvantagos assoriated with using of hart water. (a) What are scales? What ave the illetfects of seales? (0) How ean seale formation be prevented ty () Phosyhateconditioning and (i) Calgon conditioning? Distinguish between: (a) Purification and softening of water (b) Baile scale und sludgox (6) Internal and external treatment of water, ‘What do you mean by eaustic embrittlement? What measures ean be taken to contra the enustie ‘embrittlement? ‘What factors are responsible for boiler corrosion, How can the factors ean be elirinated Deacrite the procoss of water softening by permutit process (0 Pare water ‘on “Yererse oamosit” proces for desalination of hitpu/keralatechnologicaluniversity. blogspot.comWATER TREATWENT am rT 2 uM 3. 26, 1, |. What ae the methods involved for determinaticn of erporary and pe Brey discuss the proces of softening of water by means of ion-exchange proces Write down the roquiremonts fr drinking Briefly desrbe the diffrent steps invelved in the treatment of water fr damosie purpose Wat i dessin? Name the iret methods le in deat’ Dee any nein ‘Write short notes on break point chlorination. (a) What do you mean by extbonate and non-earbonate hardness of water? (¢) State harmful efete of sien prevent in water (©) Write shert notes on aeration. J. Waite different methods of storlization, How free chlorine in wator ean be estimated? ‘Write down the meth for determination of alkalinity of water. ‘The hardness of 1,€0,000 ltrs of « sample of hard water was completely removed by passing ‘through a zeolite softener. The bed an exhaustion required 500 itr of NaC solution containing 1B g Plof NaCl on regeneration. Caleulate the hardness ofthe sample water (Ans. 64.10 ppm} A.ample of bard water has a hardness of 510 ppm. Express its hardness in Clark and “French. [Mints -+ 1 ppm = 007* Clark =0.1* French) HAns 3.1 Clark and 81° Prone 200 ml ofa sample of water required 5 mi of (N/20) sodium carbonate selution for complete precipitation of ealeium ion (Ca!) and calcium carbonate. Calelate the hardness in pn Ans, 62.5 ppm Inn experiment fe the determination of hardness of « sample of water, the fllwing results are obtained: ‘Volume of sample wator = 100 ee. (8) Na,CO, elution is added to it = 20 ce, (38), ind ki rd 0, From thoso data calulate the harness of wator in grains por gallon Given, 1 = 154 grain, [Ans 699 grain of XO.) A precipitate of 0.110 of ealeium oxalate (CaC,0,H,0) was odtainod from n 230 ml of water ‘sample, Express the content of ealeium in (Ans, 120 ppm) Analyst of «sample cf water revenled the content of disslved salts in the following data MglHCO,), = 22 ng t! MgCl, = 30 mg > CaCl, = 85 mg 080, = 28 mg Find the temporary and permanent hardness of the water sample. ans. 15.07 ppm: 128.75 ppm) bhttp/keralatechnologicaluniversity blogspot.com378 10. n ENGNEERING CHEMISTRY 1,00,000 litre of water contain the following: HCL = 7.8 mg F; ALGO), = 34.2 mg PM; MgC, = 9.5 mg hl; NaCl= 29.25 mg P. CCaleulate the amounts of ime and soda tobe required fer softening the water. (Ans. Lime=3.7 kg: Soda =5.3 ke] ‘The total hardness of 1,000 litres of water was completely removed by a zeolite snftner. The eave sotener required 30 litzes of NaCl solution, containing 15 g of NaC! fr regeneration. CCalelate the hardnoss of water. (Ans. 2846 pp] CCeulate the lime and soda required to soften 10,000 litres of water sample having ealeium hhardness = 450 mg as CaCO,; magnesium = 150 mgt as CaCO, and total alkalinity = 400 mg P as CxCO, Plime = 1.11 g soda = 10.5 bg) CCeleulate the hardsess of « water sample, whose 10 ml regured 20 ml of EDTA, 20 ml of calcium chloride olution. whuse strength is equivalent o 1.5 g of calcium carbonate per litre required 30 smlof EDTA seltion. (200 prem CCaleulate the cost of lime and soda required for sftening 1 million litres of a water sample containing. Mg(HCO,), = 78 mg t!: MgSO, =120 mg M; CaSO, = 68 mg F; CaCl, = 111 mg H The cost of lime of #0% purity ix Rs, 200 per metric tn and that af soda of 80% purty is Rx 12,000 per metric tn, http:/Keralatechnologicaluniversity.blogspot. com