Water Treatment Technology PDF

 Introduction  For the existence of all living things water is essential. Without water we cannot survive. Almost all human activities domestic, agricultural and industrial demand use of water.  Water from any source has to be treated before its use. The treatment to which...

 Introduction  For the existence of all living things water is essential. Without water we cannot survive. Almost all human activities domestic, agricultural and industrial demand use of water.  Water from any source has to be treated before its use. The treatment to which it is subjected depends upon its use.  If it has to be used for drinking purposes, the treatment would include removal of objectionable colour, taste and pathogenic micro organisms, whereas the water for industrial use require the removal of dissolved salts if it is used for steam generation.  In this chapter we will discuss about the analysis of water and treatment of water for its industrial and domestic use.  Sources of water  Surface water :-The water which comes from surface through rain. eg :- rain water, river water & sea water.  Underground water:- This water comes from rain that falls on earths surface & then goes into the ground water & travel down the impervious (cracks) layers of earth, thus forming ground water. eg :- spring water & well water.  Impurities in Water Silica, clay etc CO2, O2, H2S Bacteria & other Micro-organisms like viruses & fungi Carbonates, Bicarbonates, Chlorides & Sulphates of Ca, Mg.  Types Of Water HARD WATER SOFT WATER  HARD WATER SOFT WATER Does not form lather with soap Forms lather with soap easily easily Contains dissolved salts of Ca & Does not contain dissolved salts Mg of Ca & Mg More wastage of time & fuel as Less wastage of time & fuel boiling temp. of water gets increased due to impurities More consumption of soap by Less consumption of soap by soft hard water water  Hardness of water  Hardness is the soap consuming capacity of water  Hardness of water is due to the presence of Ca and Mg salts in it. Other ions responsible for hardness are Al3+, Fe3+ and Mn2+  If Ca and Mg salts are present in water then they react with the soluble sodium soap to form insoluble salts calcium and magnesium.  2C17H35COONa + CaSO4--- (C17H35COO)2Ca + Na2SO4 Sodium stearate Insoluble salt  2C17H35COONa + MgCl2--- (C17H35COO)2Mg + 2NaCl  Types Of Hardness TEMPORARY PERMANENT  Temporary Hardness  Caused by the presence of dissolved bicarbonates of Ca, Mg. This hardness is also known as alkaline hardness.  Easily removed by heating : Heat  Ca(HCO3)2 CaCO3 + H2O + CO2 Heat  Mg(HCO3)2 Mg(OH)2 + CO2  Permanent Hardness  Due to the presence of SO4, Cl2 & NO3 of Ca & Mg  Cannot be removed by simply boiling.  Special methods like lime soda process, zeolite process, ion-exchange method are used for the removal of permanent hardness.  Degree of Hardness in terms of CaCO3 equivalent  Hardness is expressed in terms of CaCO3 equivalents.  Reasons for choosing CaCO3 as the reference standard for calculating hardness of water is :  mol. wt. is 100 that makes mathematical calculation easier.  The most insoluble salt and can be easily precipitated in water treatment process.  How to calculate harness in terms of CaCO3 equivalent Hardness in terms of CaCO3 equivalents Mol. Mass of CaCO3 = wt. of hardness producing sub. X Mol. Mass of hardness producing substance  Units of Hardness a) parts per ppm parts of CaCO3 equivalent hardness per 106 parts of million water. b) Milligrams per Mg/l no. of milligrams of CaCO3 equivalent hardness per litre litre of water. c) Degree Clark °Cl parts of CaCO3 equivalent hardness per 70,000 parts of water.. d) Degree French °Fr parts of CaCO3 equivalent hardness per 105 parts of water. Relation between various units of hardness 1ppm = 1mg/l = 0.1°Fr = 0.07 °Cl  Boiler feed water  For steam generations, boilers are used  if hard water is fed to the boiler, various problems are faced by boiler :  Scale and Sludge formation  Priming and Foaming  Boiler corrosion  Caustic embrittlement  Sludge & Scale  Continous evaporation of water takes place & conc. of dissolved salts gets increased & at saturation point forms ppts. on the inner walls of the boiler.  Sludge : If loose & slimy precipitates formed.  Scale : If sticky, hard & adherent coat formed.   Sludge Scale  Sludge  FORMATION:  Where flow of water is slow  At colder region  By substances which have greater solubility in the hot water.  MgCO3, MgCl2, CaCl2, MgSO4 etc.  DISADVANTAGES:  Poor conductor of heat hence more consumption of time and fuel.  Disturbs functioning of boiler & settles in the regions of poor water circulation.  PREVENTION :  By using soft water  By using blow down pipe operation.  FORMATION : Scale  Decomposition of Ca(HCO3)2 : Ca(HCO3)2 CaCO3 + H2O + CO2 Soft Scale CaCO3 + H2O Ca(OH)2 + CO2  Deposition of CaSO4 : Soluble in cold water As temp. solubility of CaSO4 hard scale  Hydrolysis of Mg salts MgCl2 + 2H2O Mg (OH)2 + 2HCl  Presence of (SiO2)  Scale  DISADVANTAGES  Fuel Wastage  Lowering of boiler safety  Decreased efficiency  Danger of explosion  Removal  Using wire brush  By using chemicals : CaCO3 scales by 5-10% HCl CaSO scales by EDTA 4  Blow down pipe operation  By giving thermal shocks  Scale  Prevention  External Treatment By using soft water  INTERNAL TREATMENT Colloidal conditioning :  Addition of organic substances such as tannin, Agar- Agar Phosphate conditioning : o CaCl2 + Na3PO4 Calcium phosphate + 6NaCl Carbonate conditioning o CaSO4 + Na2CO3 CaCO3 + Na2SO4 Calgon conditioning o CaSO4 + calgon Soluble complexes of Ca ions Treatment with sodium aluminate : o NaAlO2 + 2H2O Al(OH)3 + NaOH o MgCl2 + NaOH Mg(OH)2 + NaCl 2CaSO4 + 6(NaPO3) calgon Soluble complexes of Ca ions  Difference between Sludge & Scale Sludge Scale Soft, loose & slimy precipitates. Hard deposits. Non-adherent deposits & can be easily Stick very firmly to the inner surface of boiler removed. and are very difficult to remove. Formed by substances like CaCl2, MgCl2, Formed by substances like CaSO4, Mg(OH)2, MgSO4 & MgCO3. CaCO3 & CaSio3. Formed generally at colder portions of the Formed generally at heated portions of the boiler. boiler. Decrease the efficiency of boiler but are Decrease the efficiency of boiler & chances of less dangerous. explosions are also there.  Priming and Foaming  Priming  process of making wet steam when some of liquid particles are carried along with steam  Cause  Presence of dissolved salts  high steam velocity  Sudden boiling  Sudden increase in steam production  Foaming  Formation of bubbles in the boiler continuously  Cause  presence of oil that reduces the surface tension  Priming & Foaming  Disadvantages  Reduce the efficiency  Difficult to maintain proper pressure  Wastage of fuel  Actual water level can not be accessed  Prevention  Removal of priming foaming substances  Removal of Scale & sludges  Avoid rapid changes in steaming rate  Change of boiler water from time to time  Using antifoaming agents e.g. castor oil  Addition of a chemical NaAlO2 to remove salts  Boiler Corrosion  The chemical or electro-chemical eating away of metal by its environment in a boiler  Cause Dissolved Oxygen :  2Fe +2H2O + O2 2Fe(OH)2 + O2 2(Fe2O3.2H2O) Dissolved CO2 :  CO2 + H2O H2CO3 Acids from dissolved salts :  MgCl2 + 2H2O Mg(OH)2 + 2HCl  Fe + HCl FeCl2 + H2  FeCl2 + 2H2O Fe(OH)2 + 2HCl  Boiler Corrosion  Disadvantages  Shortening of boiler life  Leakages of joints and rivets  Increased cost of repairs and maintenance  Removal of boiler corrosion : Removal of O2 : 2Na2SO3 + O2 2Na2SO4 N2H4 + O2 N2 + 2H2O Removal of CO2 : 2NH4OH + CO2 (NH4)2CO3 + H2O Removal of acids : By adding alkali  Caustic Embrittlement  formation of brittle and irregular hair cracks in the boiler shell due to the accumulation of caustic substances Heat Na2CO3 + H2O 2NaOH + CO2 Fe + 2NaOH Na2FeO2 + H2 Sodium ferroate  Causes  Sodium carbonate is used in softening of water by lime soda process, due to this some sodium carbonate maybe left behind in the water.  As Conc. of NaOH increases, water flows into minute hair cracks.  Water get evaporated and NaOH increases further and react with iron of boiler, hence cause Embrittlement.  Process  Na2CO3 used for softening of water & some of which remain unreacted Na2CO3 + H2O 2NaOH + CO2 Fe + 2NaOH Na2FeO2 + H2 Sodium ferroate 3Na2FeO2 + 4H2O 6NaOH + Fe3O4 + H2  As Conc. of NaOH increases, water flows into minute hair cracks by capillary action.  As water evaporates, conc. of NaOH increases further and react with iron of boiler, (thereby dissolving Iron of boiler as Sodium ferroate), hence cause Embrittlement.  This causes embrittlement of boiler parts such as bends, joints, reverts etc.  prevention :  Use of Na3PO4 instead of Na2CO3  By adding tanin & lignin that blocks the hair cracks  By adding NaSO4 that also blocks the cracks  Softening Methods The following methods are used :  Lime soda Process  Zeolite softening process  Ion exchange process 28  Lime – Soda Process  Treatment of water with calculated amount of lime Ca(OH)2 & Soda (Na2CO3) which results in the formation of insoluble ppts. of Ca & Mg that can be removed by filteration. Cold Lime Soda  Types Hot Lime Soda Cold lime soda process Hot Lime Soda Process Cold Lime-Soda Method Hot Lime-Soda Method Carried out at room temperature. Carried out at high temperature, almost at b. pt. of water. Slow process Fast process Coagulant Al2(SO4) is added No need of Coagulant. Dissolved gases are not removed. Dissolved gases are removed. water obtained is of hardness 60 water obtained is soft of hardness 15-20 ppm. ppm. Advantages of Lime – soda process:  Economical  Process increses pH value of the treated water, thereby corrosion of the distribution pipes is reduced.  Mineral content of water is reduced  pH of water raises thus reducing content of pathogenic bacteria  iron & Mn: removed. Disadvantages of Lime – soda process:  Huge amount of sludge is formed and its disposal is difficult  Due to residual hardness, water is not suitable for high pressure boilers  2. ZEOLITE OR PERMUTIT PROCESS Zeolite - hydrated sodium alumino silicate, capable of exchanging reversibly its sodium ions for hardness- producing ions in water. The general chemical structure of Zeolite: Na2O.Al2O3.xSiO2.yH2O (x = 2-10 and y = 2-6) Micro pores of Zeolite 43 Porous structure of Zeolite  Two types: (i)Natural Zeolite: non-porous. e.g., Natrolite (ii)Synthetic zeolite: porous & possess gel structure. They are prepared by heating together china clay, feldspar and soda ash. Zeolites possess higher exchange capacity per unit weight 44 than natural zeolite. Process: Hard water is percolated at a specified rate through a bed of zeolite, kept in a cylinder. The hardness-causing ions are retained by the zeolite as CaZe and MgZe; while the outgoing water contains sodium salts. To remove temporary hardness Na2Ze + Ca(HCO3)2 CaZe + 2NaHCO3 Na2Ze + Mg(HCO3)2 MgZe + 2NaHCO3 To remove permanent hardness Na2Ze + CaCl2 CaZe + 2NaCl Na2Ze + MgCl2 MgZe + 2NaCl  REGENERATION OF ZEOLITE: Atthis stage, the supply of hard water is stopped and exhausted zeolite is reclaimed by treating the bed with concentrated NaCl solution. CaZe or MgZe + 2NaCl Na2Ze + CaCl2 (exhausted zeolite) (Brine) (Reclaimed zeolite) (washings) 47  LIMITATIONS OF ZEOLITE PROCESS  If the water is turbid ---- then the turbidity causing particles clogs the pores of the Zeolite and making it inactive  The ions such as Mn2+ and Fe2+ forms stable complex Zeolite which can not be regenerated that easily as both metal ions bind strongly and irreversibly to the zeolite structure.  Any acid present in water (acidic water) should be neutralized with soda before admitting the water to the plant, since acid will hydrolyze SiO2 forming silicic acid 48  ADVANTAGES:  Residual hardness of water is about 10 ppm only  Equipment is small and easy to handle  Time required for softening of water is small  No sludge formation and the process is clean  Zeolite can be regenerated easily using brine solution  Any type of hardness can be removed without any modifications to the process. DISADVANTAGES :  Soft water contains more sodium salts than in lime soda process  It replaces only Ca2+ and Mg2+ with Na+ but leaves all the other ions like HCO3– and CO32- in the softened water (then it may form NaHCO3and Na2CO3 which releases CO2 when the water is boiled and causes corrosion)  It also causes caustic embitterment when sodium carbonate hydrolyses to give NaOH  ION EXCHANGE PROCESS:  Ionexchange resins are insoluble, cross linked, long chain organic polymers with a microporous structure, and the functional groups attached to the chain is responsible for the “ion-exchange” properties.  Acidic functional groups (-COOH, -SO3H, etc.) exchange H+ with other cations.  Basic functional groups (-NH2=NH etc.) exchange OH- with other anions. 50  Classification of Resins  A. Cation-exchange Resins(RH+) : Strongly acidic (SO3-H+) and weakly acidic (COO-H+) cation exchange resins 51 2. Anion Exchange resin (ROH-) – Strongly basic (R4N+OH-) and weakly basic (RNH2+OH-) anion exchange resins  Process or Ion-exchange mechanism involved in water softening Reactions occurring at Cation exchange resin 2 RH+ + Ca2+ (hard water) R2Ca2+ + 2 H+ 2 RH+ + Mg2+ (hard water) R2Mg2+ + 2 H+ Reactions occurring at Anion exchange resin 2 ROH- + SO42- (hard water) R2SO42+ + 2 OH- 2 ROH- + Cl- (hard water) R2Cl- + 2 OH- At the end of the process H+ + OH- H2 O  Regeneration of ion exchange resins Regeneration of Cation exchange resin R2Ca2+ + 2H+ (dil. HCl (or) H2SO4) 2 RH+ + Ca2+ (CaCl2, washings) Regeneration of Anion exchange resin R2SO42- + 2OH- (dil. NaOH) 2 ROH- + SO42- (Na2SO4, washings) Advantages 1. The process can be used to soften highly acidic or alkaline waters 2. It produces water of very low hardness of 1-2ppm. So the treated waters by this method can be used in high pressure boilers Disadvantages 1. The setup is costly and it uses costly chemicals 55 2. The water should not be turbid and the turbidity level should not be more than 10ppm  Softening of water by Mixed Bed deioniser Description and process of mixed bed deionizer 1. It is a single cylindrical chamber containing a mixture of anion and cation exchange resins bed 2. When the hard water is passed through slowly in this bed the cations & anions of the hard water comes into contact with the two kind of resins many number of times. 3. Hence, it is equivalent to passing the hard water many number of times through a series of cation and anion exchange resins. 4. The soft water from this method contains less than 1ppm of dissolved salts and hence more suitable for boilers Hard water c a c a Anion exchange resin c Mixed bed a deionizer a Mixed resin bed a c a cc Cation exchange 56 resin Demineralised water  Regeneration of mixed bed deionizer 1. When the bed (resins) are exhausted or cease to soften the water, the mixed bed is back washed by forcing the water from the bottom in the upward direction 2. Then the light weight anion exchanger move to the top and forms a upper layer above the heavier cation exchanger 3. Then the anion exchanger is regenerated by passing caustic soda solution (NaOH) from the top and then rinsed with pure water 4. The lower cation exchanger bed is then washed with dil.H2SO4 solution and then rinsed. 5. The two beds are then mixed again by forcing compressed air to mix both and the resins are now ready for use Low NaOH density resin c a c a c a c a c aaca aa a a a a cRegenerated c Mixed bed c Exhausted Back washed a a deionizer a a Mixed bed a Mixed bed a a cccccc deionizer a c a cc c a cc c a c c 57 Back Compressed air wash High water density resin Domestic water treatment  Drinking Water Specification  Colorless & odorless; good in taste  Turbidity should be less than 10 ppm  Low nitrate, nitrite (