CHE124 Unit 1 Water Treatment PDF

Unit 1 Water and its Treatment Some facts about water Sources of Water Oceans and seas contain 96.5% water, and snow and groundwater hold 1.74% and 1.7%, respectively.  Types of Water  Soft Water  Hard Water Hard Water 2C17H35COONa + CaCl2 (C17H35COO)2Ca + 2NaCl Sod....

Unit 1 Water and its Treatment Some facts about water Sources of Water Oceans and seas contain 96.5% water, and snow and groundwater hold 1.74% and 1.7%, respectively.  Types of Water  Soft Water  Hard Water Hard Water 2C17H35COONa + CaCl2 (C17H35COO)2Ca + 2NaCl Sod. Stearate From hard water Cal. Stearate 2C17H35COONa + MgSO4 (C17H35COO)2Mg + Na2SO4 Hard Water Formation  Hardness of Water DEFINITION: “ The property of water to form an insoluble curd with soap instead of lather. In other words hardness is the soap destroying property due to the presence of Bicarbonates, Sulphates and Chlorides of Calcium and Magnesium.”  Using hard and soft water Soft water forms a lather easily with soap. Hard water needs more soap to form a lather, adding additional cost to cleaning processes. This is because dissolved chemicals in the hard water react with soap to form a scum. Hardness of water: Presence of Ca+2 and Mg+2 ions in water and hard water prevents the formation of lather with soap. Engineering Chemistry Copyright  2011 Wiley India Pvt. Ltd. All rights reserved. In hard water the Ca+2 and Mg+2 ions replaces the Na+ ion from fatty acid salt (in the soap) and make water insoluble fatty acid salt, thus prevent the formation of foam. From soap From hard water Water soluble Water insoluble  Types of hardness Temporary Hardness Calcium Hardness (Carbonated hardness) Total Hardness Permanent Hardness Magnesium Hardness (Non carbonated hardness)  Carbonate hardness – also called temporary water hardness is caused by the presence of carbonates, hydrocarbonates of calcium and magnesium. Noncarbonate hardness – caused by the presence of Ca2+, Mg2+ of other cations and anions like: Al3+, Fe2+, Fe3+, Cu2+, Cl-, SO42-, NO3-, which do not decompose and do not precipitate during boiling water. General water hardness – is the summary content of calcium, magnesium and other ions  Temporary hardness Rainwater becomes slightly acidic as carbon dioxide from the air dissolves in it, forming carbonic acid. Limestone contains calcium carbonate. As the slightly acidic rainwater trickles through rocks, the calcium carbonate reacts to form soluble calcium hydrogen carbonate carbonic calcium calcium carbon +  + + water acid carbonate hydrogen dioxide H2CO3 (aq) + CaCO3 (s) carbonate  Ca(HCO 3)2 (aq) + CO2 (g) + H2O (l) Types of Hardness Temporary hardness: due to presence of bicarbonates and carbonate of calcium and magnesium; can be Removed by boiling. Permanent hardness: due to presence of chlorides and sulphates of calcium and Magnesium; cannot be removed by boiling; removed by softening agents. Engineering Chemistry Copyright  2011 Wiley India Pvt. Ltd. All rights reserved.  Temporary hardness of water (carbonate or alkaline hardness)  due to presence of bicarbonates of Ca & Mg Ca(HCO3)2 Mg(HCO3)2  removal of temporary hardness Boiling Ca(HCO3)2 CaCO3 + H2O + CO2 Mg(HCO3)2 Mg(OH)2 + 2CO2  Permanent hardness of water  due to presence of sulphates & chlorides Ca & Mg MgSO4 MgCl2 CaSO4 CaCl2  removal of Permanent hardness Use of Softening agents Ion exchange method Zeolite/Permutit method etc. CaSO4 + Na2CO3 CaCO3 + Na2SO4 MgCl2 + Na2CO3 MgCO3 + 2NaCl Harmful effects of hard water Degree of hardness: Expressed in terms of CaCO3 equivalent (in mg/l) Hardness of substance in terms of CaCO3 = Strength of substance in mg/l * Chemical equivalents of CaCO3/Chemical equivalents of substance Or Hardness of substance in terms of CaCO3 = Strength of substance in mg/l * Molar mass of CaCO3/Molar mass of substance Degree Clarke ( 0 Cl) = It’s number of equivalent of CaCO3 present in 70,000 parts of water Degree Feench (0F) = It’s number of equivalent of CaCO3 present in 10 5 parts of water Relation 1 ppm = 1 mg/l = 0.10F = 0.070Cl Engineering Chemistry Copyright  2011 Wiley India Pvt. Ltd. All rights reserved.  Degree hardness of water Srength of substance Chemical equivalent producing hardness (mg/l) x of CaCO3 Equivalent of CaCO3 = Chemical equivalent of hardness producing substance  Units of hardness Parts Per million (ppm) Milligrams per liter (mg/l) ? Calculate the temporary hardness in terms of calcium carbonate equivalents in a water sample containing calcium bicarbonate (12.2 mg/l). Given that at. Wt. of Ca=40 amu, O=16 amu, C=12 amu, H=1 amu. Q1 What is the temporary , permanent and total hardness of a 1L of water sample containing 12.2 mg of Ca(HCO3)2 and 5.6 mg of MgSO4 (M.W=120)? Q2 A water sample contains 200 mg of Ca(HCO3)2 per liter. Calculate the hardness in terms of CaCO3 equivalents in mg/l, Degree Clark and degree French? Q3 How many grams of MgCl2 (M.W.=95) dissolved in water will give hardness of 76ppm ? ? Calculate the temporary hardness, permanent magnesium hardness, total permanent hardness & total hardness in terms of calcium carbonate equivalents in a water sample containing calcium bicarbonate (12.2 mg), magnesium bicarbonate (8.2 mg), magnesium sulphate (5.6 mg), magnesium chloride (6.2 mg), calcium sulphate (10.3 mg) & sodium sulphate (7.5 mg). Given that at. Wt. of Mg=24 amu, S=32 amu, Cl=35.5 amu, Ca=40 amu, O=16 amu, C=12 amu, H=1 amu.  Lecture 3 Determination of hardness of EDTA method Complexometric Titration HOOC H2C CH2 COOH N CH2 CH2 N HOOC H2C CH2 COOH EDTA: Ethylenediamine tetraacetic acid  Used for determining the conc. of hardness causing agents Form complexes with metal ions viz. Ca2+, Mg2+ How to determine hardness of water What we need ? ? EDTA solution Hard water (Temporary/Permanent) Erichrome black-T (Indicator-blue, org. dye) Procedure Take known quantity of hard water Add few drops of EBT indicator EDTA added  M2+ EBT complex EDTA wine red colourless Titration M2+ EDTA complex Free EBT colourless blue  Use of ammonia-ammonium chloride buffer  A 100 ml sample of water require 13.5 ml of 0.02 M EDTA for titration using EBT as indicator. Another 100 ml of water from the same source was boiled and precipitate removed by filtration, the filtrate requires 6 ml of 0.02 M EDTA for titration. Calculate the total hardness, permanent hardness & carbonate hardness of water sample. Alkalinity of water Alkalinity: ability of water to neutralize acids Alkalinity of water is due to presence of carbonates, bicarbonates & hydroxides of sodium, potassium, magnesium & calcium, alone or in combination. * However hydroxide and bicarbonate doesn’t exist together. Types CO2 alkalinity: 4.2-4.5 pH CO2 & bicarbonate alkalinity:4.2-4.5 & 8.2-8.4 pH Bicarbonate & carbonate alkalinity: 8.2-8.4 & 9.6 pH Hydroxyl alkalinity: above 9.6 pH Significance:  Lowers the acidity of water  Its amount determines its suitability for irrigation processes or waste water treatment Estimation of Alkalinity:  titration against standard acid solution, in presence of suitable indicators.  volume of acid used up to the phenolphthalein end point marks neutralization of HCO3- ions, P-alkalinity.  volume of acid used up to methyl orange end point indicates OH- ions neutralization, M-alkalinity. OH- H H2O CO3- H HCO3- HCO3- H H2CO3 Procedure  Water sample x ml  Add 2-3 drops of phenolphthalein indicator (pink colour)  Titrate it against standard acid solution till colour get disappear  Volume of titre (V1) is noted  Add 2-3 drops of Methyl orange indicator to solution  Titrate it against standard acid solution till the pink colour develops  The titre value is noted (V2) Phenolphthalein Methyl Orange Formula to calculate * x is conc.or strength  A 100 ml water sample required 25 ml of 0.05N Sulphuric acid for phenolphthalein end point and another 10 ml for methyl orange end point. Determine the nature and amount of alkalinity present in the water. Effect and significance of Alkalinity Alkalinity is important for fish and aquatic life because it protects or buffers against rapid pH changes. Large amount of alkalinity imparts bitter taste in water. The resultant precipitate can corrode pipes and other accessories of water distribution systems. Alkalinity as carbonate and bicarbonate of saline water is very important in tertiary recovery processes for recovering petroleum. The alkalinity value is necessary in the calculation of carbonate scaling tendencies of saline waters. The alkalinity acts as a pH buffer in coagulation and lime- soda softening of water. In wastewater treatment, alkalinity is an important parameter in determining the amenability of wastes to the  Boiler Feed Water Water Absorb heat Expand as steam (carry heat) Eg : power generation, space heating, sterilization, drying mpurities: hardness, dissolved gases, some dissolve salts metal etc,. Requirements depends upon: quantity of feed water, design of boiler, other operating parameters. Low pressure fire-tube boiler Sectioned fire- tube boiler from a DRB Class 50 locomotive. Characteristics and specification of boiler feed water: Impurities should not be exceeding tolerance limit Feed water should be pretreated if it is not meeting the standards/ requirements Dissolved oxygen below 0.02 mg/L Total hardness below 0.5 ppm Fe below 0.03 mg/L, Cu below 0.01 mg/L & Silica 5 mg/L P-alkalinity below 0.5 ppm & M-alkalinity below 1 ppm. Boiler Trouble/ Problem  Precipitation of impurities as scale (Ca(HCO-3) 2, CaSO4, Mg(HCO-3) 2 , MgCl2) or sludge (MgCl2 , Mg CO3 , MgSO4, CaCl2) Scales: Ca(HCO-3)2 CaCO3 + CO2 + H2O Mg(HCO-3) 2 Mg(OH)2 + 2 CO2 Effects of Scales Danger of explosion Lowering the boiler efficiency Waste of fuel Expenses Effects of Sludges:  Colder part may get plugged Wastage of fuel Effect on efficiency Expenses Priming & Foaming Wet steam/ carryover Priming: large drops of water into outlet steam. occur due to: impurities in BFW improper constructed boiler avoided by: lowering impurities, proper construction, maintain proper level of water and slow discharge of steam  Foaming: persistent bubble on boiler water surface Occur due to: alkalis, oils, fats, organic matter & suspended solids Avoided by: antifoaming agents, removal of oils & greases, removal of clay & suspended particles, removal of silica with ferrous sulphate etc. Caustic Embrittlement  Boiler material become brittle due to exposure to caustic solution H2O + Na2CO3 CO2 + NaOH Alkaline water steeps inside the cracks, NaOH react to BFW surface and cause embrittlement 2Fe + 2NaOH + O2 NaFeO2 + H2  May cause explosion Prevention  Using Sod. Phosphate  Adding lignin & tanning  Addition of Sod. Sulphate to avoid cracks Maintaing pH between 8 & 9 Boiler Corrosion Decay process due to dissolve gases 2Fe + H2O + O2 2Fe(OH)2 4Fe(OH)2 + O2 2[Fe2O3.2H2O] Softening Methods: External Treatment: Lime soda process Ion exchange process Zeolite process Internal Treatment: Treatment via chemicals (colloidal, calgon, carbonate & phosphate condition)  Lime: to remove temporary hardness Removal of Carbonate Hardness CaCO3 is precipitated out of the water (sludge) and it’s filtered off. Removal of Noncarbonate Hardness Lime is used to remove temporary hardness + Mg-permanent hardness, Soda Ash is used mainly to remove permanent hardness. Soda ash: to remove Ca permanent hardness Na2CO3 + CaCl2 CaCO3 + 2NaCl Na2CO3 + CaSO4 CaCO3 + Na2SO4  Lime-Soda Softening method Batch Softening ( Lime and Continuous Softening ( Lime soda and soda Both are mixed together with are added to hard water into hard water tank. After different compartment.) completing of reaction, the sludge is filtered off) Advantage: It not only reduce hardness but also reduce the TDS, alkalinity. Disadvantage: It cannot remove the water hardness completely. Ion exchange process Also known as demineralization of water Ion exchange resins are high molecular weight cross-linked, porous polymers  Ion exchange property is due to acidic (R-SO3H, R-COOH) or basic (R-N- OH, R-NH3OH or R-R’-NH2OH) functional groups. Acidic gr. exchange H+ or Na+ Basic gr. Exchange OH- Regeneration Advantage:  easy to operate and control  works well for variety of hardness of water  takes less space, better quality and more economical Disadvantage:  corrosion may occur when cation exchange takes place with H exchanger  Na exchanger when used TDS, alkalinity & silica are not reduced  Ion exchange material should not be allowed to get contaminated Internal Treatment for Softening Water: (Carried out inside boiler) Complexation Calgon (Sodium Hexametaphosphate) Treatment Carbonate Treatment Lime requirement = [ temp. hardness of Ca + 2. temp hardness of Mg + Permanent hardness of Mg]. Molar mass of Ca(OH)2/Molar mass of CaCO3 Soda requirement = [ Permanent hardness]. Molar mass of Na2CO3/Molar mass of CaCO3 Q. Calculate the lime and soda required for softening 50,000 l of water containing the following salts: Ca(HCO3)2 = 8.1 mg/l, Mg(HCO3)2 = 7.5 mg/l and CaSO4 = 13.6 mg/l, MgSO4 = 12 mg/l, MgCl2 = 2 mg/l Reverse Osmosis Process It results in conc. Of all solutes In one compartment and all pure Water in another Electro Dialysis (ED) is a membrane process, during which ions are transported through semi permeable membrane, under the influence of an electric potential. The membranes are cation (+) or anion (-) selective, which basically means that either positive ions or negative ions will flow through. Water Disinfection Process Properties: Should kill microorganism Inexpensive Non toxic to human Should provide protection against any contamination Bleaching Powder: CaOCl2 + H2O Ca(OH)2 + Cl2 Cl2 + H2O HCl + HOCl  Chlorination Most common Employed directly Advantages: low cost & effective stable, small space required work at range of temperature doesn’t introduce any impurity Disadvantages: c Chlorine residue could be harmful to environment bad taste and odour imparts irritation in mucus membrane not effective at higher pH  Chloramine (ClNH2) 2NH3 + Cl2  2ClNH2 + H2 ClNH2 + H2O  HOCl + NH3 HOCl  HCl + [O]