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Water Treatment

Water Treatment. Sources of water 1.Surface water- rivers, lakes, reservoirs etc. 2.Underground water – wells and springs 3.Rain water 4. Sea water. Surface water. River water – dissolved minerals Cl - , SO 4 2- , HCO 3 - of Na+, Mg 2+ , Ca 2+ and Fe 2+

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Water Treatment

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  1. Water Treatment Sources of water 1.Surface water- rivers, lakes, reservoirs etc. 2.Underground water – wells and springs 3.Rain water 4. Sea water

  2. Surface water • River water – dissolved minerals Cl-, SO42-, HCO3- of Na+, Mg2+, Ca2+ and Fe2+ suspended impurities- Organic matter, sand, rock composition is NOT constant – dep on the contact with soil. • Lake water: High in organic and less in minerals. composition is constant.

  3. Rain water – pure form dissolved organic and inorganic particles and dissolved industrial gases CO2, NO2,SO2 etc • Underground water- free from organic impurities due to filtering action of the soil • Sea water– very impure; too saline for industrial use except cooling

  4. Impurities in water • Suspended impurities inorganic (clay, sand) organic (oil,plant, and animal matter) • Colloidal impurities- finely divided silica and clay • Dissolved impurities – salts and gases • Microorganisms – bacteria, fungi and algae

  5. Hardness of water • Hardness prevents the lathering of soap. due to the presence of salts of Ca, Mg, Al, Fe and Mn dissolved in it. Soap – Na or K salts of long chain fatty acids C17H35COOH 2C17H35COONa + CaCl2→ (C17H35COO)2Ca↓ + 2NaCl 2C17H35COONa + MgSO4→ (C17H35COO)2Mg↓ + Na2SO4

  6. The Cleansing Action of Soap 12.8

  7. Hard Water Does not produce lather with soap Contains Ca and Mg salts Soap is wasted and cleaning quality is depressed Boiling point elevated, more time and fuel for cooking Soft Water Produces lather easily with soap Does not contain dissolved Ca and Mg salts Cleaning quality of soap not depressed. Less fuel and time required for cooking • Hard Water Does not produce lather with soap Contains Ca and Mg salts Soap is wasted and cleaning quality is depressed Boiling point elevated, more time and fuel for cooking • Soft Water Produces lather easily with soap Does not contain dissolved Ca and Mg salts Cleaning quality of soap not depressed. Less fuel and time required for cooking

  8. Types of Hardness • Temporary Hardness- caused by dissolved bicarbonates of Ca and Mg Also known as ‘alkaline or carbonate hardness’ • Permanent Hardness – dissolved Cl- and SO42- of Ca, Mg, Fe and Al etc

  9. Temporary Hardness caused by dissolved bicarbonates of Ca and Mg Temporary hardness can be removed by boiling of water Ca(HCO3)2→ CaCO3↓ + H2O + CO2↑ Mg(HCO3)2→ Mg(OH)2↓ + 2 CO2↑ Also known as ‘alkaline or carbonate hardness’ Determined by titration with HCl using methyl orange as indicator

  10. Permanent Hardness CaCl2, MgCl2, CaSO4, MgSO4, FeSO4, Al2(SO4)3 Cannot be destroyed on boiling the water Also known as non-carbonate or non alkaline hardness non alkaline hardness = Total hardness – alkaline hardness

  11. Advantages Tastes better Ca in water helps produce strong teeth and bones Hard water coats lead pipes with layer of insoluble CaCO3, preventing any poisonous lead dissolving in drinking water Disadvantages no taste, produces scum with soap Boiler feed water should be free from hardness or even explosions can occur Hard Water

  12. Degree of Hardness • Hardness is expressed as equivalent amount (equivalents) of CaCO3 Reason: Molar mass is exactly 100, and is the most insoluble salt that can be precipitated in water treatment. Equvalents of CaCO3 = ( mass of hardness producing substance in mg/L) x100 / (eq.wt of substancex2) units – mg/L = ppm parts of CaCO3 equivalents in hardness in 106 parts of water

  13. Equivalent weight • Eq. wt = Molar mass/ no of charge on ion CaCO3 MM/2 NaCl MM/1 AlCl3 MM/3 Al2(SO4)3 MM/6

  14. Example 1: • A water sample contains 408 mg of CaSO4 per liter. Calculate the hardness in terms of CaCO3 equivalents Hardness = mg/L of CaSO4 x 100/MM(CaSO4 ) = 408 mg/L x 100/136 = 300 mg/L = 300 ppm

  15. Example 2 • How many grams of MgCO3 dissolved per liter gives 84 ppm of hardness? Hardness = mg/L of MgCO3 x 100/MM(MgCO3) 84 ppm = ppm of MgCO3 x 100/84 ppm of MgCO3 = 84 ppm x (84/100) = 70.56 ppm = 71 mg/L

  16. Calculation of hardness caused by each ion. Na+ - 20 mg/L Ca2+ - 15 mg/L Mg2+ - 10 mg/L Sr2+ - 2 mg/L Al3+ - 0.3 mg/L Equvalents of CaCO3 = ( mass of hardness producing substance in mg/L) x100 / (eq.wt of substancex2) Cation Eq.wt Hardness Ca2+ 40.0/2 37.5 Mg2+ 24.4/2 41.0 Sr2+ 87.6/2 2.3 Al3+ 27.0/3 1.7 Total hardness = 82.5 ppm

  17. Potable Water (Drinking water) • Colorless and odorless; good in taste • Turbidity should be less than 10 ppm • No objectionable dissolved gases like H2S or minerals such as Pb, As , Cr, Mn salts. • Alkalinity should not be high; pH 7.0 – 8.5 • Total hardness less than 500 ppm • Free of harmful microorganisms. • Cl-, F-, and SO42– less than 250, 15 and 250 ppm, respectively

  18. Methods of disinfection of water 1. Bleaching powder (CaOCl2) CaOCl2+H2O → Ca(OH)2 + HCl + HOCl Enzymes of microorganism get deactivated by HOCl • Excess imparts bad taste and smell • Not stable during storage • Introduces Ca to water and thus increases hardness

  19. 2. Chlorination • Commonly used disinfectant in water used directly as a gas or conc. solution. It produces HOCl, a powerful germicide. 0.3 0.5 ppm chlorine is sufficient

  20. 3. Disinfection by ozone • O3→ O2 + O oxygen atom is a powerful oxidizing agent. 2 – 3 ppm is injected 10 – 15 min contact time Expensive method

  21. Alkalinity • The capacity of water accept H+ is called alkalinity • The basic species responsible are • HCO3- + H+→ H2O • CO32- + H+ → HCO3- • OH- + H+ → H2O Different from basicity; high pH pH is an intensity factor alkalinity is a capacity factor 1.00x10-3 M NaOH - pH=11;neutralize 1.00x10-3 mole acid 0.100 M NaHCO3 - pH = 8.34, 0.100 mole acid

  22. Alkalinity of water is attributed to presence of i. caustic alkalinity (due to OH- and CO32- ions) ii. Temporary hardness (due to HCO3- ions) i. [OH-] + [H+] → H2O -P -M ii. [CO32-] + [H+] → [HCO3- ] -P -M iii. [HCO3- ] + [H+] → H2O + CO2 -M P = OH- + ½ CO32- M = OH- + CO32- + HCO3-

  23. Biological oxygen demand (BOD) • BOD is the quantity of dissolved O2 required by aerobic bacteria for oxidation of organic matter under aerobic conditions. source of effluentBOD(ppm) Domestic sewage 320 Cow shed sewage 3010 Paper mill 8190 BOD indicator of organic pollutants

  24. Chemical oxygen demand (COD) • Defined as the oxygen consumed in the oxidation of organic and oxidizable inorganic matter. Use a strong oxidizing agent like K2Cr2O7 COD > BOD (O2 is a weak oxidizing agent) COD test does not differentiate between bio-inert and bio degradable materials

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