Associate Professor (H & S) WATER UNIT IV Dr.JYOTSNA CHERUKURI Associate Professor (H & S)

CONTENTS Sources and impurities of water Hard and soft water Units for expressing hardness Types of hardness Estimation of hardness Disadvantages of usage of hard water Boiler troubles Softening of water Treatment of water used for municipal supply

Sources of water Surface water Rain water River water Lake water and sea water Underground water Spring water and Well water

Impurities present in water Suspended impurities Mainly present in river water Inorganic matter like clay and sand, Organic matter like animal and vegetable matter Suspended matter is responsible for turbidity of water. Dissolved impurities These include inorganic salts like carbonates, chlorides, sulphates of calcium and magnesium, salts of sodium and potassium, other salts like iron, manganese,silica and alumina. Salts of calcium and magnesium cause hardness to water.

Also dissolved gases like CO2, N2, O2,and H2S. Dissolved gases causes corrosion in boilers. Colloidal impurities Very finely divided silica, clay, aluminium hydroxide. Micro-organisms include protozoa and some forms of vegetable life.

Hard water and soft water Hard water is one which does not produce lather with soap solution readily, but forms a curdy precipitate. Hard water contains dissolved calcium and magnesium salts. Soft water gives lather easily on shaking it with soap solution. Dissolved salts are not present in soft water.

Units of hardness Hardness is expressed as Parts per million (ppm) Milligrams per liter (mg/l) Clarke’s degree (oCl) Degree French (oFr) 1ppm = 1 mg/l = 0.1oFr = 0.07 oCl 1mg/l = 1 ppm = 0.1oFr = 0.07 oCl 1 oCl = 1.433oFr = 14.3 ppm = 14.3 mg/l 1 oFr = 10 ppm = 10 mg/l = 0.7 oCl

Types of hardness Hardness is of two types Temporary hardness Permanent hardness Total hardness = temporary hardness + permanent hardness Caused by the presence of dissolved bicarbonates of calcium, magnesium and heavy metals. Can be removed by mere boiling of water

Permanent hardness Caused by the presence of dissolved chlorides, sulphates and nitrates of calcium, magnesium and heavy metals. Cannot be removed by mere boiling of water Chemical treatment processes like lime-soda, zeolite and ion exchange process can remove this hardness

Estimation of hardness Three methods of estimation Soap titration method O.Hehner’s alkalimetric method EDTA method

Soap titration method (Clark’s method) Principle: The hardness is estimated by adding a soap solution of a definite strength to a known volume of a water sample until a permanent lather is formed on shaking. O.Hehner’s alkalimetric method The basic principle is finding the alkalinity of water before boiling and that left after boiling.

EDTA method (Complexometric method) Principle: When hard water comes in contact with EDTA at pH 9-10 (NH4Cl + NH4OH), the calcium and magnesium ions forms stable, colorless complex with EDTA. Ca 2+ or Mg 2+ + EBT Ca-EBT or Mg-EBT (blue) (wine red complex) Ca-EBT or Mg-EBT + EDTA Ca-EDTA or Mg-EDTA (colorless complex) + EBT (blue)

Disadvantages of usage of hard water Water finds great use in a number of industries. Hard water if is used leads to: Paper industry: paper with smooth and glossy finish is not obtained Dyeing industry: incorrect shade and spots on fabric Sugar industry: sugar does not crystallize and is deliquescent Steam production: Boiler troubles

Boiler troubles Carry over: priming & foaming Boiler corrosion Caustic embrittlement Scale and Sludge formation Carry over The phenomenon of carrying water along with impurities by steam is called carry over which is caused by priming & foaming

Priming and foaming Priming Process of wet steam formation is called priming. This is caused by Presence of large amounts of dissolved salts High steam velocities Improper boiler design Sudden boiling High level of water in boilers Avoiding all the above factors in the boilers will prevent priming

Foaming Production of persistent foam or bubbles in boilers leads to foaming. This is due to the presence of substances like oils and alkalies in the boiler feed water. Foaming decreases the life time and efficiency of the machinery It can be prevented by: Adding antifoaming agents Removing oils from boiler water by adding alum or sodium aluminate By blow down operation

Boiler corrosion This is due to the decay or deterioration of the boiler material by direct or electrochemical attack of its environment. It is caused by the presence of Dissolved oxygen Dissolved carbon dioxide and Presence of acids or acidic salts It results in shortening the boiler life, leakage from joints, increased cost of repairs and maintenance.

Boiler corrosion can be prevented by mechanical deaeration,preheating and chemical treatment. Caustic embrittlement Develops if water of high alkalinity is used in boilers operating at high pressures. It can be prevented by Maintaining the pH of water Using sodium phosphate as softening agent instead of sodium carbonate Adding colloidal agents like tannin or lignin, agar-agar etc.

Scale and sludge formation Scales are: Hard deposits Stick firmly to the inner surfaces of boilers Difficult to be removed Formed at heated portions of the boiler Formed by substances like CaSO4,Mg(OH)2etc. Decrease the efficiency of the boiler and chances of explosion are possible

Sludges are: Sludges are soft loose and slimy precipitates Non-adherent deposits Can be removed easily with a wire brush Formed at cooler portions of the boiler Formed by substances like CaCl2,MgCl2,MgSO4 etc. Decrease the efficiency of the boiler and less chances of explosion

Prevention of sludge formation Using water free from dissolved salts By frequent blow-down operation Removal of sludge from time to time Prevention of scale formation This involves 2 types of treatment: Internal treatment External treatment

Internal treatment This involves addition of proper chemicals to the boiler water to precipitate scale forming impurities in the form of sludge, which can be removed by blow down operation. It can be done by the following methods: Colloidal conditioning Phosphate conditioning Carbonate conditioning Calgon conditioning Treatment with sodium aluminate Electrical conditioning Radioactive conditioning.

Colloidal conditioning: addition of kerosene Colloidal conditioning: addition of kerosene. tannin, agar-agar etc to avoid scale formation in low pressure boilers. Phosphate conditioning: complexation of calcium salts with sodium phosphate in boilers operating at high pressures. Carbonate conditioning: strong and adherent scales formed due to CaSO4 are avoided by adding sodium carbonate. Calgon conditioning: sodium hexameta phosphate (calgon) forms soluble complex compounds with CaSO4(stong scale)

Treatment with sodium aluminate: sodium aluminate functions as a coagulant and removes scale forming salts as loose precipitates. Electrical conditioning: electrical discharges prevent scale forming particles from sticking to the walls of the boiler. Radioactive conditioning: radiation emitted from radioactive salts in the form of tablets prevent scale formation.

External treatment This is accomplished by removing hardness producing constituents from water. The techniques employed are: Lime-Soda process Zeolite process and Ion-exchange process This treatment is done before the water is fed into the boilers.

Lime-Soda process Principle: conversion of salts of calcium and Magnesium into insoluble substances which settle down as sludges and can be easily removed. This conversion is brought about by adding lime and sodium carbonate (soda). The process is of two types: Cold lime-soda process (carried at room temperature) Hot lime-soda process (carried at temperatures of 80-150oC)

Advantages: Process is economical Removes hardness causing salts and also iron and manganese salts Pathogenic bacteria is reduced due to alkaline nature of water rendered by adding lime & soda. Disadvantages: Skilled supervision is necessary Disposal of large quantities of sludge Water still contains residual hardness of 15-50ppm.

Zeolite or Permutit process Zeolites are hydrated sodium aluminium silicates,Na2O.Al2O3.xSiO2.yH2O. Principle: The hardness causing salts are retained by the zeolites as CaZe and MgZe and an equivalent amount of sodium salts are introduced into water. Limitations: Raw water should not contain turbidity, colored ions and mineral acids, as all these interfere in effective softening of water

Advantages: Requires less skilled assistance No sludge disposal problems Residual hardness in the water is 10 ppm Disadvantages: Treated water contains more amounts of sodium salts Results in caustic embrittlement Replaces only Ca and Mg ions only Zeolite bed is regenerated by treating with 10% brine solution (NaCl)

Ion-Exchange Process Principle: The exchange of the cations and anions of the dissolved salts with H+ & OH- ions of the ion-exchangers which are insoluble, cross linked long chain organic polymers with micro-porous structure. Exchangers are of two types: Cation Exchangers (capable of exchanging H+ ions) Anion Exchangers (capable of exchanging OH- ions)

Advantages: Highly acidic or alkaline water can be treated Residual hardness of treated water is 2 ppm Disadvantages: Ion exchange resins are very expensive Turbidity more than 10 ppm interferes with the process Always the water should be passed first through the cation exchanger and then only through the anion exchanger…… Regeneration: cation exchanger with dilute acids (HCl or H2SO4) and anion exchanger with NaOH.

Desalination of brackish water Sea water containing excess salts cannot be used for industrial purposes and hence subjected to desalination, which is done by: Electrodialysis Reverse osmosis Electrodialysis Principle: Ions present in saline water migrate towards their respective electrodes through ion selective membranes under the influence of applied emf.

Reverse Osmosis Principle: When two solutions of unequal concentrations are separated by a semi permeable membrane usually the solvent flow takes place from dilute to concentrated side. But if a pressure is applied the reverse happens i.e. the solvent flows from concentrated side to the diluted side. This is the process of reverse osmosis and is also termed as hyper-filtration or super-filtration of water.

Reverse Osmosis

Treatment of water for municipal supply The various stages involved in treatment are: Screening: to remove floating matter Aeration: increase the oxygen content of water Sedimentation with coagulation: to remove the suspended and colloidal matter Filtration: removal of colloidal and suspended impurities not removed by sedimentation, by using rapid and slow sand filters

Screening Boiling

Sterilization & Disinfection: This is done by: Boiling Passing ozone Chlorination using chloramines and bleaching powder The amount of chlorine required to kill bacteria and to remove organic matter is called break point chlorination. Over chlorination after break point is removed by dechlorination by addition of SO2 and sodium thiosulphate.

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